METACARPAL SYNOSTOSIS

Metacarpal synostosis is a rare congenital hand malformation usually identified in children, which is characterized by the partial or complete coalescence of ≥2 adjacent metacarpals. The condition is best described as a failure of differentiation that causes this metacarpal union, and it is most common between the ring and little metacarpals, although cases involving other metacarpal pairs have also been described. Metacarpal synostosis may present as an isolated deformity or concomitantly with other abnormalities such as central polydactyly, radial/ulnar deficiencies, cleft hand, or Apert’s syndrome.  The presence or absence of other conditions will influence both symptom presentation and treatment recommendations. Treatment may not be necessary for isolated, mild cases of metacarpal synostosis in which no functional limitations are present, while surgical intervention is more likely needed for more advanced cases in which function and/or cosmesis are negatively affected.^1-3

Pathophysiology

• The metacarpals may be fused into a common mass, or different metacarpals may be fused in the same hand.⁴
• When the ring and little metacarpals are involved, the anomaly occurs bilaterally in 60-80% of cases.⁴
• For these patients, the metacarpal ends are fused proximally, but are often widely separated at the distal end, which prevents the little finger from being brought parallel to the rest of the hand.⁴
• It was originally believed that isolated metacarpal synostosis resulted from a sporadic inheritance pattern that caused a failure of differentiation between adjacent metacarpals; however, recent data suggest that genetic inheritance may play a larger role than previously thought.¹
• Genetic sequencing and splicing has helped to describe an X-linked recessive inheritance pattern, which involves a nonsense mutation in exon 3 of FGF16 on chromosome Xq21.¹
• Other cases have suggested familial inheritance with an autosomal dominant pattern.²
• The synostosis typically involves the ring and little metacarpals, but it has also been identified in the thumb and index metacarpals, as well as the long and ring metacarpals.^1,4
• The metacarpals may be fused into a common mass, or different metacarpals may be fused in the same hand.⁴
• When the ring and little metacarpals are involved, the anomaly occurs bilaterally in 60-80% of cases.⁴
• For these patients, the metacarpal ends are fused proximally, but are often widely separated at the distal end, which prevents the little finger from being brought parallel to the rest of the hand.⁴

Related Anatomy

• Metacarpals
• Phalanges
• Physes
• Epiphysis

Classification Systems

• Despite the rare incidence of metacarpal synostosis, there are several classification systems used to describe the condition.
• One of the earliest systems from Buck-Gramcko and Wood created the following 3 categories based on the extent of the synostosis:
  • Type I: the coalition occurs only at the base of the metacarpal
  • Type II: the synostosis extends up to half the length of the metacarpal
  • Type III: the synostosis extends more than half the length of the metacarpal and often the entire length of the bone⁴
  • This classification system is useful to determine the extent of metacarpal involvement, but has been criticized for not correlating with functional deformity or providing treatment guidelines.²
  • Another system developed by Foucher et al⁵ uses the shape of the synostosis, degree of metacarpal hypoplasia, deformity of the digits distal to the synostosis, web involvement, and growth direction of the epiphysis, as follows:
    • I-shaped: one enlarged metacarpal
      • Id: 2 distinct metacarpal joints
      • If: only 1 fused metacarpal joint
• U-shaped: epiphysis parallel
  • Us: symmetrical length of metacarpals
  • Ua: asymmetrical length of metacarpals
  • Ut: 2 metacarpals fused tightly
• Y-shaped: divergent epiphysis
  • Ys: symmetrical length of metacarpals
  • Ya: asymmetry of the branches
• k-shaped: convergent metacarpals and short little metacarpal⁵
• Although this scheme is considered intuitive, it may fall short of classifying all patients due to the limitation of this kind of pictographic classification system and the diversity of the deformities.³
• In response to the shortcomings of these classification methods, a different system was proposed by Liu et al³ that focused exclusively on synostosis of the ring and little metacarpals based on 2 key pathological features: the ring-little intermetacarpal angle (IMA) and the presence of severe shortening of the little finger.
  • Type A (narrow IMA type): IMA is less than normal, ranging from 0° to normal
  • Type B (wide IMA type): degree of IMA is larger than normal
  • Type C (reverse IMA type): IMA is a negative value
  • Each type includes 2 subtypes—1 and 2—based on the presence of severe shortening of the fifth ray

Incidence and Related Conditions

• Metacarpal synostosis is a very rare condition. Its incidence has been found to range from 0.02-0.07%, but this may be an underestimation because many names have been used to describe the deformity in the past.¹

Differential Diagnosis

• Syndactyly
• Polydactyly
• Apert’s syndrome
• Symbrachydactyly
• Oligodactyly
• Hypoplastic thumb
• Ulnar clubhand
• Clinodactyly
• Brachymetacarpia
• Radial-humeral synostosis
• Cleft hand 
• Congenital dislocated elbow
• Triphalangeal thumb