Anatomy and Common Injuries of the Equine Forelimb
Anatomy of the Forelimb[edit | edit source]
An understanding of equine anatomy is essential in veterinary practice to ensure that an accurate diagnosis is made and, therefore, that the correct management option is selected.
Normal Anatomy[edit | edit source]
The forelimb (also known as the thoracic limb) in the horse is adapted for extension and ground covering. This limb carries 55 to 60 percent of the horse’s body weight, and a large proportion of the rider’s weight as well.
Skeletal System[edit | edit source]
The shoulder girdle (scapula, coracoid, clavicle) is greatly reduced in the equine limb. The clavicle is only present as a fibrous strip in the brachiocephalicus muscle.
- Wide, semilunar scapular cartilage
- Palpable tuber on the scapular spine
- No acromion
- Short and thickened
- Has a deep bicipital groove
Radius and ulna:
- Only the radius supports the humerus at the elbow joint
- The ulna is fused to the radius
- Proximally, the ulna reaches to the fifth rib
- The proximal row of carpal bones consists of the radial, intermediate, ulnar and accessory carpal bones (medial to lateral)
- The distal row consists of the first, second, third and fourth carpal bones. The first bone is small and inconsistent
Metacarpal (MC) bones:
- Only MC2, MC3, MC4 are present
- MC2 and MC4 are reduced and are also known as splint bones. They are connected to MC3 by fibrous tissue
- MC3 (also called the “cannon bone”) is well developed and carries all of the horse’s weight
- The proximal phalanx (P1) is the longest of the phalanges
- The middle phalanx (P2) is half the length of PI. It has a large flexor tuberosity to which the superficial digital flexor tendon attaches
- The distal phalanx (P3 or the “coffin bone”) is a spongy bone to which the deep digital flexor tendon attaches
- The proximal and distal sesamoid bones are clinically important. The proximal bones articulate with M3 while the distal bone (i.e. the navicular) lies within the hoof and contacts the middle and distal phalanges
Normal Anatomical Variations[edit | edit source]
There are a number of normal anatomical variations between horses that are visible on radiography. If these features are misinterpreted, inappropriate management may be prescribed, which can have a significant impact on a patient’s outcome. Some examples of normal variations in the equine front limb are:
- Variable conformation in the dorsodistal aspect of the middle phalanx (P2) and extensor process of the distal phalanx (P3). Some horses have smooth-rounded surfaces, whereas others have pointed margins with sharp contours, which can be misdiagnosed as degenerative changes
- Some horses may have a crescent-shaped lucent region in their navicular, which needs to be distinguished from flexor cortical lysis
- Some skeletally mature horses may have an incomplete closure of the ossification centre between the ulna styloid process and distal radius. This can be mistaken for a fracture of the distal radius
- The concave margin on the glenoid notch of the scapula appears as an ovoid lucent region on x-ray, which can be misdiagnosed as a subchondral bone defect or cyst
- Some horses have an incidental variation on the mid dorsal aspect of the medial trochlear ridges of their talus - it can be flattened or focally concave. This needs to be distinguished from osteochondrosis
- The trochlear ridges of the femur of juvenile horses have irregular articular margins, which can be challenging to interpret when there is a suspicion of joint sepsis
Muscles of the Thoracic Limb[edit | edit source]
The main function of the thoracic limb muscles is to provide propulsion and coordinate movements during locomotion. The following table is adapted from research by Payne and colleagues on the extrinsic thoracic limb muscles' role in locomotion in horses.
|Pectoralis transversus||Abduction / adduction, protraction|
|Pectoralis profundus||Abduction / adduction, retraction|
|Serratus ventralis cervicis||Retraction, anti-gravity|
|Serratus ventralis thoracis||Anti-gravity, protraction|
|Brachiocephalicus and omotransversarius||Protraction|
|Subclavius||Abduction / adduction, retraction|
|Trapezius||Abduction / adduction, retraction|
Thoracic limb muscles also support the thorax, largely by two muscular corsets:
- Serratus corset consisting of the cervical and thoracic ventral serratus
- Pectoral corset consisting of the ascending pectoral and subclavius muscles
It is important to note that horses do not have any collateral shoulder ligaments, so their muscles essentially act as collateral ligaments to stabilise the joint.
The key muscles of the arm and elbow joint are:
- Biceps brachii and brachialis for elbow flexion
- NB biceps brachii is also a shoulder extensor
- Triceps brachii for elbow extension
Common Forelimb Injuries[edit | edit source]
Subsolar Abscess (Foot Abscess)[edit | edit source]
Horses who develop subsolar abscesses typically present with acute onset lameness that may be moderate to severe. The horse may also present with concurrent cellulitis in its affected limb. Subsolar abscesses are common in horses of all ages and breeds and there is no specific gender predisposition.
Diagnostic features include:
- Bounding distal pulse
- Warmth in the foot
- Positive hoof tester examination
- Radiographs - these may show gas in the foot, but can also be used to rule out fracture
Typically the abscess is drained to reduce pressure and the foot is bandaged. Treatments may include a poultice, warm bath, and epsom salts. Physiotherapy is mainly focused on reducing associated muscle spasm, usually in the contralateral shoulder and diagonally opposite hind limb.
Solar Bruising[edit | edit source]
Solar bruises are contusions caused by impact, often associated with riding on hard, rocky ground.
Diagnostic and clinical features include:
- Increased digital pulses
- Responsive to hoof testers
- It is also important to check if the horse’s shoes are too small, or if there is a long toe / low heel conformation, flat or thin soled feet
NB solar bruising can resemble laminitis (see below) and may be bilateral (if it is caused by a shoeing problem).
- Eliminating the underlying cause (e.g. shoeing problems, addressing foot imbalances)
- Phenylbutazone (NSAID)
- Use of a caustic agent to harden the sole (iodine, formalin, phenol, keratex)
- In acute cases associated with severe lameness, box rest and cold water immersion may be necessary
Physiotherapy focuses on:
- Managing any associated issues
- Owner education
- Retraining and returning to fitness
[edit | edit source]
The navicular (i.e. the distal sesamoid bone) is a boat-shaped bone that is situated at the palmar aspect of the distal interphalangeal joint.
The navicular bone:
- Acts as a fulcrum to reduce strain on the deep digital flexor tendon (DDFT)
- Provides a gliding surface for the DDFT through the fibrocartilage covered flexor surface
- Withstands a significant compressive force on its distal third from the DDFT - particularly during the propulsion phase of stride
Navicular syndrome is a common cause of forelimb lameness in horses. This lameness is often bilateral. Navicular syndrome is actually an umbrella term that is used to describe a number of conditions that cause palmar foot pain and lameness.
Advances in diagnostic testing (including the use of diagnostic analgesia, biomarkers, biomechanics and diagnostic imaging techniques), have improved our understanding of navicular syndrome. However, identifying the specific dysfunction in the foot remains challenging because of the “packed arrangement” of joints, tendons, ligaments and entheses.
Pathologies that may cause symptoms associated with navicular syndrome include:
- Navicular disease - i.e. alterations in the navicular bone (such as oedema, vascular stasis, subchondral bone changes, changes in the flexor surface etc)
- Desmititis (i.e. inflammation) of the collateral ligaments, impar ligament or distal annular ligament
- Tendonitis of the DDFT (at the insertion, and palmar, or proximal to the navicular bone)
- Synovitis in the distal interphalangeal joint or navicular bursa
- Cystic lesion in the second phalanx
Diagnosis[edit | edit source]
The following tests can be used to help diagnose navicular syndrome: 
- Straight line lameness assessment and lunging - lameness may be exacerbated by hard surfaces
- Small circles on rough, uneven ground (to assess the collateral ligaments)
- Circulation test:
- Horses should show an increase in temperature in their heel region of 1 degree after 15 minutes of exercise
- Distal limb flexion:
- The horse may have a positive flexion test
- Wedge test / frog test:
- The horse stands on a small wedge, board or hoof knife to load its collateral ligaments
- Nerve block with radiographs
- MRI is considered the gold standard method to diagnose navicular syndrome
Treatment[edit | edit source]
Treatment depends on the underlying disorder, but options include:
- Offloading the heel
- Controlled exercise therapy for primary tendon lesions or collateral desmitis in conjunction with:
- Corrective shoeing
- Potentially stem-cell therapy
Specific medical management may include:
- Systemic NSAIDs
- Intrasynovial corticosteroids in the DIP joint or navicular bursa
- Navicular bursa injection with other anti-inflammatories
- Bisphosphonates (NB: the use of bisphosphonates is controversial, as it affects osteoclast activity)
- Navicular bursoscopy for DDFT debridement
- Palmar digital neurectomy (PDN)
- NB: PDN is not recommended for horses with core or linear lesions of the DDFT as they tend to experience ongoing lameness or early recurrent lameness
It is important to remember that with navicular syndrome, the horse should continue to do regular work - this condition does not tend to get better with rest.
Laminitis[edit | edit source]
Laminitis in horses is a complex, multifactorial disease. It is: “a complicated interrelated sequence of inflammatory and vascular events that affect the laminar tissues of the foot and result in varying degrees of breakdown of the interdigitation of the primary and secondary epidermal and dermal laminae of the foot”.
It can be acute or chronic and develops when there is disruption in the blood flow to both the laminae (NB: laminae structures secure the coffin bone to the hoof wall). Inflammation may weaken the laminae and, in severe cases, the bone and hoof wall may separate. If this happens, the coffin bone may rotate and / or “sink” down and eventually penetrate the sole. This results in abnormal hoof growth.
- Gastrointestinal dysfunction
- Placenta retention
- Overfeeding - particularly of grain
- Excessive weight-bearing
- Insulin dysregulation
- Pituitary pars intermedia dysfunction
- Cushing’s syndrome
- Equine metabolic syndrome
- Corticosteroid use
- Severe pain in the feet - may be in front feet only or all feet
- Bounding digital pulses
- Heat (near the coronary band)
- Reluctance to move
- The horse may also stand with a “sawhorse stance” and shift its weight to off-load the painful areas
- Signs of chronic laminitis include:
- Dished foot
- White line widening
- Chronic abscess
- Recurrent lameness
Diagnosis[edit | edit source]
- Patients with laminitis are sensitive to hoof testers
- Positive basisesamoid nerve block
- Radiological changes:
- Most changes appear because of movement of the distal phalanx in the sagittal and / or frontal planes
- This movement can occur at any point around the bone
- Dorsal rotation is the most common movement, followed by symmetrical distal displacement (i.e. sinking)
- Most horses probably have both rotation and sinking, but one tends to predominate
- Most changes appear because of movement of the distal phalanx in the sagittal and / or frontal planes
Treatment[edit | edit source]
Treatment varies based on the presentation - “sinkers” tend to have a poorer prognosis. Medical management focuses on analgesics and antithrombotic agents. It aims to reduce toxins and control mobility. While medical management is the main focus of treatment, physiotherapy can be used to reduce and release spasm. Therapists can also introduce graded weight-bearing protocols based on the veterinarian’s recommendations.
Tendinopathy[edit | edit source]
Tendon disease of the distal portion of the equine limb is challenging to treat. Ageing and exercise are considered important risk factors for tendon injury, but current understanding of the biological processes associated with tendinopathy remains incomplete.
Three tendons that may be affected in the forelimb are:
- Superficial digital flexor tendon (SDFT) - attaches proximally to the short pastern (P2). It flexes the fetlock and pastern, but not the coffin joint
- Deep digital flexor tendon (DDFT) - attaches to the sole surface of the coffin bone (P2). It flexes the whole digit and fetlock joint. Tendinopathy of the DDFT is often associated with persistent or recurrent lameness
- Suspensory ligament - in horses, the interosseous medius muscle has atrophied and functions as a ligament. It consists of muscular bundles, tendinous bundles and fat bundles. The flexor tendons slide over the suspensory ligament. Thus, the suspensory ligament is the primary structure supporting the fetlock
Tendonitis / desmitis has four phases of repair:
During the first 8 weeks after injury, the injured tissues are made up of a fibrovascular mass of immature collagen fibres. The injured tissue’s strength improves threefold between weeks 8 and 12. During this stage, it is important to load the tendon.
It is recognised that inflammation and fibrosis play a key role in healing generally, but while they have been studied in animal models of tendinopathy, knowledge about their role in horses is limited. A review by Dakin notes that preventing chronic inflammation is an important part of tendon repair. Thus, a short course of anti-inflammatory medications may be beneficial in the acute phase, but long-term use should be avoided in horses who develop tendinopathy.
Osteoarthritis[edit | edit source]
Animals with OA present with stiffness or lameness. This lameness may be caused by both joint pain and restricted range of motion.
The aetiology of OA is varied, but trauma and associated synovitis are key causes in young horses. There may also be a history of overuse and other conformational issues that result in abnormal biomechanical forces on the horse’s joints / cartilage.
The inflammatory process associated with OA usually starts in the synovium, cartilage, joint capsule or subchondral bone.
Medications for OA in horses include:
- Phenylbutazone (PBZ)
- Flunixin (Banamine)
- Intra-articular steroids
- Methylprednisolone acetate
- Hyaluronic acid
- Polysulphated glycosaminoglycans (PSGAGs)
- Oral glucosamine / chondroitin sulphate
It has been proposed that nutraceuticals (i.e. a food that is said to provide medical / health benefits) may be beneficial for OA. However, evidence for their efficacy is poor. Physiotherapy and rest are, however, recommended as supporting treatments for OA.
Shoulder Lameness[edit | edit source]
A number of shoulder conditions can cause lameness in horses, including:
- Osteochondritis dissecans
- Subchondral bone cysts
- Septic arthritis
- Soft tissue damage
- Collateral instability of the shoulder joint
However, a study by Dyson from 1986 found that around 40 percent of horses referred for shoulder problems actually had other causes of lameness including: olecranon fracture, biceps brachii strain, sprain of the medial collateral ligament of the elbow, degenerative joint disease, high suspensory desmitis, fetlock joint arthrosis, navicular disease etc.
Heterotopic ossification in a horse’s shoulder may also cause a hopping-type of forelimb lameness. While this condition (also known as myositis ossificans) is well-recognised in human patients, it has not been widely described in horses.
Summary[edit | edit source]
- Having an understanding of equine anatomy is essential to ensure accurate diagnosis and management of conditions of the equine forelimb
- Various conditions can cause lameness in the forelimb of horses. Much of the management for these conditions is medical. Physiotherapy tends to be focused on addressing secondary issues associated with these conditions
- For more information on the physiotherapy assessment and treatment of the equine forelimb, please click here
References[edit | edit source]
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