Purpose

Trochlear dysplasia is a known risk factor for patellar instability, which has also been associated with patellofemoral cartilage damage, and ultimately osteoarthritis of the patellofemoral joint. Particularly patients with high-grade trochlear dysplasia also show other pathological patellofemoral anatomical parameters, such as an increased tibial-tubercle-trochlear-groove distance (TTTG). Yet, there is paucity in the literature regarding risk factors for patellofemoral cartilage lesions in patients with trochlear dysplasia in the setting of patellar instability. The purpose of this study was therefore to identify risk factors in this patient cohort to guide surgical decision-making when surgical correction is indicated

Methods

This study evaluated 363 knees with trochlear dysplasia, who were scheduled to undergo surgery for the treatment of patellar instability at a single institution. All patients presented with a true lateral radiograph and preoperative MRI. Various patellofemoral parameters were assessed including patella morphology (Wiberg type, patella width, thickness, angle), patella height and axial positioning (Caton-Deschamps index, patellotrochlea index, patella tilt, Merchant’s congruence angle), trochlea morphology (Dejour type, trochlea sulcus angle and depth, supratrochlear spur height), quadriceps vector (TTTG, tibial-tubercle-posterior cruciate ligament distance (TTPCL), sagittal TTTG) and femorotibial rotation. All MRIs were assessed for full-thickness cartilage lesions in the patellofemoral joint.

Results

Of the included 363 knees, 91 (25.1%) showed full-thickness cartilage defects on the patella, while 21 (5.8%) had trochlea cartilage damage. Patients with trochlear dysplasia type B and D showed the highest prevalence of patellofemoral cartilage among all patients (patella defect: A: 16.1%, B: 28.7%, C: 20.2% and D: 29.7%, p=0.082; trochlea defect: A: 0%, B: 6.6%, C: 2.1% and D: 12.9%, p=0.003). Patients with cartilage defects on the patella showed increased age (p>0.001) and BMI (p=0.007), greater patella width (p=0.003) and angle (p=0.004), increased patellotrochlea index (p=0.023) and Merchant’s congruence angle (p=0.004), and a larger supratrochlear spur (p=0.001). Patients with cartilage defects on the trochlea were also older (p=0.008), had a greater patella angle (p=0.002) and tilt (p<0.001), an increased Merchant’s congruence angle (p<0.001), a larger supratrochlear spur (p=0.015), a greater TTTG (p=0.013) and sagittal TTTG (p=0.009) and an increased femorotibial rotation (p=0.002). Among all patients, patella Wiberg type 1 had significantly more often patella defects than other types of patella morphology (39.6% vs. 24.3% vs. 17.2%, p=0.023), this was even more pronounced in patients with high-grade trochlear dysplasia (44.2% vs. 25.6% vs. 17.0%, p=0.011). Trochlear cartilage lesions did not show any association with patella morphology (n.s.). Also, recurrent patella dislocations were not associated with the incidence of patellofemoral defects (n.s.).

Conclusion

Only a minority of patients with trochlear dysplasia, particularly with Dejour type B and D, present with full-thickness patellofemoral cartilage lesions, whereby most occur on the patellar. Yet, both patellar and trochlear defects are associated with patella and trochlea morphology, and patellar positioning. Trochlear lesions also seem to be affected by the coronal and sagittal quadriceps vector as well as femorotibial rotation.