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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 a little more than half the speed (980 m/sec) of normal vitreous and can attenuate the returning sound wave during ultrasonography so much that a good echoes are difficult, if not impossible, to obtain.

• 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 with 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 IOL Master is relatively easy:

• Under the menu heading, Axial Length Measurement, 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 to the center of the macula, giving the refractive, rather than the axial length. This is especially important for eyes that have posterior staphyloma.

If an IOL Master is not available, the next best approach would be, prior to IOL placement, have the retina specialist first remove the silicone oil. The axial length is then measured in the usual way and 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 of time after cataract surgery, an adjustment to 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 it is facing 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 loose between a 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 very helpful for these cases as it is able to automatically compensate for the higher index of refraction of silicone oil in the vitreous cavity when doing these special 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, and with the vitreous cavity filled with silicone oil, the additional power needed for a convex-plano PMMA intraocular lens is typically between +3.0 D to +3.5 D.

For more information on this topic, the following references are helpful:

1. 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.

2. Byrne SF: A-scan Axial Length Measurements - A Handbook for IOL Calculations. Mars Hill, Grove Park Publishers, 1995.

3. Hoffer KJ: Ultrasound velocities for axial length measurement. J Cataract Refract Surg 1994; 20: 554.