The most important finding of this study is that the treatment of RPD with genu valgum by CWDFO combined with MPFLR is effective.The procedure can obtain radiological correction of the patellofemoral instability and genu valgum, improvement in clinical scores.
RPD is connected to various anatomic abnormalities, for instance, trochlear dysplasia, femoral malrotation, tibial tubercle lateralization, and genu valgum. Because of the countless risk factors for patellar instability, a multitude of techniques have been introduced to solve this problem; the optimal procedure, still, has caused a heated debate .
It is now well established that TTM and isolated MPFLR are effective procedures to treat RPD[15–17], but these approaches are also imperfect. In terms of MPFLR, Shah et al[18] revealed an overall cumulative complication rate of 26.1% when this operation is adopted. Common complications included fracture of patellar, residual instability, pain and flexion loss. Up to 38–40% of cases were reported having anterior knee pain in those underwent MPFLR[19]. In additional, Gobbi et al[20] suggested patellar shift and tilt correction will not improve significantly when performing isolated MPFLR in patients with RPD, while patellar maltracking is regard as a pathoanatomic risk factor for RPD[21, 22].
Several authors stated that because of altered contact pressures, TTM is followed by osteoarthritis of the patellofemoral and tibiofemoral joints. Kuroda et al[5] reported in the cadaveric study that TTM significantly increased the contact pressure of patellofemoral compartment and the medial tibiofemoral compartment. Lobner et al[23] found patients treated by TTM revealed an increased damage of retropatellar cartilage and worsen pain during activity. Mani et al[24] considered the pressure applied to tibiofemoral cartilage could be altered by tibiofemoral kinematic changes after TTM in their controlled laboratory study.
Axis deviations of the lower extremity such as genu valgum is one of further risk factors for failure of an isolated MPFLR[6, 7]. Because genu valgum has been regarded as one of negative factors to increase the forced shift of the patella towards the lateral side in terminal knee extension and result in a J-sign[9, 25]. The MPFL acts as a checkrein to lateral translation of the patella yet it does not pull the patella into the trochlear groove, hence isolated MPFLR should be avoided in the patients of genu valgum[26].
Those problems mentioned above could be resolved by DFO in the patients with associated genu valgum deformity.Traditionally, DFO was regarded as a surgical option for lateral tibiofemoral compartment osteoarthritis[27]. However, in recent years, it has been commonly accepted that correction of genu valgum could reduce the lateral vector applied to the patella, thereby gaining stabilization of the patella and decreasing the risk of RPD[11]. It is of great importance because genu valgum is an accepted risk factor for RPD, and also very common among teenagers. Nha et al[11] investigated the outcomes of 14 patients who underwent CWDFO over a mean follow-up period of 30 months. The mLDFA changed significantly from 83 to 89. The average distance of the PLS declining from 13.5 mm to 2.0 mm. All patients' subjective symptoms were significantly improved after the operation, and there was no RPD. Dickschas[28] studied 18 patients who underwent CWDFO over an average follow-up period of 44 months, and no redislocation was found after surgery. The VAS score for anterior knee pain improved from 5.6 to 2.1.
DFO alone or in combination with different soft tissue procedures for RPD has been reported. The treatment of RPD with CWDFO was first reported by Omidi et al[29] in 2009. The author found that the correction of genu valgum deformity alone can improve the patellofemoral congruence angle and reduce the probability of patellar dislocation. Since then, many studies on this procedure have been reported. Chang et al[27] examined 10 patients who underwent CWDFO with lateral retinacular release and tightening of the medial retinaculum over an average follow-up period of 20 months. The postoperative KSS and Kujala scores were significantly improved. Patellar dislocation did not recur. Purushothaman et al[30] reported the case of an RPD patient treated by OWDFO combined with MPFLR who achieved satisfactory postoperative results at the 1-year follow-up examination. Frings J [22] reported the results of 12 patients with RPD of genu valgus treated with CWDFO combined with MPFLR and TTM. The VAS score, Kujala score, Lysholm score and Tegner score were significantly improved compared with those before the operation, and there was no RPD. We believe that DFO alone only converts complex patellar dislocation to simple patellar dislocation in RPD patients with genu valgum deformity. Without associated soft tissue repair, MPFLR failure and femoral trochlear abnormalities are potential risks for RPD. Furthermore, MPFLR results in higher functional scores than medial retinacular constriction[31]. Therefore, we chose to perform CWDFO combined with MPFLR in such patients, as previously described, and our results verify the reliability of this procedure.
McWalter et al[4] stated that varus or valgus deformities of the leg axis could affect patellar tracking remarkably. Specifically, it has a strong impact on the patellar tilt, yet no significant difference was found in the level or slope of lateral translation[4]. However, Nha et al [11] found that CWDFO could decreased the mean distance of PLS reduced from 13.5 mm to 2.0 mm. Similarly, Our finding is consistent with Nha’s conclusions in this respect. Since MPFLR does not improve patellar tilt or shift[20],the rationale for DFO maybe the reduction of the Q angle and medialization of the patellar tendon insertion [32].
Some authors stated that a TT-TG distance > 15 mm is a risk factor for RPD[33] and a realignment procedure is recommended to be performed if TT-TG distance > 20 mm [34]. However, to date, there is still no clear evidence of whether TTM is necessary when performing MPFLR in patients with RPD. According to the findings of Ostermeier et al[35], TTM showed no significant relief of ligament loading and stabilizing effect on patellar movement. Matsushita et al[36] demonstrated that it yielded similar clinical results regardless of TT-TG distance greater than 20 mm or not if isolated MPFLR performed in RPD. They deduced that a TT-TG distance > 20 mm may not be an absolute operative indication for TTM. Interestingly, none of patients performing TTM, but Nha et al[11] found that DFO could affect the TT-TG distance, which declined from 20.4 mm preoperatively to 13.5 mm. None of our patients underwent TTM in our study, however, in contrast to the results reported by Nha et al, we found no significant change in the TT-TG distance between before and after surgery. We attributed this finding to the level of the osteotomy area in our study. Our osteotomies were all proximal to the femoral trochlea and therefore did not affect the relative relationship of the femoral trochlea and the tibial tubercle.
This study also has limitations. First, this study was retrospective and did not have a control group performing other surgical techniques such as isolated MPFLR, TTM, just to name a few, but as Nha[11] mentioned, CWDFO may be regarded as one of the surgical options for RPD with genu valgum since there were the remarkable improvement in clinical outcomes and radiological results. Second, the present study had a relatively small sample size and short follow-up period, However, a 2-year follow-up period might be enough to prove an acceptable clinical outcomes. Further investigations with a larger sample size and longer follow-up period are needed.