Proximal Sesamoid BonesEdit
Proximal sesamoid bones are small but strategically important elements of limb anatomy in many mammals. In horses, they are the paired, rounded bones located just behind the fetlock joint, embedded within the suspensory apparatus and the supporting ligaments that help convert tendon force into efficient forward motion. Although diminutive, these bones play a major role in distributing load, guiding tendon pull, and stabilizing the joint during weight-bearing activities such as galloping or jumping.
Across species, the presence and arrangement of proximal sesamoid bones reflect a common evolutionary solution to redirect mechanical effort through a joint. In humans, for example, there are proximal sesamoid bones at the metatarsophalangeal joint of the big toe, where they function as pulleys for tendon mechanics and contribute to push-off during walking and running. The exact morphology and number of proximal sesamoid bones vary, but in all cases they interact closely with tendons, ligaments, and the surrounding joint surfaces to modulate movement and protect soft tissues from excessive wear. fetlock deep digital flexor tendon proximal sesamoidean ligaments sesamoid bone
Anatomy and Variation
Proximal sesamoid bones are commonly described as two distinct structures, known as the medial and lateral PSBs, which sit behind the joint they support. In the horse, they lie ventral to the fetlock joint and are integral to the function of the suspensory ligament-proximal sesamoidean ligaments complex. The PSBs serve as fulcrums for the pull of the long flexor tendons, helping to redirect force from the limb into propulsion. Their vascular supply and surface features can influence their susceptibility to injury, and variations in size or shape can occur among individuals or breeds.
In humans, the proximal sesamoid bones (often paired under the big toe) contribute to load distribution under the first metatarsophalangeal joint and help protect the plantar tissues during toe-off. The exact orientation and articulation differ from the equine arrangement, but the basic principle—serving as a pulley-like element to optimize tendon mechanics—is shared. first metatarsophalangeal joint sesamoid bone
Biomechanics and Function
The primary biomechanical role of the proximal sesamoid bones is to act as pulleys that increase the mechanical advantage of the associated tendons, most notably the flexor tendons that traverse the palmar or plantar aspect of the limb. By providing a stable point of leverage, the PSBs help convert tendon tension into forward propulsion while reducing shear forces on the joint surfaces. The surrounding ligaments—collectively referred to as the proximal sesamoidean ligaments—stabilize the bones within the joint region, distributing loads during high-speed activities and reducing the risk of soft-tissue damage. proximal sesamoidean ligaments suspensory ligament deep digital flexor tendon
Clinical Significance
In veterinary medicine, proximal sesamoid bones are a focal point for lameness and joint pathology, particularly in high-performance horses. Fractures of the proximal sesamoid bones are among the common causes of acute lameness and can result from racing, jumping, or traumatic overloading. The prognosis for return to prior performance depends on fracture type (simple vs. comminuted), whether the suspensory apparatus is damaged, and the presence of accompanying soft-tissue injuries. Conservative management may be attempted for non-displaced injuries, but isolated or complex fractures frequently require surgical consultation. Other conditions, such as sesamoiditis or osteoarthritis of the PSB region, can produce chronic pain and reduced mobility even without discrete fractures. bone fracture lameness sesamoiditis osteoarthritis
In humans, proximal sesamoid pathology is far less common but can include fractures or stress reactions related to overuse, or degenerative changes in the joint that affect toe-off mechanics. Proper diagnosis often involves imaging studies and a careful assessment of tendon and joint function. osteoarthritis bone fracture
Diagnosis and Imaging
Evaluation typically begins with clinical examination and targeted imaging. In horses, radiographs (X-rays) are standard for identifying fractures of the PSBs and for assessing alignment with the fetlock joint. Advanced imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI), provide more detailed views of fracture geometry, subtle avulsions, and associated soft-tissue injuries to the suspensory ligament complex. Ultrasound can be useful for evaluating surrounding ligaments and tendinous structures. In humans, imaging follows similar principles, with X-ray, ultrasound, or MRI employed to assess suspected fractures or inflammatory changes in the MTP region. bone fracture ultrasound magnetic resonance imaging
Treatment and Management
Management strategies reflect injury severity and species. For minor, non-displaced failures, rest and controlled activity, analgesia, and careful monitoring may suffice. More substantial fractures, or injuries involving the suspensory apparatus or articular surfaces, often require surgical intervention and a multidisciplinary approach involving surgeons, radiologists, and rehabilitation specialists. Therapeutic goals include restoring joint stability, minimizing pain, and re-establishing functional tendon mechanics. Prognosis varies with injury pattern and the effectiveness of rehabilitation. surgery rehabilitation analgesia
In humans, treatment ranges from conservative management with rest and physical therapy to surgical fixation or, in selected cases, removal of proximal sesamoid fragments if pain persists and joint mechanics are compromise. Decisions balance pain relief, functional outcome, and the risks of altered tendon function or joint instability. physical therapy surgical fixation sesamoidectomy