In terms of surgery layout, ideally there should be a 60cm space between the patient's head and the work units. This will enable the dentist to sit in an 11 to 12 o'clock position. There should also be a 25-30cm free space between the midline of the patient chair and the assistant's worktable. Ideally the latter should be moveable and there should be a minimum of 10cm clearance below the work top so it can be retracted without the need to remove any materials or instruments lying on top of it. The assistant's worktable should be about 75cm above the floor, 80cm if the dentist is more than 180cm (5'11") tall. The worktops of the workstation itself should be about 95cm above the floor.
Correct positioning of the assistant's worktable is very important. Correctly sited it gives the assistant the necessary space to work comfortably within, but keeps all the instruments and materials in easy reach, without having to move away from the patient. It also means the dentist can easily pick up materials and instruments from it with their left hand.
By positioning this worktable between the dentist and assistant it means the assistant can easily pick up the instruments (and materials) and hand them to the dentist's right hand.
The clinic's most important drawer should be located immediately above the free space associated with the assistant's worktable. The drawer is used to store small, frequently used materials. Its capacity can be increased by using inserts with small subdivisions.
However, some procedures require lots of different materials, some of which can be bulky. These are ideally kept on larger 'procedure trays', which can be stored elsewhere within the cabinetry and only brought out when required. Trays about 40cm x 36cm are ideal.
Some dentists might prefer a stationary assistant's worktable. In this situation a table measuring approximately 80cm x 50cm with a set of drawers underneath is recommended. The drawers should be positioned on the right side leaving at least 30cm free leg room under the left side.
Alternatively a mobile unit might be preferred. In this case an 80cm wide table top mounted above a 50cm deep element is advisable.
For dentists who prefer to pick up their own instruments an instrument tray can be located on the right hand side of the delivery unit. However, a tray located in this position cannot also be reached by the assistant.
Operator's seating position
It is sometimes necessary for dentists, hygienists and their clinical assistants to lean forward when working. If this is achieved by twisting and/or bending forward the incumbent risks injury by trapping the anteriors of the intervertebral discs. To prevent this the incumbent should ensure their thighs are angled forward and the angle of the thighs to the torso exceeds 90º (preferably about 110º). This balanced sitting position is dynamic, providing freedom of movement and maintaining a healthy back (fig 1).
Traditionally saddle stools have been used to achieve an angled sitting position. However, since almost all saddle stools provide angled support to the ischial tuberosity ('bones where you sit') it is easy to slide off the chair and 'hit' the saddle stool's central part (fig 2). Although impractical for both sexes this can be particularly unpleasant for men.
Therefore it is recommended to use an operator's chair which provides horizontal support to the ischial tuberosity, so the incumbent does not slide forward (fig 3). The seat should also be tiltable, allowing the incumbent to sit comfortably in a balanced position with angled thighs, even when the backrest is not in use. If the assistant is 12-15cm shorter than the dentist then a chair with a ring-shaped footrest is recommended.
Ideal operating positions
Good working positions are achieved using a combination of sitting position, line of sight, position of the dentist, position of the patient's head, and the angle with which the clinician holds the hand instruments or handpieces (fig 4).
Working distance and patient chair height
Many dentists work with the patient chair situated so low that it results in a poor working position. The dentist (and sometimes, the clinical assistant) bends down to see clearly, leading to a bent back and spinal column and resulting in potential injuries (fig 5).
However when dentists work in a balanced sitting position (slightly angled, with spinal curvature preserved) their eyes are naturally positioned downward. The more downward the eyes are directed, the better the sitting position (fig 6).
To achieve the most ergonomically efficient working position it is necessary to adjust the patient chair so there is a suitable distance between the patient's mouth and the dentist's eyes. This distance varies depending on the particular requirements for visual precision and the dentist's technique for carrying out the work in question.
Most dentists find that 35cm is the correct distance for precision work, with this distance being slightly shortened for others. Certain types of work require less visual precision, allowing the working distance to be extended by an additional 10cm.
To achieve the optimised working position adjust the patient chair so the dentist and assistant can sit comfortably and see well. This means that dentists who are taller than about 165-170 cm will be working with their lower arms angled upwards. This is not a problem if the dentist is working with good finger support and holds a well-balanced handpiece, so that it is pointing downward at an appropriate angle. This allows the dentist to keep their shoulders lowered.
Provided the patient chair's highest position is 90cm, this will allow even the tallest of dentists to maintain a good sitting position without having to bend over to see clearly.
Dentists should also work in a midline stance (mediosaggital plane) without turning or twisting their torso, neck, or head.
Line of sight and optimised sitting position
The line of sight into a patient's mouth is determined by the direction of the tooth surface or cavity in which the dentist is working. For example, the line of sight for working on the right side of a tooth will be from the right (in other words, from about the nine to 10 o'clock position) with the patient's head tilted slightly to the left (fig 7). This is the dentist's sitting position.
The line of sight for working on the left side of a tooth will be from the left and can be achieved by the dentist sitting in the 11 to12 o'clock position, with the patient turning their head slightly to the right (fig 8).
Occlusal surfaces in the mandible can be seen easily from the 11 o'clock position, which is also used for mirror work on the mandible. The stated sitting positions assume the dentist does not turn their back, neck, or head to the side but works ergonomically, in a midline stance.
Dentists therefore need only to work in the 9 to 12 o'clock positional range. This is made easier with slim backed patient chairs, which allow the foot controller to be placed on the left side of the chair's base (fig 9). This results in an excellent sitting position. The dentist can activate the foot controller's pedals using their right foot from the 9, 10, or 11 o'clock positions and potentially using their left foot from the 12 o'clock position.
When the dentist and assistant sit with angled thighs in a balanced sitting position, they can both sit alongside the patient, taking the 9 to 10 o'clock and the 11 to 12 o'clock positions with their legs interwoven.
Four handed dentistry
Working successfully with an assistant (four-handed dentistry) enables the dentist to operate with undisturbed concentration on the work at hand.
The assistant uses their left hand to take a hand instrument from the instrument tray, which is ideally positioned between the dentist and the assistant. The instrument is extended out toward the dentist, who does not need to look away from the patient's mouth. The assistant then takes hold of the suction tube and simultaneously moves the three-in-one syringe forward for use by the dentist if required. The cavity can now be rinsed and dried.
The assistant keeps the mirror dry and free from condensation by blowing air on it, allowing the dentist to see when working with the mirror and spray.
The dentist can easily switch between using the handpiece and hand instruments as appropriate. This enables the dentist to focus on the task at hand without looking or reaching away.
The assistant changes burs, stones, diamonds and so on as necessary.
Working without an assistant
The dentist places the hand instrument tray behind and to the left of the patient's head, just as they would when working with an assistant. The dentist can then take the hand instruments with their left hand (fig 10).
After carrying out the instrument exchange, the dentist can work with the instrument using their right hand. This left-to-right exchange takes about 0.7 seconds.
If a dentist works solely on their own and never needs an assistant to help hand them instruments, they can use an instrument tray to the right of the handpiece delivery unit.