20 Development of plate fixators: current status and perspectives
Plate fixators have already replaced conventional plating systems in fracture treatment to a large extent. Internal fixators with angular stable locking screws provide for stable fixation, especially in osteoporosis or in highly instable fractures. “Biological” internal fixation can be achieved by application of a suitable implant: reduction should be performed as far as possible indirectly without exposure or direct manipulation of the fragments followed by bridging fixation without contouring the implant to the shape of the bones at the fracture site. The fixation distant from the fracture with angular stable bolts combined with the elasticity of the implant across the fracture zone stimulates osteogenesis. In angular stable systems fracture healing generally progresses via callus formation so that primary and secondary cancellous bone grafting is now seldom needed ( Fig 20-1 ). The most widely used system, and the one applied by the authors, is that, which currently only permits unidirectional locking of the screws in the plate. Therefore, the anatomical alignment of the screws, especially in areas adjacent to the joints, is of great importance. For this reason, special precontoured plates for different regions of the skeleton have already been developed. Their design is anatomically adapted and allows for optimal insertion of the fixation screws in the joint segment.
Due to the long lever arms and the heavy loading stress on the implant, osteotomies around the knee have always been problematic regarding stability and bone healing. Therefore, it seemed reasonable to transfer the approved principles of fracture stabilization with plate fixators to osteotomy procedures around the knee joint. The first operative technique to be developed was open-wedge valgization osteotomy of the proximal tibia. TomoFix MPT (medial proximal tibia) developed by Alex E Staubli and co-workers, Luzern, is finding ever increasing acceptance worldwide due to its outstanding biomechanical properties [1, 2]. The work of the AO Knee Expert Group has helped to design special plate fixators which meet the anatomic requirements of the specific skeletal region and the type of osteotomy.
2 Proximal lateral tibia
2.1 Closed-wedge valgization osteotomy
Closed-wedge valgization osteotomy of the proximal tibia has been a standard technique for many years. Various implants have been developed for this procedure, but the incidence of instability and loss of correction with nonangular stable implants is significant. Mechanical problems can be expected especially if a fracture of the medial cortex occurs intraopera- tively which subsequently leads to loss of support on the medial side. Pape carried out an RSA analysis and noticed up to 3 mm interfragmental instability in the first three postoperative weeks for corrections of more than 8 mm that were stabilized with nonangular stable plates [3, 4]. However, the mechanical stability of this type of osteotomy can be markedly improved by application of a plate fixator. Van Heerwaarden, using the TomoFix LPT (lateral proximal tibia), found interfragmental movement of less than 0.3 mm in his RSA analysis, ie, ten times less. This indicates that the application of a special plate fixator can clearly increase the stability of closed-wedge tibial valgization osteotomy. The authors recommend that a biplanar cutting technique should be used for this procedure in the manner that it is advised for medial open-wedge osteotomy of the tibia. The authors only perform closed-wedge osteotomy in exceptional cases because of the risk of damage to the extensor muscles and the peroneal nerve, which make the medial technique preferable in most cases.
2.2 Open-wedge varization osteotomy
Posttraumatic valgus deformities, eg, after fracture of the lateral tibial head, sometimes require open-wedge varization osteotomy of the lateral tibia. This can be achieved by application of the TomoFix LPT. The technique is similar to that for the medial tibia. In general, fibula osteotomy is required and can be performed either in the diaphyseal mid-third or in the region of the fibula neck (with exposure of the peroneal nerve). In our cases, we always inserted autogenous cancellous bone graft into the osteotomy gap ( Fig 20-2 ).
3 Proximal medial tibia
3.1 Open-wedge valgization osteotomy
The technique of open-wedge osteotomy of the proximal tibia is becoming increasingly established. Advantages of this technique are simple approach, preservation of lateral muscle insertions and the peroneal nerve, and minor morbidity of this procedure. TomoFix medial proximal tibia (MPT) provides optimal biomechanical properties for this procedure and the complication rate is correspondingly very low [1, 2, 5, 6]. The implant is relatively bulky, but this is of limited importance in percutaneous implantation technique. The plate shaft, however, may cause symptoms at the tibia in small patients. The current design is probably too large for Asian patients, in particular. This is why two modifications have been developed. The new standard plate has conventional locking head holes and not combination holes in the distal part of the plate so that the shaft is designed narrower in this area. In addition, the edges of the entire implant are rounded in an effort to reduce local soft-tissue irritation. Furthermore, a plate with overall smaller dimensions was developed for small people and especially for the Asian population. It is possible to apply this small implant in patients with up to 75 kg body weight. With these new designs, suitable implants should be available for all anatomical conditions.