Etiology

• Stress or fatigue fractures.
• Repetitive high loading of condyle, ie race training leads to subchondral bone remodeling and areas of weakness in the bone in the distal condyle.
• Acute loading of the bone subsequently in training or racing can lead to fracture(s), which propagate for varying distances proximally through the bone.

Predisposing factors

General

• Racing and training at high speed.

Specific

• Thoroughbred racehorses.

Pathophysiology

• Unbalanced loading of limbs during racing on circular tracks → forces concentrated on a condyle → accumulation of concussive injury → fatigue failure of bone → fracture.
• Intense remodeling activity has been observed at the fracture site distally and in Thoroughbred racehorses in training. This is more common in the palmar lateral condyle.
• Scanning electron microscopic studies have confirmed consistently branching microcracks in the thickened compact bone of the palmar/plantar condylar groove of the distal cannon bone in Thoroughbreds in training. Accumulation and coalescence of these microcracks produce a longer and wider dominant defect, which can then propagate into a larger fracture.
• Subchondral bone remodeling can also lead to cartilage and subchondral defects especially in the palmar surface of the condyle, which complicate the repair and worsen the prognosis.
• Acute failure without pre-existing bone remodeling may occur due to unusual loading of the condyles at high speed.
• As these fractures are more common in 2 or 3-year-old horses it is considered that in these horses they may represent failure of adaptation rather than cumulation fatigue.
• Other injuries may occur simultaneously, eg axial sesamoid fracture Proximal sesamoid: fracture, soft tissue structures supporting the metacarpophalangeal joint, eg collateral ligaments and suspensory ligaments.
• Lateral condylar fractures are the most common in the foreleg (approximately 90%); they rarely spiral and may extend a variable distance from the joint surface towards the lateral cortex:
  • Those that extend only 1-2 cm proximally can be difficult to identify.
• Fractures may be unicortical or bicortical, incomplete or complete, non-displaced or displaced:
  • Fractures that extend into the diaphysis of the metacarpus or metatarsal bone are described as propagating and can be either in a sagittal or parasagittal plane, or spiral into oblique or frontal planes.
  • Propagating fractures are more likely to be medial.
  • Comminution occurs only in complete fractures.
  • Fractures usually start adjacent to the sagittal ridge but occasionally they are nearer the abaxial margin. The latter are usually shorter in length.
  • Fracture classification is important for treatment selection and long-term prognostication.
• Bilateral spontaneously occurring fractures and those involving both fore and hindlimb are occasionally recorded.
• Sagittal ridge and medial condylar fractures tend to variably extend proximally into the diaphysis, spiraling or dividing into a Y-shape, thereby increasing the risk of a complete fracture.
• Fractures of the third metatarsal bone are less common (half the incidence of forelimb). Many medial MT3 fractures have a tendency to spiral.
• See Musculoskeletal: fracture.