Designing Medical Products for Usability

Ergonomics dictate interface between hand and surgical tool

by Kent Ritzel, Metaphase Design Group, Inc.

The Bayer Microlet is operated using gross motor functions to make it easier for diabetics to use.

An opportunity to improve the usability of medical products through implementation of ergonomics has existed since medical practioners chose to use implements not designed for carpenters. Through proper ergonomics, manufacturers can achieve a person/product interface that enhances intuitiveness, comfort and convenience. Since most medical equipment, tools and devices involve the hand to a greater or lesser degree, a clear understanding of the anthropometry and function of the hand is necessary to maximize medical product utility.

The magnificent hand

Many argue that, next to the brain, the hand is the most complicated organ we possess. Consider what it affords. With more than 2,000 touch receptors on each fingertip, we are capable of sensing an individual raised dot no higher than 3 microns, differences in textural fields or roughness that is no greater than 75 nanometers high, and differences as subtle as 0.23mm in the shape of a grip.

As a mechanical manipulator, we have 32 degrees of freedom (direction of movement) in our hand and wrist. In comparison, a simple hinge has two degrees of freedom -- open or closed, just as the joints in the fingers possess with extension and flexion. Along with the physical design of the hand, there is the control side of the equation. The brain controls all of these joints in an eloquent learned, coordinated and precise fashion.

The complexity of the design and function of the hand presents a challenge to optimize the hand-product interface. If done properly, the design can provide sensory feedback and
communication: on versus off, hot versus cold, fluid versus air. The articulation of product form can also define functionality and features and directly influences the overall perception of product quality.

Unfortunately, there is no single cookbook of ergonomics and design guidelines for designing handheld products. What can be said with certainty is that an understanding of how the hand works will indeed provide a framework within which intelligent decisions and trade-offs can be made during product design.

The Bausch & Lomb Millennium II Ophthmalic Tool is designed to be an extension of the surgeon's hand. Grip architectures, center of gravity, surface texture and anchor points were determined through comprehensive ergonomic and design research

One of the first steps in designing a new product is to define the market. Who will use this product: patient, practitioner or care giver? The following reviews some of the basic facts in the equation of good design
for hands.

Smart versus dumb hands

Most everyone has a smart hand and a dumb hand -- a dominant hand and a non-dominant hand. Similarly, most hands have smart and dumb fingers. Thumb and pointer are the smartest digits we own. The middle finger is smart, but not quite as much as the thumb and index fingers. Ring finger and pinky are the stupid ones. This dexterous intelligence and control is evidenced time and again in the hierarchy of grasping patterns and grip architectures we use. We routinely scale digital functionality from bi-lateral to trilateral to multilateral to bi-manual grip architectures.

We can also use the index and thumb to execute a dexterous action while relegating the middle, ring and pinky fingers to hold an object, not unlike unscrewing a bottle cap while holding the bottle with the same hand. The transitions between these grip architectures are driven by many variables, primarily the environmental, task and organismic. These variables form the design constraints -- the task that needs to be performed, the environmental conditions (such as wet, cold or hot), and organismic limitations defined by what the hand itself is capable of performing.

Dumb hands are also an effect of age. Older hands are somewhat restricted in the motor function due to reduced joint motion, disease, or reaction time due to the aging process or the common onset of age-related diseases. In contrast, young hands can have a minimal level of functionality because the young owner has not yet learned to control all the degrees of freedom this complex manipulator offers. As a result locks out joints until the brain is capable of handling the complex motor coordination challenge.

A third and often overlooked form of dumb hands are those that have been consciously or unconsciously modified. Long fingernails, for example, can dramatically impact how fingers are used. As fingernails increase in length, the "attack angle" becomes flatter whereby they move from fingertip usage to a more distal use of the fingertip. As a result, the finger acts more as a lever rather than a fingertip actuator.

Designing grips

Another factor to consider in the design of handheld medical products is how the fingers are used to grasp and manipulate. Depending upon the task, our digits are used in very predictable patterns. Scale, mass and action are the primary factors that drive the emergent grip architecture used. For example, we use a bi-lateral pinch grip to manipulate a small object that is light, but once that object's mass exceeds a certain threshold, we change to a tri-lateral grip. Interestingly, we do not continue in this additive fashion. If a tri-lateral grip is insufficient due to the scale, mass or task one is trying to execute, then we typically migrate to a 5-digit multilateral grip.

Bi-lateral pinch grip

In designing products you can work the formula backward. For example, if you are designing a new urinalysis system that has knobs as one means of control, you have two primary variables to design with: diameter and torque resistance. Usually you will be sourcing an off-the-shelf knob solution and as a result will have to live with a certain rotary resistance. But through manipulations of diameter you alter the torque requirement. For design reasons you may define the knob diameter of choice which, by default, dictates the torque requirement. Now you can determine how many fingers are required to exert this level of torque and how many fingers are required to obtain a positive non-slip rotary grip so as to exert the force possible by those fingers. If the knob diameter is too small to provide surface area for all fingertips, then you will need to augment the interface by adding a textural treatment or elastomeric grip material to increase the coefficient of friction, thus allowing fewer fingers to exert the level of torque required.

Tri-lateral grip

Grip architectures can be generally categorized into a few generic categories including precision grips (as discussed in the example above), power grips and hook grips. For carry handles, the grip of choice is the hook grip. This is a unique static grip architecture. Once formed we can maintain this grip for long periods of time carrying very heavy loads, such as a suitcase. This is a direct result of one being able to lock the digits into a fixed posture and transmit the load to the stronger larger muscles of the whole arm.

Research the product and the user

An awareness of the distinct properties of the hand is an important step in developing usability and an efficient hand/product interface. But background information only gets you to the operating table, so to speak. Every new product requires design research to understand the user expectations, physical limitations, behavioral performance and environmental context. In addition to research on hand size, strength, grip architecture, joint mobility and task analysis, hand-held medical tools and devices, equipment and other products will greatly benefit from research studies designed to discover how your target audience actually uses your product. All of these results and insights are then incorporated into the design concept. The right concept and proper design improves your product's performance, functionality, coordination and precision and ultimately, improves your bottom line.

About the author: Kent Ritzel, MBA, is Executive Vice President at Metaphase Design Group, Inc., a St. Louis-based design research, ergonomics and product visualization firm specializing in hand-held and high-tech products.

For more information: Circle 523 - Metaphase Design Group, or connect directly to their website via the Online Reader Service Program at http://www.1rs.com/011md-523