Geoff Robins, BVetMed(hons) FACVSc
StLucia Surgical Services
StLucia, Queensland, Australia
Traumatic conditions of the sesamoid bones of the metacarpo/metatarso-phalangeal joints, such as fractures and dislocations, are well recognised especially in the racing greyhound. However attention has been focused on degeneration of the sesamoid bones, so called sesamoid disease, as a cause of lameness especially in the larger breeds of domestic dog, by Bennet and Kelly (1985). Another survey from the U.K. revealed a 44% incidence of sesamoid lesions in a consecutive series of 50 Rottweilers admitted for investigation of a variety of orthopaedic problems, but the authors concluded that in no case was the lameness caused by the sesamoid abnormality (Vaughan and France 1986). However a prospective study of 55 young Rottweiler pups revealed a 22% incidence of lameness attributable to sesamoid disease and a 73% incidence of radiographic changes in the sesamoid bones. It was concluded that sesamoid disease will cause clinical lameness, but subclinical disease is common (Read et al 1992). Sesamoid fractures and degenerative disease of the sesamoids are probably part of the same clinical problem.
Clinical Signs: The clinical signs of sesamoid fracture and sesamoid degeneration have many similar features especially in chronic cases and they will, therefore, be considered together. In the acute case there may be a marked lameness with reluctance to bear weight and in chronic cases the foot may be lifted from the ground immediately after the cessation of exercise. The affected joint may be thickened, especially in chronic cases, and the joint capsule may be distended with an increased volume of synovial fluid. Palpation and manipulation of the joint is usually resented, but this can be difficult to interpret because some normal dogs resent manipulation of their toes. Both flexion and extension of the affected metacarpophalangeal joint will elicit pain and sometimes crepitus. Ventral flexion of the joint is often markedly reduced.
In a survey of 25 cases of sesamoid degeneration, clinical and radiographic signs were detected in 21 Rottweilers, two Greyhounds, one Labrador Retriever and one Queensland Cattle dog. The right fore foot had a higher incidence of involvement than the left (30 sesamoids versus 19). Both fore feet were involved in 8 cases and in 16 feet more than one sesamoid was involved The hind feet were involved in three cases The high incidence in Rottweilers in this survey may reflect a predilection for sesamoid problems in this breed. It may also reflect a bias towards the breed, because many of the Rottweilers were presented for the investigation of other orthopaedic problems, such as fragmentation of the medial coronoid process of the ulna (FCP) and hip dysplasia (Robins 1986).
Radiography: Craniocaudal and oblique lateral projections using high definition x-ray film are an important part of the clinical assessment. The radiographic features vary according to the cause and chronicity of the problem. In acute cases of sesamoid fracture the bone may be divided into two or more fragments with sharp, well-defined irregular borders but the fragments may not be displaced. A divided sesamoid bone with smooth regular margins may be a congenitally malformed or bipartite sesamoid. The radiographic features of sesamoid degeneration are quite varied and at times hard to distinguish from sesamoid fractures. In many respects the changes are consistent with the general changes seen in degenerative joint disease with one major difference being the presence of periarticular calcified bodies. The size, number and location of these calcifications vary considerably, ranging from single rounded bodies to linear masses in the surrounding soft-tissues. In some cases the outline of the affected sesamoid bone disappears, being replaced by numerous large calcified bodies. Dorsal or axial displacement of the affected sesamoid has been noted in several cases. Serial radiographs in one case revealed a gradual increase in the size of the calcified bodies and in another union of the small fragments. The distal end of the adjacent metacarpal bone and proximal end of the first phalanges may be enlarged due to the presence of periarticular osteophytes.
Fragmented palmar sesamoids are a common incidental finding on radiographs of normal (sound) dogs, particularly Rottweilers and greyhounds.
Pathology and Pathogenesis: The pathogenesis of sesamoid fractures and degeneration remains unresolved. There does appear to be agreement amongst some authors that repeated stress on the palmar surface of sesamoids by the digital flexor tendons during over-extension of the metacarpophalangeal joints may result in fatigue fractures and/or degeneration of the underlying bone and surrounding soft-tissues. The high incidence of involvement of sesamoids two and seven can be explained by the slight anatomical variation, which exists in the relationship between the digital flexor tendons and the sesamoids in the second and fifth digits. In the central two digits the pressure of the flexor tendons during active extension is spread evenly across the paired sesamoids and the intersesamoidean ligaments.
However, in the second and fifth digits the flexor tendon pressure is borne solely by the flat surface of the inner (axial) sesamoids, number two and seven. This uneven distribution of forces may also be due to the divergence of the metacarpals and flexor tendons of digits two and five (see anatomy). The asymmetry of the distal end of metacarpals two and five also suggests that the axial condyle bears more load.
The possibility of vascular compromise has been suggested because of the similarities to a sesamoid condition seen in horses. The histopathology appearance of 18 affected sesamoids from 11 dogs (10 Rottweilers and 1 greyhound) and 20 normal sesamoids from three normal dogs were examined. In all the affected sesamoids there were areas of necrotic bone with empty osteocyte lacunae and no viable cells in the marrow. The extent of this bone necrosis was inversely correlated with the stage of fracture repair. In six of the sesamoids the entire bone was necrotic, with a marked distinction between the viable, apparently healthy chondrocytes of the articular cartilage and the underlying necrotic bone. In these cases, evidence of attempted fracture repair was restricted to fibrous proliferation in the fibromuscular attachments on the non-articular surface and early signs of cell division in the articular cartilage overlying the fracture.
The extent of bone necrosis seen in this series of affected sesamoid bones is supportive of the hypothesis that vascular compromise is important in the pathogenesis of this condition.
The finding of large areas of proliferating fibrous tissue and cartilage in a cleft in the bone associated with focal proliferation of articular cartilage is probably the basis for the hypothesis that sesamoid disease is associated with osteochondrosis. However, the evidence presented here suggests that these histological changes are simply a stage of fracture healing (Robins and Read 1998).
Treatment: Surgical treatment of sesamoid injuries remains controversial. It is well recognised that many dogs have advanced sesamoid pathology, but are never lame, but numerous reports document cases with chronic lameness and pain which recover once the affected sesamoid bone is removed. Pain and lameness may resolve after a prolonged period of restricted exercise. In one series the lameness resolved without surgery in more than 50% of cases. In those cases that were lame and painful for more than 6 weeks surgical excision of the affected sesamoid bones gave a satisfactory outcome (Read et al 1992).
Long term follow up (mean 3.8 yrs) was obtained from 17 Rottweilers that had undergone excision of one or more palmar metacarpal sesamoids for sesamoid disease. Of forty-six radiographically affected sesamoids involving 21 limbs, 31 had been surgically removed, with 26 of these involving sesamoids 2 and 7. Mean duration of lameness immediately after surgery was 2.7 weeks. Seventeen (86%) of the surgically treated limbs were sound at long term follow up, and four dogs had mild residual lameness. Surgically treated joints were thickened with reduced range of movement but were pain free on manipulation. Radiographs of surgically treated joints revealed minimal osteoarthritis.
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6. Robins GM and Read RA: Diseases of the sesamoid bones. In Bloomberg, Dee and Taylor (ed): Canine Sports Medicine and Surgery. Chapter 29. Saunders, Philadelphia, 1998.
Geoff Robins, BVetMed(hons) FACVSc