biomechanics of space closure in orthodonticcs / fixed orthodontics courses


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biomechanics of space closure in orthodonticcs / fixed orthodontics courses

  1. 1. Biomechanics of Space Closure INDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. Biomechanics of Space Closure Introduction Every object or free body has one point on which it can be perfectly balanced. This point is known as the center of gravity In a restrained body, such as a tooth, a point analogous to the center of gravity is used; this is called the center of resistance. By definition, a force with a line of action passing through the center of resistance produces translation. The center of resistance of a single-rooted tooth is on the long axis of the tooth, probably between one third and one half of the root length apical to the alveolar crest. For a multirooted tooth, the center of resistance is probably between the roots, 1 or 2 mm apical to the furcation.
  3. 3. Biomechanics of Space Closure Relevance of the center of resistance First, the position of the center of resistance varies with root length The tooth movement resulting from a force delivered at the bracket depends upon the distance of the line of action of the force from the center of resistance; Identical forces applied to teeth with different root lengths can have different effects. A second important point is that the center of resistance varies with alveolar bone height. The movement of teeth in adults with alveolar bone loss will be different than in adolescents. Smith and Burstone AJO-DO 1984
  4. 4. Biomechanics of Space Closure The M/F ratio is the relationship between the applied force and the counterbalancing couple The type of movement is dictated by the moment to force ratio (M/F) generated by the appliance at the attachments. Typically, M/F ratios of approximately 7:1 millimeters result in controlled tipping, 10:1 millimeters result in translational movements, and values of 12:1 millimeters or greater accomplish root movement. This has important implications. It is the ratio between the applied couple and force that determines the type of tooth movement, not the absolute magnitudes.
  5. 5. Biomechanics of Space Closure These ratios are based on the assumptions that the root lengths are 12 millimeters, the distance from the bracket slot to the alveolar crest is five millimeters, the alveolar bone condition is normal, the axial inclination of the teeth is normal, and the center of resistance is located apically a distance .40 times the root length when measured from the alveolar crest to the apex. Manhartsberger, Morton, Charles J Burstone Angle 1989
  6. 6. Biomechanics of Space Closure STATICALLY DETERMINATE AND INDETERMINATE SYSTEMS Force systems can be defined as statically determinate when the moments and forces can be readily determined, measured and evaluated . Statically indeterminate systems are too complex for precisely determining all the forces and moments in the equilibrium. Usually only the direction of the net moment and the appropriate net force levels can be determined. Determinate systems in orthodontics are those in which a couple is created in one end of an attachment with only a force and not a couple at the other end .e.g. a spring which is inserted to a tube or bracket at one end and tied at the other end to only one point. WILLIAM R. PROFFIT, HENRY W. FIELDS
  7. 7. Biomechanics of Space Closure Orthodontic space closure Orthodontic space closure should be individually tailored based on the diagnosis and treatment plan. The selection of any treatment whether a particular technique, stage spring or appliance designs should be based on the desired tooth movement.
  8. 8. Biomechanics of Space Closure A well-designed appliance should exhibit three general characteristics: (1) It should deliver a known, relatively constant moment-to-force ratio over a long range of activation; (2) the resultant motion of the active unit (teeth being moved) occurs about a predictable center of rotation; and (3) the force system at the reactive unit (anchor teeth) should be known and controllable. Braun and Marcotte AJO-DO 1998
  9. 9. Biomechanics of Space Closure The six goals to be considered for any universal method of space closure: (1) Differential space closure.. (2) Minimum patient cooperation. (3) Axial inclination control (4) Control of rotations and arch width. (5) Optimum biologic response. (6) Operator convenience. To achieve controlled extraction site closure, the appliance used must deliver definable force systems regulated by the clinician and not produce closure in some ambiguous, indeterminate way. Only when force systems are definable are the dental movements predictable and treatment outcomes forecast able with confidence. CHARLES J BURSTONE AJO-DO 1982
  10. 10. Biomechanics of Space Closure Idealized objective of space closure: Space closure should result in upright well aligned teeth with parallel roots and parallel occlusal plane. Therefore some degree of bodily or even root movement is required.
  11. 11. Biomechanics of Space Closure DETERMINANTS OF SPACE CLOSURE The main factors which determine the tooth movement during space closure are: Amount of crowding: in cases of severe crowding, anchorage control is very important to maintain the extraction space for relieving the anterior crowding Anchorage using the same mechanics for different anchorage needs is very important. Traditional anchorage methods like lip bumpers, headgears, transpalatal arches may be utilized but non compliance methods for anchorage control based on biomechanics can also be used. NANDA& KULHBERG
  12. 12. Biomechanics of Space Closure Axial inclination of canines the same force /and or moment applied to teeth with different axial inclinations will result in different types of tooth movement.
  13. 13. Biomechanics of Space Closure Midline discrepancies and left right symmetry. Midline discrepancies should be corrected as early as possible in treatment as it allows the remaining space closure to be completed symmetrically. Using asymmetric mechanics can cause in unilateral anchorage loss, skewing of the dental arches, or unilateral vertical forces. Vertical dimension Control of vertical dimension is essential in space closure. Undesired vertical extrusive forces on the posterior teeth can result in increased LAFH, increased interlabial gap, and excessive gingival display. Class II elastics may potentate this problem.
  14. 14. Biomechanics of Space Closure FUNDAMENTAL CONCEPTS AND CLINICAL METHODS OF ANCHORAGE CONTROL It is the ability to prevent tooth movement of one group of teeth while moving another group of teeth. The problem of anchorage is rooted in Newton’s third law: For every action there is an equal and opposite reaction. Several anchorage control methods have been developed over the last century. The contributions of Angle, Begg , Case, Tweed and others have provided a foundation for modern orthodontic mechano therapy. Although each of them advocated different methods and philosophies, a review of their work shows a lot of similarities. ANDREW KUHLBERG, DEREK PRIEBE: SEMIN ORTHOD 2001
  15. 15. Biomechanics of Space Closure In 1907, EH Angle advocated 5 types of anchorage control. Occipital anchorage depended on the use of extra oral anchorage Intermaxillary anchorage included the use of elastics. The remaining three were dental anchorage: Simple, reciprocal and stationary methods for dental anchorage. Calvin S Case also advocated stationary anchorage methods despite his differences with Angle’s school of thought. He also described the use of extra oral and intermaxillary anchorage as well as the prerequisite that resistance to tipping movements was requisite for intra arch control. Case advocated the use of soldered firm attachment of anchorage teeth to one another to maintain their upright positions.
  16. 16. Biomechanics of Space Closure 20 years later Charles Tweed advocated similar techniques. His method of anchorage preparation against unwanted tipping and extrusive side effects were a series of tip back bends to anchor the teeth like tent stakes to resist vertical and anterior posterior displacement during intermaxillary traction. Although Tweed said his methods of anchorage preparation were more mechanical than biological, the tip back bends were a further refinement of Angle’s stationary anchorage methods. Despite his adherence to the differential force theory, PR Begg also used a similar technique for anchorage control. His tip back bend to maintain the anteroposterior position of teeth to effect preferential movement of teeth was also supplemented by initially tipping the teeth to be retracted followed by up righting them.
  17. 17. Biomechanics of Space Closure Anchorage can be classified as: A ANCHORAGE This category describes the critical maintenance of the posterior tooth position. 75% or more of the extraction space is needed for anterior retraction. Group A arches tend to be of two types: B ANCHORAGE This category describes relatively symmetric space closure with equal movement of the anterior and posterior teeth to close the space. This is the least difficult of the space closures. C ANCHORAGE This category describes non critical anchorage, where 75% or more of the space closure is achieved through mesial movement of the posterior segment; this could also be described as critical anterior anchorage.
  18. 18. Biomechanics of Space Closure Dividing the extraction space into quarters aids in visualizing the anchorage classification NANDA& KULHBERG
  19. 19. Biomechanics of Space Closure ANCHORAGE FROM A BIOMECHANICAL PRESPECTIVE: The basic techniques for anchorage control basically rely on three fundamental similarities: extra oral forces on the anchorage unit intermaxillary elastics Tipping tooth movement while simultaneously discouraging tipping of anchorage teeth. Patient compliance is mandatory for the first two techniques. Without co operation control of tooth movement is lost and the results may be compromised. The way a tooth moves is dependent on the nature of the force systems that act on it. This includes the actual force and moments at the bracket, the force distribution around the periodontal ligament,. The force distribution is a function of the centre of rotation.
  20. 20. Biomechanics of Space Closure CONTROLLED TIPPING: Is tooth movement with the center of rotation at the root apex. The resultant forces are distributed at the marginal portion of the periodontal ligament The M/F ratio is approx. 7/1 TRANSLATION or bodily movement maintains the axial inclination of the tooth and the centre of rotation is at infinity. The resultant force on the PDL is equally distributed along the pressure side of the alveolar structures. The M/F ratio is approx. 10/1 ROOT MOVEMENT or displacement of the tooth apex while the crown remains stationary occurs with a M/F ratio of approx. 12/1 here forces tend to be concentrated on the apical third of the root. Within these basics lie the fundamental principles of anchorage control.
  21. 21. Biomechanics of Space Closure What happens when a high M/F ratio is applied to the anchor teeth? An applied force causes uncontrolled tipping while the applied moment counteracts the tipping effect of the force. This applied moment acts in the opposite direction and moves the roots to the extraction site and if the magnitude further increases, tips the crown distally A low M/F ratio noticeably greater produces tipping with the crown moment How can these forces be produced clinically? If M/F ratio of posteriors> M/F ratio of the anteriors there must be either unequal forces or unequal moments Unequal forces can be produced by headgear, J hook headgear, and intermaxillary elastics. Unfortunately this is co operation dependant
  22. 22. Biomechanics of Space Closure BIOMECHANICAL STRATEGIES FOR DIFFERENTIAL SPACE CLOSURE: The ideal for system for a group A space closure would have only a force system resulting in anterior translation and no forces acting on the posterior teeth thereby maintaining perfect anchorage control. This is possible only with extraoral anchorage or if the opposite arch is used as anchorage. Two approaches: differential forces differential moment to force ratios group A requires the posterior segment to have higher M/F ratios (when the force is reduced M/F is increased) and the anterior segment to have a decrease in M/F ratios. NANDA& KULHBERG
  23. 23. Biomechanics of Space Closure Differential forces the law of static equilibrium ensures that within a single intra arch appliance the mesiodistal forces must be equal, thus forces can be increased or decreased only by utilizing extra oral forces or the opposite arch via intramaxillary elastics. These are compliance dependent methods which also have additional side effects.
  24. 24. Biomechanics of Space Closure differential moment to force ratios Increasing the posterior moment causes root movement M/F>12/1 while decreasing the anterior moment causes a tipping type of moment M/F ~7/1 If the posterior moment were large enough the M/F ratio would approach infinity consistent with the application of a pure couple on the posterior segment. this would result in rotational tooth movement about the center of resistance of the anchor teeth moving the crowns distally and increasing the size of the extraction space.
  25. 25. Biomechanics of Space Closure Differential moments are not without side effects: Unequal moments must be balanced by a third moment . This couple is a pair of vertical forces. intrusive to the anterior and extrusive to the posterior. The magnitude is dependent on the difference of the moments acting on the teeth and the distance between the anterior and posterior teeth. No matter what the strategy some biomechanical side effects will occur. The difficulty of group C anchorage mirrors that of group A. The difference is that the anterior teeth are the effective anchor unit. therefore the anterior moment is of greater magnitude and the vertical side effect is an extrusive force on the anterior teeth.
  26. 26. Biomechanics of Space Closure BIOLOGIC VARIABLES IN ANCHORAGE CONTROL AND DIFFRENTIAL SPACE CLOSURE: Optimal force is the idea that there is a force level that will promote the most efficient treatment without untoward side effects. The true mechanical parameter in tooth movement is not the magnitude of the force per se, but rather the magnitude of the stress generated by the appliance in the surrounding periodontium. Stress is defined as force per unit area (for example, gm/cm2) and strain is the unit deformation that occurs in the tissue as a result of the stress.
  27. 27. Biomechanics of Space Closure . Possible hypotheses of the relationship between stress magnitude and the rate of tooth movement are graphically represented. Quinn & Yoshikawa AJO-DO 1998
  28. 28. Biomechanics of Space Closure HYPOTHESES OF THE STRESS-MOVEMENT RELATIONSHIP (Quinn and Yoshikawa) Hypothesis 1 shows a constant relationship between rate of movement and stress. The rate of movement does not increase as the stress level is increased. The clinician operating under this assumption controls anchorage only through interarch and/or extraoral mechanics. To place more teeth into the anchorage unit or extract teeth in a more anterior position in the arch does not affect the final tooth position. stress If elastics are used for retraction, only one size is necessary. Loop designs are not critical and can be simple and uncomplicated by helices. Intrarch mechanics cannot be altered to change final tooth position. An extraction site, regardless of where it lies in the arch, is always evenly closed by retraction of the teeth anterior to it and protraction of the teeth posterior to it. The periodontium is sensitive only to force direction and not to force or stress magnitude.
  29. 29. Biomechanics of Space Closure Hypothesis 2 is more complex. The relationship here calls for a linear increase in the rate of tooth movement as the stress increases. Operating under this hypothesis, the clinician would, in order to shorten treatment time, use appliances that generated the highest stress values In this system intraarch anchorage could be manipulated by adding teeth (second molars) to the anchorage unit or moving the extraction site— for example, second versus first premolars. This would distribute the stress over a larger root surface, lowering the local stresses and slowing the rate of tooth movement. Arch wires designed for space closure would be fabricated from large, crosssection steel wire with closing loops activated to generate large stresses. The appliance that delivered the highest stresses would close extraction sites most rapidly.
  30. 30. Biomechanics of Space Closure Hypothesis 3 depicts a relationship in which increasing stress causes the rate of movement to increase to a maximum. Once this optimal level is reached, additional stress causes the rate of movement to decline This hypothesis was originally proposed by Smith and Storey. Some clinicians assume the validity of the latter part of the curve in this hypothesis where light forces move teeth "optimally" and an increase of stress slows movement. These clinicians use light forces, for example, to retract canines and prevent anchorage loss while using heavy forces to protract posterior teeth and "anchor" the canines. The orthodontist who operates under this hypothesis theoretically has a great deal of control over anchorage and final tooth position without resorting to extraoral or interarch mechanics.
  31. 31. Biomechanics of Space Closure Hypothesis 4 is a composite of some of the foregoing concepts. Here the relationship of rate of movement and stress magnitude is linear up to a point; after this point an increase in stress causes no appreciable increase in tooth movement. Because the rate of movement is dependent on changes in stress, anchorage can be controlled within the arch. Change of extraction patterns, addition of teeth to the anchorage unit, and modification of intraarch retraction mechanisms to fit the anchorage requirements are all effective means to determine the final tooth position.
  32. 32. Biomechanics of Space Closure For example, operating under this hypothesis, a clinician could enhance canine retraction by (1) extracting first premolar teeth instead of second premolars, (2) incorporating second molars into the posterior segment, (3) adjusting the stress delivered by the retraction mechanism so that the stress level at the canine would coincide with the maximal rate of movement. The stress on the posterior teeth would be distributed over a greater root area, lowering the local stresses and producing a rate of movement less than maximal.
  33. 33. Biomechanics of Space Closure EVALUATION OF HYPOTHESES None of the studies reviewed by Quinn and Yoshikawa support hypothesis 1. All of the results show that, to varying extents, changing the mean stress magnitude will produce changes in the rate of tooth movement Hypothesis 2 is difficult to disprove because most studies used only two force magnitudes and were unable to describe the behavior of the curve as the stress reached higher levels. Boester and Johnston did demonstrate, however, that in their system forces above 140 gm produced no measurable increase in tooth movement. This study, along with that of Hixon et al. which suggested a 300-gm plateau, casts serious doubts on the validity of the continuing linear relationship proposed in hypothesis
  34. 34. Biomechanics of Space Closure Hypothesis 3, the original Smith and Storey proposal, can no longer be considered viable in light of subsequent data. •A more rigorous analysis of their report shows that their data did not justify their conclusions. The canine moved further than the molar at both the high and low force levels and there is no evidence for the rate of movement to suddenly reverse as the stress levels increase past a certain "optimum" value. • Furthermore, in all the canine retraction experiments, the rate of canine movement was greater than that of the molar segment. • Because of its smaller root surface, the mean stresses on the canine can be assumed to be higher than those on the posterior unit. •At the stress levels evaluated in these experiments then, an increase in stress appeared to cause an increase in the rate of movement. The available literature suggests that hypothesis 3 may not be an accurate representation of the data.
  35. 35. Biomechanics of Space Closure The evidence for hypothesis 4 is more compelling. All studies reviewed support the idea that increasing mean stress produces a higher rate of tooth movement. Both Smith and Storey and Andreasen and Zanier demonstrated greater displacement of the posterior teeth as the force level was increased. This finding is consistent with the hypothesis since the molar segment, under less stress because the force is distributed over a larger area, would be on the ascending portion of the curve and would move at a greater rate when the stress level was increased.
  36. 36. Biomechanics of Space Closure Interestingly, neither study provided evidence of a statistically significant difference in canine movement at the two force levels. This might indicate that the canine was moving at a near maximum rate at the lower force and that increasing the force did not increase the rate of movement. These data, along with those of Boester and Johnston, Hixon et al. and Burstone and Groves, provide evidence that beyond a certain stress level increasing stress no longer alters the rate of tooth movement.
  37. 37. Biomechanics of Space Closure If hypothesis 4 with a clinically useful slope and a plateau in the 100 to 200 gm range is thought to be a valid model, there are two clinical strategies that would maximize anchorage within the arches. The first is to lower the stress delivered to the posterior teeth. This can be done by increasing the root surface area, either by incorporation of second molars into the anchorage unit or by making extractions more anteriorly in the arch. The decrease in stress obtained by increasing the root surface area of the posterior teeth will slow their rate of movement and allow more canine retraction. The same rationale makes extraction of second premolar teeth a logical choice in minimum anchorage cases.
  38. 38. Biomechanics of Space Closure The second strategy for minimizing anchorage loss is to use an appliance system that can deliver relatively continuous stresses in the range described earlier. Excessive stress produces an increase in the rate of movement of posterior teeth without increasing retraction of anterior teeth. Appliances that have a high load-deflection rate are unable to achieve a difference in the rate of tooth movement. Low load deflection-rate mechanics, on the other hand, can maintain stresses in the desired range and maximize the difference in the rate of movement.
  39. 39. Biomechanics of Space Closure DIFFERENTIAL FORCE SYSTEMS-VARIABLE MOMENTS AND FORCES The force system of an orthodontic appliance acts in all three planes and determines the type of tooth movement. ALPHA MOMENT This is the moment acting on the anterior teeth (also called anterior torque) BETA MOMENT This is the moment acting on the posterior teeth. Tip back bends placed mesial to the molars produce an increased beta moment The components of a force system for a space closure from a Sagittal view are: NANDA& KULHBERG
  40. 40. Biomechanics of Space Closure HORIZONTAL FORCES These are the mesio-distal forces acting on the teeth which are equal to each other. VERTICAL FORCES These are the extrusive intrusive forces generated because of the unequal moments. When the beta moment is greater, a intrusive forces act on the anterior teeth. And while alpha moment is greater an extrusive force acts on the anteriors while an intrusive force acts on the posteriors.
  41. 41. Biomechanics of Space Closure Center of resistance of anterior teeth during retraction The location of the center of resistance of various consolidated units of the maxillary anterior dentition was studied using a dry human skull when subject to retrusive forces. The units studied consisted of (a) two central incisors, (b) four incisors, and (c) six anterior teeth. The laser reflection technique and the holographic interferometric technique were employed to measure the displacement of the dentition to the applied forces. Vanden Bulcke, Dermaut, Sachdeva, and Burstone AJO-DO 1998
  42. 42. Biomechanics of Space Closure Results: 1. For an anterior segment comprising two central incisors, the center of resistance was located on a projection line parallel to the midsagittal plane on a point situated at the distal half of the canines. 2. For an anterior segment that included the four incisors, the center of resistance was situated on a projection line perpendicular to the occlusal plane between the canines and first premolars. 3. For a rigid anterior segment that included the six anterior teeth, the center of resistance was situated on a projection line perpendicular to the occlusal plane distal to the first premolar.
  43. 43. Biomechanics of Space Closure 4. The centers of resistance of the anterior segments incorporating two or four anterior teeth were within ± 2 mm of each other. However, inclusion of the canines in the anterior segment resulted in the center of resistance moving distally by approximately one premolar width (7 mm). This effect may have been the result of the resistance of bony structures at the level of the canines and some bending of the maxillary complex as was observed on the holograms. 5. No appreciable shift in the location of the centers of resistance of the various segments studied was detected as varying magnitudes of retractive force were applied.
  44. 44. Biomechanics of Space Closure METHODS OF CANINE RETRACTION: Friction Frictionless ( PG spring, Burstone T loop, Ricketts) METHODS OF ENMASSE RETRACTION: OF FOUR INCISORS Friction Frictionless PG retraction spring, Utility arch, Omega Loop archwire Extraoral Headgears OF SIX ANTERIORS Closing loop archwire Burstone T loop continuous archwire Opus loop INTRUSION AND RERACTION OF FOUR INCISORS Burstone’s three piece intrusion arch Rickets retraction and intrusion utility arch SIMULTANEOUS INTRUSION AND RETRACTION OF SIX ANTERIORS K-sir arch
  45. 45. Biomechanics of Space Closure The most significant distinction between the mechanics of standard edgewise and preadjusted appliances was observed during space closure. With standard edgewise appliances, rectangular archwires did not effectively slide through the posterior bracket slots because of the 1st-, 2nd-, and 3rd-order bends. The orthodontist normally used a closing loop arch, which was activated in the office by opening the closing loop and moving the archwire through the posterior bracket slots - RICHARD P. McLAUGHLIN, JOHN C. BENNETT, JCO 1998
  46. 46. Biomechanics of Space Closure The level bracket slot alignment of the new appliances allowed archwires, for the first time, to move more effectively through the posterior slots when the patient was not in the office. As a result, many orthodontists discontinued use of closing loops and began using various forms of sliding mechanics— for example, placing hooks in the anterior sections of straight archwires and tying elastics or springs to them from molar brackets
  47. 47. Biomechanics of Space Closure Closing loop arches had several disadvantages: 1. Extra wire-bending time 2. Poor sliding mechanics 3. Tendency to run out of space for activation (after two or three activations, the omega loop contacted the molar bracket and the archwire had to be adjusted or remade) 4. High initial force levels They also had advantages: 1. Precise control of the amount of loop activation (often as little as 1mm), limiting the amount of initial tipping 2. Adequate rebound time for uprighting between appointments (with minimal activations, loops closed quickly with little tipping)
  48. 48. Biomechanics of Space Closure Sliding mechanics had these advantages: 1. Minimal wire-bending time 2. More efficient sliding of archwires through posterior bracket slots 3. Sufficient space for activations But sliding mechanics at first also had disadvantages: 1. No established guidelines on amounts of force to be used during space closure 2. Tendency for initial overactivation of elastic and spring forces, causing initial tipping and inadequate rebound time for uprighting
  49. 49. Biomechanics of Space Closure To maximize the advantages and minimize the disadvantages of sliding mechanics • force levels are reduced during space closure. • Instead of springs or over activated elastics (which can produce 500g of force), single elastic modules are attached to anterior archwire hooks with ligature wires extended forward from the molars These "elastic tiebacks", when activated 2-3mm, generate about 100-150g of force. If the arches have been properly leveled, such light force allows for effective space closure; there is little tipping with subsequent binding of the archwires, and leveling is maintained .
  50. 50. Biomechanics of Space Closure 019" × .025" archwires with .022" slots provide optimum rigidity, but adequate freedom for the wires to slide through the slots. Round wires and smaller rectangular wires provided less precise control of torque, curve of Spee, and overbite. Hooks of .024 " stainless steel or .028 " brass are soldered to the upper and lower archwires
  51. 51. Biomechanics of Space Closure Effects of Overly Rapid Space Closure Space closure typically occurs more easily in high-angle patterns with weak musculature than in low-angle patterns with stronger musculature. The rate of closure can be increased, particularly in high-angle cases, by slightly raising the force level or using thinner archwires. However, more rapid space closure can lead to loss of control of torque, rotation, and tip.
  52. 52. Biomechanics of Space Closure Loss of torque control results in upper incisors being too upright at the end of space closure with spaces distal to the canines and a consequent unaesthetic appearance. The lost torque is difficult to regain. Also, rapid mesial movement of the upper molars can allow the palatal cusps to hang down, resulting in functional interferences, and rapid movement of the lower molars causes "rolling in"
  53. 53. Biomechanics of Space Closure Reduced rotation control can be seen mainly in the teeth adjacent to extraction sites, which also tend to roll in if spaces are closed too rapidly Reduced tip control produces unwanted movement of canines, premolars, and molars, along with a tendency for lateral open bite. In high-angle cases, where lower molars tip most freely, the elevated distal cusps create the possibility of a molar fulcrum effect
  54. 54. Biomechanics of Space Closure In some instances, excessive soft-tissue hyperplasia occurs at the extraction sites ,this is not only unhygienic, but it can prevent full space closure or allow spaces to reopen after treatment. Local gingival surgery may be necessary in such cases.
  55. 55. Biomechanics of Space Closure Inhibitors to Sliding Mechanics Proper alignment of bracket slots is essential to eliminate frictional resistance to sliding mechanics. The common procedure is to use .018" or .020 " round wire for at least one month before placing .019"´.025" rectangular wires. Leveling and aligning continues for at least a month after insertion of the rectangular wires, and that space closure cannot proceed during that period.
  56. 56. Biomechanics of Space Closure Therefore the rectangular wires are tied passively for at least the first month, until leveling and aligning is complete and the archwires are passively engaged in all brackets and tubes Conventional elastic tiebacks are than placed ,In some cases, this phase takes three months.
  57. 57. Biomechanics of Space Closure There are three primary sources of friction during space closure First-order or rotational resistance at the mesiobuccal and distolingual aspects of the posterior bracket slots is produced by rotational forces on the buccal aspects of the posterior teeth. The most effective way to counteract this resistance is to apply intermittent lingual elastic forces— one month from cuspid to first molar, the next month from cuspid to second molar.
  58. 58. Biomechanics of Space Closure Second-order or tipping resistance at the mesio-occlusal and distogingival aspects of the posterior bracket slots is caused by excessive and overactivated tieback forces, which lead to • tipping of the posterior teeth, • inadequate rebound time to upright these teeth, • and a resultant binding of the system. The importance of light forces (50150g) and minimal activation length (to provide time for uprighting) cannot be overemphasized.
  59. 59. Biomechanics of Space Closure Third-order or torsional resistance occurs at any of the four areas of the bracket slot where the edges of the archwire make contact. Like tipping resistance, this is produced mainly by excessive and overactivated tieback forces, which cause the upper posterior lingual cusps to drop down and the lower posterior teeth to roll in lingually
  60. 60. Biomechanics of Space Closure Problems During Space Closure Since forces are directed from the first molars to anterior hooks on the archwire, small spaces occasionally open between the first and second molars. This can be managed in one of three ways: A damaged lower premolar or first molar bracket, either from careless use of biting sticks during bonding or from improper diet, can hinder space closure Interference from opposing teeth sometimes restricts lower arch space closure, particularly if bracket placement was incorrect or a full-unit Class II molar relationship existed.
  61. 61. Biomechanics of Space Closure As spaces close, the distal ends of the archwires will protrude more and more, and these protruding wires will tend to become bent gingivally by chewing forces Certain tissue factors can hinder full space closure with any kind of mechanics. Soft-tissue build-up can result from poor plaque control or overly rapid space closure. The alveolar cortical plate, mesial to the lower first molars, tends to narrow after extraction of the second premolars, especially in lower-angle situations. Retained roots, ankylosed teeth, and bone sclerosis are other possible factors to be considered.
  62. 62. Biomechanics of Space Closure Constant attention is required to prevent any of the following inhibiting factors: 1. Inadequate leveling, resulting in archwire binding 2. Posterior torque such that torquing and sliding cannot occur simultaneously 3. Blockage of the distal end of the main archwire by a ligature wire 4. Damaged or crushed brackets that bind the main archwire 5. Soft tissue resistance from build-up in extraction sites 6. Cortical plate resistance from a narrowing of the alveolar bone in extraction sites 7. Excessive force, causing tipping and binding 8. Interferences from teeth or the opposing arch 9. Insufficient force
  64. 64. Biomechanics of Space Closure Friction is a function of the relative roughness of two surfaces in contact. It is the force that resists the movement of one surface past another and acts in a direction opposite the direction of motion. VARIABLES AFFECTING FRICTIONAL RESISTANCE DURING TOOTH MOVEMENT PHYSICAL BIOLOGICAL ARCHWIRE SALIVA LIGATION PLAQUE BRACKET ACQUIRED PELLICLE ORTHODONTIC APPLIANCE CORROSSION NANDA& KULHBERG
  65. 65. Biomechanics of Space Closure PHYSICAL BIOLOGICAL Saliva ARCHWIRE crossectional size/shape Plaque material Acquired pellicle surface texture Corrosion stiffness LIGATION ligature wires elastomerics self ligating brackets BRACKET material manufacturing process slot width and depth first/second/third order bends ORTHODONTIC APPLIANCE interbracket distance level of bracket slots between adjacent teeth forces applied for retraction
  66. 66. Biomechanics of Space Closure The segmental arch technique as developed by Burstone utilizes T loop space closure springs for anterior retraction, symmetric closure or posterior protraction. The segmental T loop as described by Burstone is one of the most versatile space closure devices available. One of the main principles of the segmental arch technique is considering the anterior segment and posterior segment as one large tooth respectively. The right and left buccal units are connected by a transpalatal arch forming one big posterior unit. The basic configuration of the TMA loops consists of a .017X.025” TMA wire. CHARLES J BURSTONE AJO-DO 1982
  67. 67. Biomechanics of Space Closure The rate of decay of the force applied by a spring is called the load-deflection rate, and it averages 33 Gm. per millimeter in the Burstone’s T loop. The low load-deflection rate is important in this spring, since it enables the orthodontist to deliver optimal magnitudes of force.
  68. 68. Biomechanics of Space Closure High-load deflection springs as vertical loops dissipate force rapidly; hence, one must activate to very high force levels in order to produce any significant tooth movement. Since the load-deflection rate is so high, it would be impossible for a clinician to activate the loop to produce an optimum magnitude of force. To deliver 200 Gm. of force, the required activation would be 0.2 mm. Not only is it practically impossible to activate such a small distance, the force of 200 Gm. would be dissipated rapidly over the remaining 0.2 mm. of activation. Thus, orthodontists who use high-force load-deflection mechanisms must use high force values that have undesirable sequelae, which include anchorage loss, pain, and undermining resorption.
  69. 69. Biomechanics of Space Closure In contrast, a retraction spring with a low load-deflection rate of 33 Gm. per millimeter allows for the delivery of optimal force levels, since an error in activation of 1 mm. results in an error of only 33 Gm. Furthermore, as teeth move distally, the reduction in force is small, giving greater constancy of force at optimal levels. Early in treatment, the posterior teeth are joined together to form a posterior anchorage unit. The anchorage unit consists of the right and left posterior teeth which are connected by a buccal stabilizing segment and a transpalatal lingual arch in the maxillary arch and a low lingual arch in the mandibular arch During space closure, it is to be considered that there are only two teeth — an anterior tooth comprising the incisors and the canines which have been connected and a posterior tooth which includes molars and premolars
  70. 70. Biomechanics of Space Closure . The attachment on the posterior tooth (segment) is a 0.018 by 0.025 inch auxiliary tube on the first molar, and the one on the anterior tooth (segment) is an auxiliary vertical tube on the canine bracket
  72. 72. Biomechanics of Space Closure To understand its design one must first understand its passive form of the spring and then its activation. In the passive state there are no moments or forces acting on it. In its active state it applies a force system on the teeth, The activation of a spring requires forces and moments to engage the spring in its brackets and tubes.
  73. 73. Biomechanics of Space Closure Neutral position: The neutral position in an activated loop is found by applying the activation moments and without any horizontal forces. In other words the ends are twisted to bring the each attachment to its horizontal position. in this position the spring has zero horizontal force The horizontal force is got by pulling the spring open from this position.
  74. 74. Biomechanics of Space Closure Differential moments are obtained by the principle of off center V bends which results in unequal moments. the closer the V bend is to the tooth the higher the moment. the segmented T loops approximated a V bend. Clinically the spring needs to be positioned at least 1-2mm closer to one side than another to obtain a moment differential.
  75. 75. Biomechanics of Space Closure With the introduction of beta-titanium wire (TMA), it has been possible to simplify the design so that a T loop by itself will have a relatively low load-deflection rate and a large maximum springback. The heavier base arch which fits into the auxiliary tube of the first molar is important, since it allows positive orientation of the spring and, more significantly, it is capable of withstanding, without permanent deformation, the higher moments that are needed for anchorage control. Furthermore, the use of a heavier base arch tends to increase the moment-to-force ratio on the anterior teeth, since any bending in the occlusally positioned part of the spring tends to minimize this ratio. To aid the clinician in achieving the proper angulation, templates are used. Rather than to measure the angles, it is more expeditious to duplicate the shape of the spring from a template.
  76. 76. Biomechanics of Space Closure The T loop described in Biomechanics by Nanda is designed for an activation upto 6 full 6mm activation tooth movement occurs in three phases: tipping, translation and root movement. For a symmetric centered spring an initial activation produces a M/F ratio of 6/1 which results in tipping movement of the teeth into the extraction space.
  77. 77. Biomechanics of Space Closure With 2mm deactivation or spring activation = 4mm the M/F ratio is 10/1 which results in translation of the segments towards each other. With 1-2mm space closure (spring activation =2mm) the M/F ration increases to 12/1 and higher resulting in tooth movement. Clinically the spring should not be re activated till all three phases are complete.
  78. 78. Biomechanics of Space Closure OFF CENTERED T LOOPS This demonstrates another method that may be used for controlling the forces and moments produced by segmented 0.017 ´ 0.025-inch TMA T-loop springs or closing loops in general. Previously, the approach described for achieving differential alpha/beta moments with segmented T-loops used asymmetric angulations of the preactivation bends. However, with this method the moment differential does not remain constant with spring activation, i.e., the moment differential is dependent on both spring activation and the differences in the preactivation angulations. Andrew Kuhlberg, Charles J. Burstone AJO-DO 1998
  79. 79. Biomechanics of Space Closure Off-center positioning maintains the constancy of the moment differential throughout the range of spring deactivation (space closure). This concurs with Burstone and Koenig who demonstrated a moment differential and vertical forces with off-center vertical loops.
  80. 80. Biomechanics of Space Closure The effect of off-center placement of T-loops with a standard shape at a standardized activation and interbracket distance. A centered T-loop produces equal and opposite moments with negligible vertical forces. Off-center positioning of a T-loop produces differential moments. More posterior positioning produces an increased beta moment. More anterior positioning produces an increased alpha moment. A standard shaped T-loop can be used for differential anchorage requirements by altering the activation and mesialdistal position of the spring.
  81. 81. Biomechanics of Space Closure The center position of the spring can be found by: distance = (interbracket distance –activation)/ 2 where distance = length of the anterior and posterior arms (distance from the center of the T loop to either the anterior or posterior tubes) interbracket distance=distance between the canine and molar brackets. Activation= millimeters of activation of the spring
  82. 82. Biomechanics of Space Closure With the use of a vertical tube at the canine a 90 degrees gingival bend at the calculated distance eases placement and monitoring throughout space closure The T loop is places in the molar auxiliary tube and then inserted into the canine bracket. The distal end is pulled back until it is the desired length which results in the desired activation.
  83. 83. Biomechanics of Space Closure Force systems related to type A, B, and C extraction site closure
  84. 84. Biomechanics of Space Closure MAXIMUM POSTERIOR ANCHORAGE: (Group A anchorage) The T loop is positioned closer to the posterior segment (1-2 mm off centering) is sufficient . activation of 4 mm is necessary. This reduces the horizontal forces without altering the moment differential. The force system acting on the anterior segment favors tipping. The moment difference remains as the space closes and the spring deactivates. The spring must be re activated when 2 or less mm of activation remains.
  85. 85. Biomechanics of Space Closure Because the beta moment is greater than the anterior moment a vertical intrusive force acts on the anterior teeth which can exaggerate the tipping tendency and steepen the occlusal plane. Similarly the posterior occlusal plane can be steeped buy the extrusive force. Maintaining adequate horizontal force helps to reduce this effect. A High pull headgear can also be used to control the posterior occlusal plane. It is likely that root correction will be required at the end of space closure.
  87. 87. Biomechanics of Space Closure MAXIMUM ANTERIOR ANCHORAGE :( Group C anchorage) This is the most difficult of all space closures. The increased alpha moment has a tendency to deepen the overbite. The loop must be placed 12mm closer to the anterior teeth. Care must be taken that the wire segment achieves full bracket engagement because play can reduce the moment differential. Space closure with tipping of the buccal segments will occur.
  88. 88. Biomechanics of Space Closure the activation must be around 4mm and should be activated every 2mm. The major side effects are loss of anchorage and extrusion of the anteriors. Class III elastics or protraction headgear may help in the protration of the upper buccal segments. For mandibular molars class II elastics may help.
  89. 89. Biomechanics of Space Closure Segmental T loop space closure principles can also be applied to space closure on a continuous arch. the force system is not as well defined a the segmental but careful use of the alpha and beta moments helps to achieve comparable results especially for group B and C anchorage cases. For group A cases high pull headgear is necessary to control tooth position. T loops one on each side are made using preformed arch wires .017 X . 025 TMA or .016 X .022 Stainless steel arch wire.
  90. 90. Biomechanics of Space Closure The activations given are for TMA wires and the Stainless steel wires activation is reduced by half. The T lops are made 6-7mm tall and 10mm wide and are positioned distal to the cuspids. Desired alpha and beta moments are place anterior and posterior to the T loop vertical legs. Recommended beta activations for A, B, and C anchorages are 40 degrees, 30 degrees and 20 degrees.
  91. 91. Biomechanics of Space Closure After the activations are placed the loops should be open approximately 2mm before placing in the mouth. the wire is inserted into the molar auxiliary tube and ligated to the anterior teeth. The t loop bypasses the premolars brackets and is not inserted in them. For TMA loops the activation can be 3mm distal to the molar tube which gives it a range of force of 250-300gms. The patient should be monitored but no further activations are necessary for 2-3 months. Too frequent reactivation can prevent root movement and cause excessive tipping
  92. 92. Biomechanics of Space Closure CONTROL OF THE MECHANICAL SIDE EFFECTS OF SPACE CLOSURE: From the Occlusal view the first order side effect of rotation of the molars and canines are observed. Rotation of the molars can be prevented by use of a transpalatal arch. (rectangular wires only not round wires TPA) Control of canine rotation can be achieved by a variety of techniques.
  93. 93. Biomechanics of Space Closure For enmasse retraction a rigid anterior segment can reduce this tendency. A canine bypass connecting he canines but bypassing the incisors can also control rotation. thirdly a anti rotation bend can be incorporated into the spring. With asymmetric space closure vertical forces may be produced. these may produce undesired extrusive or intrusive tooth movements. these vertical forces may also produce third order side effects. With group A space closure the third order side effect on the canine is troublesome the intrusive force causes a buccal flaring which increases the overjet at the canine and/or increases the intercanine width.
  94. 94. Biomechanics of Space Closure CONTROL OF THE SIDE EFFECTS OF SPACE CLOSURE: Careful monitoring is essential during space closure. A frequently overlooked side effect of space closure is the first order side effects. The mesially directed buccally located force of the molar may lead to the erroneous supposition that there is anchorage loss. Distalization is not necessary . A mesially out directed force is all that is needed to regain the original molar position. A transpalatal arch provides an excellent mean to prevent this or actively corrects it.
  95. 95. Biomechanics of Space Closure CORRECTION OF THE SIDE EFFECTS Tipping of the anterior and posterior teeth into the extraction space Increase the alpha and beta moments Flaring of the anterior teeth Reduce the alpha moment or increase the distal activation Mesial in rotation of the buccal segments Mesial out rotation of the palatal arch, archwire or lingual arch Excessive lingual tipping of the anterior teeth Increase the alpha moment
  96. 96. Biomechanics of Space Closure Rotating moments caused by buccal forces during extraction site closure.
  97. 97. Biomechanics of Space Closure The OPUS loop was designed to deliver an inherent M/F ratio sufficient for enmasse space closure via translation of teeth of average dimensions with no bone loss. Because its inherent M/F ratio is high enough no preactivation bends is needed before insertion The neutral position is the passive position of the spring as it sits before insertion. Simple cinch back activations can take care of the tooth movement thresholds to meet anchorage objectives. Raymond E Siatkowski Semin Orthod 2001 AJO-DO 1997
  98. 98. Biomechanics of Space Closure No closing loop design previously has been capable of delivering at constant M/F of 8.0 to 9.1 mm most having inherent M/F of 4-5 mm or less. To achieve net translation, orthodontists have had to add residual moments to the closing loop arch wire with angulation bends (gable bends) anterior and posterior to the loop, a posterior gable bend and angulations within the loop, or a posterior gable bend and anterior wire-bracket twist (anterior root torque).
  99. 99. Biomechanics of Space Closure Adding these residual moments has several disadvantages: 1. The teeth must cycle through controlled tipping to translation to root movement to achieve net translation (lower Young's Modulus materials go through fewer of these cycles for a given distance of space closure). 2. The correct residual moments are difficult to achieve precisely in linear materials. 3. The resulting ever-changing PDL stress distributions may not yield the most rapid, least traumatic method of space closure. If a closing loop design capable of achieving inherent, constant M/F of 8.0 to 9.1 mm without residual moments were available, en masse space closure with uniform PDL stress distributions could be achieved. Such a mechanism would be less demanding of operator skill to apply clinically and might provide more rapid tooth movement with less chance of traumatic side effects
  100. 100. Biomechanics of Space Closure The opus loop achieves a M/F ratio of 8-9.1mm without addition of activation bends in the loop or archwire itself. Therefore its neutral position is the same as the inactivated position before it was tied into the brackets. Having the loops neutral position accurately allows known forces systems to be applied to the teeth via simple cinch back activations.
  101. 101. Biomechanics of Space Closure The apical horizontal leg is 10mm long, The ascending legs at an angle of 70 degrees to the plane of the brackets The apical helix is on the leg ascending from the anterior teeth, (that ascent must begin within 1.5mm posterior to the most distal bracket of the anterior teeth being retracted) The spacing between the ascending legs especially the apical loops legs must be 1mm or less All these dimensions are critical to the performance of the loop. Clinically comfort bends are not necessary.
  102. 102. Biomechanics of Space Closure Being free of residual moments, the design can produce a true rest period when deactivated and therefore could be used with future technology to produce intermittent force systems during space closure. Wire bracket play numbers as given in the figure shows that it is important that sufficient lingual twist exists in the arch wire engaging the incisors so that bracket wire play is reduced for axial control of the incisors.
  103. 103. Biomechanics of Space Closure It is appropriate to begin with a straight wire and bend the arch wire in a torquing turret to achieve incisor axial inclination control by inducing wire twist ("lingual root torque") just enough to eliminate labiolingual wire-bracket play in the incisor brackets. The amount of such twist is dependent on the wire/bracket sizes and slot torque used A torquing turret has been designed for use with TMA wire. Maximum incisor twist is appropriate for posterior protraction
  104. 104. Biomechanics of Space Closure The advantage of having the opus loop formed in 17X25 TMA is that it provides a relatively long range of activation; unfortunately it is difficult to bend the wire with sufficient incisor torque to reduce the wire play. It is difficult to contour the loop for comfort on one side without altering the other side also and a large stock of wires is necessary for preformed wires. This can be over come by having: An anterior wire of Niti alloy with two separate 17X25 TMA posterior segments, which are attached by a Forestadent cross tube This bialloy has the following advantages: Infrequent activations Ease of comfort bending Incisor axial inclination control
  105. 105. Biomechanics of Space Closure
  106. 106. Biomechanics of Space Closure Melsen refinement of the Quinn Yoshikawa model of tooth movement: The OSTEOLOGIC graphic form is the theoretical explanation for the mode of action of the opus loop arch wire. It relates the orthodontic force systems to the stresses in the PDL rather than the strains. It examines the rate of tooth movement as the loop deactivates
  107. 107. Biomechanics of Space Closure When a force system is applied on a tooth initially after a quiescent period, the initial rate of tooth movement corresponds to no 2 on the diagram. This model is valid only for uniform stress on the PDL as produced by translation and not tipping followed by up righting. These arch wires by definition are activated far less than the systolic blood pressure at which hyalinization is supposed to occur. Larger cinch back activations shift the curve to the right and a lesser activation shifts the curve to the left
  108. 108. Biomechanics of Space Closure The various possible activations of the opus lop cinch back as a function of time is shown in the figure. Group B anchorage Curve 1: anteriors retract Curve 2: posteriors protract Group A anchorage Curve 2: anteriors retract Curve 3: posteriors little change Group C anchorage Curve 4: anteriors no change Curve 1 posteriors protract
  109. 109. Biomechanics of Space Closure DISADVANTAGES OF THE OPUS LOOP Although less so than with other closing loop designs, Opus loops do have the potential to steepen the cant of occlusal plane in the maxillary arch and flatten it in the mandibular arch. Although steepening occlusal plane can be useful for overtreatment of Class III relationships (and flattening occlusal plane for Class II relationships), that potential should be monitored for possible intervention. Such intervention could be reducing maximum activation force levels or using an occipital headgear with short and high outer bows to generate a moment tending to flatten maxillary occlusal plane. For the most severe anchorage required to achieve treatment goals, second molars, if available, could be included with the posteriors and/or a Combi headgear used. For less severe or moderate anchorage, the canines could be incorporated with the anteriors
  110. 110. Biomechanics of Space Closure The configuration for posterior protraction The closing loop arch wire generates the moments required and some of the protraction force. Most of the protraction force is generated by the large anterior moment and by the intermaxillary elastics to a rigid rectangular arch wire in the opposing arch. Intermaxillary Niti closed coil springs capable of delivering 150 gm force can be substituted for the elastics. The potential exists for changing occlusal plane in the opposing arch. Should such cant changes begin to be observed, the intermaxillary force can be reduced.
  111. 111. Biomechanics of Space Closure In group C anchorage cases, class III elastics with a force of 150gms/side from the opposite arch which has a rigid rectangular stainless steel archwire can be used. Another alternative is to use TP 256 torquing auxiliary which when overlaid over the closing loop provides an additional protraction force to the posteriors. It has the following advantages: The clinician is free to continue treatment in the lower arch Undesired vertical forces from the elastics are not a problem Posterior arch width increases are not a problem when using a TMA wire
  112. 112. Biomechanics of Space Closure SEGMENTED ARCH TECHNIQUE FOR SPACE CLOSURE IN ADULTS: The advantages of the T-loop design over a vertical loop is that the T-loop produces a higher M/F ratio, a lower loaddeflection rate, and delivers a more constant force and M/F ratio Often in adult patients, where no growth is anticipated, extraction therapy is performed. The situation is often complicated through loss of bone. In order to maintain an assumed stress magnitude and distribution under the condition of reduced bony support area, force magnitude must be reduced and the M/F ratio must be increased. The necessity of producing a lower load deflection rate in such cases suggests the use of a wire with lower stiffness Manhartsberger, Morton, Charles J Burstone Angle 1989
  113. 113. Biomechanics of Space Closure With a change in the center of resistance the M/F ratio must be modified thus, in adult patients with periodontal loss, higher M/F values must be attained. To obtain higher M/F ratios a number of strategies can be employed. The loop can be made as long as possible in an apical direction. By increasing the height of the loop to 11 millimeters, one will approximately double the M/F ratio. However, there are limitations to how far apically the loop can extend before irritation is produced in the mucobuccal fold. Another approach is to increase the amount of wire placed gingivally at the top of the loop, as in the T-loop . Increasing the gingival length of wire (dimension G) increases the M/F ratio and reduces the load deflection rate.
  114. 114. Biomechanics of Space Closure The variation of the center of resistance with differing levels of bony support.
  115. 115. Biomechanics of Space Closure The T-loop dimensions (H=7mm, D=2mm, G =10mm).
  116. 116. Biomechanics of Space Closure TMA T-loop with gradual curvature angulation. T-loop with concentrated angulation. A .016² × .025² TMA T-loop activated five millimeters with gradual curvature bends in comparison to a .017² × .025² TMA T-loop activated seven millimeters produced a 47 percent lower horizontal force, and a 23 percent higher M/F ratio. The actual spring must be individualized for each patient by altering wire cross-section, angulation, and activation.
  117. 117. Biomechanics of Space Closure The clinical discipline of orthodontic space closure requires complete understanding of its complexities Just as an orthodontist is provided with innumerable alternatives of appliance systems, it is imperative to be aware it the merits as well as the limitations of the techniques involved Mechanical as well as biologic factors must be considered in selecting the appliance best suited for the patient
  118. 118. Biomechanics of Space Closure Thank you For more details please visit m