Os sesamoideum proximale mediale
The proximal sesamoid bones are a pair of small, triangular bones located at the palmar (forelimb) or plantar (hindlimb) aspect of the fetlock joint. They act as a pulley for the superficial and deep digital flexor tendons and provide attachment points for the suspensory ligament and the distal sesamoidean ligaments. These bones experience high tensile forces during locomotion, especially during high-speed exercise when the fetlock hyperextends.
Proximal sesamoid bone fractures and sesamoiditis are important causes of fetlock region lameness, particularly in racehorses. Fracture types include apical, midbody, basilar, and abaxial. Biaxial fractures that disrupt the suspensory apparatus are catastrophic. Sesamoiditis involves inflammation and remodeling, often associated with suspensory ligament traction. Diagnosis uses clinical examination, nerve blocks, and radiographs; advanced imaging is sometimes needed. Prognosis depends on fracture configuration and associated soft-tissue damage.
## Overview Sesamoiditis is chronic inflammation and remodeling of the proximal sesamoid bones from repetitive loading and suspensory ligament traction. It is common in young racehorses and sport hor
## Overview Suspensory branch desmitis is inflammation or tearing of one of the branches of the suspensory ligament just above the fetlock. It is common in racehorses and eventers. Signs include acut
## Overview Fractures of the proximal sesamoid bones range from small apical or basilar chips to catastrophic biaxial fractures in which both sesamoids break and the fetlock collapses. Chip fractures
Acute lameness with fetlock swelling or heat after fast work needs immediate evaluation. Chronic low-grade fetlock lameness should also be investigated. Protective boots can help reduce risk in some disciplines.
## Overview Sesamoiditis is chronic inflammation and remodeling of the proximal sesamoid bones from repetitive loading and suspensory ligament traction. It is common in young racehorses and sport horses. Signs include low-grade forelimb lameness, pain on palpation of the sesamoids, a positive flexion test, and enlarged, remodeled sesamoids on radiographs. ## Causes Repetitive strain at the suspensory insertion. ## Diagnosis Radiographs showing sesamoid changes, correlated with clinical signs. Ultrasound or scintigraphy may add information. ## Treatment Rest, anti-inflammatories, and gradual return to work. Shock wave or regenerative therapies are sometimes used. Prognosis for high-level performance is often guarded. ## Recovery and Prognosis Guarded for return to previous athletic level in many cases. ## Prevention Careful training of young horses and early management of suspensory soreness. Signs include: chronic low-grade forelimb lameness; pain on sesamoid palpation; positive flexion test; enlarged remodeled sesamoids on radiographs
## Overview Suspensory branch desmitis is inflammation or tearing of one of the branches of the suspensory ligament just above the fetlock. It is common in racehorses and eventers. Signs include acute lameness, swelling, heat, and pain over the affected branch. Ultrasound shows fiber disruption. Foot imbalance is a frequent predisposing factor. ## Causes Overload, often with foot imbalance. ## Diagnosis Palpation and ultrasound. ## Treatment Rest, foot balancing, and sometimes shock wave or regenerative therapy. Recovery takes months. ## Recovery and Prognosis Fair to good. Re-injury risk is higher with marked fibrosis or fetlock hyperextension. ## Prevention Correct foot balance and progressive training. Signs include: acute lameness; swelling, heat, and pain over the branch just above the fetlock; ultrasound fiber disruption
## Overview Fractures of the proximal sesamoid bones range from small apical or basilar chips to catastrophic biaxial fractures in which both sesamoids break and the fetlock collapses. Chip fractures cause lameness and effusion and are often removed arthroscopically. Biaxial fractures are usually fatal on the racetrack. ## Causes Hyperextension injury during high-speed work. ## Diagnosis Radiographs. The clinical appearance of a dropped fetlock indicates severe disruption. ## Treatment Arthroscopic removal for chips. Surgical repair or euthanasia for more severe fractures depending on configuration. Biaxial fractures are almost always fatal. ## Recovery and Prognosis Good for small chips. Poor to grave for comminuted or biaxial fractures. ## Prevention Monitoring for early suspensory or sesamoid changes in racehorses. Signs include: lameness and fetlock effusion with chips; catastrophic biaxial fracture causes the fetlock to drop to the ground
Proximal sesamoid bone fractures and sesamoiditis are important causes of fetlock region lameness, particularly in racehorses. Fracture types include apical, midbody, basilar, and abaxial. Biaxial fractures that disrupt the suspensory apparatus are catastrophic. Sesamoiditis involves inflammation and remodeling, often associated with suspensory ligament traction. Diagnosis uses clinical examination, nerve blocks, and radiographs; advanced imaging is sometimes needed. Prognosis depends on fracture configuration and associated soft-tissue damage.
Acute lameness with fetlock swelling or heat after fast work needs immediate evaluation. Chronic low-grade fetlock lameness should also be investigated. Protective boots can help reduce risk in some disciplines.
See the Proximal Sesamoid Bone (Left Forelimb, Medial) on our interactive 3D horse model. Open 3D Model