Summary

Comparison of vastus medialis anatomy in MRI between patellar instability patients and controls.

Abstract

Objective

The vastus medialis is considered an important stabilizer muscle of the patella. Surprisingly, few studies have described its anatomy using magnetic resonance imaging (MRI) scans. The aim of this study is to compare the anatomy of the vastus medialis by MRI in patients with patellar instability and controls.

Methods

The Patellar Instability group consisted of patients with at least one episode of complete patellar dislocation. The Control group consisted of patients with anterior cruciate ligament injury, meniscal injury or sprains. Patients with patellofemoral pain were not included. The groups were matched for sex, and patients younger than 15 years were excluded. All measurements were performed by one examiner. A second examiner repeated the measurements in 30 cases to evaluate the inter-observer agreement (intraclass correlation coefficient - ICC). The anatomy of the vastus medialis was examined using the following measurements: distance between the most distal part of the origin of the vastus medialis in the femoral diaphysis and the medial femoral condyle (longer distances indicated more vertical and fewer oblique fibers); distance from the proximal pole of the patella to the most distal insertion of the muscle and its ratio to the length of the articular surface of the patella (greater ratios indicated greater medial patellar coverage by the muscle); and a qualitative description of the insertion position of the muscle fibers (directly in the patella or only in the medial retinaculum). The hypotheses were that in the Patellar Instability group, the insertion of the vastus medialis would be more proximal and more closely related to the retinaculum than directly related to the patella. In addition, the most distal origin of the muscle in the femur would be more proximal, suggesting more vertical and fewer oblique fibers.

Results

A total of 78 knees were included in the Control group, with a mean age of 30.2 ± 7.8 years, and 78 knees were included in the Patellar Instability group, with a mean age of 25.6 ± 7.5 years (p = 0.001). Both groups had 48 (61.5%) women. The distances of the vastus origin to the condyle were 27.52 mm ± 3.49 and 26.59 mm ± 3.43 (p = 0.041), the distances from the proximal pole of the patella to the most distal muscle insertion were 17.59 mm ± 5.54 and 15.02 mm ± 4.18 (p < 0.001), and the ratios of this distance to the joint surface length were 0.586 ± 0.18 and 0.481 ± 0.13 (p < 0.001) in the Control and Patellar Instability groups, respectively. In 75.6% of the patients in the Patellar Instability group, the insertion of the vastus was in the ligament and not in the patella, compared to 52.6% in the Control group (p = 0.003, odds ratio = 2.8). The ICC for the distance from the proximal pole of the patella to the most distal insertion of the muscle was 0.688 (good, 95% CI 0.43-0.84), and that for the distance from the vastus origin to the condyle was 0.59 (fair, 95% 0.29-0.78). The Kappa statistic for the insertion site of the vastus was 0.8 (excellent).

Conclusion

The distal insertion of the vastus medialis differed significantly in patients with patellar instability, with a more proximal insertion and less patellar coverage relative to controls, and insertion of the muscle fibers was more frequently found in the retinaculum instead of directly to the patella in the Patellar Instability group.Level of evidence: IIC