Axial length following implantation of posterior chamber lenses

We used ultrasound to measure axial length in 66 eyes before and three months after extracapsular cataract extraction with implantation of a posterior chamber lens. An ultrasound velocity of 2,718 m/s for poly(methyl methacrylate) was assumed. The intraindividually measured difference (postoperative--preoperative axial length) ranged from -0.4 mm to 0.6 mm and averaged 0.11 mm. The lengthening of the eye was statistically significant (P less than .001; two-tailed, paired t-test). Statistical analysis of the regression equation between the preoperative and the postoperative axial lengths disclosed that this correlation was not more accurate than simply substituting the two measurements. Assuming no real change in axial lengths, our data suggest an ultrasound velocity of 1,679 m/s for the natural lens or an average velocity of 1,560 m/s for the phakic eye. We conclude that the preoperatively measured axial length may be used as a fairly accurate estimate of the postoperative distance in intraocular lens power calculation formulas.