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From an ergonomic standpoint, the operator stool is the most important chair in the treatment room. Patients come and go, but the dental assistant is there throughout the day, so proper and safe positioning of the body should be paramount. Poor fit or adjustment of the stool can lead to low back, neck, or shoulder pain. Naturally, the stool should adjust to support the body in a neutral posture, but with the wide variety of body sizes and heights among dental practitioners, certain stool types will fit individuals better than others.
When it comes to the selection process, the vast array of designs and features on the market today can make it a perplexing project. Numerous features must be considered and evaluated for their fit to a particular body type. These include cylinder height, seat contour, armrests, seat depth, tilt, backrest, wheel casters, and stool style. Of these, the two most poorly understood ergonomic features are the backrest and armrest—and understandably so. Dental stool models claiming to be “ergonomic” run the gamut from backrests with armrests, to backrests without armrests, to armrests without backrests, to no backrests and no armrests. To add to the conundrum, there is a dearth of dental studies regarding the benefits (or lack thereof ) of these two features on the health of the dental practitioner. But what is certain from the literature is that these features definitely have an impact on health, so they deserve more in-depth evaluation.
The Backrest
A primary function of the backrest is to help maintain the lumbar lordosis in sitting by maintaining contact with a convexity on the backrest called the lumbar support.1,2 This is an especially important feature with a non-tilting seat pan, which tends to flatten the low back curve. When the lumbar curve is allowed to flex (as in slouching), the iliolumbar ligaments and other dorsal soft structures are strained and weakened, predisposing them to injury.2-5 Slouching also disengages the posterior vertebral facets, which play a primary role in supporting the spine when standing. When these are disengaged, the load is transferred to the spinal disc. The disc nucleus is then pushed backward, and can eventually result in a disc bulge or herniation.6 Some secondary benefits of the lumbar support are to reduce lumbar disc pressure, decrease low back and referred leg pain, reduce forward head posture, and decrease muscle activity (rest the back muscles).7-10
The lumbar support portion of the backrest should be convex from top to bottom to conform to the natural lordosis of the low back. It is the portion that supports the low-back curve and need only be eight inches or so in height to provide adequate support (Figure 1 and Figure 2). Many dental stools have no recognizable lumbar support at all. The prominence of the lumbar support can impact disc pressure. Sizes between 1 cm and 5 cm in thickness have been evaluated, with the largest benefits gained from the larger (3 cm to 5 cm) thickness.7,11 Very large backrests should be carefully evaluated to ensure adequate convexity of the lumbar support to properly support the low back and also ensure that the upper portion of a large backrest does not push the thoracic spine forward. [Note: Use of a prominent lumbar support can worsen pain symptoms among patients with spinal stenosis or spondylolisthesis.]
Because of the nature of their work, dental assistants cannot always maintain contact with the lumbar support of the stool and intermittently must lean slightly forward to view the tooth surface. Spinal disc pressure is highest in this type of anterior sitting,11 which is why it is imperative that dental assistants learn to properly stabilize and protect their low back with their internal core stabilizing muscles (primarily the transverse abdominal muscles) whenever they leave the backrest of the stool. When properly used, these muscles have been shown to reduce low back pain.12 An excerpt from the author’s book, Practice Dentistry Pain-Free: Evidence-Based Strategies to Prevent Pain and Extend Your Career, describes how to use these muscles in the operatory to prevent low back pain whenever leaning forward. The exercise stabilizes and protects the lumbar spine by engaging an individual’s “internal backbelt” (Table 1).
Shorter dental assistants may find that seat pans that are too deep inhibit contact with the backrest. In these cases, a shorter-style seat pan should be considered. Finally, because lumbar disc pressure is lower in standing than in unsupported sitting, dental assistants should also try to stand periodically when resting against the backrest is not possible.7
The backrest should adjust up and down, as well as toward and away from the dental practitioner’s back. Although studies show that the lowest lumbar disc pressure and muscle activity is with the backrest reclined to about 110° to 120°, it is not possible to practice dentistry in this position.7,11 In occupations where upright posture is necessitated, sitting with the thighs sloping slightly downward can help minimize disc pressure and muscle activity. Because static seated postures should be avoided, it may be beneficial to readjust the backrest and seat pan tilt periodically to move the work load from tissue to tissue to minimize microtrauma.11,13
The shape of the backrest is an especially important consideration. A tall backrest that extends above the lower edge of the scapulae can minimize the benefits of the lumbar support by exerting pressure on the shoulder blades, and should, at its highest point, be 6 cm below the lower edge of the scapulae.2 Because the ribcage is a rigid structure, back support above the lower edge of the shoulder blade is unnecessary, and in dentistry it can inhibit shoulder movement. Wide backrests should be avoided because these can inhibit spinal movement and lateral movement of the arms and shoulders, which is especially important in the prevention of low back pain syndromes.6
No Backrest
Operators often ask why some stools (primarily saddle-style stools) are considered ergonomic when they have no backrest. When sitting in a horse saddle, the operator’s pelvis (the foundation of a seated posture), is in a near-neutral position, as in standing. This pelvic position allows the spinal curves to balance more easily in proper alignment and reduces muscle strain, which is why backrests are considered optional on saddle stools. By design, saddle stools will place more compression on the peritoneal area, and should be carefully evaluated for suitability to the operator.
Armrests
Studies support the use of armrests in the prevention of neck, shoulder, and low back pain.7,14-16 Armrests have been shown to decrease muscle activity in the neck and shoulders of dental practitioners, especially in the upper trapezius on the handpiece, or dominant side.15 Elbow rests also reduce activity in the rhomboid, and thoracic and cervical erector spinae muscles.14
There are several styles of armrests on the market today: fixed but movable, swiveling, pivoting, and telescoping armrests. Armrests should be highly adjustable to provide support to the operator in a neutral working posture.6 Different styles tend to be more appropriate for different team members. So which style of armrest is right for the individual? Because dentists tend to work in one position longer than dental assistants, fixed, movable armrests are preferred by many dentists, especially endodontists. This type of armrest system is highly adjustable in multiple directions: width, height, forward/backward, and the armrest itself tilts for optimal hand positioning (Figure 3). Once the desired position is found, the knobs can be tightened to secure the settings. Swiveling armrests (Figure 4) move freely with the operator, which many dental practitioners find favorable. One drawback may be if the armrests swivel too freely, the operator cannot find it when needed. A very popular and effective style among dental practitioners is the telescoping/hydraulic armrest which moves in/out and forward/backward with the operator and can be set at a specific height and resistance (Figure 5). A pivoting, movable armrest (Figure 6) uses a small circular pad on the armrest, which moves with the operator.
Proper adjustment of the armrests is essential in preventing neck and shoulder problems. Adjusting the armrests too high can actually worsen neck pain, as it puts the levator scapulae muscle on slack and can cause neck stiffness and pain at the crook of the neck and shoulder. Muscle activity is lowest when the elbow rests are set slightly low. Proper width adjustment (between armrests) is also important, as armrests set too far apart will cause abduction of the arms. Positioning the armrests too far forward can encourage leaning forward, compromising operator posture.
From an ergonomic standpoint, it is desirable to operate with two armrests; however, if there is a confined operatory space and the dental assistant finds it difficult to maneuver a chair with armrests around the patient, a unilateral armrest fixed to a counter may be considered as an alternative.17 Dental practitioners who operate with the left arm resting have been shown to have less pain than those who do not.18 These armrests are available in a variety of heights to adjust to multiple users and situations.
Conclusion
To be clear, armrests are not an automatic solution for all operators with neck pain. Depending upon the etiology of the pain, taking the arm weight off the neck and shoulders may or may not reduce the symptoms. During in-office consultations, the author performs a specific physical therapy test to determine if individuals with neck pain would benefit from armrests.
Operators should always request trialing a stool for 1 to 2 weeks before purchasing it to evaluate the comfort and benefits of the armrest, backrest, and other features. Consider alternating between a stool with armrests and backrest and a different style type stool in another operatory to prevent overworking one area of the body. It pays to give your muscles a rest.
References
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2. Snijders DJ, Hermans PF, Nieseing R, et al. The influence of slouching and lumbar support on iliolumbar ligaments, intervertebral discs and sacroiliac joints. Clin Biomech. 2004;19(4):323-329.
3. Adams MA, Dolan P. Time-dependent changes in the lumbar spine’s resistance to bending. Clin Biomech. 1996;11:194-200.
4. Fujiwara A, Tamai K, Yoshida H, et al. Anatomy of the iliolumbar ligament. Clin Orthop Relat Res. 2000;380:167-172.
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11. Harrison DD, Harrison SO, Croft AC, et al. Sitting biomechanics part 1: review of the literature. J Manipulative Physiol Ther. 1999;22(9):594-609.
12. Richardson C, Jull G, Hodges P, Hides J. Therapeutic exercise for spinal segmental stabilization in low back pain. London, England: J Churchill Livingstone; 1999:4-5.
13. McGill SM. Low back disorders—evidence-based prevention and rehabilitation. Champaign, Ill: Human Kinetics; 2002: 175-177.
14. Schuldt K. On neck muscle activity and load reduction in sitting postures. An electromyographic and biomechanical study with applications in ergonomics and rehabilitation. Scand J Rehab Med Suppl. 1988;19:1-49.
15. Parsell DE, Weber MD, Anderson BC, Cobb GW. Evaluation of ergonomic dental stools through clinical simulation. Gen Dent. July/August 2000;440-444.
16. Andersson BJG, Ortengren R, Nachemson A, Elfstrom G. Lumbar disc pressure and myoelectric back muscle activity during sitting II. studies on an office chair. Scand J Rehabil Med. 1974;6:115-121.
17. Valachi B. Practice Dentistry Pain-Free: Evidence-Based Strategies to Prevent Pain and Extend Your Career. Portland, Ore: Posturedontics Press; 2008:70.
18. Rundcrantz B, Johnsson B, Moritz U. Cervical pain and discomfort among dentists. Epidemiological, clinical and therapeutic aspects. Swedish Dental Journal. 1990;14:71-80.
