After approval by our institutional review board (No. 08-001342), all patients with a history of primary MPFL reconstruction at our institution from 2005 to 2013 were identified in a surgical database with Current Procedural Terminology and International Classification of Diseases codes. Patients were included if they (1) underwent primary MPFL reconstruction at our institution for recurrent patellar instability after failed nonoperative treatment, (2) had pre- and postoperative lateral knee radiographs within 1 year of surgery, and (3) were >=18 years old at surgery. Patients were excluded if they had (1) concomitant cruciate ligament reconstruction, (2) multiligament knee injury, (3) no lateral knee radiographs, (4) tibial tubercle-trochlear groove (TT-TG) measurement >20 mm, and (5) moderate trochlear dysplasia (Dejour type C or greater) and were (6) <18 years old at the time of surgery. A total of 32 patients satisfied these criteria and were included in the study.
Chart review was performed to verify patient information (age, sex, surgical characteristics) and pre- and postoperative lateral knee radiographs. All lateral radiographs were performed with the knee positioned in 20  to 30  of flexion and the epicondyles perpendicular to the image receptor. Lateral radiographs were assessed for patellar height with the Caton-Deschamps, Blackburne-Peel, and Insall-Salvati ratios. Two authors (J.M.W. and R.A.C.:an orthopedic sports fellow and a chief resident, respectively) calculated these ratios to ensure interrater reliability (Figure 1). Means of the measures between the 2 observers were calculated and used to compare pre- and postoperative patellar height. Mean values of patellar height in the cohort were compared with values reported in the literature as normal. Femoral tunnel location was plotted in relation to Schottle's point on a true lateral femoral radiograph. A position within 7 mm of this point was considered anatomic as described by Servien et al. 
Isolated MPFL reconstruction was performed at a single institution by 1 of 3 senior authors (A.J.K., D.L.D., M.J.S.) on patients with a clinical history of RPFI and a positive apprehension test result. Diagnostic arthroscopy was performed in all cases, with grading of the cartilage status of both the undersurface of the patella and the femoral trochlea. Mechanical chondroplasty of unstable cartilage was performed when indicated.
After diagnostic arthroscopy, the graft (11 semitendinosus autografts, 2 quadriceps tendon autografts, 11 gracilis autografts, and 8 gracilis allografts) was prepared with a running whipstitch placed in both free ends. A 4-cm incision was made over the medial aspect of the patella. Sharp dissection was carried down to the medial retinacular layer, where the interval between medial layer 2 (retinaculum) and medial layer 3 (capsule) was then defined for graft passage. A small shallow trough was then created in the superomedial aspect of the patella, in the area of the native MPFL origin. The graft was then secured to the patella with two 2.8-mm FASTak suture anchors (Arthrex) or two 3.5-mm biocomposite SwiveLock anchors (Arthrex), spaced 1 cm apart. The femoral insertion point was identified by fluoroscopy, according to the method described by Schottle et al. An incision was made at this designated point, and sharp dissection was performed down to the medial aspect of the distal medial femoral condyle. The graft was then looped to create a double-bundle construct, and the free ends were passed from the patella to the femur. The graft was placed around the femoral guide wire as the knee was brought through an arc of motion to test for relative graft isometry from 0  to 90 . The diameter of the doubled graft determined the diameter of the tunnel that was drilled; typically, a 6-mm socket was made by advancing a cannulated drill over the guide wire to a depth of 30 mm. With the knee held in 30  of flexion, a 6 x 23-mm bioabsorbable interference screw (Arthrex) was then used to secure the 6-mm graft in the femoral socket. A layered closure was then performed, sterile dressings applied, and a knee immobilizer initially provided.
Postoperative rehabilitation consisted of a knee immobilizer for 2 weeks and partial weightbearing with crutches for 6 weeks. Range-of-motion exercises were performed immediately and without restriction. Full weightbearing, full knee range of motion, and progressive strength and proprioceptive training were allowed after 6 weeks.
The interclass correlation coefficient was used to quantify interrater reliability for each measure of patellar height. Adequate correlation was accepted as an interclass correlation coefficient >0.5; good, >0.7; and excellent, >0.8. Paired Student t tests were used to evaluate for postoperative changes in each continuous measure of patellar height. The McNemar test for paired categorical data was used to compare the number of patients with patella alta pre- versus postoperatively. All statistical analyses were performed with SAS Statistical Discovery JMP (v 7.0; SAS Inc). Differences in the means of the groups preoperatively and at final follow-up were evaluated, and significance was determined with an alpha level of 0.05.