Glycosaminoglycans of human rotator cuff tendons: changes with age and in chronic rotator cuff tendinitis

Catabolism of aggrecan, decorin and biglycan in tendon

We have examined the catabolism of the proteoglycans aggrecan, decorin and biglycan in fresh tendon samples and in explant cultures of tissue from the tensional and compressed regions of young and mature bovine tendons. A panel of well-characterized antibodies that recognize glycosaminoglycan or protein (linear or neoepitope) sequences was used to detect proteoglycans and proteoglycan degradation products that were both retained within the tissue and released into the culture medium. In addition, a reverse-transcriptase-mediated PCR analysis was used to examine the mRNA expression patterns of tendon proteoglycans and aggrecanases. The results of this study indicate a major role for aggrecanase(s) in the catabolism of aggrecan in bovine tendon. The study also provides a characterization of glycosaminoglycan epitopes associated with the proteoglycans of tendon, illustrating age-related changes in the isomers of chondroitin sulphate disaccharides that remain attached to the core protein glycosaminoglycan linkage region after digestion with chondroitinase ABC. Evidence for a rapid turnover of the small proteoglycans decorin and biglycan was also observed, indicating additional molecular pathways that might compromise the integrity of the collagen matrix and potentially contribute to tendon dysfunction after injury and during disease.

The bursal and articular sides of the supraspinatus tendon have a different compressive stiffness

OBJECTIVE

To measure the compressive stiffness of the supraspinatus tendon and to determine whether regional difference exists in the bursal and articular side of the tendon.

DESIGN

Indentation testing was performed on both the bursal and articular sides of the supraspinatus tendon, focused on the 'critical area', where rotator cuff tears often occur.

BACKGROUND

When the supraspinatus tendon wraps around the humeral head or is under impingement condition, compressive force on the tendon surface is expected. Therefore, compressive stress has been recently considered to be one of the important factors associated with the cuff tear. The mechanical properties would be essential for analytic modeling of stress distribution.

METHODS

Indentation tests were performed at 15 locations on the bursal and articular surfaces of the supraspinatus tendon. A mathematical model with exponential relationship was used to describe the measured force-deformation relationship and to calculate the compressive stiffness of the supraspinatus tendon.

RESULTS

The over-all initial stiffness on the bursal and articular sides of the tendon was significantly different. On the bursal side, the anterior third had a significantly higher initial stiffness than the other thirds on average. On the articular side, initial stiffness at location 10 mm proximal to the greater tuberosity was significantly higher than the rest on average.

CONCLUSIONS

The compressive stiffness of the supraspinatus tendon was found to be non-homogenous throughout the structure.RelevanceNon-homogenous compressive stiffness of the supraspinatus tendon would affect the load transmission within the tendon, which might be associated with the potential mechanism of tear. Such characteristics needs to be considered when performing finite element modeling of stress fields in the tendon.

Lysylhydroxylation and non-reducible crosslinking of human supraspinatus tendon collagen: changes with age and in chronic rotator cuff tendinitis

OBJECTIVES

To investigate age related and site specific variations in turnover and chemistry of the collagen network in healthy tendons as well as the role of collagen remodelling in the degeneration of the supraspinatus tendon (ST-D) in rotator cuff tendinitis.

METHODS

Collagen content and the amount of hydroxylysine (Hyl), hydroxy-lysylpyridinoline (HP), lysylpyridinoline (LP), and the degree of non-enzymatic glycation (pentosidine) were investigated in ST-D and in normal human supraspinatus (ST-N) and biceps brachii tendons (BT-N) by high-performance liquid chromatography.

RESULTS

In BT-N, tendons that served as control tissue as it shows rarely matrix abnormalities, pentosidine levels rise linearly with age (20-90 years), indicating little tissue remodelling (resulting in an undisturbed accumulation of pentosidine). A similar accumulation was observed in ST-N up to 50 years. At older ages, little pentosidine accumulation was observed and pentosidine levels showed large interindividual variability. This was interpreted as remodelling of collagen in normal ST after age 50 years because of microruptures (thus diluting old collagen with newly synthesised collagen). All degenerate ST samples showed decreased pentosidine levels compared with age matched controls, indicating extensive remodelling in an attempt to repair the tendon defect. Collagen content and the amount of Hyl, HP, and LP of ST-N and BT-N did not change with age. With the exception of collagen content, which did not differ, all parameters were significantly (p < 0.001) lower in BT-N. The ST-D samples had a reduced collagen content and had higher Hyl, HP, and LP levels than ST-N (p < 0.001).

CONCLUSIONS

Inasmuch as Hyl, HP, and LP levels in ST-N did not change with age, tissue remodelling as a consequence of microruptures does not seem to affect the quality of the tendon collagen. On the other hand, the clearly different profile of post-translational modifications in ST-D indicates that the newly deposited collagen network in degenerated tendons is qualitatively different. It is concluded that in ST-D the previously functional and carefully constructed matrix is replaced by aberrant collagen. This may result in a mechanically less stable tendon; as the supraspinatus is constantly subjected to considerable forces this could explain why tendinitis is mostly of a chronic nature.

Rotator cuff defect healing: a biomechanical and histologic analysis in an animal model

Rotator cuff tears are one of the most common causes of pain and disability in the upper extremity. With the use of an animal model, we studied the healing response of a controlled defect in the normal supraspinatus tendon and in a tendon with a reduced intrinsic healing capacity. In 36 Sprague-Dawley rats, defects (2 mm x 2 mm) were created in the supraspinatus tendons bilaterally. To model a tendon with an intrinsically reduced capacity to heal, the tissue adjacent to the defect area in the left shoulder was treated with in situ freezing. The contralateral tendon was not frozen. After 3 (n = 12), 6 (n = 12), and 12 (n = 12) weeks, animals were killed and underwent histologic (n = 4 from each group) and biomechanical (n = 8 from each group) evaluation. An additional group of untreated animals served as a normal control group. On histologic evaluation 78% of tendons had persistent defects (defined as incomplete closure of the defect site). Over time, the tissue from both groups demonstrated an improved histologic grade but did not reach normal levels, even at 12 weeks. No histologic differences were found between defect healing in normal tendons and in those treated with in situ freezing. On biomechanical evaluation there were also no significant differences between treatment groups. Over time, an improvement occurred in tissue properties, indicating that some healing of the defects had occurred. However, these tissue properties remained an order of magnitude lower than those of normal control tendons. These findings indicate that there is an active but inadequate repair response to the defect in the rat supraspinatus tendon, which is not significantly worsened by in situ freezing of the tissue around the defect. This model has applications toward the study of techniques to improve or accelerate cuff defect healing.

Rotator cuff pathology in athletes

The rotator cuff is the primary dynamic stabiliser of the glenohumeral joint and is placed under significant stress during overhead and contact sports. Mechanisms of injury include repetitive microtrauma, usually seen in the athlete involved in overhand sports, and macrotrauma associated with contact sports. Rotator cuff injury and dysfunction in the overhand athlete may be classified based on aetiology as primary impingement, primary tensile overload, and secondary impingement and tensile overload resulting from glenohumeral instability. A thorough history and physical examination are paramount in the evaluation, classification and treatment planning of the athlete with rotator cuff pathology. Imaging studies are a helpful adjunct to the history and physical. Athletes with primary impingement are usually middle aged or older and often have chronic shoulder pain and sometimes weakness associated with overhand sporting activities. Night pain is typical of full thickness rotator cuff tears. Impingement signs are positive and strength of elevation and external rotation are often limited. They usually respond to a nonoperative rehabilitation programme centred on decreasing inflammation, restoring range of motion and strengthening the rotator cuff and scapular stabilisers. Depending on the degree of cuff pathology, acromioplasty, debridement of partial cuff tears, and repair of full thickness tears are usually successful in those who fail a rehabilitation programme. Overhand athletes with cuff pathology secondary to subtle anterior instability are usually young and complain of pain and decreased throwing velocity. Instability may be so subtle that it is only detectable through a positive relocation test on examination. The majority of these athletes do not have a Bankart lesion on magnetic resonance imaging or arthroscopic examination. Arthroscopic examination usually demonstrates anterior capsular laxity (positive 'drive-through' sign), as well as superior-posterior labral and cuff injury typical of internal impingement. If rehabilitation alone is not successful, a capsulolabral repair followed by rehabilitation may allow the athlete to return to their previous level of competition. The athlete with an acute episode of macrotrauma to the shoulder resulting in cuff pathology usually presents with pain, limited active elevation and a positive 'shrug sign'. Arthroscopy and debridement of thickened, inflamed or scarred subacromial bursa with cuff repair or debridement as indicated is usually successful in those who do not respond to a rehabilitation programme.

Biology of the rotator cuff tendon

Tendons are complex composite material composed primarily of water, collagen, proteolycans, and cells, designed to transmit tensile loads from muscle to bone. Although rotator cuff tendons differ in many ways from other tendons in the body, a knowledge of basic tendon structure and function is helpful in understanding rotator cuff tendon biology, injury, and repair. In addition to type I collagen, rotator cuff tendons contain small amounts of type III collagen, which play a role in healing and repair. In comparison with other tendons, the increased glycosaminoglycan and proteoglycan content seen in rotator cuff tendons may be adaptive, pathologic, or both. The etiology of rotator cuff pathology is probably related to trauma, aging, and degeneration. As our understanding of these processes increases, we will be able to develop and implement improved preventative and therapeutic interventions for rotator cuff pathology.

Tenascin-C and human tendon degeneration

We investigated the distribution of tenascin in supraspinatus tendons to determine whether an alteration in tenascin expression was associated with human tendon degeneration. Tenascin was present in all of the tendons studied, although with two distinct patterns of expression. First, tenascin was associated with organized, fibrous regions of the tendon matrix that were typical of the normal tendon structure. This distribution is consistent with a role for tenascin in collagen fibril organization, perhaps maintaining the interface between fibrils and adjacent structures. Second, although tenascin was generally absent from poorly organized matrix in degenerate tendons, it was strongly associated with some rounded cells in disorganized fibrocartilaginous regions that were more abundant in pathological specimens. Tenascin was also found around infiltrating blood vessels, with more intense staining associated with a mononuclear cell infiltrate. Western blotting of tendon extracts showed differences in tenascin isoform expression, with only the small (200-kd) tenascin isoform found in normal tendons. Degenerate tendons also expressed the 300-kd isoform, consistent with a role for the larger tenascin isoform in tendon disease, potentially stimulating tenocyte proliferation, cell rounding, and fibrocartilaginous change. Proteolytic fragments of tenascin were detected but only in ruptured tendons, an indication of matrix remodeling in degenerate tendons, with fragment sizes consistent with the activity of matrix metalloproteinase enzymes.

Proteoglycans of human rotator cuff tendons

Rotator cuff and biceps tendons that appeared grossly normal were procured from adult cadavers without a history of shoulder problems. These tendons were analyzed for the amount and type of glycosaminoglycan, type of proteoglycan, and histology. When compared with the distal/tensional region of biceps tendon, the glycosaminoglycan content of supraspinatus, infraspinatus, and subscapularis tendons was 2.5-fold higher and the glycosaminoglycan content of the proximal/compressed region of biceps tendon was 3-fold higher. The ratio of hyaluronic acid to chondroitin sulfate/dermatan sulfate in all three cuff tendons was approximately 1. Rotator cuff tendons contained large proteoglycan similar to aggrecan, as demonstrated by sodium dodecyl sulfate-polyacrylamide gel migration elution from Sepharose CL-4B, and content of both chondroitin sulfate and keratan sulfate chains. Both decorin and biglycan were also present, as demonstrated by migration in sodium dodecyl sulfate-polyacrylamide gels and core protein immunoreactivity. In contrast decorin was the only proteoglycan prominent in distal/tensional regions of biceps tendon. Histological analysis showed layers of loosely organized alcian blue-stained material running between the longitudinal collagen fiber bundles. The proteoglycan content of rotator cuff tendons was similar to fibrocartilage in tendons that have been subjected to compressive loads in situ. This suggests that cells of normal adult rotator cuff tendons have adapted to loads distinct from pure tension. However, the histological organization did not resemble mature fibrocartilage. The increased amount of proteoglycan in rotator cuff tendons may serve to separate and lubricate collagen bundles as they move relative to each other during normal shoulder motion.

Prevalence and possible pathological significance of calcium phosphate salt accumulation in tendon matrix degeneration

OBJECTIVES

To investigate the prevalence of calcium phosphate mineral salt accumulation in degenerative supraspinatus 'tendinitis' compared with a normal sample of human tendons, and to determine whether there is an association of calcium salt deposition with pathological changes in the tendon extracellular matrix.

METHODS

Cadaver tendons (supraspinatus and common biceps tendons, n = 96) and fragments of supraspinatus tendons obtained during shoulder surgery (n = 31) were analysed for calcium content by atomic absorption spectroscopy, phosphorous content using a spectrophotometric assay, and matrix composition (collagen, glycosaminoglycans and DNA) using standard biochemical techniques.

RESULTS

We established baseline values of calcium concentration in macroscopically normal cadaver tendons (mean 1.1 (SD 0.35) micrograms/mg dry wt, n = 60) and found that 33% (nine of 27) of ruptured tendons from patients with 'degenerative tendinitis' contained an excess of calcium (more than 2SD greater than the normal sample mean). Five of these specimens had increased concentrations of phosphorous and calcium:phosphorous (molar) ratios consistent with a variety of possible calcium crystals, including calcium pyrophosphate, hydroxyapatite, and tricalcium phosphate, in addition to mixed or amorphous calcium phosphate deposits. Four of these specimens contained normal concentrations of phosphorous, consistent with deposits of calcium oxalate or calcium carbonate, although this was not confirmed biochemically. In contrast, surgical specimens (n = 4) from patients with 'calcifying tendinitis' (radiographically detected calcium deposits) all contained salts with a mineral composition consistent with hydroxyapatite. The presence and identity of crystal deposits was subsequently confirmed in five specimens by radiographic microanalysis. Analysis of the tendon matrix demonstrated a number of significant differences between normal and degenerate (ruptured) tendons, including a reduction in collagen content, an increase in sulphated glycosaminoglycans (predominantly dermatan sulphate) and an increase in DNA (cellular) content. However, there were no significant differences between degenerate tendons that were 'calcified' and those degenerate specimens that contained normal concentrations of calcium.

CONCLUSIONS

Although there was a relatively high prevalence of calcium salts in degenerate tendons, which might contribute to the pathological process (such as increased matrix collagen degradation), these data are consistent with the hypothesis that 'dystrophic calcification' of degenerate tendon matrix is a pathological entity distinct from cell mediated 'calcifying tendinitis'. Calcification is probably one possible outcome (or end point) of chronic tendon injury, although the possibility exists that in many cases, the presence of calcium salts may contribute to the tendon matrix degeneration.

The distal tendon of the biceps brachii. Structure and clinical correlations

The structure and blood supply of 42 distal biceps tendons were investigated by means of light and electron microscopy as well as by immunohistochemistry. Possible structural causes for the rupture of the tendon are discussed. The distal biceps tendon wraps around the radius during pronation of the forearm. In this area the tendon is exposed to pressure and shearing forces in addition to those caused by tension. Two fibrocartilaginous areas were regularly observed. Large chondrocyte-like cells were found inside the fibrocartilage. As an expression of strain, the extracellular matrix is rich in acidic glycosaminoglycans and stains intensely with toluidine blue at pH 1. Electron microscopy showed a granular pericellular matrix that increases in size towards the gliding surface. Type I collagen is the main component of the distal biceps tendon. Type II collagen is found in tendon fibrocartilage but not in traction tendons. The gliding surface of the tendon is made up of reticular fibres that are equivalent to type III collagen. Monoclonal antibodies revealed the presence of dermatan-sulfate, keratansulfate and chondroitin-4- as well as chondroitin-6-sulfate. Blood vessels are usually absent in fibrocartilage, as was shown with a polyclonal antibody against the basement membrane component laminine. There are significant differences between the extracellular matrix of traction and gliding tendons, which may be responsible for the location of tendon rupture.

Human shoulder tendon biopsy samples in organ culture produce procollagenase and tissue inhibitor of metalloproteinases

OBJECTIVE

To investigate the production of the matrix metalloproteinase (MMP), collagenase (MMP-1), and its natural inhibitor, the tissue inhibitor of metalloproteinases (TIMP) by diseased human tendon samples in organ culture.

METHODS

Portions of tendons were excised from the shoulders of patients undergoing shoulder surgery, classified as either proximal to the lesion (abnormal) or distal to the lesion (normal) according to their macroscopic appearance at surgery, and placed in organ culture for periods of up to 28 days. The release of collagenase and TIMP activity in the conditioned culture medium was measured.

RESULTS

Procollagenase and TIMP were both produced by all the tendon samples for an extended period of time. The levels of enzyme and inhibitor varied between patients, but in most of them TIMP levels were greater than collagenase levels. In one sample of calcified tendon, procollagenase levels were greater than those of TIMP. The mean level of collagenase produced by tendon proximal to the lesion and tendon distal to the lesion were not significantly different (95.2 (SD 106.8) U/g and 34.0 (45.3) U/g, respectively), while the corresponding figures for TIMP were 109.7 (62.3) U/g and 53.0 (27.9) U/g (p = < 0.05), although there was considerable variation in some samples. Western blotting and collagen fragment analysis confirmed that the collagenolytic activity detected was attributable to the metalloproteinase fibroblast collagenase (MMP-1).

CONCLUSIONS

Tendon tissue can actively secrete procollagenase, an enzyme that, once activated, is capable of remodelling collagen, the major connective tissue component of tendon. Collagenase is produced even in unstimulated cultures, although the concentrations of TIMP are usually greater than that of collagenase in most samples. Some activation of collagenase appeared to have occurred. These results indicate that tendon tissue cells are capable of producing a remodelling response, even in end stage tendon disease.