Introduction
Trochlear dysplasia is a primary risk factor for patellar instability and refers to loss of the osteochondral constraint of the patella in the trochlear groove. Although this deformity is often quantified on axial imaging, the assessment of trochlear morphology on sagittal imaging has not commonly been described. The aim of this study was to describe measurements of trochlear length on sagittal MRI in patients with and without patellar instability, and to correlate trochlear length with measurements of trochlear dysplasia.
Methods
Knee MRIs of patients with a history of patellar instability were compared with age and gender-matched controls. Trochlear length was measured on sagittal MRI as the extension of the cartilage proximal to a plane through the center of the intercondylar notch. Length was quantified in mm, as well as using the trochlear alpha angle, with a line between the femoral shaft axis with a line between the middle of the notch and the proximal extension of the trochlea (smaller angle indicating more proximal extension of the trochlea). Measurements were performed at the center of the knee and center of the medial and lateral condyles, and compared between symptomatic and asymptomatic knees. Stepwise multiple regression analysis was performed to identify the relationship between each measurement and standard measurements of trochlear dysplasia, including sulcus angle, lateral trochlear inclination (LTI) and trochlear depth.
Results
48 knees were included in this study (36 female, 12 male, age 21.3+/-4.9years), of which 24 were symptomatic. At the center of the knee, the trochlea extended 13.5+/-3.3mm proximal to the middle of the notch in the symptomatic group vs 10.9+/-2.0mm in the control group (p=0.002). Similarly, trochlear alpha angle was smaller in the symptomatic group at the center of the knee (68.4+/-4.4 degrees vs 70.7+/-3.3 degrees), indicating a more proximal extension of the trochlea.
Stepwise multiple regression demonstrated independent relationships between central trochlear length with sulcus angle (coeff -0.17, p=0.046) and trochlear depth (coeff 0.30, p<0.001) (r=0.64, R2=0.41, p<0.001).
Conclusion
In this study of trochlear morphology, the central trochlea was found to extend 2.6mm more proximally in knees with patellar instability, and trochlear length correlated with measurements of dysplasia. As assessment of the trochlea and surgical treatments are often based on the proximal entry point of the cartilaginous trochlea, further studies are needed to identify the impact this difference may have on the measurement, diagnosis and treatment of trochlear dysplasia.