The principal finding of the present study is that direct reinsertion of the MPFL with a suture anchor at the patellar border can obtain good imaging and functional outcome for the first APD in adolescents. Acute dislocations may cause avulsion of the medial patellofemoral ligamentous complex from the patella. Patients with APD have a high probability of MPFL injury, which mainly limits the lateral displacement of the patella. Yamamoto [6] first published his article on the arthroscopic repair of the MPFL for APD. Fukushima and Satterfield [7-9] presented cases of APD in a skeletally immature patient treated with arthroscopic medial patellofemoral ligamentous complex repair using suture anchors, this is amenable to direct primary repair to prevent recurrent instability and recreates normal anatomy and function. Ren et al. [10] reported that 12 patients with an avulsion fracture of medial retinaculum from the edge of the patella were repaired with suture anchor under ultrasonic positioning. The affected limb recovered quickly, the knee function was satisfactory, and there was no recurrence of patellar instability. Mariani et al. [11] treated 17 patients with first APD by repairing MPFL at the medial edge of the patella through a patellar tunnel. No redislocation occurred during follow-up. The Lysholm and Kujala scores after operation were significantly higher than those before operation. Fourteen out of 17 patients were able to return to sports at the same level as before. Toritsuka et al. [12] repaired MPFL patellar insertion point with suture anchor in 7 patients with primary patellar dislocation combined with MPFL patellar avulsion fracture. All patients recovered to the activity level before injury at the last follow-up, and there was no redislocation or subluxation. The above follow-up studies showed good postoperative function and no patellar instability. Relevant meta-analysis studies show that, the postoperative redislocation rate of surgical treatment is significantly better than that of conservative treatment. It is suggested that surgical treatment should be performed for the first patellar dislocation. Although reconstruction and repair are not discussed separately in the surgical treatment group, it seems that either type of surgery is superior to conservative treatment [3, 5]. The incidence of redislocation caused by surgical treatment of primary APD (25%) was significantly lower than that of conservative treatment (36.4%) [13]. In a prospective randomized controlled study, the re-dislocation rate in the repair group (22%) was significantly lower than that in the external fixation group (43%) in children with primary traumatic APD [14].
However, this technique may not apply to all traumatic MPFL injuries with APD. One may achieve stable fixation of the MPFL by minimally invasive means provided several prerequisites were met such as proper patient selection, and exact localization of the lesion. Patients in the present study were confirmed as the primary APD which had a high healing rate and low tension when the MPFL was directly restored to its anatomical position. Accurate evaluation of the injury site of the medial retinaculum in APD played an important role in treatment decision-making. At present, the location of MPFL injury in APD is still controversial. Recent reports have indicated that patellar side avulsion has an appreciable incidence, contrary to previous studies which have characterized the injury as principally femoral side. The percentage of MPFL tears at the adductor tubercle was 73% of the cases studied [15]. Askenberger et al. [16] presented a study to describe MPFL injuries in the skeletally immature patient by MRI and to compare the results with the injury pattern found at arthroscopic surgery. The MRI scans showed an isolated MPFL injury at the patellar attachment site (60%), a multifocal injury (35%), an injury at the femoral site (4%), and no injury (1%). Arthroscopic surgery disclosed an isolated MPFL injury at the patellar site (81%) and a multifocal injury (18%). Skeletally immature children are more prone to sustaining an MPFL injury at the patellar attachment site. Another study reported by Seeley et al aimed to characterize the patterns of MPFL on MRI following primary APD in pediatric patients. MRI demonstrated MPFL injury in 87 patients (78.4%). MPFL injury was present at an isolated patellar insertion (31%), an isolated femoral insertion (14%), and more than one location was present (33%) [17]. Kepler et al. [2] reported that 61% of MPFL injuries occurred at the patellar attachment and 12% at the femoral attachment. Most studies showed that MPFL injury after patellar dislocation mainly occurs at the patellar side, which is consistent with our clinical data.
Clarifying the injury location has important clinical guiding significance for the selection of treatment methods and surgical approaches. Evaluating the location of avulsion fracture at the medial edge of the patella is of guiding significance for the selection of anchor insertion point and puncture point of the medial retinaculum. Anatomical studies showed that MPFL was mainly located in the middle and upper part of the patella, and MPTL / MPML was mainly attached to the lower part of the patella [18]. Mochizuki et al. [19] reported that 84% of the patellar avulsion fractures were located in the inferomedial patellar border, which consisted of MPTL/MPML attachments but not the MPFL attachments that mainly limits the outward movement of the patella. This strange phenomenon may be caused by many factors, such as injury energy, buckling, and torsion direction. Among APDs, non-contact knee sprain in flexion and valgus is the leading mechanism. When the knee flexion is greater than 45°, excessive rotation, valgus, and inclination of the patella will cause excessive tension of MPTL and MPML at the medial lower edge of the patella. It is consistent with the mechanism that most of the patients with primary APD included in this study have a history of non-collision, flexion, and torsion injury [20]. However, only repairing the secondary limiting structures of MPTL and MPML located at the medial lower edge of the patella may not be enough to limit patellar Exodus, and MPFL is the main binding force to limit patellar exodus. Therefore, in this study, two anchors were placed at the trisection of the medial edge of the patella, and the medial retinaculum was sutured evenly. The follow-up results showed that there was no recurrent patellar instability. Biomechanical studies have shown that the repair of MPFL with suture anchor suture can almost produce similar contact stress and joint kinematics compared with the reconstruction of MPFL with tendon transplantation [21]. Some studies have shown that the incidence of re-dislocation after MPFL repair is equivalent to or better than that of allograft reconstruction [22]. Reconstruction has higher requirements for the surgeon, requires additional grafts, and may damage the epiphysis of children [23].
After the medial retinaculum was repaired and pre-knotted and tightened, we observed the trajectory of a patellofemoral joint under arthroscopy, and then decided whether to release the lateral retinaculum. The purpose was to avoid the high pressure between the patella and medial femoral condyle and the significant increase of medial instability caused by excessive release and increase the risk of iatrogenic medial dislocation. Or insufficient release caused excessive suture tension of medial anchor and instability of the patellofemoral joint, resulting in failure of healing of medial retinaculum reinforced suture [24]. The advantages of this operation were anatomical repair of MPFL insertion and plication for medial patellar retinaculum and simultaneous treatment of other structural injuries in the joint. The potential advantages of arthroscopic surgery for patellar dislocation are less trauma, less operative complications, and more intuitive release of lateral retinaculum to accurately adjust patellar trajectory under monitoring. Our follow-up results were satisfactory, and there was no recurrence and re-dislocation. This study also has some limitations. Firstly, due to the strict selection criteria, the number of patients included is limited. Secondly, there is a lack of an effective control group. Despite these limitations, this study hopes that surgeons will pay attention to the absence of MPFL avulsion fracture of the patella and the possibility of arthroscopic-assisted suture anchoring in the treatment of this injury.