Silicone oil inside an eye.
Silicone oil is sometimes temporarily placed in the vitreous cavity for recurrent retinal detachments in eyes with proliferative vitreoretinopathy, proliferative diabetic retinopathy, cytomegalovirus retinitis, giant retinal tears, and following perforating injuries. Axial length measurements by ultrasound of an eye in which the vitreous cavity has been filled with silicone oil is an exercise with many potential pitfalls, especially if the silicone oil has become emulsified.
There are presently two viscosities of silicone oil in use:
- 1,000 mPa.s. Silicone oil (Silikon, Alcon Laboratories, Ft. Worth, Texas) slows sound waves to approximately half the speed (980 m/sec) of normal vitreous. It can attenuate the returning sound wave during ultrasonography to such an extent that obtaining good echoes becomes difficult, if not impossible.
- 5,000 mPa.s. Silicone oil (ADATO SIL-ol 5000, Bausch & Lomb Surgical, San Dimas, California) has a somewhat higher density and slows sound waves to approximately 1,040 m/sec. Typically, when ultrasound measurements are made through silicone oil, hugely erroneous axial lengths (such as 35 mm) are displayed.
Until the introduction of partial coherence interferometry using the Zeiss IOL Master, accurate ultrasound measurements of the axial length of the human eye with silicone oil in place were difficult and complex. Each component of the eye had to be individually measured (usually at 1,532 m/sec) and the true axial length calculated using the velocity conversion equation (TAL = Vc / Vm x AAL) for the lens thickness and the vitreous cavity.
In contrast, using partial coherence interferometry to measure eyes containing silicone oil in the vitreous cavity with the IOLMaster is relatively straightforward:
- Under the menu heading, Axial Length Settings, select either “Silicone Filled Eye”, or “Silicone Filled Eye – Aphakic”.
What was once a time-consuming, difficult, and sometimes inaccurate measurement by ultrasonography is now simple and highly reproducible.
Phakic axial length measurement of an eye
with the vitreous cavity filled with silicone oil.
As long as the patient can see well enough to look directly at the small, red fixation light, the measurement will be taken to the center of the macula, providing the refractive, rather than the axial, length. This is especially important for eyes that have posterior staphyloma.
If an IOLMaster is not available, the next best approach would be, before IOL placement, to have the retina specialist first remove the silicone oil. The axial length is then measured in the usual way, and the intraocular lens power can then be calculated.
Adjustments to intraocular lens power
If the silicone oil is to remain in the eye for an extended period after cataract surgery, an adjustment to the intraocular lens power must be made.
Holladay and others have recommended that biconvex intraocular lenses should not be placed in patients who have silicone oil in the vitreous cavity. Instead, these patients should have a PMMA convex-plano lens, with the plano side oriented so that it faces towards the vitreous cavity and preferably over an intact posterior capsule. This approach prevents the silicone oil from altering the refractive power of the posterior surface of the intraocular lens. By contrast, a +20.00 diopter biconvex intraocular lens could lose between one-third and half of its refractive power if it comes into contact with silicone oil. PMMA lenses are a first choice, and silicone lenses should be avoided. The Holladay IOL Consultant is particularly helpful for these cases, as it can automatically compensate for the higher index of refraction of silicone oil in the vitreous cavity when performing these specialized IOL power calculations.
The additional power that must be added to the original IOL calculation for a convex-plano IOL (with the plano side facing towards the vitreous cavity) is determined by the following relationship, as described in 1995 by Patel and confirmed by Meldrum:
- Ns = refractive index of silicone oil (1.4034).
- Nv = refractive index of vitreous (1.336).
- AL = axial length in mm.
- ACD = anterior chamber depth in mm.
Additional IOL power (diopters) = ((Ns – Nv) / (AL – ACD)) x 1,000
For an eye of average dimensions, with the vitreous cavity filled with silicone oil, the additional power required for a convex-plano PMMA intraocular lens is typically between +3.0 D and +3.5 D.
- Axial Eye Length Measurements (A-Scan Biometry) in Byrne SF, Green RL (eds): Ultrasound of the Eye and Orbit. St. Louis, Mosby, Second Edition, 2002.
- Byrne SF: A-scan Axial Length Measurements – A Handbook for IOL Calculations. Mars Hill, Grove Park Publishers, 1995.
- Hoffer KJ: Ultrasound velocities for axial length measurement. J Cataract Refract Surg 1994; 20: 554.
