All patients underwent a standard rehabilitation protocol. This included toe-touch weightbearing with crutches and a hinged knee brace locked in extension for 6 weeks postoperatively. Physical therapy was initiated 1 week postoperatively to begin knee ROM in the hinged knee brace. Knee ROM began at 0° to 30° and advanced to 0° to 90° by 6 weeks postoperatively. At 6 weeks postoperatively, the crutches and knee brace were discontinued. At 4 months after surgery patients were allowed to run and sporting activity was allowed at 6 months if patients demonstrated clinical stability. A graduated physical therapy program was used to allow return to sports. Patients were on average followed at 6-week intervals until 6 months after surgery and then 6 months after full return to sport.
Postoperative notes and imaging were reviewed for presence of complications defined as recurrent dislocation, recurrent subluxations, fractures, infection, or arthrofibrosis. These complications were identified by chart review by the senior surgeon (NKP) and study personnel (EH) not involved in clinical care of the patients or through patient-reported complications by phone/email. Recurrent subluxation or dislocation was patient-reported at the time of the clinic visit and/or follow phone/email. Fractures were defined as any cortical disruption in the femur or patella that required treatment (change in postoperative protocol), infection requiring treatment (antibiotics and/or return to the operating room), or arthrofibrosis (stiffness that necessitated a change in the postoperative protocol or manipulation under anesthesia).
Ninety-two percent (23 of 25) of patients reported no instability episodes at latest followup and had returned to their prior activity levels. Two patients sustained repeat dislocations, one who had trochlear dysplasia. No patients had recurrent subluxations.
Four of 25 patients (16%) developed postoperative complications. Two patients sustained repeat dislocations mentioned previously, one patient sustained a patella fracture as a result of a fall 6 months after surgery, and one patient underwent reoperation 17 months after surgery to remove a biointerference screw on the femoral side that was causing her irritation. No patients had an infection or developed arthrofibrosis.
MPFL reconstruction is commonly used to treat patellofemoral instability both in children and adults. Allograft tissue can be used for ligament reconstruction, and although it has been noted to be used successfully in adults [8, 45], it is unclear if the use of allograft tissue in MPFL reconstruction will work as well in children. This of particular concern given the high risk of failure of allograft tissue in anterior cruciate ligament reconstruction in this population [6, 9, 15]. In a cohort of children and adolescents, we found that allograft MPFL reconstruction was associated with only an 8% rate of recurrent instability.
This study has a number of limitations. First, this was a relatively small sample size of patients without a control group of patients who underwent surgery with autograft tissue for comparison. Therefore, we cannot be certain whether this approach would be better or inferior to other approaches. Our study can be considered preliminary and may be used to help sample size calculations for future comparative trials between allograft MPFL reconstruction and other approaches; we believe such studies will be important in answering the questions posed in this article. Second, we did not collect patient-reported outcome scores. This is an important limitation, because without validated outcomes tools, our assessment is limited to the relatively basic endpoints of recurrent instability and major complications. Future studies on this topic should gather patient-reported outcome scores, because those will help us to compare this approach with other available options in a more nuanced manner, particularly as it relates to the preservation of autogenous tissue and postoperative pain and function. Future studies should also look at the specific risk factors for recurrent instability and will require multicenter efforts. Third, because allograft tissue can attenuate over time [7, 14], longer term followup of our patients will be important to ensure that these reconstructions remain durable in this population of young patients even given our mean followup of 2 years. Fourth, during this time period, not all patients underwent reconstruction for instability, which does represent a degree of patient selection bias, yet our indications were reasonable and consistent; the senior author (NKP) suggested reconstruction to patients who had recurrent dislocation or subluxation after 6 weeks of bracing, physical therapy, and activity modification, if they were noted to have a torn or attenuated MPFL on MRI. During that period, this was the only surgical technique the surgeon used to treat traumatic patellar instability. Finally, the risk of growth disturbance in performing the procedure is important for the clinician to consider. We had only nine patients with open physes at the time of the procedure with a mean age of the patients of 14 (± 2) years. Therefore, although these patients are skeletally immature, they represent a group that was nearing skeletal maturity. We also did not obtain long-standing films unless an abnormality was noted clinically as a result of the advanced age of the patients. No patients developed any clinically significant growth abnormalities (ie, leg length discrepancies or angular deformities). We do agree that a larger study with a much younger population of patients could address the risk of growth disturbance using this technique in a more skeletally immature population.
We found a low risk of repeat instability using allograft gracilis for MPFL reconstruction in children and adolescents with traumatic patellar instability, we believe comparable to other studies about allograft MPFL reconstructions in adults. In adults, Calvo Rodríguez et al.  compared 13 knees that underwent MPFL reconstruction with hamstring autograft versus 16 knees with allograft and found no recurrent dislocations in either group. Weinberger et al.  performed a systematic review comparing autograft and allograft and found no difference between either graft choice (recurrence rate of 5.7% for autograft and 6.7% for allograft) . The observed risk of recurrence in our study seems similar to those of those other studies, which have examined allograft tissue in the adult population. Our results likewise seem similar to MPFL reconstruction with autogenous tissue in children and adolescents. Lind et al.  reported a 20% redislocation rate in their series of 20 children between the ages of 8 and 16 years with the utilization of autogenous tissue. Parikh et al.  in a series of 179 knees found a redislocation rate of 4.6%, and Nelitz et al.  in their series of 21 knees had a 9.5% rate of continued apprehension after reconstruction with autogenous tissue. Even in the context of trochlear dysplasia (present in 21 of our 25 knees), which places additional strain on the MPFL as a result of lack of bony constraint, our recurrence rates were low. Only one of our patients with a recurrence had trochlear dysplasia. Other authors have reported a correlation between worse outcomes and degree of trochlear dysplasia after MPFL reconstruction [36, 44]. However, because allograft tissue can attenuate over time [7, 14], longer term followup of our patients will be important to ensure that these reconstructions remain durable in this population of young patients.
The use of allograft tissue in our study resulted in an overall complication rate of 16%, which seems similar to what has been reported for both autogenous and allograft tissue in adults as well as autograft tissue in the pediatric and adolescent populations. Calvo Rodríguez et al.  noted complications in 19% of adults treated with allograft tissue, including one revision procedure resulting from poor anchor placement, one patellar fracture, and one patient who developed postoperative arthrofibrosis. Singhal et al.  examined nine different studies representing 320 MPFL reconstructions done with hamstring autograft and found a complication rate of 12.5%, including cases of arthrofibrosis and patellar fracture. In the pediatric and adolescent populations, Parikh et al.  noted an overall complication rate of 16.2% including eight cases of arthrofibrosis, six patellar fractures, and five cases of prolonged pain. As a result, taking overall complication rate into consideration, allograft tissue utilization in our study was similar to what has been reported for autograft and allograft in adults as well as autograft in pediatric patients. It is important to note that all allografts are not equivalent and that preparation differences may make historical allograft literature less applicable than the historical autograft literature. It is also important to note that rehabilitation protocols differed not only between these studies, but also with the protocol used at our institution. This can be a factor that accounts for the difference in the incidence of complications between the different papers.
In this small case series, we found that MPFL reconstruction using allograft tissue in children and adolescents resulted in a low risk of recurrent instability, perhaps comparable to what has been published by others who have used autograft tissue. Longer followup is needed, because in some orthopaedic applications, allograft ligaments have been observed to attenuate over time [7, 14]. Future studies will be performed with a control group of patients using autogenous tissue with patient-reported outcomes to further understand graft choice in the pediatric and adolescent populations.
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