Faced with this task in the summer of 2003, I sensed that my new company could do a better job in designing dental stools by incorporating modern ergonomic principals.
In large part, dental stools had followed the same design characteristics since sit-down dentistry was developed in the 1960s. Round seats, deep seat pans, horizontally curved back supports all were hold-overs from a time when little was known about the stress put on the body while working in a seated position. Since that time, however, there have been many definitive studies by doctors and ergonomic scientists such as Mandal, Keegan, Burandt, Grandjean, Lueder, et al., that suggest we could do a better job in designing and utilizing a stool in the dental-operating environment.
I came to appreciate proper seat design in a prior career, professional automobile racing. Many years of racing at over 230 MPH, in durations of 24 hours on the Le Mans Grand Prix circuit in Europe and North America convinced me that a proper fitting seat is directly related to performance, maintaining concentration, and lessening fatigue, not to mention what happens to your body by the extreme forces placed on it when you crash.
In the Lola EX 257 Le Mans prototype race car that I drove, every driver had a custom made seat that perfectly matched the contours of his or her body from the knee to the base of the neck. These specialized seats were constructed of poured foam that enveloped the body as it cured, and while it cured to a rock-hard surface, it was very comfortable because it matched the body’s contours exactly.
These seats were routinely replaced three times a season at a cost of $3,000. each. Although I had many crashes in my racing career, I was rarely hurt, in large part because of the design of the seat. I was also able to win many races because a properly designed seat allowed me to perform at my best. Most dental professionals do not have to perform at over 200 MPH or over a 24-hour duration at one time, the principals of good ergonomic seating translate into the same benefits of excellent performance and physical well being in the dental operatory.
Most traditional dental stools on the market prior to 2004 had been designed with a man’s measurements in mind—they did not seem to fit women very well. I started hearing from my women dentist friends, specifically the leadership of the American Association of Women Dentists and several of the dentists behind the industry publication, Woman Dentist Journal that they needed something different in their offices. It made immediate sense to me.
At our new company, we set a goal to bring modern ergonomics into the dental environment and started the process by watching hundreds of female hygienists, dental assistants, and dentists in their work environment. It quickly became clear to us that a better design—a more ergonomically designed stool was needed for women.
Research at that time told us that although women dentists made up less that one-fifth of the dentists practicing in the United States, dental school enrollments painted an entirely different picture for the future. Realizing that the time was rapidly approaching when 50% of the practicing dentists would be women, it was clear that this segment of the market deserved an operating stool that was designed with their particular body type in mind.
Practicing dentistry requires the operator to sit in two primary positions, the active position in which the operator is working on the patient, and the passive position in which the operator is engaged in activities such as conversing with patients or assistants, prep work, and viewing film. Most stools facilitate only the passive seated position without a forward-tilt motion. They do not allow the user to move forward unless the user bends at the waist (Figure 1).
This position loads the discs with pressure. Also needing to get a better visual plane, the user may gain additional height by slightly elevating the body, transferring pressure to the legs so that the user is almost approaching a supported standing position.
Currently, most seat pans in the industry are too deep for the average woman’s measurements. When used by a woman, this overly deep seat promotes a situation that we call “perching,” or sitting on the leading edge of the seat pan. When a user perches on the front of the seat, (1) she has no back support, because no back rest will travel that far forward, (2) she restricts the blood flow to the lower leg because her thigh is only intersecting the front edge of the seat pan, and (3) she creates sore pressure points on her tailbone and hip joints because all of her weight is concentrated on a small area and not spread out evenly over the entire seat pan. All of these conditions are magnified when the user leans forward into the active position to work.
When we surveyed hundreds of female dental professionals in their work environment, we also saw a slumping of the lower lumbar region of the user’s back from no back support, we saw feet that could not be placed flat on the floor because the stool would not go low enough, and we heard complaints of feet tingling from a lack of proper blood flow.
Once we analyzed the problems women experience when seated on a stool originally designed for a man, we made up multiple prototypes of seat pan and back designs and sent them out to offices where women dentists worked. One dentist who helped us immensely during our testing phase, Dr. Lori Trost, tested over 36 different seat/back combinations. Dr. Trost makes a good point when she says, “We always felt like there could be a better choice for operatory seating because we were never very comfortable using the traditional stool designs. They just did not seem to fit right. The testing took a lot of effort and time, but in the end, it was certainly worth it.”
After almost a year of collecting data, the most popular designs were brought back to our facility to be made into stools that could go to trade shows for further testing. After all the data was analyzed, one seat shape and two back shapes that fit all of our desired design criteria were overwhelmingly popular.
In this new design for women dentists, we shortened the seat pan from the traditional 17 inches to 12 inches to allow the user to sit back in the chair, we sculpted out the area in the back of the seat pan to relieve pressure on the tailbone, we sculpted out the front sides of the pan, similar to an old-fashioned bicycle seat to relieve pressure under the thigh and increase blood flow to the lower leg, and we put a tilting mechanism on the stool to allow the seat pan to tilt forward, thereby allowing the user to maintain a moderate level of lordosis in the lower back, instead of a kyphosis positioning where the inter-vertebral discs are pinched. In other words, this means that the user maintains the natural curve of the lower spine when seated, as A.C. Mandal recommends in his seminal work on ergonomics, Homo Sedens (the seated man).
Epidemiological studies are showing an increase in chronic back problems linked directly to the occupational demand to sit for extended periods of time. When compared to the standing position, the seated position puts twice as much pressure on the inter-vertebral discs, even when sitting up perfectly straight.
In order to understand this phenomenon, one must observe the anatomical changes that take place when a person moves from a standing to a sitting position. The findings suggest that the uneven vertebral pressure is a result of exaggerated pelvic rotation.
Note the lordosis curve in the standing Figure A and how the curve is beginning to straighten (kyphosis) in Figure B.
Any time the upper leg is brought upward, thus bending the hip joints 60 degrees or greater, the pelvis rotates inward, and pulls the vertebral column out of its protective lumbar curve.
Sit-down dentistry increases the likelihood that the practitioner will develop soreness in the lumbar region as a result of this positioning. As is shown in the following picture, the lordosis curve is taken out of the lumbar region when the person is seated through a rotation of the pelvis, thus putting painful stress on the inter-vertebral discs.
In order to help maintain the lordosis curve, we also put an extra-thick backrest on the stool to allow the user to bring the rest all the way up into their lower lumbar region, and we designed an extra short cylinder so a shorter user can get the stool low enough to put both feet flat on the floor.
The seat shape closely resembles a “saddle” style. A saddleshaped seat eliminates undue pressure on the leg while facilitating a declining leg position to make it possible to maintain the proper lumbar curve. The two available backrest shapes include a heavily padded “lumbar” back style and the other style resembles a “pear” shape with a heavily padded lumbar bolster.
This seat, with its option of two back styles, became our C70D series of women’s dentist stools. Although the exercise started out as a way to design a better stool for female dentists, its design attributes cross over to many male dentists as well, as 40% of our sales of this design are to men dentists. As Lisa Broering, OTR/CHT, a nationally recognized expert on ergonomics aptly states, “The important point is to look at the hip to knee ratio for shorter users and fit the seat pan to that measurement. A shorter seat pan also allows the user to get in closer to the patient chair, eliminating the need to bend farther forward to access the oral cavity.”
Next up was an assistant’s stool. When it comes to the traditional “round seat” assistant’s stools, we saw the majority of users perching on the front of the stool seat. Besides making the stool “tippy” and forcing the user to keep at least one foot on the floor instead of the footring to maintain balance, the positioning on the seat puts excessive pressure on the body where it intersects the front edge of the seat. To correct this situation, (1) we shortened the seat pan to keep the user from perching on the front by making it oval instead of round, (2) we sculpted out an area in the back of the seat to relieve pressure on the tailbone, (3) we put an extra thick lumbar bolster back on the stool to provide proper back support, and (4) we repositioned the attachment point of the cylinder to make the stool stable, even when the user has both feet up on the footring. We have two series of assistant stools that incorporate these ergonomic attributes, with slightly different size seats and backs. We still make the traditional round assistant’s stool for customers who want that style, but the large majority of our assistant stool orders are for our C50ABT and C60ABT stools that incorporate these ergonomic modifications.
Deserving a stool designed specifically for their specialized motions, hygienists also got the appropriate consideration. When we watched hygienists work on a traditional maleoriented dental stool, we saw the user perching on the front of the stool, with no back support, cutting off circulation to the lower legs, and sometimes hooking their foot on one of the stool legs to keep the stool from sliding out from under them. It was obvious that the seat pan was too deep for the hygienist’s anatomy and some of the time the stool would not go low enough for them to place both feet flat on the floor. Our C30HS stool for hygienists starts with a tilting, shallower seat pan to allow the user to sit further back or fully on the seat, thereby spreading their weight over a larger area so as to not cause undue pressure points. We relieved the area directly under the tailbone to reduce the pressure that spreads up through the spine into the lower back. We doubled the thickness of the back rest to provide increased back support to the lumbar region, and we designed a shorter cylinder to allow a lowered seating position for shorter hygienists.
The development of dental stools “designed by women for women” has been long overdue. This new design direction demonstrates Crown Seating’s commitment to a dynamic, progressive, and intelligent customer base, and is a direct response to the request for more ergonomic and specialized seating. We are grateful to the many women dentists, assistants, and hygienists who contributed their time, input, and energy into helping us learn more about how they work, and what they need. As a company, Crown Seating looks forward to continuing to improve our stool designs for both men and women.
The author, Steve Knight, currently owns two Colorado-based dental equipment manufacturing companies, Crown Seating LLC (dental stools) and DNTLworks Equipment Corporation (portable and mobile dental equipment). Now retired from professional racing, Steve won the American Le Mans Championship in 2002.