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Rectification Maps: A New Method for Describing Residual Limb and Socket Shapes

John A. Sidles, Ph.D.
David A. Boone, B.S., C.P.
Jon S. Harlan, B.S.M.E.
Ernest M. Burgess, M.D.

Introduction

The skill of a prosthetist, and his clinical effectiveness, are in part determined by his ability to manipulate the subtle shape differences between a below-knee residual limb and a prosthetic socket which fits the residual limb. These differences are collectively termed the "rectification."¹

As students, prosthetists learn mainly by apprenticeship, assisted by semi-quantitative "rules-of-thumb." Their skills improve with practice, as they intuitively assimilate their clinical experience. In general, skills are acquired and taught in a largely non- quantitative manner.

The goal of our research is emphatically not to replace these practices, which are time tested and proven successful at training prosthetists. Rather, we hope to supplement traditional methods with more quantitative techniques as suggested by Fernie² and Klasson. ³

To be of clinical use, a quantitative description of a residual limb must be presented in a form which can be readily assimilated. In this paper we present a technique for achieving this goal. A description of the technical details of our computations will be published elsewhere.

Details

Quantitatively comparing three-dimensional shapes is necessarily a challenging task. Large tables of numbers are needed to describe complex shapes, yet such tables cannot be readily assimilated. In this paper we present a graphics-based method.

Figure 1 shows our approach to the problem, illustrated in two-dimensions. The basic idea is simple. The shape of the residual limb and the socket are measured. The two shapes are placed in a specified alignment, which is chosen as one suitable for comparison. For each point of the surface of the residual limb, the distance to the nearest point on the socket is computed. These separation distances are used to generate a "rectification map," which is a three-dimensional picture of the residual limb, with the degree of rectification encoded as a color or pattern change.

Implementing these ideas is rather complicated. In particular, the alignment used to compute the rectification map is convention- dependent. Two reasonable choices are:

1. choose the alignment which minimizes the amount of material added or removed, or
2. choose an alignment which preserves anatomic landmarks known to be invariant (e.g. the tibial crest).

In this paper, we choose the first alternative. Technical details of the algorithms for computing rectification maps will be described elsewhere. In the near future, we hope to add to our software provisions by which a prosthetist can interactively specify the anatomic landmarks which he favors, thus customizing the mapping process to suit his understanding of the rectification process.

Results

Figure 2 and Figure 3 show representative three-dimensional rectification maps. Both show an anterior view of the cast of a below-knee residual limb, and the rectifications shown are those of sockets fitted to the subject. In one case, the socket was fitted using conventional hand-modification techniques (Figure 2) ; in the other case using the University College London (UCL) system for automated design and fabrication of sockets (Figure 3) .^1,4 Both sockets gave a reasonably satisfactory short term fit, though they have not yet been evaluated for long term success. Dark shading shows negative rectification (residual limb shape pushed in to get socket shape), light patterned areas are the oppo site, and white regions are neutral, per the legend.

Inspection of the maps clearly shows that very different rectification strategies have been employed. Relative to the hand-modified socket, the UCL socket shows a broader negative rectification over the patella and patellar tendon, greater relief over the fibula, with the relief extending over a greater area, and additional rectification lateral to the distal tibial crest. Positive rectification over large areas both proximally and distally in the UCL socket may indicate a less intimate fit to the residual limb.

Thus, the rectification map achieves its primary goal: to allow prosthetists to readily and quantitatively assess differences in rectification technique. An equally important factor is ease of access. A high-quality user interface is emerging as one of the most important factors in computer technology. We implemented our software using the Apple Macintosh iconic user interface. Figure 4 shows a typical screen as presented to the user. Multiple windows show multiple views of the same or different maps, in black-and-white or user-specified color, from any of eight different perspectives. A zoom feature allows closer inspection of all or part of a given image. Comments can be entered and stored for each individual image. Printing is supported, and in general the program is a standard Macintosh implementation.

At present, the software is limited to displaying pre-computed rectification maps. Our next software upgrade will allow interactive realignment and recomputation of the maps. The program can be used immediately, with no formal training necessary, provided the user is familiar with the Macintosh conventions.

Discussion

Ironically, current prosthetic practices use the most advanced materials, while teaching, design, and fabrication techniques lag behind in use of technical innovation. Progress in these areas can benefit from careful application of modern computer technologies, with due respect for the subtleties of prosthetic practice.

We anticipate that the rectification map techniques presented here will find application in:

1. teaching: students can study the rectifica tion techniques used by successful prosthetists, or their own attempts,
2. clinical studies: rectification maps can be used to quantify the degree of rectification employed,
3. research: rectification maps can be used to develop a quantitative description of methods of rectification, and
4. fabrication: rectification maps can be used to help develop improved design tools for the automated design and manufacture of mobility aids.

At the present time three bioengineering research centers in this country are conducting a large scale field study of 200 below-knee amputees. Both the UCL and MERU systems are being evaluated. These centers are the Prosthetics Research Study (University of Washington, Seattle); Prosthetics Research Laboratory (Northwestern University, Chicago); Howard A. Rusk Institute of Rehabilitation Medicine (New York University).

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