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A Two-Dimensional Mathematical Model to Simulate the Effects of Knee Center Misalignment in Lower-Limb Orthoses
Date Issued
01-01-2021
Author(s)
Bapat, Ganesh M.
Indian Institute of Technology, Madras
Abstract
Introduction Pistoning is relative sliding motion between an affected limb and its corresponding externally fit assistive device (like an orthosis). Pistoning causes skin problems, pain, and discomfort to the orthosis user. Misalignment of an orthotic joint with respect to the anatomical joint is one of the causes of pistoning motion, and the current knowledge pertaining to the effects of orthotic knee center (OKC) misalignments in lower-limb orthoses is limited. This work quantifies the effects of OKC misalignment in terms of relative motion between the limb and the orthosis and predicts locations of resultant pressure point on the limb. Method A two-dimensional link segment model that simulates relative motion between the limb and orthosis was developed. The OKC was systematically misaligned in the anterior-posterior (A-P), proximal-distal (P-D) directions, and their combination to simulate orthosis/thigh strap sliding and identify pressure points on the thigh. Simulations were performed for stand-to-sit activity and walking with a knee-ankle-foot orthosis. Results It was found that OKC misalignment causes increased A-P and P-D relative motions with an increase in misalignment distance for the stand-to-sit activity. The A-P and P-D relative motions are of greater concern for activities involving increased knee flexion such as sitting, squatting, and kneeling as compared with walking with an orthosis. Although the A-P and P-D relative motions during walking are of small magnitude, they occur with reversals in the direction and are repetitive in nature, which may cause skin problems and discomfort due to recurring pressure points. Conclusions The model provides a means to study the consequences of misalignment and insights for orthosis modification for improved comfort. A software simulation tool based on the presented model can serve as an educational and training tool in prosthetics and orthotics courses for creating awareness about the importance of proper alignment of orthotic knee joints. Study of misalignments of this nature will also guide fabrication and fitting of lower-limb orthoses/exoskeletons.
Volume
33