Summary

Morphological characteristics of habitual patellar dislocation comparing with traumatic patellar dislocation were shallow sulcus, increased TT-TG, medial hypoplasia of femoral condyle, and patella infla.

Abstract

Introduction

Habitual patellar dislocation is defined as a condition with the patella always dislocating laterally during each knee movement. It occurs commonly in flexion but rarely in extension. It is not fully elucidated why the patella dislocates whenever the knee is flexed unlike traumatic patellar dislocation which often occurs in knee extension. Our hypothesis is that there are some morphological differences between habitual patellar dislocation knee and traumatic patellar dislocation knee.

Materials And Methods

Eleven knees with habitual patellar dislocation were retrospectively compared with 168 knees with traumatic patellar dislocation. Mean ages at examination were 28 years old (range, 12 to 58; 8 women, 3 men) and 22 years old (range, 12-67; 112 women, 56 men), respectively. Standard knee x-rays and computed tomographic scans were obtained. Sulcus angle, congruence angle, lateral tilt angle, type of trochlear dysplasia (Dejour’s classification), lateral deviation of the tibial tubercle (Tbial Tubercle-Trochlear Groove distance; TT-TG distance), and patellar height (Insall-Salvati ratio) were compared between habitual and traumatic patellar dislocation knee.

Results

Habitual patellar dislocation knees had higher mean sulcus angle (162° ± 12° vs 147° ± 7°; P < .01), higher congruence angle (58° ± 19° vs 13° ± 22°; P < .01), higher lateral tilt angle (37° ± 26° vs 3° ± 10°; P < .01), higher TT-TG distance (20 ± 4 mm vs 17 ± 4 mm; P < .01), and lower Insall-Salvati ratio (0.8 ± 0.1 vs 1.1 ± 0.1; P < .01) compared with traumatic dislocation knees. Of the 11 habitual patellar dislocation knees, 10 were classified as Dejour type C (91%) and 1 as type D (9%) while, of the 168 traumatic patellar dislocation knees, 18 were classified as Normal (11%), 80 as type A (48%), 30 as type B (18%), 28 as type C (17%), and 12 as type D (7%) (P < .01).

Discussion

Various pathological factors have been described in the pathogenesis of habitual dislocation of patella, including quadriceps contracture, weakness of the vastus medialis, trochlear dysplasia, insufficiency of the MPFL, patella alta, abnormal Q angle and genu valgum. However, it has not been known why the patella dislocates whenever the knee is flexed because there has been no comparative study to clarify the pathogenesis. This is the first study to clarify the morphological characteristics of habitual patellar dislocation comparing with traumatic patellar dislocation. Based on the current results, all of habitual dislocation knees had shallow sulcus, increased TT-TG, medial hypoplasia of femoral condyle, and patella infla. With these anatomical abnormalities, habitual dislocation of the patella would occur when the contracture of extensor muscles or structure was added.

Conclusion

Habitual dislocation knees had shallow sulcus, increased TT-TG, medial hypoplasia of femoral condyle, and patella infla compared with traumatic patellar dislocation knees.