Os tarsi fibulare (Calcaneus)
The calcaneus (also called the fibular tarsal bone) is the largest bone in the hock (tarsus). It projects upward and backward to form the prominent “point of the hock” (tuber calcanei). This projection serves as the major lever arm for the common calcanean (Achilles) tendon, which includes the superficial digital flexor tendon and the gastrocnemius. On its medial side, the sustentaculum tali supports the talus and creates a groove for the deep digital flexor tendon as it passes through the tarsal sheath. Because the point of the hock is relatively unprotected, the calcaneus is vulnerable to direct trauma, most often kicks from other horses. The thin soft-tissue covering means fractures can become open, increasing the risk of infection in the nearby calcaneal bursa or tarsal sheath. The bone’s role in the stay apparatus and propulsion makes injuries here performance-limiting.
Traumatic fractures (especially from kicks) are the most common problem. Severe fractures can displace the superficial digital flexor tendon and involve the sustentaculum tali. Open or comminuted fractures with bursa involvement carry a poor prognosis. Diagnosis is usually straightforward on radiographs; skyline and flexed views help assess the sustentaculum. Treatment ranges from fragment removal (sometimes arthroscopically) to lag-screw or plate fixation. Stress fractures of the calcaneus are less common but reported in racehorses.
## Overview Calcaneal fractures are breaks of the bone that forms the point of the hock. They usually result from external trauma such as a kick. Because the bone has little soft-tissue cover, fractu
## Overview Tarsal sheath effusion, known as thoroughpin, is fluid accumulation in the sheath surrounding the deep digital flexor tendon as it passes through the hock. It appears as a soft swelling o
## Overview Fractures of the sustentaculum tali (the medial shelf of the calcaneus that supports the deep digital flexor tendon) cause lameness and swelling on the medial hock. They can damage the te
## Overview Capped hock is distention of the subcutaneous bursa or formation of an acquired bursa over the point of the hock (tuber calcanei). It results from repetitive trauma such as kicking stall
## Overview Curb is soft-tissue swelling on the plantar distal aspect of the hock. Historically it referred to enlargement of the long plantar ligament, but similar swelling can arise from superficia
Watch for sudden severe lameness after a known or suspected kick, swelling at the point of the hock, or reluctance to bear weight. Any open wound over the point of the hock needs immediate veterinary attention because of the risk of joint or sheath infection. Persistent swelling or heat after a kick warrants radiographs.
## Overview Calcaneal fractures are breaks of the bone that forms the point of the hock. They usually result from external trauma such as a kick. Because the bone has little soft-tissue cover, fractures may be open and can displace the superficial digital flexor tendon or involve the sustentaculum tali and tarsal sheath. Moderate to severe lameness is typical. ## Causes Direct trauma from kicks or impact against fixed objects is the primary cause. High-energy injuries can produce comminuted or open fractures. ## Diagnosis Acute hind-limb lameness after trauma, swelling at the point of the hock, and pain on palpation are suggestive. Standard and skyline radiographs confirm the fracture configuration. Ultrasound evaluates tendon displacement and synovial involvement. ## Treatment Options include fragment removal, lag-screw fixation, or plate fixation depending on configuration. Open or comminuted fractures involving the calcaneal bursa carry a poorer prognosis. Conservative management with casting may succeed for selected incomplete, nondisplaced fractures. ## Recovery and Prognosis Prognosis for athletic function is fair to guarded and depends on fracture type, displacement, and synovial sepsis. Many horses return to pasture soundness; fewer return to high-level performance after complex repairs. ## Prevention Minimizing situations that lead to kicks or falls reduces risk. Signs include: moderate to severe hindlimb lameness; swelling at the point of the hock; pain on palpation; possible open wound; displacement of the superficial digital flexor tendon in severe cases
## Overview Tarsal sheath effusion, known as thoroughpin, is fluid accumulation in the sheath surrounding the deep digital flexor tendon as it passes through the hock. It appears as a soft swelling on both sides of the hock. Many cases are idiopathic and cause no lameness. Others indicate deeper pathology. ## Causes Idiopathic, trauma, or associated tendon or hock disease. ## Diagnosis Clinical appearance and ultrasound to assess the tendon. ## Treatment None if the horse is sound. Address underlying tendon injury if present. ## Recovery and Prognosis Excellent for idiopathic cases. ## Prevention None specific. Signs include: soft swelling on both sides of the hock; may or may not cause lameness
## Overview Fractures of the sustentaculum tali (the medial shelf of the calcaneus that supports the deep digital flexor tendon) cause lameness and swelling on the medial hock. They can damage the tendon sheath and lead to chronic tenosynovitis. Diagnosis is by radiographs or computed tomography. ## Causes Trauma. ## Diagnosis Radiographs, often with special views or computed tomography. ## Treatment Surgical removal of fragments or stabilization when possible. Conservative management for non-displaced fractures. Prognosis is guarded because of tendon-sheath involvement. ## Recovery and Prognosis Guarded. ## Prevention Trauma avoidance. Signs include: medial hock swelling and lameness; radiographic fracture of the sustentaculum tali; possible deep digital flexor tendon sheath effusion
## Overview Capped hock is distention of the subcutaneous bursa or formation of an acquired bursa over the point of the hock (tuber calcanei). It results from repetitive trauma such as kicking stall walls or trailer doors. The swelling is usually cosmetic and not associated with lameness unless infection or deeper synovial involvement occurs. ## Causes Repeated contusion or pressure is the primary cause. A single severe blow can also initiate the swelling. Penetrating wounds may introduce infection. ## Diagnosis The characteristic soft or firm enlargement exactly over the point of the hock is diagnostic. Ultrasound distinguishes simple bursal fluid from abscess or involvement of the calcaneal bursa. ## Treatment Protect the area from further trauma. Cold therapy, bandaging, and anti-inflammatories reduce acute swelling. Chronic fibrous enlargements often persist. Infected bursae require drainage and antibiotics. ## Recovery and Prognosis Prognosis for soundness is excellent when the swelling is purely subcutaneous. Cosmetic blemishes frequently remain. Septic cases have a more guarded outlook. ## Prevention Padding stall walls, using hock boots during transport, and addressing behavioral kicking reduce recurrence. Signs include: soft or firm swelling over the point of the hock; usually no lameness; heat in acute cases; possible draining tract if infected
## Overview Curb is soft-tissue swelling on the plantar distal aspect of the hock. Historically it referred to enlargement of the long plantar ligament, but similar swelling can arise from superficial or deep digital flexor tendinitis, periligamentous inflammation, or a combination. It is seen most often in racehorses, particularly Standardbreds, and may or may not cause lameness. ## Causes Conformational faults (sickle or curby hocks), trauma, or overload during training. ## Diagnosis Visible and palpable thickening distal to the point of the hock. Ultrasound identifies the specific structures involved. ## Treatment Rest, anti-inflammatory medication, and controlled exercise. Corrective shoeing addresses conformational contributors. Most cases resolve without long-term effects. ## Recovery and Prognosis Good. Recovery typically takes weeks to a few months. ## Prevention Attention to conformation and gradual conditioning. Signs include: firm swelling on the back of the distal hock; variable heat and lameness; thickening of plantar soft tissues
Traumatic fractures (especially from kicks) are the most common problem. Severe fractures can displace the superficial digital flexor tendon and involve the sustentaculum tali. Open or comminuted fractures with bursa involvement carry a poor prognosis. Diagnosis is usually straightforward on radiographs; skyline and flexed views help assess the sustentaculum. Treatment ranges from fragment removal (sometimes arthroscopically) to lag-screw or plate fixation. Stress fractures of the calcaneus are less common but reported in racehorses.
Watch for sudden severe lameness after a known or suspected kick, swelling at the point of the hock, or reluctance to bear weight. Any open wound over the point of the hock needs immediate veterinary attention because of the risk of joint or sheath infection. Persistent swelling or heat after a kick warrants radiographs.
See the Calcaneus / Fibular Tarsal Bone (Left) on our interactive 3D horse model. Open 3D Model