Plasticity of muscle architecture after supraspinatus tears.

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 Sprague-Dawley rats (mass, 400-450 g), and the 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. 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 physiological cross-sectional area at 2 weeks, but physiological cross-sectional area 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.

Tendon properties remain altered in a chronic rat rotator cuff model.

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.

Alterations in function after rotator cuff tears in an animal model.

HYPOTHESIS: This study examined the effect of multiple rotator cuff tendon tears on shoulder function in an animal model. MATERIALS AND METHODS: 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 through ambulatory parameters (paw and stride measures) and range of motion prior to tendon detachment and at various time points after tendon detachment. RESULTS: Ambulatory parameters and total range of motion, representing measures of shoulder function, were significantly altered with rotator cuff tears. The addition of a second torn rotator cuff tendon (infraspinatus or subscapularis)had further detrimental effects on animal shoulder function compared to uninjured control. DISCUSSION: This study demonstrated functional changes in a rat rotator cuff model. Many of the permanent functional changes were likely present because the required motion used for those actions can no longer be performed. For parameters that were transient, compensation with another limb or subsidence of pain may have occurred. CONCLUSION: The findings in this study are consistent with the alterations in shoulder function observed with rotator cuff and other shoulder injuries in the human. Future studies using this model can begin to examine the root of the functional differences, whether it is pain, mechanical deficiency, or a combination of both, which cannot be fully studied clinically [corrected].

Mechanical properties of the long-head of the biceps tendon are altered in the presence of rotator cuff tears in a rat model.

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 + infraspinatus or supraspinatus + 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 approximately 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.

After rotator cuff tears, the remaining (intact) tendons are mechanically altered.

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 used an animal model of multiple rotator cuff tendon tears to investigate alterations in the remaining (intact) tendon mechanical properties at 4 and 8 weeks after injury. Twenty-four rats served as uninjured controls, whereas 72 were divided among 3 tendon detachment groups: supraspinatus tendon detachment, supraspinatus + infraspinatus tendon detachment, and supraspinatus + subscapularis tendon detachment. The remaining (intact) rotator cuff tendons had decreased mechanical properties in the presence of rotator cuff tears. The remaining (intact) subscapularis and infraspinatus tendon cross-sectional areas increased, whereas tendon modulus decreased after tears of both 1 and 2 tendons. The remaining (intact) tendon cross-sectional areas continued to increase with time after injury. These alterations could potentially lead to further tendon damage and tear progression.

Use of small intestine submucosa in a rat model of acute and chronic rotator cuff tear.

Augmentation materials for rotator cuff tears, such as small intestine submucosa (SIS), have been used with the goal of improving outcome. Knowledge is limited on the use of SIS in animal models of acute and chronic rotator cuff tears. We hypothesized that the use of SIS in the surgical management of full thickness supraspinatus tears would improve histologic and biomechanical properties. Results show temporal improvements in several histologic parameters. Both acute and chronic injuries repaired with SIS have similar and increased mechanical properties respectively, compared to those repaired without SIS. In general, acute repairs with SIS were comparable to acute repairs without SIS. In chronic repairs, the use of SIS significantly reduced the cross sectional area of the healing tendon and increased the modulus. These results provide information on the use of SIS for rotator cuff repairs.

Inflammatory and angiogenic mRNA levels are altered in a supraspinatus tendon overuse animal model.

Shoulder overuse injuries, especially those to the supraspinatus tendon of the rotator cuff, are common musculoskeletal disorders. Unfortunately, little is known about the disease etiology and pathogenesis. The objective of this study was to determine the levels of specific inflammatory and angiogenic markers in a rat supraspinatus tendon overuse injury model. We hypothesized that inflammation would not be present early in the overuse protocol. Conversely, we hypothesized that the overuse protocol would result in increased angiogenesis early. Increases in five-lipoxygenase activating protein, cyclooxygenase-2, vascular endothelial growth factor, and von Willebrand factor were evaluated by use of reverse transcription-polymerase chain reaction from 1 day through 16 weeks of treadmill running (overuse protocol). These results provide important information on the role of angiogenesis and inflammation in the disease process. Future studies will further evaluate the mechanisms of the disease process as well as potential targeted treatment modalities.

The effect of overuse activity on Achilles tendon in an animal model: a biomechanical study.

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.

Supraspinatus tendon organizational and mechanical properties in a chronic rotator cuff tear animal model.

Rotator cuff tears of the shoulder are a common cause of pain and disability. The successful repair of rotator cuff tendon tears depends on the time from onset of injury to the time of surgical repair. However, the effect of time from injury to repair remains poorly understood. A rat model was used to investigate the supraspinatus tendon organizational and mechanical property changes that occur with time post-injury to understand the natural injury response in the absence of repair. It was hypothesized that increased time post-injury would result in increased detrimental changes to tendon organizational and mechanical properties. Tendons were detached at the insertion on the humerus without repair and the quantitative organizational and mechanical properties were analyzed at 1, 2, 4, 8, and 16 weeks post-detachment. Tendon detachment resulted in a dramatic decrease in mechanical properties initially followed by a progressive increase with time. The quantitative collagen fiber orientation results provided corroborating support to the mechanical property data. Based on similarities in histology and mechanical properties to rotator cuff tears in humans, the animal model presented here is promising for future investigations of the tendon's natural injury response in the absence of repair.