- Email: [email protected]
Error of calibration in ophthalmic calipers: a source of significant clinical errors
ORIGINAL ARTICLE
Error of calibration in ophthalmic calipers: a source of significant clinical errors Mishari Mohammed Dahrab, MD, FRCSC, FRCS(Glasg), G. Robert LaRoche, MD, FRCSC ABSTRACT ● RÉSUMÉ Objective: Length measuring instruments are frequently used in ophthalmic surgery practice. For all subspecialties, calipers need to be accurate. This study was carried out to identify errors of calibration in ophthalmic calipers as a potential source of significant clinical errors. Design: This study is a descriptive research. Methods: All Castroviejo calipers free of any visible damage and available to the ophthalmic surgeons in the operating room suites of our 2 affiliated hospitals were included. The caliper scale readings were compared to measurement markings on a standardized ruler at screening points of 1, 5, 10, and 15 mm on the ruler. Any caliper with a discrepancy of 0.5 mm or more at any set of these screening points went on to having further analysis. Results: Seventy-one calipers were examined, of which 30 (42%) showed at least 1 caliper scale reading discrepancy of ⱖ0.5 mm as compared to ruler measurements. Errors of at least 1 mm were found in 6 of 30 calipers (20%). The majority of calipers underestimated lengths 22/30 (73%), whereas 27% overestimated. Conclusions: With close to half of the calipers inducing a 0.5 mm or more error, and with 20% of these at least 1 mm, significant clinical consequences could ensue: for example, in follow up of glaucomatous corneas in children, in measurements for anterior chamber intraocular lens sizing, in certain refractive surgery techniques, pars-plana sclerotomies, and intravitreal injection sites, or in measuring amounts in strabismus to name a few. Errors in calibration of ophthalmic calipers must be acknowledged and avoided. Objet : Les instruments de mesure de la longueur sont utilisés souvent dans la pratique chirurgicale ophtalmique. Cette étude a pour objet d’identifier les erreurs de calibration des étriers ophtalmiques, sources potentielles d’importantes erreurs cliniques. Nature : Recherche descriptive. Méthodes : Ont été inclus tous les étriers Castroviejo libres de tout dommage et accessibles aux chirurgiens ophtalmologistes dans les salles d’opération de nos deux hôpitaux affiliés. Les lectures des échelles des étriers ont été comparées au marquage des mesures aux points de contrôle de 1, 5, 10 et 15 mm d’une règle standardisée. Tout étrier ayant un écart de 0,5 mm ou plus à tout ensemble de points de contrôle a été soumis à d’autres analyses. Résultats : 71 étriers ont été examinés et 30 (42 %) d’entre eux avaient au moins une mesure avec un écart de ⱖ 0,5 mm en regard de l’échelle de lecture. Des erreurs d’au moins 1 mm ont été constatées dans 6 des 30 étriers (20 %). La majorité des étriers sous-estimaient des longueurs 22/30 (73 %), alors que 27 % les surestimaient. Conclusions : Le fait que près de la moitié des étriers suscitaient des erreurs de 0,5 mm ou plus et que 20 % de celles-ci étaient d’au moins de 1 mm, d’importantes conséquences cliniques pouvaient s’ensuivre: par exemple, dans le suivi de cornées glaucomateuses chez les enfants, dans la mesure de la taille de la lentille intraoculaire de la chambre antérieure, dans certaines techniques chirurgicales réfractives, dans la sclérostomie par la pars plana, sites d’injections intravitréennes et des mesures se rapportant au strabisme, pour n’en nommer que quelques-unes. Les erreurs de calibration des étriers ophtalmiques doivent être reconnues et évitées.
With new advances in ophthalmic instrument technology, calipers of many types have nevertheless continued to be necessary in many aspects of ophthalmic care. The various types of these length measuring instruments fall into 2 categories: fixed or moveable scales. Examples of fixed scale calipers are the Stahl caliper, Braunstein fixed caliper, incision gauge, and scleral ruler. The latter is of note as it measures arc length as opposed to chord lengths measured by all the others. Chord length measures distance between 2 points as a straight line. Because the globe is curved, readings of chord length for large arc length measurements (⬎9.0 mm) will result in errors. For that purpose, scleral rulers are preferred to measure accurately the true arc length of the globe when the two have the exact same radius of curvature.1 As for moveable scales, hinged calipers and bulkier sliding vernier calipers are the commonest
forms. The hinged Castroviejo caliper has become particularly popular with its relative small size, ease of use and readability. Length measuring instruments are frequently used in ophthalmic surgical practice, and for all applications there is a need for accuracy: from measuring white to white corneal diameter for anterior chamber intraocular lens sizing, follow-up of congenital glaucoma, sizing limbal relaxing incisions in refractive surgery, indicating the site for pars plana sclerotomy/intravitreal injection, and measuring amounts in strabismus surgery.2-6
CASE REPORT During the examination of a congenital pediatric cataract patient under anesthesia, a Castroviejo caliper (Fig. 1)
From Dalhousie University, Halifax, NS.
Quebec City, Que., June 26 –29, 2010.
Originally received Jan. 12, 2011 Final revision Apr. 29, 2011 Accepted Jun. 17, 2011 Correspondence to G. Robert LaRoche, IWK Health Centre, PO Box 9700, Halifax, NS B3K 6R8, Canada; email: [email protected] Poster presentation at the Canadian Ophthalmological Society Meeting in
Can J Ophthalmol 2011;46:510 –512
510
CAN J OPHTHALMOL—VOL. 46, NO. 6, DECEMBER 2011
0008-4182/11/$-see front matter © 2011 Canadian Ophthalmological Society. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.jcjo.2011.09.007
Error of calibration in ophthalmic calipers—Dahrab and LaRoche surements on their scales against the markings of a reference ruler. Screening measurements were 1, 5, 10, and 15 mm on the ruler and were compared to the expected corresponding caliper reading. These distance points were considered important as they lie in our surgically relevant range (1-15 mm). Caliper errors only present in distances ⬎15 mm were not examined nor considered relevant. Any caliper with one or more discrepancies ⱖ0.5 mm at any of the set screening distances of the Codman ruler was removed from our surgical instrument set and further tested with 1 more set of verifications at 1, 3, 5, 7, 10, 12, 15, 17, and 20mm ruler measurements. With calipers scale readings having only 1 mm increments, we estimated a 0.5 mm scale reading as a visually determined half point between 2 1-mm increment markings. Points ⱕ0.5 mm visually but not quite at 1 mm increment, were noted as 0.75 mm and 0.25 mm, respectively. This was done to create a useable categorization of these measurements rather than an unrealistic linear set of data.
RESULTS
Fig. 1—Caliper reading 15 mm while measuring 13 mm on standard office ruler.
was used to measure corneal diameter. All other factors being normal in the patient, the measurement of an abnormally large corneal diameter in both eyes was somewhat surprising. The caliper used was therefore “calibrated” against a Codman ruler (Codman Inc, Raynham, MA) as well as a surgical metallic ruler. Both showed a discrepancy of ⬎1 mm on the Castroviejo caliper scale. Inspection of the caliper showed no structural defects. This occurrence lead to the evaluation of all Castroviejo calipers used in both adult and pediatric ophthalmology services at our 2 affiliated teaching hospitals. We report the results of this evaluation and discuss their significance.
METHODS All Castroviejo calipers that were available to the ophthalmic surgeons in the operating room suites at both locations of our 2 affiliated teaching hospitals at Dalhousie University in Halifax, Nova Scotia, Canada were included. Calipers were to be free of any signs of damage or missing parts and likely to be used without hesitation if requested by a surgeon during a routine case. All Castroviejo calipers were initially screened by comparing the indicated mea-
In total, 71 calipers were examined. 30 (42%) showed at least 1 point of discrepancy of ⱖ0.5mm on the caliper scale measurement between 1-15 mm. Once caliper discrepancy was present at a certain set point, it persisted on all subsequent measurements or increased. Eleven calipers had ⬎0.5 mm error (Table 1). There were no caliper errors ⬎0.5 mm at points below 7 mm but 9 (30%) had 0.5 mm of error at or the below 5 mm ruler measurement point. An error of at least 1 mm was identified in 20% (6/30). These measurement errors were identified between the 7 mm and 15 mm scale readings of the calipers. Overall, the majority of all the errors were underestimation of lengths rather than overestimation (73% vs 27%).
DISCUSSION As a result of this project, many calipers in the service were removed because of a ⬎0.5 mm error within a surgically relevant range (1-15 mm) on the caliper scale. This was done after consensus determination by our surgeons that caliper measurements discrepancy ⬎0.5 mm were significant enough to lead to potentially harmful surgical decisions. These decisions included: poorly fitted anterior chamber lens implants, inaccurate identification of pars plana approach resulting in retina or crystalline lens damage, erroneous results in examination under anesthesia in congenital glaucoma, and inaccurate measurements during strabismus surgery. Erroneous caliper measurements can be compounded when the same caliper is used multiple times on the same patient i.e., bilateral strabismus surgery, or different calipers used on separate occasions: one that overestimates whereas the second underestimates giving a false indication of significant change. Although none of the calipers had ⬎0.5 mm of discrepancy at measured points CAN J OPHTHALMOL—VOL. 46, NO. 6, DECEMBER 2011
511
Error of calibration in ophthalmic calipers—Dahrab and LaRoche Table 1—Amount of caliper error as measured to the ruler at each screening point Ruler (mm) Caliper
1
3
5
7
10
12
15
17
20
1 2 4 5 6 7 15 18 20 21 30
0 0 0 0 0 0 0 0 0 0 0
⫺0.25 0 ⫺0.25 ⫺0.25 0 ⫺0.25 0 0 0 0 ⫹0.5
⫺0.50 ⫺0.50 ⫺0.50 ⫺0.25 ⫺0.25 ⫺0.50 0 ⫹0.25 0 0 ⫹0.5
⫺0.50 ⫺0.50 ⫺0.75 ⫺0.50 ⫺0.50 ⫺0.50 0 ⫹0.75 ⫺0.25 ⫹0.50 ⫹1
⫺0.75 ⫺0.50 ⫺1 ⫺0.50 ⫺0.50 ⫺0.75 ⫹0.25 ⫹0.75 ⫺0.50 ⫹1 ⫹1.25
⫺0.75 ⫺0.50 ⫺1 ⫺0.50 ⫺0.50 ⫺0.50 ⫹0.5 ⫹1 ⫺0.75 ⫹1 ⫹1.50
⫺1 ⫺0.75 ⫺0.75 ⫺0.75 ⫺0.75 ⫺1 ⫹0.75 n/a ⫺0.50 ⫹1 ⫹2
⫺1 ⫺0.75 ⫺0.75 ⫺1 ⫺0.75 ⫺0.75 ⫹1 — ⫺0.50 ⫹1.25 ⫹2.25
⫺1 ⫺1 ⫺1 ⫺1 ⫺1 ⫺0.75 n/a — ⫺0.50 n/a n/a
The (⫹) sign indicates over-estimation and the (⫺) sign indicates under-estimation. 254 ⫻ 190 mm (96 ⫻ 96 DPI). Note: n/a, not available.
below 7 mm, at such a short measuring distances even a 0.5 mm offset can become critical. Finding a significant number of calipers with measurement discrepancy has resulted in our change of practices the first of which being the identification of the erroneous calipers, with removal of the ones considered potential harmful. Second, we are now regularly testing our calipers against ruler measurements. Indeed, an erroneous caliper is only problematic when it is used as the sole measuring device and not calibrated to a ruler. Otherwise, it becomes nothing more than an expensive geometry compass. We also feel this calibration is required at every caliper measurement point before use to ensure accuracy. Indeed, after the completion of this study, calipers that initially passed our screening exam later developed significant measurement errors, highlighting that calipers are constantly at risk of being erroneous and vigilance in detection is key. This suggests that calipers are exposed to wear and tear, affecting their measuring accuracy. It is likely due to the hinge mechanism, but we recognize our protocol did not address this particular issue as our sample consisted mostly of already well used instruments. This project did not seek to thoroughly evaluate the mechanical reasons for the hinged caliper discrepancies. However, their discovery is significant because these measurement errors can lead to harmful clinical and surgical decisions. Therefore, errors in calibration of ophthalmic calipers must be acknowledged and avoided.
512
CAN J OPHTHALMOL—VOL. 46, NO. 6, DECEMBER 2011
CONCLUSION Even the simplest of instruments needs rigorous quality control to avoid significant surgical and diagnostic errors. Therefore, screening all hinged calipers to identify potentially harmful discrepancy measurements is encouraged; and so is the consistent use of ruler measurements to identify proper scale readings. In sensitive measurements, a non-hinged caliper could be more accurate.
Disclosure: The authors have no proprietary or commercial interest in any materials discussed in this article. REFERENCES 1. Clark R, Rosenbaum A. Instrument-induced measurement errors during strabismus surgery. J AAPOS. 1999;3:18-25. 2. Tower P. Increased accuracy in squint surgery. AMA Arch Ophthalmol. 1950;44:395-8. 3. Pop M, Payette Y, Mansour M. Predicting sulcus size using ocular measurements. J Cataract Refract Surg. 2001;27:1033-8. 4. Khng C, Fine IH, Packer M, Hoffman RS. Improved precision with the millimeter caliper for limbal relaxing incisions. J Cataract Refract Surg. 2005;31:1671-2. 5. Kaufmann C, Peter J, Ooi K, Phipps S, Cooper P, Goggin M. Limbal relaxing incisions versus on-axis incisions to reduce corneal astigmatism at the time of cataract surgery. J Cataract Refract Surg. 2005; 31:2261-5. 6. Thompson JT, Michels RG. Comparison of chord length and scleral arc length when placing sutures for episcleral exoplants. Retina. 1985; 5:225-6.
Error of calibration in ophthalmic calipers: a source of significant clinical errors Mishari Mohammed Dahrab, MD, FRCSC, FRCS(Glasg), G. Robert LaRoche, MD, FRCSC ABSTRACT ● RÉSUMÉ Objective: Length measuring instruments are frequently used in ophthalmic surgery practice. For all subspecialties, calipers need to be accurate. This study was carried out to identify errors of calibration in ophthalmic calipers as a potential source of significant clinical errors. Design: This study is a descriptive research. Methods: All Castroviejo calipers free of any visible damage and available to the ophthalmic surgeons in the operating room suites of our 2 affiliated hospitals were included. The caliper scale readings were compared to measurement markings on a standardized ruler at screening points of 1, 5, 10, and 15 mm on the ruler. Any caliper with a discrepancy of 0.5 mm or more at any set of these screening points went on to having further analysis. Results: Seventy-one calipers were examined, of which 30 (42%) showed at least 1 caliper scale reading discrepancy of ⱖ0.5 mm as compared to ruler measurements. Errors of at least 1 mm were found in 6 of 30 calipers (20%). The majority of calipers underestimated lengths 22/30 (73%), whereas 27% overestimated. Conclusions: With close to half of the calipers inducing a 0.5 mm or more error, and with 20% of these at least 1 mm, significant clinical consequences could ensue: for example, in follow up of glaucomatous corneas in children, in measurements for anterior chamber intraocular lens sizing, in certain refractive surgery techniques, pars-plana sclerotomies, and intravitreal injection sites, or in measuring amounts in strabismus to name a few. Errors in calibration of ophthalmic calipers must be acknowledged and avoided. Objet : Les instruments de mesure de la longueur sont utilisés souvent dans la pratique chirurgicale ophtalmique. Cette étude a pour objet d’identifier les erreurs de calibration des étriers ophtalmiques, sources potentielles d’importantes erreurs cliniques. Nature : Recherche descriptive. Méthodes : Ont été inclus tous les étriers Castroviejo libres de tout dommage et accessibles aux chirurgiens ophtalmologistes dans les salles d’opération de nos deux hôpitaux affiliés. Les lectures des échelles des étriers ont été comparées au marquage des mesures aux points de contrôle de 1, 5, 10 et 15 mm d’une règle standardisée. Tout étrier ayant un écart de 0,5 mm ou plus à tout ensemble de points de contrôle a été soumis à d’autres analyses. Résultats : 71 étriers ont été examinés et 30 (42 %) d’entre eux avaient au moins une mesure avec un écart de ⱖ 0,5 mm en regard de l’échelle de lecture. Des erreurs d’au moins 1 mm ont été constatées dans 6 des 30 étriers (20 %). La majorité des étriers sous-estimaient des longueurs 22/30 (73 %), alors que 27 % les surestimaient. Conclusions : Le fait que près de la moitié des étriers suscitaient des erreurs de 0,5 mm ou plus et que 20 % de celles-ci étaient d’au moins de 1 mm, d’importantes conséquences cliniques pouvaient s’ensuivre: par exemple, dans le suivi de cornées glaucomateuses chez les enfants, dans la mesure de la taille de la lentille intraoculaire de la chambre antérieure, dans certaines techniques chirurgicales réfractives, dans la sclérostomie par la pars plana, sites d’injections intravitréennes et des mesures se rapportant au strabisme, pour n’en nommer que quelques-unes. Les erreurs de calibration des étriers ophtalmiques doivent être reconnues et évitées.
With new advances in ophthalmic instrument technology, calipers of many types have nevertheless continued to be necessary in many aspects of ophthalmic care. The various types of these length measuring instruments fall into 2 categories: fixed or moveable scales. Examples of fixed scale calipers are the Stahl caliper, Braunstein fixed caliper, incision gauge, and scleral ruler. The latter is of note as it measures arc length as opposed to chord lengths measured by all the others. Chord length measures distance between 2 points as a straight line. Because the globe is curved, readings of chord length for large arc length measurements (⬎9.0 mm) will result in errors. For that purpose, scleral rulers are preferred to measure accurately the true arc length of the globe when the two have the exact same radius of curvature.1 As for moveable scales, hinged calipers and bulkier sliding vernier calipers are the commonest
forms. The hinged Castroviejo caliper has become particularly popular with its relative small size, ease of use and readability. Length measuring instruments are frequently used in ophthalmic surgical practice, and for all applications there is a need for accuracy: from measuring white to white corneal diameter for anterior chamber intraocular lens sizing, follow-up of congenital glaucoma, sizing limbal relaxing incisions in refractive surgery, indicating the site for pars plana sclerotomy/intravitreal injection, and measuring amounts in strabismus surgery.2-6
CASE REPORT During the examination of a congenital pediatric cataract patient under anesthesia, a Castroviejo caliper (Fig. 1)
From Dalhousie University, Halifax, NS.
Quebec City, Que., June 26 –29, 2010.
Originally received Jan. 12, 2011 Final revision Apr. 29, 2011 Accepted Jun. 17, 2011 Correspondence to G. Robert LaRoche, IWK Health Centre, PO Box 9700, Halifax, NS B3K 6R8, Canada; email: [email protected] Poster presentation at the Canadian Ophthalmological Society Meeting in
Can J Ophthalmol 2011;46:510 –512
510
CAN J OPHTHALMOL—VOL. 46, NO. 6, DECEMBER 2011
0008-4182/11/$-see front matter © 2011 Canadian Ophthalmological Society. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.jcjo.2011.09.007
Error of calibration in ophthalmic calipers—Dahrab and LaRoche surements on their scales against the markings of a reference ruler. Screening measurements were 1, 5, 10, and 15 mm on the ruler and were compared to the expected corresponding caliper reading. These distance points were considered important as they lie in our surgically relevant range (1-15 mm). Caliper errors only present in distances ⬎15 mm were not examined nor considered relevant. Any caliper with one or more discrepancies ⱖ0.5 mm at any of the set screening distances of the Codman ruler was removed from our surgical instrument set and further tested with 1 more set of verifications at 1, 3, 5, 7, 10, 12, 15, 17, and 20mm ruler measurements. With calipers scale readings having only 1 mm increments, we estimated a 0.5 mm scale reading as a visually determined half point between 2 1-mm increment markings. Points ⱕ0.5 mm visually but not quite at 1 mm increment, were noted as 0.75 mm and 0.25 mm, respectively. This was done to create a useable categorization of these measurements rather than an unrealistic linear set of data.
RESULTS
Fig. 1—Caliper reading 15 mm while measuring 13 mm on standard office ruler.
was used to measure corneal diameter. All other factors being normal in the patient, the measurement of an abnormally large corneal diameter in both eyes was somewhat surprising. The caliper used was therefore “calibrated” against a Codman ruler (Codman Inc, Raynham, MA) as well as a surgical metallic ruler. Both showed a discrepancy of ⬎1 mm on the Castroviejo caliper scale. Inspection of the caliper showed no structural defects. This occurrence lead to the evaluation of all Castroviejo calipers used in both adult and pediatric ophthalmology services at our 2 affiliated teaching hospitals. We report the results of this evaluation and discuss their significance.
METHODS All Castroviejo calipers that were available to the ophthalmic surgeons in the operating room suites at both locations of our 2 affiliated teaching hospitals at Dalhousie University in Halifax, Nova Scotia, Canada were included. Calipers were to be free of any signs of damage or missing parts and likely to be used without hesitation if requested by a surgeon during a routine case. All Castroviejo calipers were initially screened by comparing the indicated mea-
In total, 71 calipers were examined. 30 (42%) showed at least 1 point of discrepancy of ⱖ0.5mm on the caliper scale measurement between 1-15 mm. Once caliper discrepancy was present at a certain set point, it persisted on all subsequent measurements or increased. Eleven calipers had ⬎0.5 mm error (Table 1). There were no caliper errors ⬎0.5 mm at points below 7 mm but 9 (30%) had 0.5 mm of error at or the below 5 mm ruler measurement point. An error of at least 1 mm was identified in 20% (6/30). These measurement errors were identified between the 7 mm and 15 mm scale readings of the calipers. Overall, the majority of all the errors were underestimation of lengths rather than overestimation (73% vs 27%).
DISCUSSION As a result of this project, many calipers in the service were removed because of a ⬎0.5 mm error within a surgically relevant range (1-15 mm) on the caliper scale. This was done after consensus determination by our surgeons that caliper measurements discrepancy ⬎0.5 mm were significant enough to lead to potentially harmful surgical decisions. These decisions included: poorly fitted anterior chamber lens implants, inaccurate identification of pars plana approach resulting in retina or crystalline lens damage, erroneous results in examination under anesthesia in congenital glaucoma, and inaccurate measurements during strabismus surgery. Erroneous caliper measurements can be compounded when the same caliper is used multiple times on the same patient i.e., bilateral strabismus surgery, or different calipers used on separate occasions: one that overestimates whereas the second underestimates giving a false indication of significant change. Although none of the calipers had ⬎0.5 mm of discrepancy at measured points CAN J OPHTHALMOL—VOL. 46, NO. 6, DECEMBER 2011
511
Error of calibration in ophthalmic calipers—Dahrab and LaRoche Table 1—Amount of caliper error as measured to the ruler at each screening point Ruler (mm) Caliper
1
3
5
7
10
12
15
17
20
1 2 4 5 6 7 15 18 20 21 30
0 0 0 0 0 0 0 0 0 0 0
⫺0.25 0 ⫺0.25 ⫺0.25 0 ⫺0.25 0 0 0 0 ⫹0.5
⫺0.50 ⫺0.50 ⫺0.50 ⫺0.25 ⫺0.25 ⫺0.50 0 ⫹0.25 0 0 ⫹0.5
⫺0.50 ⫺0.50 ⫺0.75 ⫺0.50 ⫺0.50 ⫺0.50 0 ⫹0.75 ⫺0.25 ⫹0.50 ⫹1
⫺0.75 ⫺0.50 ⫺1 ⫺0.50 ⫺0.50 ⫺0.75 ⫹0.25 ⫹0.75 ⫺0.50 ⫹1 ⫹1.25
⫺0.75 ⫺0.50 ⫺1 ⫺0.50 ⫺0.50 ⫺0.50 ⫹0.5 ⫹1 ⫺0.75 ⫹1 ⫹1.50
⫺1 ⫺0.75 ⫺0.75 ⫺0.75 ⫺0.75 ⫺1 ⫹0.75 n/a ⫺0.50 ⫹1 ⫹2
⫺1 ⫺0.75 ⫺0.75 ⫺1 ⫺0.75 ⫺0.75 ⫹1 — ⫺0.50 ⫹1.25 ⫹2.25
⫺1 ⫺1 ⫺1 ⫺1 ⫺1 ⫺0.75 n/a — ⫺0.50 n/a n/a
The (⫹) sign indicates over-estimation and the (⫺) sign indicates under-estimation. 254 ⫻ 190 mm (96 ⫻ 96 DPI). Note: n/a, not available.
below 7 mm, at such a short measuring distances even a 0.5 mm offset can become critical. Finding a significant number of calipers with measurement discrepancy has resulted in our change of practices the first of which being the identification of the erroneous calipers, with removal of the ones considered potential harmful. Second, we are now regularly testing our calipers against ruler measurements. Indeed, an erroneous caliper is only problematic when it is used as the sole measuring device and not calibrated to a ruler. Otherwise, it becomes nothing more than an expensive geometry compass. We also feel this calibration is required at every caliper measurement point before use to ensure accuracy. Indeed, after the completion of this study, calipers that initially passed our screening exam later developed significant measurement errors, highlighting that calipers are constantly at risk of being erroneous and vigilance in detection is key. This suggests that calipers are exposed to wear and tear, affecting their measuring accuracy. It is likely due to the hinge mechanism, but we recognize our protocol did not address this particular issue as our sample consisted mostly of already well used instruments. This project did not seek to thoroughly evaluate the mechanical reasons for the hinged caliper discrepancies. However, their discovery is significant because these measurement errors can lead to harmful clinical and surgical decisions. Therefore, errors in calibration of ophthalmic calipers must be acknowledged and avoided.
512
CAN J OPHTHALMOL—VOL. 46, NO. 6, DECEMBER 2011
CONCLUSION Even the simplest of instruments needs rigorous quality control to avoid significant surgical and diagnostic errors. Therefore, screening all hinged calipers to identify potentially harmful discrepancy measurements is encouraged; and so is the consistent use of ruler measurements to identify proper scale readings. In sensitive measurements, a non-hinged caliper could be more accurate.
Disclosure: The authors have no proprietary or commercial interest in any materials discussed in this article. REFERENCES 1. Clark R, Rosenbaum A. Instrument-induced measurement errors during strabismus surgery. J AAPOS. 1999;3:18-25. 2. Tower P. Increased accuracy in squint surgery. AMA Arch Ophthalmol. 1950;44:395-8. 3. Pop M, Payette Y, Mansour M. Predicting sulcus size using ocular measurements. J Cataract Refract Surg. 2001;27:1033-8. 4. Khng C, Fine IH, Packer M, Hoffman RS. Improved precision with the millimeter caliper for limbal relaxing incisions. J Cataract Refract Surg. 2005;31:1671-2. 5. Kaufmann C, Peter J, Ooi K, Phipps S, Cooper P, Goggin M. Limbal relaxing incisions versus on-axis incisions to reduce corneal astigmatism at the time of cataract surgery. J Cataract Refract Surg. 2005; 31:2261-5. 6. Thompson JT, Michels RG. Comparison of chord length and scleral arc length when placing sutures for episcleral exoplants. Retina. 1985; 5:225-6.