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Rotator cuff tears are disabling conditions that result in changes in joint loading and functional deficiencies. Clinically, damage to the long-head of the biceps tendon has been found in conjunction with rotator cuff tears, and this damage is thought to increase with increasing tear size. Despite its importance, controversy exists regarding the optimal treatment for the biceps. An animal model of this condition would allow for controlled studies to investigate the etiology of this problem and potential treatment strategies. We created rotator cuff tears in the rat model by detaching single (supraspinatus) and multiple (supraspinatus þ infraspinatus or supraspinatus þ subscapularis) rotator cuff tendons and measured the mechanical properties along the length of the long-head of the biceps tendon 4 and 8 weeks following injury. Cross-sectional area of the biceps was increased in the presence of a single rotator cuff tendon tear (by $150%), with a greater increase in the presence of a multiple rotator cuff tendon tear (by up to 220%). Modulus values decreased as much as 43 and 56% with one and two tendon tears, respectively. Also, multiple tendon tear conditions involving the infraspinatus in addition to the supraspinatus affected the biceps tendon more than those involving the subscapularis and supraspinatus. Finally, biceps tendon mechanical properties worsened over time in multiple rotator cuff tendon tears. Therefore, the rat model correlates well with clinical findings of biceps tendon pathology in the presence of rotator cuff tears, and can be used to evaluate etiology and treatment modalities. Keywords: shoulder; rotator cuff; biceps; animal model; tendon injury Rotator cuff tears are painful and disabling conditions that result in changes in joint loading and functional deficiencies.1 Shoulder injuries rank third in musculoskeletal clinical visits after back and neck pain, and tears of the rotator cuff are thought to occur in up to 50% of the population over age 65.2 Although most rotator cuff tears occur in the supraspinatus tendon, only half of those tears are isolated in that one tendon.3 Clinically, damage to the long-head of the biceps tendon has been found in conjunction with rotator cuff tears, and this damage is thought to increase with increasing tear size. 4 Clinical observations have described the biceps tendon as being widened and flattened at the time of rotator cuff repair.5 Biceps tendon pathology is thought to be a significant source of pain, and can be treated with tenotomy or tenodesis even when the rotator cuff tear is too massive for repair.Unfortunately, most clinical studies are unable to address the underlying cause of biceps tendon changes in the presence of rotator cuff tears in a controlled manner, and cadaveric studies cannot monitor the injury process with time. Currently, no animal models are available on this topic. Therefore, our objective was to develop and utilize such an animal model to investigate the mechanical property changes in the long-head of the biceps te...

Musculoskeletal soft tissue injuries from athletic activities are common in the rotator cuff tendons, lateral epicondyle of the elbow, the patella tendon, and the Achilles tendon. Despite the fact that the Achilles tendon is the largest and strongest tendon in the human body, it is frequently injured in the athletic setting. To study the etiology and pathogenesis of Achilles tendon injuries, our goal was to develop a model of Achilles tendon overuse by evaluating the Achilles tendons from animals subjected to the exercise protocol previously described as overuse for the supraspinatus tendon. We hypothesized that the same exercise protocol would produce injuries to the Achilles tendon as demonstrated by changes in the cross-sectional area and biomechanical properties. While a significant injury was induced into the supraspinatus tendon, we found no changes in the Achilles tendons of these exercised animals based on gross observation, geometric measurements, and mechanical testing analyses. Although surprising, there are many possible explanations for these findings including differences in potential injury mechanisms, functional capabilities of the differing tendons, and other factors.

Rotator cuff tears are a common problem presenting with loss of shoulder function, such as reduced range of motion and inability to perform daily activities. Unfortunately, most animal models of shoulder injuries do not examine shoulder function as a result of rotator cuff injury. This study examined the effect of rotator cuff tears on shoulder function in an animal model. Forty-eight Sprague-Dawley rats were divided into uninjured control, supraspinatus tendon detachment, supraspinatus+infraspinatus tendon detachment, or supraspinatus+subscapularis tendon detachment groups. Functional assessment was determined with ambulatory parameters (paw and stride measures) and range of motion prior to tendon detachment and at various time points after tendon detachment. We found that measures of shoulder function were significantly altered with rotator cuff tendon tears. The addition of a second tendon detachment had additional detrimental effects on animal shoulder function. These findings are consistent with alterations in shoulder function observed clinically with rotator cuff injuries.

Study Design Controlled Laboratory Study Objectives To measure the architectural properties of rat supraspinatus muscle after a complete detachment of its distal tendon. Methods Supraspinatus muscles were released from the left humerus of 29 rats (Sprague-Dawley, mass 400 - 450 g) and animals were returned to cage activity for 2 weeks (n = 12), 4 weeks (n = 9), or 9 weeks (n=8) before euthanasia. Measurements of muscle mass, pennation angle, fiber bundle length (sarcomere number), and sarcomere length permitted calculation of normalized fiber length, serial sarcomere number, and physiological cross-sectional area (PCSA). Results Coronal oblique sections of the supraspinatus confirmed surgical transection of the supraspinatus muscle at 2 weeks, with reattachment by 4 weeks. Muscle mass and length were significantly lower in released muscles at 2 weeks, 4 weeks, and 9 weeks. Sarcomere lengths in released muscles were significantly shorter at 2 weeks but not different by 4 weeks. Sarcomere number was significantly reduced at 2 and 4 weeks, but returned to control values by 9 weeks. The opposing effects of smaller mass and shorter fibers produced significantly smaller PCSA at 2 weeks, but PCSA returned to control levels by 4 weeks. Conclusions Release of the supraspinatus muscle produced early radial and longitudinal atrophy of the muscle. The functional implications of these adaptations would be most profound at early time points (particularly relevant for rehabilitation), when the muscle remains smaller in cross-sectional area and, due to reduced sarcomere number, would be forced to operate over a wider range of the length-tension curve and at higher velocities, all adaptations resulting in compromised force generating capacity. These data are relevant to physical therapy because they provide tissue-level insights into impaired muscle and shoulder function following rotator cuff injury.

Although presumed, damage in the remaining (intact) rotator cuff tendons in the presence of an isolated supraspinatus tendon tear or multiple tendon tear has not been well studied. This study utilized an animal model of multiple rotator cuff tendon tears to investigate alterations in the remaining (intact) tendon mechanical properties at 4 and 8 weeks post-injury. Twenty-four animals served as uninjured controls, while seventy-two were divided among the tendon detachment groups (supraspinatus tendon detachment, supraspinatus+infraspinatus tendon detachment, supraspinatus+subscapularis tendon detachment). We found the remaining (intact) rotator cuff tendons have decreased mechanical properties in the presence of rotator cuff tears. Remaining (intact) subscapularis and infraspinatus tendon cross-sectional area increased, while tendon modulus decreased after both one and two tendon tears. Additionally, the remaining (intact) tendon cross-sectional areas continued to increase with time post-injury. These alterations could potentially lead to further tendon damage and tear progression.

Background Chronic rotator cuff tears are often associated with pain or poor function. In a rat with only a detached supraspinatus tendon, the tendon heals spontaneously which is inconsistent with how tears are believed to heal in humans. Questions/purposes We therefore asked whether a combined supraspinatus and infraspinatus detachment in the rat would fail to heal and result in a chronic injury in the supraspinatus tendon. Methods We acutely detached the supraspinatus and infraspinatus tendons in a rat model. At 4, 8, and 16 weeks post-detachment, biomechanical testing, collagen organization, and histological grading were evaluated for the detached supraspinatus and infraspinatus tendons and compared to controls. Results In the detached supraspinatus tendon, area and percent relaxation were increased at all time points while the modulus and stiffness were similar to those of controls at 4 and 8 weeks. Collagen disorganization increased at late time points while cellularity increased and cells were more rounded in shape. In the detached infraspinatus tendon, area and percent relaxation were also increased at late time points. However, the modulus values initially decreased followed by an increase in both modulus and stiffness at 16 weeks compared to control. In the detached infraspinatus, we also observed a decrease in collagen organization at all time points and increased cellularity and a more rounded cell shape. Conclusions Due to the ongoing changes in mechanics, collagen organization and histology in the detached supraspinatus tendon compared to control animals at 16 weeks, this model may be useful for understanding the human chronic tendon tear. Clinical Relevance This rat rotator cuff chronic model can be used to test hypotheses regarding injury and repair mechanisms that cannot be addressed in human patients or in cadaveric studies.