assessment of visumax femtosecond laser accuracy and precision of flap thickness and centration dan...
TRANSCRIPT
Assessment of VisuMax Femtosecond
Laser Accuracy and Precision of Flap
Thickness and Centration
Dan Z Reinstein MD MA(Cantab) FRCSC1,2,3,4
Timothy J Archer, MA(Oxon) DipCompSci(Cantab) 1
Marine Gobbe, MST(Optom) PhD 1
1. London Vision Clinic, London, UK2. St. Thomas’ Hospital - Kings College, London, UK3. Weill Medical College of Cornell University, New York4. Centre Hospitalier National d’Ophtalmologie, (Pr. Laroche), Paris, France
©DZ Reinstein [email protected]
Financial Disclosure
The author acknowledges a financial interest in the Artemis™
Many aspects of the Artemis™ technology are patented. Patents are administered by the Cornell Research Foundation. This intellectual property has been licensed by Cornell to ArcScan Inc, a company in which the author has a financial interest.
The author is a consultant for Carl Zeiss Meditec (Jena, Germany)
©DZ Reinstein [email protected]
PURPOSETo measure: – Accuracy
– Reproducibility– Flap Centration (corneal vertex centration intended)
of VisuMax central flap thickness
Accuracy refers to the closeness of the measurement to the actual value
Precision refers to the distribution of values obtained when making multiple measurements of the same object under specified conditions
Reproducibility refers to the distribution of measurements made of multiple objects intended to have a single measurement value
In determining the reproducibility of flap thicknesses produced by a femtosecond laser, the precision of the measuring tool MUST be 2 SD smaller than the reproducibility of flap thickness provided by the femtosecond laser.
For example, let’s assume that Instrument 1 has a precision (i.e SD) of 1 µm and Instrument 2 has a precision of 10 µm. Also assume that Flap 1 was 105 µm thick and Flap 2 was 115 µm thick.
95 104 105 106 114 115 116 125
Instrument 1
Instrument 2
Instrument 1 would measure Flap 1 in the range 104-106 µm and so can distinguish between flaps which differ in thickness by 3 µm. The 1 µm precision adds an error of ±1 µm to the process of measuring flap thickness precision.
Instrument 2 does not have sufficient precision to measure a population with a reproducibility <10 µm
±1 µm
±10 µm
±10 µm
±1 µm
Instrument 2 would measure Flap 1 in the range 95-115 µm and Flap 2 in the range 105-125 µm. The overlap of these ranges demonstrates that Instrument 2 may not be able to distinguish between the two flaps.
Carl Zeiss MeditecJena, Germany
©DZ Reinstein [email protected]
METHODS: VisuMax Femtosecond Laser
• Tissue disruption is reduced to submicron volume by tighter focusing and lower energy. The lower energy reduces the risk of an inflammatory response compared to other femtosecond lasers.
• Suction is applied to the cornea rather than the sclera, which means that the suction required is low and the increase in IOP is low. This also means that there is no risk of corneal shift during suction.
• The contact glass has a curved surface, so that the eye does not need to be applanated to a flat surface. The contact glass is available in 3 sizes according to the limbus diameter.
• This results in no vision loss during suction and the patient is able to fixate which helps to achieve optimal flap centration.
• There is minimal to no opaque bubble layer (OBL) remaining after bilateral sequential flap creation; eye tracking can be effected without delay to begin excimer laser ablation immediately after flap creation.
• The manifest refraction is entered to focus the fixation target for each eye individually.
• The keratometry is also entered to calculate the laser focus depth in the peripheral cornea.
Cross-section of the contact glass
©DZ Reinstein [email protected]
Artemis very high-frequency digital ultrasound arc-scanner
• The Artemis uses a 50 MHz VHF ultrasound transducer• Immersion scanning means the tear-film is not incorporated into
measurements and there is no contact of the transducer with the eye• Arc-scan mechanism enables maximum perpendicularity of the
transducer to the corneal surface to minimize refractive errors to the ultrasonic signal in the peripheral cornea
• Digital signal processing used to significantly reduce noise and enhance signal-to-noise ratio – has been shown to double resolution and increase measurement precision by a factor of 3 compared with analog processing1
• Patient fixation beam is coaxial with the infra-red camera, the corneal vertex and the centre of rotation of the scanning system so that each scan plane can be centered on the corneal vertex
• Meridional B-scans of the cornea enable localisation of the epithelium, Bowman’s, the flap interface and the back surface
• Thickness measurements made by computer-analysis of peaks on the I-scan trace – each peak provides a surface localization of 0.87 µm2
• Axial resolution of 21 µm enables measurement of layers thicker than 21 µm, ie sufficient to distinguish epithelium, flap, stroma and cornea
• 3D layered pachymetry calculated by interpolation between multiple meridional scans with a precision < 1.0 µm2,3
1. Reinstein DZ, Silverman RH, Rondeau MJ, Coleman DJ. Epithelial and corneal thickness measurements by high-frequency ultrasound digital signal processing. Ophthalmology 1994;101(1):140-6.2. Reinstein DZ, Silverman RH, Raevsky T, et al. Arc-scanning very high-frequency digital ultrasound for 3D pachymetric mapping of the corneal epithelium and stroma in laser in situ keratomileusis. J Refract Surg 2000:414-30.3. Reinstein DZ, Silverman RH, Trokel SL and Coleman DJ. Corneal pachymetric topography. Ophthalmology 1994:432-8.
Surface localization:0.87 µm
ArcScan Inc Evergreen, Colorado
©DZ Reinstein [email protected]
METHODS: Population
24 eyes of 12 patientsAge– median 30 years– mean 31.6 ± 7.5 years – range 24 to 52 years
BSCVA– 100% 20/20– 63% 20/16
Spherical equivalent– mean -3.60 ± 1.61 D– range -1.00 to -6.38 D
Cylinder– mean -0.80 ± 0.55 D– range 0.00 to -2.00 D
Artemis B-Scan (above) of VisuMax Flap 6 months post LASIK. Edge detection by I-scan digital signal processing
(red outline, below) based on raw scan data
VisuMax Flap Settings– Intended thickness 110 µm– Flap diameter 8.5 mm– Hinge 5.0 mm– Sidecut 110
©DZ Reinstein [email protected]
METHODS: Flap Thickness Measurement
Flap Thickness Measurement
i) Epithelial changes are known to occur after LASIK,2 therefore postop flap thickness measurements are not valid. Addition of the preoperative epithelium to the stromal component of the flap provides a closer representation of the original flap at the time of creation.
ii) Artemis I VHF digital ultrasound scans are performed before and 3 months after treatment to ensure no remaining edema in the stromal component of the flap.
+Pre-op
Post-op3 months
Epithelial thickness
Stromal component of the flap
Original flap thickness
1. Reinstein DZ, Sutton HF, Srivannaboon S, Silverman RH, Archer TJ, Coleman DJ. Evaluating microkeratome efficacy by 3D corneal lamellar flap thickness accuracy and reproducibility using Artemis VHF digital ultrasound arc-scanning. J Refract Surg. 2006;22:431-440.
2. Reinstein DZ, Srivannaboon S, Silverman RH, Coleman DJ. The accuracy of routine LASIK; isolation of biomechanical and epithelial factors. Invest Ophthalmol Vis Sci. 2000;41(Suppl):S318.
©DZ Reinstein [email protected]
Comparison of Methods of Flap Thickness Measurement
• Artemis Reinstein Flap Thickness– Method: 1 µm precision localization of flap interface,
flap thickness calculated as stromal component of the flap plus preoperative epithelium
– Sources of error:• Instrument flap thickness measurement precision of 1.4 µm1
• Potential post-operative stromal thickness changes
• Intraoperative Handheld Ultrasound– Method: subtract intraoperative residual stromal bed thickness measurement from corneal
thickness measurement– Sources of error:
• Instrument corneal thickness measurement precision of about 6 µm2
• Instrument residual stromal bed thickness measurement precision (not published, but likely >6 µm)• Stromal hydration during surgery• Misalignment of probe location for corneal and residual bed measurements
• Optical Coherence Tomography– Method 1: direct measurement of flap thickness by automated computer algorithm– Method 2: direct measurement of flap thickness by manual placement of measuring tool on
OCT B-scan image– Sources of error:
• (Method 1) Instrument central flap thickness measurement precision of 6.5 µm3 • (Method 2) Instrument flap thickness measurement precision of 6.5 µm added to manual measurement
precision : flap tool only allows flap measurement to the nearest ±6 µm (12 µm increments)• (Method 2) Intra-observer error of flap interface location• Postoperative epithelial changes not accounted for, so flap thickness will be overestimated• Potential post-operative stromal thickness changes
1. Reinstein DZ, Silverman RH, Raevsky T, et al. Arc-scanning very high-frequency digital ultrasound for 3D pachymetric mapping of the corneal epithelium and stroma in laser in situ keratomileusis. J Refract Surg 2000:414-30.2. Yaylali V, Kaufman SC and Thompson HW. Corneal thickness measurements with the Orbscan Topography System and ultrasonic pachymetry. J Cataract Refract Surg 1997:1345-50.3. Li Y, Netto MV, Shekhar R, et al. A longitudinal study of LASIK flap and stromal thickness with High Speed Optical Coherence Tomography. Ophthalmology 2007;114(6):1124-32
Most accurate method of determining original flap thickness produced by flap creating device
©DZ Reinstein [email protected]
Distance Corneal Vertex – Flap Centre:
Horizontal Offset = 0.5 x (VN – VT)
Vertical Offset = VI – (0.5 x FD
METHODS: Flap Centration Measurement
VN
I
T
FD: Flap Diameter
VN T
I
F D
Total offset = Centration of the Flap relative to the Corneal Vertex (CV)
= (Horizontal Offset 2 + Vertical Offset 2)
C
Flaps were intended to be centered on the corneal vertex. The patient aligns their eye naturally to the corneal vertex by focusing on an internal fixation target.
©DZ Reinstein [email protected]
RESULTS: Central Flap Thickness
Intended flap thickness = 110.00 µm
Average flap thickness = 112.31 µm
Accuracy = +2.31 µm
Reproducibility (SD) = 7.89 µm
Minimum flap thickness = 102.61 µm
Maximum flap thickness = 132.94 µm
Range = 30.34 µm
Range from Intended 110 µm Flap Thickness
Percentage Eyes
Within 2 µm 25%
Within 5 µm 54%
Within 10 µm 88%
©DZ Reinstein [email protected]
RESULTS: Flap Centration
Average = 0.32 mm
Standard Deviation = 0.17 mm
Minimum = 0.13 mm
Maximum = 0.83 mm
Range from Intended Flap Centration
Percentage Eyes
Within 0.25 mm 50%
Within 0.50 mm 87%
Within 0.75 mm 96%
Distance Corneal Vertex - Flap Centre
DISCUSSIONThe VisuMax femtosecond laser system was found to produce very accurate and highly reproducible flaps, well centered to the corneal vertex
Accuracy = +2.31 µm
Reproducibility = 7.89 µm
Flap centration = 0.32 mm
The Artemis VHF digital ultrasound arc-scanner has a flap thickness measurement precision of 1.4 µm and therefore provided sufficient precision to determine flap thickness reproducibility as high as 2.8 µm (= 2 x 1.4).
Paradoxically, pachymetry by devices with lower measurement precision may give falsely high flap thickness reproducibility results. If the measurement precision of the instrument being used is unable to discriminate between flaps of similar thickness, the random error associated with each flap thickness measurement could incorrectly cluster the measured values resulting in a falsely high flap thickness reproducibility.