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Research Papers

An Inexpensive Weight Bearing Indicator With Load Range Capability for Rehabilitation of Patients With Lower Extremity Injuries

[+] Author and Article Information
Daniel F. Walczyk, William T. Ziomek

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

J. Med. Devices 3(3), 031001 (Aug 28, 2009) (10 pages) doi:10.1115/1.3212557 History: Received March 18, 2008; Revised July 19, 2009; Published August 28, 2009

This paper investigates the mechanical behavior and design of a patent pending device called a load range weight bearing indicator (LWBI), which provides upper and lower range indication to patients with lower extremity injuries as part of a partial weight bearing rehabilitation. The LWBI consists of two opposing stacks (a.k.a. double stack) of snap domes—bistable mechanical elements that snap through only when a threshold weight is applied—sandwiched between a load transfer plate and base plate. The mechanical behavior of a LWBI has been characterized by testing single and double stacks of snap domes in a rigid aluminum fixture using a universal testing machine. Single stacks of two to eight snap domes each exhibited very predictable and repeatable buckling behavior (i.e., stack buckling load is simply the sum of individual snap dome buckling loads) when deflected at speeds typical for patients walking with a regular gait. The double stack configuration only works when supporting legs of the opposing snap dome stacks are offset by half the angle between adjacent legs. The lower load stack buckles first, while the higher load stack buckles at its threshold load because of the very low force required to keep the lower load stack collapsed. While the presence of a spacer has little effect on the double stack buckling behavior under controlled rate deflection in a precision test fixture, it was required for proper functioning of a LWBI prototype probably because of looser dimensional tolerances. The type of substrate that snap dome stacks are in contact with has little effect on the buckling loads as long as the material is not too soft. Finally, the speed of deflection within the expected range of ambulating patients has an insignificant effect on the LWBI’s buckling behavior. A LWBI prototype was designed based on the observed characteristics of the snap dome double stack with a spacer plate between the upper and lower load stacks. The prototype was installed in a recess in the insole of a biomechanical shoe beneath the patient’s heel. The shoe with LWBI was tested by various subjects pushing on a force plate and the upper and lower buckling loads were clearly indicated to the subject by audible and tactile click and measured as ground reaction force versus time. Future work will focus on further testing of the device and refinement of the design for various medical appliances.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Isometric and top and side views of a typical snap dome (44) and (b) side view of a typical WBI configuration with two stacked snap domes in unbuckled (solid lines) and buckled (dashed lines) positions

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Figure 2

(a) Side view schematic of a load range weight bearing indicator with individual components defined and (b) the four possible snap dome stack configurations

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Figure 3

(a) Isometric view of a test fixture used for obtaining load versus displacement measurements of single or double snap dome stacks and (b) transparent top view showing the 45 deg offset stacks

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Figure 4

(a) Instron Model 4444 universal testing machine with (b) close-up of the snap dome test fixture and load cell

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Figure 5

Pictures of (a) the Vernier® force plate and the LABPRO ® DAQ device and (b) a typical cam walker boot used for patients requiring PWB

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Figure 6

Force versus displacement plot for dome No. 5 and run No. 1

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Figure 7

Ground load versus time for a test subject wearing a cam walker boot with a WBI installed walking over the Vernier force plate

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Figure 8

Schematic showing the order of each stack and the letter identifier given to each

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Figure 9

Schematic shown proper orientation and stack ordering for the six double stack arrangements

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Figure 10

Load versus displacement plot for double stack A_D

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Figure 11

Plot of buckling load versus deflection speed for double stack A_D

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Figure 12

(a) CAD model of the top hat LWBI design without snap dome double stack shown and (b) components comprising the actual prototype

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Figure 13

(a) Apex ambulator shoe (right) and (b) top view of installed LWBI prototype

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Figure 14

Example force versus time plots for: (a) subject 1 for lower buckling load and (b) subject No. 4 for higher buckling load

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