Macroscopic 'degeneration' of equine superficial digital flexor tendon is accompanied by a change in extracellular matrix composition

Comparison of equine tendon- and bone marrow-derived cells cultured on tendon matrix with or without insulin-like growth factor-I supplementation

OBJECTIVE

To compare in vitro expansion, explant colonization, and matrix synthesis of equine tendon- and bone marrow-derived cells in response to insulin-like growth factor-I (IGF-I) supplementation.

SAMPLE

Cells isolated from 7 young adult horses.

PROCEDURES

Tendon- and bone marrow-derived progenitor cells were isolated, evaluated for yield, and cultured on autogenous cell-free tendon matrix for 7 days. Samples were analyzed for cell viability and expression of collagen type I, collagen type III, and cartilage oligomeric matrix protein mRNAs. Collagen and glycosaminoglycan syntheses were quantified over a 24-hour period.

RESULTS

Tendon- and bone marrow-derived cells required 17 to 19 days of monolayer culture to reach 2 passages. Mean ± SE number of monolayer cells isolated was higher for tendon-derived cells (7.9 ± 0.9 × 10(6)) than for bone marrow-derived cells (1.2 ± 0.1 × 10(6)). Cell numbers after culture for 7 days on acellular tendon matrix were 1.6- to 2.8-fold higher for tendon-derived cells than for bone marrow-derived cells and 0.8- to 1.7-fold higher for IGF-I supplementation than for untreated cells. New collagen and glycosaminoglycan syntheses were significantly greater in tendon-derived cell groups and in IGF-I-supplemented groups. The mRNA concentrations of collagen type I, collagen type III, and cartilage oligomeric matrix protein were not significantly different between tendon- and bone marrow-derived groups.

CONCLUSIONS AND CLINICAL RELEVANCE

In vitro results of this study suggested that tendon-derived cells supplemented with IGF-I may offer a useful resource for cell-based strategies in tendon healing.

Tendon structure changes after maximal exercise in the Thoroughbred horse: use of ultrasound tissue characterisation to detect in vivo tendon response

Investigations into the response of the superficial digital flexor tendon (SDFT) of the Thoroughbred horse to mechanical stimuli have been limited to in vitro cell culture studies focused primarily on gene expression of critical matrix proteins. It is uncertain how well in vitro outcomes translate to the tendon of the horse during exercise. The current study examined changes in tendon structure in response to maximal exercise using ultrasound tissue characterisation (UTC) to scan the SDFT prior to and after competitive racing. UTC uses contiguous transverse ultrasound images to assess the dynamics of the echopattern, which has a close relationship with changes in the 3-D ultra-structure of the tendon. Using UTC, it was possible to detect subtle changes in the dynamics of the echopattern, with a reduction in pixels that represent aligned and integer collagen tendon bundles on days 1 and 2 post-race when compared to pre-race (P<0.05). The echopattern of these tendons returned to baseline on day 3. This change in echopattern was not seen in control horses. It was concluded that short-term changes in the SDFT following maximal exercise could be detected using UTC.

The effect of intralesional injection of bone marrow derived mesenchymal stem cells and bone marrow supernatant on collagen fibril size in a surgical model of equine superficial digital flexor tendonitis

REASONS FOR PERFORMING STUDY

Collagen fibril size is decreased in repair tissue following tendon injury compared to normal tendon matrix in horses. Mesenchymal stem cells have been suggested to promote regeneration of tendon matrix rather than fibrotic repair following injury, although this concept remains unproven.

OBJECTIVES

To explore the hypothesis that implantation of autologous mesenchymal stem cells derived from bone marrow into a surgically created central core defect in the superficial digital flexor tendon (SDFT) of horses would induce the formation of a matrix with greater ultrastructural similarities to tendon matrix than the fibrotic scar tissue formed in control defects.

METHODS

Tissue was collected 16 weeks after induction of injury and 12 weeks after treatment from normal and injured regions of control and treated limbs of 6 horses and examined using transmission electron microscopy. Collagen fibril diameters were measured manually with image analysis software and surface areas calculated. Three parameters assessed for normal and injured tissue were mass average diameter (MAD), collagen fibril index (CFI) and the area dependent diameter (ADD).

RESULTS

Normal regions from both treated and control limbs displayed higher MAD and CFI values, as well as a characteristic bimodal distribution in fibril size. Injured regions from both treated and control limbs displayed significantly lower MAD and CFI values, as well as a unimodal distribution in fibril size. There were no significant differences between treated and control limbs for any of the parameters assessed.

CONCLUSIONS

Intralesional injection of autologous bone marrow derived mesenchymal stem cells had no measurable effect on the fibril diameter of collagen in healing tissue in the SDFT of this experimental model 16 weeks after injury.

POTENTIAL RELEVANCE

Favouring matrix regeneration over fibrotic repair may not be the mechanism by which autologous mesenchymal stem cells assist healing of tendon injury.

Concerted and adaptive alignment of decorin dermatan sulfate filaments in the graded organization of collagen fibrils in the equine superficial digital flexor tendon

The equine superficial digital flexor tendon (SDFT) has a graded distribution of collagen fibril diameters, with predominantly small-diameter fibrils in the region of the myotendinous junction (MTJ), a gradual increase in large-diameter fibrils toward the osteotendinous junction (OTJ), and a mixture of small- and large-diameter fibrils in the middle metacarpal (MM) region. In this study, we investigated the ultrastructure of the SDFT, to correlate the spatial relationship of the collagen fibrils with the graded distribution. The surface-to-surface distances of pairs of fibrils were found to be almost constant over the entire tendon. However, the center-to-center distances varied according to fibril diameter. Decorin is the predominant proteoglycan in normal mature tendons, and has one dermatan sulfate (DS) or chondroitin sulfate (CS) filament as a side chain which is associated with the surfaces of the collagen fibrils via its core protein. We identified a coordinated arrangement of decorin DS filaments in the equine SDFT. The sizes of the decorin DS filaments detected by Cupromeronic blue staining showed a unique regional variation; they were shortest in the MM region and longer in the MTJ and OTJ regions, and a considerable number of filaments were arranged obliquely to adjacent collagen fibrils in the MTJ region. This regional variation of the filaments may be an adaptation to lubricate the interfibrillar space in response to local mechanical requirements. The results of this study suggest that the MTJ region, which receives the muscular contractile force first, acts as a buffer for mechanical forces in the equine SDFT.

Cycle-dependent matrix remodeling gene expression response in fatigue-loaded rat patellar tendons

Expression profiling of selected matrix remodeling genes was conducted to evaluate differences in molecular response to low-cycle (100) and high-cycle (7,200) sub-failure-fatigue loading of patellar tendons. Using our previously developed in vivo patellar tendon model, tendons were loaded for 100 or 7,200 cycles and expression of selected metalloproteinases (MMPs), tissue inhibitors of metalloproteinases (TIMPs), and collagens were quantified by real-time RT-PCR at 1- and 7-day post-loading. Expression profiles were also obtained from lacerated tendons as an acute injury model. The high-cycle group showed upregulation of TIMP-1, -2, Col3a1, and Col5a1, and downregulation TIMP-4 at both time points, upregulation of MMP-2 at 7-day post-loading and downregulation of MMP-13 and -14 at 1-day post-loading, suggesting overall repair/remodeling. In contrast, the low-cycle loaded group showed upregulation of MMP-2, -3, -13, and Col12a1 at both time points, upregulation of TIMP-1, -2, -3, Col3a1, and integrin β1 and downregulation of integrin α11 at 1-day post-loading and upregulation of Col1a1 at 7-day post-loading, consistent with a hypertrophic (adaptive) pattern. Lacerated tendons showed a typical acute wound response with upregulation of all examined remodeling genes. Differences found in tendon response to high- and low-cycle loading are suggestive of the underlying mechanisms associated with a healthy or damaging response.

Prostaglandin E(2), collagenase, and cell death responses depend on cyclical load magnitude in an explant model of tendinopathy

Tendinopathy is a significant clinical problem that can result from repetitive activity. While the precise etiology of this condition remains unclear, the cellular response to cyclical loading is believed to have a contributory role to the pathology of tendinopathy. This study examined the short-term biochemical response of avian flexor digitorum profundus tendon to repetitive cyclic loadings of varying magnitude. An in vitro tendon explant model was utilized to apply four levels of haversine tensile stress (peak stress of 0, 3, 12, and 18 MPa) at 1.0 Hz, 8 hr/day for 3 days. The 12 and 18 MPa levels were known to cause significant mechanical damage based on previous work. Tissue media was recovered and analyzed for prostaglandin E(2) (PGE(2)), lactate dehydrogenase (LDH, measure of cell death), and collagenase levels. Tissue samples were recovered and analyzed for cell viability, total collagen, and sulfated glycosaminoglycan content. Collagenase, LDH, and PGE(2) levels were found to be influenced by loading magnitude (p < 0.05) with higher levels being present at higher load magnitudes. Varying cyclical load magnitude caused minimal compositional changes as collagen content and glycosaminoglycan did not change. These results indicate that elevated cyclical mechanical loading of tendon quickly results in altered biochemical tissue responses indicative of tissue injury. More sustained cyclical loading over time may be required for these initial responses to induce more dramatic tissue changes as observed in clinical tendinopathy.

Glycan profiling of a defect in decorin glycosylation in equine systemic proteoglycan accumulation, a potential model of progeroid form of Ehlers-Danlos syndrome

Defects in glycosylation of decorin can result in systemic hereditary disease. A mutation in the galactosyl transferase I gene is the underlying defect of a progeroid form of Ehlers-Danlos syndrome. We have previously described pathological changes in equine systemic proteoglycan accumulation (ESPA, formerly degenerative suspensory ligament desmitis) as consisting of excessive presence of decorin and other proteoglycans in organs and structures with a high content of connective tissue. Using liquid chromatography/mass spectrometry, and one- and two-dimensional immunoblotting we have determined that decorin from ESPA-tendons had a higher molecular weight than decorin from non-affected control tendons. Glycosaminoglycan structure and monosaccharide composition were determined with HPLC analysis of chondroitinase ABC-digested glycosaminoglycans and gas chromatography/mass spectrometry. This analysis revealed an increase in the total content of sulfated disaccharides, particularly due to enhanced sulfation at 6-position of N-acetyl galactosamine (GalNAc) with a subsequent decrease in the ratio of 4-sulfation to 6-sulfation disaccharides in the ESPA decorin. The ESPA-affected decorin also exhibited altered biological activity resulting in (1) diminished binding of TGFbeta1 (and of anti-decorin antibody) to ESPA decorin, and (2) increased expression of TGFbeta1 in ESPA tissues.

Aspartic acid racemization and collagen degradation markers reveal an accumulation of damage in tendon collagen that is enhanced with aging

Little is known about the rate at which protein turnover occurs in living tendon and whether the rate differs between tendons with different physiological roles. In this study, we have quantified the racemization of aspartic acid to calculate the age of the collagenous and non-collagenous components of the high strain injury-prone superficial digital flexor tendon (SDFT) and low strain rarely injured common digital extensor tendon (CDET) in a group of horses with a wide age range. In addition, the turnover of collagen was assessed indirectly by measuring the levels of collagen degradation markers (collagenase-generated neoepitope and cross-linked telopeptide of type I collagen). The fractional increase in D-Asp was similar (p = 0.7) in the SDFT (5.87 x 10(-4)/year) and CDET (5.82 x 10(-4)/year) tissue, and D/L-Asp ratios showed a good correlation with pentosidine levels. We calculated a mean (+/-S.E.) collagen half-life of 197.53 (+/-18.23) years for the SDFT, which increased significantly with horse age (p = 0.03) and was significantly (p < 0.001) higher than that for the CDET (34.03 (+/-3.39) years). Using similar calculations, the half-life of non-collagenous protein was 2.18 (+/-0.41) years in the SDFT and was significantly (p = 0.04) lower than the value of 3.51 (+/-0.51) years for the CDET. Collagen degradation markers were higher in the CDET and suggested an accumulation of partially degraded collagen within the matrix with aging in the SDFT. We propose that increased susceptibility to injury in older individuals results from an inability to remove partially degraded collagen from the matrix leading to reduced mechanical competence.

Comparative rheumatology: what can be learnt from naturally occurring musculoskeletal disorders in domestic animals?

Examples of naturally occurring musculoskeletal disorders are extremely common in veterinary species and provide a valuable comparative research resource, which can provide compelling comparative data on the aetiopathogenesis and treatment of many common human musculoskeletal diseases. In particular, orthopaedic diseases are a common morbidity in both dogs and horses. In this review, we give an overview of the common musculoskeletal diseases encountered in these species: for instance, tendon and ligament injuries, arthropathies and stress fractures, as well as an insight into the basic biology of these conditions. In doing so, we aim to demonstrate the similarities and differences between these disorders and similar conditions in man.

Maturational alterations in gap junction expression and associated collagen synthesis in response to tendon function

Energy-storing tendons including the equine superficial digital flexor tendon (SDFT) contribute to energetic efficiency of locomotion at high-speed gaits, but consequently operate close to their physiological strain limits. Significant evidence of exercise-induced microdamage has been found in the SDFT which appears not to exhibit functional adaptation; the degenerative changes have not been repaired by the tendon fibroblasts (tenocytes), and are proposed to accumulate and predispose the tendon to rupture during normal athletic activity. The anatomically opposing common digital extensor tendon (CDET) functions only to position the digit, experiencing significantly lower levels of strain and is rarely damaged by exercise. A number of studies have indicated that tenocytes in the adult SDFT are less active in collagen synthesis and turnover than those in the immature SDFT or the CDET. Gap junction intercellular communication (GJIC) is known to be necessary for strain-induced collagen synthesis by tenocytes. We postulate therefore that expression of GJ proteins connexin 43 and 32 (Cx43; Cx32), GJIC and associated collagen expression levels are high in the SDFT and CDET of immature horses, when the SDFT in particular grows significantly in cross-sectional area, but reduce significantly during maturation in the energy-storing tendon only. The hypothesis was tested using tissue from the SDFT and CDET of foetuses, foals, and young adult Thoroughbred horses. Cellularity and the total area of both Cx43 and Cx32 plaques/mm(2) of tissue reduced significantly with maturation in each tendon. However, the total Cx43 plaque area per tenocyte significantly increased in the adult CDET. Evidence of recent collagen synthesis in the form of levels of neutral salt-soluble collagen, and collagen type I mRNA was significantly less in the adult compared with the immature SDFT; procollagen type I amino-propeptide (PINP) and procollagen type III amino-propeptide (PIIINP) levels per mm(2) of tissue and PINP expression per tenocyte also decreased with maturation in the SDFT. In the CDET PINP and PIIINP expression per tenocyte increased in the adult, and exceeded those in the adult SDFT. The level of PINP per mm(2) was greater in the adult CDET than in the SDFT despite the higher cellularity of the latter tendon. In the adult SDFT, levels of PIIINP were greater than those of PINP, suggesting relatively greater synthesis of a weaker form of collagen previously associated with microdamage. Tenocytes in monolayers showed differences in Cx43 and Cx32 expression compared with those in tissue, however there were age- and tendon-specific phenotypic differences, with a longer time for 50% recovery of fluorescence after photobleaching in adult SDFT cells compared with those from the CDET and immature SDFT. As cellularity reduces following growth in the SDFT, a failure of the remaining tenocytes to show a compensatory increase in GJ expression and collagen synthesis may explain why cell populations are not able to respond to exercise and to repair microdamage in some adult athletes. Enhancing GJIC in mature energy-storing tendons could provide a strategy to increase the cellular synthetic and reparative capacity.

Comparison of equine tendon-, muscle-, and bone marrow-derived cells cultured on tendon matrix

OBJECTIVE

To compare viability and biosynthetic capacities of cells isolated from equine tendon, muscle, and bone marrow grown on autogenous tendon matrix.

SAMPLE POPULATION

Cells from 4 young adult horses.

PROCEDURES

Cells were isolated, expanded, and cultured on autogenous cell-free tendon matrix for 7 days. Samples were analyzed for cell viability, proteoglycan synthesis, collagen synthesis, and mRNA expression of collagen type I, collagen type III, and cartilage oligomeric matrix protein (COMP).

RESULTS

Tendon- and muscle-derived cells required less time to reach confluence (approx 2 weeks) than did bone marrow-derived cells (approx 3 to 4 weeks); there were fewer bone marrow-derived cells at confluence than the other 2 cell types. More tendon- and muscle-derived cells were attached to matrices after 7 days than were bone marrow-derived cells. Collagen and proteoglycan synthesis by tendon- and muscle-derived cells was significantly greater than synthesis by bone marrow-derived cells. On a per-cell basis, tendon-derived cells had more collagen synthesis, although this was not significant. Collagen type I mRNA expression was similar among groups. Tendon-derived cells expressed the highest amounts of collagen type III and COMP mRNAs, although the difference for COMP was not significant.

CONCLUSIONS AND CLINICAL RELEVANCE

Tendon- and muscle-derived cells yielded greater cell culture numbers in shorter time and, on a per-cell basis, had comparable biosynthetic assays to bone marrow-derived cells. More in vitro experiments with higher numbers may determine whether tendon-derived cells are a useful resource for tendon healing.

Physical activity: does long-term, high-intensity exercise in horses result in tendon degeneration?

This study explores the hypothesis that high-intensity exercise induces degenerative changes in the injury-prone equine superficial digital flexor tendon (SDFT), but not in the rarely injured common digital extensor tendon (CDET). The horse represents a large-animal model that is applicable to human tendon and ligament physiology and pathology. Twelve age-matched female horses undertook galloping exercise three times a week with trotting exercise on alternative days (high-intensity group, n = 6) or only walking exercise (low-intensity group, n = 6) for 18 mo. The SDFT, suspensory ligament, deep digital flexor tendon, and CDET were harvested from the forelimb. Tissue from the mid-metacarpal region of the right limb tendons was analyzed for water, DNA, sulfated glycosaminoglycan and collagen content, collagen type III-to-I ratios, collagen cross-links, and tissue fluorescence. Left limb tendons were mechanically tested to failure. The analyses showed matrix composition to have considerable diversity between the functionally different structures. In addition, the specific structures responded differently to the imposed exercise. High-intensity training resulted in a significant decrease in the GAG content in the SDFT, but no change in collagen content, despite a decrease in collagen fibril diameters. There were no signs of degeneration or change in mechanical properties of the SDFT. The CDET had a lower water content following high-intensity training and a higher elastic modulus. Long-term, high-intensity training in skeletally mature individuals results in changes that suggest accelerated aging in the injury-prone SDFT and adaptation in the CDET.

The basic science of tendinopathy

Tendinopathy is a common clinical problem with athletes and in many occupational settings. Tendinopathy can occur in any tendon, often near its insertion or enthesis where there is an area of stress concentration, and is directly related to the volume of repetitive load to which the tendon is exposed. Recent studies indicate tendinopathy is more likely to occur in situations that increase the "dose" of load to the tendon enthesis - including increased activity, weight, advancing age, and genetic factors. The cells in tendinopathic tendon are rounder, more numerous, and show evidence of oxidative damage and more apoptosis. These cells also produce a matrix that is thicker and weaker with more water, more immature and cartilage-like matrix proteins, and less organization. There is now evidence of a population of regenerating stem cells within tendon. These studies suggest prevention of tendinopathy should be directed at reducing the volume of repetitive loads to below that which induces oxidative-induced apoptosis and cartilage-like genes. The management strategies might involve agents or cells that induce tendon stem cell proliferation, repair and restoration of matrix integrity.

Evaluation of a new strategy to modulate skeletal development in Thoroughbred performance horses by imposing track-based exercise during growth

REASON FOR PERFORMING STUDY

No data exist on the intensity of exercise required or on possible harmful effects of increasing exercise in foals over the natural level when free at pasture.

OBJECTIVES

To investigate whether an increase in workload over free pasture exercise in the period from directly after birth to the start of training is tolerated by Thoroughbred (TB) foals without increasing injury rate or producing other undesired side effects.

METHODS

Thirty-three TB foals were allocated to one of 2 exercise groups directly after birth. One group (PASTEX) was raised on pasture and the other (CONDEX) kept under identical circumstances, but was additionally subjected to an exercise protocol of gradually increasing intensity. Foals were monitored periodically and scored for the presence of clinical signs related to the musculoskeletal system (joint effusion, pain at flexion, occurrence of physeal swelling), and radiographs taken at the end of the conditioning phase. Also, behavioural studies were performed to detect any changes in behaviour related to the exercise programme. Cortisol levels were measured in both groups, to assess the level of stress.

RESULTS

Workload in the CONDEX group was significantly higher than in the PASTEX group (approximately 30%). Conditioning increased the likelihood for joint effusion in the antebrachiocarpal joint, but reduced tarsocrural effusion and physeal swelling at the lateral distal radius, the third metacarpal bone (medial aspect) and lateral and medial aspects of the third metatarsal bone.

CONCLUSIONS

The 30% increase in workload did not affect the animals' welfare, effects of conditioning exercise on clinical musculoskeletal health were few and there were no adverse effects.

POTENTIAL RELEVANCE

This study supports the feasibility of imposing early conditioning exercise in horses and is a benchmark for its effects on the development of equine musculoskeletal tissues.

Monitoring the fate of autologous and allogeneic mesenchymal progenitor cells injected into the superficial digital flexor tendon of horses: preliminary study

Autologous mesenchymal progenitor cells (MPCs) purified from bone marrow aspirates are being used in the treatment of superficial digital flexor tendon (SDFT) injuries in the horse with promising results. In this study the fate of autologous and allogeneic MPCs following injection into the SDFT was monitored by stable transfection of MPCs with green fluorescent protein (GFP). Small lesions were created manually in one forelimb SDFT of 2 horses and injected with autologous MPCs, allogeneic MPCs or bone marrow supernatant alone. Post mortem examinations performed after 10 or 34 days revealed GFP labelled cells located mainly within injected lesions, but with a small proportion integrated into the crimp pattern of adjacent healthy areas of tendon. Furthermore, there was no visible cell mediated immune response to allogeneic MPCs in either of the host horses.

The influence of exercise during growth on ultrasonographic parameters of the superficial digital flexor tendon of young Thoroughbred horses

REASON FOR PERFORMING STUDY

Conditioning by early training may influence the composition of certain musculoskeletal tissues, but very few data exist on its effect during growth on tendon structure and function.

OBJECTIVES

To investigate whether conditioning exercise in young foals would lead to any ultrasonographically detectable damage to the superficial digital flexor tendon or an increase in cross-sectional area (CSA).

METHODS

Thirty-three Thoroughbred foals reared at pasture were allocated to 2 groups: control (PASTEX) allowed exercise freely at pasture; and CONDEX, also at pasture, began conditioning exercise from mean age 21 days over 1030 m on a purpose-built oval grass track, for 5 days/week until mean age 18 months. Foals were observed daily, and underwent orthopaedic examination monthly. Ultrasonographic images of the superficial digital flexor tendon (SDFT) at the mid-metacarpal level of both forelimbs were obtained in all foals at ages 5, 8, 12, 15 and 18 months. CSA was validated (r(2) = 0.89) by determining CSA from digital photographs of the transected SDFT surface from 12 of the horses necropsied at age 17.1 months.

RESULTS

here was no clinical or ultrasonographic evidence of tendonopathy in either group and the greatest increase in mean CSA in both groups occurred between age 5 and 8 months. Across all age categories, there was no significant difference in mean CSA between the left and right limbs, or colts and fillies; there was a trend towards a larger CSA in the CONDEX group (P = 0.058).

CONCLUSIONS

There was no conclusive evidence for a structural adaptive hypertrophy of the SDFT, probably because the regimen was insufficiently rigorous or because spontaneous pasture exercise may induce maximal development of energy storing tendons.

POTENTIAL RELEVANCE

A moderate amount of early conditioning exercise against a background of constant exercise at pasture is not harmful to the development of the flexor tendons.

The pathobiology of exercise-induced superficial digital flexor tendon injury in Thoroughbred racehorses

Despite the high incidence of superficial digital flexor tendon (SDFT) injury in racehorses, the pathobiology of the condition is not clearly defined. The SDFT improves locomotor efficiency by storing elastic energy, but as a result it has low mechanical safety margins. As with the Achilles tendon in humans, rupture during athletic activity often follows accumulation of exercise and age-induced degenerative change that is not repaired by tenocytes. There is limited understanding of tenocyte biology and pathology, including responses to high mechanical strains and core temperatures during exercise. Unfortunately, much of the current information on SDFT pathology is derived from studies of collagenase-induced injury, which is a controversial model. Following rupture the overlapping phases of reactive inflammation, proliferation, remodelling and maturation do not necessarily reconstitute normal structure and function, resulting in long-term persistence of scar tissue and high re-injury rates. Tissue engineering approaches are likely to be applicable to SDFT lesions, but will require significant advances in cell biology research.

Tendon matrix composition and turnover in relation to functional requirements

Tendons are dense regular connective tissue structures that are defined based on their anatomical position of connecting muscle to bone. Despite these obvious commons features tendons from different locations within the body show remarkable variation in terms of their morphological, molecular and mechanical properties which relates to their specialized function. An appreciation of these differences is necessary to understand all aspects of tendon biology in health and disease. In our work, we have used a combination of mechanical assessment, histological measurements and molecular analysis of matrix in functionally distinct tendons to determine relationships between function and structure. We have found significant differences in material and molecular properties between spring-like tendons that are subjected to high strains during locomotion and positional tendons which are subjected to much lower strains. Furthermore, we have data to suggest that not only is the matrix composition different but also the ability of cells to synthesize and degrade the matrix (matrix turnover) varies between tendon types. We propose that these differences relate to the magnitude of strain that the tendon experiences during normal activities in life. Tendon cells may be preprogrammed during embryological development for the strain they will encounter in life or may simply respond to the particular strain environment they are subjected to. The elucidation of controlling mechanisms resulting in tendon cell specialization will have important consequences for cell based therapies and engineering strategies to repair damaged tendons.

Achilles tendinosis: changes in biochemical composition and collagen turnover rate

BACKGROUND

Understanding biochemical and structural changes of the extracellular matrix in Achilles tendinosis might be important for developing mechanism-based therapies.

HYPOTHESIS

In Achilles tendinosis, changes occur in biochemical composition and collagen turnover rate.

STUDY DESIGN

Descriptive laboratory study.

METHODS

From 10 patients undergoing surgery for Achilles tendinopathy, 1 tendinosis biopsy specimen and 1 biopsy specimen of macroscopically healthy tendon tissue adjacent to the lesion were collected. Furthermore, biopsy samples were collected from 3 donors with asymptomatic Achilles tendons. Water content, collagen content, percentage of denatured collagen, amount of lysine hydroxylation, number of enzymatic and nonenzymatic crosslinks, matrix metalloproteinase activity, and matrix metalloproteinase and collagen gene-expression levels were analyzed.

RESULTS

In tendinotic lesions, the water content was highest, and collagen content was subnormal with higher amounts of denatured/damaged collagen. Low pentosidine levels in tendinotic tissue indicated the presence of relatively young collagenous matrix. More hydroxylated lysine residues were present in tendinotic samples, but enzymatic crosslinks revealed no differences between tendinotic, adjacent, and healthy samples. In tendinotic specimens, matrix metalloproteinase activity was higher, matrix metalloproteinase gene-expression profile was altered, and collagen type I and III gene expression were upregulated.

CONCLUSION

In Achilles tendinosis, the collagen turnover rate is increased, and the natural biochemical composition of the collagenous matrix is compromised.

CLINICAL RELEVANCE

Although tendon tissue directly adjacent to an Achilles tendinosis lesion looks macroscopically healthy, histological and biochemical degenerative changes in adjacent tissue are evident, which may have implications for surgical interventions.

Gap junction protein expression and cellularity: comparison of immature and adult equine digital tendons

Injury to the energy-storing superficial digital flexor tendon is common in equine athletes and is age-related. Tenocytes in the superficial digital flexor tendon of adult horses appear to have limited ability to respond adaptively to exercise or prevent the accumulation of strain-induced microdamage. It has been suggested that conditioning exercise should be introduced during the growth period, when tenocytes may be more responsive to increased quantities or intensities of mechanical strain. Tenocytes are linked into networks by gap junctions that allow coordination of synthetic activity and facilitate strain-induced collagen synthesis. We hypothesised that there are reductions in cellular expression of the gap junction proteins connexin (Cx) 43 and 32 during maturation and ageing of the superficial digital flexor tendon that do not occur in the non-injury-prone common digital extensor tendon. Cryosections from the superficial digital flexor tendon and common digital extensor tendon of 5 fetuses, 5 foals (1-6 months), 5 young adults (2-7 years) and 5 old horses (18-33 years) were immunofluorescently labelled and quantitative confocal laser microscopy was performed. Expression of Cx43 and Cx32 protein per tenocyte was significantly higher in the fetal group compared with all other age groups in both tendons. The density of tenocytes was found to be highest in immature tissue. Higher levels of cellularity and connexin protein expression in immature tendons are likely to relate to requirements for tissue remodelling and growth. However, if further studies demonstrate that this correlates with greater gap junctional communication efficiency and synthetic responsiveness to mechanical strain in immature compared with adult tendons, it could support the concept of early introduction of controlled exercise as a means of increasing resistance to later injury.

Tendons and ligaments are anatomically distinct but overlap in molecular and morphological features--a comparative study in an ovine model

Tendons and ligaments are similar in composition but differ in function. Simple anatomical definitions do not reflect the fact individual tendons and ligaments have unique properties due to their adaptation to a specific role. The patellar tendon is a structure of particular clinical interest. A null hypothesis was declared stating that the patellar tendon is not significantly different in terms of matrix composition and collagen fibril diameter to other tendons. The lateral and medial collateral ligaments (LCL, MCL), anterior and posterior cruciate ligaments (ACL, PCL), together with the long digital extensor, superficial digital extensor, and patellar tendons (LDET, SDFT, PT) were harvested from three cadaveric ovine hindlimbs. The extracellular matrix was assessed in terms of water, collagen, and total sulphated glycosaminoglycan (GAG) content. The organization of the collagen component was determined by an ultrastructural analysis of collagen fibril diameter distributions, together with values for the collagen fibril index (CFI) and mass-average diameter (MAD). There were significant differences between ligaments and tendons. The PT had a bimodal collagen fibril diameter distribution with CFI 72.9%, MAD 202 nm, water content 53.1%, GAG content 2.3 microg/mg, and collagen content 73.7%, which was not significantly different from the other tendons. The results of this study support the null hypothesis suggesting that the patellar tendon is similar to other tendons and demonstrate that tendons have different characteristics to ligaments.

The response of bone, articular cartilage and tendon to exercise in the horse

Horses can gallop within hours of birth, and may begin training for athletic competition while still growing. This review cites studies on the effects of exercise on bone, tendon and articular cartilage, as detected by clinical and research imaging techniques, tissue biochemical analysis and microscopy of various kinds. For bone, alterations in bone mineral content, mineral density and the morphology of the mineralized tissue are the most common end-points. Apparent bone density increases slightly after athletic training in the cortex, but substantially in the major load paths of the epiphyses and cuboidal bones, despite the lower material density of the new bone, which is deposited subperiosteally and on internal surfaces without prior osteoclastic resorption. With training of greater intensity, adaptive change is supervened by patho-anatomical change in the form of microdamage and frank lesions. In tendon, collagen fibril diameter distribution changes significantly during growth, but not after early training. The exact amount and type of protracted training that does cause reduction in mass average diameter (an early sign of progressive microdamage) have not been defined. Training is associated with an increase in the cross-sectional area of some tendons, possibly owing to slightly greater water content of non-collagenous or newly synthesized matrix. Early training may be associated with greater thickness of hyaline but not calcified articular cartilage, at least in some sites. The age at which adaptation of cartilage to biomechanical influences can occur may thus extend beyond very early life. However, cartilage appears to be the most susceptible of the three tissues to pathological alteration. The effect of training exercise on the anatomical or patho-anatomical features of connective tissue structures is affected by the timing, type and amount of natural or imposed exercise during growth and development which precedes the training.

Nucleotide structure of equine platelet-derived growth factor-A and -B and expression in horses with induced acute tendinitis

OBJECTIVE

To characterize the nucleotide sequence of equine platelet-derived growth factor (PDGF)-A and -B and analyze temporal expression of these genes in equine tendon after induced tendinitis injury. Animals-18 mature horses.

PROCEDURES

Genes for equine PDGF-A and -B were reverse transcribed and sequenced from synovial tissue mRNA obtained from a 3-year-old horse. Collagenase-induced lesions were created in the tensile region of the superficial digital flexor tendon in 14 horses; 3 horses served as uninjured control animals. Tendons were harvested and total RNA was isolated from experimental horses 1, 2, 4, 8, and 24 weeks after collagenase injection. Temporal gene expression for PDGF-A and -B was determined by use of quantitative PCR analysis.

RESULTS

Equine PDGF-A shared 83.8% sequence and 87.5% peptide homology with human PDGF-A, with a discrepancy of 70 bp from the human sequence. Equine PDGF-B was similar in length to the human gene, sharing 90.3% and 91.7% nucleotide and peptide identity, respectively. Expression of PDGF-A mRNA in collagenase-induced tendinitis lesions was unchanged, compared with expression for normal control tendon, and remained steady throughout the 24-week study. Expression of PDGF-B mRNA decreased over time, and the expression at 24 weeks was significantly reduced, compared with expression in normal and acutely injured tendon.

CONCLUSIONS AND CLINICAL RELEVANCE

Injured tendon mounts a minimal constitutive PDGF-A or -B mRNA response. Serial exogenous treatment with either PDGF isoform within the first 2 to 4 weeks after tendon injury may bolster the meager PDGF paracrine-autocrine intrinsic response to injury.

Structure and component alteration of rabbit Achilles tendon in tissue culture

The aim of this study was to investigate alterations of cultured tendon tissues to determine whether tissue culture is a useful method for biological analyses of the tendon. Tendon tissues for tissue culture were isolated from Achilles tendons of rabbits. The tendon segments were placed one segment per well and incubated in growth medium consisting of Dullbecco's modified Eagle's medium supplemented with 5% fetal bovine serum at 37 degrees C in a humidified atmosphere with 5% CO(2) for various periods. The alignment of collagen fibrils was preserved for 48 h, but tendon structure has disintegrated at 96 h. Alcian blue staining and gelatine zymography revealed that proteoglycan markedly diminished and that matrix metalloproteinase (MMPs) activity was upregulated sharply at 72 and 96 h. The ratio of collagen fibrils with large diameter had increased and the mean diameter and mass average diameter value had reached maximum at 48 h. The values then decreased and mean diameters at 72 and 96 h were significantly different from that at 48 h. At 96 h, the ratio of collagen fibrils with small diameters had increased and collagen fibrils with large diameters had disappeared. These findings indicate that structural alteration is possible to be induced by disintegration of collagen fibrils and disappearance of glycosaminoglycans from extracellular matrix (ECM), subsequent of upregulation of MMPs activity. Although the study period is limited, the tissue culture method is available for investigating cell-ECM interaction in tendons.

Degenerative suspensory ligament desmitis as a systemic disorder characterized by proteoglycan accumulation

Background

Degenerative suspensory ligament desmitis (DSLD) is a debilitating disorder thought to be limited to suspensory ligaments of Peruvian Pasos, Peruvian Paso crosses, Arabians, American Saddlebreds, American Quarter Horses, Thoroughbreds, and some European breeds. It frequently leads to persistent, incurable lameness and need to euthanize affected horses. The pathogenesis remains unclear, though the disease appears to run in families. Treatment and prevention are empirical and supportive, and not effective in halting the progression of the disease. Presently, the presumptive diagnosis of DSLD is obtained from patient signalment and history, clinical examination, and ultrasonographic examination of clinically affected horses, and is confirmed at post mortem examination. Presently, there are no reliable methods of diagnosing DSLD in asymptomatic horses. The goal of this study was to characterize and define the disorder in terms of tissue involvement at the macroscopic and microscopic levels.

Results

We examined tissues and organs from 28 affected horses (22 Peruvian Pasos, 6 horses of other breeds) and from 8 control horses. Histopathological examination revealed the presence of excessive amounts of proteoglycans in the following tissues removed from DSLD-affected horses: suspensory ligaments, superficial and deep digital flexor tendons, patellar and nuchal ligaments, cardiovascular system, and sclerae. Electron microscopy demonstrated changes in diameters of collagen fibrils in the tendon, and in smooth muscle cells of the media of the aorta compatible with increased cell permeability in DSLD-affected cells. Separation of tendon extracts by gel chromatography revealed the presence of additional proteoglycan(s) in extracts from affected, but not control extracts.

Conclusion

This study demonstrates for the first time that DSLD, a disease process previously thought to be limited to the suspensory ligaments of the distal limbs of affected horses, is in fact a systemic disorder involving tissues and organs with significant connective tissue component. Abnormal accumulation of proteoglycans between collagen and elastic fibers rather than specific collagen fibril abnormalities is the most prominent histological feature of DSLD. Because of this observation and because of the involvement of many other tendons and ligaments beside the suspensory ligament, and of non-ligamentous tissue we, therefore, propose that equine systemic proteoglycan accumulation or ESPA rather than DSLD is a more appropriate name for this condition.

Thrombospondin-4 and cartilage oligomeric matrix protein form heterooligomers in equine tendon

Injuries of the equine superficial digital flexor tendon are common in racing horses. Knowledge of the tendon matrix composition is crucial to understand physiological and pathological processes in the tendon. The aim of this study was to analyze TSP-4 expressed in equine tendon. Equine tendons were extracted with 10 mM EDTA-containing buffer and TSP-4 purified with ion-exchange chromatography followed by heparin affinity chromatography. The purified TSP-4 was analyzed by one- and two-dimensional SDS-PAGE, immunoblotting, and MALDI-TOF mass spectrometry. Purified TSP-4 gave bands reacting with a TSP-4 specific antiserum, but also with an antiserum to COMP, when submitted to SDS-PAGE under nonreducing conditions. Two-dimensional SDS-PAGE (nonreducing followed by reducing conditions) and immunoprecipitation as well as MALDI-TOF mass spectrometry analysis showed that TSP-4 and COMP are both present in equine tendon and cannot be separated under nonreducing conditions despite significant differences in subunit size. This suggests that they are connected via disulfide bridges into heterooligomers.

Extracellular Matrix Compositon of the Equine Superficial Digital Flexor Tendon: Relationship with Age and Anatomical Site

The objectives of the present study were to test the hypotheses that: (1) the composition of the extracellular matrix of the equine superficial digital flexor tendon (SDFT) shows great functional similarities with articular cartilage, i.e. that significant differences exist in biochemical composition of differently loaded areas (which in the case of tendons may be more apparent as tendon shows more obvious differences than cartilage); and (2) that, as in articular cartilage, no substantial alterations in biochemical composition take place during ageing once adulthood has been attained. Tendon samples were taken from 60 adult slaughter horses from a central area at cross-section in the mid-metacarpal region and at the height of the proximal sesamoid bones (sesamoid region) of the SDFT. Contents of collagenous and non-collagenous components were determined. None of the parameters were correlated with age in either region, except for a significant increase in pentosidine cross-links with age in the sesamoid region. Between the two anatomical regions, there were significant differences in all variables, except for hydroxylysylpyridinoline cross-links. It was concluded that in the equine SDFT, similar to articular cartilage, most molecular parameters are not influenced by age in mature horses, indicating a low remodelling rate. Tendon composition is clearly different between regions, apparently reflecting different specific modes of biomechanical loading at the points sampled.

Cloning and expression of type III collagen in normal and injured tendons of horses

OBJECTIVE

To clone the 5' end of type III collagen and describe its pattern of mRNA and protein expression in normal and healing tendons in horses.

ANIMALS

14 healthy adult horses.

PROCEDURE

The tensile region of collagenase-injured superficial digital flexor tendons was harvested at intervals from 1 to 24 weeks after injury. Total RNA was reverse-transcribed into cDNA for cloning and sequencing of type III collagen. Equine-specific nucleic acid probes were developed and used for northern blot analysis and in situ hybridization. Type III collagen protein and cyanogen bromide-cleaved collagen peptides were assessedby gel electrophresis.

RESULTS

Type III collagen mRNA expression and protein content increased immediately after injury and remained increased. Type III collagen was localized to the endotenon in normal tendon and in injured tendon at 1 week. At 8 and 24 weeks, expression became more widely distributed throughout the tendon parenchyma. Injured tendon contained 6 times more type I than type III collagen mRNA. Quantities of type III collagen protein were maximal in the first 4 weeks after injury (approx 33%) and then began to decrease.

CONCLUSIONS AND CLINICAL RELEVANCE

Type III collagen expression is increased initially in endotenon and subsequently in parenchyma of healing tendon; however, type III remains the minor collagen throughout the healing process. The role of type III collagen in tendon healing is not fully elucidated.

Metabolism and composition of the canine anterior cruciate ligament relate to differences in knee joint mechanics and predisposition to ligament rupture

PURPOSE

The objective of this study was to determine whether differences in the composition and metabolism of the extracellular matrix (ECM) in canine anterior cruciate ligaments (ACLs) might relate to mechanical properties of the canine knee. Variations in ACL biochemistry and knee mechanics could account for divergent predispositions to ligament rupture.

METHODS

Eleven knee joints were obtained from both cadaveric Labrador Retrievers (rupture predisposed) and ex-racing Greyhounds (non-rupture predisposed). Anterioposterior laxity and tensile testing determined mechanical properties of the knee joints and ACL samples respectively. The thermal properties of the collagenous matrix were determined by differential scanning calorimetry (DSC) and the biochemical properties by measuring collagen content, collagen cross-links, glycosaminoglycan (GAG) levels, matrix metalloproteinase-2 (MMP-2), tissue inhibitors of metalloproteinase (TIMP).

RESULTS

The anterioposterior laxity was significantly greater (p = 0.04) in the Labrador Retriever knee joints, and their ACLs tended to be weaker (p = 0.06). Greater collagen turnover was demonstrated by significantly higher (p = 0.02) concentrations of pro-MMP-2, and lower enthalpy of denaturation (p = 0.05) in Labrador Retriever ACLs.

CONCLUSIONS

The different metabolism of the collagenous matrix in the ACLs of dogs predisposed to rupture was related to greater knee joint laxity and lower ligament material properties (ultimate tensile stress). This may be suggestive of a link between ligament rupture and eventual knee osteoarthritis in both dogs and humans.

Downregulation of decorin and transforming growth factor-beta1 by decorin gene suppression in tendinocytes

Scars formed after tendonitis result in altered tissue mechanical properties after injury. The interaction of collagen molecules with decorin affects collagen fibrogenesis, and scar tissue is fragile as a consequence of a large amount of decorin in the scar. We hypothesized that scar formation could be prevented by controlling decorin expression in tendinocytes. As a preliminary experiment, we treated tendinocytes with decorin antisense oligodeoxynucleotides (ODNs). Tendinocytes were isolated from Achilles tendons of New Zealand white rabbits and treated with ODN. When tendinocytes were transfected with decorin sense ODN, there was no alteration, whereas decorin antisense ODN-treated tendinocytes showed suppression of transforming growth factor (TGF)-beta1 production. Decorin and TGF-beta1-production of tendinocytes is regulated by decorin gene suppression. The results showed that the antisense approach is an attractive therapeutic strategy not only for preventing decorin deposition in scar tissue, which decreases collagen fibril diameter, but also for controlling TGF-beta1 production, which leads to organ fibrosis.

Investigation of the composition, turnover, and thermal properties of ruptured cranial cruciate ligaments of dogs

OBJECTIVE

To assess different components of the extracellular matrix with regard to their thermal properties, composition, and turnover in ruptured cranial cruciate ligaments (CCLs) of dogs, compared with components of intact CCLs from a breed predisposed to CCL failure.

SAMPLE POPULATION

Ruptured CCLs obtained from 8 dogs of breeds predisposed to ruptured CCLs and intact CCLs from 12 cadaveric Labrador Retrievers.

PROCEDURE

Ruptured and intact CCLs were analyzed for water content; collagen content and collagen cross-links were evaluated via hydroxyproline and amino-acid analyses, respectively. Glycosaminoglycan (GAG) content was analyzed via dimethylmethylene blue and uronic acid assays. Matrix metalloproteinases (MMPs)-2 and -9 and the tissue inhibitors of metalloproteinases (TIMPs)-1 and -2 were detected via gelatin SDS-PAGE zymography and reverse gelatin zymography. Thermal analysis of ligaments was performed by use of differential scanning calorimetry.

RESULTS

Ruptured CCLs had significantly higher lamounts of immature cross-links, total and sulfated GAGs, and water content, compared with that of the intact ligaments. Compared with intact CCLs, concentration of pro-MMP-2 was significantly higher in ruptured CCLs; the maximum temperature of collagen denaturation was significantly lower in the ruptured CCLs.

CONCLUSIONS AND CLINICAL RELEVANCE

The extracellular matrix of ruptured CCLs had an increased matrix turnover indicated by increased collagen and GAG synthesis, compared with that of intact CCLs. Although the extracellular matrix changes may have occurred before ligament rupture, it is possible that these observed changes may be part of a reparative process after rupture.

Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Structural changes in loaded equine tendons can be monitored by a novel spectroscopic technique

This study aimed to investigate the preferential collagen fibril alignment in unloaded and loaded tendons using elastic scattering spectroscopy. The device consisted of an optical probe, a pulsed light source (320-860 nm), a spectrometer and a PC. Two probes with either 2.75 mm or 300 microm source-detector separations were used to monitor deep and superficial layers, respectively. Equine superficial digital flexor tendons were subjected to ex vivo progressive tensional loading. Seven times more backscattered light was detected parallel rather than perpendicular to the tendon axis with the 2.75 mm separation probe in unloaded tendons. In contrast, using the 300 microm separation probe the plane of maximum backscatter (3-fold greater) was perpendicular to the tendon axis. There was no optical anisotropy in the cross-sectional plane of the tendon (i.e. the transversely cut tendon surface), with no structural anisotropy. During mechanical loading (9-14% strain) backscatter anisotropy increased 8.5- to 18.5-fold along the principal strain axis for 2.75 mm probe separation, but almost disappeared in the perpendicular plane (measured using the 300 microm probe separation). Optical (anisotropy) and mechanical (strain) measurements were highly correlated. We conclude that spatial anisotropy of backscattered light can be used for quantitative monitoring of collagen fibril alignment and tissue reorganization during loading, with the potential for minimally invasive real-time structural monitoring of fibrous tissues in normal, pathological or repairing tissues and in tissue engineering.

Are the material properties and matrix composition of equine flexor and extensor tendons determined by their functions?

REASONS FOR PERFORMING STUDY

Injury to the superficial digital flexor tendon (SDFT) is common in competition horses. The SDFT contributes to locomotory efficiency by storing energy; such tendons have low safety margins. Tendons which merely position the limb, including the opposing common digital extensor tendon (CDET), are rarely injured. The current failure of strategies to prevent or effectively treat injury to the SDFT indicates the importance of understanding how it differs from tendons which are not injury-prone.

HYPOTHESIS

That the structural and material properties and matrix composition of the SDFT and CDET differ, reflecting their specific functional requirements in vivo.

METHODS

Forelimb tendons were harvested from 26 mature horses and loaded to failure prior to matrix composition analysis of specimens.

RESULTS

The SDFT had a significantly higher cross-sectional area, structural stiffness, failure load and failure strain and a lower elastic modulus than the CDET (P < 0.0001).

CONCLUSIONS

The SDFT has conflicting requirements for strength and elasticity; although as a whole it is a stiffer structure than the CDET, differences in the matrix molecular composition including water and total sulphated glycosaminoglycan contents allow it to remain more elastic as a material.

POTENTIAL RELEVANCE

Further information on how the two tendons attain these different properties may be of use in the development of prevention and treatment strategies for SDFT rupture.

Computerized ultrasonographic tissue characterization of equine superficial digital flexor tendons by means of stability quantification of echo patterns in contiguous transverse ultrasonographic images

OBJECTIVE

To describe a method of computerized ultrasonographic tissue characterization that includes structures below the size limits of resolution in equine superficial digital flexor tendons.

SAMPLE POPULATION

2 damaged and 2 structurally normal superficial digital flexor tendons.

PROCEDURE

Transverse ultrasonographic images were collected along the tendon long axis. Stability of echo pattern was quantified by means of variation in gray levels of each pixel in contiguous images and expressed as correlation, entropy, and waviness ratios.

RESULTS

Normal young and normal old tissues were characterized by high correlation and low entropy and waviness ratios. In necrotic tissue, collapsed intratendinous septa resulted in high correlation, moderate entropy, and high waviness ratios. In early granulation tissue, complete lack of bundle formation resulted in values of zero for correlation and waviness ratios; loose connective tissue matrix resulted in a high entropy ratio. In late granulation tissue, formation of new bundles resulted in a high correlation ratio; swollen intratendinous septa and incomplete organization of connective tissue matrix were reflected in high entropy and waviness ratios. In early fibrotic tissue, rearrangement of tendon bundles resulted in a correlation ratio within reference range and a slight increase in the waviness ratio; an increase in cellularity and lack of fibrillar arrangement led to an increase in the entropy ratio. In late fibrotic and scar tissues, inferior quality of repair with almost complete lack of organization was reflected in low to moderate correlation, low waviness, and high entropy ratios.

CONCLUSIONS AND CLINICAL RELEVANCE

Stability of echo patterns accurately reflects homogeneity of tendons in horses.