Incomplete seating of cement crowns


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Incomplete seating of cement crowns

  1. 1. Incomplete seating of cemented crowns A literature review JPD 1988;59(4),429-33 R. Pilo, H. S. Cardash, H. Baharav, and M. Helft Talib amin, GDC Srinagar
  2. 2. Introduction
  3. 3.  For an ideal restoration, a casting must be made to fit the prepared tooth intimately.  However cast crowns do not seat completely when cemented.  The more accurately the casting fits the prepared tooth, the more difficult it is for cement trapped between the crown and the occlusal surface of the tooth to escape.  Thus, a properly constructed fixed prosthesis may fail as a result of incomplete seating during cementation.  Most research on cementation of crowns relates seating failure to the thickness of the cement film.
  4. 4. FACTORS AFFECTING FILM THICKNESS OF CEMENT • Jorgensen investigated factors influencing the thickness of cement between the restoration and the prepared tooth. • The optimum cementation force required to reduce the film thickness of cement was 5 kg for 1 minute. • Film thickness increase with an increase in  P/L Ratio  Temperature  Smaller the tapper ; <10degrees (when occlusal surface is not perforated)
  5. 5. • According to Windler Occlusal displacement = film thickness at axial wall sine(1/2 tapper angle) Windeler AS. Powder enrichment effects on film thickness of zinc phosphate cement. J PROSTHET DENT 1979;42:299-303.
  6. 6. FILM THICKNESS MEASUREMENTS • Dies with cemented crowns sectioned and measured directly with a travelling microscope. • Indirectly by recording the distance between the margin of the casting and the finishing line of the preparation before and after cementation, or between reference marks on the casting and die. • The ADA specification for film thickness of type I zinc phosphate cement is 25 um and for type II 40 um. • Most investigators found film thickness ranging from 60um to 435um . • The wide range may relate to different methods of measurement and experimental design of the models used.
  7. 7. Filtration process results in much higher film thickness than ADA specification No. 8 for zinc phosphate cemenet During cementation cement accumulates on the occlusal surface Cement can escape through cervical margin As the crown approaches its final position this space becomes smaller The flow of the non compressible liquid cement is inhibited and seating of the crown is resisted Hydrodynamic pressure causes the cement to separate in solid and liquid phase The solid particles form a mass that only allows passageof the thinner liquid, causing further separation and filtration of the cement (FILTRATION PROCESS)
  8. 8. According to study by Hoard et al • Intra coronal pressures during crown cementation were measured with zinc phosphate cement. • Maximum pressure was recorded at the occlusal surface of a die after 2 seconds; it reduced to zero after 1 minute. • Less pressure reducing to zero was recorded at the cusp tip and at the axial walls, reducing to 25% after 1 minute. • In a comparison of different cements, the more viscous the cement, the greater was the hydraulic pressure that did not reduce to zero. • Thus, it was concluded that internal back pressure plays a limited role in preventing the complete seating of a crown • suggested that the filtration process of Jorgensen is responsible for increased film thickness of cement (accumulation of cement particles)
  9. 9. DISCREPANCIES OF MARGINAL FIT Complications caused by incomplete seating of crowns are: Creation of premature contacts Alteration of contact areas with adjacent teeth  A reduction in crown retention by 19% to 32% Discrepancies of marginal fit of the crown.
  10. 10. • Crowns may fail because of cement wash-out at the margins. • Ill-fitting margins expose large amounts of cement to the oral fluids, &-easing the rate of deposition of plaque. • Dissolution of cement in oral fluids results in marginal leakage and a rough surface that accumulates debris and bacteria. • After setting, zinc phosphate cement has been shown to elute zinc, magnesium, sodium, and phosphate ions into an aqueous solution. • Swartz et al. found that early exposure of zinc phosphate cement to water after setting increased its solubility and suggested coating the exposed surface with cavity varnish.
  11. 11. • The configuration of the finishing line of the preparation affects the width of the band of cement exposed to oral fluids. regardless of the discrepancy of seating. • Gavelis et al. confirmed that feathered edges and shoulders had the smallest marginal openings.
  12. 12. • Clinically, there is no clear correlation between the thickness of the exposed band of cement at the crown margin and the amount of cement dissolved by the oral fluids. • Cement manipulation or environmental factors localized to the tooth should be considered.
  13. 13. Techniques to improve marginal fit • Venting • Internal relief of the castings & • modified cementation techniques
  14. 14. Venting • A small hole prepared in the occlusal or axial surface of the restoration allows exit of excess cement during cementation.
  15. 15. STUDY Improved vertical seating Cooper et al 45 um Van Nortwick and Cettleman 290um Kaufman et al 175um Retention increased by 19% to 32%
  16. 16. Dimashkieh & Davies Venting of crowns is less critical when the taper angle is increased. In near parallel-sided prepared teeth, venting was necessary to avoid an excessive occlusal cement thickness. Time of final seating of 1 minute in non vented crowns was reduced to 15 seconds in vented crowns.
  17. 17. Disadvantages • An extra visit is necessary to fill the vent hole • The material used to fill the vent hole may wear or display • Marginal leakage & • Occlusal venting of ceramic or ceramometal crowns may weaken the porcelain.
  18. 18. • Vent holes in crowns may be closed with amalgam or composite. • Bassett described a technique of casting a pin that was sized to fill a hole made by a No. 699 bur. • Seberg prepared a hole in the wax pattern and cast a matching Williams pin. The pin was seated in the vent hole immediat.ely after the escape of the cement.
  19. 19. INTERNAL RELIEF • A space is, created between the casting and the prepared tooth to accommodate excess cement after closure of the marginal pathway of escape. • Pascoe demonstrated that the margins of oversized castings fit better than undersized castings after cementation. • The same effect can be obtained by relieving the internal surface of the crown.
  20. 20. A number of methods exist to achieve internal relief: Grinding the inside of the casting Internal carving of the wax pattern  Etching the internal surface of the wax restoration with aqua regia.  Electrochemical milling of the inside of the casting
  21. 21. • These methods have been rejected as inaccurate and inconsistent. It is impossible to obtain a uniform space for the cement. • These methods cannot be used for ceramometal restorations.
  22. 22. Internal relief can also be achieved by cutting internal channels in prepared abutment teeth and in the internal surface of the crowns before cementation.  Channels are 0.5 mm deep extending from the occlusal surface to within 1 mm of the cervical margin.  No significant difference was found after modification of the occlusal surface of the preparation.  This technique causes loss of adaptation of tooth & casting only in discreet areas where channels are prepared.
  23. 23. • An effective method of obtaining internal relief is die spacing. • Die spacer is an agent applied to a die to provide space for the luting agent in the finished casting. GPT 8 • A clinically acceptable relief is considered to be 20 to 40 um/ axial wall
  24. 24. THE CEMENTATION TECHNIQUE Three variations of the cementation technique have been used in an effort to obtain improved seating: Vibration Site of application of cement, and the Magnitude of cementation pressure.
  25. 25. Vibration • Oliveira et al found that vibration improved the fitting of MOD inlays by 16 um and crowns by 27 um. • Van Nortwick and Gettleman indicated that vibration applied horizontally while the cement was setting did not affect crown seating. • Koyano et al. reported that the cement film was thinner when vibrating pressure was applied. • Vertical vibration was slightly more effective than horizontal vibration but less comfortable to the patient.
  26. 26. Site of application of cement Ishikiriama et al. found that cement painted on the inner walls of the crown promoted a better fit than when the crown was completely filled with cement.
  27. 27. The flow pathway of cement and the amount of incomplete marginal seating of the restoration has been studied. Cement was applied at various locations on the internal surface of the crown and on the prepared tooth. Rimmer Y. The flow of zinc phosphate cement under a full coverage restoration according to the location of its application and its effect on the marginal adaptation. Location of Cementation Marginal Discrepancy Margin of crown 54um Apical half of axial wall of crown 106um Margin of prepared tooth 10um Apical half of axial wall of preparation 40um
  28. 28. Magnitude of cementation pressure • Jorgensen noted that an increase in load above 5 kg had little effect on the result. • Fusayama et al reported that excessive amounts of pressure (15 to 50 kg) applied during cementation produced no significant difference in the thickness of cement on the shoulders of preparations.
  29. 29. • Grajower et al reported that a pressure increase of 2 to 10 kg applied at the first try-on stage of cast full crowns caused an average apical movement by the crown of 63 um. • This is attributed to protuberances on the internal surface of the casting arresting the complete seating of the crown. • On application of pressure, these protrusions created furrows in the axial surface of the prepared tooth
  30. 30. • The average elevation of non precious metal crowns caused by cementation was found to be 54 um. • This elevation corresponds to an effective minimum cement thickness of 4.7 um at the axial walls. • On application of pressure, protrusions on the casting surface could cause high localized pressure on the cement and might therefore penetrate the cement layer.
  31. 31. • The results of Grajower et al indicate that the effective minimum cement thickness at the axial walls can be lower than 4 um and that the possibility for metal-dentin contact in certain locations cannot be excluded. • This raises a question as to the relevance of the ADA Specification No. 8. Jorgensen and Esbenser believe that the ADA Specification No. 8 relates more to the viscosity of the cement than to the film thickness.
  32. 32. • Grajower et al suggest that studies regarding cementation and crown elevation be done on teeth instead of on dies because mechanical properties of the dentin may affect the results.
  33. 33. • The amount of incomplete seating of a cemented crown is equal to the film thickness of the cement at the occlusal surface. • It is related to the cement thickness at the axial walls of the tooth preparation by the equation: Film thickness at occlusal surface = film thickness at axial wall Sine 1/2 the taper angle
  34. 34. • The Jorgensen filtration process explaining why the thickness of cement exceeds the ADA Specification No. 8 is based on two assumptions: 1. On placing the crown containing the cement on the prepared tooth, cement accumulates on the occlusal surface, and 2. Hydrodynamic pressure develops within the cement as the marginal opening prevents cement escaping.
  35. 35.  In well controlled circumstances, these two assumptions can be contradicted: 1. The amount of cement reaching the occlusal surface can be controlled and even totally prevented by applying cement only to specific areas of the crown or preparation. It is not yet clear whether complete elimination of cement at the occlusal surface is desirable inasmuch as an empty space may encourage microbial growth. 2. At the try-on stage of the crown, a space exists between it and the tooth preparation. If the volume of this space could be calculated and the correct amount of cement introduced into the space, hydrodynamic pressure would not develop. Controlling the amount of cement may well eliminate the filtration process of Jorgensen.
  36. 36. Windelers claims that the occlusal discrepancy is caused by excess cement that exceeds the space available for it and suggests the equation:  Occlusal discrepancy = film thickness on axial walls minus space available for cement sine 1/2 taper angle of the preparation • As shown by Grajower et al.a film thickness as small as 4 pm can be reached at the axial walls.
  37. 37. Fresh cement painted with a camel brush in the part of the crown to be cemented promotes a better fit than when the crown is completely filled with cement.  Influence of some factors on the fit of cemented crowns. JPD 1981 Apr;45(4):400-4.
  38. 38. • Brushed on cement on the inner surface of restoration produces a seating discrepancy one third less than that resulting from filling the crown half full and more than 2/3rd less than that resulting from filling the crown full. Tan K, ibbetson RJ: The effect of cement volume on crown seating . J Dent Res 1995;74:422
  39. 39. Seating force must be adequate to ensure complete seating of the crown but sudden excessive force may result in the elastic strain of the dentin, creating a rebound effect which results in the crown being partly dislodged due to elastic recoil.  JPD 1992; 68(3): 476-81
  40. 40. Orange wood blocks (wooden sticks or plastic wafer) used for seating crowns after cementation produced far less seating discrepancy as compared to cotton rolls that showed highest seating discrepancy. Influence of pressure and vibration during cementation. JPD 1979 41(2) 173-177
  41. 41. A delay in applying the seating force reduces the quality of cementation. A 20 second delay caused an increase in the seating gap of 0.02mm and 2um marginal gap.  JPD 1986; 55(1): 13-18
  42. 42. • The crown is inserted slowly to about half a distance, it is then withdrawn by a few millimeters and reinserted to almost the full extent of its length. The process is then repeated. Use a slight up and down movement along this path to assist the layering of the cement. When the operator no longer feels any resistance , the crown is pushed to the finish line for final seating Int J Prosthodont Restor Dent 2012: 2(2):77-81
  43. 43. Discussion  The art and cementation is to choose cement with an inherently low film thickness and use technique which allow it to escape while the crown is being seated.  Cement flow can be hindered by preparation features which cause a build up of hydrostatic pressure.  The problem can be overcome by die spacing and controlled cement application or by venting of the crown.
  44. 44. Conclusion • Research should continue to determine the correct amount of cement to place in the crown, the site of application, and the amount of cementation pressure so that space between the axial wall and the crown will accommodate the available cement. • The emphasis in the literature about incomplete seating of crowns during cementation has been placed on eliminating marginal discrepancy by venting and internal relief. • More stress should be placed on the cementation technique as a means of solving the problem of incomplete seating.