Measurement inaccuracies in X-ray planning of deformity correction using a new hexapod calibration device: an...

Hexapod systems are accurate devices to perform deformity correction. Crucial to achieving the desirable result is the precise determination of the mounting and deformity parameters. Smith + Nephew (London, UK) developed a calibration device for their hexapod system TSF (Taylor Spatial Frame) called Beacon, which is directly mounted to the reference ring. The software indicates the Beacon in X-rays and calculates the whole hexapod construct. However, we still can find inaccuracies after completing the correction plan by the software. Since the software calibrates the construct with the bone in the center of the ring, the question arises as to whether different positions of the bone within the construct influence the calculated parameters.

A tibial artificial bone (Sawbones, Vashon Island, USA) was mounted with the TSF. Iron beads were attached to define reference distances. The test setup included five different positions of the bone. X-rays and CTs (computed tomography) were performed and the distances were measured. The accuracy of the measurements of the X-rays and the CT were compared when having the bone in different positions.

Measurements were dependent on the position of the bone in relation to the center of the ring. If the bone is closer to the detector than the center of the ring, the measured distance is shorter and vice versa. Resultingly, the measured total length of the bone in X-rays varied from the true length between ± 26 mm (7%). In contrast, measurements by CT showed much less variability.

When using X-ray planning, care must be taken if the bone is not centered in the reference ring. Measurement inaccuracies of up to 7-10% may occur. CTs give more accurate measurements regardless of the bone position. These findings should be taken into account if the limb cannot be centered in the reference ring of the hexapod system.