Comparison of Pentacam and Orbscan IIz on Posterior Curvature Topography Measurements in Keratoconus Eyes

doi:10.1016/j.ophtha.2006.03.046

Ophthalmology

Volume 113, Issue 9, September 2006, Pages 1629-1632

Presented as a poster at: American Academy of Ophthalmology meeting, October 2005, Chicago, Illinois.

https://doi.org/10.1016/j.ophtha.2006.03.046 Get rights and content

Purpose

To compare the measurements of posterior elevation above the best-fit sphere and of pachymetry in established keratoconus patients using 2 different technologies—Scheimpflug imaging with the Pentacam and scanning slit combined with Placido imaging with Orbscan IIz.

Design

Retrospective comparative case series.

Participants

Patients with confirmed keratoconus by biomicroscopy and Placido topography who had corneal topography scans by the Pentacam and Orbscan IIz on their visit from April 2004 to April 2005. These scans were obtained routinely for patient diagnosis, treatment, and progression.

Methods

The mean difference, standard deviation (SD), and 95% limits of agreement (LOA) were calculated, and Bland–Altman plots were constructed for best-fit sphere radius of curvature, posterior elevation above the best-fit sphere, and pachymetry of the thinnest point.

Results

Thirty-six eyes of 29 patients were analyzed. The average radius for posterior best-fit sphere was 5.97 μm (range, 4.69–6.79) for the Pentacam and 6.00 μm (range, 4.97–6.55) for Orbscan IIz. The mean difference (Pentacam − Orbscan IIz) for the radius generated for the best-fit sphere was −0.03±0.22 SD with a 95% confidence interval (CI) of −0.11 to 0.04 and a 95% LOA of −0.46 to 0.40 (P = 0.362). The mean posterior elevation by best-fit sphere fixed to the apex was 34.86 μm (range, 3–120) for the Pentacam and 48.50 μm (range, 11–118) for Orbscan IIz. The mean difference was −13.64±26.08 SD (95% CI, −22.46 to −4.81; 95% LOA, −64.75 to 37.48; P = 0.003). The average thinnest point for the Pentacam was 443.6 μm (range, 164–587), and that for Orbscan IIz was 445.9 μm (range, 204–597). The mean difference in the thinnest point for the Pentacam and Orbscan IIz was −2.28±35.55 (95% CI, −14.31 to 9.75; 95% LOA, −71.95 to 67.39; P = 0.703).

Conclusion

Both the Pentacam and Orbscan IIz determine similar thinnest points but have a measurable difference in posterior elevations above the best-fit sphere, despite similar radii of curvature. This difference may be important in the screening of patients for refractive surgery to avoid surgery on patients with early keratoconus. This study cannot determine if the Pentacam is underestimating the posterior vault or if Orbscan is overestimating this height, and further studies with a standardized test object are needed.

Section snippets

Materials and Methods

The Pentacam and Orbscan IIz databases were searched retrospectively from April 2004 to June 2005 for all patients with the diagnosis of keratoconus confirmed by either of the two cornea specialists at the University of Iowa Hospital and Clinics, Iowa City, Iowa. Twenty-nine patients with the diagnosis of keratoconus who had corneal topography from both systems were identified. Only patients who had both scans ordered and performed for diagnosis, treatment, and progression were included. Data

Results

The patient population consisted of 13 females and 16 males with the diagnosis of keratoconus. The average age was 46 years (range, 23–78). Thirty-five of the cones were located in the inferior temporal quadrant and 1 in the inferior nasal quadrant.

The average of the radius generated for the best-fit sphere for the Pentacam was 5.97 μm (range, 4.69–6.79), and that for Orbscan IIz was 6.00 μm (range, 4.97–6.55). Figure 1 shows the Bland–Altman plot for the mean difference of the radius of the

Discussion

Corneal topography often aids in the diagnosis of keratoconus. The characteristic stromal thinning in keratoconus corresponds to an increased posterior elevation above the best-fit sphere on corneal topography. The Pentacam Comprehensive Eye Scanner is based on Scheimpflug technology, and the Orbscan is based on a scanning-slit beam with a Placido disk. Because Orbscan derives the posterior elevation map mathematically, there had been caution raised regarding the posterior curve in the past

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Manuscript no. 2005-1173.

Supported by the University of Iowa Department of Ophthalmology Resident and Fellow Research Fund.

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Original ArticleComparison of Pentacam and Orbscan IIz on Posterior Curvature Topography Measurements in Keratoconus Eyes

Purpose

Design

Participants

Methods

Results

Conclusion

Section snippets

Materials and Methods

Results

Discussion

J Cataract Refract Surg

J Cataract Refract Surg

J Cataract Refract Surg

Surv Ophthalmol

Measuring agreement in method comparison studies

Stat Methods Med Res

Original Article
Comparison of Pentacam and Orbscan IIz on Posterior Curvature Topography Measurements in Keratoconus Eyes