AVOIDING EXTENSOR MECHANISM PROBLEMS: BUT WHEN THEY HAPPEN

Abstract

Avoidance of extensor mechanism complications is best obtained by using components with an appropriately designed femoral trochlea and patellar component and where internally rotated components are avoided. Residual valgus deformity may also lead to patellar dislocation or more subtle subluxations, which may manifest as pain along the medial patellar retinaculum. Because rotational abnormalities are difficult to detect on plain x-ray, CT scans are a useful way to diagnose this problem. Peripatellar crepitation may cause symptoms and can be avoided by aggressive peripatellar synovectomy at the time of surgery. In its most severe form, the patellar clunk syndrome, most commonly seen in posterior stabilised knees, arthroscopic debridement of the offending fibrous nodule may be needed. Patellar fracture is best treated on the basis of residual extensor mechanism function. Maintenance of active extension following fracture (with no loss of component fixation) is a good sign that conservative therapy will yield better function than ORIF. Loss of extension indicates that surgery will be required. In this setting restoration of extensor continuity is more important than retention of the patellar component. Patellar tendon avulsion can be avoided by careful attention to the tendon during surgical exposure. Intraoperative repair alone is rarely successful and should be reinforced by semitendonosis, or fascia lata grafting. Late rupture may be treated by this type of grafting alone. But if the patella is necrotic or the failure results in a high riding patella, mechanism transplant is preferred. In this setting attention to fixing the graft in full extension is mandatory to prevent severe extensor lag, as the allograft will stretch out overtime.