The term osteosynthesis was coined by Albin Lambotte a Belgian surgeon regarded universally as the father of the modern internal and external fixation. He devised an external fixator and numerous different plates and screws.
Robert Danis as surgeon in Brussel published two books on osteosynthesis in 1932 and 1949.
A young swiss surgeon E. muller read his second book and he drew around himself a group of interested swiss surgeons and in 1958, at an historical weekend meeting in chur they decided to form a study group concerning issue of internal fixation of bone- the Arbeitgemeinschaft fur Osteosynthe-sefragen, or AO.
Bone plates are like internal splints holding together the fractured ends of a bone.
A bone plate has two mechanical functions. It transmits forces from one end of a bone to the other, bypassing and thus protecting the area of fractures. It also holds the fracture ends together while maintaining the proper alignment of the fragments throughout the healing process.
A neutralization plate acts as a ""bridge". It transmits various forces from one end of the bone to the other, bypassing the area of the fracture. Its main function is to act as a mechanical link between the healthy segments of bone above and below the fracture. Such a plate does not produce any compression at the fracture site.
The most common clinical application of the neutralization plate is to protect the screw fixation of a short oblique fracture, a butterfly fragment or a mildly comminuted fracture of a long bone, or for the fixation of a segmental bone defect in combination with bone grafting.
A compression plate produces a locking force across a fracture site to which it is applied. The effect occurs according to Newton's Third Law (action and reaction are equal opposite). The plate is attached to a bone fragment. It is then pulled across the fracture site by a device, producing tension in the plate. As a reaction to this tension, compression is produced at the fracture site across which the plate is fixed with the screws. The direction of the compression force is parallel to the plate.
The mechanical function of this plate, as the name suggests, is to strengthen (buttress) a weakened area of cortex. The plate prevents the bone from collapsing during the healing process. It is usually designed with a large surface area to facilitate wider distribution of the load.
A buttress plate applied a force to the bone which is perpendicular (normal) to the flat surface of the plate.
The fixation to the bone should begin in the middle of the plate, i.e. closest to the fracture site on the shaft. The screws should then be applied in an orderly fashion, one after the other, towards both ends of the plate.
A representative clinical example of a buttress plate is the T-plate used for the fixation of fractures of the distal radius and the tibial plateau.
- Its a self compression plate due to the special geometry of screw holes which allow the axial compression.
Dynamic compression principle: The holes of the plate are shaped like an inclined and transverse cylinder. Like a ball, the screw head slides down the inclined cylinder. Because the screw head is fixed to the bone via the shaft, it can only move vertically relative to the bone. The horizontal movement of the head, as it impacts the angled side of the hole, results in movement of the bone fragment relative to the plate and leads to compression of the fracture.
Axial compression result from the an interplay between screw hole geometry and eccentric placement of the screw in the screw hole. The screw hole is a combination of incline and horizontal cylinder which permits the downward and the horizontal movement of a sphere the screw hand. Sideway movement of screw head is impossible. The aim is to position the screw head at the intersection of inclined and the horizontal cylinder. At this point screw head has a spherical contact in the screw hole which result in the maximum stability without completely blocking the horizontal movement of the screw.
3. Plate hole distribution (extended middle segment)
The structure of a limited-contact dynamic compression plate. LC-DCP
In the dynamic compression plate (A), the area at the plate holes is less stiff than the area between them. During bending, the plate tends to bend only in the areas of the hole. The limited-contact dynamic compression plate (B) has an even stiffness without the risk of buckling at the screw holes.
The LC-DCP (limited contact DCP) is a further development of the DCP is used for the same indications as the DCP, but the improved design offers additional advantage.
The evenly distributed undercuts reduces the contact area between bone and plate to a minimum. This significantly reduces impairment of the blood supply of the underlying cortical bone undercuts also allow for the formation of a small callusbridge.
The enlarged cross section at the plate holes and the reduced cross section between holes offer a constant degree of stiffness along the long axis of the plate.
The engineering principle of the tension band is widely used in fracture fixation. It applies to the conversion of tensile forces to compression forces on the convex side of an eccentrically loaded bone.
Hands-on experience suggests that, in the humerus, screws grip seven cortices on each side of the fracture ; in the radius and the ulna, five; in the tibia, six, and in the femur, seven.
Type of Plate No. of Holes No. of Cortices Bones Small 3.5 6 holes` 5 to 6 Cortex Clavicle Narrow 4.5 8 holes 7 to 8 Cortex Femur Narrow 4.5 7 holes 7 to 8 Cortex Tibia Narrow 4.5 8 holes 7 to 8 Cortex Humerus Small 3.5 6 holes 5 to 6 Cortex Forearm
Used were the exact and complex contouring is required. eg. Pelvis, Distal Humerus, Clavicle.
Reconstruction plates are thicker than third tubular plates but not quite as thick as dynamic compression plates. Designed with deep notches between the holes, they can be contoured in 3 planes to fit complex surfaces, as around the pelvis and acetabulum. Reconstruction plates are provided in straight and slightly thicker and stiffer precurved lengths. As with tubular plates, they have oval screw holes, allowing potential for limited compression.
limited stability. The thin design allows for easy shaping and is primarily used on the lateral malleolus and distal ulna. The oval holes allow for limited fracture compression with eccentric screw placement.
The basic principle of LCP is its angular stability whereas stability of conventional plate osteosynthesis relies on the friction between the plate and bone.
The principle of fixation of LCP is screw locking.
The functional LCP screw is like that of external fixator pins, that is why they are called as internal fixator.
LCP provides the relative stability.
# heals by the callus formation (Secondary Healing).
The mechanical principle of a locked screw plate. (A) The plate sits slightly of the bone. (B) Tightening of the screw locks the screw head within the plate. The plate is not drawn toward the bone and there is no compression b/w the bone and the plate. The flux is bone/ screw/ plate/ screw/ bone.
Maintenance of primary reduction Once the locking screws engage the plate, no further tightening is possible. Therefore, the implant locks the bone segments in their relative positions regardless of degree of reduction. Precontouring the plate minimizes the gap between the plate and the bone, but an exact fit is not necessary for implant stability. This feature is especially advantageous in minimally or less invasive plating techniques because these techniques do not allow exact contouring of the plate to the bone surface. Bridge/Locked Plating Using Locking Screws • Screws lock to the plate, forming a fixed-angle construct. • Bone healing is achieved indirectly by callus formation when using locking screws exclusively.
Stability under load By locking the screws to the plate, the axial force is transmitted over the length of the plate. The risk of a secondary loss of the intraoperative reduction is reduced. Blood supply to the bone Locking the screw into the plate does not generate additional compression. Therefore, the periosteum will be protected and the blood supply to the bone preserved.
Used to cut threads in the bone of same size as the screw.
Have 3 flutes.
Entire far cortex must be taped.
Taping should be done manually.
Two turn forward and half turn in the reverse direction.
Cancellous Tap :
- Short and wide threads slightly smaller than the screw.
- One near cortex tapping is required.
DIFFERENT AO SCREWS 1.5 mm Cortex Screw 2.0 mm Cortex Screw 2.7 mm Cortex Screw MINI SCREW 4.0 Canceleous Screw -Partially Threaded. -Fully Threaded 3.5 mm Cortical Screw SMALL FRAGMENT SCREW 3.5 Cannulated Screw 4.0 mm Cannulated Screw 6.5 Cannulated Screw CANNULATED SCREW SYSTEM Malleolar Screw 4.5 6.5 mm Cancellous Screw 4.5 mm Cortex Screw LARGE STANDARD SCREWS.
THE LAG SCREW A lag screw is the most effective way to achieve compression between two bone fragments; it pulls the fragments together producing pressure across the fracture line. It achieves this by providing purchase on the distal fragment while being able to turn freely in the proximal. If the screw threads engage both cortices, the fragments remain apart like two nuts on the same bolt.
Top: Biomechanics of cannulated and noncannulated screws. Bottom: Ideally, lag screw fixation produces maximum interfragmentary compression when the screw is placed perpendicular to the fracture line.
Optimal inclination of the screw in relation to a simple fracture plane.
The dynamic hip screw (DHS) implant system has been designed primarily for the fixation of trochanteric fractures. It may also be used for certain subtrochanteric fractures as well as for selected basi-cervical femoral fractures.
The implant is based on the sliding nail principle which allows impaction of the fracture. This is made possible by the insertion of a wide diameter screw into the femoral head. A side plate, which has barrel at a fixed angle is slid over the screw and fixed to the femoral shaft.
The dynamic condylar screw (DCS) is similar to the DHS in its design and concept. The fixed angle between plate and barrel is 95 and the plate is contoured to fit the lateral surface of the distal end of the femur.
The main indications are fractures of the distal femur and inter-condylar fractures. It may also be used for certain intertrochanteric fractures and very proximal subtrochanteric fractures. Impaction of compression of the fracture is achieved by using the compression screw.
The most complication is failure of the implant. A wire put under pure tension is very strong. However, if bending forces are added, it will break by fatigue quite rapidly. As this also holds true for plates, it appears essential that in the diaphysis, the tension side of the bone is known and the opposite cortex is able to withstand the compression forces.
An external fixator is a device placed outside the skin which stabilizes the bone fragments through wires or pins connected to one or more longitudinal bars/ tubes.
Biomechanical Aspects :
Components of standard external fixators
Pins (Schanz screws/Steinmann pins)
Stainless steel tubes or carbon fibre rods
A variety of clamps to fasten pins/wires to tubes/rods
A variety of clamps to fasten pins/wires to tubes/rods.
Clamps to connect tubes/rods to tubes/rods
There are a variety of pins and wires available:
Steinmann pins for bilateral frames
Schanz screws, either self drilling or requiring pre drilling
Schanz screws with small diameter tips for use in small bones.
2.0 and 1.8 mm K-wires (#olives) for ring fixator.
Threaded K-wires for small external fixators.
The standard tubular system is employed for treatment of fractures in large bones, for arthodesis, and for bone lengthening and transport systems. The small external fixator is used mainly for fractures or distal radius and forearm as well as for fractures in children and adolescents.
Insufficiently stable external fixation may delay fracture healing and lead to pin loosening. However, too much stiffness or rigidity of the external fixator construct may also delay fracture healing, especially in open fractures. In the management of such fractures it may be necessary to "dynamise" an initially quiet stable configuration or add stability in case of pin loosening.