Equis ISSN 2398-2977

SDFT: tendinitis

Synonym(s): Bowed tendon

Contributor(s): Graham Munroe, Vetstream Ltd, Chris Whitton


  • Cause: exercise at speed, fatigue.
  • Acute tendinitis is a medical emergency.
  • Signs: mild to severe lameness (forelimb > hindlimb)   Musculoskeletal: gait evaluation  ; heat, palpable pain, swelling on palmar/plantar aspect of metacarpus or metatarsus (mid to lower third)   Musculoskeletal: physical examination - adult  .
  • Diagnosis: ultrasound   Ultrasonography: flexor tendon  .
  • Treatment: cold hosing, anti-inflammatories, rest and controlled exercise program, medical treatments, surgery.
  • Prognosis: guarded for return to same level of athletic performance.
Print off the Owner factsheet on Tendon injuries to give to your clients.



  • Exercise.
  • Aging.
  • Direct trauma, eg kicking, interference injuries and blunt trauma.
  • Severe direct compression injuries from over-tight bandage or poultice.
  • Incorrect use of physiotherapy techniques, eg ultrasound.
  • Possible genetic factors as yet undetermined.

Predisposing factors

  • Prolonged and/or strenuous exercise at fast gaits over a period of time leads to exercise-induced weakening of extracellular collagen matrix and microtears within the tendon.
  • Mechanical triggers lead to increased stress on the tendon and subsequent tendinitis occurring either at normal or abnormal exercise levels.


  • Known factors include excessive heat generation within the tendon core during exercise, age-related degeneration of tendon substance, and inappropriate training intensity for the individual horse.
  • Postulated factors include variations in the vascular flow within parts of the tendon, hormone-related biochemical changes in the tendon matrix, and imbalance in the synthesis and degradation of extracellular tendon matrix proteins.
  • Specific trigger factors include:
    • The speed at which the horse exercises (much more common in fast gaited and galloping breeds).
    • Training methods.
    • Fatigue, particularly towards the end of exercise.
    • Loss of co-ordination of gait.
    • Ground surface conditions.
    • Jumping exercise.
    • Abnormal shoeing or foot shape, particularly dorsopalmar/plantar and mediolateral foot imbalance   Foot: trimming and balancing  .
    • Weight carriage, either of horse and/or rider.


  • Multifactorial:
    • Cyclical fatigue.
    • Previous injury.
  • Cumulative cyclical fatigue microdamage of tendon matrix and failure of intrinsic repair mechanisms   →   accumulation of damage and fatigue of the tendon. Tendinitis is initiated by maximal loading either during a normal period of exercise or where sudden supramaximal loading occurs due to specific trigger factors.
  • In addition to the tendon matrix alterations, the longitudinally-arranged Type 1 collagen fibers which are normally present in tendons is increasingly replaced by Type 3 which are poorly aligned. These latter fibers have a decreased elasticity and resistance to cyclic strain. Mechanical stretching of the tendon, either at normal or abnormal levels, can then lead to straining or even rupture of collagen fibers.
  • Local direct trauma usually   →    localized lesions with paratendinitis extending to tendinitis in more severe cases.
  • Inflammation is initiated after platelets are released and degranulate within the hemorrhage that occurs from the endotendon capillaries:
    • Cellular invasion (neutrophils, macrophages, monocytes).
    • Increased blood flow   →   edema.
    • Release of proteolytic degradative enzymes   →   removes damaged tissue but also destroys surrounding normal tendon.
    • Pain, heat and swelling.

Intratendinous degenerative changes may occur in horses with little or no evidence of inflammation and tendinitis.

  • Repair phase:
    • Strong angiogenesis response.
    • Fibroblast accumulation (local tenocytes and migratory cells) begins around 4 days after injury, peaks at 3 weeks   →   synthesis of Type III collagen and small diameter disorganized fibrils.
    • Degree of fibroplasia depends on severity of injury and exacerbation of injury by premature exercise.
    • Fibroplasia at periphery of tendon can   →   adhesion formation either with the paratendon tissues or tendon sheath wall   →   reduced function.
    • Resultant 'scar' is weaker and less elastic than original tendon.
  • Remodeling phase:
    • Begins several months after injury and can last for up to 18 months.
    • Collagen transforms from Type III to Type I although fibrils still not arranged in functional longitudinal pattern.
    • Functional loading is required to encourage haphazard scar tissue to mature into Type I collagen with stable cross-links, and therefore controlled exercise should be introduced at an early stage following as short a period of initial rest as the severity of the injury dictates.
    • Some Type III persists and the healed tendon is strong but has reduced elasticity and resistance to cyclic tensile strains.
    • Poorer blood supply in the tendon sheath areas of the tendon may lead to poorer healing in these regions.


  • From injury to repair: 6-24 months.


This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login


This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login


This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login


This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Further Reading


Refereed papers

  • Recent references fromPubMed andVetMedResource.
  • Estrada R Jet al(2014)Comparison of healing in forelimb and hindlimb surgically induced core lesions of the equine superficial digital flexor tendon.Vet Comp Orthop Traumatol27(5), 358-365PubMed.
  • Hu A J & Bramlage L R (2014)Racing performance of Thoroughbreds with superficial digital flexor tendonitis treated with desmotomy of the accessory ligament of the superficial digital flexor tendon: 332 cases (1989-2003).J Am Vet Med Assoc244(12), 1441-148PubMed.
  • Tully L Jet al(2014)Polymorphisms in TNC and COL5A1 genes are associated with risk of superficial digital flexor tendinopathy in National Hunt Thoroughbred racehorses.Equine Vet J46(3), 289-293PubMed.
  • Welsh C Eet al(2014)Estimates of genetic parameters of distal limb fracture and superficial digital flexor tendon injury in UK Thoroughbred racehorses.Vet J200(2), 253-256PubMed.
  • Carvalho Ade Met al(2013)Equine tendonitis therapy using mesenchymal stem cells and platelet concentrates: a randomized controlled trial.Stem Cell Res Ther4(4), 85PubMed.
  • Montgomery L, Elliott S B & Adair H S (2013)Muscle and tendon heating rates with therapeutic ultrasound in horses.Vet Surg42(3), 243-249PubMed.
  • Smith R Ket al(2013)Beneficial effects of autologous bone marrow-derived mesenchymal stem cells in naturally occurring tendinopathy.PLoS One8(9), e75697PubMed.
  • Tipton T E, Ray C S & Hand D R (2013)Superficial digital flexor tendonitis in cutting horses: 19 cases (2007-2011).Am Vet Med Assoc243(8), 1162-1165PubMed.
  • Caniglia C J, Schramme M C & Smith R K (2012)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.Equine Vet J44(5), 587-593PubMed.
  • Godwin E Eet al(2012)Implantation of bone marrow-derived mesenchymal stem cells demonstrates improved outcome in horses with overstrain injury of the superficial digital flexor tendon.Equine Vet J44(1), 25-32PubMed.
  • Marr C M & Bowen I M (2012)Does firing have a valid place in the treatment of superficial digital flexor tendon injury in the 21st century?Equine Vet J44(5), 509-510PubMed.
  • Reardon R Jet al(2012)Risk factors for superficial digital flexor tendinopathy in Thoroughbred racehorses in hurdle starts in the UK (2001-2009).Equine Vet J44(5), 564-569PubMed.
  • Whitlock Det al(2012)Possible role of carpal hyperextension in superficial digital flexor tendinopathy.Equine Vet J44(5), 559-563PubMed.
  • Witte T H, Yeager A E & Nixon A J (2011)Intralesional injection of insulin-like growth factor-I for treatment of superficial digital flexor tendonitis in Thoroughbred racehorses: 40 cases (2000-2004).J Am Vet Med Assoc239(7), 992-997PubMed.
  • Bosch Get al(2010)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.J Orthop Res28(2), 211-217PubMed.
  • O'Meara Bet al(2010)An investigation of the relationship between race performance and superficial digital flexor tendonitis in the Thoroughbred racehorse.Equine Vet J42(4), 322-326PubMed.
  • Thorpe C T, Clegg P D & Birch H L (2010)A review of tendon injury: Why is the equine superficial digital flexor tendon most at risk?Equine Vet J42(2), 174-180PubMed.
  • Chesen A Bet al(2009)Tendinitis of the proximal aspect of the superficial digital flexor tendon in horses: 12 cases (2000-2006).J Am Vet Med Assoc234(11), 1432-1436PubMed.
  • Moraes J Ret al(2009)Effects of glycosaminoglycan polysulphate on the organisation of collagen fibres in experimentally induced tendonitis in horses.Vet Rec165(7), 203-205PubMed.
  • Patterson-Kane J C & Firth E C (2009)The pathobiology of exercise-induced superficial digital flexor tendon injury in Thoroughbred racehorses.Vet J181(2), 79-89PubMed.
  • Dyson S (2007)Superficial digital flexor tendon injuries in teenage and older horses.Equine Vet Educ19(4), 187-188VetMedResource.
  • Pickersgill C H, Marr C M & Reid S W J (2001)Repeatability of diagnostic ultrasonography on the assessment of the equine superficial digital flexor tendon.Equine Vet J33(1), 33-37PubMed.
  • Hogan P M, Bramlage L R (1995)Transection of the accessory ligament of the superficial digital flexor tendon for treatment of tendinitis - long term results in 61 Standardbred racehorses (1985-1992).Equine Vet J27, 221-226PubMed.

Other sources of information

  • Ross M Wet al(2011)Superficial Digital Flexor Tendonitis.In:Diagnosis & Management of Lameness in the Horse. Eds: Ross M W & Dyson S J. Saunders, Missouri. pp 706-726.