Bone spavin, also known as Degenerative Joint Disease (DJD) of the hock (tarsus), is a common cause of equine lameness.

It is caused by repeated concussion and rotational forces on the hock joint, as well as excessive forces on the adjoining ligaments.

As a wear and tear condition, bone spavin is characterized by narrowed joint spaces and bone spurs. [1][2] It is frequently bilateral and accompanied by lameness, with a decreased range of motion. [1]

It is mostly seen in mature equine athletes participating in sports that involve: [1][2][3]

  • A lot of hock flexion, such as dressage
  • Repeated jarring, such as jumping
  • Rotational compressive forces, such as reining
  • Repeated cyclical concussion, such as Thoroughbred or Standardbred racing

Bone Spavin in Horses

Two major factors lead to the development of bone spavin in horses:

  1. Cartilage compression affecting the distal tarsal bones: the central and third tarsals. Known as ‘cuboid’ bones, these are stacked one above the other. With continued, repetitive compression, the cartilage degenerates and becomes crushed, leading to narrowed joint spaces. These then fill with bone.
  2. Uneven loading leads to excessive compression of the cartilage and underlying bone on one side of the hock, with strain on the soft tissue structures located on the other side. This can lead to remodelling and the development of exostoses (bone spurs) which are outgrowths of bone. Pressure on the soft tissue structures can also lead to exostoses. [2]

Bone Spavin in Horses Illustration of Hock Joint
Illustration: Dr. Ana Mesa, Mad Barn Inc. 2022

Conformational Factors

Additionally, sickle hocks and cow hocks are conformational factors that predispose to bone spavin and degenerative joint disease. [1][2][3]

Sickle-hocked horses have excessive angulation in the joints and stand with the hind feet well forwards, which creates excessive pressure down the plantar aspect (back) of the tarsus.

Cow-hocked horses have hocks that angle inwards and feet that angle outwards, leading to added pressure on the medial aspect of the tarsus.

Tarsal Joint Disease

The hock consists of five joints:

  1. The tarsocrural joint (also known as the tibiotarsal) articulates the tibia and the talus.
  2. The centrotarsal joint(proximal intertarsal) articulates the tarsus and central tarsal bone.
  3. The distal intertarsal joint articulates the central and third tarsal bones.
  4. The tarsometatarsal joint articulates the lowest hock bone and the third metatarsal (cannon).
  5. The talocalcaneal joint articulates the talus and the calcaneal bone (which forms the point of hock).

Most of the hocks’ movement occurs in the tarsocrural joint, while the other joints are almost immobile. Synovial effusion is often seen around the tarsocrural joint, if the horse is developing osteoarthritis, but not around the central tarsal and distal intertarsal.

Location-wise, degenerative joint disease mostly involves the lower joints of the hock. Sometimes it affects the proximal joints, particularly the tarsocrural joint. [1]

Types of Tarsal DJD

Different types of degenerative joint disease (DJD) may develop, including:

  1. True bone spavin involves the radiographically visible osteoarthritis of the distal tarsal joints.
  2. Blind or occult spavin is not radiographically visible, although lesions may be present in the articular cartilage (this is a post mortem finding).
  3. Blood spavin indicates a significant swelling that causes distension of the saphenous vein where it runs over the joints.
  4. High spavin is osteoarthritis in the proximal tarsal joints. [1]
  5. In young horses, ‘juvenile spavin‘ is often associated with subchondral bone issues, such as osteochondrosis. [2]

Metabolic bone disease may also contribute to DJD, due to mineral imbalances or deficiencies, protein excess or deficiency.

Endocrine imbalances linked to hypothyroidism or gestational immaturity can also lead to incomplete ossification of the central and third tarsal bones. This can lead to collapse or wedging and subsequent abnormal loading. [1][2][4]

Bone spavin may also be secondary to direct trauma, or damage affecting the hock’s ligaments or other soft tissue structures, with inflammation leading to secondary cartilage damage. [3]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn Equine Nutrition Consultants

Pathophysiology

Bone spavin starts to develop on the dorsomedial (inner, frontal) aspects of the hock joints, progressing primarily to the front. [3]

Early radiographic changes may show cyst formation involving the adjacent subchondral bone. As this progresses, irregular atrophy occurs in the subchondral bone, which makes the joint spaces appear wider.

A local periosteal reaction then occurs. This involves the cells of the periosteum – a membrane covering the bone – slowly forms bone. This progresses at different speeds depending on the individual – in some, ankylosis (bony fusion) between the third and central tarsal can occur within a year. At this point, the lameness is resolved. [1]

With computed tomography imaging (CT scans), it has become apparent that multiple pathologies can be present in different tarsal joints at the same time as radiographically identifiable osteoarthritis, including: [5]

  • Subchondral cyst-like lesions
  • Osteolytic lesions
  • Sclerosis
  • Lesions in the tarsometatarsal interosseous ligament

Changes in subchondral bone thickness have also been identified in horses with hock pain and radiographic changes. [6]

Signs & Symptoms

Horses with bone spavin usually become gradually lame over a period of time, often showing a mild lameness that may shift from one leg to the other.

In mild cases, horses can warm up out of the lameness. However, the lameness may become more consistent over weeks and months, and the horse shows a growing reluctance to work.

The horse may be reluctant to pivot or circle in the direction of the most affected limb, with a tendency to place greater weight on the least affected limb.

The lameness may improve with medication, only to worsen once the drugs are stopped. In severe cases, the horse may get worse as exercise continues.

Lameness generally worsens with work over a few days, then resolves with rest. [1][2][3]

Changes in Gait

The lameness is commonly seen as a ‘hip hike‘. During the cranial phase of the most affected limb, the sounder limb pushes both hips upwards so that the less sound limb can swing forward with reduced joint flexion.

The pain caused by flexion leads to a lower foot flight arc and a shorter cranial phase to the stride, with a tendency to land toe-first. This causes the toe to become worn on the most affected side, or both sides if the condition is bilateral. [1][2]

The foot or shoe may also show more wear on the outside, as the horse is trying to avoid pressure on the medial aspect of the joint. [1]

Sometimes, the horse can be observed lifting its hind feet and flexing its limbs while at rest. [1]

Bone Remodelling

As bone spavin mostly develops in the medial aspect of the hock, this may be where a bony enlargement is seen on the lower hock. This may be harder to identify when the horse is a large-boned type.

With the distal tarsal joints affected, there is not usually any effusion. If bog spavin (swelling of the tibiotarsal joint) is also present, the tarsocrural joint may be involved, or there is a further problem. [2]

Diagnosis

Consult with your veterinarian if your horse is showing signs of lameness. Diagnosis of bone spavin may be based on:

  • Presence of lameness with lowered foot flight, toe wear, and possibly outside wear on hoof or shoe
  • Marked positive response to flexion text
  • Nerve blocks

Visual Assessment

Caution is needed when assessing the signs of lameness, as hock and stifle issues can present similarly.

Enlargem