Lens and Cataract Surgery
Lens and cataract surgery examines biometry and intraocular lens power as much as the operation itself. Board questions often present an axial length, a keratometry reading and a target refraction, then ask which formula you would trust.
Core rules
- Biometry: optical low-coherence reflectometry is standard; contact ultrasound is reserved for dense cataracts. Measure axial length, K, anterior chamber depth and lens thickness; repeat if the two eyes differ by more than 0.3 mm.
- Formulas: SRK/T for average eyes (22.0–24.5 mm), Hoffer Q for short eyes, Haigis or Barrett Universal II for long eyes and for variability in effective lens position. The A-constant is lens-specific and must be adjusted for surgeon technique.
- Post-refractive surgery: standard keratometry overestimates corneal power after myopic LASIK and underestimates it after hyperopic LASIK. Use the ASCRS post-refractive calculator or measured posterior corneal power; otherwise the eye lands hyperopic after myopic treatment.
- Phacoemulsification: 5.0–5.5 mm continuous curvilinear capsulorhexis, hydrodissection, divide-and-conquer or chop, low power with high vacuum. Posterior capsule rupture is managed by stopping phaco, anterior vitrectomy, and a sulcus or anterior chamber lens.
- Endophthalmitis: intravitreal vancomycin 1 mg/0.1 mL plus ceftazidime 2.25 mg/0.1 mL after vitreous tap; vitrectomy if vision is light perception only.
- Paediatric cataract: operate early for dense unilateral opacity (before 6–8 weeks) to prevent irreversible amblyopia and leave the posterior capsule open.
Worked example
A 60-year-old myope had LASIK 15 years ago and now has a dense cataract. Axial length is 26.8 mm, automated K is 41.0 D and SRK/T suggests a 14.0 D lens. The K reading is artificially steep because the central cornea was flattened and the keratometric index does not account for the changed anterior-to-posterior corneal ratio. Using the ASCRS calculator with the pre-LASIK refraction and the change in refraction gives a power about 2 D higher. Choosing the unadjusted 14.0 D would leave the patient hyperopic.
Traps
Trusting a single K reading after refractive surgery, ignoring a short eye when selecting a formula, and implanting an IOL in the bag after capsule rupture.