Comparison of Wheelchair Cushion Characterization Tests for Loaded Contour Depth and Overload Deflection: ISO vs. CMS Coding Verification Requirements

RESNA 28th Annual Conference - Atlanta, Georgia

Marissa L. Ammer1, Patricia E. Karg, MS, David M. Brienza, PhD

1University of Pittsburgh, Rehab. Science and Technology, 5044 Forbes Tower, Atwood and Sennott Streets, Pittsburgh, PA 15260


Fifty-three seat cushions were subjected to both the ISO and the CMS \loaded contour depth and overload deflection tests. The tests are similar, but differ in that the ISO methodology results are reported on a linear scale, and the CMS test results in a pass/fail based on contact of the trochanter buttons on the jig. The results establish a baseline against which individual cushions can be compared. The study also revealed unexpected differences between the two tests. Cushions that demonstrated greater (or less) than 40mm of loaded contour using the ISO methodology do not necessarily pass (or fail) the 40mm CMS verification requirement as would be expected. The same was shown to be true for 25mm of loaded contour and the 25mm CMS jig.


Maintaining tissue integrity while seated is related to the ability of a seating surface to immerse and envelop the pelvis and accommodate movement without creating harmful overload conditions. Two groups have developed procedures to qualitatively and quantitatively measure immersion. Specifically, an International Organization for Standardization (ISO) working group is developing a voluntary standard for wheelchair seat cushions that includes the Loaded Contour Depth (LCD) and Overload Deflection test (1). Based in part on the ISO work, the Centers for Medicare and Medicaid Services (CMS) published the Policy Article for Wheelchair Seating containing CMS coding verification requirements for wheelchair seat cushions (2).

Figure 1 d. Loaded contour jig - from CMS supporting documentation (Click image for larger view)
Follow d-link for description

The ISO protocol is a quantitative measure of immersion for loading conditions using a so-called loaded contour jig (LCJ) (Fig. 1); there is no pass/fail criteria. The test also suggests that cushions should accommodate an increase in load of 33% with at least 5mm more of immersion for overload, after rounding to the nearest 5mm. The CMS policy specifies the use of the ISO LCJ, changing its name to a CLI (contour loading jig) and added a qualitative performance requirement: a cushion passes the loading segment of the test if the trochanters of the CLI contact the cushion. CMS maintains the requirement of 5mm of overload, after rounding. The CMS Policy also requires the cushion pass the loaded contour and overload test after being subjected to 12-18 months simulated use. This paper does not address this portion of the testing.

The apparatus used for the ISO testing include the seat cushion thickness measurement rig and the LCJ (loaded contour jig). However, there is only one jig with a 40mm difference in height between the trochanters and ischial tuberosity (IT) (Fig. 1). The CMS policy expanded on the 40mm jig from ISO to also include a jig with 25mm between the heights of the trochanters and ITs. The 40mm jig is used for either the skin protection (K0652, K0653) or skin protection and positioning (K0656, K0657) seat cushions. The 25mm jig is used for both general use (K0650, K0651) and positioning (K0654, K0655) seat cushions (2).


The procedures for both protocols are essentially the same. The CMS protocol refers to the ISO draft for jig dimensions and cushion preconditioning protocol. The preconditioning involves a cushion being subjected to 830±10N for 120 to 180 seconds using the Rigid Cushion Loading Indenter (RCLI) and loading rig while the cushion is on a horizontal support surface. The load is removed for 120 seconds and reapplied for another 120 to 180 seconds. The cushion is then given 300±20 seconds to recover unless known recovery characteristics warrant more time; up to 60 minutes.

The loaded contour depth and overload deflection test consists of several components all occurring on a flat, horizontal, level surface. If a cushion has a curved base it is stabilized before any testing is performed. The ISO protocol calls for a cushion thickness measurement to be taken at 125±25mm from the rear edge of the cushion while applying 1.5±0.5N with a circular foot. The value is recorded to the nearest millimeter. For flat cushions the measurement is taken at the midline, for contoured cushions the measure is taken at the lateral edge. Both protocols then call for the appropriate jig, based on cushion width and/or coding, to be aligned on the cushion. The CMS policy refers to the CLI being placed between 11 and 15cm from the rear edge of the cushion with the ITs aligned with the analogous part of the cushion. ISO refers to a LCJ positioned at 125±25mm from the rear edge of the cushion, with the ITs at the location specified by the manufacturer. Both ISO and CMS then load the cushion with 135±5N or 140N, respectively. After 300 seconds the vertical distance between the horizontal support surface and the inferior LCJ surface is recorded to the nearest millimeter for ISO while CMS notes whether or not the trochanters contact the cushion. The overload condition is then applied for 60±5sec with the change in vertical distance from the loaded condition being recorded to the nearest millimeter for both procedures. However, while ISO calls for the load to be increased to 180±5N, CMS adds 47N (10 pounds). The load is then removed from the cushion, the cushion is given 300±10sec to recover, and the procedure for both load and overload is repeated two more times for a total of three trials. Load and overload values are then reported after being rounded to the nearest 5mm. The CMS protocol requires that the cushion pass all three trials.


A set of 53 unique cushions from six manufacturers was tested in accordance with both the ISO and CMS protocols. The cushion sample included cushions composed of foam and viscofoam as well as combinations of materials: foam and gel; foam, gel, and air; viscofoam and air; viscofoam and gel. A total of 191 trials were performed. Multiple trials were performed on the cushions to meet the protocol requirements and some cushions were tested with the LCJ/CLI at multiple locations. 25.1% of the total sample was collected using a 25mm jig; 74.9% used a 40mm jig. 29.8% of the trials used a jig indicated for a 40-41cm wide cushion. 70.2% used a jig for 46-48cm wide cushions. No other cushion sizes have been tested. 78.0% of the trials experienced contact between the trochanters and the cushion under loading conditions. Finally 94.2% passed the overload condition of at least 3mm of additional immersion.

Table 1: Results of 40mm jigs: n = 143
No Contact with Load: n = 36
  Cushion height [mm] Loaded Contour Depth [mm] Overload Deflection [mm]
Range 73 – 102 31-48 2-5
Mean 89.8 +6.1 37.4 +4.7 3.6 +0.7
Contact with Load: n = 107
Range 72 – 127 37 – 93 2 – 7
Mean 91.7 +15.2 54.0 +14.5 3.5 +0.9

The set of trials that utilized a 40mm jig (n=143) was broken into two categories: cushions that had contact with loading conditions and those that did not. The results are shown in Table 1. The data from testing with the 25mm jigs (n=48) was similarly categorized (Table 2).

Table 2: Results of 25mm jigs: n = 48
No Contact with Load: n = 6
  Cushion height [mm] Loaded Contour Depth [mm] Overload Deflection [mm]
Range 73 – 75 24 – 25 2-5
Mean 73.0 +1.1 24.5 +0.5 3.7 +0.5
Contact with Load: n = 42
Range 55 – 94 24 – 51 2 – 5
Mean 73.6 +11.3 24.8 +6.6 3.6 +0.8


140 of the 191 trials passed the CMS test for the code specified by the manufacturer, having contact with load and at least 3mm of overload. The CMS policy allows testing of any cushion size. The fact that most cushions used a 46-48cm jig and the remainder used a 40-41cm jig indicates that the manufacturers want to ensure that their most popular cushions qualify for Medicare reimbursement.

The data in the tables present several points of interest. First, there is significant overlap in the range of loaded contour depths, suggesting that the raw measurement alone does not indicate whether or not contact will occur. Secondly, a large range is present for cushions that experienced contact. The maximum value is two times larger than necessary for both the 25mm and 40mm jigs. The extreme numbers have two main cause: one deals with the manufacturer’s choice of coding, for example, there were 6 trials with the 25mm jig that could have passed with a 40mm jig. However, in an attempt to have a line of cushions representing the four types of codes mentioned above, the manufacturer chose a code inconsistent with the cushion’s performance characteristics. The other possibility involves the lateral edges of the cushion being built up. While allowing the cushion to pass easily, it can be concluded that the cushion design can be altered to the idiosyncrasies of the test.

Other occurrences exist that can be attributed to how well the cushion was suited for the test. One trial with both the 25mm and 40mm jig had contact with load but had less than 25mm or 40mm, respectively, of immersion. Two trials with 25mm jigs had at least 25mm of immersion but failed to contact under loading conditions. Also, 14 trials with a 40mm jig measured at least 40mm of immersion upon loading but did not contact. As a cushion with significant lateral pelvic support can give values in excess of twice what is required, a cushion with little to no lateral pelvic support will struggle to pass.

Another point of interest is the comparison of where the cushion height measurement is taken for the ISO test to where the trochanters make contact with the cushion. As previously stated, the cushion height is usually taken at the lateral edge as most cushions exhibit some contour. However, the jig does not typically span the entire width of the cushion, placing the trochanter buttons inside of any lateral support located along the extreme lateral edge. Therefore, 40 mm of loaded contour depth as measured by the ISO method does not always translate into contact of the trochanters for the 40 mm CMS test, which was the intent.

As the CMS policy designates pass/fail conditions, it needs to be very clear about what passes. Contact was interpreted to be any part of the trochanters bilaterally contacting any part of the cushion, but this condition was occasionally difficult to determine. A more precise definition of contact would be useful to avoid confusion.


  1. ISO/DIS 16840-2 2004: Wheelchair Seating – Determination of physical and mechanical characteristics of devices to manage tissue integrity – Seat Cushions
  2. Article for Wheelchair Seating – Policy Article – Effective July 2004 (A17918)


This research was supported by the NIDRR Rehabilitation Engineering Research Center on Wheeled Mobility, Grant No. H133E99001