Mechanical factors in the initiation and propagation of tears of the rotator cuff. Quantification of strains of the supraspinatus tendon in vitro

Differential strain has been proposed to be a causative factor in failure of the supraspinatus tendon. We quantified the strains on the joint and bursal sides of the supraspinatus tendon with increasing load (20 to 200 N) and during 120 degrees of glenohumeral abduction with a constant tensile load (20 to 100 N). We tested ten fresh frozen cadaver shoulders on a purpose-built rig. Differential variable reluctance extensometers allowed calculation of the strain. Static loading to 100 N or more increased strains on the joint side significantly more than on the bursal side. During glenohumeral abduction an increasing and significant difference in strain was measured between the joint and bursal sides of the supraspinatus tendon, which reached a maximum of 10.6% at abduction of 120 degrees. The joint side strain of 7.5% reached values which were previously reported to cause failure. Differential strain causes shearing between the layers of the supraspinatus tendon, which may contribute to the propagation of intratendinous defects that are initiated by high joint side strains.     

Acromial spur: relationship to aging and morphologic changes in the rotator cuff

This study's objective is to elucidate the relationship between acromial spur formation and rotator cuff pathology or aging. The subjects comprised 1029 shoulders in control, cadaveric, and operative groups. A radiograph in the supraspinatus outlet view was taken in all subjects. The lengths of the incident spurs were measured and classified into 3 sizes (small, <5 mm; medium, <10 mm; and large, > or =10 mm). The rotator cuff was macroscopically classified as normal or as having bursal-side fraying, joint-side tear, intratendinous tear, bursal-side tear, complete tear limited to the supraspinatus tendon, or massive tear. In the control group, the overall incidence of spurs and spur size increased with advancing age (P < .001), but the majority of spurs were small (<5 mm). In the cadaveric group, the overall incidence did not correlate with advancing age (P > .05). However, spur size increased with age in subjects aged 50 years or older (P < .001). The incidence of spurs in rotator cuffs with bursal-side tears was highest and was significantly higher than that in normal cuffs and cuffs with bursal-side fraying and intratendinous tears. We observed 40% of medium spurs and 69% of large spurs in cases with bursal-side tears, complete tears limited to the supraspinatus tendon, and massive tears. In the operative group, the overall incidence correlated to advancing age (P < .001), reaching 80% or more in subjects aged 30 years or older. In addition, the size of spurs was larger than that in the other 2 groups in all age groups with few exceptions (P < .05). Small spurs were associated with advancing age. Morphologic change to the bursal side of the rotator cuff may enhance spur growth. The presence of a small spur has no diagnostic value for rotator cuff tears. Spurs measuring 5 mm or more, however, are of diagnostic value because of their high rate of association with bursal-side tear, complete tears limited to the supraspinatus tendon, or massive tears.     

Histologic and biomechanical characteristics of the supraspinatus tendon: Reference to rotator cuff tearing

A bursal- or joint-side incomplete thickness tearing of the rotator cuff is clinically important, because it is known that this tearing has the potential to develop into a complete tendon disruption. Normal cadaveric supraspinatus tendons were analyzed histologically and biomechanically to clarify the differences in pathomechanical causation of bursal- and joint-side incomplete tears. Histologically, the bursal-side layer was composed of tendon bundles with a decreasing muscular component toward the insertion. The joint-side layer was a complex of tendon, ligament, and joint capsule without transitional areas. Biomechanically, the bursal-side layer had greater deformation and tensile strength. When each layer was divided into three portions of equal length, the middle segment of the bursal-side layer elongated the most, whereas the entire joint-side layer increased evenly in length. We conclude that the joint-side layer is more vulnerable to a tensile load than the bursal-side layer.     

Strain distribution due to propagation of tears in the anterior supraspinatus tendon

Rotator cuff tears are a significant clinical problem. Tears in the anterior supraspinatus might behave differently compared to central tears due to differences in regional structural properties. The objective of this study was to determine strain distributions for anterior supraspinatus tendon tears and the relationship to tear propagation during cyclic loading. It was hypothesized that highest maximum principal strain would be posterior to the tear, and tears would propagate in the direction of the maximum principal strain. Eight human cadaveric supraspinatus tendons with surgically created small tears in the anterior third were tested with increasing levels of cyclic loads. The position of strain markers was recorded on the bursal surface of the tendon to calculate strain. Tendons reached a 2 cm critical tendon retraction at 580 ± 181 N. Largest strains were found medial and posterior to the tear (26.1 ± 9.4%). In five tendons, the strain direction for the initial (114 ± 28°) and final loading sets (86 ± 20°) indicated the strain direction shifted from an anterior to posterior orientation (p < 0.01), corresponding to the direction of tear propagation. Based on the results, anterior supraspinatus tears would remain isolated to the supraspinatus tendon during activities of daily living. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1283–1289, 2014.     

Inhomogeneous mechanical behavior of the human supraspinatus tendon under uniaxial loading.

Disorders of the rotator cuff, particularly tears of the rotator cuff tendons, cause significant shoulder disability. Among numerous factors thought to be responsible for the initiation and progression of supraspinatus tears are those related to the tendon's biomechanical properties. We hypothesized that in supraspinatus tendons subjected to tensile loading a strain gradient (difference) exists between the articular and bursal tendon surfaces, that regional strain differences exist on each of these two tendon surfaces, and that tendon surface strains vary with glenohumeral abduction. To test these hypotheses, the intrinsic inhomogeneous deformational characteristics of the articular and bursal surfaces of eight intact human cadaveric supraspinatus tendons were studied at three glenohumeral abduction angles using a novel multiple strain measuring system which simultaneously recorded surface marker displacements on two opposing soft tissue surfaces. Under applied tensile loads, the articular surface exhibited greater strain at 22 degrees (7.4+/-2.6% vs. 1.3+/-0.7%, p=0.0002) and 63 degrees (6.4+/-1.6% vs. 2.7+/-1.2%, p=0.0001) whereas the bursal surface exhibited greater strain at 90 degrees (7.6+/-2.8% vs. 4.9+/-0.4%, p=0.013). At all abduction angles, insertion strains were higher than those of the mid-tendon and tendon-muscle junction regions. The existence of inhomogeneous surface strains in the intact supraspinatus tendon demonstrates that intratendinous shear occurs within the tendon. The higher strain on the articular side of the tendon, especially at the insertion region, suggests a propensity for tears to initiate in the articular tendinous zone.     

The effect of rotator cuff tear size on shoulder strength and range of motion

STUDY DESIGN: Prospective cohort study. OBJECTIVES: To determine the effect of rotator cuff tear size on shoulder strength and range of motion. BACKGROUND: Patients with rotator cuff pathology typically present with weakness and motion loss in various motions. The extent to which the presence of a rotator cuff tear and the size of the tear affect strength and range of motion is not well understood. METHODS AND MEASURES: Sixty-one patients scheduled for surgery, with a diagnosis of a rotator cuff tear and/or subacromial impingement, underwent examination for shoulder pain, function, range of motion, and strength. The extent of rotator cuff pathology was documented during subsequent surgery (presence of tear, tear size, tear thickness). RESULTS: There were 10 massive tears, 15 large tears, 13 medium tears, 12 small tears, and 11 rotator cuffs without a tear. Patients had marked weakness in abduction strength at 90 degrees and 10 degrees of abduction, in external rotation strength at 90 degrees, and in the "full can test" (all, P<.0001). Marked range of motion losses in shoulder flexion and external rotation at 0 degrees and 90 degrees abduction (all, P<.001) were also observed. Abduction strength deficit at 10 degrees was affected by rotator cuff tear size (P<.0001). Twenty of 25 patients with large or massive tears had deficits greater than 50%, compared with only 1 of 11 patients with no tear, 2 of 12 patients with a small tear, and 5 of 13 patients with a medium tear (P<.0001). Other strength and range of motion deficits or indices of pain and function were unaffected by tear size. CONCLUSIONS: Weakness of greater than 50% relative to the contralateral side in shoulder abduction at 10 degrees of abduction was indicative of a large or massive rotator cuff tear.     

Mechanical environment of the supraspinatus tendon: a two-dimensional finite element model analysis

We performed 2-dimensional finite element model analysis to estimate the mechanical environment of the supraspinatus tendon. The geometric shape of the finite element model was determined by magnetic resonance imaging of a normal human shoulder obtained at 0 degrees, 30 degrees, and 60 degrees of abduction, whereas the histologic location of noncalcified and calcified fibrocartilage was determined from a cadaveric specimen. The supraspinatus tendon was pulled proximally with the force of 10 N at 0 degrees, 53 N at 30 degrees, and 115 N at 60 degrees of abduction. The area of high principal stress maximum was observed on the articular side of the supraspinatus tendon, which shifted toward the insertion as the arm was abducted. High stress concentration on the articular side of the supraspinatus tendon near its insertion during arm elevation may explain the frequent occurrence of rotator cuff tears at this site.     

"The color test": An intraoperative staining test for joint-side rotator cuff tearing and its extension

The joint-side rotator cuff tear and its extension are usually diagnosed by arthrography. However, it is not always possible at surgery to determine the exact site and extension of the tear when one is viewing the apparently intact bursal floor. "The color test," an intraoperative staining test with indigo carmine solution or methylene blue solution, was devised and has been helpful in the identification of the tear in 11 (65%) of 17 joint-side rotator cuff tears. The test was also useful in detecting concomitant intratendinous lamination in pinhole and very small (less than 1 cm) full-thickness tears. The tendon sheath for the long head of the biceps brachii muscle and the rotator interval are two sites that are normally stained. They should serve for surgical orientation. An 18-month follow-up indicates that injecting the dyes into the joints of Japanese monkeys appears to be harmless.     

Intrinsic healing capacity and tearing process of torn supraspinatus tendons: In situ hybridization study of α1(I) procollagen mRNA

To determine the healing potential and healing process of torn supraspinatus tendons, in situ hybridization was used to localize cells containing α1 type-I procollagen mRNA. Biopsy specimens of torn supraspinatus tendons from 19 patients with complete-thickness tears and 13 patients with incomplete-thickness tears were obtained during surgery. Four macroscopically normal supraspinatus tendons were obtained to serve as normal controls. Specimens were fixed in 10% buffered formalin and embedded in paraffin. A 22-mer oligonucleotide probe was labeled with digoxigenin and used as an in situ marker. The labeled cells were mainly composed of tenocytes and undifferentiated mesenchymal cells. In complete-thickness tears, the labeled cells at the proximal tendon stumps in the specimens that were obtained less than 4 months after trauma were significantly more abundant than in the specimens obtained 4 months or more after trauma. However, the number of labeled cells was maintained at the torn portion even in long-standing incomplete-thickness tears. The labeled cells at the margins of concomitant intratendinous extensions of the tears were detected even in the long-standing tears. The intratendinous extensions exhibited more labeled cell than were bursal-side or joint-side layers of the tendon substance in the incomplete-thickness tears (p < 0.05). The torn supraspinatus tendon may possess an intrinsic healing capability in the intermediate and late phases of tendon healing. Incomplete-thickness tears and concomitant intratendinous extensions can continue to rupture after the initial injury.     

Pathology and pathogenesis of bursal-side rotator cuff tears viewed from en bloc histologic sections

Histologic sections from 12 en bloc surgical specimens were studied in order to clarify the pathogenesis of bursal-side rotator cuff tears (BSRCTs). The specimens consisted of the bony insertion, the partially torn area, and the musculotendinous junction of the supraspinatus tendon. There were eight men and four women, with an average age of 48.7 years. Trauma was noted in three instances. All patients exhibited clinical evidence of subacromial impingement, which was confirmed at surgery. Histologic sections were stained with azan or hematoxylin and eosin. All the tears developed from within 1 cm of the insertion. The depth of the tears varied from a superficial flap to a nearly full-thickness tear. Microscopically, the sections demonstrated several abnormalities in addition to degeneration. At the site of insertion and in the distal stump, local disruptions of the normal four layers of enthesis and areas of hypervascularity were observed in all sections. The proximal stumps were rounded, retracted, and avascular, with abundant chondrocytes. No active repair was noted in the proximal stumps. The pathogenesis of BSRCTs appears to be related to: (1) a combination of aging and precarious vascularity of the tendon, (2) repetitive movements of the arm at above-horizontal levels, (3) injury, and (4) especially subacromial-tendon impingement.     

Contribution of the passive properties of the rotator cuff to glenohumeral stability during anterior-posterior loading.

The passive properties of the rotator cuff have been shown to provide some stability during anterior-posterior (AP) translation. However, the relative importance of the rotator cuff to joint stability remains unclear. The purpose of this study was to quantify the force contributions of the rotator cuff and of capsuloligamentous structures at the glenohumeral joint during AP loading. We hypothesized that the rotator cuff acts as a significant passive stabilizer of the glenohumeral joint and that its contribution to joint stability is comparable to the contribution made by the components of the glenohumeral capsule. A robotic/universal force-moment sensor testing system was used to determine both the multiple "degrees of freedom" joint motion and the in situ force carried by each soft tissue structure during application of an 89N AP load at 4 abduction angles. The percent contribution of the rotator cuff to the resisting force of the intact joint during AP loading was significantly greater during posterior loading (35% +/- 26%) than during anterior loading at 60 degrees of abduction (P < .05). The contribution of the rotator cuff (i.e., 29% +/- 16% at 30 degrees of abduction) was found to be significantly greater than the contributions of the capsule components during posterior loading at 30 degrees, 60 degrees, and 90 degrees of abduction (P < .05). However, no differences could be found between the respective contributions of the rotator cuff and the capsule components during anterior loading. The results support our hypothesis and suggest that passive tension in the rotator cuff plays a more significant role than other soft tissue structures in resisting posterior loads at the glenohumeral joint. The important role of the rotator cuff during posterior loading may be a result of the thin posterior joint capsule compared with the anterior capsule, which has several thickenings. This information increases our understanding of posterior stability at the glenohumeral joint during clinical laxity tests.     

Clinical and structural outcomes of nonoperative management of massive rotator cuff tears

BACKGROUND: The natural history of massive rotator cuff tears is not well known. The purpose of this study was to determine the clinical and structural mid-term outcomes in a series of nonoperatively managed massive rotator cuff tears. METHODS: Nineteen consecutive patients (twelve men and seven women; average age, sixty-four years) with a massive rotator cuff tear, documented by magnetic resonance imaging, were identified retrospectively. There were six complete tears of two rotator cuff tendons and thirteen complete tears of three rotator cuff tendons. All patients were managed exclusively with nonoperative means. Nonoperative management was chosen when a patient had low functional demands and relatively few symptoms and/or if he or she refused to have surgery. For the purpose of this study, patients were examined clinically and with standard radiographs and magnetic resonance imaging. RESULTS: After a mean duration of follow-up of forty-eight months, the mean relative Constant score was 83% and the mean subjective shoulder value was 68%. The score for pain averaged 11.5 points on a 0 to 15-point visual analogue scale in which 15 points represented no pain. The active range of motion did not change over time. Forward flexion and abduction averaged 136 degrees; external rotation, 39 degrees; and internal rotation, 66 degrees. Glenohumeral osteoarthritis progressed (p = 0.014), the acromiohumeral distance decreased (p = 0.005), the size of the tendon tear increased (p = 0.003), and fatty infiltration increased by approximately one stage in all three muscles (p = 0.001). Patients with a three-tendon tear showed more progression of osteoarthritis (p = 0.01) than did patients with a two-tendon tear. Four of the eight rotator cuff tears that were graded as reparable at the time of the diagnosis became irreparable at the time of final follow-up. CONCLUSIONS: Patients with a nonoperatively managed, moderately symptomatic massive rotator cuff tear can maintain satisfactory shoulder function for at least four years despite significant progression of degenerative structural joint changes. There is a risk of a reparable tear progressing to an irreparable tear within four years.     

A modification of the relocation test: arthroscopic findings associated with a positive test

Thirteen overhand-throwing athletes who failed at least 3 months of physical therapy while restricted from throwing and who demonstrated a positive modified relocation test at 90 degrees, 110 degrees, and/or 120 degrees were clinically and arthroscopically examined. On arthroscopic examination, all patients demonstrated articular surface pathology. Eleven patients had fraying of the undersurface of the rotator cuff, and 10 patients had fraying of the posterosuperior labrum. With 90 degrees shoulder abduction, 8 patients had rotator cuff contact with the posterosuperior labrum. When the shoulder was abducted to 110 degrees, all patients demonstrated such contact. At 120 degrees of abduction, 12 patients revealed contact. The presence or absence of pain during the modified relocation test correlated with the presence or absence of cuff contact with the posterosuperior labrum 79% of the time. Six patients had a positive modified relocation test at all levels of abduction. These patients included 2 with SLAP lesions, 1 with a Bankart lesion, and 1 with a complete tear of the rotator cuff.     

Passive tension and gap formation of rotator cuff repairs

The objectives of this study were to quantify the relationship between passive tension of rotator cuff repairs and arm position and to examine the effect of this tension on repair gap formation. Five patients undergoing open surgical rotator cuff repair of the supraspinatus tendon were recruited. Tendon tension was recorded as the supraspinatus was advanced into a bone trough and secured. The relationship between arm position and repair tension was then measured. Standardized rotator cuff tears were created in 3 cadaveric shoulders and repaired by use of the intraoperative technique. The difference in tension measured between 0 degrees and 30 degrees abduction was statically applied for 24 hours and the gap formation measured. Repair tension increased with advancement of the supraspinatus tendon into the bone trough. Abduction reduced the repair load. The mean reduction in load by 30 degrees abduction was 34 N. Twenty-four hours of 34-N loading caused gap formation of 9 mm in cadaveric rotator cuff repairs. Passive tension in surgically repaired rotator cuffs may contribute to repair failure and can be modified by arm positioning.     

Biomechanical tests for type II SLAP lesions of the shoulder joint before and after arthroscopic repair

Superior labral anterior-to-posterior (SLAP) lesions can cause shoulder pain partly by causing glenohumeral instability. The purpose of this study was to examine the effect of a simulated type II SLAP lesion and subsequent repair on glenohumeral translation of the vented shoulder. In eight cadaver joints, a robotic/UFS testing system was used to measure joint translation by applying an anterior, posterior, or inferior load of 50 N to each shoulder. The "apprehension tests" for anterior and posterior instability were simulated by applying an anterior load of 50 N with an external rotation torque of 3 Nm or a posterior load of 50 N with an internal rotation torque of 3 Nm. Each loading condition was applied at 30 degrees and 60 degrees of glenohumeral abduction with a constant joint compressive load (44 N) to the intact, simulated SLAP lesion, and repaired shoulder. Repair of the type II SLAP was then performed by placing a Suretac through the labrum both anterior and posterior to the biceps anchor and testing was repeated. ANOVA was used to compare translation of the intact joint, the joint after the type II SLAP lesion had been simulated, and after repair. At 30 degrees of abduction, anterior translation of the intact vented shoulder joint from anterior loading was 18.7+/-8.5 mm and increased to 26.2+/-6.5 mm after simulation of the type II SLAP lesion ( p< or =0.05). The arthroscopic repair did not restore anterior translation (23.9+/-8.6 mm) to the same degree as the intact joint ( p> or =0.05). At 60 degrees of abduction, anterior translation of 16.6+/-9.6mm in the intact joint was not significantly increased at 19.4+/-10.1 after simulation of the type II SLAP lesion ( p=0.0527). AP loading also resulted in inferior translation. At 30 degrees of abduction it was 3.8+/-4.0 mm in the intact joint and increased to 8.5+/-5.4 mm after the type II SLAP lesion ( p< or =0.05. After repair the inferior translation decreased significantly to 6.7+/-5.3 mm ( p< or =0.05). Although inferior translations were less at 60 degrees of abduction, results were similar to those at 30 degrees after repair. There were no significant increases in translation after SI/AP combined external rotation torque or posterior-anterior combined internal rotation torque loading.In this study the repair of a type II SLAP lesion only partially restored translations to the same degree as an intact vented shoulder joint. Therefore, improved repair techniques or an anteroinferior capsulolabral procedure in addition to the type II SLAP lesion repair might be needed to restore normal joint function.     

Acute rotator cuff tear: Do we miss the early diagnosis? A prospective study showing a high incidence of rotator cuff tears after shoulder trauma

The aim of this study was to evaluate the epidemiologic data of patients with an acute soft-tissue injury to the shoulder, with an emphasis on improving the early diagnosis of acute rotator cuff tears. This study included 104 patients, with a median age of 49 years (range, 19-75 years). The patients were evaluated clinically and with ultrasonography at a median of 13 days (range, 3-49 days) after the injury. A total of 60 patients (58%) had some degree of cuff lesion on the ultrasonographic examination. Of these patients, 33 (32%) had a full-thickness rotator cuff tear, 14 (13%) had a partial-thickness cuff tear in the tendon substance, and 13 (13%) had a partial cuff tear at the insertion site on the major tubercle. The injury mechanism or activity at the moment of injury did not correlate with the presence of a rotator cuff lesion, but we found a strong age correlation, with a prevalence of any rotator cuff tear, above 50%, for patients aged above 50 years and with a prevalence of full-thickness tears of 50% in the groups aged 50 to 59 years and aged 60 to 69 years. In conclusion, this study found a high incidence of rotator cuff lesions by further evaluation of patients undergoing consultation at the emergency department, with an inability to perform active abduction above 90 degrees and normal radiographs, after an acute shoulder trauma.     

In situ transtendon repair outperforms tear completion and repair for partial articular-sided supraspinatus tendon tears

This biomechanical study compared 2 repair techniques for high-grade, partial, articular-sided supraspinatus tendon tears of the rotator cuff: transtendon in situ repair and tear completion with repair. Standardized, 50% partial, articular-sided supraspinatus lesions were created in 10 pairs of matched fresh, frozen cadaveric shoulders: 10 underwent partial lesion repair with an in situ transtendon technique using 2 suture anchors. In the contralateral 10 shoulders, the partial lesion was converted to a full-thickness tear and repaired with a double-row technique, using 4 suture anchors. Cyclic loading to failure of the supraspinatus tendon was performed using a material testing machine. Gap formation was measured for each rotational position and each incremental load. The in situ transtendon repair had statistically significant less gapping (P = .0001) and higher mean ultimate failure strength (P = .0011) than the double-row repair. In situ transtendon repair was biomechanically superior to tear completion for partial, articular-sided supraspinatus tears.     

Stress distribution in the supraspinatus tendon with partial-thickness tears: an analysis using two-dimensional finite element model

The purpose of this study was to clarify the stress distribution in the rotator cuff tendon with 3 types of partial-thickness tears by use of 2-dimensional finite element analysis. In the finite element model of the normal human shoulder, a tendon defect was created on the articular surface, on the bursal surface, or in the midsubstance close to the insertion. A tensile force was applied to the proximal end of the tendon, and the stress distribution was calculated. In all 3 types of tears, a high stress concentration appeared around the articular surface at the insertion and at the site of the tear, which extended proximally. The maximum value of the von Mises stress increased with the presence of a partial-thickness tear. With the arm in abduction, a high stress concentration was also observed around the site of the tear. These biomechanical conditions of the supraspinatus tendon may eventually lead to a full-thickness tear at the critical zone.     

Histological analysis of the coracoacromial arch: Correlation between age-related changes and rotator cuff tears

The purpose of this study was to analyze age-related changes in the coracoacromial arch and correlate these degenerative changes with rotator cuff tears. We obtained 80 shouldes from 40 cadavers. The mean age at death was 58.4 years. We performed a gross examination of the rotator cuff and the acromion and histological examination of the coracoacromial ligament. The statistical significance of any difference for each group considered was determined by Student's t-test. The rotator cuff was normal in 66 specimens; there was an articular-side partial tear in 4 cases, a bursal-side partial tear in 6 cases, and a full-thickness tear in 4 cases. Age was correlated with increasing incidence and severity of cuff tears. We noted age-related degenerative changes in the coracoacromial ligament, degeneration of the acromial bone-ligament junction, and acromial spur formation. Anterior acromial spur was not related to the morphology of the acromion. We observed an increased incidence of bursal-side and complete cuff tears when the acromion was curved or beaked. Degenerative changes in the undersurface of the acromion were also present when the rotator cuff was normal. Bursal-side and complete cuff tears were associated with severe degenerative changes in the acromion in 100% of cases. Articular-side cuff tears were not related either to acromial morphology or degenerative changes in the coracoacromial arch. The association between cuff tears and acromial spur was more evident in the presence of a type III acromion. Our results would suggest that the incidence and severity of rotator cuff tears are correlated with aging and with the morphology of the acromion. Rotator cuff tears that involve the bursal side are often associated with changes in the coracoacromial ligament and the undersurface of the acromion. However, degenerative changes in the coracoacromial arch are always related to aging, also in the presence of a normal rotator cuff. Articular-side partial tears do not cause damage to the undersurface of the acromion.     

Structural properties of the subscapularis tendon.

The subscapularis muscle is an important mover and stabilizer of the glenohumeral joint. The purpose of this study was to measure regional variations in the structural properties of the subscapularis tendon in two joint positions. Subscapularis tendons from cadaveric shoulders were divided into four sections superiorly to inferiorly and tested to failure at 0 or 60 degrees of glenohumeral abduction. Arm position had a significant influence on stiffness in the inferior and superior portions (p < 0.05). The inferior region showed a higher stiffness in the hanging-arm position (0 degrees) than at 60 degrees of abduction (27.4+/-17.7 compared with 9.5+/-5.9 N/mm). Meanwhile, stiffness of the superior portion was higher at 60 degrees of abduction than in the hanging-arm position (208.7+/-60.9 compared with 147.2+/-32.3 N/mm). In the hanging-arm position (0 degrees) and at 60 degrees of abduction, the superior and midsuperior portions failed at significantly higher loads (superior: 623.2+/-198.6 and 478.2+/-206.6 N at 0 and 60 degrees of abduction, respectively; midsuperior: 706.2+/-164.6 and 598.4+/-268.4 N, respectively) than did the inferior portion (75.1+/-54.2 and 30.3+/-13.0 N, respectively). Likewise, stiffness of the superior and midsuperior portions was significantly higher than that of the inferior region in both positions. Higher stiffness and ultimate load in the superior tendon region may explain the infrequent extension of rotator cuff tears into the subscapularis tendon. Conversely, the significantly lower ultimate load and stiffness in the inferior tendon region could facilitate anterior dislocation of the humeral head when this portion stabilizes the joint in a dislocated position. Therefore, repair of torn inferior portions of the subscapularis tendon should be considered in surgery for glenohumeral instability.