Changes in Rotator Cuff Muscle Volume, Fat Content, and Passive Mechanics After Chronic Detachment in a Canine Model

Safran, Ori MD; Derwin, Kathleen A. PhD; Powell, Kimerly PhD; Iannotti, Joseph P. MD, PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.D.02421
Scientific Articles

Background: Long-standing tears of the rotator cuff can lead to substantial and perhaps irreversible changes in the affected rotator cuff muscles. We developed a chronic rotator cuff tear in a canine model to investigate and quantify the time-related changes in passive mechanics, volume, and fat of the infraspinatus muscle. We hypothesized that infraspinatus muscle stiffness would increase, volume would decrease, and fat content would increase at twelve weeks following tendon detachment.

Methods: The right infraspinatus tendon of eight adult mongrel dogs were surgically detached from the proximal part of the humerus. The uninvolved left shoulder served as a control. Muscle volume changes were quantified with use of magnetic resonance imaging. At twelve weeks, the passive mechanical properties of the chronically detached and control muscles were determined intraoperatively with use of a custom-designed device. Intramuscular fat was evaluated histologically at the time that the animals were killed.

Results: After twelve weeks of detachment, the stiffness was significantly increased in the detached infraspinatus muscles relative to that in the controls (p < 0.0001). Magnetic resonance image analysis demonstrated that the detached muscle volumes decreased by an average of 32% in the first six weeks and remained constant thereafter. Intramuscular fat increased significantly in the detached muscles and to a greater extent in the lateral regions (p < 0.05).

Conclusions: The chronically detached muscle is not merely a smaller version of the original muscle but, rather, a different muscle. The detached muscle becomes stiffer, and the passive loads required to repair it can become excessive. A significant reduction in muscle volume occurs within days to weeks following tendon detachment (p < 0.0001). The nonuniformity of changes in muscle fat suggests that fat content should be used cautiously as an indicator of muscle quality.

Clinical Relevance: Clinically, chronic large rotator cuff tears are observed to have a qualitatively shorter and stiffer muscle-tendon unit than normal. We developed a chronic rotator cuff model to quantitatively investigate changes in the detached infraspinatus muscle. The passive mechanical properties of a chronically torn rotator cuff muscle-tendon unit may be a useful predictor of repairability and clinical outcome.

Author Information

1 Department of Orthopaedic Surgery and the Orthopaedic Research Center (O.S., K.A.D., K.P., and J.P.I.) and the Department of Biomedical Engineering (K.A.D., K.P., and J.P.I.), The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195. E-mail address for J.P. Iannotti:

Article Outline

Chronic rotator cuff tears are a frequent cause of morbidity in the adult population. Surgical repair of chronic tears is indicated when conservative treatment has failed to decrease the symptoms1. Recent studies have noted that large chronic cuff tears failed to heal after repair in twenty-four of 100 shoulders2, twenty-seven of fifty shoulders3, and ten of twenty-seven shoulders4. Several factors have been suggested to be responsible for this high failure rate. These include the size of the tear5,6, time from the injury to the repair7, tendon quality8, muscle quality9, biologic healing response10,11, and surgical technique12,13.

Clinically, chronic large rotator cuff tears are observed to have a qualitatively shorter and stiffer muscle-tendon unit than normal. Changes to the muscle, including atrophy14,15, fat accumulation14-17, and an increase in fibrous tissue15,18, have been reported following rotator cuff detachment in animal models. These data suggest that the difficulty in obtaining an intact rotator cuff repair, on the basis of imaging criteria, is related to a change in the quality and quantity of the involved muscle.

We developed a chronic tear of the rotator cuff tendon in a canine model to investigate and quantify the time-related changes in the passive mechanics, volume, and fat of the infraspinatus muscle. Specifically, we hypothesized that infraspinatus muscle stiffness would increase, volume would decrease, and fat content would increase at twelve weeks following tendon detachment in the canine model.

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Materials and Methods

All procedures were performed in accordance with the standards of the American Association for the Accreditation of Laboratory Animal Care. Eight adult (one to three years old) mixed-breed dogs, including six females and two males, weighing an average (and standard deviation) of 28.2 ± 2.8 kg were used. To simulate a rotator cuff tendon tear, we surgically detached the right infraspinatus tendon. To prevent early spontaneous reattachment of the tendon and to facilitate tendon identification, the detached tendon was wrapped with a polymer membrane. Postoperatively, the dogs were allowed free cage activity. The uninvolved left shoulder served as a control. At twelve weeks, the dogs underwent a second surgery and were immediately killed. Changes in the passive mechanics, volume, and fat content of the detached and control muscles were quantified.

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First Surgery

The dogs initially were anesthetized with an intravenous dose of thiopental (20 mg/kg) to effect. Benzathine penicillin and procaine (20,000 IU/kg) were administered intramuscularly at induction. The dogs were intubated orotracheally and maintained on isoflurane in oxygen, titrated to effect (0.5% to 5%). In a sterile surgical field, a 7-cm-long incision was made between the acromion medially and the greater tuberosity laterally through skin and subcutaneous tissue. The interval between the two heads of the deltoid muscle was developed, exposing the infraspinatus insertion into the proximal part of the humerus. The tendon was then sharply detached from its insertion. The tendon and the lateral portion of the muscle were freed from the surrounding tissues. After release of the tendon, its edge retracted approximately 1 cm (an average of 11 ± 1 mm) from the greater tuberosity. The free end of the infraspinatus tendon was then wrapped with polyvinylidene fluoride (Durapore 7 SVLP; Millipore, Bedford, Massachusetts) with a 125-μm thickness and a pore size of 5 μm in the first three dogs and with PRECLUDE membrane (W.L. Gore and Associates, Flagstaff, Arizona) in the last five dogs, to prevent spontaneous reattachment and to facilitate the identification of the tendons during the second surgery. We used two different membranes because of the availability of these materials. The membranes were sutured to the tendon with a 4-0 monofilament nonabsorbable suture. The wound was irrigated and closed in two layers with use of 3-0 absorbable suture. A light compressive bandage was applied. Postoperative analgesia consisted of buprenorphine (0.3 mg) administered subcutaneously on the day of surgery and the following day if indicated. Following surgery, the involved shoulders were not immobilized and the dogs were allowed free cage activity.

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Second Surgery

The second surgery was performed twelve weeks after the first surgery. The dogs were anesthetized, and the shoulder was prepared in a sterile fashion. The left (control) infraspinatus tendon was approached and detached from its humeral insertion as described above. The right (tendon-released) shoulder was approached with use of the previous incision, and the infraspinatus tendon, wrapped with the membrane, was isolated. On both sides, the tendon and the lateral portion of the infraspinatus muscle were freed from surrounding tissues. The passive mechanical properties of the left and right infraspinatus tendon-muscle units were then measured as described in detail below. The dogs were administered intravenous pancuronium bromide (0.1 mg/kg) five minutes prior to the mechanical tests to ensure complete muscle relaxation. The dogs were then killed with an intravenous lethal injection of pentobarbital sodium with phenytoin (Beuthansia-D; Schering-Plough Animal Health, Union, New Jersey). The infraspinatus muscles of both shoulders were dissected for analysis of their physical properties (volume, length, and weight) and fat content.

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Passive Mechanics

A custom-designed tissue-tension device was used intraoperatively to stretch the infraspinatus muscle-tendon unit (Fig. 1). The device consisted of a large, threaded bore that translated linearly within a mating cylinder by means of a manual crank and gear mechanism. The device was secured to the humerus by a separate cleat attached to the bone with two 2.5-mm screws. A ball-type joint in the device allowed its orientation to be further adjusted along the line of action of the infraspinatus muscle during testing. The device components were made of type-6061-T6 aluminum or type-316 stainless steel and could be autoclaved. Linear digital calipers (Mitutoyo America, Aurora, Illinois) mounted on a stage that resided outside the sterile field were used to monitor displacement. The calipers interfaced with the translating inner bore of the device by means of a steel cable. Load was monitored by a 250-N (50-lb) load cell in series with the inner bore and residing entirely within the device housing (Honeywell Sensotec, Columbus, Ohio).

For tissue tension measurements, a number-1 Dacron suture was passed through the infraspinatus tendon stump with use of a Krackow stitch19. The free ends of the sutures were then attached to the tension device. The device was secured to the humerus and was manually cranked at a displacement rate of approximately 0.4 mm/sec. Load and displacement were continuously monitored throughout the test by a personal computer. Each infraspinatus muscle-tendon unit was passively stretched laterally along its line of action, 15 to 20 mm from the point at which it began resisting load. Tests were repeated three to four times for each muscle.

Load and displacement data were processed with use of an algorithm to iteratively fit the data from 2-mm displacement to the data point at which the R2 value of a linear least-squares fit was maximized. The remaining portion of the data was fit with a cubic equation, maintaining point and slope continuity with the linear fit. The point bounding the linear and cubic fit regions was designated as the inflection point, and the inflection load and inflection displacement were defined at this point. The slope of the linear fit was defined as stiffness-1, and the slope of the cubic fit at 15 mm was defined as stiffness-2. The 15-mm point was used because we had recorded data at 15 mm for all data sets. Load-displacement data were normalized by the initial muscle physiological cross-sectional area and the initial muscle length, respectively, to yield engineering stress-strain data. Stress-strain data were analyzed in a similar manner to that used for the load-displacement data, providing an inflection stress, inflection strain, modulus-1, and modulus-2.

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Volume Measures

The tendon-released shoulders were scanned by magnetic resonance imaging just prior to the detachment surgery and every two weeks thereafter. A final scan was performed just prior to the second surgery. The control shoulder was scanned only prior to the detachment and the second surgery. The shoulders were scanned with a 1.5-T magnetic resonance scanner with use of a T1 sequence with 4-mm wide oblique sagittal views to facilitate the differentiation of the infraspinatus muscle from the adjacent muscles. The dogs were positioned in a lateral decubitus position with the scanned shoulder facing down in order to minimize breathing artifacts. The entire infraspinatus muscle was scanned. The magnetic resonance scans were analyzed with use of custom image-analysis software. The perimeter of the infraspinatus muscle was traced in each image slice by two independent observers, and the muscle cross-section area was calculated. For each magnetic resonance scan sequence, a total muscle volume was estimated by summing the slice cross-sectional areas and multiplying by the slice width (4 mm)20. Total muscle volume was calculated by averaging the muscle volume obtained by each observer.

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Fluid-Displacement Volume Measures

At the time that the animals were killed, the intact infraspinatus muscles were dissected and submersed in a graduated cylinder containing saline solution. Care was taken not to submerse the tendon stump. The volume change was recorded as the muscle volume. Direct muscle volume measures were compared with the volume results on magnetic resonance imaging and were used to validate the magnetic resonance imaging method.

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Muscle Fat Content

At the time that the animals were killed, 2-mm-thick transverse sections of tissue were cut from the medial, central, and lateral regions of the harvested infraspinatus muscles. Tissue sections were fixed in 10% neutral buffered formalin for five days. The samples were then post-fixed with a solution of 2% osmium tetroxide and 5% potassium dichromate for two weeks14. They then underwent copious water washes and were dehydrated through ascending series of alcohols, embedded in paraffin, and cut into 5-μm sections. The sections were viewed under a light microscope (Olympus BH-2; Olympus America, Melville, New York) and digitized at four times magnification with use of a camera (CoolSNAP-Pro 24-bit; Media Cybernetics, Silver Spring, Maryland) and frame grabber (Prior ProScan; Prior Scientific, Rockland, Massachusetts). The infraspinatus cross section was traced to identify a region of interest for image analysis. With use of custom software, the cross-sectional area was calculated and the area of fat, stained black, was segmented. The ratio of fat to infraspinatus area constituted the percentage of intramuscular fat for each muscle section.

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Statistical Methods

For tissue tension data, differences between sides were assessed with use of repeated-measures analysis of variance techniques to account for the within-subject correlation. For all other parameters, where a single measurement per subject was available for analysis, paired t tests or Wilcoxon signed-rank tests were used, as appropriate, for comparisons of the paired data. Results were considered significant if the p value was ≤0.05.

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Physical Properties of the Infraspinatus Muscle

The physical properties of the control and detached infraspinatus muscles are shown in Table I. Volume decreased by a mean of 33% (p < 0.0001); weight, by 34% (p < 0.0001); and muscle length, by 16% (p < 0.0001) in the detached muscle during the twelve-week period. The average gap distance (and standard deviation) between the detached tendon and the greater tuberosity was 34 ± 4 mm.

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Passive Mechanics

Three distinct regions were noted in the passive load-displacement response for both control and detached infraspinatus muscle-tendons: an initial linear portion21; a second linear portion of lesser slope than the first; and a nonlinear portion of increasing slope21. Because the initial linear portion of the data was somewhat variable among the samples and was confined to the very early phase of muscle-loading (typically contained within the first 1 to 2 mm of stretch), we chose not to include it in our analysis. As described in the Materials and Methods section, only the data after the 2-mm displacement point were fit with our algorithm.

Accordingly, the passive mechanical properties of the control and detached muscles are displayed in Table II, and representative load-displacement curves for both groups are shown in Figure 2. The inflection load was not different between the control and detached muscles (p = 0.74); however, the inflection displacement significantly decreased from a mean (and standard error) of 10.2 ± 0.33 mm in the control muscle to 6.8 ± 0.33 mm in the detached muscle (p < 0.0001). The chronically detached infraspinatus was significantly stiffer than the control, with a twofold increase in stiffness-1 (p < 0.0001) and a fourfold increase in stiffness-2 (p < 0.0001). The results were similar for the normalized stress-strain data sets. The inflection stress was not significantly different between the control and detached muscles (p = 0.13); however, inflection strain was significantly less in the detached infraspinatus (p = 0.0039). The chronically detached muscles had significantly higher moduli than the controls, with a twofold increase in modulus-1 (p < 0.0001) and a fourfold increase in modulus-2 (p < 0.0001).

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Volume Measures

The volume change as measured by magnetic resonance imaging for a chronically detached infraspinatus muscle is shown in Figure 3. Detached muscle volume decreased by an average (and standard deviation) of 32.4% ± 4.6% at six weeks (p < 0.0001) and by 31.3% ± 5.7% at twelve weeks (p < 0.0001) relative to the initial muscle volume. No significant volume change occurred in the detached infraspinatus between six and twelve weeks after detachment (p = 0.43). No change in volume occurred in the control muscle during the twelve-week period (p = 0.74). Agreement in the muscle volume data from the two independent image analysts was high (correlation coefficient = 0.84), with no significant difference between the data sets (p = 0.49). Furthermore, the magnetic resonance imaging data on the muscle volume at twelve weeks were highly correlated to the data on the fluid-displacement volume (Spearman correlation coefficient = 0.93).

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Muscle Fat Content

The intramuscular fat content in the control and detached muscles is shown in Figures 4 and 5. Intramuscular fat increased significantly in the detached infraspinatus compared with that in the control with respect to each muscle region as well as the average of all regions (p < 0.05). The average fat content was 0.8% ± 0.2% in the control and 6.8% ± 1.5% in the detached muscles. There were also significant differences in the fat content among the different regions of the detached muscles (p < 0.02). The fat content was an average of 9.1% ± 1.9% in the lateral regions, 7.0% ± 1.3% in the central regions, and 4.3% ± 2.4% in the medial regions. No regional differences in fat content were found in the control infraspinatus muscles.

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Our results demonstrated that the chronically detached infraspinatus muscle changed significantly during the detachment period, becoming stiffer, smaller, and infiltrated with fat. In addition, we showed that the shape of the passive load-displacement curves of chronically detached muscles was qualitatively similar to that of the controls and other normal muscles22. However, the curves were shifted “up and to the left,” indicating a decrease in the low stiffness region and an overall increase in stiffness of the detached muscles. These results are similar to the increased passive elastic stiffness that has been reported for muscles immobilized in shortened positions23,24. The fundamental consequence of this shift is that the detached muscle generates higher loads than normal muscle does for the same amount of stretch.

The chronically detached muscle is not merely a smaller version of the original muscle but, rather, a different muscle. After the load-displacement mechanical data were normalized by the muscle dimensions, the chronically detached muscles were shown to have decreased inflection strain and increased moduli compared with control (normal) infraspinatus muscles. In other words, the material properties or quality of the detached muscles had changed. It is important to note that canine infraspinatus tendon and number-1 Dacron suture were mechanically tested in pilot studies and were shown to be two to three orders of magnitude stiffer than the infraspinatus muscle. Hence, the passive mechanical properties measured in the present study essentially reflect the properties of the infraspinatus muscle. This approximation has been made in previous studies of passive muscle mechanics25.

Changes in the passive mechanics of lower limb muscles have been attributed to two major factors: the connective tissue component within and surrounding the muscle, which increases in volume during muscle atrophy18,26, and the noncontractile proteins of the sarcomeric cytoskeleton25. More specifically, titin, a giant protein associated with the myosin and actin filaments in the sarcomere, has been implicated in playing a major role in passive muscle mechanics25,27. In our animal model, whether the change in the muscle material properties arises from a change in the connective tissue component or a change in other muscle components, such as titin, is a subject for further research. Furthermore, whether this muscle material change is reversible with simple muscle reattachment should be investigated and has important clinical implications.

Our study is the first, as far as we know, to report on changes in longitudinal muscle volume over a chronic detachment period. Comparable reductions in muscle volume have been demonstrated in lower-limb unloading models, such as lower-limb suspension, muscle immobilization, and Achilles tenotomy28-30. Volume reduction was more pronounced in the antigravity muscles than in their antagonists and varied in intensity between species30. An initial decrease in sarcomere length and subsequently in sarcomere number is believed to be responsible for the observed reduction in muscle volume18,30,31. It is assumed that a reduction in sarcomere number is a remodeling response to optimize the contractile function of the remaining sarcomeres in the shortened muscles30.

The detached muscle reached a new steady-state volume at six weeks in our animal model. However, it is important to note that at the time that the animals were killed, we observed that the detached infraspinatus tendon (together with the polymeric membrane) had scarred to the joint capsule. Therefore, we cannot conclude that the muscle was fully unloaded during the entire detachment period. Reformation of the distal muscle attachment to surrounding connective tissues and subsequent reapplication of tension has been shown to decrease muscle atrophy30. Hence, it is difficult to speculate whether muscle volume would have decreased further in our model had the muscle been more fully unloaded throughout the twelve-week period.

We found a significant increase in the fat content of chronically detached muscles at twelve weeks. In a chronic supraspinatus model in rabbits, intramuscular fat content increased from 1.3% in normal muscle to 5.4% in detached muscle after twenty-four weeks14; however, no increase in fat was found at twelve weeks16. In a chronic infraspinatus model in sheep, fat content increased from 0.47% in normal muscle to 5.9% at six weeks and 6.2% at eighteen weeks in the detached muscle17. All of these studies evaluated fat content at one location in the involved muscle. Our study demonstrated that muscle fat changed nonuniformly in the detached muscle, with the greatest changes occurring in the lateral portion. Clinically, this means that muscle fat should be used cautiously as an indicator of muscle change because the lateral muscle fat content does not represent the entire muscle. Furthermore, subtle changes in the biopsy site could yield significantly different fat percentages within the same muscle.

We suggest that the repairability of a chronic rotator cuff tendon tear and the likelihood of achieving a successful clinical outcome depend on the passive loads generated during repair. Passive load is influenced not only by the muscle stiffness but also by the distance required to reattach the tendon to bone. We represented the loads required to repair the control and detached infraspinatus muscle-tendons in our animal model by plotting the passive muscle load as a function of the gap closure distance (Fig. 6). We approximated 100% of the gap distance as corresponding to a retraction distance of 10 mm in the control group and 35 mm in the chronically detached group. These are distances between the end of the tendon and the greater tuberosity and represent the distance the tendon would need to be pulled, in the acute and chronic tear conditions, to reach its attachment site at the time of rotator cuff repair. One can see that the stretch required to repair the chronically detached muscle would load the muscle into the high-stiffness portion of the loading curve, even beyond the region that we achieved with our device. By extrapolation, the passive loads required to repair the detached muscle-tendon could reach at least 70 N. Together with any additional loads generated postoperatively from passive shoulder motion and active muscle contraction, these excessive loads could precipitate a failed repair.

In summary, we developed a canine model of a chronic rotator cuff tear to investigate and quantify the changes in the passive mechanics, volume, and fat of the detached infraspinatus muscle. We found that the infraspinatus muscle becomes significantly stiffer, smaller, and infiltrated with fat within twelve weeks after detachment in the canine model. We demonstrated that the chronically detached muscle is not merely a smaller version of the original muscle but, rather, a different muscle. The passive loads required to repair the detached muscle can become quite large and may be important in determining the ability to repair the tendon to its original site of attachment or for that tendon to heal after repair.▪

NOTE: The authors acknowledge the helpful contributions of David Hicks, MD, Hakan Ilaslan, MD, and Jean Tkach, PhD.

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from the Sukenik Family Foundation and DePuy Orthopaedics. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Department of Orthopaedic Surgery, the Orthopaedic Research Center, and the Department of Biomedical Engineering, The Cleveland Clinic Foundation, Cleveland, Ohio

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