Patellar complications comprise a major portion of total knee complications.
Patellofemoral dysfunction may be related to a variety of problems involving the patella including:
- Instability and malalignment
- Extensor tendon mechanism disruption
- Component wear and loosening
Patellar instability and malalignment result in tracking abnormalities which cause pain and premature component wear. Tracking problems arise from improper surgical placement of components or failure to perform lateral retinacular release when necessary. Subluxation of the patellar component, usually lateral, is easily identified on patellofemoral radiographs.
Rupture of either the patellar or quadriceps tendon is a devastating complication causing absence of knee extension. Lateral radiographs demonstrate patella alta with quadriceps tendon rupture and patella baja with patellar tendon rupture. Adjacent soft-tissue swelling with loss of defined tendon shadow is also present.
Component wear and subsequent loosening occur in up to 5-11% of metal-backed patellar components.18 For this reason, polyethylene-backed patellar components are now in use. Loosening is less frequent and when it does occur it is usually related to malalignment or improper cement technique.
Patellar osteonecrosis results in fragmentation and marked resorption. Interruption of blood supply during arthrotomy, patellar realignment surgery, and lateral release of patella have been implicated as risk factors by interrupting blood supply.19
Baseline and 18-month follow-up radiographs demonstrating resorption of the patella related to avascular necrosis
Diagram of blood supply to patella
Sequential radiographs with fracture of superior pole of patella followed by superior dislocation of the polyethylene patellar components (arrow)
Laterally dislocated patella