PRODUCT DEVELOPMENT INSIGHT
Popular television shows portray the changing demographics of people in medicine. In weekly episodes, gender-balanced ensembles of attractive women and men portray doctors, each taking their turn at cardiopulmonary resuscitation (Clear!...Thump!). Although the scenes are make believe, they reflect a genuine trend in the United States and abroad.
Indeed, the American Medical Association's latest figures indicate that 26.6% of all U.S. doctors are women, up dramatically from 7.6% in 1970.1 Obstetric, gynecologic, and pediatric specialties are reportedly drawing the most women, with 74.5%, 69.1%, and 51.9%, respectively, of all new residents in 2004 being women.2 Even more telling, about 50% of all U.S. medical school applicants are now women.3 This suggests that the total proportion of practicing female doctors will grow dramatically in the coming years, eventually achieving parity with male doctors.
The demographic shift has significant implications for surgical instrument designers. Surgical instruments have traditionally been designed for men. Therefore, the demographic shift calls for designers to broaden their product requirements. They must address the needs of women in addition to men. According to many women surgeons who have struggled with instruments designed almost exclusively for men, the changes are overdue and there are continuing consequences. Viviane Connor, MD, codirector of the section of minimally invasive gynecology at Cleveland Clinic (Weston, FL), observes, “I've never seen a surgical instrument that was too small for my hand. The average new instrument is still too large and bulky for the average female surgeon. Many require the application of excessive force. Some stretch a woman's hand to the limit, or even require the use of two hands.”
Size and Strength Comparisons
To date, women have had to cope with instruments designed for hands that are, in most cases, larger and stronger than theirs. As shown in Figure 1, the 5th-percentile woman's hand is considerably smaller than the 95th-percentile man's hand in every dimension. The differences are even more pronounced at the 1st and 99th percentiles, where thumb widths, for example, range from a mere 0.6 to 1.2 in.—twice as wide (see Table I).
Strength differences are also dramatic.4 The 2.5th-percentile woman's squeezing grip strength, for example, is 53 lbf, while the 97.5th-percentile man can exert 147 lbf. For the same individuals, fingertip pinch strengths range from 7.5 to 30 lbf. As a rule, designers can figure that the average woman's hands, and upper extremities in general, are about half as strong as the average man's, but the difference is exaggerated at the extremes, as illustrated by the earlier data.
Unfortunately, data for individuals at the extremes are spotty, especially for women. Therefore, designers interested in estimating extreme values can use the formulas presented in Table II. However, for those formulas to work, designers must be able to obtain mean (M) and standard deviation (SD) values for a given measurement.
Figure 1. Comparison of 5th-percentile female and 95th-percentile male hand length.
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Consequences of Poor Ergonomics
Keeping the biomechanical differences between men and women in mind, myriad usability problems can arise from women using oversized tools. As Connor described, women with average to small-sized hands need to stretch them to grip widely spread handles. Small-diameter fingers will float in finger holes, potentially causing instability and physical discomfort. Instruments requiring an especially firm squeeze might place repetitive stresses on weaker hands. For example, a tool contoured for a comparatively large hand could contact a small hand in the wrong spot, also causing unsteadiness and possibly pain.
A recent survey sponsored by Ethicon collected hand size and strength data from the participating surgeons. The data suggest that almost 50% of all surgical instruments feel too large to grip comfortably and feel improperly contoured to the hand. Moreover, respondents with particularly small hands opined that 80–90% of all surgical instruments are not ergonomically suitable for their particular use. In addition, many tools require the surgeons to apply forces that are difficult for them to deliver. Although the data sample from the survey is not statistically significant given the small number of measurements, it does illustrate the range of female hand size and strength for a specialized user population that is underrepresented in available databases.
Table I. Comparative data for small women and large men. Data source: “Humanscale 6b—Hands and Feet” (Cambridge, MA: MIT Press, 1981).
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Susan Johnson, MD, a gynecologist practicing at Legacy Portland Hospitals (Portland, OR), agrees that women make adjustments to ill-fitting instruments. “Most surgical devices are designed with men in mind, which places women at a disadvantage,” she says. She worries that women with particularly small hands might turn away from a career in surgery because the tools of the trade are generally designed for larger hands.
Working with oversized tools can make any task more difficult. This is certainly not to say that women surgeons are less effective when forced to use oversized surgical instruments— humans are generally good at adjusting to design shortcomings. Still, logic suggests that task performance peaks when tools are properly sized to the user's body. Moreover, device design should adapt to people, not the other way around.
Designing devices to adapt to people is what user-centered design is all about. The philosophy holds that a design should be based on user needs rather than the demands of a particular technology or designer's preferences. Many successful products in many different industries have arisen from clearly defined user requirements. For example, the Oxo can opener is notable for its soft-feeling handle and smooth, almost effortless operation. Originally it was geared toward use by individuals with arthritic hands. In other words, it was designed for hands possessing limited strength and range of motion. However, the product satisfied not only the needs of customers with hand impairments, but also people with normal use of their hands. Therefore, the product had universal appeal and stands as an exemplar of universal design.
Table II. Formulas for calculating percentiles, where M = mean and SD = standard deviation.
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Concepts like the one described here can be borrowed and expanded upon when designing surgical instruments. A user-centered design process should lead directly to designs that accommodate both men and women. The designs should require little, if any, physical adjustment. In cases where one tool simply cannot accommodate both large- and small-handed individuals, it would be appropriate to produce two or more models. Of course, working with accurate human factors data regarding men and women's needs is essential.
The data comparisons presented earlier in this article represent the tip of the iceberg. This kind of data—called anthropometrics—is available in many publications and software applications.
One of the more popular, albeit simplified, sources of data is called Humanscale. It is a set of nine plastic charts and associated booklets (see Figure 2). Simply set one of the charts' rotating plastic dials on “small woman” and it provides various body size and strength values for the 5th-percentile female. Although academics may seek a more extensively referenced data source, Humanscale is nonetheless a useful tool. It provides a good starting place for designers interested in creating products that fit intended users' physical characteristics.
A more basic, practical, and reasonably effective approach is to enlist the support of people who represent the physical extremes. For example, within a company that employs dozens or hundreds of people, it would probably be easy to find individuals with very small hands and others with extremely large ones. Although they might be perfectly pleased with their own hands, designers may think of them as worst cases. Designers can use them as reference points for judging the fit and usability of instruments at the extremes.5
Figure 2. Example of a chart from “Humanscale 6b—Hands and Feet” (Cambridge, MA: MIT Press, 1981). This chart depicts basic grips and wrist pivots, among other data.
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Going in the other direction in terms of rigor and complexity, designers might also use advanced software tools to support their product development efforts. Notable software applications include ManneQuinPro, JACK, and the Santos Hand. The latter can simulate the movement of a hand based on a computer model incorporating 25 degrees of freedom (meaning that it can be moved in 25 different ways, e.g., up, down, left, right, etc.).
Conducting special studies is another way to gather pertinent anthropometric data. For example, as part of the aforementioned survey of female surgeons, researchers recorded data during various dimensional and strength measurements. Key data (measured and calculated) for this subpopulation are presented in Table III. However, it should be noted that the sample size was quite small.
The female surgeons were asked to pinch a force gauge with what they considered to be a comfortable amount of pressure for a short period (5 seconds). The results showed that the surgeons used about two-thirds of their maximum pinching force. However, when the surgeons were asked to grip a force gauge with a comfortable amount of pressure for a short period (4 seconds), they used a bit less than half of their maximum gripping force. These comfortable force levels would undoubtedly decline as the force application duration increases.
Table III. As part of a survey of female surgeons, data were gathered for various dimensional and strength measurements (data are rounded to tenths).
Applying the Data
If designing for the average user causes misfit designs, disregarding the needs of women altogether is even worse. A surgical instrument designed specifically for the average man's hand probably would not meet the needs of large- or small-handed individuals, male or female. But many more women, perhaps even a large majority, would have to cope with physical incompatibilities.
The best approach is actually to design in a gender-neutral manner. Designers should try to accommodate hands ranging widely in size and strength, rather than men versus women. After all, some men have small hands too—5 out of 100 men have shorter hands than 3 out of 4 women (7.2 versus 7.3 in.).6 Depending on the surgical instrument, one solution might cover the full range of users. Alternatively, there may be a need for two, three, four, or even more models of the same basic instrument to accommodate all users. For example, there are at least eight different sizes of surgical gloves (sizes 5.5 to 9).
The consequence of designing explicitly for an average-sized individual—male or female—is that the resulting product may accommodate very few users.
Figure 3. A hemostatic forcep. Small differences in finger-hole size could affect ease of use.
Consider the nonmedical example of jewelry. A size 8 ring may fit only an average person and be too loose or too tight for people with slightly narrower or thicker fingers. Therefore, U.S. adult ring sizes commonly range from 4 (113¼16 in.) to 13 (215¼16 in.). Accordingly, the key is to create medical device designs that work for the widest possible range of individual characteristics without compromising performance. Obviously, a ring needs to fit snugly but not too tightly, so rings need to be made in many sizes. But consider the finger holes in a hemostatic forcep (see Figure 3). A small difference in finger circumference could have a significant effect on the fit. Notably, the index finger circumferences (measured at the proximal interphalangeal joint) of the female surgeons surveyed averaged 2.35 in., but varied over a range of 0.44 in.
Gender-biased products are prevalent in society. Some products are definitely geared toward either men or women. Particularly large or small individuals can confirm there is also a bias toward average-sized people. This explains why tall men bang their heads on aircraft ceilings and small women cannot reach the overhead luggage compartments. The common solution to such problems is to duck down or ask for help. However, user adjustments to improperly sized medical products can make for less-than-optimal performance and customer dissatisfaction. Moreover, nobody wants his or her surgeon struggling to grasp an ill-fitting instrument. Establishing appropriate anthropometric requirements at the start of new product development efforts can help solve this problem. Given proper data and follow-up user testing with representative users, the chance of producing ill-fitting surgical tools will be greatly reduced. Manufacturers should remember that half of the new doctors in the United States will be women in the not-too-distant future. In response, they must design products that will accommodate both ends of the user hand-size spectrum.
Michael Wiklund is president of Wiklund Research & Design Inc. (Concord, MA). Jim Rudnick works in research and development at Stryker (Kalamazoo, MI), and Jessica Liberatore is an industrial designer with Ethicon (Somerville, NJ).
2. “Appendix II. Graduate Medical Education,” Journal of the American Medical Association 297, no. 9 [online] (2005): 1129–1131; available from Internet: www.ama-assn.org/ama/pub/category/12915.html.
3. “Table 2—Women Medical School Applicants,” in Women in U.S. Academic Medicine Statistics and Medical School Benchmarking 2004–2005 [online] (Washington, DC: Association of American Medical Colleges, 2006); available from Internet: www.ama-assn.org/ama/pub/ category/12913.html.
4. “Humanscale 4a—Human Strength” (Cambridge, MA: MIT Press, 1981).
5. Michael Wiklund, “Defining and Designing for Worst-Case Users,” Medical Device & Diagnostic Industry 28, no. 7 (2006): 52–59; available from Internet: www.devicelink.com/mddi/archive/06/07/ 012.html.
6. “Humanscale 6b—Hand Length” (Cambridge, MA: MIT Press, 1981).