The clinical significance of this case underscores the difficulties inherent in early diagnosis of habitual patellar dislocation even using the skyline view. Although the patella had looked odd to her father due to tilting compared with contralateral side, it had been radiologically confirmed to be on the trochlea at 1 year prior to the referral. This case also demonstrates the time course from the initial patellar dislocation to habitual dislocation, which has not been reported previously. It implies that first-time patellar dislocation can gradually lead to habitual dislocation as well as recurrent dislocation.
Previous articles have reported that 40 to 60% of patients with first-time patellar dislocation have advanced to recurrent dislocation [8, 9] due to several predisposing factors, including patella alta, abnormal patellar morphology, trochlear dysplasia, increased Q angle with lateralized tibial tuberosity, genu valgum, ligament hyperlaxity, external tibial torsion, and increased femoral anteversion [8], [10–13]. In contrast with recurrent dislocation, the factors contributing to the onset of habitual dislocation and its time course from initial dislocation have not been elucidated despite previous reports of an association with quadriceps fibrosis due to muscle injections [14–16], quadriceps contracture [17], and abnormal attachment of the iliotibial tract to the patella [18], none of which were applicable to our case. Among the various parameters evaluated in our case, modified Q angle, sulcus angle, and lateral deviation angle showed aberrations. Further studies, using either meta-analysis or cases series, are needed to determine the factors that predispose an initial dislocation to become a habitual dislocation. Such research will allow more effective treatment of patellar dislocation by predicting the course after first-time dislocation.
In addition to the lateral reticular release, MPFL reconstruction was applied to our case. Since the epiphyseal plate was still open, the femoral attachment site was shifted from the original point to a point just posterior to medial femoral epicondyle and distal to the adductor tubercule and growth plate. Considering the length patterns reported previously [19], it is suggested that the reconstructed ligament be slightly loose in flexion. We have confirmed that the patella in our patient is stable in the femoral groove with the knee in greater than 60° of flexion. At 2 years after surgery, our patient is capable of bending her knee fully without fear of dislocation. Since the patient is in her growth spurt, we will continue close follow-up to ensure a good clinical outcome for our patient.
We emphasize that, had it been suspected 1 year earlier that this case would eventually lead to habitual dislocation, the lateral retinacular release would not have been required [1, 20]. Thus, the possibility of habitual dislocation should be considered if patella appears odd and different from the opposite side in extension, even if the radiological examinations do not show apparent dislocation.