Recently, I had a case where the dog was not weight bearing on their front leg. The dog was holding the front foot off the ground when they were standing. The dog’s forelimb had been x-rayed and the veterinary diagnosis was a soft tissue injury.

On palpation, the dog exhibited sensitivity in the caudal (back) part of the forearm, from the carpal joint to the forepaw, and the palmar (ground facing) aspect of the forepaw.

Often treatments focus on the shoulder, elbow, hip, and stifle joints however the dog’s forearm has complex musculoskeletal anatomy worthy of consideration when a dog presents with forelimb lameness. This part of the forelimb plays an important role in stabilising the dog during stance and movement.

Bony structures of the dog’s carpal joint

The carpal joint in the dog which is roughly comparable to the human wrist comprises a number of joint surfaces including:

• Proximal (antebrachiocarpal) joint extends from the distal (lower) part of the radius, ulna and trochlea to the proximal (upper) part of the proximal (or first row) of carpal bones.

• Middle carpal joint is located between the two rows of carpal bones.

• Distal or carpometacarpal joint is located between carpus and metacarpus.

• Intercarpal joint between carpal bones.

The carpal joint (comprising the above joint surfaces) is a hinge joint. As such it flexes and extends and has limited lateral movement. The greatest amount of movement occurs in the proximal and middle carpal joints.

Muscles of the dog’s front leg

Canine forelimb muscles can be grouped based on their activity during motion.

• Group one includes the muscles of the shoulder (Infraspinate, Triceps) and forearm muscles (Extensor carpi ulnaris and the Flexor carpi ulnaris). This group of muscles are active from touchdown in the gait until the middle of the stance phase. Their purpose is to stabilize the joints during touchdown and compensate for any unevenness in the ground.

• The second muscle group of the forelimb includes the chest muscles (Pectoral), shoulder muscles (Teres Major, Deltoid) and forearm muscles (Radial Carpal Flexor). This group of muscles activates during the second half of the stance phase. Their role is to support the limb as it retracts and prepares for lift off.

• The third and final muscle group comprises shoulder muscles (Ventral Serrated, Supraspinate, Deltoid, Brachial biceps muscles) and forearm muscles (Deep digital flexor). During the stance phase of the dog’s gait, this group of muscles stabilise the joints while the limb is weight bearing.

Nerves of the dog’s front leg

The muscles of the dog’s forelimb are innervated by a network of nerves. Nerves supplying the forelimb originate in the brachial plexus which is located ventrally (underneath) on the branches of the last three cervical vertebrae and thoracic vertebrae 1 and 2.

The radial nerve innervates the brachial tricep muscle which maintains the elbow joint extended to bear the dog’s weight when they are standing. This nerve also innervates extensor muscles of the dog’s forearm which are required to stabilise the dog on touchdown. When the radial nerve is damaged around the elbow joint then the dog will not be able to weight bear normally and they may bear weight on the dorsal (top) side of their forepaw.

Ligaments of the canine forearm

The palmar carpal fibrocartilage comprises many small ligaments. It attaches to the carpal bones and forms the smooth surface of the carpal canal which is embedded within the flexor retinaculum (see below).

The carpal canal encloses the digital flexor tendons, blood vessels and nerves. Within the carpal canal, the tendons are enclosed within a synovial sheath. Synovial tendon sheaths are elongated synovial sacs whose purpose is to reduce friction on tendons as they pass over bony or fibrous surfaces during movement. The outer layer of the synovial tendon sheath is the means by which blood and nerves are supplied to the muscle tendon.

The flexor retinaculum is a modification of the carpal fascia. It attaches to the accessory carpal bone (which projects at the back of the carpal joint). The flexor retinaculum also forms part of the carpal canal.

Muscles in the dog’s forearm

The muscles at the back (caudal aspect) of the dog’s forearm flex the carpal joint and digits (toes) of the dog’s forepaw. These muscles include the following:

Radial carpal flexor muscle

This muscle originates at the medial epicondyle at the inside surface of the humerus. It forms a flat tendon at the radius and extends through the carpal canal. At the metacarpus, the tendon splits and inserts into metacarpal II and III. The radial carpal flexor muscles flexes the carpal joint and is active during the stance phase to stabilise the carpus and to provide forward thrust during the mid to late stance phase.

Superficial digital flexor

This muscle originates at the medial epicondyle at the inside surface of the humerus. It forms a large elliptical shaped tendon and at the metacarpus splits into four to insert into the second to fifth metacarpophalangeal joints. This muscle flexes the metacarpophalangeal and interphalangeal joints of the four principle digits.

Ulnar carpal flexor muscle

This muscle has two muscle bellies: Ulnar head which originates at the ulna and the humeral head which originates on the medial epicondyle. This muscle flexes the forepaw when the forelimb is moving away from the body.

Deep digital flexor muscle

This muscle has three muscle bellies that fuse into one deep digital flexor tendon. The tendon crosses the carpal joint and at the metacarpus it splits into a smaller tendon that inserts into the first digit. Thereafter, the main tendon divides into four and insert into the distal phalanges II – IV.

The anatomy of the dog’s forearm and paw is quite complex so careful observation and palpation is required to plan an effective treatment plan.

Full Stride provides remedial canine massage treatments in Brisbane, Australia.

Until next time, enjoy your dogs.

Sources:

Evans, H & de Lahunta, A, 2013, Miller’s Anatomy of the Dog (4th ed), Elsevier Saunders, Missouri.

Fischer, M.S & Lilje, K. E, 2014 Dogs in motion, 2nd edition, VDH Service GmbH, Dortmund, Germany

Kainer, Robert A & McCracken, Thomas O 2003, Dog Anatomy: A Coloring Atlas, Telon New Media, Wyoming USA.