Intraocular lens calculation formulas for new intraocular lens implants

Several different formulas are available for preoperative calculation of the required implant power for a desired postoperative refraction. However, the application of both theoretical and statistically derived regression formulas to the new generation of soft intraocular lens implants poses several difficulties. In this paper the calculation of an A constant for a specific intraocular hydrogel lens implant, as well as the derivation of a universal theoretical formula, is described. The theoretical formula can be applied to other implant styles with various optical configurations and composed of different biomaterials. The SRK and theoretical formulas have been applied retrospectively to a series of patients receiving an intraocular hydrogel lens implant. A comparison shows that both perform satisfactorily in predicting the desired postoperative refraction.     

Intraocular lens implants and risk of endophthalmitis

AIM—To investigate the possible association between the use of three piece foldable silicone polypropylene (SPP) intraocular lenses (IOLs) and an increased risk of postoperative endophthalmitis.
METHODS—A retrospective analysis was conducted of all cases of postoperative endophthalmitis following phacoemulsification surgery in a single unit over a 3 year period. The incidence of postoperative endophthalmitis in eyes with SPP IOLs was compared with the incidence in eyes with single piece polymethylmethacrylate (PMMA) IOLs.
RESULTS—772 cataract extractions by phacoemulsification were performed. One (0.16%) of the 622 patients with PMMA IOLs developed endophthalmitis. Excluding one patient who had aplastic anaemia, five (3.33%) of 150 patients with SPP IOLs developed endophthalmitis. The relative risk for postoperative endophthalmitis associated with the use of the SPP IOL compared with the PMMA IOL was 20.1 (p=0.015).
CONCLUSION—This study adds further evidence to the concept that SPP IOLs can be a significant risk factor in the development of postoperative endophthamitis.

Keywords: cataract surgery; phacoemulsification; intraocular lenses; endophthalmitis     

Intraocular lens materials and styles: A review

Biomaterial science has lead to the development of a variety of foldable intraocular lens (IOL) biomaterials. This literature review examines these lenses from both a basic science and a clinical perspective. By most parameters, hydrogel, soft acrylic and silicone IOL are better than polymethylmethacrylate (PMMA) lenses.
Plate haptic silicone IOL have the lowest incidence of cystoid macula oedema and posterior capsule opacification, but these lenses require an intact anterior capsularhexis and posterior capsule. Yttrium aluminium garnet (YAG) laser capsulotomy must be delayed at least 3 months to avoid posterior lens dislocation. Silicone has the lowest threshold for YAG laser damage of all IOL materials and also adheres irreversibly to silicone oil with subsequent optical impairment. Three piece silicone IOL with polypropylene haptics have a higher incidence of decentration, pigment adherence and capsule opacification compared with PMMA haptics. Hydrogel lenses are very biocompatible and resistant to YAG laser damage, but pigment adheres to the surface more readily than PMMA. Soft acrylic IOL unfold slowly, resulting in controlled insertion, but it is possible to crack the lens and some lenses develop glistenings due to water accumulation.
There are significant socioeconomic implications to the large differences in posterior capsule opacification rates between the various biomaterials and the lens styles.     

Intraocular lens implants for uniocular cataracts in childhood.

We report the outcome of intraocular lens implantation in 20 children with visually significant cataracts (seven traumatic, 13 non-traumatic). Six patients had anterior and 14 had posterior chamber implants. The mean age of the whole group at the time of surgery was 5.9 years (range 0.3 to 15.1 years), while the mean period of follow-up was 2.4 years (range 0.8 to 5.9 years). Postoperatively 10 patients developed a transient fibrinous uveitis, four required lens repositioning, one needed lens removal, and eight required posterior capsulotomy. Complications warranting secondary surgical procedures occurred predominantly in eyes with posterior chamber implants. Co-operation with conventional amblyopia treatment was satisfactory in eight out of 16 patients. Postoperatively nine out of 18 patients had peripheral fusion, four patients regained visual acuities of better than 6/9, and visual acuity did not improve beyond 3/60 in six cases. In 19 eyes the optical pathway to the retina is clear and the implants are stable with no evidence of persisting inflammation. Contact lenses remain the initial treatment of choice in infancy, but modern intraocular lenses are well tolerated and have a role in the visual rehabilitation of patients with contact lens and probable contact lens failures and older children with uniocular cataracts.     

Intraocular lens implants (pseudophakoi) and steelwire sutures: a contraindication for MRI?

The strong magnetic field of the super-conducting MRI (Magnetic Resonance Imaging) apparatus could cause problems in the presence of metallic foreign material, such as the metal clips and loops of intraocular lenses and steel as suturing material. If the magnetic field were to induce movement this could be dangerous for the eye. In this study these materials were tested with the 0.5 Tesla MRI and a 1.0 Tesla permanent magnet. Intraocular lenses with a platinum clip, or metallic or plastic loops, and 5 × 0/6 × 0 steelwire used as suturing material experienced no magnetic movement or change of position in the strong magnetic field. Neither did these foreign materials have any effect on the images obtained (spin-echo pulse sequence, 0.5 T Gyroscan). No artefacts were produced, the normal MRI picture of the T₁ -T₂ calculated images, made with a special deticated eye surface coil, were obtained.     

Intraocular lens implants for macular protection. Nine years of follow-up

PURPOSE: To evaluate the advantages of macular protection by modifying light transmission of intraocular lenses in cataract surgery. PATIENTS AND METHODS: The modified transmission intraocular lens (IOL) blocking the visible spectrum up to 450 nm was used. Forty patients with serous drüsen were operated on with modified transmission IOL as were 40 patients with the same IOL with conventional transmission up to 380 nm. Visual acuity, microperimetry and color vision were recorded for 9 years. RESULTS: The visual acuity in the modified transmission group improved one line or more than in the other group with conventional transmission. The risk of exudative macular degeneration was 12% in the modified transmission group and 34% in the conventional transmission group. CONCLUSION: Modifying transmission up to 450 nm seems very important to protect vision in macular degenerative patients.     

Intraocular lens complications

The authors implanted intraocular lenses (IOL) in 780 eyes and considered complications of this surgery, dividing the complications into intraoperative and postoperative, the latter early and late. The most frequent intraoperative complications were: hyphema, vitreous injury, pupil deformation with an IOL in the anterior chamber (AC), and hyphema and corneal lesions with an IOL in the posterior chamber (PC). Among the early postoperative complications of an IOL in the AC the authors noticed striate keratopathy and uveal flare and postoperative transitory ocular hypertension and striate keratitis with an IOL in the PC. Moreover, in late postoperative complications in AC implantation they observed: cystoid macular edema, bullous keratitis, retinal detachment, late uveitis, opacification of posterior capsule, eccentric displacement of the IOL, and less frequently, cystoid macular edema in PC implantations. The authors prefer IOL implantation in the PC after extracapsular cataract extraction, since this surgery causes less problems in the cornea, vitreous, and retina.