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Middle East African Journal of Ophthalmology Middle East African Journal of Ophthalmology
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Year : 2019  |  Volume : 26  |  Issue : 1  |  Page : 17-22  

Improved refractive outcomes of small-incision extracapsular cataract surgery after implementation of a biometry training course

1 Fred Hollows Foundation New Zealand, Auckland; Department of Ophthalmology, New Zealand National Eye Centre, University of Auckland, New Zealand
2 Department of Ophthalmology, New Zealand National Eye Centre, University of Auckland, New Zealand
3 Pacific Eye Institute, Suva, Fiji
4 Fred Hollows Foundation New Zealand, Auckland, New Zealand

Date of Web Publication24-Apr-2019

Correspondence Address:
Dr. Jay J Meyer
Department of Ophthalmology, Private Bag 92019, University of Auckland, Auckland
New Zealand
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/meajo.MEAJO_3_18

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PURPOSE: To determine whether a biometry training course could improve refractive outcomes of patients undergoing manual small-incision extracapsular cataract surgery (SICS).
MATERIALS AND METHODS: This was a prospective, interventional, cohort study at the Pacific Eye Institute, Fiji. SICS refractive outcomes were evaluated before and after a structured biometry teaching course. Eyes that underwent evaluation and subsequent SICS with placement of a posterior chamber intraocular lens (IOL) were included. Axial length measurements were obtained using A-scan applanation ultrasound and keratometry with a handheld keratometer. Main outcome measures included mean absolute prediction error of IOL calculations, percentage of eyes within ±0.5 D and ±1.0 D of intended spherical equivalent, and proportion of eyes with ≥6/18 uncorrected visual acuity.
RESULTS: A total of 240 eyes were analyzed: 120 eyes before and 120 eyes after the structured biometry training. The mean absolute prediction error was 50% lower following the training (1.13 ± 0.84 D pre vs. 0.56 ± 0.44 D post; P < 0.001). A higher percentage of the eyes had a postoperative spherical equivalent within ±0.5 D (26.7% pre vs. 52.5% post; P < 0.001) and ±1.0 D (55.0% pre vs. 90.0% post; P < 0.001) of the intended target. A higher proportion of the eyes achieved ≥6/18 uncorrected visual acuity (77.5% pre vs. 91.7% post, P = 0.004), while the proportion with ≥6/18 corrected visual acuity was similar (94.4% pre vs. 98.3% post; P = 0.28).
CONCLUSIONS: A structured biometry training course may improve the accuracy of preoperative IOL calculations to achieve the postoperative refractive target. Ophthalmology training programs should include structured biometry teaching in their curricula.

Keywords: Biometry, cataract, cataract surgery, phacoemulsification

How to cite this article:
Meyer JJ, Vellara HR, Bhikoo R, Sefo LA, Lolokabaira S, Murray NL, McGhee CN. Improved refractive outcomes of small-incision extracapsular cataract surgery after implementation of a biometry training course. Middle East Afr J Ophthalmol 2019;26:17-22

How to cite this URL:
Meyer JJ, Vellara HR, Bhikoo R, Sefo LA, Lolokabaira S, Murray NL, McGhee CN. Improved refractive outcomes of small-incision extracapsular cataract surgery after implementation of a biometry training course. Middle East Afr J Ophthalmol [serial online] 2019 [cited 2020 Aug 13];26:17-22. Available from: http://www.meajo.org/text.asp?2019/26/1/17/256971

   Introduction Top

Cataract is the leading cause of blindness worldwide and alone accounts for half of all cases.[1] While cataracts may be treated and vision improved by surgery, the surgical outcomes are suboptimal in some regions of the world.[2],[3],[4],[5] Surgical complications and inadequate optical correction have been identified as major causes of poor visual outcomes.[6] In many parts of the world, patients cannot afford spectacles after cataract surgery, and therefore, uncorrected postoperative refractive error limits their potential vision. As such, it is important to obtain accurate biometry measurements and have a wide range of available intraocular lens (IOL) powers to deliver the best possible uncorrected vision following surgery.

Certainly, there is a paucity of research describing methods to improve refractive outcomes, particularly in the developing world. Indeed, most available studies focus on methods to improve refractive outcomes in societies where biometry is performed using the latest generations of optical biometers and cataract surgery is performed using phacoemulsification or femtosecond laser-assisted techniques.[7],[8],[9],[10] These studies are not relevant in many parts of the world where optical biometers cannot penetrate more advanced, dense cataracts, and extracapsular cataract extraction techniques are routinely employed.

In regions where dense cataracts are frequently encountered, A-scan ultrasound and manual or handheld keratometry are typically used to obtain biometry measurements for IOL power calculation. It has been demonstrated in a developed world setting that a standardized biometry teaching program results in improved refractive outcomes.[11] Therefore, we developed a prospective study to determine whether a standardized biometry teaching program in a developing region, where ultrasound axial length measurements and small incision extracapsular cataract extraction are routinely performed, would result in improved refractive outcomes.

   Materials and Methods Top

Patients undergoing cataract evaluation and surgery were prospectively enrolled in a study to evaluate the outcomes of cataract surgery at the Pacific Eye Institute located in Suva, Fiji. This study was approved by the University of Auckland Ethics Committee (012435) and the Fiji Ministry of Health Ethics Committee (2014.59.CD), and informed consent was obtained from participants.

Preoperative, intraoperative, and postoperative data were recorded for consecutive patients undergoing cataract surgery. The eyes that received an anterior chamber IOL were excluded from the study. Only one eye from each participant was included. After data were collected for an initial cohort of patients under the prevailing biometry assessment conditions (Group 1), formal didactic and practical teaching sessions regarding biometry technique, and IOL selection were administered to all participating surgeons, consisting of consultants (n = 5) and registrars (n = 10). The registrars had previously never received formal didactic training on biometry but had been taught by consultants and fellow trainees how to perform A-scan measurements and keratometry. Two of the five consultants reported prior formal didactic training in the distant past.

The major teaching points of the sessions included: A-scan measurement acquisition and interpretation, keratometry measurement acquisition and interpretation, IOL formula selection, and IOL power selection. In addition, guidelines regarding acceptable measurements were given. For axial length measurements, it was recommended that both eyes always be measured, and for each eye, the mean of at least 5 measurements within 0.1 mm (if possible) be used. In addition, measurements should be repeated/verified if the quality of ultrasound spikes is poor, if mean values of the right and left eyes are not within 0.3 mm of each other, or if axial length is <22 mm or >25 mm. For keratometry, it was recommended that measurements be repeated/verified if mean values are <40 D or >47 D, if astigmatism >2.5 D, or if total difference in mean measurements between the two eyes exceeds 1.5 D. It was recommended that the average of at least 3 keratometry measurements be used.

At this center, each surgeon routinely performs biometry measurements for all patients; (s) he will operate upon. Each surgeon received hands-on practical sessions where their axial length and keratometry measurement techniques were directly observed and instructions to improve their techniques were provided. The most frequent errors in measurement acquisition/technique and data interpretation were noted. Following the teaching, data were prospectively recorded for a second cohort of patients (Group 2), for comparison at 1–3 months (Group 2A), and 6 months later, or 9–10 months after the teaching (Group 2B), to test for persistence of effect.

All eyes underwent manual small incision extracapsular cataract surgery (SICS) with a temporal or superior (if unable to be performed temporally) scleral tunnel performed by a consultant ophthalmologist or registrar with consultant supervision. The technique involved a limbal conjunctival peritomy, followed by a straight or frown-shaped scleral incision located approximately 3 mm posterior to the limbus. A sclerocorneal tunnel was fashioned with an external opening of 5–8 mm (depending on the size of the nucleus) and wider internal opening of up to ~ 9 mm. Trypan blue dye was used to stain the anterior capsule in cases with a poor red reflex. A paracentesis was created and the anterior chamber was filled with viscoelastic (sodium hyaluronate 1.4%, Aurolab). A large, continuous, curvilinear capsulorrhexis or can-opener capsulorrhexis was created, depending on the surgeons' preference and experience. Hydrodissection and/or viscodissection was performed to prolapse the lens into the anterior chamber. Viscoelastic was injected above and below the nucleus which was extracted using a vectis. A Simcoe cannula was then used to remove cortical lens material. Viscoelastic was used to fill the capsular bag followed by placement of the IOL within the capsular bag (or sulcus if unable to place in bag). The viscoelastic was removed, conjunctiva reapproximated with diathermy, and cefuroxime given intracamerally. Sutures were not routinely used/required for wound closure. A FH106 (Fred Hollows Intraocular Lens Laboratory; Kathmandu, Nepal) or Aurolens S3600 (Aurolab; Langenhagen, Germany) polymethylmethacrylate (PMMA) posterior chamber IOL was implanted in all cases. Emmetropia or slight myopia (−0.5 D or less) was targeted for all eyes.

The recorded measurements included axial length as measured by A-scan applanation ultrasound using an Accutome® A-scan Plus and keratometry measurements taken using a Nidek KM-500 handheld autokeratometer. The implanted IOL type/power was used to calculate the predicted postoperative spherical equivalent, and this was compared to the actual postoperative spherical equivalent to determine the prediction error of the formula. The predicted postoperative refractive error was calculated using the SRK/T formula.[12] The postoperative subjective refraction was performed between 1 and 3 months postoperatively. The refraction was performed by an experienced optometrist or ophthalmologist who did not perform the cataract surgery, with autorefraction performed before refinement by subjective refraction as required. The presence of postoperative complications, including posterior capsular opacification (PCO) graded I-IV and presence of cystoid macular edema, were also recorded.

Statistical analysis was performed using SPSS version 23 statistical software (SPSS Inc., Chicago, IL, USA) and Microsoft Office Excel 2010 (Redmond, Washington, USA). Visual acuity data were log-transformed (LogMar) where indicated for statistical analysis. The Student's t-test was used to compare means of normally distributed data and the Fisher's exact test to compare categorical data.

   Results Top

A total of 240 eyes were analyzed: 120 eyes in the preteaching cohort, Group 1, and 120 eyes in the postteaching cohort, Group 2. Eleven additional eyes with anterior chamber lenses placed due to inadequate capsular support were excluded (5 preteaching and 6 postteaching). There were no significant differences in the mean age, gender, diabetes status, preoperative best-corrected visual acuity, axial length, and average keratometry values between the two cohorts [Table 1]. The mean keratometric astigmatism was higher in the preteaching cohort (0.90 D vs. 0.76 D; P = 0.01). The mean time to postoperative refraction was 32.3 days in the preteaching cohort Group 1 and 37.0 days for the postteaching cohort Group 2 (P = 0.03). There were no differences in the percentages of patients with postoperative macular edema (7.5% Group 1 vs. 5.0% Group 2; P = 0.60) or with PCO Grades III-IV (7.5% Group 1 vs. 3.3% Group 2; P = 0.25).
Table 1: Preoperative characteristics of the study participants

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The mean postoperative spherical equivalent was −0.02 ± 1.00 D (range −4.0 to +2.5 D) in Group 1 and −0.28 ± 0.66 D (range −2.125 to +1.5 D) in Group 2 [P < 0.001, [Figure 1]. The mean absolute value of the spherical equivalent was higher in Group 1 compared to Group 2 (1.15 ± 0.85 D vs. 0.55 ± 0.47 D; P < 0.001). The mean postoperative refractive astigmatism was not significantly different in Group 1 (0.84 ± 0.78 D; range 0–4.0 D) compared to Group 2 (0.73 ± 0.60 D; range 0–3.5 D) (P = 0.21). A higher proportion of eyes achieved ≥6/18 uncorrected visual acuity following the biometry course (77.5% Group 1 vs. 91.7% Group 2, P = 0.004), while the proportion of eyes with ≥6/18 corrected distance visual acuity was similar between the two groups (94.4% pre vs. 98.3% post; P = 0.28).
Figure 1: Box plots of the postoperative refractive spherical equivalent before and after biometry teaching. The box represents the central interquartile range, the line within the box the median, and the upper and lower lines (whiskers) represent data within 1.5 times the interquartile range with outlying points displayed

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The mean absolute prediction error of the IOL calculations was reduced by 50% following the biometry teaching (1.13 ± 0.84 D pre vs. 0.56 ± 0.44 D post; P < 0.001). Following the teaching, a higher percentage of eyes had a postoperative spherical equivalent within ± 0.5 D (26.7% pre vs. 52.5% post; P < 0.001) and ± 1.0 D (55.0% pre vs. 90.0% post; P < 0.001) of the predicted postoperative spherical equivalent [Figure 2].
Figure 2: Percentage of eyes with a postoperative refractive spherical equivalent within specified ranges of the predicted postoperative spherical equivalent

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Data for Group 2 were collected following surgeries performed 1–3 months after the teaching (Group 2A, n = 80 eyes) and 9–10 months after the teaching (Group 2B, n = 40 eyes). There was no significant difference in the mean absolute prediction error between Group 2A (0.52 ± 0.42 D) and Group 2B (0.64 ± 0.47 D; P = 0.16).

During the practical, hands-on, teaching, the most frequent errors in measurement acquisition/technique observed during A-scan acquisition were: excessive corneal compression, off-axis measurements, gain set too high, and incorrect automatic gate position relative to anatomical A-scan spikes. Techniques were refined during the hands-on session by demonstrating a two-handed technique of resting the hand that holds the probe on the patient's cheek while using the second hand/finger to steady the probe. During applanation, it was emphasized to gently touch the cornea and maintain the probe position at the center of the cornea directed at the macula. To maintain the beam on the macula, patients were instructed to look directly at the red light on the probe with the eye being measured (if vision adequate to visualize the light) or to focus with the contralateral eye on a point straight ahead.

The most common errors in A-scan interpretation were due to lack of scrutiny of measurements and acceptance of measurements with suboptimal quality or high standard deviations. Guidelines regarding when to repeat measurements and settings/features of the A-scan machine were reviewed.

The most frequent errors in keratometry were measurement of axial length before keratometry and acceptance of variable measurements. It was recommended that keratometry always be performed before axial length measurement to avoid disruption of the ocular surface from the A-scan probe and guidelines regarding when to repeat measurements were reviewed.

   Discussion Top

Accurate biometry and selection of the optimal IOL power are necessary to achieve the best possible refractive outcomes following cataract surgery. This study demonstrated that a focused didactic and practical training course significantly improved the accuracy of IOL calculations to achieve the targeted postoperative refractive spherical equivalent. Following the teaching, there was a 50% reduction in the prediction error of the IOL calculations.

A postoperative refractive outcome of >85% of the eyes within 1.0 D of the targeted refraction following phacoemulsification has been suggested as a benchmark standard in a large study performed in the UK.[13] While there are no such benchmarks following SICS, this standard was met following the biometry course with 90% of the eyes falling within 1.0 D of the targeted spherical equivalent. The World Health Organization has promoted a standard of “good” uncorrected visual acuity of ≥6/18 in 80% of postoperative patients.[14] Following the biometry course, there was improvement in the percentage of the eyes at this level of visual acuity with the postteaching cohort (91.7%) reaching this standard.

Before the biometry course, the refractive outcomes were typically more myopic than targeted and were more variable. This was likely due to the excessive and variable corneal indentation during A-scan applanation, resulting in a falsely short axial length and higher IOL power selection. This error was frequently noted during the observation phase of the biometry teaching. To reduce this error, instruction was given regarding proper technique during applanation along with critical interpretation of the biometric data and repetition or exclusion of measurements as needed. Errors in axial length measurement have the largest effect on the prediction error, as a 1.0-mm error in measurement can result in a final error of nearly 3 D.[15]

One previous study examined the effect of a teaching program on the refractive outcomes of cataract surgery performed by trainees in the United States.[11] In that retrospective study, immersion A-scan ultrasound was used to obtain axial length measurements before phacoemulsification. The authors reported that the percentage of the eyes with a refractive spherical equivalent within ±1.0 D improved from 75.3% to 94.2% following the institution of a teaching program, although the two study cohorts were separated in time by two years. In our study, postoperative data for the postteaching cohort began to be collected within 2 months following teaching, so it may be less probable that the improvement in outcomes was due to other factors.

While others have reported lower prediction error than achieved in our study,[16],[17],[18],[19] it is notable that those studies used immersion A-scan or partial coherence interferometry, which have a higher accuracy and repeatability than applanation A-scan ultrasound.[20],[21],[22] This is primarily due to the variable corneal indentation that inevitably occurs during applanation. Those studies also used phacoemulsification which may improve the refractive outcomes due to a more predictable capsulorrhexis and subsequent effective IOL position. SICS employs a large diameter capsulorrhexis that may be continuous or noncontinuous, as in the case of a can-opener technique. The large capsulorrhexis may limit the predictability of the effective lens position of an in-the-bag IOL. In addition, when placing a one-piece PMMA with a large or poorly demarcated capsulorrhexis edge, it is possible for one or both haptics to be inadvertently placed in the sulcus. These factors likely limited the ability to achieve the predicted refractive target in this study, even after the teaching module.

It is unknown how long the effect of the single teaching session will result in improved refractive outcomes. However, when Group 2 was further analyzed according to the two subgroups, the effect appeared to persist during the study period or up to 10 months following teaching. Since the surgeons were not aware that additional data would be collected after 6 months on another subset (Group 2B), it is unlikely that the persistence of improved refractive outcomes was due to the Hawthorne/observer effect that occurs when a subject's (surgeon's) behavior is modified due to awareness of the study.

We cannot be sure that the improvement in refractive outcomes can be entirely attributable to the teaching intervention as there are other limitations of this study that may have affected the results. PCO is commonly observed after SICS and may have limited the accuracy of refraction in each group. In addition, the eyes in each group were not excluded on the basis of any preexisting ocular comorbidities or postoperative complications such as cystoid macular edema, which could have limited the accuracy of the refractive data. In addition, there is known variability and inaccuracy in achieving a final manifest refraction, with a reported standard deviation of ±0.4 D.[23]

   Conclusions Top

In summary, a short biometry instruction course may significantly improve the refractive outcomes of patients following SICS. This training is likely to be most beneficial when given in locations where such training is deficient, and patients do not have ready access to spectacles postoperatively. It is recommended that ophthalmology teaching programs include biometry teaching in their curricula for training. The International Council of Ophthalmology residency curriculum includes the following regarding biometry teaching objectives: “Describe basic diagnostic tools used in refractive surgery, including topography, pachymetry, and biometry; and interpret result.”[24] We also recommend practical hands-on teaching sessions, particularly in areas of the world where surgeons are involved in data acquisition by performing their own measurements. A biometry instruction course may also be beneficial for nurses or technicians who perform measurements although we did not specifically study this group.

Financial support and sponsorship

JJM and RB were supported, in part, by a research stipend from the Fred Hollows Foundation, New Zealand

Conflicts of interest

There are no conflicts of interest.

   References Top

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  [Figure 1], [Figure 2]

  [Table 1]


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