instrumental strategies

Measure Corneal Biomechanics

Device assesses refractive surgery candidates and monitors glaucoma.

SCOTT F. LEE, O.D. AND ELLA FAKTOROVICH, M.D.

Research has revealed that central corneal thickness (CCT) isn't the only corneal factor that influences intraocular pressure (IOP) measurement accuracy and refractive surgery patient selection. The biomechanical properties of the cornea appear to play an even greater role than thickness alone. Measurement parameters, such as corneal hysteresis (CH) and corneal resistance factor (CRF), are indications of the cornea's biomechanical properties and have been shown to be clinically useful in refractive surgery screening, glaucoma management and IOP assessment.^1-5 One diagnostic device enables you to assess IOP, CH and CRF simultaneously: The Ocular Response Analyzer (ORA), from Reichert Ophthalmic Instruments.

Like a non-contact tonometer, the ORA applies force to the cornea via a brief air puff, causing it to become slightly concave prior to returning to its normal curvature. The instrument's infrared detection system records this movement, resulting in a signal that depicts both the air pulse pressure the puff exerts (green curve) and corneal movement (red curve). The red curve's first peak coincides with the air pressure required to applanate the cornea on its way to becoming concave (P1). The red curve's second peak represents the air pulse pressure on the cornea as it returns to its normal convex shape. (P2). If the cornea moves abnormally or its physical nature is atypical, the red peaks may be significantly low, wide, or the signal may be otherwise irregular. These findings may indicate the presence of corneal conditions, such as keratoconus or Fuchs' Dystrophy, for example.

Ocular Response Analyzer HEIGHT: 17.0 inches WIDTH: 9.5 inches LENGTH: 13.5 inches WEIGHT: 30 lbs. COST: $28,500, including installation and training.

Here, we explain the benefits of the device for glaucoma patients and those interested in refractive surgery.

ORA patient benefits

The ORA features the following helpful measurements:

► Intraocular pressure-corneal compensated (IOPcc). Because the device provides measurements of the corneal biomechanical properties, you can obtain an IOP measurement independent of these properties. This has enabled us to obtain post-refractive surgery IOP measurements that aren't artificially lower or higher than presurgery measurements, design the most appropriate glaucoma treatment plans and effectively monitor the results of the chosen treatment.

► CH. This is the difference between P1 and P2 values. It's a measurement of corneal viscous damping properties (i.e. energy absorption capabilities). CH values in normal subjects typically range from about eight to 14.¹ A low CH value indicates a cornea less able to absorb (damp) the air pulse' energy, indicating a possible corneal disorder. For instance, a keratoconus patient may have a CH value less than eight.¹ Low CH values may also be present in patients who have glaucoma and appear to be an indication of progression in the disease that is independent of other risk factors.⁶

► CRF. This is a measurement of the cumulative effects of the cornea's viscous and elastic resistance when the air puff applies pressure to the cornea. Normal values for CRF are similar to those for CH.¹ In weak corneas, such as those with keratoconus, CRF values are lower than CH values.¹ In glaucoma patients, CRF values are close to normal, though, as mentioned above, CH values are low. The bottom line: A direct correlation between CCT and CRF is not present. In other words, a thick cornea doesn't necessarily mean the CRF will be high, and vice versa.

► Goldmann correlated IOP (IOPg). This provides a traditional IOP measurement. Comparing this data with the IOPcc data is helpful because the difference between the two measurements gives you significant insight into how the biomechanical properties are affecting the traditional measurement.

If you want to strengthen your ability to make the best decisions concerning glaucoma care and refractive surgery candidate selection, I suggest you consider the ORA. Its corneal biomechanical property readings in addition to the traditional reading have fine-tuned our diagnostic and treatment approaches and increased patient satisfaction. OM

1. Touboul D, Roberts C, Kerautret J, et al. Correlations between corneal hysteresis, intraocular pressure, and corneal central pachymetry. J Cataract Refract Surg. 2008 Apr;34(4):616-622.

2. Kirwan C, O'Keefe M, Lanigan B. Corneal hysteresis and intraocular pressure measurement in children using the Reichert Ocular Response Analyzer. Am J Ophthalmol 2006 Dec;142(6):990-2.

3. Pepose JS, Feigenbaum SK, Qazi MA, et al. Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic and noncontact tonometry. Am J Ophthalmol. 2007 Jan;143(1):39-47.

4. Luce DA. Determining in vivo biomechanical properties of the cornea with an Ocular Response Analyzer. J Cataract Refract Surg. 2005 Jan;31(1):156-62.

5. Luce D. Methodology for cornea compensated IOP and corneal resistance factor for the Reichert Ocular Response Analyzer. Paper presented at: the ARVO 2006 Annual Meeting; April 30-May 4; Fort Lauderdale, Fla.

6. Congdon NG, Broman AT, Bandeen-Roche K, et al. Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol. 2006 May; 141(5):868–75.