[Biomechanika osteosyntézy periprotetických zlomenin femuru po aloplastice kyčelního kloubu].

Periprosthetic femoral fractures around the femoral stem of a hip prosthesis constitute a serious challenge. The number of these fractures, associated with a high failure rate and a high number of overall complications, including higher mortality, has been on an increase worldwide. Stable femoral stems are indicated for osteosynthesis, while in case of loosening the method of choice is the replacement by a revision implant. The aim of osteosynthesis of periprosthetic femoral fractures is a stable fixation with soft tissue preservation which results in faster union allowing the patient to return to pre-injury activities. Biomechanical studies on cadavers or on synthetic models and computer simulations make it possible to evaluate the fixation strength in various types of implants in dependence on fracture characteristics and bone quality, but cannot be substituted for clinical trials since there is no direct proportion between fixation rigidity and fracture union. Fundamental principles that shall be followed in osteosynthesis of periprosthetic fractures can be deduced from the available studies. Proximal fixation by screws or a combination of screws and cables are biomechanically more advantageous than the Ogden fixation by cerclage wires or cables. Bicortical fixation enabled by state-of-the-art implants of LAP-LCP or NCB type represents a significantly more stable construction compared to monocortical fixation and led to reduced use of structural allografts. Better stability can be achieved by "double plating" technique which is applied especially in revision surgeries, but also in osteoporotic periprosthetic fractures with a defect zone. In these cases, osteosynthesis with long plates is recommended, bridging the entire femur, i.e. distally with femoral condyles fixation since this prevents the risk of a fracture below the plate. In shorter plates, this risk created by stress concentration at the end screw is amplified when a distal bicortical locking screw is used. That is why it is beneficial to reduce this stress by a monocortical screw or with the use of a conventional screw. Adherence to the principles regarding the position, type and number of screws constitutes the key parameter of successful osteosynthesis of periprosthetic fractures. Key words: hip joint, arthroplasty, periprosthetic fractures, biomechanics of osteosynthesis.