Background
Recurrent patellar dislocation (RPD) is a complex condition, and multiple contributing factors for patellar instability have been identified, including patella alta, genu valgum, disrupted or weaken medial soft tissue, trochlear dysplasia, increased tibial tuberosity and the trochlear groove (TT-TG) distance, and rotational malalignment of the femur or tibia [
1‐
5]. Specially, osseous deformities in the coronal and axial plane, such as genu valgum and torsional deformities of the lower extremity, are now considered to be associated with adverse effects on patellofemoral instability [
6,
7]. Herein, the increased femoral anteversion angle (FAA) and genu valgum are thought to create a sustaining lateralizing force vector applied on the patella, which might increase excessive loading forces on the reconstructed graft and even lead to patellar redislocation [
8,
9].
At present, there is still much controversy in the surgical techniques of RPD combined with knee valgus deformity. In recent years, various surgical techniques for addressing RPD with genu valgum have been described [
10‐
12]. Generally, the MPFL is the major stabilizer which restricts lateral patellar displacement from zero to thirty of knee flexion, and MPFL reconstruction has been validated as a reliable surgical procedure for treating recurrent patellar instability. Nevertheless, isolated MPFL reconstruction might not be sufficient in patients with increased femoral anteversion and genu valgum, as it does not address the underlying lateralizing force vector acting on the patella [
13].
Although it has been shown that genu valgum combined with excessive femoral internal torsion are primary risk factors for RPD, it is rarely corrected by surgery simultaneously. Despite studies that confirm the association between mechanical malalignment of multi-plane and patellar instability, there is a paucity of studies published regarding clinical and radiographic results after MPFL reconstruction combined with supracondylar biplanar FDO procedure in this population.
The purpose of this study was to a) analyze the clinical and radiographic results of the MPFL reconstruction combined with biplanar supracondylar FDO procedure, b) to evaluate the differences between pre- and post-operative knee function and radiographic results including effects on patellar parameters and alignment correction, c) to assess complications associated with bone healing, soft tissue irritation, wound infection, and recurrence of dislocation. It was our hypothesis that patients with RPD associated with increased FAA and genu valgum treated with biplanar supracondylar FDO and MPFL construction can prevent patellar re-dislocation, achieve satisfactory clinical and radiographic results in the short-term follow-up period.
Discussion
In this retrospective pilot study, the most important finding in the present study is that the treatment of RPD with an increased FAA (> 25°) and genu valgum using MPFL reconstruction combined with supracondylar biplanar FDO is effective, with no reported re-dislocation of the patella. Through the abovementioned combined surgery, radiological correction of the patellofemoral instability, excessive femoral anteversion and genu valgum could be achieved, and significant improvements of clinical outcomes could be obtained. Generally, the presence excessive femoral anteversion and genu valgum are known risk factors for patellar dislocation [
8,
9], and cause many clinical manifestations, including anterior knee pain, patellofemoral instability, and gait disturbance [
25]. Therefore, surgery treatment of RPD aims to correct maltracking of the knee extensor mechanism, which is benefit for restoring a normal mechanical environment of the patellofemoral joint in this population.
Patellar instability associated with genu valgum treated by supracondylar distal femoral osteotomy have been reported in several studies [
26‐
28]. Nha et al. [
10] demonstrated the satisfactory improvement of knee function of 14 patients (23 knees) who underwent closing-wedge distal femoral osteotomy without MPFL reconstruction. Similarly, Swarup et al. [
11] demonstrated that lateral opening wedge distal femoral osteotomy combined with lateral retinacular release yield satisfactory clinical results in this population. However, the clinical significance and potential advantages of MPFL reconstruction combined with biplanar supracondylar FDO procedure in RPD with increased FAA and genu valgum have not yet been identified. In this retrospective study, MPFL reconstruction combined with biplanar supracondylar FDO procedure achieved significant functional improvement after surgery in knee function scores (VAS, Kujala score, IKDC score, Lysholm score, and Tegner scores) and satisfactory radiographic outcomes (FAA, TT-TG, mLDFA, aFTA, and mechanical axis) in patellar instability with increased FAA and genu valgum, and no recurrence of dislocation cases had been found within the follow-up period.
Several orthopaedic surgeons have emphasized the role that osseous deformities of the axial and coronal plane acts as a significantly higher risk factor for patellofemoral maltracking. Dejour et al. [
14] identified that patients with patellofemoral instability had a higher value of FAA comparted to healthy controls (15.6 vs. 10.8) following CT evaluation. Similarly, Zhang et al. [
29] have reported that the adverse effects of increased femoral internal torsion on reconstructed MPFL, especially in patients when the FAA greater than 30°, which could be partially explained by the fact that the excessive lateralizing force vector acting on the patella due to the increased Q angle [
25]. Recently, biomechanical studies further demonstrated that the adverse effect of isolated MPFL reconstruction for patellar instability associated with increased FAA. Kaiser et al. [
30] revealed that isolated MPFL reconstruction for patellar instability is insufficient for higher degrees of FAA, which indicated that increased FAA may result in a persistent lateral force vector on the patella.
Due to these abnormal biomechanics of osseous deformity, it is vital to identify these underlying predisposing risk factors and to early make intervention for RPD. Despite isolated anatomical MPFL reconstruction is considered to be a standard treatment for patellofemoral instability with satisfactory results [
31,
32], subsequent studies have demonstrated that a high rate of subjective dissatisfaction in patients with increased femoral internal torsion [
33]. Supracondylar FDO as an isolated procedure has been shown good clinical outcomes for RPD with increased FAA [
34,
35]. However, isolated FDO procedure at the distal femur may increase the risk of graft failure when ignoring the correction of the knee valgus deformity, because the laterally-oriented vector forces applied to patellofemoral joint which can result in excessive tension into the reconstructed MPFL graft [
5,
9].
Recently, there has been a great focus of investigating the effect of derotation femoral osteotomy on the changes of coronal alignment. Nelitz et al. [
36] reported that FDO procedure tend to result in an increased valgus angulation in the frontal plane due to a decreased mLDFA. Similarly, Konrads et al. [
37] also identified that supracondylar femoral external osteotomy would lead to valgus deformity of the coronal limb alignment, which may be attributed to the reorientation of the femoral antecurvature and the femoral neck. Despite a biplanar supracondylar DFO has been performed in the correction of valgus deformity and excessive femoral internal torsion simultaneously, none of this group of patients showed signs for delayed union or non-union of the osteotomy, which was comparable to Imhoff et al. [
34] on a combined varus and external rotation producing distal femoral osteotomy.
There are several limitations in this study. The main limitation of this study is the small sample. It has to be highlighted that patients underwent this combined surgery have to be selected depending on rigorous inclusion criterions. In addition, the retrospective study design of the present study and the small number of patients included should be considered when interpreting our results. Future studies with larger patient cohorts are needed to further confirm the clinical outcome of this combined surgery. Therefore, as this was a pilot study and only a limited number of patients were available, no power analysis was performed. Furthermore, a missing comparative group of patients who treated by other surgical techniques. However, considering that the significant functional improvement and absence of re-dislocation, FDO combined with MPFL reconstruction may be a treatment option for RPD with increased FAA (FAA > 25°) and genu valgum.
Finally, second-look arthroscopic evaluation was not performed to evaluate the changes of trochlear and retropatellar cartilage.
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