Joints and Ligaments - Horse Anatomy
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The joint capsule attaches a very short distance from the periphery of the articular surfaces. The intertubercular (bicipital) bursa lies between the humeral tubercles, cushioning the bicipital tendon. The bursa and tendon are held in place by the transverse humeral retinaculum, running between the greater and lesser tubercles of the humerus.
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. It fuses with the collateral ligaments. Paired collateral ligaments attach the epicondyles to the tuberosities of the radius and ulna.
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. Collateral ligaments extend from the radius to the metacarpal bones on the medial and lateral aspect of the carpus. The carpal canal houses both the superficial and deep digital flexor tendon within a common synovial sheath.
Metacarpophalangeal (Fetlock) Joint
The fetlock joint allows flexion and extension movements. This joint is associated with a pair of 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. It has an extensive pouch under the extensor tendon continuing a quarter the length up the cannon bone. The collateral ligaments bind the metacarpal bone to the proximal phalanx, with a deep branch attaching to the sesamoid bone. The palmar/intersesamoidean ligament is a mass of fibrocartilage that embed the sesamoid bones. The palmar aspect of it forms a groove for the deep flexor tendon to run in. The collateral sesamoid ligaments connect the outer aspect of the sesamoids to the proximal phalanx and there is a distinct branch that attaches to the cannon bone. The 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 palmer 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. It lies 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. Finally, the palmar annular ligament wraps around the sesamoids, flexor tendons and their associated sheaths.
Proximal Interphalangeal (Pastern) Joint
The pastern joint 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. The joint capsule is simple and attaches by the articular margins of the two phalanges. 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.
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.
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.
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.
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.