Stephanie M. Perry

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.

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 ∼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.

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.

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].

Erratum to: “Alterations in function after rotator cuff tears in an animal model” [J Shoulder Elbow Surg 2009 Mar-Apr;18(2):296-304.]

The above-mentioned article published in the MarchApril issue with an incorrect abstract. The correct abstract appears below. 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, supraspinatusþinfraspinatus tendon detachment, or supraspinatusþsubscapularis 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