Traditional ultrasonic pachymeters work by providing the user with a number representing the thickness of the human cornea. Until recently, physicians had no way to confirm the accuracy of that number. As a result, physicians found pachymeters to have poor reproducibility. All that has changed with the
PalmScan P2000E High-Definition portable pachymeter.
This study, the first of its kind, evaluates the technique of direct corneal flap measurements when using femtosecond lasers.
The PalmScan P2000E Pachymeter has set a new standard in pachymetry. It is the first and only Pachymeter designed to capture the actual Corneal Waveform TM. The Corneal Waveform is the ultra-high precision echogram of the corneal stroma using 264 MHz sampling speed to digitally reconstruct the cornea. By having the echogram of the cornea, the user is assured of the validity of their capture. Additionally, corneal waveform has the benefit of providing details about the corneal anatomy.
Figure 1: Normal human corneal waveform measuring 575 micron
A valid corneal waveform has anterior and posterior corneal spikes that are perpendicular, without any stromal echoes. See figure 1. Sub-optimal corneal waveforms may have additional spikes due to bad coupling or wide anterior or posterior spikes due to misalignment. The corneal thickness is calculated internally based on the corneal waveforms, however, the physician can manually recalculate it by placing the cursors on the peaks of the anterior and posterior corneal spikes.
Prior to the creation of the PalmScan, the flap thickness was measured during LASIK via the subtraction method. This entailed performing a pre-flap pachymetry, a post flap corneal bed measurement, and then subtracting the pre-flap measurement from the corneal bed. The authors found a fundamental shortcoming in subtraction pachymetry to calculate Femtosecond flap thickness. First, since the cornea is a meniscus, one must touch the exact location on the cornea before and after the flap is created; second the corneal bed measurement is subjective to change with the moisture state of the cornea, the authors found that the dry corneal bed measures differently then normal cornea; and finally there is no objective way for the surgeon to validate the corneal thickness measured by a conventional Pachymeter.
In contrast, by measuring the planar femtosecond flap directly, lateral displacement of the probe is no longer a factor. The measurement of the planar flap in any location should be the same.
Figure 2: This figure shows the cornea waveform after the femtosecond laser flap cut, this is used to directly measure the flap thickness (FT) of 99 micron and bed thickness (BT) of 475 and total corneal thickness (CT) of 574 micron.
This figure shows the cornea waveform after the femtosecond laser flap cut, this is used to directly measure the flap thickness (FT) of 99 micron and bed thickness (BT) of 475 and total corneal thickness (CT) of 574 micron.
Results:
The authors studied flaps of 17 eyes which were made using the Intralase laser (calibrated by the company) set at 100 micron depth. The author used the PalmScan P2000E pachymeter with FS software and a 50 MHz transducer to simultaneously measure the flap and bed thickness after creation of the flap. A Sonoguage pachymeter was used to measure central corneal thickness prior to making an incision and after lifting the flap and subtracting the corneal thickness from the bed thickness to measure the flap thickness to obtain the subtraction thickness for comparison.
As can be seen by the results, the average flap thickness measured using PalmScan is within 3.06 micron of the expected value, while the results obtained from the subtraction method were over 12 microns off. The standard deviation for the direct flap measurement was 4.9 microns while the subtraction was over twice that at 10.7 microns.
Conclusion:
Until now, surgeons had no way to calibrate their femtosecond lasers independently and often found that the subtraction method did not correlate well with the laser depth setting. Instead, they had to rely on the factory representative to make sure that their laser was properly calibrated. With corneal waveform technology, the user can be assured that the femtosecond laser is properly calibrated at all times. In addition more accurate corneal flap thickness is measured to ensure that adequate amount of corneal bed remains after laser ablation.
This study demonstrates corneal waveform pachymetry’s superior accuracy in measuring corneal thickness and corneal structures as compared to conventional pachymeters which lacks corneal waveform technology.
Optical coherence tomography is usually used after the LASIK is done and patient is back for a postop evaluation. OCT systems are not suitable for intra-operative flap thickness determinations because the femtosecond laser makes a temporary corneal opacity. If you really wanted to use the OCT during the surgery, you would have to cut the flap, wait a while for corneal opacity to resolve, get the patient out of OR bed and have him sit in front of the OCT Then after is OCT is completed, the patient would have to be repositioned for the ablation
The PalmScan can take a measurement right after the flap is cut. Corneal opacity has no effect on the measurement made by PalmScan. The PalmScan is portable and can be brought to any OR. Otherwise you need to get one OCT for each OR if you want to go that route. In addition, a typical OCT system can cost 10 to 20 times more than a PalmScan. Of course, every surgeon wants to make sure that their IntraLase is in calibration and is cutting the flap to the correct depth. This is done very easily with PalmScan during the surgery when needed most. OCTs will give good results after the cornea has healed and the patient is seen on follow-up.
Richard Eisner, MD is a board certified ophthalmologist practicing refractive surgery at 125 Plantation Center Dr., Suite 250, Macon, GA 31210.
Rafi Israel MD is a board certified ophthalmologist and the director of Beverly Hills Eye Institute located at 9025 Wilshire Blvd #209 Beverly Hills, CA 90211, phone 310-276-3450, Fax 310-276-3548. He is an associated clinical professor of ophthalmology at Cedar Sinai Medical Center in Los Angeles, CA. and co-inventor of PalmScan.