Recommendations on Standards for the Design of Medical Diagnostic Equipment for Adults with Disabilities, Advisory Committee Final Report

5.8.2 Knee and Toe Clearance Recommendations under Breast Platform Recommendations

Description: The knee and toe clearance under the breast platform applies to mammography equipment for patients seated in a wheelchair. Knee and toe clearances are critical where patients seated in a wheelchair need to position their knees and toes next to or underneath a component of the diagnostic equipment. The proposed provision requires the depth of wheelchair spaces to include knee and toe clearance. Given the unique use of mammography equipment, the provision also contains a specific depth requirement under breast platforms.

NPRM Proposed Provision: M303.2.4 Knee and Toe Clearance. Wheelchair spaces shall include knee and toe clearance complying with M303.2.4. The depth of the wheelchair space shall include knee and toe clearance of 17 inches minimum and 25 inches maximum. Knee and toe clearance under breast platforms shall be 25 inches deep.

M303.2.4.1 Toe Clearance. Toe clearance shall be provided at a height of 9 inches above the floor to a depth of 6 inches maximum.

M303.2.4.2 Knee Clearance. Knee clearance shall be provided at a depth of 11 inches minimum and 25 inches maximum at 9 inches above the floor and at a depth of 8 inches minimum at 27 inches above the floor. Between 9 inches and 27 inches above the floor, the knee clearance shall be permitted to reduce at a rate of 1 inch in depth for every 6 inches in height.

NPRM Preamble Discussion: The dimensions for toe clearance in M303.2.4.1 and knee clearance in M303.2.4.2 are based on the 2010 Standards and are shown in the second column of the table below. The Wheeled Mobility Anthropometry Project showed that these dimensions do not accommodate many people in the sample and recommended alternative dimensions that would accommodate 95 percent of the people in the sample. The alternative dimensions recommended by Wheeled Mobility Anthropometry Project are below in the last column of the table. The Access Board is considering requiring in the final standards the dimensions for toe clearance and knee clearance recommended by the Wheeled Mobility Anthropometry Project.

Table 5.8.2(a)
Recommended Dimensions for Knee and Toe Clearances

The Committee recommends increasing the overall knee and toe space to a minimum 28 inches deep, from the initially proposed 25-inch absolute dimension; increasing the knee clearance directly underneath the breast platform to a minimum of 18 inches; and assuring unobstructed floor space in front of the base support at a minimum of 17 inches deep.

Rationale for recommendation

Each of these measurements creates a more accessible piece of imaging equipment while balancing the physical attributes and operation of the equipment.

The M303.2.4 proposed NPRM provision and the dimensions recommended by the Wheeled Mobility Anthropometry Project are based on an anthropomorphic model that defines knee and toe clearances based on measurements from the tip of a person’s foot. Industry suggested that using an anthropomorphic model that defines knee and toe clearances based on measurements from a person’s abdomen would be more appropriate because mammography procedures require a person’s chest to be flush with the front of the breast platform. Industry also indicated that the Committee would need to consider the range of travel of the breast platform and the impact that it has on the knee and toe space. The Committee reviewed the relevant anthropomorphic modelsand industry input on how these elements affect one another from a technical perspective. Below is an overview of the rationale for each of the final recommended dimensions affecting the knee and toe clearance underneath the breast platform. Figure 5.8.2 illustrates each of the critical dimensions. Further discussion of each of these dimensions follows in sections 5.8.2.1 – 5.8.2.7.

Figure 5.8.2 Overview of the Critical Dimensions that Define the Knee and Toe Space Underneath the Breast Platform.
(SOURCE: Hologic, Inc.)

Table 5.8.2(b) Critical Equipment Dimensions with the relevant Anthropomorphic Data

Section 5 References

^v IDeA Center at the University of Buffalo, New York: The Wheeled Mobility Anthropometry Project, Final Report, pages 89-92.

^vi D’Souza, et al. “Clearance Space Envelopes of Wheeled Mobility Device Users for Computer Workstations”. Proceedings of the Human Factors and Ergonomics Society Annual Meeting 2012.

^vii D’Souza, Steinfeld. “Revised Knee Clearance Depth and Footrest Clearance dimensions for sizing of mammography equipment.” Memorandum. April 23, 2013.

5.8.2.1 Height of Breast Platform at Time of Measurement
Recommendation

The Committee recommends measuring all knee and toe clearances when the top of the breast platform is set at 34 inches above the floor.

Rationale for the recommendation

The shape of the breast platform affects the shape of the knee and toe space. This shape varies across different pieces of equipment, and breast platforms do not have standard features that can be used to define the required knee clearance space underneath the breast platform. However, the top surface of the breast platform is a well-defined surface, and defines where the patient’s chest will be during the exam. When the top of the breast platform is at the height of the patient’s breasts, there must be sufficient clearance at the height of the patient’s knees, as is shown in Figure 5.8.2.1.

Figure 5.8.2.1 Illustration of the relationship between the breast platform height and the clearance required at the patient’s knees.
(SOURCE: Hologic, Inc.)

As indicated by the graphics key, the dotted lines reference the current ADA-ABA standard. Industry has laid an example mammography image over the relevant anthropomorphic modelvv.

The NPRM proposed measuring the knee clearance under the breast platform at 27 inches above the ground. Industry started with this dimension and determined the proportionate height of the breast platform to be 34 inches. The Committee agreed to this as a “set point” (Identified as “Breast Platform Height” in Figure 5.8.2.1). These dimensions (27 and 34-inch heights) are the same as those defined by the current 2010 Standards to accommodate wheelchair users at desks, which is outlined by the dotted lines in Figure 5.8.2.1.

The 27-inch and 34-inch dimensions help to define the breast platform requirements for a person with deep knees and a tall wheelchair. When the breast platform lowers to accommodate patients in shorter wheelchairs, there will be ample knee clearance for those patients. Conversely, as the breast platform moves up for patients who have longer torsos, the breast platform will just be moving away from their knees and legs, thus giving them ample space as well.

By defining the height of the breast platform as the “set point” for the other dimensions, all clearances have a static measurement point that assures maximum clearances during operation of the equipment. If there were not a defined method of measuring the clearances in relationship to the breast platform, then it would be possible for inaccessible equipment configurations to meet the clearance requirements.

5.8.2.2 Knee Clearance Depth at 27” Above the Ground

The Committee recommends increasing the knee clearance at 27 inches above the ground to 18 inches minimum.

5.8.2.3 Overall Knee and Toe Clearance Recommendation

The Committee recommends increasing the overall knee and toe space to 28 inches minimum.

Rationale for the recommendation

See Section 5.8.2.4 below.

5.8.2.4 Unobstructed Floor Space Recommendation

The Committee recommends the unobstructed floor space in front of the base support be a minimum of 17 inches deep.

Rationale for the recommendation

The overall knee and toe clearance and unobstructed floor space dimensions dictate how close a patient in a wheelchair can get to the mammography equipment. As was discussed above, the Committee considered anthropomorphic data that defined knee and toe clearances from a person’s abdomen. To consider overall knee and toe clearance, the Committee used female-only anthropomorphic data that defined abdomen-toe depths to ensure that a patient’s toes would not hit the gantry. To consider unobstructed floor space, the Committee used anthropomorphic data that defined abdomen-caster edge depths to ensure that the front caster wheels on a patient’s wheelchair would not hit the front of the base support.

The base support affects how close the patient can get to the mammography machine because it intrudes into the floor space. The purpose of the base support is to eliminate any instability caused by the weight of the cantilevered c-arm. Moving the c-arm further away from the gantry would increase the overall knee and toe clearance, but would also require an increase in the size of the base support in order to stabilize the equipment. The simultaneous increase in the size of the base support will minimize any gains from increasing the knee and toe clearance, since the base support obstructs the floor space. The Committee considered this balance when making final decisions about overall knee and toe space and unobstructed floor space.

While discussing these issues, the Committee also considered positioning methods for patients using wheelchairs. During a mammography procedure, the patient’s chest is flush with the front edge of the breast platform and the technician positions the patient’s breast on the breast platform. In order to image as much of the breast as possible, patients cannot be seated in the “natural” sitting position. They will need to sit up straight in order to situate as much breast tissue as possible on the breast platform. For patients seated in a wheelchair, a common positioning technique is to place towels, pillows, or other positioning aids behind the patient’s back in order to sustain an upright pose during imaging. This upright pose is necessary regardless of how close the patient can naturally get to the breast platform. The pose reduces the abdomen-toe and abdomen-caster edge dimensions by a couple of inches.

To accommodate 95% of female wheelchair users, the anthropomorphic data shows that 30.5 inches and 19.5 inches are the necessary minimum overall knee and toe clearance and unobstructed floor space dimensions, respectively. [Data is below in Tables 5.8.2.4(a) and 5.8.2.4(b)]. Members agreed on reducing these dimensions by 2.5 inches to adjust for the upright pose and to ensure technical feasibility and equipment safety. The final Committee recommendation is to require a minimum of 28 inches overall knee and toe clearance and a minimum of 17 inches unobstructed floor space. Looking purely at the anthropomorphic data, these dimensions accommodate 80% - 90% of the population. In adjusting for the needed upright pose, the dimension works for 95% of the population. Members recognized that these dimensions are an optimal blend of accessibility and technical feasibility .

Table 5.8.2.4(a). Abdomen Depth Data^vii to determine overall knee and toe clearance

Table 5.8.2.4(b) Abdomen-Caster Edge Depth^vii to determine unobstructed floor space 

Section 5 References

vii D’Souza, Steinfeld. “Revised Knee Clearance Depth and Footrest Clearance dimensions for sizing of mammography equipment.” Memorandum. April 23, 2013.

5.8.2.5 Toe Height Recommendation

The Committee recommends increasing the toe height to 18 inches minimum, measured above the ground when the top of the breast platform is 34 inches above the ground.

Rationale for the recommendation

The NPRM defined the toe clearance at a height of 9 inches. However, as the breast platform moves up and down, it may intrude on the toe space. Looking further at female-specific toe height data, the 95th percentile toe height is 13 inches for manual wheelchair users and 16.1 inches for power wheelchair users.^vi As such, the subcommittee recommended that the toe height be set to 18 inches (when the breast platform is at the 34 inch height discussed above). This higher 18-inch measurement (compared to the originally proposed 9-inch toe height) would also minimize the chance of the breast platform hitting patients’ toes when the breast platform moves to some of its lower heights.

A toe space dimension usually defines a static unobstructed space. However, in the case of mammography, as the breast platform moves up and down, the toe space changes. The 18-inch toe height is defined when the breast platform is at 34 inches above the ground. When the breast platform lowers to its required minimum height of 26 inches, the resulting toe height is 10 inches above the ground. When a patient needs the breast platform to be this low, she is likely to be lower to the ground and thus likely to have her feet closer to the ground as well. As such, a 95th percentile toe height (13 – 16.1 inches) does not need to be accommodated when the breast platform is at this minimum height. However, it is important to provide extra toe space for patients who are seated close to the ground but have their footrests positioned at more of an angle. The toe height defined here will provide this extra accommodation. Providing this dimension helps define the outer limits of the breast platform configuration in a way that maximizes the dimensions for accessibility.

Section 5 References

^vi D’Souza, et al. “Clearance Space Envelopes of Wheeled Mobility Device Users for Computer Workstations”. Proceedings of the Human Factors and Ergonomics Society Annual Meeting 2012.

5.8.2.6 Clearance Depth at Toe Height above the ground Recommendation

The Committee recommends 22 inches minimum clearance depth at toe height.

Rationale for recommendation

See Table 5.8.2(b).

5.8.2.7 Base Support Allowance Recommendation

The Committee recommends allowing a base support that obstructs the clear floor space if it fits within the allowed base support volume. Base supports can be a maximum of 1½ inches high. An additional sloped region above the base support is permitted at a depth of 25 inches from the front edge of the breast platform at 1½ inches above the floor, and can extend to a height of 4 inches above the floor at a depth of 28 inches from the front edge of the breast platform. Figure 5.8.2.7(a) illustrates this configuration below.

Figure 5.8.2.7(a) Illustration of final committee recommendation on proposed base support configuration. 
(SOURCE: Hologic, Inc.)

Rationale for the recommendation

The base support is of fundamental importance to mammography equipment and provides structural support, seismic stability, and installation safety. It does obstruct the floor space in front of the gantry and, thus, may limit how close a wheelchair can get to the equipment. To respond to this issue, industry proposed a configuration that would cause minimal obstruction to the floor space in front of the gantry and would allow footrests to ride over it.

To discuss the maximum base support height, the sub-committee looked at anthropomorphic data regarding footrest heights. The footrest height data measures the height from the floor to the top surface of the footrest at its proximal outside corner. To determine the necessary clearance for the footrests, the Committee used the footrest height data and subtracted the thickness of the footrests (~0.5 inch). The ~0.5inch thickness is the recommendation from the data supplied by Steinfeld and D’Souza7. The anthropomorphic data, as well as the calculated footrest clearances, is below in Tables 5.8.2.7(a) and 5.8.2.7(b).

Table 5.8.2.7(a). Anthropomorphic data on measured footrest heights

 Table 5.8.2.7(b). Actual footrest height clearances

Allowing a maximum base support height of 1.5 inches will provide room for the structural components necessary for an effective base support design and will also be accessible by around 92% of manual chair users and over 95% of power chair users.

Committee members noted that allowing the base support to encroach into the floor space creates an exception from the 2010 Standards. Since mammography equipment bolts or affixes to the floor of the facility, it becomes a fixture to the building. The Department of Justice considers fixtures to fall under the architectural elements covered by the 2010 Standards. The provisions for clear floor space generally do not allow an encroachment even with a maximum height of 1½ inches for the base support. However, the Committee agreed that since the base support is critical to the equipment’s stability and installation safety, the support must be configured to allow for maximum wheelchair accessibility.

The Committee also discussed adding an allowance for a sloped region at the base of the gantry. Many of the mammography machines today have important informational displays at the base of the gantry, on top of the base support (shown as the checkered region in Figure 5.8.2.7(b)) that display information such as compression force and compression thickness.

Figure 5.8.2.7(b) Checkered region shows location of informational displays and other mechanical and electrical components on many mammography machines.
(SOURCE: Hologic, Inc.)

The Committee discussed the additional sloped region. Several members felt strongly that the additional sloped region must be justified by structural stability since displays could be located elsewhere. Members commented that not all equipment has displays in this region, so it seemed possible for industry to redesign the displays to create less interference.

Industry input indicated that customer feedback was one of the major drivers to position the displays at that location. Technologists have reported that while positioning the patient’s breast, they are naturally looking downward, and by having the display at the base of the gantry, they could watch the compression force and breast thickness readout while positioning the patient. They suggested this location for the display as a method for improving workflow. On other machines, this region has other mechanical and electrical components that are not necessarily there for structural stability, but are important to the function of the equipment.

The Committee refined the configured allowance for the sloped region from 7 inches down to a height of 4 inches above the floor at a depth of 28 inches from the front edge of the breast platform. Industry worked with Committee members to optimize the accessibility of the base support configuration, shown in Figure 5.8.2.7(a) above. The Committee agreed to accept the refined configuration during the full discussion of the report with no reference to structural stability.

Figures 5.8.2.7(c) and 5.8.2.7(d) illustrate this region overlaid with the anthropomorphic data for manual and motorized wheelchairs, respectively. Steinfeld and D’Souza supplied these charts plotting their study’s findings on the depth of participant’s footrests and caster wheelsvii. The black lines show the areas with no interference between the footrests and caster wheels and the base support interface. However, based on the technical feasibility and clinical use considerations discussed in the above sections, industry proposed the overlay configuration in red in the figures.

Figure 5.8.2.7(c) Final Committee recommendation on allowable base support configuration overlaid with anthropomorphic data for manual wheelchairs. 
(SOURCE: Hologic, Inc.)

The black lines show the areas with no interference between the footrests and caster wheels and the base support interfacevii. However, based on the technical feasibility and clinical use considerations discussed in the above sections, industry proposed the overlay configuration in red.

Figure 5.8.2.7(d) Committee recommendation on allowable base support configuration overlaid with anthropomorphic data for power wheelchairs. 
(SOURCE: Hologic, Inc.)

The black lines show the areas with no interference between the footrests and caster wheels and the base support interface. However, based on the technical feasibility and clinical use considerations discussed in the above sections, industry proposed the overlay configuration in red.

The green dots in the scatter plot illustrations represent observed values for footrest height (on the vertical axis), and footrest depth (on the horizontal axis) measured for manual wheelchair users to the abdomen point. The footrest heights do not account for the 0.5-inch footrest thickness, so the required footrest clearances are 0.5 inch lower than the footrest heights plotted here. The footrest depth is the horizontal distance from the abdomen to the distal (forward-most) edge of the footrest. The person’s toes, however, will extend beyond the front edge of the footrests, so the Committee reviewed different data to determine the overall knee and toe clearance (see Section 5.8.2.3 above).

5.8.2.7.1 Mammography Chair Footrest Recommendation

The Committee recommends that the mammography chair meet the elements of M302 with the additional requirement that any footrests must accommodate and ride over the base support.

Rationale for the recommendation

The mammography chair is an essential piece of equipment allowing persons who are unable to stand or balance to sit for the diagnostic procedure. The chair must meet the elements of M302 with the additional requirement that any footrests must accommodate and ride over the base support.

5.8.2.8 Knee and Toe Clearance Recommendations Summary

Ultimately, all of the knee and toe clearance recommendations in sections 5.8.2.1 – 5.8.2.7 are summarized below, in Figure 5.8.2.8, which shows the measurements needed to determine if a certain piece of mammography equipment is accessible. For such a measurement, the top of the breast platform will be moved to 34 inches above the ground, and then the clearance dimensions will be measured just as they would be on any stationary piece of equipment, with allowance for slopes where shown.

Figure 5.8.2.8 Summary of Knee and Toe Clearance Measurements. 
(SOURCE: Hologic, Inc.)

The top of the breast platform would be moved to 34 inches above the ground, and then the clearance dimensions would be measured just as they would be on any stationary piece of equipment, with allowance for slopes where shown.