Background
Arthroscopic superior capsular reconstruction (ASCR), for the treatment of irreparable rotator cuff tears (RCTs), aims to restore the stability and physiological kinematics of the shoulder. ASCR has been shown to produce excellent clinical outcomes; however, graft tear rates can range from 4.2% up to 75%. A key factor that may affect the outcome of ASCR is the position of the shoulder during graft fixation. The role of this positioning was previously evaluated for a full-thickness tear of the supraspinatus tendon (SSP), but no data is available for more extensive RCTs. The aim of this study was to evaluate the influence of the positioning of the graft in ASCR on shoulder stability considering different patterns of RCTs. The hypothesis was that ASCR alone would not increase the stability of the shoulder for preoperative IRCTs extending beyond the supraspinatus.
Methods
A 3-D musculoskeletal model of the upper limb was modified to account for the fixation of the graft in ASCR. The RCTs were modelled assuming the 4 most common types of shoulder preoperative conditions for ASCR. Full-thickness tears of the following rotator cuff tendons were assumed: (1) SSP; (2) SSP and Infraspinatus (ISP); (3) SSP and Subscapularis (SC); and (4) SSP, ISP and SC. For each pattern of RCT, the corresponding muscle of the torn tendon was removed from the model. The single-piece fascia lata graft was modelled as a set of four parallel segments. The material properties of the graft were defined based on previous experimental data of fascia lata graft constructs. A biomechanical parameter was used to evaluate the shoulder stability: the glenohumeral joint reaction force. Muscle and joint reaction forces were estimated through inverse dynamic analyses considering experimental data collected in the biomechanics laboratory. Analysis of variance (ANOVA) and Tuckey’s test were used to compare the shoulder stability between pre- and postoperative conditions. The significance level was set to p < 0.05.
Results
For an isolated SSP tear, shoulder stability significantly improved compared with the preoperative condition, regardless of the shoulder position of fixation (95% confidence intervals, p<0.001). For the second pattern of RCT (SSP and SC tear), a significant loss in stability was observed for high abduction angles of fixation (p<0.001), compared with the preoperative condition. Regarding the patterns of RCT (3) and (4), the postoperative condition was better than the preoperative condition; however, this improvement was not significant.
Conclusion
ASCR for isolated SSP tears improved shoulder stability, regardless of the shoulder position of fixation. For tears that extended to the SC and ISP tendons, ASCR did not significantly improve shoulder stability, and the graft did not restore the anterior and posterior stability lost with the RCT.
Clinical Relevance: This study supports the relevance, for an isolated SSP tear, of the position of the shoulder during graft fixation for shoulder stability after ASCR. New studies should investigate the necessity and relevance of concomitant procedures to restore the action of the remaining torn rotator cuff tear tendons.