Equis ISSN 2398-2977

Accessory carpal: fracture

Contributor(s): Steve Adair, Patrick Colahan, Chris Whitton, Graham Munroe




  • Trauma:
    • Falls particularly after jumps where the horse lands on a flexed forelimb.
    • Hyperextension on landing after jumps.
    • Direct injury from kicks or blows, eg polo mallets.

Predisposing factors


  • Although fractures can occur in any plane of the bone, the most common configuration is in the vertical frontal plane through the lateral groove formed by the long tendon of the ulnaris lateralis muscle.
  • Once the fracture has occurred the constant pull of the flexor muscles on the fracture fragments causes distraction and instability with subsequent fibrocartilaginous non-union.


  • The exact mode by which trauma leads to accessory bone fractures is presently unclear although there are a number of theories.
  • External direct trauma is possible but skin/hair damage in the area is rare in fracture cases.
  • 'Bowstring' effect of ulnaris lateralis, flexor carpi ulnaris and flexor tendons with the leg partially flexed and fully loaded has been suggested.
  • The ‘nutcracker effect’ can occur where the accessory carpal bone is caught between the third metacarpal bone and the radius during a fall on the flexed limb. Contact lesions have been noted on the caudal aspect of the radius in some cases


  • Most cases present as acute onset lameness but some may be presented with chronic lower-grade lameness when there are complications such as carpal sheath constriction or tenosynovitis Carpus: sheath tenosynovitis.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Bonilla A G & Santschi E M (2015) Comminuted fracture of the accessory carpal bone removed via an arthroscopic-assisted arthrotomy. Can Vet J 56 (2), 157-161 PubMed.
  • Lang H & Nixon A J (2015) Arthroscopic removal of discrete palmar carpal osteochondral fragments in horses: 25 cases (1999-2013). J Am Vet Med Assoc 246 (9), 998-1004 PubMed.
  • Wright I M & Minshall G J (2014) Frontal plane fractures of the accessory carpal bone and implications for the carpal sheath of the digital flexor tendons. Equine Vet J 46 (5), 579–584 PubMed.
  • Munroe G A et al (1997) Surgical treatment of a comminuted articular fracture of the accessory carpal bone in a thoroughbred horse. Vet Rec 141 (2), 47-49 PubMed.
  • Rijkenhuizen A B et al (1994) Accessory carpal bone fractures in the horse. Vet Q 16 (Suppl 2), 101-103 PubMed.
  • Barr A R et al (1990) Fractures of the accesssory carpal bone in the horse. Vet Rec 126 (17), 432-434 PubMed.
  • Easley K J et al (1981) Evaluation of a surgical technique for the repair of equine accessory carpal bone fractures. JAVMA 178 (3), 219-223 PubMed.

Other sources of information

  • Ruggles A (2012) Carpus. In: Equine Surgery. 4th edn. Eds: Auer J A & Stick J A. Elsevier Saunders, USA. pp. 1347–1362.
  • Ross M W (2011) The Carpus. In: Diagnosis and Management of Lameness in the Horse. 2nd edn. Eds: Ross M W & Dyson S J. Elsevier Saunders, USA. pp. 426–449.
  • Higgins J L, Spike-Pierce D L & Bramlage L R (2010) Racing prognosis of Thoroughbred yearlings with dorsal proximal accessory carpal bone fragments. In: Proc Am Assoc Equine Pract. pp 402.