A. J. MACGREGOR, D.D.K., M.CH.D., F.D.S.R.C.S. and G. E. TOMLINSON, B.SC.

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1 British Jotrrd of Oral Surgery 17 ( ) AN APPARATUS FOR MEASURING THE FORCES OF DENTAL EXTRACTION A. J. MACGREGOR, D.D.K., M.CH.D., F.D.S.R.C.S. and G. E. TOMLINSON, B.SC. Leeds Dental Hospital and the Department of Medical Physics, Leeds General InjYrmary Summary. An apparatus is described which measures compressive and rotational forces during the extraction of teeth. It has been of value in propaedeutic and clinical instruction. Introduction Dental extraction by means of forceps is a skill which traditionally is learnt by clinical apprenticeship. The most that can be done propaedeutically is to show the student how to hold the forceps, where to stand or sit in relation to the patient and the kind of manipulations appropriate to each tooth. Mannikins or phantom heads are of limited value as it has been found impossible so far to simulate the mechanical relationship of tooth to bone. If dental students learn a good basic technique, it is probably because during their training they extract a large number of teeth. The student s first extractions are however always a problem which causes anxiety because the teacher cannot communicate the strength required for the individual movements of the forceps. Apparently there has been only one previous attempt to measure the forces of extraction. Charlton and Griffith (1971) reported in a brief abstract what Charlton described in more detail in his thesis (Charlton, 1970), an account of a resistive strain gauge recording system which they had used to measure pulling forces. Sixty-five experienced dentists were asked to use what they thought was the effort involved in removing a tooth, to pull a steel post from a core. The results showed that the forces used ranged from 4 to 24 kg, the average being kg. The participants found it extremely difficult to sustain these forces and they complained of cramp and of the forceps slipping from the hand. When they were asked to pull as hard as they could, the readings went up to 32 kg. Charlton and Griffiths admitted that their methods were not a close approximation to those used in the clinic. Indeed they withdrew one of their dentists from the trial because he did not feel he could exercise the force he would normally use in extracting difficult teeth. They intended to go on to a clinical study but they have not published further in this field. The problem has been under consideration at Leeds where it was felt at that time the necessary electronic components were not available. An apparatus was eventually constructed in It has been most interesting to use and appears to have been of value in teaching. Design of apparatus The electrical resistance of a strain gauge varies when it is distorted and this property can be used to measure distortions in the handles of forceps caused by extracting teeth. (Received 19 December 1977; accepted 9 February 1978) 71

72 BRITISH JOURNAL OF ORAL SURGERY It was necessary to select suitable forceps, strain gauges, and to decide what measurements to attempt (e.g. gripping, twisting) and therefore where to place the gauges.

2 72 BRITISH JOURNAL OF ORAL SURGERY It was necessary to select suitable forceps, strain gauges, and to decide what measurements to attempt (e.g. gripping, twisting) and therefore where to place the gauges. Changes in the electric resistance had to be amplified and displayed. The forceps chosen were Ash, catalogue numbers 136 and 137; these are known colloquially as Guy s Universal Forceps as they were designed by William Guy of Edinburgh, the 136 to extract all upper teeth, the 137 all lowers, thus avoiding the necessity of changing forceps more than once. In his time, general anaesthesia for dentistry could be brief and turbulent and it was necessary to remove teeth quickly. Tinsley Telcon 8/350/EC/i I foil strain gauges were small enough when suitably protected to withstand use in the laboratory and clinic. They were placed on the handles on milled flat areas from which grooves extended to receive the electrical cables (Fig. 1). The positions chosen were the result of a compromise since if they were placed too near the joint the strain could be expected to be non-uniform, and if too far away, direct hand pressure and temperature would have caused unpredictable effects. Connections were made with two, six-way cables to a bridge amplifier which produced a voltage proportional to the strain measured. Each amplifier contains a 1.2 volt bridge supply, a differential amplifier with a gain of 100 followed by an amplifier with a variable gain. An additional pre-set control is provided to add or subtract a small amount of signal to compensate for any cross sensitivity between different types r BRIDGE SUPPLY FIG. 1. Lower forceps showing positions of strain gauges. I STRAIN GAUGES DIFFERENTIAL GAIN INDICATOR AMPLIFIER FIG. 2. Block circuit diagram.

MEASURING THE FORCES OF DENTAL EXTRACTION 73 of strain. Each set of amplifiers were enclosed in a plastic box with a dual stabilised power supply. The block circuit diagram is shown in Fig.

3 MEASURING THE FORCES OF DENTAL EXTRACTION 73 of strain. Each set of amplifiers were enclosed in a plastic box with a dual stabilised power supply. The block circuit diagram is shown in Fig. 2, and the actual apparatus is shown in Fig. 3. The output was displayed on a chart recorder, as it was thought important to have a permanent record not only for calibration but also for illustration. Considerable attention was paid to the protection of the electrical components. In addition to being cemented in recesses, the gauges were waterproofed with Araldite and then the whole area filled and coated with silicone rubber. The upper forceps were wired to record gripping loads and the lowers were wired for both gripping and rotation. FIG. 3. Apparatus showing the forceps, amplifier and chart recorder. FIG. 4. Calibration of the forceps. The display on the chart recorder is calibrated with the reading on the balance.

4 74 I3RITISH JOURNAL OF ORAL SL RGtRY The apparatus was calibrated using a pair of levers with a ratio of 10: 1 so that with a O-10 kg spring balance attached to one end a O-100 kg force could be applied between the jaws of the forceps. Cross sensitivity between the gauges was adjusted so that there was minimum deflection on the rotation output when maximum grip was applied. The handles of the forceps were gripped with a clamp and the tension of the spring balance was increased to a known value. The gain on the amplifier was then adjusted to give a required deflection on the chart recorder. The gain was checked up to values equivalent to 100 kgs at the jaws of the forceps. The arrangement is shown in Fig. 4. Clinical experience The apparatus in its present form has been used for operations by staff and students, for extractions under local anaesthesia, open general anaesthesia, and endotracheal anaesthesia. The compressive forces employed for 260 teeth are shown in Table 1. The highest was 132 kilograms for a resistant upper canine: the lowest was 24 kilograms for an upper second premolar. The rotational forces employed in the removal of the lower teeth is shown in Table II. The forces are measured in kilogram centimetres and vary from 7 on a lower central incisor to 65 on a lower second molar. There was some surprise that the compressive forces were so high. Non-physicists may have difficulty in understanding the concept. It is as if a weight were attached to the lower blade of a pair of forceps and the upper kept still as the handles are compressed. The weight is equal to the compressive force. It is important to note that this is not a crushing pressure which would depend on the area of the forceps blades in contact Table I Compressive loads in Kilograms S.I. Units = $ Newtons. Uppers Lowers Tooth I No. of teeth Maximum load Minimum load Mean load Table II Rotational loads in Kilograms per centimetre S.I. Units = $ Newtons. Tooth Lowers No. of teeth Maximum load Minimum load Mean load

5 MEASURING THE FORCES OF DENTAL EXTRACTION 75 with the tooth. If it is remembered that the average man weighs about 80 kilograms then the maximum loads applied can be seen to be considerable. Even with the advantage of leverage, forces of over 120 kilograms require the operators to be physically strong. When the apparatus was first introduced, dentists were asked to grasp a piece of wood with the pressure they thought they would apply to a tooth during its extraction. Almost invariably the pressure they used was too low. The same could be expected for pulling forces. It is therefore likely that what Charlton and Griffith s dentists thought was their maximum force was what they normally used. What they thought was probably an underestimate. The curves produced by the recorder were instructive. There seemed to be two distinct patterns. In one there was an initial notch showing that the operator was finding the best place to attach the forceps to the tooth and perhaps flexing his muscles prior to maximum effort (Fig. 5). This pre-tensing is well known in golf as the preliminary waggle or press forward. Not all golfers do it, nor do all extractors of teeth and immediate application of pressure gives a second pattern. Compression was sometimes applied in steps with accompanying rotation. When the maximum compression is applied, rotation is often applied separately as shown in Fig. 6. A typical beginner s effort in extracting a lower molar is shown in Fig. 7. There are two initial movements as the fit of the forceps is tested then gripping and slight relaxation and the compression is literally stepped up. There is some accompanying rotation. The tooth is actually removed quite quickly; characteristically the time taken for a relatively easy extraction is much longer and can be over 20 seconds. The students have accepted the apparatus very well and it is in regular use in the extraction room. It has been found, contrary to expectation, that there is little chance of a beginner applying an excessive compressive force to a tooth. Reading the chart output encourages them to make the necessary effort. It is noteworthy that some use the correct patterns of force almost without instruction. The introductory course is where the apparatus finds its chief use and, as it is expensive and would be difficult to repair or replace, it is always supervised. Clearly not all of the forces of extraction are measured, in particular pushing and some rotating movements, which are important, are not recorded. It is technically difficult to place strain gauges to record these movements and it is impossible to measure them all simultaneously merely for want of space. Gripping was measured as it was thought to be a parameter of all other forces. km o_ / FIG. 5. Characteristic sets secs curves, one of which shows a notch due to muscle flexing while fitting the forceps to the tooth.

6 76 BRITISH JOURNAL OF ORAL SURGERY kg/cm ROTATION kg/cm ROTAT ION 10 I 10 I 0 10 &Is 80 kls secs secs 0 FIG. 6 (left). Compression and rotation during the extraction of a lower molar by a skilled operator. FIG. 7 (right). A beginner s attempt (see text). There is also a great number of variables in which an interest could be taken. How does the force vary for similar teeth with different designs of forceps? How does the performance of the forceps vary from one phantom head to another? It is obvious that the one used at Leeds did not compare with the real thing, but it is now possible to be certain that students go through the proper motions before taking them up to the clinic. Charlton, G. (1970). Design of posts and cores for porcelain jacket crowns. M.D.% Thesis. Bristol University Dental School. Charlton, G. & Griffiths, M. J. (1971). Investigation of the forces required to extract maxillary incisors and canines. Journal of Dental Research, 50, 1200.

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