Bilateral symmetry of the human metacarpal: implications for sample size calculations

The aim of this study was to assess the three-dimensional mechanical symmetry of the human second metacarpal and provide sample size estimates for future mechanical intervention studies of the metacarpal.

Bone densitometry and digital image analysis were used to assess the morphometric, geometric and densitometric symmetry of the second human metacarpal.

An assessment of the left-right mechanical symmetry of the human metacarpal is important in considering the suitability of using the contralateral metacarpal as a control and in providing sample size calculations for future studies involving a mechanical intervention to the metacarpal such as implantation of a metacarpophalangeal prosthesis.

Metaphyseal sectional areas, diaphyseal cortical sectional areas, second moments of area, average periosteal and medullary radii and bone densities were measured at nine transverse levels for each of seven pairs of index metacarpals using computed tomography and bone densitometry. Polar Fourier regression was used to assess the morphometry of sectional periosteal and endosteal boundaries. Differences between clinically important left-right parameters were assessed.

Mean differences between clinically important left-right parameters were small (<3%) and similar to the degree of experimental precision. There were strong significant left-right correlations for the morphometric, geometric and densitometric parameters considered, indicating a high degree of bilateral mechanical symmetry.

The contralateral bone is a suitable control for mechanical intervention studies of the human metacarpal, and the use of bilateral pairing results in an important reduction in sample size.

Responses to mechanical interventions on the human metacarpal, such as implantation of a metacarpophalangeal prosthesis, are generally unknown. The degree of left-right mechanical symmetry in the human metacarpal provides a measure of the advantage of using paired design studies to address these questions.