Postero-anterior radiography of the wrist: scapholunate ratios and joint projection shape analysis.

A normal database of qualitative and quantitative data, obtained from standard posteroanterior wrist radiographs has been established which might be useful in clinical practice. Variations in the parameters obtained could provide an explanation for the variable carpal kinematics and multiple concepts of carpal instability described in the literature. Selected landmarks were digitized on 80 posteroanterior wrist radiographs of asymptomatic volunteers. Besides classical parameters, measurement of carpal dimensions and angular relationships were also taken. Two new parameters, the scapholunate ratios, are described which evaluate the relative lengths of the frontal projections of the scaphoid and lunate: these ratios might prove useful in the diagnosis of carpal instability. The shapes of the frontal projections of the radiocarpal, midcarpal and STT joint spaces were also analysed. Average values for parameters already reported in the literature agreed with previous findings. Some parameters, however, displayed important individual variations, the standard deviations ranging from 1 to 3 mm for dimensions and from 3 to 13 degrees for angular measurements. The posteroanterior and anteroposterior scapholunate ratios averaged 1.41 +/- 0.21 and 0.66 +/- 0.13 respectively. The potential clinical benefit of these ratios is discussed. Different types of joint space projection shapes could be determined. With respect to the difficulty of obtaining reliable lateral wrist radiographs, compared to posteroanterior radiographs, it is suggested that the clinical significance of the proposed parameters must be evaluated.