Measuring Mandibular Ridge Reduction on Oblique Cephalometric Radiographs Ronald E. G. Jonkman, Marinus A. J. van Waas, Jack Plooij, Warner Kalk
Dept. of Oral Function and Prosthetic Dentistry (Head: Prof. W. Kalk, D.D.S., PhD. and Prof. A. F. Kfiyser, D.D.S., PhD.), TRIKON: Institute for Dental Clinical Research, University of Nijmegen, The Netherlands
Submitted 12.4. 1990; accepted 19.6. 1990
Introduction
Mandibular ridge reduction after extraction of teeth is a major problem for both patient and dentist. It is because of this problem, that during recent decades the role of the complete (immediate) overdenture has been emphasized more and more (LaVere and Krol, 1973; Crum, 1980; Toolson and Smith, 1983). In order to investigate the ef- fects of overdentures a study was done in which patients were treated at random either with complete immediate dentures or with complete immediate overdentures. One of the purposes of this study was to measure differences in the mandibular alveolar ridge reduction in both groups. Several radiographic techniques can be used for measuring this reduction. Wical and Swoope (1974), Van Waas (1979, 1983), and Swart andAllard (1985) measured on panoram- ic radiographs. The method is simple, but a disadvantage is the inaccuracy due to the large and irregular horizontal and vertical magnification on the X-rays. It can, however, be used in longitudinal studies with large numbers of patients (Van Waas, 1983). Others, for instance Tallgren (1957, 1972), Carlsson and Persson (1967), Atwood and Coy (1971), Crum and Roo- ney (1978), Tallgren et al. (1980), Bras et al. (1985), Frei- hofer and Hoppenreys (1986), Habets et al. (1987), used la- teral cephalometric radiographs. This method is more re- producible and there is little (about 5 %) horizontal and vertical magnification (Carlsson, 1967; Mercier et al. 1982). A disadvantage of the method is the overprojection. For that reason the method cannot be used for measuring ridge reduction in overdenture patients, because the left and right sides of the alveolar ridges and the abutment teeth are superimposed. Cartwright and Harvold (1954), Barber et al. (1961), Win- ter et al. (1974), Peterson and Slade (1977), Steen (1984), and Freihofer and Hoppenreys (1986) modified the lateral cephalometric technique. Contrary to the usual technique, where the patient's head is fixed in a cephalostat perpendi- cularly to X-ray beams, the head is fixed at an oblique angle.
Summary
The aim of this study was to test the accuracy of a set- up for measuring alveolar ridge reduction of totally ed- entulous mandibles and of mandibles with abutment teeth for overdentures, on oblique lateral cephalomet- ric radiographs. For that purpose, a cephalostat was modified so that the patient's head - in particular the mandible - could be fixed in a reproducible manner. Four radiographs were used for measuring all parts of the mandible. To assess the accuracy of the method, two randomly chosen parts of the mandible of 20 pa- tients were X-rayed twice. The radiographs were traced independently by two observers and the height of the mandible was measurd along a line drawn perpendicu- larly to the tangent of the lower border of the mandible on these X-rays. The method appeared to be simple, less time-consuming, and accurate. The average stand- ard deviation of the measurements of the height of the mandible was 0.15 millimetres. When measuring ridge reduction, the standard deviation was 0.21 millimetres.
In this manner all segments of the mandible can be ex- posed without overprojection on a series of four X-rays. This method is called the "oblique" lateral cephalometric technique. Steen (1984) studied the accuracy of this meth- od, using a unit with electromagnets for the fixation of the mandible and a plotter for analyses of the surface areas of the mandible. His method appeared to be very accurate and highly reproducible. A disadvantage of the method is, however, the great amount of time required to install a pa- tient reproducibly as well as the complicated and time-con- suming way of measuring the mandibular reduction. In this article a new method is described, using a cephalo- stat with nasal and chin support, which can be adjusted in- dividually and reproducibly, and a simple method of mea- suring. The method and the accuracy of the method are presented.
Material and Methods
At the Dental School of Nijmegen a Philips Rotalix ROT 35010 X-ray machine and an Evald type E1-302 cephalostat (Miiller, Denmark) are used for lateral cephalometric X-rays. The distance between Rotalix and radiograph is 4.75 metres; the distance between the centre of the cepha- lostat and the radiograph is 13 centimetres. Between the Rotalix and the patient a lead screen with a diaphragm of 25 x 8 centimetres is situated at 3 metres from the Rotalix. For our purpose the cephalostat was modified as follows: A stainless steel bar with a nasion bar and chin support was made and connected to the cephalostat (Fig. 1). The nasion bar and chin support could be adjusted horizontally and ro- tated around the vertical bar. The latter allowed vertical ad-
28 J. Cranio-Max.-Fac. Surg. 19 (1991) R.E. G. Jonkman et al.
Fig. 1 A patient installed in the modified cephaiostat by means of earplugs, nasion bar and chin support.
justment. The whole cephalostat can be rotated around a vertical axis. All adjustment possibilities were calibrated. For taking the X-rays, the patient's head was fixed with the nasion bar and ear plugs in a way that the Frankfort hori- zontal plane was parallel to the floor. Then the chin sup- port and the other bar were adjusted, when the mandible was in the rest position in relation to the maxilla. The va- lues which could be read off on each adjusting bar were re- corded in the first session, in order to replace the patient in the same position during the following sessions. The ce- phalostat is installed at 30 ° and 45 ° angles left and right re- spectively to the meridian of the X-ray beam. In this way the anterior and posterior se~nents on both sides of the mand- ible can be seen on four radiographs (Figs. 2 a -c ) . For measuring the height of the mandibles two observers separately traced the borders of the mandibles on each radiograph. A tangent line was drawn on each tracing through the "symphysis" and the posterior angle of the ho- rizontal and vertical part of the mandible (line A, Figs. 3 a -b) . Those points could be marked as the equiva- lents of Menton and Gonion. Perpendicular to this line, a line was drawn along the anterior part of the mandible (line B, Figs. 3 a -b) . At 3 and 3.5 centimetres from this line, two lines were drawn perpendicular to line A for the anterior segments of the mandible, and at 3.5 centimetres from line B and from that line every 5 millimetres onward, six lines also perpendicular to the line A for the posterior segments of the mandible. The distances between the upper and low- er border of the mandible were measured along each line. Both observers measured the distances with a caliper scaled in tenths of a millimetre.
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................ f i l m
Fig. 2 a A schematic representation of a mandible rotated in oblique angles of 30 ° and 45 ° to the meridian of the X-ray beam. In this way the anterior and posterior parts of a mandible, respectively, are ex- posed.
Fig. 2 b An oblique cephalometric radiograph of an anterior part of a mandible, made at a 30 ° angle.
Fig. 2 c An oblique cephalometric radiograph of a posterior part of a mandible, made at a 45 ° angle.
In order to determine the reproducibility of this method, 20 edenmlous patients were asked to participate in the test procedure. Two randomly chosen parts of the mandible were X-rayed by one of the two operators at the first ses- sion (films used: Dupont-Cronex-4-Safety). The recorded installation data of this session were used a week later by the other operator for making a duplicate X-ray. The Rota- lix was adjusted at 120 kiloVolts, the amount of milliam-
Measuring Mandibuk, r Ridge Reduction on Oblique Cephalornetric Radiographs J. Cranio-Max.-Fac. Surg. 19 (1991) 29
J ,../2
A
B
Fig, 3 a A tracing of an anterior part of a mandible with the reference lines A, a tangent line at the lower border of the mandible and B, a line drawn perpendicular to A. Dorsal to line B, two perpendicular lines (1 and 2) drawn for measuring the mandibular height.
B
Fig. a b A tracing of a posterior part of a mandible with the reference lines A, a tangent line at the lower border of the mandible and B, a line drawn perpendicular to A. Dorsal to line B, six perpendicular lines (1-6) drawn for measuring the mandibular height.
pere-seconds varied between 64-100 and the exposure time varied between 0.25-0.40 seconds. The latter varia- tions depended on the thickness of the mandible and its surrounding (soft) tissues.
Results
In this set-up it took 6 minutes to record the first installa- tion data. The reinstallation took 1.5 minutes. For assess- ment of the errors made in the whole procedure the ANO- VA was used. With the use of the estimated variances the standard deviation of the distances per line was calculated. The values given in Table 1 are the standard deviations (s) of the measurements along the lines on the tracings of the X-rays of the anterior and posterior segments of the mandi- bles separately. The average of all of these standard devia- tions was 0.15 millimetres. The values producing this aver- age did not show a great variance. There were no signifi- cant differences of the values of the measurements of both anterior and posterior parts of the mandibles. When mea- suring ridge reduction, which means the differences of the heights of the mandible on two radiographs, the error of
that measurement can be calculated (]/~s). This gives 0.21 millimetres.
Table 1 Standard deviations (s) of height of the mandible along the lines on the tracings of the anterior and posterior segments of the mandibles, in millimetres. The lines are numbered 1 and 2 for the an- terior parts of the mandible and from 1 to 6 for the distal parts starting at Menton.
The method described for measuring ridge reduction of ed- entulous mandibles and mandibles with abutment teeth, us- ing the oblique lateral radiograph technique, proved to be accurate, easy and quick to perform. The mean error of 0.15 millimetres is minimal, taking into account that this er- ror is composed of the errors made in making the radio- graphs, tracing and measuring, as well as the inter-observer error. However, four radiographs have to be used in order to get around overprojection of the ridge and the abutment teeth. This gives a higher radiation dose in comparison with other techniques, but is unavoidable. Steen (1984) needed 20 minutes for the first set-up. With the method described 6 minutes were required. He mea- sured the dimensions of areas of the mandible on tracings of oblique lateral radiographs and plotted the areas with the use of a computer. The standard deviation(s) of a sec- tion in his research was 3.8 square millimetres. The stand- ard deviation of the vertical component of the area was 0.15 millimetres. This is the same result found in this study with a simple modification of the cephalostat and a simple way of measuring. Taking the large individual range of ridge reduction in indi- viduals into account (Van Waas, 1983) an error - ex- pressed in the standard deviation - of about 0.21 millime- tres in measuring ridge reduction is acceptable.
Conclusions
The present technique can be used for measuring alveolar ridge reduction in edentulous as well as in overdenture pa- tients individually. The accuracy of 0.21 millimetres is ac- ceptable for studies related to small numbers of patients and with small differences in the height of the mandible.
Acknowledgment
We wish to express our gratitude to Mr. J. Albers and Mr. F. Eijkholt of our Technical Services Department for the construction of the modified cephalostat.
30 J. Cranio-Max.-Fac. Surg. 19 (1991) R. E. G. Jonkman et al.: Measuring Mandibular Ridge Reduction
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R. E. G. Jonkman, D.D.S. UniversiZy of Nijmegen, Dental School Department of Oral Function and Prosthetic Dentistry P.O. Box 9101 NL-6500 HB Nijrnegen The Netherlands
