A sesamoid bone is a bone embedded within a tendon or muscle. Sesamoids act like pulleys, providing a smooth surface for tendons to slide over, increasing the tendon’s ability to transmit muscular forces. For example, in humans, the patella (kneecap) is the largest sesamoid bone. The proximal sesamoid bones in horses are two small bones located behind the fetlock of each leg. The proximal sesamoids serve the very important function of anchoring the suspensory ligaments and tendons, allowing the hoof and fetlock to move properly. The location and anatomy of the proximal sesamoids make them vulnerable to high impact injuries such as fractures or related soft tissue injuries, both which can be often very difficult to treat or even catastrophic.
Proximal sesamoiditis is a condition that occurs when the proximal sesamoid bones become inflamed and cannot function properly. Affected horses are typically young to middle age and will often present with acute onset of lameness and swelling. Pain and heat are often evident upon palpation and flexion of the fetlock joint. In some cases, the intersesamoidean ligament, a dense ligament that firmly secures the proximal sesamoid bones together along their axial (center) aspect, can also become inflamed and may fray, detach, or rupture.
Research has demonstrated Friesians may have a relatively high incidence of a rare condition in horses called “axial sesamoiditis with intersesamoidean ligament desmitis”. This medical definition basically means the center area where the two proximal sesamoid bones meet, along with the ligament that secures them together, becomes inflamed and diseased. The primary theory for the increase of this disease in Friesians is that increased tendon and ligament elasticity in Friesians may lead to increased fetlock hyperextension and subsequent inflammation of the proximal sesamoids and associated ligaments.
Research conducted by Utrecht University in the Netherlands indicates that axial sesamoiditis with intersesamoidean ligament desmitis in Friesian horses generally carries a poor prognosis. The study followed 12 Friesians with the disease through a variety of treatments such as arthroscopy, tenoscopy, steoclast inhibitors, IRAP, PRP, orthopedic shoeing, radial shock wave therapy, stall rest, corticosteroids and NSAIDs.
However, no single method of treatment appeared to have a better outcome compared to the others. When it comes to treatment, the study indicated a poor prognosis for Friesians, with a return to light riding often being the best achieved result. Axial sesamoiditis with intersesamoidean ligament desmitis should be considered a differential diagnosis of severe hindlimb fetlock lameness in Friesian horses.
Note: Axial sesamoiditis is also sometimes referred to as “osteitis of the proximal sesamoid bones”.
1. Macroscopic view of the proximal sesamoid bones and the intersesamoidean ligament of a diseased and a non-diseased Friesian horse. Note the focal loss of bone at the proximo-axial aspects of the proximal sesamoid bones (open arrows) and concurrent partial rupture of the intersesamoidean ligament (white line marked area).
Photo credit: Utrecht University.
2. Histopathology of the interface of the proximal sesamoid bones and the intersesamoidean ligament a diseased and a non-diseased Friesian horse. Note the multifocal to coalescing inflammation of the intersesamoidean ligament which is characterized by abundant fibroblasts, lymphocytes, and plasma cells in the diseased horse. The transition from ligament to bone (arrow) is very irregular in the diseased horse and compared to the non-diseased horse, there is a decrease of surface area where ligament tissue merges to bone in the diseased horse. Remaining adjacent bone shows increased osteoclastic bone resorption (osteoclasts marked with asterisks). Bone marrow was hypercellular due to invasion of lymphocytes and plasma cells.
Photo credit: Utrecht University.
Brommer, H., Voermans, M., Veraa, S. et al. Axial osteitis of the proximal sesamoid bones and desmitis of the intersesamoidean ligament in the hindlimb of Friesian horses: review of 12 cases (2002-2012) and post-mortem analysis of the bone-ligament interface.
Le Roux C, Carstens A. Axial sesamoiditis in the horse: A review. J S Afr Vet Assoc. 2018;89(0):e1-e8. Published 2018 Mar 29. doi:10.4102/jsava.v89i0.1544
M.E. Verkade, E. Hazeleger, C.H.A. van de Lest, W. Back. Biochemical differences between distal limb extensor and flexor tendons among equine breeds selected for racing and sport. The Veterinary Journal, Volume 262, 2020