Patellofemoral instability after total knee arthroplasty

Preview

Patellofemoral instability after total knee arthroplasty (TKA) is a serious complication that has significant impact on knee function. The underlying causes are usually related to the surgical technique. Most often, patellar instability results from component malposition, limb malalignment, improper patellar preparation, or soft-tissue imbalance. Understanding the root causes helps to prevent these complications as well as to manage them successfully.

Factors determining patellofemoral stability

Patellofemoral (PF) instability after TKA has been reported in up to 20% of TKAs [1]. 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].

Figure 1. The Q-angle is the angle between a line from the ASIS to the midpoint of the patella (dotted line) and the line from the tibial tubercle to the midpoint of the patella. By moving the patella laterally (b), the CQ-angle decreases to zero, whereas the RFQ-angle becomes negative. When the patella is moved medially (c) both CQ and RFQ become larger (more positive). Abbreviations: ASIS: anterior superior iliac spine; CQ: clinical Q-angle; PA: patella; RF: rectus femoris; RFQ: RF-Q-angle; TT: tibial tubercle. Redrawn from source: Freedman BR, Brindle TJ, Sheehan FT. Re-evaluating the functional implications of the Q-angle and its relationship to in-vivo patellofemoral kinematics. Clin Biomech (Bristol, Avon). 2014;29(10):1139–1145.

Guillermo Bonilla, clinical professor for orthopedic surgery at the Universidad de los Andes in Bogota, Colombia, emphasizes: “In order to adequately treat problems arising from patellar instability, it is essential to understand the underlying biomechanical mechanisms.” So, let us consider and analyze the major factors that may lead to patellar instability.

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 [4]. 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 [5], and loss of bone stock in the distal lateral condyle are at risk [3]. 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 [4].

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.

Figure 2. Internal rotation of the tibial component forces the tibia into external rotation during knee flexion, increasing the Q-angle and leading to lateral patellar tracking and subluxation. Diagram on the right showing that rotating the implant (sagittal axis dotted line) internally (medially) relative to the tibia (sagittal axis continuous line) displaces the tibial tubercle laterally and consequently the patella as well. Adapted from: Malo M, Vince KG. The unstable patella after total knee arthroplasty: etiology, prevention, and management. J Am Acad Orthop Surg. 2003;11(5):364–371.