This study investigated the effects of restoring the balance of anterior and posterior cuff forces on shoulder function in a chronic rat rotator cuff tear model and is the first to investigate repair of two-tendon rotator cuff tears in an animal model. Using quantitative ambulatory measures, we found that the kinetics of an infraspinatus only repair mirrored that of a two-tendon repair and were significantly different than the kinetics of a simulated chronic tear in which neither tendon was repaired (). These results support our hypothesis and other clinical observations that restoring the balance of anterior-posterior forces by repair of the infraspinatus only is sufficient to improve shoulder function to a level similar to repairing the supraspinatus and infraspinatus together.
Comparison of kinetic variables between groups.
At nearly every time point post-repair surgery, the medial/lateral, braking, and propulsion forces of the infraspinatus only repair animals were significantly different than those forces of the no repair group. Furthermore, kinetics of the infraspinatus repair group mirrored that of the two-tendon repair group, with differences found at only a minimal number of time points. Propulsion and braking forces, which are measured in the forward flexion plane in the rat, correlate to human abduction due to differences in the orientation of the scapula. This is often used as an objective measurement of shoulder function in humans. We observed no differences between the infraspinatus repair and two-tendon repair groups for these ambulatory parameters, and by day 10 to 14, saw no differences between the braking and propulsion values for the two repair groups and baseline ambulatory values measured before repair surgery. This suggests that a partial repair of the infraspinatus only would be adequate to provide a clinically significant improvement in shoulder function for both repair groups. These differences persisted until the last time point recorded after repair surgery.
The step width can be used as an indicator of shoulder function by quantifying the amount of contralateral limb compensation to provide stability for the injured limb. The step widths of the infraspinatus only repair and two-tendon repair groups were no different than baseline at any point after surgery and were significantly higher than the no repair group up to 10 days after repair surgery. The decreased step width in the animals in which no tendon was repaired indicates “tripoding” and decreased shoulder function, while the step widths of the infraspinatus repair and two-tendon repair groups were no different than baseline values. Previous studies investigating the ambulatory parameters in animals undergoing acute repair of supraspinatus detachments showed no difference in step width at any time point when compared to uninjured controls.23
These results indicate that in the no repair group, animals needed more contralateral limb compensation and stabilization during the gait cycle compared to the repair groups.
Clinically, these results suggest that a patient with a chronic, two-tendon tear with retraction of the supraspinatus may benefit from a partial repair. Previous clinical case series showed improvements in shoulder function after partial repair.20–21
Burkhart et al demonstrated an improvement of an average of 90° of shoulder elevation (from 60 to 150°) after partial repair, with 13 of 14 patients satisfied with their results. They concluded that partial repair of massive irreparable cuff tears and creation of a “functional cuff tear” is mechanically superior to tendon transposition.21
Duralde et al similarly demonstrated an improvement in active elevation from 114 to 154° with 22 of 24 patients satisfied after partial repair of a massive rotator cuff tear.20
These clinical results suggest that shoulder elevation in the functional range can be restored with restoration of the posterior cuff; however, long term implications on repair integrity are unknown. Using ultrasound and standard radiographic evaluation, Keener et al demonstrated that a significantly greater amount of proximal humeral migration occurred in tears involving the infraspinatus than isolated tears of the supraspinatus, and tear size was the only independent predictor of proximal humeral migration25
. These data support our results and suggest that massive tears that extend to involve the infraspinatus are more likely to disrupt normal glenohumeral kinematics.
Our study has several limitations. While we previously showed that biomechanical, geometric, and histological properties remain altered in this chronic rat rotator cuff model through the time points studied, it does not exactly replicate the human condition. Based on similar anatomy including tendon insertions and bony anatomy, the kinematics of the rat shoulder would most likely replicate the human most closely of all non-human primate animal models; however, in our study, improved rat shoulder function after partial repair may not necessarily translate to a similar degree of improved human shoulder function. Nonetheless, our results demonstrate the concept of functional consequences of these conditions. The tendon is acutely detached from its insertion site and therefore does not result from chronic degeneration that is usually seen in humans; therefore, the environment and healing potential of the repaired tendons in this model may be different. Also, scar tissue forms between the detached tendon edges and the insertion site, which is common among all animal models, but does not occur in humans. This scar tissue could account for the apparent improved kinetics seen in the group with no tendon repair. In addition, not all kinetic parameters reached uninjured values by the end of the study; therefore, the clinical significance of improved shoulder function that does not reach baseline can be questioned. Kinetic data only rather than kinematic data additionally were reported, as we were unable to track shoulder position with enough accuracy to discriminate between groups.
Future investigations include studies investigating the effect of activity after a partial repair. Although restoration of the balance of forces restored shoulder function in this study, the long-term implications of repairing only the posterior rotator cuff musculature are not well described. Previous studies showed that larger tears of the supraspinatus lead to progression of the tear anteriorly and/or posteriorly, but whether these data apply to a “functional rotator cuff tear” created by a partial repair is unknown.26–28
Other future studies may include developing methods to accurately track shoulder position to determine shoulder kinematics. Also, the effect of restoring the balance of rotator cuff forces on the glenoid cartilage and intact tendons, such as the long head of the biceps tendon, remain interesting questions to be further investigated.
This study demonstrates that restoring the balance of anterior-posterior rotator cuff forces is sufficient to restore shoulder function in a rat rotator cuff model of a chronic, massive two-tendon tear. Ground reaction forces and step width show that animals in which only the infraspinatus was repaired have improved function when compared to animals in which neither tendon was repaired. Additionally, the kinetic data from an infraspinatus-only repair were not different from those from a two-tendon repair, indicating that repairing the posterior cuff musculature restores shoulder function to a level similar to that of a two-tendon repair. These results support use of a partial repair after a two tendon tear when necessary to restore adequate shoulder function. We utilize a controlled in vivo model system to quantitatively demonstrate the importance of rotator cuff musculature balance. This animal model of a two-tendon repair model can be used in future studies investigating massive rotator cuff tears.