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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Orthop Res. Author manuscript; available in PMC 2014 May 1.
Published in final edited form as:
Published online 2014 January 25. doi:  10.1002/jor.22586
PMCID: PMC4006275
NIHMSID: NIHMS569197

Disruption of the Anterior–Posterior Rotator Cuff Force Balance Alters Joint Function and Leads to Joint Damage in a Rat Model

Abstract

The rotator cuff assists in shoulder movement and provides dynamic stability to the glenohumeral joint. Specifically, the anterior–posterior (AP) force balance, provided by the subscapularis anteriorly and the infraspinatus and teres minor posteriorly, is critical for joint stability and concentric rotation of the humeral head on the glenoid. However, limited understanding exists of the consequences associated with disruption of the AP force balance (due to tears of both the supraspinatus and infraspinatus tendons) on joint function and joint damage. We investigated the effect of disrupting the APforce balance on joint function and joint damage in an overuse rat model. Twenty-eight rats underwent 4 weeks of overuse to produce a tendinopathic condition and were then randomized into two surgical groups: Detachment of the supraspinatus only or detachment of the supraspinatus and infraspinatus tendons. Rats were then gradually returned to their overuse protocol. Quantitative ambulatory measures including medial/lateral, propulsion, braking, and vertical forces were significantly different between groups. Additionally, cartilage and adjacent tendon properties were significantly altered. These results identify joint imbalance as a mechanical mechanism for joint damage and demonstrate the importance of preserving rotator cuff balance when treating active cuff tear patients.

Keywords: force couple, rotator cuff, animal model

Rotator cuff disease is a common condition that can lead to significant pain and dysfunction. Cuff tears are prevalent with repetitive overhead activities, and when left untreated can dramatically alter joint mechanics in a complex, multifactorial manner leading to shoulder injury. Tears often begin isolated to the supraspinatus tendon, and over time may progress posteriorly to complete ruptures of the supraspinatus and infraspinatus tendons. This progression may result in abnormal glenohumeral joint kinematics,1,2 which can lead to significant joint damage including articular cartilage degeneration35 and injury to the adjacent tendons, such as the biceps6 and subscapularis.7 However, the mechanical mechanisms by which tendon injuries lead to permanent damage are not well-defined.

Two mechanical mechanisms are likely responsible for the initiation and progression of shoulder joint damage following tendon injury: Tendon overload/overuse and joint imbalance. We previously studied the first mechanism (overuse) by examining a single injury (two-tendon tears involving the supraspinatus and infraspinatus) and evaluating the effect of different activity levels (cage activity vs. overuse).8 Results demonstrated that overuse activity in the presence of cuff tears led to damage to the adjacent joint structures. In the current study, we focused on the second mechanism (joint imbalance) by examining a single activity level (overuse) with two different injuries (supraspinatus tendon tear only and tears of both the supraspinatus and infraspinatus tendons).

Glenohumeral joint stability depends on a dynamic balance between rotator cuff forces, the subscapularis anteriorly and the infraspinatus posteriorly.8 An intact cuff stabilizes the joint and acts as a “suspension bridge,” providing concavity compression and a stable fulcrum to allow for concentric rotation of the humeral head on the glenoid.9 In the presence of an isolated supraspinatus tear, this anterior–posterior (AP) force balance remains intact. However, a cuff tear involving the supraspinatus and infraspinatus may disrupt the normal balance of forces, resulting in loss of a stable fulcrum and abnormal loading. This may also alter joint contact forces with increased risk for cartilage degeneration and injury to the surrounding structures.

Cadaver studies demonstrated the importance of AP rotator cuff forces in preserving joint balance.10,11 However, the in vivo cause and effect relationships that mechanical alterations have on glenoid cartilage and adjacent tendons are unknown. Previous studies also demonstrated the importance of restoring rotator cuff force balance to improve joint function.12 However, no studies have evaluated the consequences associated with disrupting the AP force balance on joint function and joint damage, particularly with overuse. Therefore, we investigated the effect of disrupting the AP force balance on joint function and damage using a clinically relevant overuse model. We hypothesized that a disrupted AP force balance (supraspinatus and infraspinatus tear) would result in decreased joint function and inferior adjacent tissue (glenoid cartilage, biceps, and subscapularis tendon) properties compared to an intact AP balance (supraspinatus only tear). Specifically, we expected a more laterally directed force, decreased propulsion force, decreased braking force, and decreased vertical force, based on previous studies of shoulder injury using this model, which may indicate an altered loading environment.13

METHODS

Study Design

Following a 2 wk training period, 28 eight adult male Sprague-Dawley rats (400–450 g) underwent 4 wks of overuse (downhill,10°) treadmill running at 17 m/min for 1 h/day, 5 days/wk to induce a tendinopathic condition in the supraspinatus tendon.14 The animals were then randomized into two surgical groups: Detachment of the supraspinatus only (SO tear) or detachment of the supraspinatus and infraspinatus (SI tear) as described previously.4,15 Briefly, with the arm in external rotation, a 2cm skin incision was made followed by blunt dissection down to the rotator cuff musculature. The cuff was exposed, and the tendons were visualized at their humeral insertions. The supraspinatus and infraspinatus tendons were separated from the other cuff tendons before sharp detachment at the insertion on the greater tuberosity. Any remaining fibrocartilage at the insertion was left intact, and the detached tendons were allowed to freely retract. The overlying muscle and skin were closed.

Animals were returned to 1 wk of cage activity and gradually returned to the overuse protocol over 2 wks, followed by another 5 wks of overuse activity. All animals were euthanized 8 wks following surgery. For histology (n = 4), tissues were harvested immediately and fixed in formalin. The remaining 10 animals were frozen (−20°C) until the time of mechanical testing.

Quantitative Ambulatory Assessment

To assess joint function, forelimb gait, and ground reaction forces were recorded using an instrumented walkway16 1 day preoperatively (baseline) and at 3, 7, 14, 28, 42, and 56 days postoperatively. Force data, including medial/lateral, braking, propulsion, and vertical forces were collected for each walk. Temporal spatial parameters were used to calculate step length and width. Parameters were averaged across ≥2 walks on a given day/animal and normalized to body weight.

Tendon Mechanical Testing

The animals were thawed, and the scapula and humerus were grossly dissected with the long head of the biceps (LHB) and subscapularis tendons intact. Tendons were then fine dissected under a microscope to remove secondary soft tissue and to separate the upper and lower bands of the subscapularis tendon. Tendon testing was performed as previously described.17,18 Briefly, stain lines, for local optical strain measurement (at insertion and mid-substance), were placed on the LHB and upper and lower bands of the subscapularis tendons. Cross-sectional area was measured using a custom laser device. The scapula and humerus were embedded in holding fixtures using PMMA, gripped with cyanocrylate annealed sand paper, and immersed in PBS at 37°C. Tensile testing was performed with a preload to 0.08 N, preconditioning (10 cycles of 0.1–0.5N at a strain rate of 1%/s), stress relaxation to 4% (LHB) or 5% (subscapularis) strain at a rate of 5%/s for 600 s, and ramp to failure at 0.3%/s. Stress was calculated as force/initial area, and 2D Lagrangian optical strain was determined from stain line displacements that were measured from images using custom texture tracking software.

Cartilage Mechanical Testing

Following completion of LHB testing, the glenoid cartilage was prepared for testing by sharply detaching the LHB at its insertion on the superior rim of the glenoid using a scalpel blade. The glenoid was then preserved by wrapping in soft tissue and refreezing (−20°C). For cartilage thickness measurement,4 each scapula was thawed and immersed in PBS containing a protease inhibitor cocktail (5 mM Benz-HCl, 1 mM PMSF, 1 M NEM) at room temperature. Specimens were scanned at 0.25 mm increments using a 55 MHz ultrasound probe (Visualsonics, Inc., Toronto, Ontario, Canada) in plane with the scapula. Captured B-mode images of each scan were manually segmented (three times and averaged) by selecting the cartilage and bony surfaces of the glenoid. The 3D surface positions were reconstructed with a custom program (MATLAB, MathWorks, Inc., Natick, MA) and used to determine cartilage thickness maps. Each map was divided into six regions (center (C), posterior–superior (PS), posterior–inferior (PI), anterior–superior (AS), anterior–inferior (AI), and superior (S)), and an average thickness computed for each region. Following scanning, specimens were again preserved by wrapping in soft tissue and refreezing (−20°C) until mechanical testing.

For cartilage mechanical testing,4 each scapula was thawed and immersed in PBS containing the protease inhibitor cocktail at room temperature. Utilizing a 0.5 mm diameter non-porous spherical indenter tip, indentation testing was performed as previously described.4 Briefly, a preload (0.005 N) was set followed by 8 step-wise stress relaxation tests (8 µm ramp at 2 µm/s followed by a 300 s hold). The scapula was repositioned for each localized region using angular, rotational, and linear stages such that the indenter tip was perpendicular to the cartilage surface. Cartilage thickness was determined by identifying the indentation location on each thickness map. Equilibrium elastic modulus was calculated4 at 20% indentation and assuming Poisson’s ratio (υ = 0.30).

Histology

Histologic assessment was performed as previously described.19 Briefly, rotator cuff samples were left intact as bone-tendon-muscle units and pinned to prevent tissue contracture during paraffin processing. For the scapula, the bony glenoid origin of the LHB was resected, and the bone-tendon-muscle units were pinned and processed separately from the remaining glenoid cartilage. All samples went through a fixation and decalcification process followed by standard paraffin processing. Sagittal sections (7 µm) were collected, and tendon samples were stained with hematoxylin-eosin (H&E); cartilage samples were stained with Safranin O, Fast Green, and Iron Hematoxylin. H&E stained sections were imaged at the insertion site and mid-substance of each tendon using a microscope (Nikon Eclipse 90i, Melville, NY) at 200× magnification. Due to the unique LHB anatomy, the mid-substance region was further subdivided (intra-articular space, proximal groove, and distal groove). Cell density (cells/mm2) and cell shape (aspect ratio; 0–1 with 1 being a circle) were quantified using a bioquantification software system (Bioquant Osteo II, Nashville, TN). Cartilage sections were imaged at 100× magnification in five regions (C, PS, PI, AS, and AI) corresponding to the indentation locations (with the exception of the superior region that was removed along with the biceps tendon, and graded using a Modified Mankin Score (including scores for cellularity, structure, matrix staining). Due to the presence of an intact tidemark, scoring for this category was removed. Scoring was performed by three blinded investigators, and the mode of these three values taken as the score for each specimen. Cell density was also quantified using bioquantification software.

Statistics

For the ambulatory assessment, significance was assessed using a two-way ANOVA with repeated measures on time with follow-up t-tests between groups at each time point. Due to the nature of measuring rat ambulation, data points were occasionally absent (~7%) for a specific animal on a specific day. Therefore, multiple imputation (Markov chain Monte Carlo (MCMC) method) was conducted on the ambulation data to allow for a repeated measures analysis. Tissue mechanics and tendon histology were assessed using oneand two-tailed t-tests, respectively. For cartilage thickness and scoring, median grades were compared between groups using a Mann Whitney test. For all comparisons, significance was set at p < 0.05 and trends defined as p < 0.1.

RESULTS

Ambulatory Data

Large rotator cuff tears (SI tear) significantly affected medial/lateral, propulsion, braking, and vertical ground reaction forces compared to the SO tear group (Fig. 1), with both groups returning toward pre-injury values over time. Lateral forces were significantly increased in the SI tear group at early times (3, 7, and 14 days), with a trend at 56 days. Also, propulsion force was significantly decreased in the SI tear group at all times, while braking force was significantly increased at early times (3, 7, and 14 days); vertical ground reaction force was significantly decreased in the SI tear group at early times (7 and 14 days).

Figure 1
(A) Medial/lateral, (B) propulsion, (C) braking, and (D) vertical ground reaction forces for the supraspinatus and infraspinatus (SI) tear group and supraspinatus only (SO) tear group. Results showed significant differences between groups at most time ...

Cartilage Properties

Eight weeks post-injury the SI tear group had a significantly decreased equilibrium elastic modulus in the C region of the glenoid cartilage compared to the SO tear group (Fig. 2). A similar trend toward decreased modulus was observed in the PS and S regions. No group differences in cartilage thickness were observed in any region, except for a trend toward increased thickness in the PS and S regions in the SI tear group compared to the SO tear group (Fig. S1). No differences were found for modified Mankin scores between injury types in any region (Table 1). Cell density decreased significantly in the SI tear group compared to the SO tear group in the AI region, with a similar trend in the AS region (Table 1).

Figure 2
Glenoid cartilage modulus at 6 glenoid regions (center (C), anterior–superior (AS), anterior–inferior (AI), posterior–superior (PS), posterior–inferior (PI), and superior (S)). Modulus values were significantly lower (* ...
Table 1
Cartilage Histology

Tendon Properties

Eight weeks post-injury, tendon elastic modulus decreased in the SI tear group compared to the SO tear group for both the lower and upper subscapularis mid-substance regions (Fig. 3). Modulus increased in the SI tear group in the insertion site of the upper band of the subscapularis (Fig. 4). No group differences were found for the LHB tendon modulus. A trend toward decreased mid-substance area was observed in the LHB tendon of the SI tear group compared to SO tear (Fig. S3), while no differences in area were observed at the insertion of any tendon (Fig. S4). Increased cell density was found in the SI tear group at the insertion site of all tendons (Fig. 5, Table 2) and at the mid-substance of the upper and lower bands of the subscapularis. Both the upper and lower subscapularis tendons of the SI tear group had a less rounded cell shape at the tendon mid-substance compared to the SO tear group.

Figure 3
Tendon mid-substance modulus for the adjacent intact tendons (LHB, long head of the biceps; upper sub, upper subscapularis; and lower sub, lower subscapularis) of SI and SO groups. The upper and lower subscapularis in the SI group had a significant decrease ...
Figure 4
Tendon insertion modulus for the adjacent intact tendons (LHB, long head of the biceps; upper sub, upper subscapularis; and lower sub, lower subscapularis) of the SI and SO groups. The upper subscapularis in the SI group had a significant increase in ...
Figure 5
Tendon histology (subscapularis and biceps) was quantified for cell shape and cell density at the insertion and mid-substance locations. A representative image for the biceps insertion is displayed. Cell density was significantly increased (*p < ...
Table 2
Tendon Histology

DISCUSSION

Our results demonstrate that disruption of the AP force balance due to two-tendon rotator cuff tears, in combination with overuse activity, leads to alterations in shoulder function and cartilage and tendon mechanical properties. Previous clinical and animal studies demonstrated alterations in shoulder function following cuff tears;1,2,20 however, in vivo shoulder function has never been compared. We hypothesized that a two-tendon tear would diminish shoulder function. Consistent with our hypothesis, joint function was significantly altered. Specifically, medial/lateral forces shifted from a medially directed force to a more laterally directed force in the SI tear group compared to the SO tear group. Also, braking force increased while propulsion force decreased in the SI tear group compared to the SO tear group. Lastly, vertical forces were diminished at early times following injury in the SI tear group compared to the SO tear group.

Previous studies demonstrated similar changes in forces following shoulder injury in this animal model, which may indicate an altered loading environment.13 Forward flexion in the rat may correlate to human glenohumeral abduction while medial and lateral motions may correlate to internal and external rotation, due to the orientation of the scapula in the rat and motion of the humerus in the plane of the scapula.21 At longer time points, animals have likely achieved sufficient compensation from the surrounding musculature, and therefore some alterations in ambulation were no longer present. The detached rotator cuff tendons may also have spontaneously healed,22 thereby restoring some function. These forces together create a resultant force oriented in the posterior–superior direction, suggesting that tears of the supraspinatus and infraspinatus tendons lead to a net increase in posterior–superior joint forces during ambulation. This load alteration may place the joint at increased risk for injury to adjacent structures.

Interestingly, mechanical changes were also observed in the glenoid cartilage. Significantly decreased cartilage modulus was observed in the center region, with similar trends in the superior and posterior–superior glenoid in the SI tear group, indicative of altered cartilage loading, probably as a result of increased humeral head translations and decreased joint stability, due to disruption of the AP force balance following SI tear. The location of these changes contrast to a cadaver study that observed changes in anterior–superior glenoid loading with simulated cuff tears.23 The location-specific cartilage changes may be related to the differences in loading patterns for a quadruped animal. Taken together with shoulder function, these results indicate that detachment of the posterior and superior dynamic restraints (infraspinatus and supraspinatus, respectively) lead to increased joint loading, which ultimately manifests as altered cartilage mechanics. The decreases in cell density in the anterior region (superior and inferior) of the SI tear group may indicate decreased cell activity due to altered joint loading and is consistent with a shift in posterior–superior loading of the glenoid.

Combined infraspinatus–supraspinatus cuff tears also led to mechanical changes in the adjacent intact subscapularis tendon. Following an SI tear, the tendon modulus decreased at the mid-substance of the lower and upper subscapularis and increased at the insertion of the upper subscapularis. Histologic changes were also observed in these regions. The location-specific mechanical response to cuff tears in the upper subscapularis tendon may be related to differences in the loading environments. Due to its anatomic location, the subscapularis insertion may experience subcoracoid compressive loading following disruption of the AP force balance.24 These findings support previous studies that examined overuse effects following rotator cuff tears;13,19 in the presence of an isolated supraspinatus tear, overuse activity did not compromise subscapularis tendon mechanical properties compared to cage activity. This may be due to the presence of an intact AP force balance between the subscapularis and infraspinatus. However, when examining overuse effects after a two-tendon tear, alterations were observed in the subscapularis tendon. The differential response likely results from the force balance disruption due to the SI tear, resulting in abnormal loading and compromised adjacent tendon properties.

Interestingly, no mechanical changes were observed in the LHB tendon. However, increased cell density in the SI tear group at the insertion site suggests increased cell activity. Full-thickness tears are commonly associated with LHB tendon pathology, causing pain and dysfunction. The LHB tendon is thought to act as a humeral head depressor and joint stabilizer.23 Previous studies in this animal model identified alterations in LHB tendon mechanical properties following cuff tears, suggesting alterations in its loading environment.18 But no study evaluated the effect of tear size on tendon damage. Our results suggest that the damage magnitude does not differ between an isolated supraspinatus tear and a tear affecting both the supraspinatus and infraspinatus.

Clinically, our results highlight the importance of an intact AP force balance to avoid or minimize long-term joint damage. Previous studies using this animal model demonstrated the importance of restoration of the AP force balance to improve joint function.12 Previous cadaveric studies showed that alterations in tendon loading (through simulated tears and/or altering applied forces) result in abnormal joint mechanics due to disruption of the normal force couple.10,23 However, these studies were not designed to address cause and effect relationships that these alterations have on the associated structures. Therefore, we used an established animal model to elucidate the mechanisms governing the relationship between tendon injury and joint damage. This clinical scenario can only be addressed using an animal model, in which the nature of the tendon tears, post-operative activity, and time from injury can be controlled and evaluated over time. Our results identify joint imbalance as a mechanism for damage, highlighting its clinical consequences. It also provides justification for prospective clinical studies into the effect of tendon injuries on joint damage, specifically in active patients, and those with multiple cuff tears. Treatment strategies targeting restoration of joint balance would likely improve function and prevent or delay secondary joint injury; however, future research is required before definitive recommendations could be made.

This study has several limitations. Although the rat shoulder has similar bony architecture and soft tissue anatomy as the human, the use of a quadruped animal does not replicate the human condition and therefore loading patterns may not exactly translate to the same patterns in humans. However, the rat shoulder shares functional similarities in that during forward locomotion, the supraspinatus passes repetitively under the acromial arch, similar to what occurs in humans during repetitive overhead activity.14 The cuff tear model also uses acute surgical detachment of the cuff tendons, thus not exactly replicating clinical tears. However, the animals did receive an overuse treadmill protocol for 4 wks prior to detachment, which creates a tendinopathic condition and provides for an acute on chronic injury condition, similar to the clinical condition. Despite these limitations, our results demonstrate that mechanical and structural consequences are observed in the shoulder joint following two-tendon cuff tears.

Our results helps define the in vivo mechanical processes by which overload, due to disruption of the rotator cuff force balance, leads to joint damage. This information may provide a framework for physicians to better advise patients regarding long-term consequences on adjacent joint tissues for both isolated supraspinatus and combined supraspinatus-infraspinatus rotator cuff tears. Future studies will examine tissue biological properties in each tear scenario in order to further understand joint changes.

Supplementary Material

Supplementary Figure 1

Supplementary Figure 2

Supplementary Figure 3

Supplementary Figure 4

Supplementary Figure Legends

ACKNOWLEDGMENTS

This study was funded by NIH/NIAMS (R01AR056658) and the Penn Center for Musculoskeletal Disorders (P30AR050950). The authors o thank Lena Edelstein, Andrew Dunkman, Liz Feeney, Benjamin Freedman, and Corinne Riggin for their contribution to the overuse protocol and Rameen Vafa and Aricia Shen for their contributions to histology.

Footnotes

Conflicts of interest: none.

SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article at the publisher’s website.

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