Licensed Professional Engineers
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 77: Summer 2020
Mobility Aid Accidents
By John L. Ryan, P.E.
Mobility aids are devices that help people with difficulty walking whether due to age, injury, or disease, continue to be able to move around. These devices include a variety of styles for specific applications. Users of these devices rely on the stability and strength of the mobility aids, and trust that the device they are using is safe, and will continue to be safe. Due to the difficulties that users experience that require the use of mobility aids, users are often less able to protect themselves in the event of a device failure or tipover, and can be more likely to suffer more significant injuries. The weight of the user can often be a factor as well, as due to a lack of mobility some users can be near the weight limit of these devices or even over the weight limit. This issue of Forensic Clues will examine the different types of mobility devices, some common failure modes, and how we can help you with your client’s accident case.
Walkers, Canes, Crutches
Mobility devices without wheels include walkers, canes, and crutches which are typically support aids that users lean on to help them walk. Walkers typically have four legs to them, with some designs having two legs. The four-legged versions typically have two legs with wheels, and two legs with rubber stoppers on them. Canes and crutches have singular legs.
Wheelchairs / Powered Scooters
Wheelchairs are one of the most common types of mobility aids, and are mobile chairs that can be self-propelled or motor-driven, for the purpose of locomotion of the user. Wheelchairs provide full support for the weight of the individual, and motion is generated by someone pushing it, the user rolling the large wheels to provide locomotion, or with a motor of some sort. These usually have some sort of braking system.
Rollators are a type of walker with wheels on each leg. They are sometimes equipped with a sitting surface to rest on, that is not intended to be used while the unit is in motion. Rollators provide stability, balance, and support for the user. While normally equipped with four wheels there are three-wheel versions. They usually have brakes on two or more of the wheels. Rollators are at times combined with a transport chair.
The primary difference between a rollator, and a rollator/transport chair is that the rollator/transport chair is designed to allow motion of the device with the user in it, compared to a rollator being designed to help the user walk, and stop to rest when needed.
A knee scooter is typically a four-wheeled device that allows a user to place one or both of his or her knees on the device to facilitate movement. It can have brakes, as well as all-terrain versions with larger wheels.
Some devices are not intended to help facilitate movement, but rather stationary support. These can include lift assist devices intended to help a person to their feet from a seated position, or can include chairs that are intended for bath use. There are also bath transfer chairs and benches which have a seated position on the exterior of the bath as well as in the bath itself, to help with transitioning between the bath and the floor.
What Can Go Wrong?
Mobility aids typically have designs that make them stowable, involving folding or retractable legs and other structural members. While this facilitates transport in vehicles, this also introduces a failure mode of the device collapsing. If these devices had fixed structural members, this failure mode would be eliminated. Collapse can occur due to the failure of connections that keep the device in the weight-supporting mode, including bolts that fail, are loose, or fall out. Other connection failures can include pins that are spring-loaded, and come out of their corresponding hole, or is assembled such that the pin is not engaged. Some designs are safer than others, with failures of connections or loading in an atypical manner not causing collapse. Collapse can be due to design failure, failure to maintain, incorrect assembly, or human error.
Tipover is a very common mode of failure with mobility devices. Tipovers can occur to either side, to the front, or to the rear. While some tipovers are unavoidable, many are due to designs that are not adequately stable. Manufacturers at times design their products to meet industry standards, which does not necessarily mean that the design will be safe, due to arbitrary industry standards, standards that do not encompass all conditions a mobility aid will face, and corrupt standards that are developed by product manufacturers to protect themselves. One accident we see often is tipovers due to hitting a bump. This scenario is not adequately covered by industry standards, as standards call for static (not-moving) stability tests unless the device is an electric-powered wheelchair. Tipovers can be due to a poor, unstable design, overreaching on the part of the user, a user being overweight, being on an angled surface, the occupant holding extra weight, or hitting a bump.
Brake failure can result in collision or tipover accidents, and can be due to a failure to maintain, incorrect assembly of the mobility device, design defects, or due to the brakes being inadequate to slow or stop the device.
Another common cause of mobility device accidents is structural failure of the device, which can include the main structural components collapsing, buckling, or rupturing. This can be due to design defects resulting in inadequate strength, the weight of the user being greater than the rated strength of the device, deterioration of the device, or loose connections that result in unanticipated forces that the structure cannot withstand.
Incorrect assembly is often noted prior to use, but can go undetected, and can include a variety of failures such as not using all bolts or other connectors, or having components reversed or mounted in the incorrect location.
User Exceeding Capacity of Device
We often see devices fail with users that are either slightly under, or slightly over the rated capacity of the device. While a lay person may think that a failure of a device when used by someone who his ten pounds heavier than the rated capacity is the user’s fault, this does not correlate with engineering design principles, that require a factor of safety for any design where the health and welfare of an individual is at stake. A failure of a device due to the user being heavy, is a failure to properly design the device with adequate factors of safety. Dynamic loading occurs whenever motion is involved, such as hitting a bump in a wheelchair. This bump can magnify the weight of the user over four times, resulting in potential failure of the device if it does not have adequate factors of safety. This literally means that a 300-pound user can generate 1200 pounds of force when motion is involved.
How We Can Help
At MASE, we can determine if a an accident involving a mobility aid is due to a design defect, failure to maintain, or human error. We offer full service mechanical engineering expert witness services. The first step is a free review of basic case information and photographs, at which point you will be informed of our thoughts on the case, the mode of failure if identifiable at this point, and the validity of a potential liability or premises claim. Data and conclusions can be drawn from the failed device itself, and exemplars are often purchased, examined, and tested.
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