Precision of surface measurements for below-knee residua

To determine the absolute and relative precision of geometric measurements made of below knee (BK) residua and their BK plaster positive casts using calipers, electromagnetic digitizer, optical surface scanner (OSS), and spiral x-ray computed tomography (SXCT).

The experimental measurement protocol for a single measurement session was as follows: Dot markers were placed on the residuum, and volume and distances were measured using water displacement and calipers; residuum was measured using electromagnetic digitizer; residuum was scanned using three-dimensional (3D) OSS; a negative plaster cast of subject's residuum was made; the residuum was scanned using SXCT scanner. These steps were repeated at a second measurement session. Plaster positive casts were constructed and subsequently measured using the same protocol.

Thirteen adult below-knee amputee volunteers (subjects) participated in the study, and nine subjects returned for a second measurement session. The study group consisted of 9 men and 4 women; 10 Caucasians and 3 African Americans.

Distance measurements for all measurement devices were repeatable within 1% in vivo and within 0.5% on plaster casts; and volumes were within 1% in vivo and within 0.1% on plaster casts. Distance measurements for each device were precise within 3% in vivo and within 1% on plaster casts; and volumes were within 5% in vivo and within 6% on plaster casts when compared with caliper and water displacement measures.

These measurement systems were found to be substantially equivalent in terms of repeatability and precision for measurement of lower extremity residua.