Metacarpal synostosis

Metacarpal synostosis is the congenital union between two or more adjacent metacarpals. Most occur physiologically as anatomical variants and are often found incidentally, typically seen between the 4^th or 5^th metacarpals and rarely between the 3^rd and 4^th metacarpals ¹.

Epidemiology

The prevalence of metacarpal synostosis is rare, as it is typically an incidental finding on hand x-rays. The prevalence is estimated at ~0.05% (range 0.02-0.07%) ^4,5.

Clinical presentation

Majority of metacarpal synostosis are discovered incidentally and are asymptomatic, however for some people, this may be associated with syndactyly and affect the overall function of the hand ⁴. Clinical evaluation of the hand may demonstrate ulnar deviation of the fifth finger, decreased range of movement, metacarpal hypoplasia or clinodactyly ⁴.

Pathology

Metacarpal synostosis can be either partial or complete, depending on the degree of involvement of the affected metacarpals ^ref.

Etiology

During embryological development of the metacarpals, sporadic inheritance is though to lead to failure of differentiation between adjacent metacarpals and as such synostosis. isolated synostosis causes adjacent metacarpal bones to not differentiate, which are though to be as a result of sporadic inheritance patterns ¹. Both autosomal dominant and X-linked recessive inheritance patterns have been documented ¹.

Associations

• Apert syndrome ¹

• syndactlyl ¹

• cleft hand ¹

• polydactyly ¹

Classification

Numerous classification systems exist for metacarpal synostosis. With the first described by Buck-Gramcko and Wood in 1992 based on the length of synostosis ²:

• type 1: coalition at the base of the metacarpal

• type 2: synostosfis extending up to half the metacarpal length

• type 3: synostosis extending more than half the metacarpal length

Additionally, newer classification systems consider the shape of synostosis, extent of metacarpal hypoplasia, deformity of digits distal to synostosis, involvement of finger webbing, and the growth direction of epiphysis ³.

Treatment and prognosis

Majority of metacarpal synostosis are asymptomatic and do not require treatment. The main surgical treatment is by osteotomy and introduction of a spacer to seperate the metacarpals ⁴. Other surgical treatment options include fixation, tendon transposition, lengthening or ligamentous reconstruction ^ref.

See also

• carpal coalition

• synostosis

• coalition