Our results demonstrate a strong association between trochlear morphology, specifically lateral trochlear inclination, and PFJ structural damage. Knees with low lateral trochlear inclination (flattened lateral trochlea) had more than two times the odds of lateral PFJ cartilage damage and BMLs compared with knees with high lateral trochlear inclination. These results support the hypothesis that those knees with flattened lateral trochlea are at increased risk for structural damage in the PFJ. As the knee begins to flex from an extended position, the patella will glide inferiorly and enter the trochlear groove. When the lateral facet is flattened, the patella will be more likely to be displaced laterally due to the lateral force vector acting on the patella as a result of the angle of pull of the quadriceps muscle (13
). Thus, knees with a trochlea that lacks a lateral “barrier” may be subject to increased shear forces between the patella and trochlea leading to cartilage damage and BMLs in the PFJ. We found no association between medial trochlear inclination and structural damage in either the lateral or medial PFJ. These results suggest that the inclination of the medial trochlear facet may not be as important in the stability of the PFJ as the lateral trochlear facet.
Previous studies have investigated the association between trochlear morphology and PFJ OA (8
). To our knowledge no other study has investigated this question with as much detail in the measurement of our outcomes (cartilage damage and BMLs). Ali et al found the mean lateral trochlear inclination in knees from subjects > 40 years old with no and severe cartilage damage to be 22.8 and 14.7 degrees, p=0.007 (9
). However, they did not define cartilage damage in specific anatomic regions of the PFJ (i.e. medial vs. lateral). Additionally, they found no differences in the mean sulcus angle between the groups. Our results also demonstrate a strong association between lower lateral trochlear inclination and lateral PFJ cartilage damage and BMLs, and also with medial PFJ cartilage damage. We also found an association between higher sulcus and cartilage damage and BMLs in lateral PFJ, albeit not as strong as lateral trochlear inclination. Trochlear angle, the third measure of trochlear morphology, also demonstrated a strong association with lateral PFJ cartilage damage ().
The association between quartiles of trochlear angle (TA) and patellofemoral joint (PFJ) cartilage damage and bone marrow lesions.
Both lateral trochlear inclination and trochlear angle use the posterior condylar line as a reference line, whereas the sulcus angle measures the angle between the medial and lateral trochlear facets. The sulcus angle may be similar in one knee that has a flattened medial facet and normal lateral facet and another that has a flattened lateral facet and normal medial facet. Despite finding an association between all three measures of trochlear morphology and structural damage, lateral trochlear inclination demonstrated the strongest association, as the results were similar when also accounting for the effect of the trochlear angle. Therefore, this variable should be included in future studies investigating PFJ pathology and can be easily measured to help identify patients at risk for PFJ OA.
Lateral trochlear inclination was also associated with medial PFJ cartilage damage, although not as strongly as lateral PFJ cartilage damage or BMLs. The mechanism for this damage may be different than for that of lateral PFJ damage. As the knee extends in knees with low lateral trochlear inclination, the patella is likely to be displaced laterally due to the shallow proximal trochlea (18
). As the knee begins to flex again, the patella, starting from a lateral position, would need to re-engage the trochlea and glide medially to center itself in the trochlea. When this occurs, there is potential that the patella may compress the medial PFJ while it is reengaging in the trochlea. Additionally, studies have demonstrated that the patella does displace medially in the initial degrees of flexion (19
). However, it is unknown if this medial displacement increases forces and stress in the medial PFJ.
We recognize limitations in this current study. First, using a cross-sectional design we cannot infer causality from our results. However, reverse causation is unlikely as the shape of the femoral trochlea likely does not change over time and structural damage in the PFJ would not cause changes in trochlear morphology. Second, the slice chosen to measure trochlear morphology could affect our results. We chose the slice where the posterior femoral condyles projected most posteriorly, which corresponds to the proximal trochlea and allows for visualization of both the medial and lateral trochlear facets on one slice. Finally, we acknowledge that measures of trochlear morphology can also be assessed from lateral or tangentional (skyline) radiographs. However, a tangentional radiograph is difficult to reproduce consistently (23
) and also only allows for visualization at one level of the trochlea. Using MRI we were able to focus on the proximal trochlea and also assess differences in the inclination of the lateral and medial trochlea, separately. It is also unknown if MRI measures of trochlear morphology correlate better with clinical findings (i.e. pain, function, etc.) than radiographic measures.
In summary, low lateral trochlear inclination was strongly associated with lateral PFJ cartilage damage and BMLs, and with medial PFJ cartilage damage. Additionally, the sulcus angle, which is most commonly reported in the literature, was not as strongly associated with structural damage as lateral trochlear inclination. Lateral trochlear inclination is easily measured from MRI and can be used to identify individuals at risk for knee OA.