Surgical treatment for recurrent patellar dislocation with severe torsional deformities: Double-level derotational osteotomy may not have a clear advantage over single-level derotational osteotomy in improving clinical and radiological outcomes

Habitual patellar dislocation exhibits less severe rotational deformities but poorer trochlear development and higher incidence of patella baja compared with recurrent patellar dislocation in skeletally mature patients

PURPOSE

This study compared the radiological characteristics between habitual and recurrent patellar dislocation in skeletally mature patients.

METHODS

From 2017 to 2019, 77 skeletally mature patients with habitual patellar dislocation were surgically treated at a single institution and reviewed retrospectively. A total of 55 knees from these patients were included in the habitual patellar dislocation group. During the same period, 55 knees with recurrent patellar dislocation were randomly selected from 242 patients and included in the recurrent patellar dislocation group. Various bony deformities were measured and compared between the two groups. Additionally, a subgroup analysis was conducted among patients with habitual patellar dislocation, comparing those with and without an 'invisible patella' observed on true lateral views with 30° of knee flexion.

RESULTS

The femoral anteversion angle (21.8° vs. 26.3°, p = 0.041), tibiofemoral rotation angle (9.7° vs. 12.4°, p = 0.042) and external tibial rotation angle (24.3° vs. 29.6°, p = 0.001) in the habitual patellar dislocation group were significantly lower than those in the recurrent patellar dislocation group. 54% of knees in the habitual patellar dislocation group had a patella baja, and this was in sharp contrast to the recurrent patellar dislocation group in which none of the knees had a patella baja. 49.1% of knees in the habitual patellar dislocation group showed 'invisible patella' at 30° of knee flexion, and knees with 'invisible patella' had significantly higher tibial tubercle-trochlear groove (TT-TG) distance (30.4 vs. 19.8, p < 0.001) and tibiofemoral rotation angle (13.2° vs. 6.4°, p < 0.001) than knees with a visible patella.

CONCLUSIONS

A distinct difference in bony anatomical features was observed between habitual and recurrent patellar dislocation in skeletally mature patients. Habitual patellar dislocation exhibited less severe rotational deformities of the lower extremity but showed poorer trochlear and patellar development, a larger TT-TG distance and a higher incidence of patella baja compared with recurrent patellar dislocation.

LEVEL OF EVIDENCE

Level III.

Editorial Commentary: Surgical Treatment Algorithm for Patellofemoral Recurrent Instability

As a frame of reference, the patella can be "out" of the trochlea owing to extensor mechanism malalignment, such as an increased tibial tuberosity (TT)-trochlear groove (TG) distance, or the trochlea can be "out" of the patella if the TG is misplaced owing to increased internal femoral torsion. Both lead to an increased Q angle and patellofemoral maltracking. Medial patellofemoral ligament reconstruction acts as a medial checkrein against instability, but it does not solve maltracking in every case. TT medialization is recommended for an increased TT-TG distance, and TT anteromedialization additionally diminishes patellofemoral contact pressure. TT distalization can address patella alta. Trochleoplasty is a direct treatment for trochlea dysplasia. Varus osteotomy in cases of valgus deformity, and derotational femoral osteotomy in cases of increased internal femoral torsion, can realigning the trochlea with the patella. Derotational osteotomy addresses the misplaced trochlea-and not the patella or extensor mechanism. Cases of increased external tibial torsion (ETT) and foot pronation both result in compensatory increased hip internal rotation leading to knee internal rotation and lateral shear forces on the patella. In such cases, derotational supra-TT osteotomy provides better results than medialization because medialization could increase ETT and exacerbate symptoms. Supra-TT osteotomy results in TT medialization, whereas infra-TT osteotomy does not, so patients with increased ETT and an increased TT-TG distance may require supra-TT derotational osteotomy whereas those with increased ETT and a regular TT-TG distance may require infra-TT derotational osteotomy-if tibial derotational osteotomy is to be performed. Indications for medial patellofemoral ligament reconstruction, various TT transfers, trochleoplasty, varus osteotomy, and derotational osteotomy (with rotational population means and thresholds for surgical treatment) are finally presented, in addition to alert signs such as a squinting pattella, a high grade J sign, a type 2 extended Q angle described by Manilov, and the inverted proximal humerus sign.