Joints and Ligaments - Horse Anatomy

Thoracic Limb

Shoulder Joint

The shoulder joint is the articulation between the glenoid cavity of the scapula and the head of the humerus. In the horse, lateral and medial movements of this joint are impossible due to the shape of the humeral head; movement is therefore limited to flexion and extension. The joint is strengthened by the medial and lateral glenohumeral ligaments. There is also an additional coracohumeral ligament between the supraglenoid tubercle and the greater tubercle of the humerus. In the horse, there is no sheath surrounding the bicipital tendon; instead there is an intertubercular bursa. This bursa lies between the humeral tubercles, cushioning the bicipital tendon, but does not communicate with the cavity of the shoulder joint. The bursa and tendon are held in place by the transverse humeral retinaculum, running between the greater and lesser tubercles of the humerus.

Elbow Joint

The articulation between the humeral condyle, trochlear notch of the ulna and the radial head. The joint capsule attaches to the articular surface of the condyle, the periphery of the olecranon fossa and the articular cartilage of the trochlear notch of the ulna. Ligaments of the elbow include:

• Lateral (radial) collateral ligament: Attaches to the lateral humeral epicondyle, the caudal (ulnar) branch is absent in the horse.
• Medial (ulnar) collateral ligament: Attaches to the medial hymeral epicondyle and inserts on the ulnar and radius. The cranial part of the ligament is the remnant of the pronator teres muscle in horses.

Carpal Joint

The carpal joint is a compound joint composed of:

The antebrachiocarpal joint between the radius/ulna and the proximal carpal bones

The middle carpal joint between the two rows of carpal bones

The carpometacarpal joint between the distal carpal bones and the proximal metacarpals

The joint is a synovial joint, comprising a common outer fibrous capsule and three inner synovial pouches, one for each joint. The fibrous joint capsule is strengthened by the extensor retinaculum on the dorsal aspect and the flexor retinaculum on the palmar aspect. The carpal canal houses both the superficial and deep digital flexor tendon within a common synovial sheath.

Ligaments supporting the carpus include:

• Long lateral collateral ligament: Attaches to the lateral styloid process of the radius, then divides into a superficial branch and two deep branches. The superficial branch attaches on the lateral metacarpal (splint) and the two deep branches insert on the ulnar carpal and 4th carpal bones.
• Long medial collateral ligament: Attaches to the medial styloid process of the radius then runs to insert on the medial metacarpal (splint) bone. A deep branch then detaches to insert on 2nd carpal bone.
• Short ligaments: Join neighbouring carpal bones either within the same row, or in the adjacent row.

Metacarpophalangeal (Fetlock) Joint

The fetlock joint is the articulation between the trochlea of metacarpal III (carpal bone), the proximal phalanx and the proximal sesamoid bones; it allows flexion and extension movements. The joint capsule is divided into a dorsal and palmar pouch. The dorsal pouch extends between metacarpal III (cannon bone) and the common digital extensor tendon. The palmar pouch lies between metacarpal III (cannon bone) and the suspensory ligament. It has an extensive bursa under the common digital extensor tendon, continuing a quarter of the length up metacarpal III. This joint is associated with a pair of proximal sesamoid bones on the palmar surface. Between the sesamoids, in a groove, lies articular cartilage which articulates with the sagittal ridge of metacarpal III. The joint capsule runs between all bones in the joint and attaches to the articular cartilage of the sesamoids.

Ligamentous support:

• Collateral ligaments bind the metacarpal bone to the proximal phalanx, with a deep branch attaching to the sesamoid bone.
• Palmar/intersesamoidean ligament is a mass of fibrocartilage that embeds the sesamoid bones. The palmar aspect of it forms a groove for the deep flexor tendon to run in, allowing frictionless movement.
• Collateral sesamoid ligaments connect the outer aspect of the sesamoids to the proximal phalanx and there is a distinct branch that attaches to metacarpal III.
• Distal sesamoid ligaments are a collection of ligaments that are very marked in the horse and connect the distal surface of the sesamoids to the palmar aspect of the phalanx. These ligaments include the short ligaments, cruciate ligaments, oblique ligament, straight ligament and the palmar annular ligament.
  • The short ligaments connect the axial base of the sesamoids to the lateral/medial aspect on the palmar margin of the proximal phalanx. These are the deepest of the ligaments.
  • The cruciate ligaments are paired ligaments from the base of the sesamoids that diagonally cross to attach to the proximal phalanx. They lie superficial to the short ligaments.
  • The oblique ligament is a triangular shaped ligament, the base of which attaches to the sesamoids and their interconnecting ligament and the point attaches to the rough palmar aspect of the proximal phalanx. It is superficial to the cruciates.
  • The straight ligament proximally attaches like the oblique, but attaches as a flat band to the edge of the complementary fibrocartilage of the proximal end of the middle phalanx.
  • The palmar annular ligament wraps around the sesamoids, flexor tendons and their associated sheaths.

Proximal Interphalangeal (Pastern) Joint

The pastern joint, between the trochlea of the proximal phalanx and the proximal end of the second phalanx, allows flexion and extension movements. The palmar border of the middle phalanx has a complementary fibrocartilage that extends the proximal articular surface of the middle phalanx. It is sometimes called the middle scutum as it provides a gliding surface for the deep digital flexor tendon. The joint capsule is simple and attaches by the articular margins of the two phalanges; blending with the common digital extensor tendon dorsally, the collateral ligaments lateral and medially and the straight sesamoidean ligament on the palmar aspect.

The collateral ligaments connect the distal end of the proximal phalanx to the proximal end of the middle phalanx. They lie in a vertical direction rather than along the bone axis. The fibres of the collateral sesamoid ligaments of the distal interphalangeal joint run with it, attach to the middle phalanx and then continue on to their distal attachments. The palmar ligaments connect the palmar aspect of the distal half of the proximal phalanx to the complementary fibrocartilage of the middle phalanx. These restrict dorsiflexion of the joint. A proximal digital annular ligament wraps around the superficial surface of the flexor tendons and their sheaths at the level of the proximal phalanx. There is a distal one also exists that is attached to the proximal phalanx, it is superficially associated with the digital cushion and is connected to the ergot by the thin fibrous ligament of the ergot.

Distal Interphalangeal (Coffin) Joint

The coffin joint allows extension and slight flexion movements. The navicular bursa lies between the navicular bone and the deep flexor tendon. The joint capsule attaches to the articular periphery. There is a dorsal recess under the extensor tendons and a palmar one that runs to about half the height of the middle phalanx. It dips between the distal phalanx and sesamoid and bulges laterally where it is related to the collateral ligaments. The collateral ligaments connect the distal part of the middle phalanx to the sides of the proximal distal phalanx. The distal sesamoid ligaments connect the distal border of the navicular bone to the flexor surface of the distal phalanx. The collateral sesamoid ligaments are fibroelastic ligaments that attach to the proximal phalanx with the collateral ligaments of the pastern joint and insert to the proximal border of the navicular bone. The ungual/lateral cartilages have a series of ligaments going to the medial/lateral surfaces of the three phalanges and distal sesamoid. There is also a fibrous band connecting the inner surfaces of the cartilages by crossing the superficial aspect of the deep digital flexor tendon.

Pelvic Limb

Sacroiliac Joint

In horses the short branch of the dorsal sacroiliac ligaments connects the sacral tuberosity to the spinous processes of the sacrum. The sacrotuberous ligament is a broad sheet-like ligament, which extends between the transverse processes of the first caudal vertebrae and the dorsal border of ischium and ilium. In this ligament are the lesser and greater ischiatic foramen, these are present to allow for blood vessels, nerves and tendons.

Coxafemoral/Hip Joint

The horse has a limited range of hip movement compared to the dog. This is mainly restricted to flexion and extension and is a result of the conformation of its femoral head, intra-articular ligaments and a large muscle mass around the joint. The ligament of the femoral head extends from the acetabulum to the fovea on the femoral head. Unlike other species, the horse also has an extra ligament present in the joint; the accessory ligament. The accessory ligament arises from the tendon of insertion of the rectus abdominis muscle and to a lesser extent external abdominal oblique muscle as part of the prepubic tendon and inserts on the fovea. Both ligaments pass through acetabular notch.

Stifle Joint

Femorotibial Joint

The femorotibial joint is formed by the femur and tibia. It is divided by menisci into proximal and distal segments. These communicate through open centres of the menisci where the condyles of the femur and tibia are in contact. The synovial membrane of the joint capsule, which is complete only in the horse, further divides the joint into medial and lateral compartments. The menisci are fibrocartilaginous structures that act as shock absorbers, reducing concussion on the joint as well as incongruency of the articular surfaces. Menisci are attached at the outer margins to the fibrous joint capsule and anchored at their ends mainly on the tibia by the cranial and caudal ligaments, but the lateral meniscus has an additional ligament attaching it to the distal femur. The cruciate ligaments in the centre of the joint are important for stifle stability.

Femoropatellar Joint

The femoropatellar joint is formed by the femur and patella. The patella is anchored to the femur by the medial and lateral femoropatellar ligaments and to the tibia by three patellar ligaments:

• Middle Patellar Ligament

The middle patellar ligament connects the the cranial aspect of the patella apex to the tibial tuberosity. It has two associated bursae; one between the ligament and the groove on the tibial tuberosity and one between the proximal part of the ligament and the patella apex. It is palpable just proximal to the tibial plateau.

• Lateral Patellar Ligament

The lateral patellar ligament attaches to the cranio-lateral aspect of the patella and to the lateral aspect of the tibial tuberosity. This arrangement is crucial for the equine 'stay' apparatus and provides a locking ability. This ligament contains tendinous tissue from the biceps femoris and tensor fasciae latae muscles.

• Medial Patellar Ligament

The medial patellar ligament connects the parapatellar fibrocartilage to the medial aspect of the tibial tuberosity. This ligament contains tendinous elements of the sartorius and gracilis muscles.

Contraction of the quadriceps while in rest enables the patella to be hooked over the trochlea of the femur due to the binding loop formed by the patella, middle and medial patellar ligaments. The femeropatellar joint capsule sometimes communicates with the lateral femerotibial cavity and usually with the medial. No communication exists between the femorotibial joints.

Tibiofibular Joint

In the horse, this joint only exists proximally because the distal end of the fibula is fused to the tibia to form the lateral malleolus.

Tarsal Joint (Hock)

The hock has four levels of articulation: tarsocrural joint, proximal intertarsal joint, distal intertarsal joint and tarsometatarsal joint. In the horse, the distal three permit almost no movement (high impact, low motion joint).

Tarsocrural Joint

In the horse, this joint is formed between the tibia and talus only, since the distal end of the fibula is incorporated in the lateral malleolus of the tibia. There is communication with the proximal intertarsal joint.

Proximal Intertarsal Joint

Proximally, there is articulation between the talus and calcaneus. Distally there is articulation between the central and fourth tarsal bones.

Distal Intertarsal Joint

Articulation between the central tarsal bone and the numbered tarsal bones.

Tarsometatarsal Joint

Articulation between the small, numbered tarsal bonesand the metatarsals.

Ligaments of the Tarsus

• Collateral ligaments:
  • Long lateral collateral: Arises from the lateral maleolus of the tibia and terminates on the proximal end of metatarsal IV (lateral splint). Between these points it attaches to the lateral tarsal bones.
  • Short lateral collateral: Deep to the long lateral collateral. It arises from the lateral maleolus of the tibia and attaches via two branches, one to the calcaneus and one to the talus.
  • Long medial collateral: Arises from the medial malleolus and terminates on the proximal end of metatarsal II (medial splint). Between these points it attaches to the medial tarsal bones.
  • Short lateral collateral: Deep to the long medial collateral. It arises from the medial malleolus and attaches via two branches, one on the calcaneus and one on the talus.
• Long plantar ligament: Extends on the plantar aspect of the hock from the calcaneal tuberosity distally to the proximal end of the third and fourth metatarsal bones and also attaches to the central and fourth tarsal bones.

References

• Budras, K. Sack, W.O., Anatomy of the Horse, 6th Edition (2012), Schlutersche Verlagsgesellschaft mbH & Co. KG