Computer-socket manufacturing error: how much before it is clinically apparent?

Abstract

The purpose of this research was to pursue quality standards for computer-manufacturing of prosthetic sockets for people with transtibial limb loss. Thirty-three duplicates of study participants' normally used sockets were fabricated using central fabrication facilities. Socket-manufacturing errors were compared with clinical assessments of socket fit. Of the 33 sockets tested, 23 were deemed clinically to need modification. All 13 sockets with mean radial error (MRE) greater than 0.25 mm were clinically unacceptable, and 11 of those were deemed in need of sizing reduction. Of the remaining 20 sockets, 5 sockets with interquartile range (IQR) greater than 0.40 mm were deemed globally or regionally oversized and in need of modification. Of the remaining 15 sockets, 5 sockets with closed contours of elevated surface normal angle error (SNAE) were deemed clinically to need shape modification at those closed contour locations. The remaining 10 sockets were deemed clinically acceptable and not in need modification. MRE, IQR, and SNAE may serve as effective metrics to characterize quality of computer-manufactured prosthetic sockets, helping facilitate the development of quality standards for the socket manufacturing industry.

Figures

Figure 1

Clinical assessment of test sockets. During evaluation, subject stood bearing weight on fitting stool.

Figure 2

Results from clinical test fitting grouped by (a) subject and (b) by central fabrication facility. *Socket was too large and needed to be reduced. ^Socket was too small and needed to be enlarged. Fab = fabrication facility, Subj = subject.

Figure 3

Mean radial error (MRE) and percentage volume error results. All 33 sockets ranked in order of lowest to highest MRE. Socket volume errors (Vol E) are expressed in percentage volume of subject’s normally used socket with brim trimmed. Fab = fabrication facility, Subj = subject.

Figure 4

Example radial error results. (a) Socket with mean radial error (MRE) > 0.25 mm clinically deemed in need of reduction and possibly shaping change (2/e). (b) Socket with MRE > 0.25 mm clinically deemed in need of enlargement and possibly shaping change (7/a). (c) Socket with MRE ≤ 0.25 mm deemed in need of sizing and possibly shaping change (10/b). x- and y-axes are radial and vertical distances, respectively, in mm. Scale range is −1.0 mm to +1.0 mm.

Figure 5

Mean radial error (MRE) results using different weighting ratios in socket alignment optimization algorithm. MRE results with 1.0:0.0 ratio are compared with results with 0.8:0.2 ratio.

Figure 6

Interquartile range (IQR) results. All 20 test sockets with mean radial error ≤ 0.25 mm are ranked in order of lowest to highest IQR. Fab = fabrication facility, Subj = subject.

Figure 7

Example interquartile range (IQR) results. (a) Socket with mean radial error (MRE) ≤ 0.25 mm and IQR > 0.40 mm clinically deemed in need of sizing and possibly shaping change (3/b). (b) Socket with MRE ≤ 0.25 mm and IQR > 0.40 mm not in need of change (1/f). Image of radial error and histogram of radial error are shown for (a) and (b).

Figure 8

Surface normal angle error results. (a) All 15 sockets with mean radial error ≤ 0.25 mm and interquartile range ≤ 0.40 mm are ranked from lowest to highest mean surface normal angle error (MSNAE). (b) Descriptions of surface normal angle distributions and their link with clinical recommendations. ant = anterior, dist = distal, Fab = fabrication facility, fib = fibular, lat = lateral, med = medial, post = posterior, prox = proximal, Subj = subject, tib = tibial.

Figure 9

Example surface normal angle error (SNAE) results: sockets with mean SNAEs greater than 4.0°. Left two panels show SNAEs (units are degrees), and right two panels show radial error (units are mm). All sockets had mean radial errors ≤ 0.25 mm and interquartile ranges ≤ 0.40 mm. (a) Socket clinically deemed in need of shaping change anterior distally and posterior proximally (8/e). Red circles highlight regions in need of shaping change. (b) Socket clinically deemed in need of shaping change at posterior aspect of fibular head (7/d). Red circles highlight regions in need of shaping change. (c) Socket clinically deemed not in need of modification (3/c).

Figure 10

Example surface normal angle error (SNAE) results: sockets with mean SNAEs less than or equal to 4.0°. Left two panels show SNAEs (units are degrees), and right two panels show radial error (units are mm). All sockets had mean radial errors ≤ 0.25 mm and interquartile ranges ≤ 0.40 mm. (a) Socket clinically deemed not in need of change (5/f). (b) Socket clinically deemed in need of shaping change at tibial tubercle (2/f). Red circle highlights region in need of shaping change.

Figure 11

Stress concentrations for concave and convex socket shaping errors. (a) Concave (pushed-in) socket shaping error focuses stresses over surface of circular pushed-in region. (b) Convex (pulled-out) socket shaping error focuses stresses at circumference of pulled-out region.