Elements of a Seating System

The discussion that follows will lead you on a course that should shed light on seating systems and their four major components: the base, seat, backrest and arm support.

The Base
The base of the chair should always utilize a 5-point system (Figure 1). The five legs on a chair will provide two main essentials: ease of movement and stability.

Figure 1. The five-point “star” base.

Image of the five-point “star” base.

The 5-point system allows you to move in varied angles and patterns around your patient’s chair with proficiency and without the possibility of destabilizing your sitting surface, which can occur with a 4-point system. The focus must be on your treatment and not on your chair. Most, if not all, employ this concept.

The Seat
Seats come in varied sizes and shapes (Figures 2 and 3). Five key issues surrounding seats include pressure reduction, shape, length, height and tilt.

Pressure reduction: First, let’s address ischial pressure reduction. Decreasing the pressure under the ischial tuberosities can be accomplished in two ways. First, a pliable surface, such as a Swiss ball, or other forgiving surfaces can be used. Applying a concept from physics, if there is no net movement of our body down into the seat, then the hard surface we sit on will direct an equal and opposite force upon us. So, if we direct a 100kg force into the firm seat, the seat will direct that 100kg force back at us. This force can be transmitted immediately up the spine resulting in an increased spinal load during sitting. In standing, the opposing force is directed up through our legs before being transmitted to the spine. Thus, there are lower disc pressures with standing; however, if the surface significantly deforms from our force directed down into it, as with sitting on a Swiss ball, our force is greater than the force pushing back up at us and the spinal load is less.

Figure 2. Flat-type seats.

Image of Flat-type seats.

Figure 3. Saddle-type seats.

Image of Saddle-type seats.

Second, an articulating surface that tilts the back part of the seat down while keeping the front part of the seat level and combines an aggressive backrest to support a lordosis akin to an angle in standing is recommended.21 Today, an articulating seat has not been designed beyond the laboratory, so one should look for a seat that not only slopes down in the front, decreasing the pressure to the distal thighs, but also slopes down in the back to reduce the ischial pressure as well. If the back part of the seat slopes downward, there must be a corresponding aggressive back support to facilitate lumbar lordosis.

We now know from science that reducing the pressure under the ischial tuberosities reduces strain on the back, and that femoral abduction and external rotation allows the pelvis to more easily attain a position whereby the contact pressure under the ischial tuberosities is reduced (i.e., an anteriorly rotated pelvis).

Shape: The next issue related to seats is shape. The flat, scooped or pan seat tends to increase the pressure under the ischial tuberosities, unless the seat is declined in the back and/or leg regions offloading the ischial tuberosities or thighs respectively (Figure 4). In addition, flat seats require one to expend more effort in assuming the tail-bone up position. Often the thighs are cradled, placing them in a more adducted and internally rotated position, whereas, the biomechanics of the straddle-sitting position give rise to structural stability.

Figure 4. Declining Seat.

Image of declining seat.

Look for a seat that allows the thighs to abduct and externally rotate, making anterior pelvic rotation easier and further stabilizing your pelvis. Assuming a wide base of support also promotes femoral external rotation, and the abduction will keep you balanced when you move around your patient’s chair. Some saddle-type seats can accomplish these goals, but be careful. Some manufactures use the term “saddle” to describe a seat that tilts, but in reality, it is flat and inhibits your thighs from automatically abducting and externally rotating.

Length: Length of the sitting surface also should be considered. The length should always allow for at least two to four inches of space between the end of the seat and the back of the knee. A longer seat length may compromise circulation and cause excess pressure to the tissue.

Seat Height: Seat height must be adjustable. At the very lowest, the seat should be adjusted so the hips and knees are equal in height, i.e., parallel to the floor. Ideally, the hips should be slightly higher than the knees, creating a trunk-to-thigh angle greater than 90° (135° is ideal). This degree of angle allows the pelvis to rotate forward with greater ease and thus create the essential lumbar curve.

Tilt: Tilt is the next popular feature in the seat. Science has demonstrated that we lose some of our lumbar curve when we sit as compared to standing,21 and the tilt feature can facilitate a greater lordosis than a flat seat. Look for a chair that allows a forward tilt of at least 15-25°, as this can be the difference in the lumbar curve in sitting as compared to standing.21 Also, tilting the seat to promote a 135° trunk-to-thigh angle will reduce the pressure in the lumbar spine by decreasing the contact area under the ischial tuberosities.16

The Backrest
Just as the shapes of seats differ, so do the shapes and types of backrests (Figure 5). The back support of the chair is key in assisting with creating a lumbar curve comparable to that in standing. The adjustability must be robust enough to meet and/or improve your own lumbar lordosis after assuming the “tail-bone up”22 posture. It should aggressively support the lumbar spine and not cradle or wrap around your trunk. The backrest height should not compromise the spine’s natural curves, but only support the middle to lower lumbar region. Many backrests on today’s chairs offer too much width and height, which can compromise spinal positioning and movement. Therefore, the adjustability should include an up and down and a front-to-back modification.

Figure 5. Back Rests.

Image of back rests.

Arm Supports
Clinical simulation suggests a benefit in using dental stools that incorporate static arm supports.23 Surface EMG studies24,25 examined the effects of forearm support on muscle activity in the shoulder and forearm during horizontal movements and subject comfort. Study results indicated that forearm supports do improve subject comfort and reduce muscle activity in the shoulder and forearm regions which can reduce fatigue, pain and possibly pathology. However, the ability of the manufacturers to deliver these components both functionally and effectively varies with design. Some designs offer only a forearm “rest” that has the potential to allow the forearm or elbow to slip off, while others offer a forearm support that cradles the forearm, eliminating the possibility of slippage. Other variations in forearm rests or supports include those that are fixed on both sides. Alternatively, one side is fixed and the other spring-loaded. This design provides stability for one arm and supported mobility for the other. Also, a telescoping feature may be offered that allows the support to move with your arms as you reach. Again, science supports this option, but effectiveness relies on selecting the appropriate design matching your needs.