Patello femoral Instability: A.Extensor mechanism imbalance: In which the lat. Retinaculum is too tight or medial soft tissues are too loose. If lat. Retinaculum is too tight. Lat release is indicated, if possible sparing the sup. Lat. geniculate A. Medial retinacular laxity may occur with postoperative rupture of the medial capsular repair. Some authors have advised closing the retinacular capsular layer with the knee in 90° of the flexion. The knee should be placed through a full range of motion after medial capsular closure.
B.Malpositioned patellar, femoral or tibial components: Also may lead to patellofemoral instability. I.Excessive lateral patellar facet resection. May lead to tilting of the patellar component. (the level of lat. Facet resection must be much shallower than the medical facet) Lateral placement of the patellar component on the cut surface of the patella. Can lead to lat. Subluxation of the patella in extension. II.Malposition of the tibial component in an internally rotated position increase the Q angle by moving the tibial tubercle laterally and the increased Q angle leads to lateral subluxation. The tibial component should be centered on the medial border of the tibial tubercle. With any deviation into external rotation.
III.Internal rotation and medial translation of the femoral component make the trochlea more medial relative to the extensor mechanism leading to lat subluxation. The posterior femoral condyles, epicondylar axis and anterposterior axis all are useful for intra operative evaluation of femoral component rotational alignment in the primary TKA. In revision arthroplasty, the position of the previous component and the epicondylar axis are the only landmarks available for this assessment. Surgical treatment of patellar subluxation is based on the cause. The components should be inspected for malposition and if necessary they should revised.
If the components are positioned appropriately surgical efforts to improve patellar tracking should proceed in stepwise fashion. 1.Lateral retinacular release should be performed first. 2.If patellar subluxation persists a proximal patellar realignment procedure should be done (advancement of vastus medialis) 3.Distal realignment procedures such as the tibial tubercle osteotomy should be undertaken with extreme caution because serious functional loss would result if nonunion occurred.
Patellar fracture: (0.5%) It is correlated with multiple factors, excessive patellar resection, vascular compromise secondary to lateral release, patellar maltracking, excessive joint line elevation, knee flexion of more than 115°, trauma, thermal necrosis of PMMA and revision. There is a statistically significant association between lateral release and patellar fracture. Nonunion and hardware failure after I.F. of patellar fracture in TKA are frequent. The recommended treatment of displaced and non displaced fractures with no extensor lag and no loosening of patellar component is non operative. If operative treatment of patellar fracture is necessary component malalignment should be corrected.
Periprosthetic patellar fractures have been classified according to the integrity of extensor mechanism and stability of the implant. Type I: –Fractures with an intact extensor mechanism and stable implant. –Should be treated non operatively with a knee immobilizer or cylinder cast for 6 weeks. Type II: –Displaced fractures with extensor mechanism discontinuity. –Should be treated operatively with tension Band wiring and retinacular repair.
Type III: –Fractures with loose patellar component. Should be treated by excision of implant, it should not be replaced because may impair fracture healing. Stable patellar component that impair fracture fixation also should be removed. Proximal or distal pole fractures should be treated with partial patellectomy and suture repair. When extreme communication or poor bone stock is seen patellectomy and extensor mechanism repair is indicated.
Patellar component failure Metal – backed patellar components failed by various mechanism: 1)Fatigue fracture of metal base plate 2)Delamination of the polyethylene 3)Failure of the ingrowth interface 4)Wear in areas of thin polyethylene Clinically, the onset of a knee effusion, patello femoral crepitus or audible squeaking and scraping suggest component failure. Early revision of the failed components is recommended to prevent extensive metallosis of the knee. Revision consists of exchange of tibial polyethylen insert, synovectomy and revision or removal of patellar component.
Patellar component loosening: Predisposing factors are: 1.Deficient bone stock 2.Component malposition and subluxation 3.Patellar fracture 4.Osteonecrosis of the patella 5.Loosening of other knee components. More symptomatic patients require revision, component removal or patellectomy depending on the status of the remaining patellar bone.
Patellar clunk syndrome: First was described by Hozack in association with P.S. knee artroplasties. A fibrous nodule forms on the posterior surface of the quadriceps tendon, just above the superior pole of the patella. This nodule can become entrapped in the intercondylar notch of the femoral prosthsis and cause the knee to POP or clunk at approximately 30 to 45 degree of knee flexion when the knee is actively extended.
Two causes for this condition have been proposed: 1)Proximal placement of patellar button. The prominent button could impinge on the quadriceps tendon with resultant fibrous tissue proliferation. 2)Femoral component design is another possible cause. Posterior stabilized femoral components with high, sharp femoral sulcus could impinge on the quadriceps tendon.
The recommended treatment is arthroscopic debridement of the nodule. Arthrotomy and nodule excision is indicated when. 1.Recurrence after arthroscopic treatment occur. 2.Loose or malpositioned patellar components that may require revision. Insall recommended a limited synovectomy of the posterior surface of the quadriceps tendon when performing a P.S. knee Arthroplasty.
Rupture of the quadriceps or the patellar tendon. (0.1% to 0.55%) 1.Quadriceps rupture is related to lateral release because of vascular compromise and extension of release anteriorly that weaken the tendon. For partial tear non operative treatment is recommended. Surgical repair is advocated for complete tears, although the results, are suboptimal. 2.Patellar tendon rupture: Is associated with previous knee surgery. Knee manipulation and distal realignment procedures of extensor mechanism.
Treatments of patellar tendon rupture after TKA including: Direct repair, augmentation with hamstring tendons or synthetic ligament substitutes. Gastrocnemius flap, and use of the extensor mechanism allograft. It is better to tension the allograft with knee in full extension. Non of these procedures has been routinely successful. If patellar bone stock allows distal primary repair seems warranted with the addition of a tension band wire from the proximal patella to the tibial tubercle or hamstring augmentation or both. When patella is absent or insufficient for distal repair extensor mechanism allograft reconstruction or gastrocnemius muscle flap should be considered.