Ophthalmic Plastic and Reconstructive Surgery Vol. 21, No. 6, pp 427–430 ©2005 The American Society of Ophthalmic Plastic and Reconstructive Surgery, Inc.
Effect of Exophthalmometer Design on Its Accuracy Yoav Vardizer,
M.D.*†,
Tos T. J. M. Berendschot,
M.D.‡,
and Maarten P. Mourits,
M.D.*
*Orbital Center, Department of Ophthalmology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands, †Department of Ophthalmology, Haemek Medical Center, Afula, Israel, and ‡University Medical Center Maastricht, Maastricht, The Netherlands.
Purpose: To investigate the effect of the design of different exophthalmometers on their measurement accuracy. Methods: Eight different exophthalmometers were tested with a specially developed calibrator by experienced orbital surgeons. Results: Exophthalmometers with one mirror and a straight footplate were found to be more accurate than others. One-mirror exophthalmometers were less accurate when assessing low (ⱕ12 mm) and high (ⱖ23 mm) exophthalmometric values. Conclusions: The design of an exophthalmometer affects its accuracy.
T
tion of relative rather than absolute exophthalmometric values. Recently, the use of a custom-made calibrator has been suggested to overcome this error.7 For our study, we manufactured such a calibrator and investigated the accuracy of eight different commonly used Hertel exophthalmometers from three manufacturers.
he orbital cavity is surrounded by bony structures on all sides, except anteriorly. Significant volumetric changes inside that space, for instance, as a result of a growing tumor, alter the position of the globe within the orbit. Therefore, the position of the globe within the orbit is an indicator of orbital disease. Both clinical1 and imaging techniques2 help the clinician to determine the axial position of the globe in relation to the orbital bony rim. The most widely used method at present is Hertel exophthalmometry, in which the position of the globe relative to the anterior lateral orbital margin is measured with the aid of a set of mirrors. When his exophthalmometer was introduced in 1905, Hertel himself questioned its ability to provide absolute measurements; he advised physicians to use his exophthalmometer only as a tool for follow-up.3 The problem of potentially inaccurate absolute measurements is increased by the fact that there are several commercially available Hertel exophthalmometers and other models.4,5 Sleep et al.6 examined the relative accuracy of several Hertel exophthalmometers by using a plastic doll head and yielded interinstrument variations of up to 3.2 mm. Because they did not have a golden standard for their measurements, it is hard to tell which of the instruments they used were accurate and how to extrapolate their results to other instruments. Therefore, their findings underline the no-
METHODS A calibrator (Fig. 1), using a modified design of Van den Bosch,7 was manufactured by screwing two sets of L-shaped aluminium 51 ST plates to a 100 x 40-mm hard plastic board (aluminium L-shaped plates were cut from a square 65-mm tube). The shorter L-shaped metal plate (height: 20 mm internal measurement) was fixed on the edge of the plastic board. The longer L-shaped metal plate was 55 mm high (internal measurement). The out-facing facet was scratched horizontally and vertically. The vertical line was at the middle of the facet (20 mm). It assisted in holding the ruler perpendicular, when making the markings to be measured. The horizontal line was scratched 20 mm from the bottom, which represented the zero point when drawing the measurements. The long panel was placed opposing the short L, so that the inner distance between the two panels was 20 mm. In this study, three lines, each in a different color (blue, red, and green), were drawn on the outer facet of the long L-shaped panels on either side of the calibrator. The lines were drawn at 7 mm, 10 mm, 18 mm, 21 mm, 24 mm, and 29 mm. The accuracy of eight different exophthalmometers (Fig. 2) was tested by three orbital surgeons who performed exophthalmometry on a daily basis for more than 10 years. The exophthalmometers were manufactured by Carl Zeiss, Keeler, and Oculus. The exophthalmometers basically differ only in their footplate design (straight or curved) and mirror design (one or two mirrors). Excluded from this study were exophthalmometers without bilateral orbital rim contact (Naugle and Luedde).
Accepted May 9, 2005. Address correspondence and reprint requests to Dr. Yoav Vardizer, Orbital Center, Department of Ophthalmology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands. E-mail
[email protected] DOI: 10.1097/01.iop.0000180066.87572.39
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Y. VARDIZER ET AL. errors are underlined. Exophthalmometers number 1 and 8 do not have the same total number of examinations as the other instruments. The thick footplate of these two instruments obscures the lower markings on the calibrator. Table 2 (available online at www.op-rs.com) shows the effect of footplate and mirror design on the accuracy of the exophthalmometers tested. Exophthalmometers with a straight footplate and one mirror are more accurate when examined on the calibrator. On a linear regression analysis (general linear model repeated measures) they share an additive effect on accuracy (curved versus straight footplate, p ⱕ 0.001; one versus two mirrors, p ⱕ 0.00). Table 3 (available online at www.op-rs.com) shows that the range in which the values were measured had no effect on the accuracy in the two-mirror group of instruments; this is because the mean error in the low and high ranges did not significantly differ from the mean error in the middle range. However, in the one-mirror group, the mean error in the low and high ranges did differ significantly from the one in the middle range. Low values appear on the one-mirror exophthalmometers to be smaller and high values to be larger than their true value. Footplate design appeared to have no effect on the relative accuracy of low as opposed to medium or high measurements.
DISCUSSION
FIG 1. Top, Schematic drawing of the modified Van den Bosch calibrator. Bottom, The modified calibrator used in this study.
The participants were unaware of the exact value of each of the colored lines on the calibrator. They were asked to measure the three colored lines on the two sides of the calibrator with every exophthalmometer. Each exophthalmometer was used three times on three different occasions. To avoid adjusting measurements to the most common previous one, no more than four different instruments were evaluated on a given day. A mean error plus twice the standard deviation was defined acceptable when it was less than or equal to 2 mm.8 The exophthalmometer features we evaluated for their effect on accuracy included straight versus curved footplates and one versus two mirrors. We also evaluated the accuracy of the exophthalmometers according to different ranges of measurements: low range, Hertel values ⱕ12 mm; middle range, 13 to 22 mm; high range, ⱖ23 mm.
RESULTS There was no difference in measurement accuracy among the three examiners (p ⱕ 0.73, Student t test). The absolute errors of examinations with each of the exophthalmometers tested are listed in Table 1 (available online at www.op-rs.com). Instruments within the range of acceptable Ophthal Plast Reconstr Surg, Vol. 21, No. 6, 2005
In this study we evaluated the accuracy of eight commonly used Hertel exophthalmometers, using a selfmade calibrator. Our calibrator is different from the one described previously7 in two important aspects: First, we elongated the inner L-shaped plate to create a gap of 35 mm (instead of 20) between the two panels (simulating a 0- to 35-mm axial gap between the orbital rim and the cornea). This change enabled us to check the entire spectrum of values applied by the wide range of instruments. Second, in contrast to the Van den Bosch calibrator,7 we did not put an 18-mm fixed mark on the external facet of the long panel. Instead, we marked colored lines by using a ruler and a permanent pen. In this way, one can choose any value within the range mentioned, and change it, if needed. We chose to measure three values on each side of the calibrator. The lower value was chosen to represent a measurement within the enophthalmos-to-normal range, the middle corresponded to mild exophthalmos, and the upper was chosen in the range of moderate to severe proptosis. Several types of errors have been described when using Hertel and other exophthalmometers.9 Some of these errors arise from the instrument itself; others from its use in practice. Errors can result from changing the position of the patient’s head,10,11 the position of gaze, from pressing more or less against the orbital rim, or from the presence of swelling at the lateral canthus. These can arise in a clinical setting but are avoided when using our calibrator.
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FIG. 2. 1, Modified Carl Zeiss Hertel exophthalmometer (two mirrors, modified by the addition of plastic foot plate rests as described in detail in Reference 8); 2, Carl Zeiss exophthalmometer (old) (two mirrors, curved footplates); 3, Carl Zeiss exophthalmometer (new) (two mirrors, curved footplates); 4, Keeler two-mirror exophthalmometer (old) (two mirrors, straight footplates), 5. Keeler one-mirror exophthalmometer (new) (one mirror, straight footplates); 6, Oculus plastic exophthalmometer (two mirrors, curved footplates); 7, Oculus plastic exophthalmometer with rim rests (one mirror, straight footplates); 8, Oculus metal exophthalmometer (one mirror, curved footplates).
We could think of two errors apart from the intraobserver and interobserver errors that cannot be eliminated when using the calibrator. The first is if the curved
footplate of the meter is poorly positioned over the short facet of the calibrator.9 This error is not present when using the straight footplate exophthalmometers. The sec-
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ond is the parallax error derived from a vertical shift of the examiner’s head position during examination. When using the newer exophthalmometers (with one mirror), this parallax error is avoided by the use of a line placed on the mirror side on the middle of the ruler. No movement of the head by the examiner during measurement may create an error when reading extremely high or low values with these instruments, as indicated by Van den Bosch.7 These assumptions were shown to influence the accuracy of the exophthalmometers examined when checked for group analysis. However, when looking at the accuracy of individual instruments, not all of the acceptable mean error exophthalmometers had these characteristics. Therefore, we are reluctant to recommend a specific exophthalmometer and prefer to stress the general design as a recommendation. Because we examined exophthalmometer accuracy in a nonclinical setting, other factors, such as the thickness of the footplate, might influence the pressure generated when placing the exophthalmometer against the orbital rim and therefore influence the clinical measurements. Our recommendations, based on our study results, are as follows. (1) When purchasing a new exophthalmometer, one may consider an instrument that has a straight footplate and one mirror, as these seem to be more accurate. (2) Instruments with one mirror are most accurate at approximately 20 mm (exophthalmic values) and least accurate for enophthalmic values. This could be adjusted if the manufacturers would create a nonlinear ruler to compensate for the angle created by the need to adjust
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the pointer and the middle of the ruler in all examinations. When using absolute rather than relative exophthalmometric values, it is crucial to calibrate the exophthalmometer results. This is especially true when using more than one type of exophthalmometer on the same patient population or in a research environment, as it would facilitate future comparisons. REFERENCES 1. Segni M, Bartley GB, Garrity JA et al . Comparability of proptosis measurements by different techniques. Am J Ophthalmol 2002; 133:813–8. 2. Kim IT, Choi JB. Normal range of exophthalmos values on orbit computerized tomography in Koreans. Ophthalmologica 2001; 215:156–62. 3. Hertel E. Ein einfaches Exophthalmometer. Arch Ophthalmol 1905;60:171–4. 4. Cole HP, Couvillion JT, Fink AJ, et al Exophthalmometry: a comparative study of Naugle and Hertel instruments. Ophthal Plast Reconstr Surg 1997;13:189–94. 5. Kratky V, Hurwitz JJ. Hertel exophthalmometry without orbital rim contact. Ophthalmology 1994;101:931–7. 6. Sleep T, Manners R. Interinstrument variability in Hertel-type exophthalmometers. Ophthal Plast Reconstr Surg 2002;18:254–7. 7. Van den Bosch WA. Normal exophthalmometry values: the need for calibrated exophthalmometers [Editorial]. Orbit 2004;23:147– 51. 8. Mourits MP, Lombardo SHC, Van der Sluijs FA, Fenton S. Reliability exophthalmos measurement and exophthalmometry value distribution in a healthy Dutch population and graves patients: an exploring study. Orbit 2004;23:161–8. 9. Davanger M. Principles and sources of error in exophthalmometry: a new exophthalmometer. Acta Ophthalmol 1970;48:625– 33. 10. Asad R, Tewari HK, Ahuja MM, Mithal A. Postural variation of exophthalmometry in Graves ophthalmopathy. Indian J Ophthalmol 1990;38:166–8. 11. Frueh BR, Garber F, Gril RL, et al Positional effects on exophthalmometer readings in Graves disease. Arch Ophthalmol 1985; 103: 1355–6.
