Biometry is traditionally carried out by A-ultrasound for the measurement of the axial length. Partial coherence interferometry biometry is a new non-invasive method for measuring the axial length. The method does not need any contact with the eye, and no local anaesthesia is required. A recent study compared this method to standard A-ultrasound axial length measurement in the evaluation of biometry with the SRK II formula. Precision of the partial coherence interferometry was estimated to be ten times more accurate than the ultrasound. This could result in a 27% improvement in the mean absolute error for postoperative refraction.

IOL power measurement and biometry is not often accurate in eyes with high myopia or high hyperopia. The use of third generation formulae e.g. Holladay I, SRK/T and Hoffer may provide more accurate results. IOL power measurement in eyes that had undergone corneal refractive surgery is also inaccurate and may result in unexpected postoperative refractive surprises. Standard calculations in these eyes may result in underestimation of the IOL power with a postoperative hyperopia.

The use of automated corneal topography to measure the corneal power may be useful in achieving better results. The measured corneal power must be corrected. The modified corneal power can be calculated by subtracting the spherical equivalent change in the corneal plane induced by the PRK from the average corneal power measured before the PRK. The use of pre-refractive surgery readings about corneal power and axial length, as well as over refraction with a hard gas permeable contact lens may also be used.

Axial length may decrease after a successful trabeculectomy operation. Biometry in phakic eyes with a history of trabeculectomy may result in unexpected postoperative refractive results (they tend to be more myopic). Measurement of axial length before the trabeculectomy is recommended, as patients may need to have cataract surgery later on.

Many patients with a history of vitreoretinal surgery and silicon oil develop postoperative cataract. Combined removal of the silicon oil and phacoemulsification surgery may be carried out in the same operation. The speed of sound is different in silicon oil compared with its speed in the vitreous. Echography of an oil-filled eye may overestimate the axial length of the eye. Unexpected postoperative refractive errors may occur if biometry is calculated using the same formula as in eyes with no silicon oil. A conversion factor (0.71 with a range of 0.70-0.73) has been developed to allow for this difference in the sped of sound. The factor will allow an accurate IOL power calculation, when used with the SRK / T formula, for patients undergoing a combined phacoemulsification and silicon oil removal.

Capsular bag distension syndrome is a newly described complication of capsulorhexis that may present with shallowing of the anterior chamber, anterior displacement of the IOL and unexpected postoperative high myopia. In this syndrome the capsule opening is occluded by the IOL optic, distension of the capsular bag then occur due to the high osmotic pressure of its contents (e.g. viscoelastic, soft lens matter or epithelial cells).

Capsular bag distension syndromes often occur 1 to 2 days after surgery. It has also been reported about 5 years after uncomplicated cataract surgery. Capsular block syndrome has often been reported with PC IOL implantation in the capsular bag. Rarely the syndrome may also occur with IOL implantation in the ciliary sulcus.

YAG laser peripheral anterior capsulotomy has been successfully performed in cases of slowly resolving myopia. Surgical removal of the viscoelastic from behind the IOL may be needed if the condition persists and if laser treatment is not possible. Care should be taken in making the capsulorhexis opening adequate and in ensuring that the viscoelastic is removed from behind the implant at the end of surgery. Entrapment of the viscoelastic material may also result in unexpected myopic shift, without increase in the IOP, due to the forward displacement of the IOL. Removal of the viscoelastic material at the end of surgery is, there, very important.