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IOL Power Determination in Eyes With Silicone Oil and Choosing The Right IOL

Problems with Choosing The Right IOL. Accurate A-scan biometry is very difficultSilicone oil has a slower sound velocity than vitreousSilicone oil produces strong sound attenuationLeaving silicone oil in or not after cataract removal will determine the final IOL power Type of IOL implanted can b

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IOL Power Determination in Eyes With Silicone Oil and Choosing The Right IOL

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    1. IOL Power Determination in Eyes With Silicone Oil and Choosing The Right IOL Chi-Wah (Rudy) Yung, M.D.

    2. Problems with Choosing The Right IOL Accurate A-scan biometry is very difficult Silicone oil has a slower sound velocity than vitreous Silicone oil produces strong sound attenuation Leaving silicone oil in or not after cataract removal will determine the final IOL power Type of IOL implanted can be critical if silicone oil is to remain

    3. Important Facts to Consider Vitreous sound velocity – 1532 m/s Average sound velocity used for routine axial length measurement – 1550 m/s Two densities of silicone oil: 1000 centistokes (cSt) – sound velocity 980 m/s 5000 cSt – sound velocity 1040 m/s

    4. If one uses the typical default sound velocity setting in an A-scanner (1550 m/s) to measure axial length of an eye containing silicone oil, the measurement will be erroneously long !!!

    5. With silicone oil inside the eye, it is best to measure with the patient seated as upright as possible especially if the vitreous cavity is only partly filled with silicone oil. Due to the lighter density of silicone oil, if patient is recumbent, silicone oil can shift anterior away from the retina leading to confusion as to the true position of the retinal spike

    6. Solutions to Overcome Measurement Problem Measure the other eye – but not always reliable Baseline axial length measurement taken prior to silicone oil implantation – often not done Use optical coherence biometry (OCB) such as IOL Master to measure axial length– can be a problem with dense cataract and most cataracts associated with silicone oil are very dense Measuring each component of the eye individually with the right corresponding velocity – can be done only if the biometer has different measuring gates and allows the sound velocity to be modified as needed

    7. Velocity Conversion Method The correct measurement of each component of the eye can be calculated by dividing the correct sound velocity (Vc) by the incorrect or apparent velocity (Vm) used for the measurement multiplied by the measurement obtained: Correct measurement = (Vc / Vm) X measurement

    8. Velocity Conversion Method step-by-step Step 1: Use aphakic sound velocity (1532 m/s) to measure the eye Step 2: Obtain the measured AC depth, lens thickness and vitreous length (AC depth is the only corrected measurement at this step) Step 3: Using the conversion method, calculated the real lens thickness and vitreous length

    9. Velocity Conversion Method Step-by-Step Step 4: Determine the true axial length by adding together the respective measurements of ACD, lens thickness and vitreous length Step 5: Use the calculated (true) axial length for IOL power calculation

    10. Example 45-year old male history of ARN left eye and s/p PPV and silicone oil implantation one year ago. Left eye vision now counting finger, while right eye 20/20 with refraction. No prior axial length measurements were obtained Axial length of both eyes were measured during pre-op

    11. Axial Length Measurements Left eye using aphakic velocity of 1532 m/s yielded an apparent ALm of 30.71 mm Right eye using average velocity of 1550 m/s yielded AL of 22.27 mm

    12. Left Eye (silicone oil) Correct ACD = 2.66 mm (velocity used 1532 m/s) Measured lens thickness (LTm) = 4.84 mm Measured vitreous length (VITm) 30.71 – 2.66 – 4.84 = 23.21 mm

    13. Example – cont. Calculated lens thickness: LTc = (Vc / Vm) x LTm LTc = (1641/1532) X 4.84 mm LTc = 5.18 mm

    14. Example – cont. Calculated vitreous length (VITc) VITc = (Vc / Vm) X VITm VITc = (980 / 1532) X 23.21 mm VITc = 14.85 mm

    15. Example – Cont. Calculated (true) axial length: ALc = ACm + LTc + VITc ALc = 2.66 + 5.18 + 14.85 ALc = 22.69 mm

    16. Final IOL Power Determination Final IOL power will depend on if silicone oil is to remain or to be removed If silicone oil is to be removed, standard IOL power calculation can be performed If silicone oil is to remain, a power adjustment must be made to prevent significant post-op hyperopia

    17. IOL Choice If Silicone Oil Is To Remain Since silicone oil has a higher index of refraction than vitreous, a higher power IOL is required to achieve the same refractive result The best implant would be a polymethylmethacrylate (PMMA) convex-plano lens with the plano side toward the vitreous cavity Intact posterior capsule is highly desirable

    18. Power must be added to the original IOL calculation for a PMMA convex-plano IOL is as follow: Power to be added = Ns - Nv X 1000 ALc – ACD Ns = Refractive index of silicone oil Nv = Refractive index of vitreous (1.4034) (1.336) ALc = True (calcuated) axial length ACD = Measured AC depth

    19. For an average-length eye with silicone oil in vitreous cavity, the typical additional power needed for a convex-plano PMMA IOL is between +3.0 D to +3.5 D For other bi-convex non-PMMA IOL, the estimated additional power could be as high as +6.0 D Avoid silicone IOLs – silicone oil can condense on the IOL surface when come in contact

    20. Other Important Numbers to Remember Sound velocity for PMMA – 2718 M/sec For Silicone implant – 980 M/sec For Acrylic implant – 2120 M/sec

    21. Axial Length Measurements in Pseudophakia PMMA AL1532 + 0.4 Silicone AL1532 - 0.6 Acrylic AL1532 + 0.2

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