Pseudo axial length increase after silicone lens implantation as determined by ultrasonic scans

Albert T. Milauskas, M.D., Sherrie Marney, C.R.A.

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Use of the A-scan to determine the eye's axial length has become a routine procedure prior to intraocular lens (IOL) implantation. Most axial length determina­tions of a patient's two eyes are quite close-a differ­ence of one diopter (D) or less. Scanning both eyes acts as a check on the reliability of the first scan. This is especially true when one eye has been implanted and the final refraction and power of the implant are known. A large variation in measurements at this point would alert the operator to a possible error.

In the course of silicone (Elastic@:!, Elastimide@:!) lens implantation and prior to surgery on a second eye , we routinely perform repeat A-scans. Surprisingly, we found large discrepancies between the two eyes with­out any significant variance from the expected refrac­tion in the pseudophakic eye. The apparent increase in axial length was approximately l.0 mm . Increases ranged from 0.70 mm to l.55 mm . This same increase was noted in both types of proprietary silicone mate­rial, RMX-l and RMX-3, manufactured by Staar Surgical Company and after the implantation ofIOLAB soft silicone lenses.

The significant aspect of this finding is the inability to compare accurately a pseudophakic axial length with a phakic axial length. The appearance of an implant made of RMX-3 material, especially the new filtered silicone with its controlled center thickness, is almost indistinguishable from polymethylmethacrylate (PMMA) lenses. It is even more difficult to recognize the posterior chamber lenses made of RMX-3 with polyimide haptics because of their similarity to the standard posterior chamber lens' appearance. It is therefore important to determine the axial length of both eyes preoperatively, and also to ascertain the type of implant material used in the case of a pseudophakic eye.

MATERIALS AND METHODS Forty-five patients who had A-scans performed

preoperatively using CooperVision A-scan instruments had repeat scans either for surgery on the fellow eye or as part of obtaining data for this investigation. The scans were done randomly by four different techni­cians. There was no preference in the type of material,

Reprint requests to Albert T. Milauskas, M.D., 39700 Bob Hope Drive, Suite 109, Rancho Mirage, California 92270.

400 J CATARACT REFRACT SURG-VOL 14, JULY 1988

Fig. 1. (Milauskas) The RMX-3 silicone block being measured with the CooperVision A-scan using the immersion technique.

RMX-1 or RMX-3, or the type of implant, one-piece silicone lens or silicone optic and polyimide hap tics, that was scanned. The preoperative axial lengths mea­sured from 2l.97 mm to 25.0l mm, with a normal distribution in between. The velocity of sound was preset in the instrument to 1,550 M/sec for the phakic eye. The velocity of sound for the pseudophakic eye with a PMMA optic was 1,548 M/sec. All the scans were done using the applanation method with the soft water, jacket-type probe.

To determine the velocity of sound in silicone, the following experiment was set up: Using a block of silicone 1 cm in thickness, the CooperVision ultrasonic probe was set at the aphakic mode with the velocity of sound at 1,550 M/sec with only two gates, one for the anterior surface and the other for the posterior surface of the silicone block. The velocity of sound in silicone was determined to be 1,049 M/sec (Figure 1). The same measurement was confirmed by the CooperVision IOL division. Their determination of sound transmis­sion through RMX-3 silicone was 1,046 M/sec. The slight discrepancy is due to measuring differences, but these two figures are virtually the same (personal communication, September 1987).

RESULTS Table 1 shows the average increase in axial length in

the seven patients who received RMX-1 silicone

Table 1. Axial length increase (pseudo) in eyes implanted with one-piece RMX-l silicone lenses.

Model Number of Eyes

Band El 7

Average Increase in Axial Length

1.08 mm

Table 2. Axial length increase (pseudo) in eyes implanted with one-piece RMX-3 silicone lenses.

Model Number of Eyes

3B 13

Average Increase in Axial Length

1.08 mm

Table 3. Axial length increase (pseudo) in eyes implanted with RMX-3 two material (silicone optics, polyimide haptics) lenses.

Model Number of Eyes

PS3 5

Average Increase in Axial Length

1.06 mm

Table 4. Axial length increase (pseudo) in eyes implanted with RMX-3 two material (silicone optics, polyimide haptics) lenses.

Model Number of Eyes

AA4203 20

Average Increase in Axial Length

0.96 mm

lenses; these were all one-piece lenses. The model designation, B or E1, refers to the length of the implant and the shape of the haptics. The average axial length of the seven patients increased l.08 mm.

Table 2 refers to the one-piece models made of RMX-3 silicone material designated 3B, 3C1, or AA-4004, with only a slight size difference. The average axial length increased l. 08 mm. Although RMX-3 is a firmer, clearer material than RMX-1, the axial length change was the same.

Table 3 shows the increase in axial length in the implant with an RMX-3 optic and polyimide loops. This lens is designated PS3, AQlOOO, AQlOlO, AQlOll, or AQ1014. The numerals refer to the haptic design­straight, angulated, notched, or with a positioning loop in the haptic. The average increase in axial length was l.069 mm.

Table 4 refers to the one-piece model AA4203 implanted in the capsular bag. This is a new lens with a controlled center thickness of l.32 mm (all lenses have the same center thickness). The average increase in axial length was 0.96 mm.

DISCUSSION The data clearly demonstrate the apparent increase

in axial length in eyes implanted with Staar silicone lenses of RMX-1 or RMX-3 material when measured with the velocity of sound used in determining pseu­dophakic axial length. Lens model differences were insignificant in affecting the apparent axial length

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change. A thorough search of the literature failed to reveal any references to this phenomenon. The expla­nation for this discrepancy is that sound travels through a denser material, such as PMMA, faster than it does through a less dense material, such as silicone. Silicone is less dense than the natural lens or a PMMA lens and therefore slows down the sound waves. Through experimentation, we have determined that the velocity of sound of silicone is approximately 1,049 M/sec. This contrasts with 2,850 M/sec for PMMA.

Since April 1985, more than 10,000 silicone lenses have been implanted and the rate appears to be increasing as new manufacturers enter the silicone lens market. Until recently, silicone lens implantation was limited to a few core investigators who did not have many lenses available to them. However, with some silicone lenses being on an "adjunct" type of core study, the implantation rate is increasing. Awareness of the pseudo increase in axial length is important because of the entry of manufacturers other than Staar into the silicone or soft lens market and because the elderly population move to areas where their medical records

may not be readily available and the possibility of silicone lens implantation· may be overlooked. The similarity of appearance oflens design and material can also complicate the situation.

Recently, we were consulted about two cases of possible lens exchange because of decentration of early silicone implants. The problem of determining the power of the PMMA implant was solved using the conversion factor determined from this study. Had this not been done, an implant power error of approx­imately 3 D would have occurred.

Repeat A-scans of eyes with silicone lens implants result in an apparent axial length increase of 1.045 mm with instruments calibrated to measure the axial length in pseudophakic eyes using the velocity of sound of 1,546 M/sec. If the instruments can not be reset to the velocity of sound of the silicone lens, 1,486 M/sec, a conversion factor of 0.96 times the longer axial length will determine the axial length to be used for compari­son. Awareness of this fact will alleviate some of the anxiety associated with obtaining axial lengths that differ significantly.

402 J CATARACT REFRACT SURG-VOL 14, JULY 1988