Determination of Static Stability of Manual Prone Carts

Pascal Malassign, MID, Clement J. Zablocki VAMC, Milwaukee WI. Thomas L. Amerson, Ph.D., Naval Submarine Medical Research Laboratory, Groton, CT. Mark Cors, Medical College of Wisconsin, Milwaukee, WI. Audrey L. Nelson, RN, Ph.D., James A. Haley Veterans' Hospital, Tampa, FL.

ABSTRACT

Because commercially available prone carts have been found by SCI users to have several deficiencies, static stability evaluation of four (4) manual prone carts was performed according to the procedure outlined in ANSI/RESNA WC-01, 1990, to determine if static stability was a factor.

Two commercially available model were evaluated, the E&J and Gendron, as well two newly designed prone carts, the SCI PC 22 and the Sammy LS. The results indicated that the prone carts differed with respect to their overall tippiness due to their dimensions and overall height. The type of load, 75 or 100 kg with humans, was also a factor of significance. The study demonstrated that all four prone carts were quite resistant to tipping and that the angles at which they tipped were far beyond those expected in normal use.

BACKGROUND

Persons with spinal cord injuries are at high risk for developing pressure ulcers because of decreased mobility, sensory deficits and injuries due to transfers and other factors. Because SCI patients sit in wheelchairs for extended periods of time, the sacral and ischial areas are the most common sites for pressure ulcers.

The healing process takes several months. During that time, the SCI patient must lie on the side or in a prone position. Either of these positions precludes the use of a wheelchair. The prone cart provides a mechanism for mobility without jeopardizing the healing process.

PROBLEM STATEMENT

The prone cart provides a valuable alternative to prolonged bedrest; however, users have found the design of the commercially available: E&J and Gendron carts to have several deficiencies such as inadequate body support for comfort and safe positioning. These models are also difficult to use for maneuvering, braking and for completing activities of daily living (ADL).

RESEARCH QUESTION

The authors hypothesized that the deficiencies found by users on the commercially available prone carts may be related to their design and measurements. As a result the study to determine static stability was conducted.

L to R: SAMMY LS, E&J, Gendron & SCI-PC 22

METHOD

Testing equipment: A rectangular testing platform (120 x 160 cm) incorporated an electric motor that adjusted the inclination up to 30 degrees. A 4 cm wooden bar was used to chock the wheels in performing the static stability evaluation. A protractor was used to record the progression of the raised platform' angles.

Performing the test: For purpose of comparison between the prone carts, the two height adjustable models were set at their highest setting, E&J at 77 cm and Gendron at 80 cm off the floor. The other two prone carts were not height adjustable. The height of the SCI-PC 22 is set at 89 cm and the SAMMY LS is set at 85 cm off the floor.

Static tipping angle rearwards, forwards and sidewards was performed with two able-bodied individuals of 100 and 75 kg respectively. Finally, the tipping angle was measured when a piece of paper could pass under the un-chocked wheels without turning them.

RESULTS

The static stability test results for each of the prone carts are presented in Table 1. Table 1.

Sidewards evaluation with the Gendron Forwards evaluation with the SCI-PC

DISCUSSION

The test for stability compared the performance of two commercially available manual prone carts, the E&J and Gendron, and two prone carts designed by the authors, the Sammy LS and the SCI-PC 22. The prone carts were tested at their highest height setting. The static tip angle was assessed for each prone cart under 6 different test configurations: Load type (75 kg and 100 kg) and Tip direction (Rearwards, Forwards, and Sideways). Since the intent was to distinguish differences in overall prone cart performance, a repeated measures ANOVA was applied to the data with the test configurations grouped by Load type. The prone carts (E&J, Gendron, Sammy LS, and SCI-PC 22) served as the repeated measures. There were no significant differences found in Load type, Prone cart type, or interactions. Given the relatively small data set used, however; the basic assumptions and requirements for an ANOVA procedure can be questioned.

The sample size was small and the distribution of the static tipping angles appeared not to support a parametric analysis; so a non-parametric statistical test was justified. The appropriate test for this condition is the Friedman two way layout (Hollander & Wolfe, 1973; Siegel, 1956). By considering the static tipping angles obtained for all 6 testing configurations for the four prone carts, an overall ranking of the "tippiness" of the prone carts was obtained.

From most stable to least stable prone cart, the data indicate the following order:

SCI-PC 22, GENDRON, SAMMY LS, E&J

CONCLUSIONS

This study demonstrated that all four prone carts were quite resistant to tipping and that the angles at which they tipped (greater than 15 degrees) were far beyond those expected in normal use (the angle of a ramp for wheelchairs is 4degrees).

Finally the authors have concluded that the deficiencies identified by SCI users regarding commercially available prone carts are related to other aspects of their design.

Testing platform

Sideward testing with the Sammy LS

REFERENCES

ANSI/RESNA Wheelchair Standards, 1990, Washington, D.C.,: RESNA PRESS.

Hollander, M., & Wolfe, D.A. (1973). Nonparametric Statistical Methods. New York: John Wiley & Sons.

Siegel, S. (1956). Nonparametric Statistics. New York: McGraw-Hill.

MalassignÆ, P., Nelson, A., Amerson, T., Determination of static stability of Bowel care shower chairs, Proceedings RESNA 94, pp. 318-320.

ACKNOWLEDGEMENTS

Support for this research was provided by the VA Rehabilitation R&D Service as part of the study # B92-522 entitled: "Toward the design of a new prone cart for SCI veterans, Pilot Study". In addition the authors are thankful to Ortho-Kinetics, Inc., for providing the testing platform.

Pascal Malassign, MID, IDSA Research Engineer, Research Service (151) Clement J. Zablocki VAMC 5000 W. National Ave., Milwaukee WI., 53295 Tel. 414.291.8069 Fax. 414.291.8077