Injury response of geriatric mouse patellar tendons

Injury adversely impacts the structure and mechanical properties of a tendon, thus causing pain and disability. Previously, we demonstrated that patellar tendons in mature (P150) and aged (P300) mice do not recover original functionality, even 6 weeks after injury, and that uninjured geriatric tendons (P570) are functionally inferior to uninjured mature tendons. In this study, we hypothesized that the repair response in injured geriatric mice would be further compromised, thus undermining patellar tendon function post‐injury. Patellar tendons from wild‐type mice were injured at 540 days. At 3 and 6 weeks post‐surgery, structural, mechanical, and biochemical analyses were performed and compared to uninjured controls. Mechanical properties of geriatric tendons failed to improve after injury. When compared to mature and aged tendons post‐injury, it was determined that at no age was there a suitable repair response. In previous studies, we were able to associate the absence of SLRPs with phenotypic changes both early and late in repair. Here we found that SLRPs were significantly decreased after injury, thus offering a possible explanation for why geriatric tendons were unable to mount an adequate repair response. Thus, we conclude that regardless of age after maturity, tendon healing ultimately results in a substandard outcome. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1256–1263, 2016.     

Scleraxis expression is coordinately regulated in a murine model of patellar tendon injury

This study investigated the expression of Scleraxis in a murine model of patellar tendon injury in which the central third of the patellar tendon was unilaterally injured. The presence of tendon pathology was assessed using dual photon microscopy, conventional histology and microCT. Tendon pathology was also quantified noninvasively over a 12‐week period using high‐frequency ultrasound and laser Doppler flowmetry. Gene expression (Scx, Tnmd, and Col1a1) was determined at defined end‐points (1, 4, 8, and 12 weeks) using qPCR on RNA from individual patellar tendons on injured and uninjured sides. There was significant development of tendon pathology as gauged by ultrasound and laser Doppler over 12 weeks. Injured tendons demonstrated significant histological and microCT evidence of pathological change, and disorganized collagen with reduced density. The expression of Scx and Col1a1 was unchanged at 1 week, significantly upregulated at 4 and 8 weeks, and had returned to baseline by 12 weeks. Tnmd expression was unchanged at 1 week, and significantly increased at 4, 8, and 12 weeks. Patellar tendon injury was associated with marked increases in the expression of Scx, Tnmd, and Col1a1. Our data suggest new roles for Scleraxis in coordinating the response to injury in the pathogenesis of tendon disorders. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:289–296, 2011     

Tendon‐derived progenitor cells improve healing of collagenase‐induced flexor tendinitis

Tendinitis is a common and a performance‐limiting injury in athletes. This study describes the value of intralesional tendon‐derived progenitor cell (TDPC) injections in equine flexor tendinitis. Collagenase‐induced tendinitis was created in both front superficial digital flexor (SDF) tendons. Four weeks later, the forelimb tendon lesions were treated with 1 × 10⁷ autogenous TDPCs or saline. Tendinitis was also induced by collagenase in one hind SDF tendon, to study the survival and distribution of DiI‐labeled TDPCs 1, 2, 4, and 6 weeks after injection. The remaining normal tendon was used as a “control.” Twelve weeks after forelimb TDPC injections, tendons were harvested for assessment of matrix gene expression, biochemical, biomechanical, and histological characteristics. DiI‐labeled TDPCs were abundant 1 week after injection but gradually declined over time and were undetectable after 6 weeks. Twelve weeks after TDPC injection, collagens I and III, COMP and tenomodulin mRNA levels were similar (p = 0.3) in both TDPC and saline groups and higher (p < 0.05) than normal tendon. Yield and maximal stresses of the TDPC group were significantly greater (p = 0.005) than the saline group's and similar (p = 0.6) to normal tendon. However, the elastic modulus of the TDPC and saline groups were not significantly different (p = 0.32). Histological assessment of the repair tissues with Fourier transform‐second harmonic generation imaging demonstrated that collagen alignment was significantly better (p = 0.02) in TDPC group than in the saline controls. In summary, treating collagenase‐induced flexor tendon lesions with TDPCs improved the tensile strength and collagen fiber alignment of the repair tissue. Study Design © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2162–2171, 2016.     

Nonsurgical treatment and early return to activity leads to improved Achilles tendon fatigue mechanics and functional outcomes during early healing in an animal model

Achilles tendon ruptures are common and devastating injuries; however, an optimized treatment and rehabilitation protocol has yet to be defined. Therefore, the objective of this study was to investigate the effects of surgical repair and return to activity on joint function and Achilles tendon properties after 3 weeks of healing. Sprague–Dawley rats (N = 100) received unilateral blunt transection of their Achilles tendon. Animals were then randomized into repaired or non‐repaired treatments, and further randomized into groups that returned to activity after 1 week (RTA1) or after 3 weeks (RTA3) of limb casting in plantarflexion. Limb function, passive joint mechanics, and tendon properties (mechanical, organizational using high frequency ultrasound, histological, and compositional) were evaluated. Results showed that both treatment and return to activity collectively affected limb function, passive joint mechanics, and tendon properties. Functionally, RTA1 animals had increased dorsiflexion ROM and weight bearing of the injured limb compared to RTA3 animals 3‐weeks post‐injury. Such functional improvements in RTA1 tendons were evidenced in their mechanical fatigue properties and increased cross sectional area compared to RTA3 tendons. When RTA1 was coupled with nonsurgical treatment, superior fatigue properties were achieved compared to repaired tendons. No differences in cell shape, cellularity, GAG, collagen type I, or TGF‐β staining were identified between groups, but collagen type III was elevated in RTA3 repaired tendons. The larger tissue area and increased fatigue resistance created in RTA1 tendons may prove critical for optimized outcomes in early Achilles tendon healing following complete rupture. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2172–2180, 2016.     

Surface modification counteracts adverse effects associated with immobilization after flexor tendon repair

Although post‐rehabilitation is routinely performed following flexor tendon repair, in some clinical scenarios post‐rehabilitation must be delayed. We investigated modification of the tendon surface using carbodiimide derivatized hyaluronic acid and lubricin (cd‐HA‐Lub) to maintain gliding function following flexor tendon repair with postoperative immobilization in a in vivo canine model. Flexor digitorum profundus tendons from the 2nd and 5th digits of one forepaw of six dogs were transected and repaired. One tendon in each paw was treated with cd‐HA‐Lub; the other repaired tendon was not treated. Following tendon repair, a forearm cast was applied to fully immobilize the operated forelimb for 10 days, after which the animals were euthanized. Digit normalized work of flexion (nWOF) and tendon gliding resistance were assessed. The nWOF of the FDP tendons treated with cd‐HA‐Lub was significantly lower than the nWOF of the untreated tendons (p < 0.01). The gliding resistance of cd‐HA‐Lub treated tendons was also significantly lower than that of the untreated tendons (p < 0.05). Surface treatment with cd‐HA‐Lub following flexor tendon repair provides an opportunity to improve outcomes for patients in whom the post‐operative therapy must be delayed after flexor tendon repair. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:1940–1944, 2012     

Cytokine-induced tendinitis: a preliminary study in rabbits.

This study was designed to determine the effects of a single injection of a species-specific preparation of cytokines into rabbit patellar tendons and to compare the results with a known model of tendinitis, the collagenase-injection model. New Zealand White rabbits were divided into two groups and two time periods (4 and 16 weeks) and injected in the midsubstance of the right patellar tendon with either cytokines or collagenase under ultrasound guidance to confirm intratendinous needle placement. The left patellar tendon was injected with 0.025 ml of saline solution and served as a control. The rabbits were returned to cage activity after injection. At death, two rabbits in each group underwent histological analysis; the remaining eight animals in each time frame were evaluated biomechanically and then biochemically with use of the patella/whole patellar tendon/tibia complex. Histologic results at 4 weeks in the tendons injected with cytokines demonstrated increased cellularity, which was resolving by 16 weeks. The matrix appeared unchanged. The tendons injected with collagenase demonstrated increased angiogenesis of the matrix, hypercellularity, and fibrosis around the tendon at 4 weeks. At 16 weeks, myxoid changes, focal fibrosis, and collagen-bundle disarray with persistent increase in cellularity were noted. Biomechanically, a significant decrease in ultimate load at 16 weeks was seen in the tendons injected with cytokines but no change was seen in cross-sectional area. The tendons injected with collagenase demonstrated a significant increase in cross-sectional area at 4 and 16 weeks compared with those injected with cytokines. Biochemically, there was no significant difference in collagen content between the two groups at 4 or 16 weeks but the tendons injected with collagenase demonstrated a significant increase in crosslinking at 16 weeks. Our conclusion is that the tendons injected with the cytokine preparation represent a model of mild, seemingly reversible tendon injury. The cytokine preparation produces no matrix damage or evidence of collagen degradation and is species specific.     

Macrophage depletion reduces cell proliferation and extracellular matrix accumulation but increases the ultimate tensile strength of injured Achilles tendons

Macrophages are present in large numbers and display specific and distinct phenotypes during the various phases of tissue repair. However, their role following tendon injury and during repair has never been investigated. We injected C57BL/6 mice daily for 4 days with liposome‐encapsulated clodronate to deplete circulating monocytes/macrophages. Placebo mice were injected with PBS. The left Achilles tendons of the mice were transversely sectioned and sutured using the 8‐strand technique. Macrophage accumulation and cell proliferation were significantly lower in the tendons of clodronate‐treated mice than in those of PBS‐treated mice on days 3 and 7 post‐injury. TGF‐β1 staining was significantly more intense in the tendons of PBS‐treated mice on day 7 post‐injury. Edema and the dry mass of the Achilles tendons were also higher in the PBS‐treated mice on days 7 and 14 post‐injury. No differences in absolute strength and stiffness were observed, but Young's modulus and maximal stress were significantly greater for tendons from the clodronate‐treated mice than those from PBS‐treated mice after 14 days of tendon repair. Overall, our findings showed that macrophages promote cell proliferation and extracellular matrix accumulation but their presence leads to inferior ultimate tensile strength of the Achilles tendons. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:279–285, 2014.     

Histological and molecular analysis of the biceps tendon long head post‐tenotomy

Tendinopathy is a vexing clinical problem as its onset and development is often asymptomatic and unrecognized until tendon rupture. While extensively studied in the rotator cuff, Achilles, and patellar tendons, no study to date has examined the histological and molecular characteristics of the tendinopathic biceps long‐head (LHB). The anatomy of the LHB is unique in that it comprises intra‐ and extra‐articular portions, each exposed to differing loading patterns. Eleven LHBs post‐tenotomy were sectioned, fixed in formalin, and stained (H&E; Alcian Blue), and gross structural organization of collagen measured using polarized light microscopy. Protein expression of intra‐ and extra‐articular portions of the tenotomized biceps for IGF‐I, collagen III, and MMP‐1, ‐2, ‐3, and ‐13 was determined with Western blot analyses. The intra‐articular LHB exhibited significantly greater histological evidence of tendinopathy inclusive of increased proteoglycan (p < 0.05) and decreased organization as measured by polarized light microscopy (p < 0.01). The intra‐articular LHB also had significantly increased expression of collagen type III (p < 0.01) and of MMP‐1 and 3 (p < 0.01, p < 0.05 respectively). No significant differences were found for IGF‐I or for MMP‐2 and ‐13. The intra‐articular LHB exhibited histological characteristics of tendinopathy. Protein expression of the intra‐articular LHB did not universally display signs of tendinopathy in comparison to the extra‐articular portion of the tendon. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1379–1385, 2009     

Effect of NKISK on tendon lengthening: An in vivo model for various clinically applicable dosing regimens

One proposed mechanism of tendon lengthening is the “sliding fibril” hypothesis, in which tendons lengthen through the sliding of discontinuous fibrils after release of decorin‐fibronectin interfibrillar bonds. The pentapeptide NKISK has been reported to inhibit the binding of decorin, a proteoglycan on the surface of collagen fibrils, to fibronectin, a tissue adhesion molecule, which are thought to play a role in interfibrillar binding. Prior investigations have demonstrated that NKISK produces in vivo tendon lengthening. This study investigates the effect of potential clinically applicable NKISK injection regimens in an in vivo model. One hundred and thirteen male Sprague‐Dawley rats were divided into 15 different treatment groups, each receiving percutaneous patellar tendon injections of NKISK, QKTSK (a “nonsense” pentapeptide), or PBS of varying volumes, concentrations, and injection schedules. Following sacrifice, the patellar tendon lengths were measured in all groups, and biomechanical testing was performed with comparisons made to the contralateral, untreated control limbs. Tendon lengthening was significantly greater (p ≤ 0.05) in nearly all NKISK‐treated tendons as compared to PBS‐ and QKTSK‐treated tendons and was dose‐dependent. Measured lengthening was less in rats whose sacrifice was delayed following the final injection of NKISK, which likely indicates recontraction of lengthened tendons, but they remained significantly longer than the controls. Biomechanical testing did not reveal significant differences in ultimate load, modulus, stiffness, or displacement. This study demonstrates that NKISK given in clinically plausible dosing regimens produces dose‐dependent tendon lengthening in an in vivo setting with minimal effects on the mechanical properties of the treated tendons. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:971–976, 2008     

Effects of platelet‐rich plasma on the quality of repair of mechanically induced core lesions in equine superficial digital flexor tendons: A placebo‐controlled experimental study

Tendon injuries are notorious for their slow and functionally inferior healing. Intratendinous application of platelet‐rich plasma (PRP) has been reported to stimulate the repair process of tendon injuries, but there is little conclusive evidence for its effectiveness. A placebo‐controlled experimental trial was performed to test the hypothesis that a single intratendinous PRP treatment enhances the quality of tendon repair, as evidenced by improved biochemical, biomechanical, and histological tissue properties. In six horses, tendon lesions were created surgically in the Superficial Digital Flexor Tendons (SDFT) of both front limbs, one of which was treated with PRP and the other with saline. After 24 weeks, the tendons were harvested for biochemical, biomechanical, and histological evaluations. Collagen, glycosaminoglycan, and DNA content (cellularity) was higher in PRP‐treated tendons (p = 0.039, 0.038, and 0.034, respectively). The repair tissue in the PRP group showed a higher strength at failure (p = 0.021) and Elastic Modulus (p = 0.019). Histologically, PRP‐treated tendons featured better organization of the collagen network (p = 0.031) and signs of increased metabolic activity (p = 0.031). It was concluded that PRP increases metabolic activity and seems to advance maturation of repair tissue over nontreated experimentally induced tendon lesions, which suggests that PRP might be beneficial in the treatment of clinical tendon injuries. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:211–217, 2010     

Tendon injury response: assessment of biomechanical properties, tissue morphology and viability following flexor digitorum profundus tendon transection.

Insertion site injuries of the flexor digitorum profundus (FDP) tendon often present for delayed treatment. Apart from gross observations made at the time of surgery, the changes that occur in the flexor tendon stump during the interval from injury to repair are unknown. These changes may include tendon softening and loss of viability, which may contribute to the poor outcomes observed clinically and experimentally. Thirty-eight FDP tendons from 23 adult dogs were transected sharply from their insertions on the distal phalanges and were not repaired. Dogs were allowed full weight bearing and were euthanized 7 or 21 days after injury. Biomechanical testing indicated that the resistance of injured tendons to pullout of a Kessler-type suture was not different from control tendons at 7 days and was increased at 21 days by 25% (p<0.05). Morphologically, at 7 and 21 days the cut surface had a smooth appearance and the end of the injured tendon was increased in thickness by 30% and 50%, respectively (p<0.05). Histologically, we observed increased cellularity and dramatic fibroblast proliferation within the injured tendon stump; there was no evidence of decreased cell viability. We conclude that during the interval from 0 to 21 days after FDP insertion-site injury, tendons cells are viable, proliferative and synthesizing new matrix. This leads to increased tendon size and enhanced resistance to suture pullout. These findings offer a scientific rationale supporting the clinical practice of surgical re-attachment within the first 3 weeks after 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.     

Changes in the expression of matrix extracellular genes and TGFB family members in rotator cuff tears

Lack of synthesis of extracellular matrix compounds may contribute to degeneration of the tendons. Thus, we aimed to evaluate the expression of extracellular matrix and TGFB family members in ruptured and non‐ruptured tendons of the rotator cuff, as well as the effect of clinical factors on gene expression in tendon samples, and the relationship between histological findings and altered gene expression. Injured and non‐injured supraspinatus tendon samples and subscapular non‐injured tendon samples were collected from 38 patients with rotator cuff tears. Non‐injured supraspinatus tendons were obtained from eight controls. Specimens were used for histological evaluation, quantification of collagen fibers, and mRNA and protein expression analyses. Increased COL1A1, COL1A2, COL3A1, COL5A1, FN1, TNC, and TGFBR1 mRNA expression was observed in the tear samples (p < 0.05). Duration of symptoms was correlated with the levels of collagen type I/III fibers (p = 0.032; ρ = 0.0447) and FN1 immunostaining (p = 0.031; ρ = 0.417). Smoking was associated with increased frequency of microcysts, myxoid degeneration, and COL5A1, FN1, TNC, and TGFB1 mRNA expression (p < 0.05). FN1 immunostaining was correlated with the number of years of smoking (p = 0.048; ρ = 0.384). Lower levels of collagen type I/III fibers were detected in samples with fissures (0 = 0.046). High frequency of microcysts was associated with increased COL5A1, FN1, and TNC expression (p < 0.05, for all comparisons). Neovascularization was associated with reduced FN1 (p = 0.035) and TGFBR1 expression (p = 0.034). Our findings show differential expression of matrix extracellular genes and TGFB family members in the degeneration process involved in rotator cuff tears. These molecular alterations are influenced by clinical factors. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2542–2553, 2018.

     

Functional tissue engineering for tendon repair: A multidisciplinary strategy using mesenchymal stem cells,bioscaffolds, and mechanical stimulation

Over the past 8 years, our group has been continuously improving tendon repair using a functional tissue engineering (FTE) paradigm. This paradigm was motivated by inconsistent clinical results after tendon repair and reconstruction, and the modest biomechanical improvements we observed after repair of rabbit central patellar tendon defects using mesenchymal stem cell‐gel‐suture constructs. Although possessing a significantly higher stiffness and failure force than for natural healing, these first generation constructs were quite weak compared to normal tendon. Fundamental to the new FTE paradigm was the need to determine in vivo forces to which the repair tissue might be exposed. We first recorded these force patterns in two normal tendon models and then compared these peak forces to those for repairs of central defects in the rabbit patellar tendon model (PT). Replacing the suture with end‐posts in culture and lowering the mesenchymal stem cell (MSC) concentration of these constructs resulted in failure forces greater than peak in vivo forces that were measured for all the studied activities. Augmenting the gel with a type I collagen sponge further increased repair stiffness and maximum force, and resulted in the repair tangent stiffness matching normal stiffness up to peak in vivo forces. Mechanically stimulating these constructs in bioreactors further enhanced repair biomechanics compared to normal. We are now optimizing components of the mechanical signal that is delivered in culture to further improve construct and repair outcome. Our contributions in the area of tendon functional tissue engineering have the potential to create functional load‐bearing repairs that will revolutionize surgical reconstruction after tendon and ligament injury. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:1–9, 2008     

Purified collagen I oriented membrane for tendon repair: An ex vivo morphological study

Injured tendons have limited repair ability after full‐thickness lesions. Tendon regeneration properties and adverse reactions were assessed ex vivo in an experimental animal model using a new collagen I membrane. The multilamellar membrane obtained from purified equine Achilles tendon is characterized by oriented collagen I fibers and has been shown to sustain cell growth and orientation in vitro. The central third of the patellar tendon (PT) of 10 New Zealand White rabbits was sectioned and grafted with the collagen membrane; the contralateral PT was cut longitudinally (sham‐operated controls). Animals were euthanized 1 or 6 months after surgery, and tendons were subjected to histological and Synchrotron Radiation‐based Computed Microtomography (SRµCT) examination and 3D structure analysis. Histological and SRµCT findings showed satisfactory graft integration with native tendon. Histological examination also showed ongoing angiogenesis. Adverse side‐effects (inflammation, rejection, calcification) were not observed. The multilamellar collagen I membrane can be considered as an effective tool for tendon defect repair and tendon augmentation. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 738–745, 2013     

Effects of lubricant and autologous bone marrow stromal cell augmentation on immobilized flexor tendon repairs

The purpose of the study was to test a novel treatment that carbodiimide‐derivatized‐hyaluronic acid‐lubricin (cd‐HA‐lubricin) combined cell‐based therapy in an immobilized flexor tendon repair in a canine model. Seventy‐eight flexor tendons from 39 dogs were transected. One tendon was treated with cd‐HA‐lubricin plus an interpositional graft of 8 × 10⁵ BMSCs and GDF‐5. The other tendon was repaired without treatment. After 21 day of immobilization, 19 dogs were sacrificed; the remaining 20 dogs underwent a 21‐day rehabilitation protocol before euthanasia. The work of flexion, tendon gliding resistance, and adhesion score in treated tendons were significantly less than the untreated tendons (p < 0.05). The failure strength of the untreated tendons was higher than the treated tendons at 21 and 42 days (p < 0.05). However, there is no significant difference in stiffness between two groups at day 42. Histologic analysis of treated tendons showed a smooth surface and viable transplanted cells 42 days after the repair, whereas untreated tendons showed severe adhesion formation around the repair site. The combination of lubricant and cell treatment resulted in significantly improved digit function, reduced adhesion formation. This novel treatment can address the unmet needs of patients who are unable to commence an early mobilization protocol after flexor tendon repair. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:154–160, 2016.     

Ablating hedgehog signaling in tenocytes during development impairs biomechanics and matrix organization of the adult murine patellar tendon enthesis

Restoring the native structure of the tendon enthesis, where collagen fibers of the midsubstance are integrated within a fibrocartilaginous structure, is problematic following injury. As current surgical methods fail to restore this region adequately, engineers, biologists, and clinicians are working to understand how this structure forms as a prerequisite to improving repair outcomes. We recently reported on the role of Indian hedgehog (Ihh), a novel enthesis marker, in regulating early postnatal enthesis formation. Here, we investigate how inactivating the Hh pathway in tendon cells affects adult (12‐week) murine patellar tendon (PT) enthesis mechanics, fibrocartilage morphology, and collagen fiber organization. We show that ablating Hh signaling resulted in greater than 100% increased failure insertion strain (0.10 v. 0.05 mm/mm, p<0.01) as well as sub‐failure biomechanical deficiencies. Although collagen fiber orientation appears overtly normal in the midsubstance, ablating Hh signaling reduces mineralized fibrocartilage by 32%, leading to less collagen embedded within mineralized tissue. Ablating Hh signaling also caused collagen fibers to coalesce at the insertion, which may explain in part the increased strains. These results indicate that Ihh signaling plays a critical role in the mineralization process of fibrocartilaginous entheses and may be a novel therapeutic to promote tendon‐to‐bone healing. © 2015 The Authors. Journal of Orthopaedic Research published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 33:1142–1151, 2015.     

Experimental treatment of tendon injury with heparin

The effect of heparin on the progress of healing in the injured tendon was examined after a standard injury had been created in the Achilles tendons of New Zealand White rabbits. Half of the rabbits were then treated by the intratendinous injection of heparin. The diameters of the injured right hind and the uninjured left hind Achilles tendon and overlying skin were measured using vernier calipers before injury and at one month after injury. At this time histological analysis was carried out on all Achilles tendons and tendon dry weight, collagen concentration and DNA concentration were measured. Orientation of collagen fibres was more pronounced in treated than untreated tendons while cellularity and blood vessel density were less pronounced. Analysis by Students t-test demonstrated a statistically significant difference between treated and untreated tendons in terms of leg diameter (p less than 0.005), dry weight (p less than 0.01) and DNA concentration (p less than 0.01).     

Tendon-derived stem cells (TDSCs) promote tendon repair in a rat patellar tendon window defect model

Injured tendons heal slowly and often result in the formation of mechanically and functionally inferior fibrotic scar tissue or fibrous adhesions. This study investigated the use of tendon-derived stem cells (TDSCs) for tendon repair in a rat patellar tendon window defect model. Fibrin glue constructs with or without GFP-TDSCs were transplanted into the window defect. The patellar tendons were harvested for histology, ex vivo fluorescent imaging and biomechanical test at various time points up to week 4. Our results showed that TDSCs significantly enhanced tendon healing as indicated by the increase in collagen production as shown by hematolxylin stain-ability of the tissue, improvement of cell alignment, collagen fiber alignment and collagen birefringence typical of tendon. The labeled cells were observed at weeks 1 and 2 and became almost undetectable at week 4. Both the ultimate stress and Young's modulus were significantly higher in the TDSCs group compared to those in the fibrin glue group at week 4. In conclusion, TDSCs promoted earlier and better repair in a rat patellar tendon window defect model.     

Tendon mineralization is accelerated bilaterally and creep of contralateral tendons is increased after unilateral needle injury of murine achilles tendons

Heterotopic mineralization may result in tendon weakness, but effects on other biomechanical responses have not been reported. We used a needle injury, which accelerates spontaneous mineralization of murine Achilles tendons, to test two hypotheses: that injured tendons would demonstrate altered biomechanical responses; and that unilateral injury would accelerate mineralization bilaterally. Mice underwent left hind (LH) injury (I; n = 11) and were euthanized after 20 weeks along with non‐injured controls (C; n = 9). All hind limbs were examined by micro computed tomography followed by biomechanical testing (I = 7 and C = 6). No differences were found in the biomechanical responses of injured tendons compared with controls. However, the right hind (RH) tendons contralateral to the LH injury exhibited greater static creep strain and total creep strain compared with those LH tendons (p ≤ 0.045) and RH tendons from controls (p ≤ 0.043). RH limb lesions of injured mice were three times larger compared with controls (p = 0.030). Therefore, despite extensive mineralization, changes to the responses we measured were limited or absent 20 weeks postinjury. These results also suggest that bilateral occurrence should be considered where tendon mineralization is identified clinically. This experimental system may be useful to study the mechanisms of bilateral new bone formation in tendinopathy and other conditions. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31:1520–1528, 2013