An Inexpensive Weight Bearing Indicator Used for Rehabilitation of Patients With Lower Extremity Injuries

[+] Author and Article Information
Daniel F. Walczyk

Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3590walczd@rpi.edu

John P. Bartlet

Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3590

J. Med. Devices 1(1), 38-46 (Aug 15, 2006) (9 pages) doi:10.1115/1.2355690 History: Received April 06, 2006; Revised August 15, 2006

Background . Partial or nonweight bearing is a useful treatment prescribed by medical professionals (e.g., orthopedists, podiatrists) for patients rehabilitating from lower extremity injuries However, there is significant variability in patient conformance with the commonly used, but very imprecise “weight scale method.” Furthermore, the few commercially available load monitoring devices are rarely used because of their high cost. Method of Approach . As a simple and inexpensive alternative to current load monitoring methods and commercial products, a new medical device called a snap dome weight bearing indicator (WBI) has been developed to gently warn a patient when they have exceeded a limited percentage of their body weight on the recovering leg or foot. When installed in a patient’s shoe or medical appliance, the device does so by utilizing the reversible buckling phenomena of a snap dome to provide a tactile and audible feedback when the prescribed weight has been exceeded. To demonstrate the feasibility of this new device, the performance of (1) the snap dome by itself and (2) several WBI designs developed were tested. The most useful of the new designs include one incorporating a heel cup for loose fitting medical appliances and a podiatric off-loading indicator (POLI). In addition, a pilot study and manufacturing cost analysis of the POLI device were performed to investigate patient usability and affordability issues. Results . The particular four-leg snap domes used in device prototypes performed quite well with regards to buckling load consistency between domes, the linearity of buckling load by stacking domes in a parallel arrangement, and buckling load repeatability of a single dome. The performance of each WBI prototype was tested with regards to load transfer, tactile, and audible feedback to the patient, patient comfort, and ease of installation. Prototype performance was generally very good or excellent except for the POLI device, which does not provide sufficient tactile or audible feedback for many patients. A costing analysis of the POLI device suggests that it can be manufactured in the U.S. for around one dollar. Conclusions . The generally positive results from performance testing of commercially available snap domes and WBI prototypes suggest that this new medical device will indeed be an inexpensive, yet effective conformance tool for orthopedists and podiatrists to use in prescribing partial or no weight bearing for a patient.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

(a) Reverse buckling phenomena of a snap dome and (b) an exploded CAD view of one version of the snap dome WBI

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Figure 2

(a) Top view of the four-leg snap dome, (b) stacking snap domes in a parallel arrangement to achieve proportionally higher snap loads, and (c) force sensor used to determine peak load prior to snap dome buckling

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Figure 3

Buckling load versus number of snap domes stacked

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Figure 4

Graph showing the repeatability of the snap dome buckling load over 500cycles

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Figure 5

From left to right, examples of a Cam walker, post-operative sandal, and cast boot

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Figure 6

(a) Simple base and (b) extended base designs for snap dome WBI

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Figure 7

(a) WBI with a large LTP and smaller inset base, and (b) the resulting concept prototype installed in a cast boot

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Figure 8

(a) and (b) Initial design iterations, and (c) final version of the heel cup prototype

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Figure 9

Heel cup LTPs installed in (a) cast boot, (b) post-op sandal, and (c) cam walker

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Figure 10

(a) Top and side views of POLI prototype, and (b) device installed in a post-op sandal

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Figure 11

(a) A picture of the Vernier Force Plate and the LabPro® DAQ device, and (b) example of a time plot of buckling load for a WBI indicator installed in a particular medical appliance

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Figure 12

(a) Sound level meter on a tripod in the anechoic chamber used for audible testing. The cast boot is resting on a piece of wood that has damping foam material underneath it. (b) A scan of the output from the sound level test of the heel cup with four snap domes.




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