TKA – Approach to Patellofemoral Instability

Factors associated with patellofemoral instability:
1. Extensor mechanism imbalance
2. Asymmetric patellar resection
3. Malposition of the components – femur, tibia, and/or patella
4. Component design – hinged components, or rotating hinge

1. Extensor Mechanism Imbalance
– excessive tightness of lateral retinaculum and associated weakness of vastus medialis (one theoretical advantage of doing a subvastus approach is that the vastus medialis is not weakened as much).
– can also occur if the sutures of the median parapatellar approach teas and the medial pull of vastus medialis is lost.

2. Asymmetric Patellar Resection
– remember that the patella is not a dome. The medial facet is quite abit more substantial than the lateral, so when you make your cut, you should be just skimming the bone on the lateral side, and taking more off medially. By taking symmetric wafer of bone, you actually cut the patella asymmetrically, and the thing may be tilted laterally.

3. Malposition of the Components – A HUGE Problem.
– Femoral Internal Rotation – (or failure to externally rotate is 3o) moves the trochlea medially allowing the patella to sublux laterally
– Tibial Internal Rotation – the tibial tubercle is then relatively lateral, moving the extensor mechanism lateral
– Patellar Component Centralization – placing the patellar component in the center of the patella is the wrong place for it to be; it should be placed about 3-4 mm medial to center, which is the median ridge of the patella. Placing it in the center of the patella actually lateralizes the patella and again, moves the extensor mechanism lateral.
4. Component Design
– in the normal knee, up to 20o of internal rotation occurs during early knee flexion – medializes the extensor mechanism.
– in rigid hinge devices, this internal rotation does not occur – can lead to patellofemoral instability
– in rotationally unconstrained implants, excessive external rotation of the tibia can occur, lateralizing the tibial tubercle

In most cases, the component design will not be an issue (or it will be obvious!) So you are left with the first three things to think about – extensor mechanism imbalance, asymmetric patellar resection, and malposition of the components. Look closely for malposition of the components!


Intraoperatively – do the lateral release and check for malposition of the components. If you’ve really fucked them up, you better revise them.
– physio for quads, external bracing, avoidance of aggravating activity
– if not working, take back for lateral retinacular release and advancement of vastus medialis (proximal realignment)
– if still not enough, consider medializing the tibial tubercle (distal realignment)
– all this time, you should be thinking that you are avoiding the real problem, and you should just get on and deal with the component malposition by revising one or all of the components.

“when there is significant component malalignment, tubercle osteotomy is less effective than component revision.”
– Lonner, JAAOS 1999

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