Quadriceps malalignment is strongly associated with recurrent patellofemoral instability in native knees

Patellofemoral instability (PFI) has been associated with multiple anatomical risk factors. The recent development of accurate quadriceps muscle measurements using three-dimensional imaging techniques have demonstrated that quadriceps alignment is highly variable and frequently leads to a lateralised force vector on the patella. This study aimed to determine the association between quadriceps malalignment and recurrent patella instability in native knees. This is a preliminary retrospective comparative cohort study where computed tomography (CT) scans of patients presenting with recurrent PFI relative to a control group of patients with medial compartment osteoarthritis were retrospectively analysed. Twenty patients were identified in each group. The quadriceps alignment was measured using the quadriceps tendon axial angle (QTAx) in both cohorts. Other commonly used anatomical measurements including the tibial tuberosity to trochlear groove distance (TT-TG), Dejour classifications, sulcus angle, trochlear groove shape and patella height were also measured and compared between groups using one-way ANOVA and Student t-tests for parametric variables and &#x3c7;test for nonparametric variables. The quadriceps tendon axial angle in the patella instability group averaged 24.8&#xb0;&#x2009;&#xb1;&#x2009;12.0&#xb0; external rotation, compared to 2.5&#xb0;&#x2009;&#xb1;&#x2009;9.4&#xb0; external rotation (p&#x2009;<&#x2009;0.001). 65% of the patients in the PFI group had trochlear dysplasia (Dejour 3.0 classification 1 to 3) compared to 10% in the control group (p&#x2009;<&#x2009;0.001). Quadriceps malalignment was strongly associated with PFI as determined by area under the curve (AUC) analysis of 0.94. PFI was also associated with trochlear dysplasia (AUC&#x2009;=&#x2009;0.75) and patella alta (AUC&#x2009;=&#x2009;0.78). Externally rotated quadriceps alignment was found to be strongly associated with the development of recurrent patellofemoral instability in native knees. This is likely due to the increased lateral force vector on the patella caused by external rotation of the quadriceps muscle around the femoral shaft. Assessment of quadriceps malalignment in patients with PFI has the potential to influence surgical decisions and determine prognosis in patients treated both surgically and non-operatively. Level III.