Intriguing Article on Prosthetic Alignment
Prosthetics & Orthotics International has done it again: the latest issue has a very thought-provoking article from Dr. Childress' group at Northwestern University that just might forever change the way we think about prosthetic alignment. [Hansen AH, Childress DS, Knox EH, Prosthetic foot roll-over shapes with implications for alignment of trans-tibial prostheses. Prosth Orthot Intl 24:205-215, 2000]
This is an extension of previous work from this laboratory in measuring the mechanical performance of various prosthetic foot mechanisms. Using a measuring apparatus of their design, they measured the deflection of commonly prescribed feet at five different orientations representative of the range of sagittal plane motion during walking.
When these data points were connected, each foot had a characteristic "signature" of motion that was similar to the well-known "rocker contours" used by orthotists and pedorthists to modify the roll-over characteristics of footwear to improve the gait of patients with various foot and ankle pathologies. Conceptually, the ankle-foot motion of a particular component could be simulated by a rigid rocker with the same roll-over shape.
This paper then notes that some of the more popular prosthetic feet have a roll-over shape that is very similar to that of the biological ankle-foot during walking. This might help explain why amputees often strongly prefer a particular component but cannot articulate why they like it: it may just "feel right".
Even more interesting, the authors go on the hypothesize that what the prosthetist does during iterative dynamic alignment trials is to position the foot in space so that its characteristic roll-over shape most closely approximates that of the amputee's sound foot.
In other words, it may be that we as prosthetists compensate in our dynamic alignment procedure for shortcomings in various feet, so that the final result is very similar even though the mechanical properties of the feet differ significantly. If so, this might explain the paradox that many gait studies have not found significant differences in the performance of some prosthetic feet even though they have substantially different functional characteristics.
These proposals make very good sense to me, based on my clinical experience, although additional studies will be needed to validate these hypotheses. But, if "optimizing roll-over shapes" is indeed what we do during alignment trials, then it may be possible to predict or measure what the "optimal" shape is for a given amputee. That would allow us to align the prosthesis initially in the optimal attitude, and should reduce both the time spent in walking trials as well as the variation between different practitioner's alignments.
It's exciting to see such a simple, rational explanation suggesting there is an underlying principle guiding what has sometimes been dismissed by non-prosthetists as "just subjective alignment changes". Prosthetists fervently believe that careful alignment is important to the amputee's quality of life; Hansen et al may have just given us the insight we need to validate this cherished assumption! Click here to read the entire article.
