Introduction

• Cause: rare event, traumatic tearing of collateral ligament, fracture of P1 or P2, loss of suspensory or palmar support structures; may occur in a dorsal, palmar, medial or lateral direction.
• Signs: non-weight-bearing lameness in fore- or hindlimbs Musculoskeletal: gait evaluation; acute - swelling over site of luxation, pain on manipulation Musculoskeletal: manipulative tests, local heat; chronic - firm swelling Musculoskeletal: physical examination - adult.
• Diagnosis: physical examination, radiography Forelimb: radiography.
• Treatment: depends on cause; may require surgical repair; casting → arthrodesis PIP joint: arthrodesis.
• Prognosis: good for light work only.

Pathogenesis

Etiology

• Trauma.

Predisposing factors

Specific

• Previous injury to support structures of distal limb.

Pathophysiology

Complete luxation

• Often associated with external severe trauma:
  • Trapped foot → twisted pulling on limb → tearing of lateral or medial collateral ligament → lateral or medial luxation.
  • Uniaxial fractures of distal P1; fracture of P2 Middle phalanx: fracture.
  • Lacerations damaging collateral ligaments, joint capsule and other supporting structures.
• More likely to be medial or lateral.
• The proximal interphalangeal joint may be open or closed at presentation, with the former decreasing the prognosis.
• Forelimbs may be more likely to be involved.
• Almost always a unilateral problem.

Subluxation

• More likely to be dorsal, palmar or plantar; relates to the position of P1 relative to P2.
• Can involve one or both limbs, fore- or hindlimb.
• Dorsal subluxation:
  • Injury to the soft tissue support structures of the fetlock joint in Thoroughbred racehorses may secondarily develop dorsal subluxation, sometimes called ‘Thoroughbred ringbone’.
  • In association with transection of a branch of the SDFT, severe injury of the oblique sesamoidean ligament, or rupture of the straight sesamoidean ligament.
  • Secondary to flexural deformity of distal interphalangeal joint Musculoskeletal: flexural deformity and possibly after true contracture of the DDFT.
  • Secondary to collateral ligament injury.
  • Suspensory ligament desmotomy +/- deep digital flexor tenotomy in treating severe flexural deformities of the fetlock joint.
  • Secondary to progressive and severe injury of the suspensory ligament and its branches.
  • May occur without any identifiable damage to the bones or soft tissue structures in the pastern or metatarsal region. This is mainly seen in the hindlimbs of young horses and lameness is either mild or absent. The dorsal subluxation is dynamic and resolves during full weight-bearing. If there are any changes within the PIP joint of osteoarthritis Musculoskeletal: osteoarthritis (joint disease) this has a negative effect on prognosis.
• Palmar/plantar subluxation:
  • Overextension of the PIP joint with tearing of palmar/plantar soft tissue support structures, eg jump from height → landing on forelimbs. Possible injuries include complete tearing of the distal sesamoidean ligaments, SDFT branch injury or a combination of soft tissue injuries and fractures.
  • Biaxial palmar/plantar eminence and comminuted fractures of the middle phalanx.
  • Most common in forelimbs and in foals/weanlings.

Timecourse

• Acute.

Diagnosis

Treatment

Outcomes