Tendons - Horse Anatomy

Introduction

Tendons are composed of bundles of collagen, predominantly type I, surrounding parallel rows of fibroblasts known as tenocytes. Tenocytes synthesize the collagen fibres that they surround. Many collagen fibres make up a fascicle. Within the fascicle, collagen fibres are parallel with a ‘crimp’ waveform. This arrangement allows lateral cohesion between fibres, preventing slippage between fibres or fibrils. The ‘crimp’ straightens when the tendon is loaded and then recoils when the load is removed; this allows the elastic function of tendons.

Thoracic Limb

Extensors

Common Digital Extensor Tendon

• Origin: Common digital extensor muscle on the lateral humeral epicondyle
• Insertion: Extensor process of distal phalanx
• Action: Extends carpus and digit

The common digital extensor tendon passes over the dorsolateral aspect of the carpus, continuing distally over the dorsal metacarpus. The branches of the interosseous muscle (suspensory ligament) join the common digital extensor tendon before its insertion on the extensor process of the distal phalanx. A minor branch inserts on the second phalanx and some fibres insert on the hoof cartilages. The common digital extensor tendon is surrounded by the extensor retinaculum and a protective synovial sheath. The tendon sheath begins approximately 10cm proximal to the carpus, extending distally to the level of the metacarpus.

Lateral Digital Extensor Tendon

• Origin: Lateral digital extensor muscle on the lateral humeral epicondyle
• Insertion: Dorsolateral aspect of proximal phalanx
• Action: Extends the carpus and metacarpophalangeal joint

The lateral digital extensor muscle arises from the lateral humeral epicondyle and forms the lateral digital extensor tendon on the lateral aspect of the antebrachium; caudal to the common digital extensor tendon. The tendon is enclosed within a synovial sheath as it extends distally over the metacarpus to insert on the dorsolateral aspect of the proximal phalanx.