Relative fixation strength of rabbit subscapularis repair is comparable to human supraspinatus repair at time 0.

BACKGROUND: Recent evidence suggests that the rabbit subscapularis tendon may be anatomically, biomechanically, and histologically suitable to study rotator cuff pathology and repair. However, biomechanical comparisons of rotator cuff repairs in this model have not been evaluated and compared to those in human cadaveric specimens.

QUESTIONS/PURPOSES: We quantified the biomechanical properties of the repaired rabbit subscapularis tendon after (1) single-row, (2) double-row, and (3) transosseous-equivalent rotator cuff repair techniques and compared the ratios of repairs to previously published data for human repairs.

METHODS: Tensile testing was performed on 21 New Zealand White rabbit subscapularis tendon-humerus complexes for single-row repair, double-row repair, and transosseous-equivalent repair (n = 7 for each group). Video digitizing software was used to quantify deformation. Load elongation data were then used to quantify structural properties. We compared the ratios of rotator cuff repairs for the rabbit data to data from human supraspinatus repair studies previously performed in our laboratory. For our primary end points (linear stiffness, yield load, ultimate load, and energy absorbed to failure), with the numbers available, our statistical power to detect a clinically important difference (defined as 15%) was 85%.

RESULTS: The ratios of single-row/double-row repair were 0.72, 0.73, 0.71, and 0.66 for human supraspinatus and 0.77, 0.74, 0.79, and 0.89 for rabbit subscapularis repair for linear stiffness, yield load, ultimate load, and energy absorbed to failure, respectively. The ratios of double-row/transosseous-equivalent repair were 1.0, 0.86, 0.70, and 0.41 for human supraspinatus and 1.22, 0.85, 0.76, and 0.60 for rabbit subscapularis for linear stiffness, yield load, ultimate load, and energy absorbed to failure, respectively. There were no differences comparing rabbit to human repair ratios for any parameter (p > 0.09 for all comparisons).

CONCLUSIONS: Subscapularis repairs in the rabbit at Time 0 result in comparable ratios to human supraspinatus repairs.

CLINICAL RELEVANCE: The biomechanical similarities between the different types of rotator cuff repair in the rabbit subscapularis and human supraspinatus at Time 0 provide more evidence that the rabbit subscapularis may be an appropriate model to study rotator cuff repairs.