Load-to-failure characteristics of patellar tendon allograft superior capsule reconstruction compared with the native superior capsule

BACKGROUND: The potential use of a patellar tendon allograft for superior capsular reconstruction has been demonstrated biomechanically; however, there are concerns regarding compromised fixation strength owing to the longitudinal orientation of the fibers in the patellar tendon. Therefore, the purpose of this study was to compare the fixation strength of superior capsule reconstruction using a patellar tendon allograft to the intact superior capsule. METHODS: The structural properties of the intact native superior capsule (NSC) followed by superior capsular reconstruction using a patellar tendon allograft (PT-SCR) were tested in eight cadaveric specimens. The scapula and humerus were potted and mounted onto an Instron testing machine in 20 degrees of glenohumeral abduction. Humeral rotation was set to achieve uniform loading across the reconstruction. Specimens were preloaded to 10 N followed by cyclic loading from 10 N to 50 N for 30 cycles, then load to failure at a rate of 60 mm/min. Video digitizing software was used to quantify the regional deformation characteristics. RESULTS: During cyclic loading, there was no difference found in stiffness between PT-SCR and NSC (cycle 1 – PT-SCR: 12.9 ± 3.6 N/mm vs. NSC: 22.5 ± 1.6 N/mm; P = .055 and cycle 30 – PT-SCR: 27.3 ± 1.4 N/mm vs. NSC: 25.4 ± 1.7 N/mm; P = .510). Displacement at the yield load was not significantly different between the two groups (PT-SCR: 7.0 ± 1.0 mm vs. NSC: 6.5 ± 0.3 mm; P = .636); however, at the ultimate load, there was a difference in displacement (PT-SCR: 20.7 ± 1.1 mm vs. NSC: 8.1 ± 0.5 mm; P < .001). There was a significant difference at both the yield load (PT-SCR: 71.4 ± 2.2 N vs. NSC: 331.6 ± 56.6 N; P = .004) and the ultimate load (PT-SCR: 217.1 ± 26.9 N vs. NSC: 397.7 ± 62.4 N; P = .019). At the yield load, there was a difference found in the energy absorbed (PT-SCR: 84.4 ± 8.9 N-mm vs. NSC: 722.6 ± 156.8 N-mm; P = .005), but no difference in energy absorbed was found at the ultimate load. CONCLUSIONS: PT-SCR resulted in similar stiffness to NSC at lower loads, yield displacement, and energy absorbed to ultimate load. The ultimate load of the PT-SCR was approximately 54% of the NSC, which is comparable with the percent of the ultimate load in rotator cuff repair and the intact supraspinatus at time zero.