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

Knee & Toe Space [M303.2.4]

Recommendation: The final subcommittee’s recommendations increased the overall knee and toe space to a minimum of 28 inches, from the initially proposed 25-inch absolute dimension. The final recommendation for knee clearance directly underneath the breast platform is a minimum of 18 inches and the final recommendation for the unobstructed floor space in front of the base support is a minimum of 17 inches. Each of these measurements creates a more accessible piece of imaging equipment balanced with the physical attributes and operation of the equipment.

Rationale

The issue of Clearance and Unobstructed Floor Space

The knee and toe clearance dimensions, as well as the unobstructed floor space dimensions, dictate how close a patient in a wheelchair can get to the mammography equipment. The initial NPRM dimensions used information for the clear space under a desk or table as the basis for the proposed regulation. Industry suggested the dimension might be different because mammography procedures require a person’s chest to be flush with the front of the breast platform. The subcommittee reviewed the existing data of wheelchair users¹ and industry input on how these elements technically affect one another. Below is an overview of the rationale for each of the final recommended dimensions affecting clearances in front of the gantry. (Table 1)

Table 1. Critical Equipment Dimensions with the relevant Anthropomorphic Data

One issue affecting how close the patient can get to the mammography machine is the base support, an essential equipment element that intrudes into the floor space. As discussed below, 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. Since the base support obstructs the floor space, resulting gains from increasing the knee and toe clearance are minimized because of the simultaneous increase in the size of the base support. The subcommittee considered this balance when making final decisions about overall knee and toe space and unobstructed floor space.

While discussing these issues, the subcommittee 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-to-toe and abdomen-to-caster edge dimensions by a couple inches. The anthropomorphic data shows that 30.5 inches and 19.5 inches are the needed overall knee and toe clearance and unobstructed floor space dimensions, respectively. (Data shown below in Tables 2 and 3).

Table 2. Abdomen Depth Data to determine overall knee and toe clearance

Table 3. Abdomen-Caster Edge Depth

 In applying the various positioning considerations along with the data, measurements of 28 inches (full knee and toe clearance above the base support) and 17 inches (clearance underneath the breast platform) would capture nearly everyone. Looking purely at the anthropomorphic data above, these dimensions accommodate ~90% of the population. In adjusting for the needed upright pose, an adjustment of at least 2.5 inches is likely. If so, the equipment accommodates 95% of the population since the difference is ~2.5 inches less than the 95th percentile of the corresponding anthropomorphic dimensions. Members recognized that these dimensions are an optimal blend of accessibility and technical feasibility² when balanced with the physical design and operation of the equipment,

Figure 4. Overview of the critical dimensions that define the knee and toe space underneath the breast platform.

Relationship of knee and toe space with breast platform

The shape of the breast platform affects the shape of the knee and toe space. Since the shape of the breast platform varies across different manufacturers and equipment, merely defining the knee clearance as the “knee clearance below the breast platform” would lose the interrelated aspect of these elements. This means a particular feature on the breast platform does not define the required location of the knee clearance. Instead, the knee space dimension is located with the platform at a specified height. 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. Ultimately, the knee dimensions must be determined when the person’s breasts are on top of the breast platform and there must be sufficient clearance at the height of the person’s knees, as is shown in Figure 6 below. By using this approach, the subcommittee was able to maximize the clearances in relationship to the equipment operations.³

The NPRM proposed measuring the knee clearance under the breast platform at 27 inches above the ground. Industry used this dimension and determined the proportionate height of the breast platform at a height of 34 inches. The subcommittee agreed to this as a “set point” (Identified as “Breast Platform Height” in Figure 5 below). Thus, when the patient’s breast is on the breast platform, there must be sufficient knee clearance at 27 inches above the ground (“Knee Height” in Figure 5 below). These dimensions (27 and 34-inch heights) are the same as is defined by the current ADA/ABA accessibility standard to accommodate wheelchair users at desks. The dotted lines in Figure 6 below illustrates this dimension.

Figure 5. Using the height of the top of the breast platform as a set point for the knee and toe clearance measurements.

Figure 6. Illustration of current ADA/ABA standard to accommodate wheelchair users sitting at desks.

The two dimensions define the breast platform requirements for a person with the deep knees and a tall wheelchair. When the breast platform lowers to accommodate shorter patients in 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.

Toe Height

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. The subcommittee recommended that the toe height be set to 16 inches. Looking further at female-specific toe height data, the 95th percentile toe height was 13 inches for manual wheelchair users and 16.1 inches for power wheelchair users. As such, the subcommittee recommended that the toe height be set to 16 inches (when the breast platform is at the 34 inch height discussed above). This higher 16-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

Again, normally a toe space dimension defines a static unobstructed space. In this case, in order to get the maximum accessibility out of all the dimensions, the final recommendation requires the dimension of toe clearance is determined at the 34-inch breast platform height (see discussion above). This higher 16 inches measurement (compared to the originally proposed 9 inches toe height) would minimize the chance of the breast platform hitting patients’ toes when the breast platform moves to some of its lower heights. Providing this dimension helps define the outer limits of the breast platform configuration in a way that maximizes the dimensions for accessibility.

When considering toe height, it is more convenient to use the anthropomorphic model that defines toe height specifically, shown below in Figure 7 for manual chairs.

Figure 7. Anthropomorphic model used to analyze toe height.

If the toe height is at 16 inches above the ground when the breast platform is at 34 inches above the ground, we are accommodating the 90-95th percentile person. The toe height is more than enough to accommodate wheelchair users since the 95th percentile toe height is 13 inches. When the breast platform lowers to a 26-inch height, then the toe clearance height would be 8 inches. This maintains proportions likely to accommodate persons needing the breast platform at the lowest height since knee height is also less.

As discussed above, the minimum range of travel for the breast platform is 26 inches to 42 inches. When the top of the breast platform is at its lowest position of 26 inches, there is no need for knee clearance at 27 inches above the ground (knees will be lower than breast height). Defining knee and toe clearances based the lowest height breast platform at 26 inches may compromise taller patients in higher wheelchairs. As such, it is more appropriate to measure the dimensions of the knee and toe space set the breast platform to a higher height to ensure that the greatest required knee and toe clearances. Then, when the breast platform lowers to accommodate smaller patients, there will be more than enough clearance available.

The diagram below illustrates the final knee and toe clearance recommendations (Figure 8).

Figure 8. Summary of knee and toe clearance recommendations.

Notes: 

^1. Where possible, the subcommittee focused only on the anthropomorphic data of women, the primary group needing mammography.

^2. The concept of technical feasibility is one found in other areas of access.  It recognizes that there may be physical factors that make create limits to what functions can be achieved even with redesign.  In mammography, the physical stability and operation of the equipment fall into this category.  The committee recognizes that future visionaries may be able to “re-vision” new devices.

^3. Given the differing shapes of mammography devices, defining the dimensions in relationship to each other with a static measurement point for the breast platform assures accessibility despite the varied shapes and dimensions of the breast platform. Otherwise,, it is quite possible to configure a piece of equipment that meets all the clearances yet does not provide accessibility if there is not a defined method of measuring the clearances in relationship to the breast platform.

^4. Measured from the point on the floor where a perpendicular line intersects the front edge of breast platform shown in Figure 10.