Preoperative quadriceps malalignment is associated with poor outcomes after knee replacement which are avoided by external rotation of the femoral component

Purpose: Lateralisation of the proximal apex of the quadriceps tendon relative to the mechanical axis or external rotation relative to the femoral shaft can be accurately measured and is strongly associated with patella maltracking. The aim of this study was to first assess the association between preoperative quadriceps tendon alignment (QTA) and the patient-reported outcomes (PROMs) of total knee replacement, and second, determine the influence of component position on outcomes in patients with preoperative quadriceps tendon malalignment (QTM).

Methods: A retrospective analysis of prospectively collected data was performed. All patients had preoperative and postoperative CT scans performed. PROMs were collected preoperatively and at 1 year postoperatively. QTA was measured by the quadriceps tendon axial angle (QTAx). The preoperative and postoperative coronal and axial alignment were measured. Femoral component rotation was measured relative to the preoperative posterior condyles.

Results: Analysis was conducted on 388 cases and the mean preoperative QTAx was 6.2° externally rotated (standard deviation 12.0°). QTM (QTAx > 14°) was identified in 76 (19.8%) patients. The diagnosis of QTM was associated with reduced patient outcomes including Forgotten Joint Score (60.2 vs. 51.2, p = 0.008), EuroQol Visual Analogue Scale (81.3 vs. 75.7, p = 0.009), KOOS-12 (80.3 vs. 73.3, p = 0.001) and reduced PASS percentages for all KOOS subscales. In patients with preoperative QTM, femoral component external rotation >2° was associated with improved PROMs when compared to patients with <2° of femoral rotation. This included a clinically significant difference in the improvement of KOOS-12 (11.7 points, p = 0.013) and improved PASS percentages in all KOOS subscales. There was no association between coronal alignment or tibial axial alignment and outcomes.

Conclusions: Quadriceps malalignment is a common cause for poorer patient outcomes following total knee replacement. This can be avoided by externally rotating the femoral component to accommodate the deformity in the extensor mechanism.

Level of evidence: Level III, case-control study.