Outcome of conservative and surgical treatment for luxations of the equine superficial digital flexor tendon from the calcaneal tuber

The objective of this study was to report the outcome of horses treated either conservatively or surgically for luxation of the superficial digital flexor tendon (SDFT) from the calcaneal tuber. Medical records of horses with diagnosed SDFT luxations from the calcaneal tuber were reviewed (1993–2015) and long-term follow-up examinations and owner questionnaires performed. Survival to hospital discharge, complications, final SDFT position, soundness and return to intended use were compared between conservatively or surgically treated horses. Following conservative treatment all horses (n = 8) survived to hospital discharge. At long-term follow-up 71.4% (5/7) had returned to intended use. The SDFT was unstable in all of them resulting in a mild mechanical lameness. Nine horses underwent surgical treatment. The SDFT was reduced and maintained in position with a synthetic mesh and sutures (n = 2), with a synthetic mesh, sutures and suture screws (n = 6) or with sutures and suture screws (n = 1). Only 66.7% (6/9) of the horses survived to hospital discharge. Fatal complications including support limb laminitis (1/9), implant infection (1/9), and support limb laminitis plus infection (1/9) occurred. All horses available for long-term follow-up (n = 5) were clinically sound and 80% (4/5) had returned to intended use. Following conservative treatment, a mild mechanical lameness will persist but will not prevent the horses from returning to their intended use. Surgical repair can result in a stable fixation of the SDFT with long-term soundness and return to intended use; however, fatal complications (support limb laminitis, infection) can occur leading to the euthanasia of the horse in the immediate post-operative period.     

Tendonitis of the branches of insertion of the superficial digital flexor tendon in horses

Objective To describe clinical findings, ultrasonographic features and outcome of injury to the branches of insertion of the superficial digital flexor (SDF) tendon in horses.
Design Retrospective study of 14 cases.
Procedure Fourteen Thoroughbred horses with tendonitis affecting the branches of insertion of the SDF tendon were examined for lameness, location and amount of swelling, and the presence of other musculoskeletal abnormalities. The flexor tendons were assessed by ultrasonographic examination, and recommendations were made for management of the cases. Outcome was assessed by re-examination of some horses, direct communication with the owner or trainer, and examination of race records.
Results The lateral SDF branch was affected in 10 horses; the medial branch in three, and both branches in one horse. Two horses had concurrent injuries to the SDF tendon in the metacarpal region of the contralateral limb. Ultrasonographic findings included swelling of the affected SDF branch, peritendinous fluid accumulation, disruption of normal fibre alignment on sagittal scan, and variable loss of echogenicity. As healing occurred, there was return of normal echogenicity, but normal fibre alignment did not return completely and apparent adhesions formed between the affected SDF branch and adjacent structures. Seven of 10 horses which returned to their previous use were able to compete without further tendon injury. Recurrence of injury occurred in one case, and another two horses developed tendonitis in the metacarpal region. One horse was retired from racing but was able to compete at dressage without recurrence of injury. Two horses were retired for breeding without returning to training, and one horse was sold and lost to follow up but did not race.
Conclusion The prognosis is fair for return to previous use following injury to the branches of insertion of the SDF tendon in athletic horses.     

Implantation of bone marrow-derived mesenchymal stem cells demonstrates improved outcome in horses with overstrain injury of the superficial digital flexor tendon

Reasons for performing study: Mesenchymal stem (progenitor; stromal) cell (MSC) therapy has gained popularity for the treatment of equine tendon injuries but without reports of long‐term follow‐up. Objectives: To evaluate the safety and reinjury rate of racehorses after intralesional MSC injection in a large study of naturally occurring superficial digital flexor tendinopathy and to compare these data with those published for other treatments. Methods: Safety was assessed clinically, ultrasonographically, scintigraphically and histologically in a cohort of treated cases: 141 client‐owned treated racehorses followed‐up for a minimum of 2 years after return to full work. Reinjury percentages were compared to 2 published studies of other treatments with similar selection criteria and follow‐up. The number of race starts, discipline, age, number of MSCs injected and interval between injury and treatment were analysed. Results: There were no adverse effects of the treatment with no aberrant tissue on histological examination. The reinjury percentage of all racehorses with follow‐up (n = 113) undergoing MSC treatment was 27.4%, with the rate for flat (n = 8) and National Hunt (n = 105) racehorses being 50 and 25.7%, respectively. This was significantly less than published for National Hunt racehorses treated in other ways. No relationship between outcome and age, discipline, number of MSCs injected or injury to implantation interval was found. Conclusions: Whilst recognising the limitations of historical controls, this study has shown that MPC implantation is safe and appears to reduce the reinjury rate after superficial digital flexor tendinopathy, especially in National Hunt racehorses. Potential relevance: This study has provided evidence for the long‐term efficacy of MSC treatment for tendinopathy in racehorses and provides support for translation to human tendon injuries.     

The propagation of induced tendon lesions in the equine superficial digital flexor tendon: An ex vivo study

Reasons for performing study: Clinical tendon lesions usually enlarge during the first days to weeks after sustaining the injury due to enzymatic and biomechanical influences. Limiting this enlargement would positively influence prognosis related to lesion size. Objectives: To investigate the effect of cyclic loading on the propagation of enzymatically and physically induced tendon lesions and to assess the effect of immobilisation thereon in an ex vivo model. Methods: Equine cadaver limbs with either physically or collagenase‐induced SDFT lesions were placed in a pneumatic loading device. Groups consisted of unloaded limbs, cyclically loaded limbs and cyclically loaded limbs with fibreglass cast immobilisation. Lesion size was evaluated ultrasonographically and macroscopically and load at failure was determined for groups where lesions enlarged. Results: Physically induced lesions did not propagate after cyclical loading and were not further investigated. Collagenase‐induced lesions propagated significantly more after cyclical loading compared to unloaded limbs. This propagation could effectively be reduced by immobilisation through a lower limb cast. Immobilisation did not influence load at failure. Conclusions: The combination of enzymatic and mechanical stimulation caused the lesions to propagate most. Casting appeared effectively to reduce the enlargement of the original tendon lesions in this ex vivo model, while it did not affect ultimate tendon strength. Potential relevance: As prognosis of tendon injuries is related to lesion size, the outcome of this study warrants further investigations in vivo.     

Relationship between the ultrasonographic findings of suspected superficial digital flexor tendon injury and the prevalence of subsequent severe superficial digital flexor tendon injuries in Thoroughbred horses: a retrospective study

The onset of severe injury to the superficial digital flexor tendon (SDFT) is extremely difficult to predict from slight changes in ultrasonographic findings in cases with no apparent clinical signs. This study investigated the relationship between an increased cross-sectional area (CSA) or edema in the subcutaneous tissue around the tendon and the subsequent onset of severe SDFT injury in Thoroughbred racehorses. Horses were classified into three groups based on ultrasound diagnosis (USD) findings: Group A included cases with enlarged tendons; Group B included cases with tendons of normal size but with prominent edema in the peritendinous tissue; and Group C (control group) included cases with no abnormal USD findings. The incidence of subsequent severe tendon injury was significantly higher in the horses in Groups A (25.7%, 28/101) and B (28.3%, 65/212) than in those in Group C (4.9%, 2/41). There were no significant differences in the median period and the median number of races from the first examination to the subsequent tendon injury between Groups A (140 days, 1 race) and B (120 days, 1 race). The results of this study revealed that horses with increased CSA and peritendinous edema are likely to suffer a subsequent severe tendon injury. Also, these two USD findings, i.e., increased CSA and peritendinous edema, indicate the risk of onset of severe SDFT injury.     

Tendonitis of branches of the superficial digital flexor tendon in Standardbred racehorses: 15 cases

This case report describes the clinical and ultrasonographic findings and performance outcome of distal tendonitis of the superficial digital flexor tendon (SDFT) in Standardbred racehorses. Cases with distal tendonitis of the SDFT in isolation (not associated with a metacarpal lesion) were reviewed retrospectively. The clinical and ultrasonographic findings and performance outcome were reviewed. The injury was located on the left forelimb in 67% (10/15) of horses and on the right forelimb in 33% (5/15). None of the horses were affected bilaterally. The lateral branch was involved in 80% of the cases (12/15) and medial branch in 20% (3/15). None of the horses were affected biaxially. Mean time between injury and first start ± s.d. was 302 ± 142 days. Returning to racing was observed in 93% of the horses (14/15), with a reinjury rate of 22% (2/9). For horses which raced after the injury, the number of starts, shows and wins was not significantly different before and after injury, regardless of the location of the lesion. Tendonitis of branches of the SDFT in isolation carries a good prognosis in Standardbred racehorses. A higher return to racing and a lower reinjury rate may be expected compared to lesions in the metacarpal region.     

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.     

Tenectomy of the superficial digital flexor tendon as a treatment of suspected septic tendinitis and tenosynovitis of the digital flexor tendon sheath followed by rehabilitation with an orthotic device

A 15-year-old Sports horse gelding was referred for nonweightbearing lameness of the left hindlimb. Septic cellulitis was diagnosed and managed medically. After 14 days, septic tenosynovitis of the digital flexor tendon sheath (DFTS), with septic tendinitis of the superficial (SDFT) and deep (DDFT) digital flexor tendons, became evident. Surgical resection of the intrathecal portion of the septic SDFT was performed. Post-operatively, a half-limb cast was placed on the operated limb for 10 weeks followed by an articulated orthotic support boot during a rehabilitation period of 6 weeks. The horse recovered and regained long-term pasture soundness. Ultrasonography demonstrated the presence of bridging connective tissue in the location of the resected SDFT.     

An investigation of the relationship between race performance and superficial digital flexor tendonitis in the Thoroughbred racehorse

Reasons for performing study: There is limited information regarding the number of races and the period for evaluation of outcome which is critical for assessment of SDF tendonitis treatments. Objective: To evaluate the re‐injury rate and racing performance of Thoroughbred racehorses that sustain SDF tendonitis in relation to matched controls in terms of number of races post treatment and maximum racing performance ratings before and after injury. Study design: Clinical records and racing histories of 401 racehorses with a first occurrence of SDF tendonitis diagnosed by ultrasonography. Controls were of the same age, sex and were horses training in the same establishment at the time of injury as the case horses and where the trainer reported that the horse had not had a previous SDF tendon injury or treatment. Results: Eighty percent of both case and control horses returned to racing after the date of injury, and the re‐injury proportion within 3 years of treatment was 53%. The difference in Racing Post Rating_(max) (RPR_(max)) and the Racing Post Rating in the race immediately before the treatment date was significantly smaller in case horses (mean = 9.6 lbs; range = 0–75) compared to control horses (mean = 17.0 lbs; range = 0–79). No significant decrease in RPR_(max) was noted post injury. No difference between case and control horses was found for return to racing and racing 3 times, but control horses were significantly more likely to compete 5 races post treatment date than case horses. Conclusion and clinical relevance: Injury was associated with an individual's pre‐injury maximum performance level and return to racing and completion of 3 races are not useful indicator of the outcome of horses with SDF tendonitis. The assessment of the outcome of horses with an SDF injury in a population of racehorses using the number of races post injury requires a minimum of 5 races post injury to be a useful indicator. Further, a re‐injury proportion in a population of horses in training for 3 years post treatment.     

Injection of the insertion of the deep digital flexor tendon in horses using radiographic guidance

Insertional tendinopathies of the DDFT have been reported both as the sole lesion and as part of a multifocal lesion (Dyson et al. 2003). Computed tomography (CT) and magnetic resonance imaging allow specific diagnosis of deep digital flexor tendon lesions within the hoof capsule; however, direct intralesional treatment of such lesions is difficult because of the hoof's rigid structure. A technique designed to mimic intralesional injection of insertional tendinopathies of the DDFT in the standing horse using radiographic guidance was assessed. Radiographic and contrast CT imaging and sectioning of the limbs confirmed accurate injection in all cases although inadvertant administration of injectate into adjacent structures was also evident.     

Effects of recombinant equine growth hormone on in vitro biomechanical properties of the superficial digital flexor tendon of Standardbred yearlings in training

OBJECTIVE: To determine whether recombinant equine growth hormone (rEGH) would alter the in vitro biomechanical properties of the forelimb superficial digital flexor tendon (SDFT) in exercising young Standardbred horses. STUDY DESIGN: Randomized complete block design. ANIMALS: Twelve Standardbred yearlings. METHODS: Horses were trained for 12 weeks on a high-speed treadmill (10% positive incline). rEGH was administered intramuscularly (IM) daily (10 microg/kg during week 4; 20 microg/kg for weeks 5-9) to 6 horses (treated group), whereas 6 horses (control group) were administered an equivalent daily volume of sterile water IM. At 12 weeks, horses were euthanatized and left forelimb SDFTs were collected and stored (-70 degrees C). A section from the mid-region of the SDFT was held in cryoclamps with a 4 cm interspace distance and distracted at 10 mm/s until failure. The variables evaluated were maximal load at yield and failure, ultimate and yield tensile stress and strain, tendon stiffness, and mode of failure. Data were analyzed using unpaired, two-tailed, Student's t-test. Statistical significance was set at P < or =.05. RESULTS: Yield and ultimate tensile stress were significantly lower in the rEGH-treated horses compared with controls. There was a trend toward increased maximal displacement, increased ultimate tensile strain, and decreased tendon stiffness in rEGH-treated horses compared with controls. Tensile stress and cross-sectional area, and tensile stress and stiffness were significantly correlated at yield and failure points. CONCLUSIONS: rEGH, administered at the manufacturer's recommended dose rates to maturing Standardbred horses in training, does not significantly augment the in vitro biomechanical properties of the forelimb SDFT. CLINICAL RELEVANCE: Administration of rEGH to young horses in training is unlikely to enhance the physiologic adaptation of the SDFT to exercise stress.     

Possible role of carpal hyperextension in superficial digital flexor tendinopathy

Reasons for performing study: The specific biomechanical circumstances that induce excessive superficial digital flexor tendon (SDFT) strain in horses are unknown. Hypothesis: Carpal joint hyperextension during axial limb loading during the middle of stance disproportionately enhances SDFT strains compared to suspensory ligament (SL) strains. Methods: Superficial digital flexor tendon and SL strains were measured in 7 cadaver limbs during in vitro loading that maintained carpal extension or allowed carpal hyperextension by constraining, or allowing rotation of, the radius during loading conditions that simulated the middle of stance at the walk. The effect of carpal hyperextension on SDFT and SL strains and joint angles was assessed using repeated measures ANOVA. Results: Limb loading generally resulted in higher SL strains than SDFT strains for both carpal extension and hyperextension loading conditions. Compared to carpal extension, carpal hyperextension resulted in increased strains in both the SDFT and the SL; however, a greater increase in strain was seen in the SDFT. On average, carpal hyperextension caused approximately 3° greater carpal extension, 1° greater metacarpophalangeal joint hyperextension, 1° greater proximal interphalangeal joint flexion and <1° greater distal interphalangeal joint flexion than did carpal extension. Conclusions and clinical relevance: Carpal joint hyperextension is likely to induce disproportionately higher strain in the SDFT than in the SL. Factors that affect carpal stability are likely to affect the risk for superficial digital flexor tendinopathy.     

Tenosynovitis of the carpal sheath of the digital flexor tendons associated with tears of the radial head of the deep digital flexor: observations in 11 horses

Reasons for performing study: Tears of the radial head of the deep digital flexor (DDF) have not previously been documented. Objectives: To describe the presentation, clinical, ultrasonographic and tenoscopic features associated with tears of the radial head of the DDF and to report the results of treatment. Hypotheses: Tears of the radial head of the DDF cause lameness and distension of the carpal sheath of the digital flexor tendons. Removal of disrupted tissue that is extruded into the sheath can result in clinical resolution and restoration of function. Methods: Case records and diagnostic images of horses in which tearing of the radial head of the DDF was diagnosed were reviewed retrospectively and follow‐up information obtained. Results: Eleven cases were identified. Clinical, ultrasonographic and tenoscopic commonality was recorded and treatment techniques were documented. All cases returned post operatively to pre‐injury levels of work. Conclusion: Tearing of the radial head of the DDF is a clinical entity with consistent diagnostic features. Tenoscopic removal of the torn tissue is associated with a good outcome. Potential relevance: Clinicians evaluating lame horses should include tearing of the radial head of the DDF as a differential diagnosis in animals with distended carpal sheaths. Tenoscopic surgery is a recommended treatment.     

A collagenase gel/physical defect model for controlled induction of superficial digital flexor tendonitis

Reasons for performing study: A consistent and clinically relevant model for the induction of core lesions confined to the mid‐metacarpal superficial digital flexor tendon (SDFT) has not been previously reported. Injection of bacterial collagenase is commonly used but often results in large, irregular and inconsistent lesions that disrupt the superficial tendon layers and epitenon. Objective: To develop and evaluate a new injection technique for collagenase induction of SDFT injury. Methods: Collagenase gel was injected into a physical columnar defect created by longitudinally placing a curved 16 gauge 8.89 cm needle in the mid‐metacarpal SDFT in a randomly selected forelimb of 10 horses. A placebo treatment injection was performed 1 week later. Serial ultrasound examinations were performed. Horses were subjected to euthanasia at 2 (n = 2), 4 (n = 2), 8 (n = 4) and 16 (n = 2) weeks post treatment injection. Post mortem magnetic resonance imaging and histological analysis were performed. Gene expression (18S, SCX, TNC, TNMD, COL1A1, COL3A1, COMP, DCN, MMP1, MMP3 and MMP13), total DNA, glycosaminoglycan and collagen content were determined for experimental tendons (n = 10) and unaffected tendons (n = 9). Results: Mid‐metacarpal SDFT core lesion induction was successful in all tendons with consistent lesion cross‐sectional area and minimal epitenon disruption. Histology confirmed loss of normal tendon architecture after tendonitis induction and subsequent healing of the tendon core lesion. Compared with gene expression in unaffected tendons, several tested genes were significantly upregulated (COL1A1, COL3A1, TNMD, SCX, TNC, MMP13), while others showed significant downregulation (COMP, DCN, and MMP3). Conclusion: Compared with the previously used direct injection of collagenase, this injection technique was easily performed and induced more consistent lesions that were mid‐metacarpal and did not disrupt the epitenon. Potential relevance: This model will allow for objective assessment of therapies for tendon regeneration in the mid‐metacarpal SDFT prior to clinical trials and routine clinical application.     

Ultrasonographic diagnosis of injuries to the deep digital flexor tendon and associated structures in the equine foot (suprasesamoidean area)

Foot pain is the most common cause of lameness in horses. In sport horses, podotrochlear syndrome (‘navicular syndrome’) is reported to be the most frequent condition affecting the front foot. Ultrasonography has the potential to detect damage to the soft tissues as well as the bone surfaces; in some clinics it has become the technique of choice for the identification and documentation of many podotrochlear injuries. The purpose of this paper is to review the main pathological conditions of the proximal part of the podotrochlear apparatus (PTA) that can be diagnosed ultrasonographically, focusing on the deep digital flexor tendon (DDFT), podotrochlear bursa (PTB) and distal digital annular ligament (DDAL). Potentially significant ultrasonographic findings of the DDFT include thickening of one or both lobes, longitudinal tears, focal or diffuse changes in echogenicity, irregularities of the dorsal border and adhesions between the DDFT and the proximal sesamoidean ligament and/or distal sesamoid bone. Deep digital flexor tendon injuries are often associated with concurrent lesions of the PTB (acute to chronic bursitis) and of the DDAL (desmopathy). Both feet should be routinely examined as lesions of the PTA are often bilateral. We currently consider that ultrasonography should be routinely employed as the primary diagnostic procedure to complement radiography of the equine foot.     

Risk factors for superficial digital flexor tendinopathy in Thoroughbred racehorses in hurdle starts in the UK (2001-2009)

Reasons for performing study: Superficial digital flexor (SDF) tendinopathy is a common injury in Thoroughbred horses racing over hurdles. Identification of risk factors may allow the introduction of measures to minimise the incidence of this injury. Objective: To identify risk factors for SDF tendinopathy in Thoroughbred horses in hurdle races in the UK. Methods: Potential risk factors for SDF tendinopathy were studied between 1 January 2001 and 31 December 2009 using a retrospective cohort study design with 1031 case starts and 168,637 control starts. Potential risk factors were screened using univariable logistic regression prior to multivariable model building. Results: In the final multivariable model, 20 statistically significant risk factors were identified. Variables found to be associated with increased odds of SDF tendinopathy included: firmer going; increased horse age at first race; having had a previous SDFT injury; and racing in the summer compared to other seasons. Variables found to be associated with decreased odds included: being trained by a more successful trainer; and having raced more frequently in the preceding 9–12 months. Conclusions: This study identified multiple risk factors associated with SDF tendinopathy, some of which are difficult to explain with the current data source. Factors resulting in increased cumulative horse and tendon fatigue (increased years in racing; carrying increased weight; and racing over longer distances), firm ground, and the presence of previous tendon injury should be considered when attempting to reduce the likelihood of sustaining SDF tendinopathy. Potential relevance: The results will facilitate the development of strategies to improve the overall safety of horses in hurdle racing in the UK.     

Deep digital flexor tendon rupture in two horses: A potential complication of comminuted second phalangeal fractures

This case report describes the history, clinical findings, treatment and outcome of 2 horses with damage and eventual rupture of the deep digital flexor tendon as a result of a comminuted second phalangeal fracture. The rupture of the deep digital flexor tendon complicated the treatment and in one horse contributed to its eventual euthanasia. The athletic outcome was limited in the second case and the lack of palmar soft tissue support likely had a significant effect on the stability and biomechanics of the repair.