Appendix 3: Commentary on Data, Models and Procedures for Design of Accessible Medical Instrumentation

Gaps between demands imposed on an individual by a task and an individual’s capabilities are often identified after the system has been designed and a user is unable to successfully perform the task, becomes injured or develops a health problem. In some cases, user trials, in which the equipment is tested by a wide range of users before it is produced, are conducted so that the final design can be modified in ways that enable all users to achieve the goals of the system. A wide range of users often are not available to try the equipment and procedures. Data, models and procedures for applying them are needed that can be used to evaluate equipment and procedures before they are produced [1]. Models should consider physical demands and capacities, such as users’ ability to see, reach, grasp and exert necessary forces without becoming injured. It also should consider sensory and cognitive abilities, such as users’ ability to detect and process information about the equipment and environment.

The evaluation and design of an examining table illustrates how models and analysis tools can be used to identify physical barriers to persons with conditions that restrict their reach and strength. An analysis of the task and equipment show that users must reach and grasp something on the opposite side of the table to pull themselves into position and to stabilize their body. Using three-dimensional biomechanical models, it can be shown that persons with spinal chord injuries, (SCIs) have reduced reach capacities compared to persons without SCIs [2]. Designers can use these models to locate handles that will enable patients with SCI to complete the transfer. In addition to reaching, it is necessary to grasp a solid object to pull against. Often this solid object is the side of the table. Figure 1 below shows a hand posture predicted for gripping the edge of a table using a three-dimensional biomechanical hand model [3]. All of the force must be exerted with the finger tips, which only have about 15% as much strength as the hand in a power grip posture. Figure 1b shows that the predicted posture for gripping a 3.5cm diameter rail is a power grip – a position of maximum hand strength. Additionally, the rail can be mounted in such a way that it can be moved closer to the opposite side of the bed for a patient with a reduced reach or limitation and it can be positioned as a guard rail to prevent persons who cannot control their posture from falling over the edge.

Figure 1: a. Biomechanical hand model shows that fingertips must support transfer loads. b. Model also shows that adjustable handle can be used to eliminate barriers due to reach and hand strength limitations.

The use of models can proactively increase the number of persons who will be able to use the medical equipment and reduce their risk of injuries and illnesses. Basic biomechanical models have been developed, but additional research including population studies are necessary to develop models for evaluating and designing all medical devices.

References:

1. Brandt, E. and Pope, A., Ed. Enabling America: Assessing the role of rehabilitation science and engineering. Washington, DC, National Academy of Press, 1997.
2. haffin, D., Woolley, C., Martin, B., Womack, N. and Dickerson, C. Reaching and Object Movement Capability in Spinal Cord Injured Population. NIDRR Workshop on Anthropometrics, Buffalo, NY, 2001.
3. Choi, J. and Armstrong, T., 3-Dimensional kinematic model for predicting hand posture during certain gripping tasks. International Society of Biomechanics, Cleveland, 2005.