The Fetlock joint

Learning outcomes

By the end of the podcast, the listener should be able to:

Identify and describe the anatomical structures that form the fetlock joint.

Explain the role of the major soft-tissue structures in stabilising the fetlock and controlling movement.

Relate fetlock anatomy and function to farriery, particularly how hoof balance and loading can influence the structures surrounding the joint.

Definition

The fetlock joint is a synovial, condylar joint located between the distal metacarpus or metatarsus and the proximal phalanx of the horse. It is commonly referred to as the metacarpophalangeal joint in the forelimb and the metatarsophalangeal joint in the hindlimb. The fetlock is a highly mobile and mechanically important joint that plays a major role in locomotion by allowing controlled flexion and extension while also absorbing and storing mechanical energy during movement. Because the joint is subjected to substantial forces, particularly during high-speed exercise, an understanding of its anatomy and biomechanics is important for veterinary professionals and farriers.

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Anatomy

The primary bony structures of the fetlock are the distal end of the third metacarpal or metatarsal bone, the proximal phalanx and the paired proximal sesamoid bones. The distal metacarpal or metatarsal bone has medial and lateral condyles that articulate with the proximal phalanx. The proximal sesamoid bones are positioned palmar or plantar to the joint and form an important part of the suspensory apparatus.

The joint is enclosed within a fibrous joint capsule, with a synovial membrane producing synovial fluid to lubricate and nourish the articular surfaces. The arrangement of the joint surfaces permits predominantly flexion and extension, although small degrees of lateral movement are possible. The fetlock therefore provides both mobility and stability, which are essential requirements for efficient locomotion.

Soft Tissue Structures of the Fetlock

Several important soft-tissue structures contribute to the stability and function of the fetlock. The suspensory ligament is particularly important. It originates from the proximal metacarpus or metatarsus and descends along the palmar or plantar aspect of the limb before dividing into medial and lateral branches. These branches attach to the proximal sesamoid bones and continue distally as the extensor branches, which join the common digital extensor tendon.

The superficial and deep digital flexor tendons also pass behind the fetlock. The superficial digital flexor tendon inserts on the proximal phalanx and middle phalanx, while the deep digital flexor tendon continues distally to insert on the distal phalanx. These tendons contribute to flexion of the digit and help support the fetlock during weight bearing.

The collateral ligaments provide medial and lateral stability to the joint. The distal sesamoidean ligaments also make an important contribution to the suspensory apparatus. These include the straight, oblique and cruciate sesamoidean ligaments. Together with the proximal sesamoid bones and suspensory ligament, they form a sophisticated supporting system that limits excessive extension of the fetlock.

The common digital extensor tendon passes over the dorsal aspect of the joint and contributes to extension of the digit. The joint capsule and associated synovial structures also provide important protection and lubrication.

Function

The fetlock has a fundamental role in the horse's locomotor system. During weight bearing, the joint undergoes considerable extension as the limb accepts the horse's body weight and the forces generated during movement. The suspensory apparatus and associated tendons resist excessive extension, preventing the fetlock from collapsing towards the ground.

The ability of the fetlock to extend and subsequently recoil also contributes to energy storage and release. Elastic structures such as the suspensory ligament and digital flexor tendons can store mechanical energy during loading and release it as the limb leaves the ground. This helps improve locomotor efficiency.

For the farrier, understanding fetlock function is particularly important because the hoof is the foundation of the limb's interaction with the ground. Changes in hoof conformation, hoof balance, breakover and loading can alter the forces transmitted through the distal limb. Consequently, appropriate trimming and shoeing should be considered as part of the wider biomechanical system rather than in isolation from the fetlock and its supporting structures.

Learning questions

Which bones form the primary articulating surfaces of the fetlock joint?

What is the role of the proximal sesamoid bones within the fetlock's supporting apparatus?

How does the suspensory ligament help prevent excessive extension of the fetlock?

What roles do the superficial and deep digital flexor tendons play in fetlock function?

How could changes in hoof balance and breakover potentially influence the forces acting on the fetlock?