Patellofemoral instability after total knee arthroplasty
Factors determining patellofemoral stability
Patellofemoral (PF) instability after TKA has been reported in up to 20% of TKAs . Most often, it is caused by technical errors during surgery [1, 2]. Given the complexity of TKA biomechanics, many technical parameters are susceptible to error. Therefore, in most cases, PF instability cannot be traced back to a single cause. More likely, there are multiple contributors.
As a general rule, any manipulation of the normal anatomical and kinematic relationships of knee structures that increases tension in the lateral retinaculum or increases the quadriceps angle, or Q-angle (Figure 1), will produce an abnormal, laterally-directed muscle vector and thus cause lateral maltracking of the patella, instability of the PF joint, and more serious patellar complications if left untreated [3, 4].
Fundación Santa Fe de Bogotá University Hospital
First and foremost, excessive valgus alignment is an important risk factor, since it leads to a mismatch of the trochlear groove and the extensor vector . This encumbers proper patellar tracking and may cause the patella to tilt, subluxate, or even dislocate. A similar mechanism takes effect when a normal Q-angle is not restored. Especially patients with severe preoperative valgus or external rotational deformity, preoperative maltracking , and loss of bone stock in the distal lateral condyle are at risk . In patients with pronounced preoperative valgus, the main culprit for this predisposition is usually the retraction of the lateral retinaculum.
Beyond the overall leg alignment, individual component positioning is the most important contributor. Internal rotation of the femoral or the tibial component [5-10] as well as medialization of the femoral component and incorrect placement of the patellar component [10, 11] appear the most obvious.
An internally rotated femoral component shifts the trochlear groove medially, thus increasing the distance to the patella, which tracks laterally relative to the femur. Through the tension exerted by the lateral retinaculum, the patella is pulled sideways. This may lead to patellar tilt, subluxation, or even dislocation .
On the other hand, an internally rotated tibial component causes the tibia to rotate externally during knee flexion (Figure 2). This drives the tibial tubercle laterally, which increases the Q-angle and thus leads to lateral tracking. Depending on the severity, this may again lead to patellar tilt, subluxation, or dislocation.
Did you know?
When rotational platforms were introduced, there were high hopes that through the inherent self-alignment, a certain degree of malrotation could be corrected for. Unfortunately, this was not the case. Biomechanical studies analyzing the effect of rotational platforms on patellar tracking in the presence of malrotated femoral components did not find a significant compensatory effect [6, 7], and neither did a large randomized clinical study .
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- Placement of the prosthetic patella
- Medialization of the femoral component
- Soft-tissue imbalance
- Other risk factors
- Diagnostic workup in PF instability
- The clinical workup
- The radiological workup
- Management algorithm
- Outcome and conclusion
Additional AO resources on this topic
Access videos, tools, and other assets to learn more about this topic.
- Video: Extensor Mechanism Problems after Total Knee Arthroplasty
- Video: Managing Deformity in Total Knee Arthroplasty
- Video: The Difficult Primary Total Knee Arthroplasty
- Video: Tips and Tricks to Improve Total Knee Arthroplasty Outcomes
- Upcoming events: AO Recon Course finder
This series of articles was created with the support of the following specialists (in alphabetical order):
Fundación Santa Fe de Bogotá University Hospital
Charité—University Medicine Berlin
Beijing Jishuitan Hospital
This issue was written by Elke Rometsch, AO Innovation Translation Center, Clinical Science, Switzerland.
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