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WHEN WHEELCHAIRS TIP BACKWARDS BEYOND THEIR STABILITY LIMITS

R. Lee Kirby, Mario DiPersio, Donald A. MacLeod

Division of Physical Medicine and Rehabilitation, Dalhousie University; Nova Scotia Rehabilitation Centre; Halifax, Nova Scotia, Canada B3H 4K4

Key words: wheelchairs, rehabilitation, safety, stability, standards

Dr. R. Lee Kirby c/o Nova Scotia Rehabilitation Centre 1341 Summer Street Halifax, Nova Scotia Canada B3H 4K4 (902) 492-6178 (phone) (902) 425-6466 (fax) KIRBY@AC.DAL.CA (e'mail)

BACKGROUND

Wheelchair-related injuries are common and often serious (1). Although such injuries are multifactorial, stability is a factor in the majority of cases. Rear stability is the angle of a tilting platform from the horizontal at which the forces under the uphill wheels become zero (2). This value is significantly lower with the wheels locked than unlocked (3), primarily due to the different axes of rotation. However, there is no available literature regarding what happens to a wheelchair once the stability limit has been exceeded. Pilot work on this subject suggested that the differences induced by locking or grasping the wheels were substantial and that these differences might have major implications for the training of wheelchair users in how to fall safely.

RESEARCH QUESTION

We hypothesized that, with the wheels locked (either by the mechanical wheel locks or by the user firmly grasping the wheels), the rear wheels rotate slowly backwards during a fall, whereas with the wheels unlocked, the rear wheels rotate quickly forwards.

METHODS

We studied 10 nondisabled volunteers in a single representative rear-wheel-drive, manually propelled wheelchair. With a 30-cm-thick foam pad behind the wheelchair, we tipped the occupied wheelchair backwards to the point where the wheelchair was balanced over the rear wheel. When the subject was ready, the investigator allowed the wheelchair to fall backward. In a randomly balanced order, this procedure was followed under three conditions: the wheels-locked (L), two-hands grasping the wheels (TH) and wheels-unlocked (UL) conditions.

From the videorecordings we derived three measures: the horizontal displacement of the rear axles, the rotation of the rear wheels and the time for the fall to occur. Matched-pairs t tests were used to compare the conditions.

The results of the empirical testing suggested a model to explain the observed differences. To evaluate the assumptions and implications of this model, we conducted brief experiments with variations in friction and rolling resistance.

RESULTS

The horizontal displacements of the rear axles are shown in Figure 1. In addition, the L and TH conditions both resulted in a rearward rotation of the rear wheels, by mean values of 43.7 and 46.8o respectively; there was no significant difference between these two conditions. The UL condition resulted in a forward rotation of the rear wheels by a mean value of 40.3o. The mean differences between the UL condition and the L and TH conditions were 84.0 and 87.1o respectively (p < 0.0001 for both).

In the L and TH conditions, the wheelchairs took a similar length of time to fall, with mean values of 1.20 and 1.12 s respectively; there was no significant difference between these two conditions. The UL condition resulted in faster fall, with a mean value of 0.67 s. The mean differences between the UL condition and the L and TH conditions were 0.52 (p < 0.0001) and 0.45 s (p = 0.0001) respectively.

In the model-validation experiments, with the wheels locked, when wedges were placed behind the rear wheels, the drop to the ground was prevented. With the wheels unlocked, when wedges were placed in front of the rear wheels, the rear wheels were prevented from rolling forward and they remained stationary during the drop. With one wheel locked and one unlocked, the locked wheel rotated backwards and the unlocked wheel forward such that the wheelchair came to rest on the floor at an angle of 45o to its initial orientation.

DISCUSSION

The hypothesis was corroborated. With the wheels locked, the rear wheels rotate slowly backwards during a fall, whereas with the wheels unlocked, the rear wheels rotate quickly forwards. The extent of these differences were highly significant, both clinically and statistically. The finding that rear tipping was prevented when wedges were placed behind the locked rear wheels has implications for static-stability testing, providing support for the recommendation of Cooper et al. (3) that no barriers should be placed downhill to the rear wheels when rear stability is measured with the wheels locked. With the wheels unlocked, wedges in front of the rear wheels prevented the "submarining" effect of the rear wheels rolling forward as the wheelchair dropped. This has implications for stability testing, suggesting that wedges should be placed uphill to the rear wheels when rear stability is measured with the wheels unlocked.

The major implication of this study relates to the training of wheelchair users in the safest course of action in the event of a rear-tipping accident. The natural reaction when tipping backwards is to turn and reach out in the direction of the fall to fend off obstacles, to avert the fall or to provide a measure of shock absorption. The disadvantage of this method is that the user can injure the upper extremity, no minor issue for a person who may need the upper limbs to propel the wheelchair and to transfer. If a user does reach out with one hand, the results of this study suggest that he or she should pull firmly on the wheel with the other hand to slow and guide the fall. Because we found that a significant yaw will occur when only one wheel is locked, if there is a preferred direction in which to turn (e.g., away from danger or toward a safer landing area), the hand chosen to grasp the wheel should be the one on the side away from the direction in which one would prefer to turn.

The results of our study suggest that it might be preferable for the user to use both hands to grasp and pull on the rear wheels as forcefully as possible, flexing the neck and trunk as well. (If the legs are flaccid and there is a risk of the knees striking the user in the face, then a thigh strap should make it unnecessary to use the hands to block the thighs.) Grasping and pulling on the wheels with both hands would have a wheel-locking effect (slowing the fall and lowering the forces on impact). Also, as we found in pilot work, some subjects are able to avert the fall altogether. The user may be able to further decrease the forces on impact to the extent that the flexor muscles of the arms, neck and trunk are able to act as shock absorbers. If there is an injury to be incurred, it is probably better to sustain a stretch injury to the muscles than an impact injury to the head.

Further clinical studies on such falling strategies are clearly needed. Even so, on the basis of this study, we suggest that users who find themselves beyond the rear-stability limits should firmly grasp and pull on the rear wheels.

REFERENCES

1. Kirby R.L., S.A. Ackroyd-Stolarz, M.G. Brown M.G. and S.A. Kirkland. Amer. J. Phys. Med. Rehabil. 73:319- 330, 1994.

2. International Organization for Standardization. Determination of static stability ISO/CD 7176-1 (E), ISO/TC 173/SC1 - 1995-05-03.

3. Cooper R.A., K.J. Stewart and D.P. VanSickle. J. Rehab. Res. Dev. 31:144-147, 1994.

ACKNOWLEDGEMENTS

We thank D. VanSickle, D. Limbert and R. Gaal for their contribution to the identification of this research question and the Medical Research Council of Canada for funding.

FIGURE LEGEND

Figure 1: Horizontial displacement of rear wheel (mm).