Comparison of High-Strength Aluminum Ultralight Wheelchairs Using ANSI/RESNA Testing Standards

Nine wheelchair specimens were tested, consisting of 3 specific models from 3 different manufacturers. The Ki Mobility Rogue (Ki Mobility; Stevens Point, WI), Invacare Top End Crossfire T7A (Invacare Corporation; Elyria, OH), and Sunrise Medical Quickie Q7 (Sunrise Medical; Fresno, CA) (Figure 1) were all subjected to the tests specified in the 2009 edition of the ANSI/RESNA testing standards. The testing order was chosen by random lottery, and the wheelchairs were tested according to this order throughout the study. The Ki Mobility Rogue (KIR), Invacare Top End Crossfire T7A (T7A), and the Quickie Q7 (Q7) wheelchair frames were made using 7000 series aluminum alloy as the primary material. The T7A was specifically constructed using 7005 aluminum and the KIR and Q7 used an unspecified 7000 series aluminum alloy, according to information found in the user manuals and brochures. These models were chosen because of their frame material and by interviewing therapists at the Center for Assistive Technology (CAT) in Pittsburgh, Pennsylvania, to see which chairs and brands they typically prescribe to end users that require this type of wheelchair. All 3 models were coded as K0005 wheelchairs according to the Healthcare Common Procedure Coding System (HCPCS).

Each wheelchair was subjected to the following test sections of the ANSI/RESNA Wheelchair Testing Standards: WC-01: Determination of static stability; WC-03: Determination of braking effectiveness; WC-05: Dimensions, mass, and maneuverability; WC-07: Method of measurement of seating and wheel dimensions; WC-08: Requirements and test methods for static, impact, and fatigue strength; and WC-15: Requirements for information disclosure, documentation and labeling. When this evaluation was performed, the latest revision of the standards was the 2009 edition, which was used for all testing. The testing regimen used for this evaluation is identical to testing in previous studies.8 The wheelchair testing order was chosen by random lottery, and this order remained consistent throughout the testing process. The ANSI/RESNA tests were performed in the following order: WC-15, WC-05, WC-07, WC-01, WC-03, and WC-08. The ANSI/RESNA standards do not require a specific order of testing except for within WC-08, so the order was chosen for efficiency.

The WC-15 testing standard evaluated the documentation and labeling that was included with each wheelchair. This included items such as the user manual, service manual, any labels on the wheelchair, and any other articles that would be provided to the end user. The test technician looked at all the documentation to ensure that critical items such as the maximum user mass, maintenance requirements and intervals, and any safety concerns were present either on a label or in the manuals.

WC-05 and WC-07 were evaluated next. These sections were used to determine dimensions of the wheelchair specimens. WC-05 primarily concerns the general dimensions and mass of the wheelchair, whereas WC-07 focuses on the seating system measurements and adjustability. WC-05 further evaluates maneuverability when performing tasks such as turning around completely or propelling through an open door. These sections provided a standard method for comparing dimensions between wheelchairs and may potentially assist a clinician with the selection of a wheelchair for an end user.

WC-01 testing determines the stability of the wheelchairs while at rest using a tilt platform to measure the tipping angle of the wheelchair in different orientations as specified in the standard (Figure 2). If the least stable configuration of the wheelchair tipped while on a level surface, the chair was rotated 180° from the direction requested for each test and then the platform was inclined until the wheelchair remained stable. The incline angle was then recorded as a negative value in the results.

Parking brake effectiveness was tested according to the WC-03 standard; the tilt platform used in WC-01 was also used here. In this test, instead of restraining the downhill wheels and allowing the chair to tip, the chair was allowed to slide or roll down the test ramp while the wheel brakes were engaged. The angle at which this movement occurred was recorded along with the type of movement. These results can be used to show whether the brakes will be overpowered first on an incline, or whether the tires will lose traction, and at what angle the event will occur.

Durability testing was the final section evaluated in this comparison. Three components make up durability testing: static, impact, and fatigue strength. According to the standard, the 3 test sections of WC-08 must be performed in this order. Static testing is conducted by placing the test wheelchair onto a platform and applying forces to different components using a pneumatic ram. After the force application, each tested component was evaluated for deformation or breakage (Figure 3).

Next, impact testing was completed. This testing applied an impact to different components of the wheelchair using a weighted pendulum. The mass of the pendulum varied based on the test, and the angle it was raised from vertical before being released depended on the weight of the wheelchair and maximum user capacity (Figure 4).

Finally, fatigue testing was performed. This testing was comprised of 2 parts: multidrum and curb drop testing. Multidrum testing placed the test wheelchair on a set of rollers that have slats mounted on them such that a slat impacts each wheel once per revolution (Figure 5). The drums were rotated such that the surface speed was 1 m/s. This test ran for 200,000 revolutions of the drums. After completing the multidrum tests, the chair was tested on the curb drop machine (Figure 6). This test raised all wheels of the chair and allowed it to freefall 50 mm to the ground. This drop was performed 6,666 times. After completing each fatigue test, the chair was examined to see whether any parts were broken or deformed. If a noncritical part failed, the part was replaced and testing continued. If a critical part (eg, the main frame) failed, then the testing was stopped and the total equivalent cycles were recorded as the number of cycles completed before failure. The number of equivalent cycles was calculated by multiplying the number of cycles completed on the curb drop by 30. This is the standard procedure when equating curb drop to double drum cycles for comparison studies. Hence, a chair needs to complete 400,000 equivalent cycles to meet the minimum requirement for ANSI/RESNA. If a wheelchair completes one round of fatigue testing without a critical failure, the process is repeated until such a failure occurs. The total number of equivalent cycles is then calculated and recorded.

Results from the ANSI/RESNA Wheelchair Testing Standards WC-01, WC-03, and WC-08 were analyzed using the IBM SPSS Statistics Version 23 software package (IBM Corporation, Armonk, NY). Static stability, parking brake effectiveness, equivalent cycles completed, and cost-benefit were specifically tested for statistical significance. Due to the small sample size, the 3 model groups, and non-normal distribution, the Kruskal-Wallis test was conducted on each data set. A post hoc analysis using the Mann-Whitney U test was then performed on the significant results to determine which wheelchair pairings were significant within the group. The significance value was set to p < .05 for all analyses.

The results from this study were also compared to the results found in published wheelchair comparison studies. Previously performed studies comparing manual wheelchairs classified as K0004 and K0005 according to the HCPCS coding system were chosen due to their similarities to the 70XX wheelchair models. Specifically, frame materials, configuration, and adjustability of components were used in the selection process. For durability comparison purposes, other wheelchair types were included to indicate how the wheelchairs tested in this comparison perform versus a broader spectrum of models and types.

The documentation and labeling of all 9 specimens were evaluated according to the WC-15 standard in ANSI/RESNA. All of the specimens tested were shipped with identical accompanying material (eg, tools and manuals) within the model groups. All labeling was also identical within models and was sufficient according to the standard.

The dimensions, mass, and maneuverability were measured for all 9 wheelchairs using the methods found in WC-05 and WC-07 of the ANSI/RESNA testing standard. Table 2 shows the mean results for each wheelchair model.

Static stability results were measured and recorded according to WC-01 of the ANSI/RESNA standard. Table 3 shows the mean, standard deviation, and statistically significant results for the wheelchair models tested. The range of adjustability was calculated by taking the difference between the most and least stable measurements for each specimen.

After completing the static stability testing for each wheelchair, the effectiveness of the parking brake was evaluated. Table 4 provides the results for this evaluation in degrees of tilt and the failure mode of the braking system. For this test, 2 failure modes were possible: either the wheels slid down the test surface, or the brakes were overpowered and the wheels turned. Table 4 shows the result for each specimen with respect to the failure mode and the mean and standard deviation of each model. No statistically significant differences were found during analysis.

The final tests performed were the static, impact, and fatigue tests. The only failure recorded throughout the static and impact testing was during the arm support resistance to downward forces test. All 3 T7A specimens failed this test due to deformation of the arm support. However, a comparison of these results to the other models was not possible because the T7A was the only model equipped with arm supports.

The durability tests showed that only the T7A model surpassed the minimum requirement of 400,000 equivalent cycles specified in the ANSI/RESNA Wheelchair Testing Standard. The T7A completed 623,408 mean equivalent cycles on the double drum and curb drop combined. The statistical analysis showed no statistical significance between the average equivalent cycles each model completed.

Cost-benefit was determined by dividing the average equivalent by the manufacturers' suggested retail price (MSRP) of each model. The units for cost-benefit are cycles per dollar, and the final values are shown in Table 5. Due to the similar prices of the 3 models, the cycles per dollar results coincide with the average equivalent cycles completed.

For the purposes of comparison, the results from previous studies were used. To simplify the following discussions, the study names have been abbreviated as follows: the comparison between 70XX series wheelchairs in this article (70XX), Gebrosky et al (LWFWII),8 Cooper et al (AUFW),18 Liu et al, 2010 (AURW),2 and Liu et al, 2008 (TURW).1 

The large range of adjustment, which is a hallmark of K0005 wheelchairs, shows that these models should be able to accommodate a wide range of potential users and reduce the risk of RSI and other adverse effects of long-term wheelchair use. The wheelchairs in this study were also more maneuverable when compared to the K0004 class wheelchairs in the LWFWII study (Table 6). The 70XX chairs had mean minimum turning diameters (averaged by model) that were 146 to 313 mm less than those of the K0004. Mean pivot widths were similarly smaller by 270 to 376 mm. The mean mass of each chair was 6.1 kg lower than the K0004 models. Better maneuverability allows the wheelchair user to maneuver into and around in smaller spaces, which may improve their ability to perform daily tasks. Lower wheelchair weight is also important, especially during solo transfers. A good example of a solo transfer benefitting from a lightweight wheelchair is transferring into a vehicle. It may be possible to lift a very light wheelchair into the vehicle with one arm while already inside, greatly simplifying the transfer process.

When considering a particular wheelchair model's performance on the static stability tests, it is important to take into account the highest angle likely to be encountered by a wheelchair user during daily activities. In the past, 7° has been used because it is the maximum allowed ramp angle for existing construction according to the Americans with Disabilities Act (ADA).19 If this value is used, all 3 tested wheelchairs can be configured to remain stable in the forward direction and rearward direction with the rear wheels unlocked. With the rear wheels locked, only the KIR can be configured to remain stable. Both the T7A and the Q7 would tip at this angle without user intervention (by shifting their body weight forward). Fortunately, because these wheelchairs are intended to be used by more active users, it is very likely that the user will be able to perform this weight-shifting task to counteract a tip. Nevertheless it is a potential safety issue and users should be informed of this when they first receive their wheelchair.

Also important with regard to static stability testing is the range of adjustment of the wheelchair. A wheelchair with a large range of adjustment allows more users to find their ideal seating position, which improves comfort and also reduces the risk of RSI when propelling. This range translates into a higher range of stability for the wheelchair, meaning higher angles in the most stable configuration and lower angles in the least stable configuration should be recorded. Based on the results in this study, all 3 models exhibit a range of stability that is not significantly different compared to other K0005 models tested previously. This means that the models tested here do not differ in any statistically significant way from the previously tested models

The fatigue testing in this comparison found a notable pattern in the ultralight wheelchairs tested with respect to failure mode. In all 9 wheelchairs tested, the failure point was related to the front caster wheels or components that connect the casters to the rest of the frame (Figure 7). Below, each wheelchair model and the failure modes found during testing are described.

Of the 3 70XX aluminum models tested, the T7A model was the only one that had more than one specimen meet the minimum ANSI/RESNA requirement of 400,000 equivalent cycles before failure. The other models only had one specimen meet the minimum requirement. T7A #3 also completed the most equivalent cycles of all specimens tested, more than doubling the second highest result. Two failures of these chairs are notable, however, and should be discussed. The first occurred on T7A #4, which caused the testing to stop much sooner than the other 2 T7A specimens. During the multidrum testing, the left caster stem failed and the testing was stopped. Upon inspection it was found that the stem had failed where it met the frame (Figure 8). After many attempts to remove the stem using a bolt extractor, it was deemed not possible through normal means to remove the broken piece. This may be due to thread locking compound or another retaining method inside the caster mount. Any further efforts such as heating the frame or destructive removal could negatively affect the aluminum frame and were deemed unsafe in a real-world situation if the frame would be returned to service, so they were not attempted. Nondestructive attempts to remove a nondamaged stem were made, but these also failed. Because of the stuck stem, the chair essentially suffered a frame failure that would have to be replaced to return to normal operation.

The second notable failure occurred during multidrum testing of T7A #1. In this case, the entire caster arm separated from the frame. This failure can be especially dangerous due to the lack of warning that a user may have in this situation, which increases the potential for injury. More concerning is the fact that the failure may be related to a poor weld process. Upon inspection, it was found that the filler rod used to weld the caster arm to the rest of the frame had not fully melted and was found nearly intact within the caster arm (Figure 9). Also, inspection of the fracture itself indicates the initial crack that led to the separation started in this area of the weld. This defect was likely due to the automated welding process used to assemble the frame and means that this could be found in other frames as well. This may increase the possibility of failure at this location. The other caster arm tube on this specimen was cut open to see if the other welds had the same defect, but nothing notable was found. The chair did surpass the minimum ANSI/RESNA requirement, so even with this defect it performed better than the average results of the study.

All 3 Q7 wheelchairs suffered failures of their front frame structure or caster assemblies, rendering them inoperable. There were, however, other issues preceding this failure that should be noted. The first 2 Q7s tested on the multidrum machine suffered failures within the first 15,000 cycles of testing. In both cases, a caster wheel and fork assembly broke away from the wheelchair due to a caster stem bolt failure. Upon inspection, it was discovered that the stem bolt was aluminum. Although this allows the chair to be lighter, it is likely that the aluminum was not strong enough to survive the standard WC-08 testing for any significant length of time. To continue testing, a replacement part was ordered. It appears that at some point the parts catalog was updated with a revised caster stem assembly, and now a steel caster stem is the only option available (Figure 10). An older parts catalog was obtained to confirm there was a change from aluminum to steel at some point in the production of the Q7. Based on this evidence, it is possible that some users had problems with the caster stem bolts, which caused the manufacturer to change the design. While it is concerning that the stem bolt failed in the first place, the manufacturer upgraded the original casters to the new design at no cost. There was no recall issued for these upgraded stem bolts, which means the end user may only receive the replacement parts after a failure occurs.

The KIR durability test results were interesting in that 1 chair survived significantly longer than the other 2. Specimen #6 completed 613,330 cycles before failure, which was the second highest result of all chairs tested. The other KIR specimens suffered similar failures at very low equivalent cycles, which is cause for concern. The reason for the failures on all 3 specimens was the caster rake angle adjustment mechanism. To enable a large range of caster rake angles, KIR wheelchairs incorporate a unique mechanism (Figure 11). Unfortunately, this mechanism was responsible for premature failures in 2 of the 3 KIR specimens tested. The design of the component that attaches to the wheelchair frame has an area where only a small cross section of material supports the entire load on the caster and any forces that may be transmitted by the caster driving over terrain. Coupled with the fact that the component is made from aluminum and that there is a large stress concentration caused by the hole that allows the internal components to pass through, the mechanism is not able to withstand significant impacts. This was demonstrated by the failures on the multidrum test (Figure 12) (Figure 13). Both specimens #2 and #8 failed in the same location. Based on the design, there appears to be a way to remove and repair the damaged component; however, when repairs were attempted, it was found that this is not the case. Although there is a bolt that goes through the wheelchair frame and the adjuster, the adjuster is also bonded into the frame using adhesive. The manufacturer was contacted, and it was determined that this part is not designed to be removed. This means that the entire frame will need to be replaced to repair a failure of the adjustment mechanism.

The equivalent cycles before a class III failure of the KIR, T7A, and Q7 were compared to previous studies using a Kaplan-Meier survival plot to visualize the results (Figure 14). The results show that the 70XX wheelchairs finished with the second highest number of cycles before all specimens failed and also ranked third out of the 6 studies for survival rate at 400,000 equivalent cycles, which is the minimum requirement for ANSI/RESNA. The survival rate at 400,000 equivalent cycles (0.44) is in between the other rigid K0005 studies (AURW = 0.58, TURW = 0.33). It is concerning, however, that less than half of the tested specimens met the minimum ANSI/RESNA requirement.

The cost-benefit results of this study were also compared to previous wheelchair comparisons to determine how the 70XX measures up when its initial cost is factored in (Figure 15). Overall, the 70XX aluminum wheelchairs (151 cycles/$) performed slightly better than other K0005 rigid models in the AURW (137 cycles/$) and TURW (84 cycles/$) studies. Only the AUFW study (673 cycles/$) performed better in the K0005 category. The data were further analyzed for statistical significance, and no significant differences were found between the 6 studies compared.

The primary limitation of this comparison study is the small sample size of wheelchairs compared. Nine wheelchair specimens were evaluated from 3 different models. The inclusion of additional specimens would improve the reliability of the data and the analysis of that data. More models would also improve the reliability of the data by providing a larger view of the K0005 class of wheelchairs. Both of these improvements are extremely cost-prohibitive, and the high cost of purchasing wheelchairs is the main reason for the small sample sizes.

With the introduction of new materials and technologies into modern wheelchairs, it is important to consider whether the benefits outweigh the costs to both the consumers and the manufacturers. Exotic materials such as high-strength aluminum alloys, titanium, and carbon fiber have properties that can greatly benefit wheelchair design; but if they are not incorporated properly to maximize usability and durability, they are not worth the added cost. This study evaluated 9 wheelchairs constructed using 70XX aluminum alloys versus the common 6061 alloy and found that there are no significant differences between durability results of this study and previous rigid aluminum wheelchairs. Furthermore, only 1 of the 3 models in this study met the minimum requirements of the ANSI/RESNA testing standards. This follows the trend in wheelchair durability seen previously where the majority of the wheelchairs tested do not meet the minimum requirements of the ANSI/RESNA standards. It may be necessary to revise the FDA's 510k approval process for manual wheelchairs to include, at the minimum, durability requirements. Over the past 3 K0005 rigid wheelchair studies, 8 out of 11 wheelchair models failed to meet the minimum specified in the ANSI/RESNA standards, which shows that there is a need for this requirement. Even more telling is the fact that this type of wheelchair fares better than K0004 wheelchairs on the durability portions of ANSI/RESNA. Setting these minimum requirements would not harm competition because all manufacturers would be required to comply. Further credibility could be lent to the submitted results by requiring independent testing, something that is not presently required of any wheelchair, either manual or powered.

The authors declare no conflicts of interest.