O2098: Deep Dive on Sulcus Angle: Optimal Axial MRI Slice Selection and Cutoff Values

Background: The sulcus angle (SA) is defined as the angle formed by the lateral and medial trochlear facets and is an important measure used to evaluate trochlear dysplasia (TD). Although classically described on axial X-rays with normal values < 145°, recent publications have extended the application of SA to computed tomography (CT) and magnetic resonance imaging (MRI) studies. The mean cartilaginous SA in patients with patellofemoral instability (PI) was found to be 156.24° (95% CI: 153.71 – 158.77) and in controls to be 141.83° (95% CI: 139.90 – 143.76) based on MRI analysis. However, these studies assess TD at just one slice of the trochlear groove (TG) even though MRIs capture several relevant axial images. Previous research has demonstrated that the proximal trochlear groove (TG) is the most important location to assess; the lack of initial engagement of the PF joint during the first few degrees of flexion leads to instability. The purpose of this study is twofold: first, to establish the optimal cutoff value of the sulcus angle measured along the cartilaginous surface of the TG at four consecutive axial images and second, to identify the optimal axial image selection.

Methods: We retrospectively reviewed patients ages 9-25 at a tertiary care center (2012-2023) with PI to isolate those with recurrent patella dislocations (RPDs); age and sex-matched controls were identified from an internal database of patients with anterior cruciate ligament (ACL) injuries. Patients with prior surgery in the affected knee and those with substandard MRI studies were excluded from analysis. The proximal TG was defined as the most proximal axial image in which the cartilaginous portions of the medial and lateral trochlea are visible. Cartilaginous SA was recorded on MRI studies at four consecutive axial images for all participants (SA1 – most proximal, SA2, SA3, SA4 – most distal). We conducted the following tests: independent samples t-tests, Pearson correlations, receiver operating characteristics curve to determine area under the curve (AUC) and odds ratios (OR) using the 25th percentile of the recurrent group as the cutoff.

Results: In total, 206 knees were included in our measurements (106 Recurrent PI, average age = 15.29 ± 2.45; 100 ACL controls, average age = 15.60 ± 1.41; p=.277). Overall average SA is 159.6° RPDs is 147.3° in controls.

Conclusion: The cartilaginous SA is greater in RPDs than in controls at all levels of the trochlear groove. The “cutoff” for pathologic SA needs to be adjusted based on the level at which the trochlea is being assessed on MRI. Cartilaginous SA is more strongly associated with PI status at the proximal-most TG than at the distal trochlear groove. Odds ratio calculations suggest that TD in RPDs is more pathological in the proximal TG and more normal distally.