Purpose
To apply arthroscopic measurements of patellar position in a clinical series and describe the relationship to clinical and morphologic risk factors for patellar instability.
Methods
Patients who were undergoing surgical stabilization for patellar instability were included in this study. Arthroscopy was performed through a standard anterolateral portal created at the level of the inferior pole of the patella. A 30 degree arthroscope was utilized, with the pump pressure set at 40 mmHg. Patellotrochlear distance was measured as previously described, as the distance between the patella and trochlea at 20 degrees knee flexion as a measure of engagement. Lateral overhang was measured as the length of the patella (in mm) overhanging the lateral border of the lateral femoral condyle. Measurements were compared between the pre and postoperative states, and assessed for relationships with sex, height, weight, age, Beighton score, TTTG distance, CDI, and sulcus angle.
Results
15 knees from 15 patients were included in this study (8 male, 7 female). Patellotrochlear distance decreased from 12.4+/-3.8mm to 4.7+/-1.5mm after patellar stabilization (p<0.001), while lateral overhang measurements decreased from 10.8+/-4.7mm to 7.5+/-1.7mm (p=0.029). Stepwise multiple regression analysis demonstrated an independent moderate relationship between patellotrochlear distance and sulcus angle (r=0.67, R2=0.45, p=0.008), while lateral overhang demonstrated an independent relationship with TTTG distance and # of dislocations (R=0.72, R2=0.51, p=0.019). AUC calculations demonstrated that patellotrochlear distance >6mm indicated an unstable patella with an AUC of 1.0 with a sensitivity of 100%, specificity 92.6%, positive predictive value (PPV) 93.8%, and negative predictive value (NPV) 100%. For lateral overhang, a measurement >10mm indicated patellar instability with AUC 0.82, with a sensitivity of 100%, specificity 53.3%, PPV 100% and NPV 63.1%.
Conclusion
Utilizing previously described arthroscopic measurement techniques, this clinical pilot study demonstrated that both patellotrochlear distance and lateral overhang could differentiate between knees before and after patellar stabilization surgery. Furthermore, patellotrochlear distance measurements were found to correlate with sulcus angle, while lateral overhang correlated with TTTG distance. Further studies with a larger series and correlating patient outcomes will help identify the optimal thresholds that can be used during intraoperative assessment during patellar stabilization surgery.