What tilt and recline angles are acceptable in motor-vehicle transportation and what are the potential negative consequences of reclining an occupant too much?
WC19 does not place any requirements on the backrest recline or seat tilt angles used to transport persons in wheelchairs. WC19 only places requirements on tilt and recline angles provided for in the wheelchair design, and on preferred and recommended tilt and recline angles that must be specified in the manufacturer’s instructions and warnings to the user .
The only design requirement that WC19 places on the wheelchair is that the backrest frame is capable of being tilted to within 30 degrees of vertical and that the seat frame can be positioned within 30 degrees of horizontal. That is, WC19 requires that the wheelchair allow the wheelchair user to attain a seated posture. Stretchers, beds, and other devices that might be used will therefore not comply with WC19.
WC19 also requires manufacturers of wheelchairs designed for use in transportation to provide instructions and warnings in the wheelchair literature that the wheelchair backrest should not be reclined more than 30 degrees to the vertical. This maximum ”recommended” recline angle is intended to help maintain effective performance of belt-type restraint systems by having the shoulder belts in close proximity to, and preferably in contact with, the chest and shoulders. When the torso is reclined, the distance between the shoulder belt and the occupant increases, and this results in increased forces on the occupant in a frontal crash. In addition, with a more reclined torso posture, the chest will tend to take more of the load than the shoulders, thereby increasing the probability of belt-induced thoracic injuries.
It is also preferable for the lap angle to be between 45 and 75 degrees relative to the horizontal, and at an angle of 30 degrees or more to the occupant’s thigh angle. These angles will help ensure that the lap belt remains low on the pelvis in a frontal crash, and that the potential for lap-belt induced injuries to the relatively soft abdomen is minimized.
WC19 does not require the manufacturer to make a recommendation about the maximum tilt or incline angle of the seat of tilt-in-space wheelchairs. While inclining a forward-facing seat has some benefit in a frontal crash in that the seat provides additional restraint for forward movement of the pelvis in a frontal crash, too much seat incline increases the possibility of the lap belt loading the softer abdomen. However, up to 30 degrees of seat incline relative to the horizontal is considered acceptable.
In some situations it may be necessary to recline a backrest and tilt a seat more than these recommended limits. This may redeuce effectivness of the belt restraint system, but sometimes a trade off is needed for postural stability or the medical needs of the patient. In these situations, it is still desirable to keep the lap angle steep and the shoulder belt as close to the upper torso as possible, perhaps by adjusting the locations of the belt anchor points more rearward in the vehicle.
It is also important to clarify the difference between seat recline and tilt angles used in the WC19 frontal impact test, and the wheelchair design and user instruction requirements noted above. When a wheelchair is tested on an impact sled according to the requirements of WC19 as specified in Annex A, the seat tilt angle and back-support angle are usually set between 5 and 15 degrees to the horizontal and vertical, respectively. Wheelchair manufacturers know that these test angles are recorded in the test report. However, just because a wheelchair has been tested with the seat adjusted to an angle of 5 to 15 degrees does not mean that these are the maximum acceptable recline or tilt angles for use in transportation. While wheelchair manufacturers may recommend recline and tilt angles in this range, they should also indicate that recline and tilt angles up to 30 degrees are acceptable.
Finally, WC19 requires that tilt-in-space wheelchairs are dynamically tested at 30 mph with the seat tilted rearward of the most-forward tilt position. Thus, the tilt locking mechanism is dynamically loaded and must demonstrate that it will not fail catastrophically in a relatively severe frontal crash. Although the same principles about maximum tilt and recline angles apply to non-WC19-compliant wheelchairs, the fact that the tilt mechanism may not be strong enough to hold up under frontal crash loading adds an additional concern when using higher seat tilt angles, especially when the tiedown straps are attached to the wheelchair base rather than the seat. If the seat-tilt locking mechanism fails, a more reclined seat will pitch forward more than a more upright seat. A sudden and large change in seat orientation due to failure of a tilt locking mechanism would be likely to reduce the effectiveness of the belt restraint system, and further increase the possibility of the lap belt loading the abdomen of the wheelchair user.