Etiology

• Performance horses → repetitive concussive forces → fatigue fracture.
• Stepped on by dam.
• Pasture accident.

Predisposing factors

General

• High speed galloping exercise, such as racing and eventing.

Specific

• Direct trauma in foals.

Pathophysiology

Chip fractures of the proximal dorsal rim

• Most common type of proximal phalangeal fracture.
  •    Hyperextension injury leading to compression of the dorsal proximal portion of P1 against the third metacarpal bone. Repeated trauma in this area leads to changes in the bone prior to fragmentation.
• Racing and other performance horses, especially Thoroughbreds.
• Intra-articular fracture, more commonly of the forelimb metacarpophalangeal joint.
• Most common location is proximal medial eminence of the proximal phalanx; less commonly lateral aspect or both positions.
• Require differentiation from rounded fragments seen in this area, particularly in young Warmblood horses which are part of developmental orthopedic disease and a separate entity. 

Axial osteochondral fragments of the proximal palmar/plantar aspect of the proximal phalanx

• Found commonly in young Standardbreds and Warmbloods, and occasionally in other breeds Proximal phalanx: fracture.
• Disputed etiology but they are thought to be avulsion fractures occurring during skeletal development.
• Commonly identified in horses not started exercise. 
• There may be no lameness, mild lameness or lameness only at high speed. Response to flexion and joint effusion is variable.
• Most commonly found in plantar medial aspect of hindlimb fetlock joints.
• Essential to assess their clinical significance as many are not causing problems. Intra-articualr analgesia Anesthesia: intra-articular is important to localize any lameness.
• If significant, removal by arthroscopic surgery Joint: arthroscopy - overview is very effective.

Major fractures

• Sagittal and sagittal-spiral fractures:
  • Mid-sagittal groove of proximal joint surface propagating distally in the sagittal or axial plane. Many propagate incompletely toward the lateral side of P1, at varying distances distally, some fracture completely through the lateral cortex, and others extend into the pastern joint. Propagation distally may be sagittal or spiral.
  • Usually minimal pre-existing changes in the fetlock cartilage and subchondral bone which improves the chances of a return to soundness after surgery. They are excellent cases for lag screw fixation. .
  • Compressive and torsional forces transmitted from the sagittal ridge of the cannon bone to the groove.
  • Forelimb > hindlimb.
  • Most commonly a racing injury, but occasionally occur at pasture.
• Smaller proximal factures of P1 include:
  • Non-articular fractures of P1 in foals usually caused by the dam. Usually Salter-Harris type 2 fractures.
  • Palmar or plantar wing fractures (young horses).
  • Medial collateral ligament avulsion fractures in young horses such as yearlings; often derived from accidents at grass.
  • Incomplete short sagittal fractures about 1 cm long; in hindlimb subjected to more stress on rising from ground → displacement more likely.
  • Incomplete dorsal plane fractures occur usually in the proximal aspect of P1 in mainly the hindlimb of racing Thoroughbreds, probably due to additional hyperextension of the fetlock joint in these limbs. Often the fracture involves more of the dorsolateral (USA) or dorsomedial (UK) portions of P1 rather than being perfectly symmetrical. 
• Complete dorsal or frontal plane fractures can occur in racehorses and pleasure horses.
• Comminuted fractures:
  • Severely comminuted fractures: release of substantial internal energy → multiple fragmentation of the phalanx.
  • Variable conformation from simple three-piece fractures to 'bag of ice' injury.