Solving The Mystery Of Vaulting For Multi-Piece Intraocular Lenses In The Eye

Multi-piece intraocular lenses (IOLs) are mostly designed with PMMA haptics attached to the optic of the lens. Unlike a single-piece, multi-piece IOLs have angulation or vaulting in the inherent design. PMMA haptics have been shown to lose some of their memory. This could possibly affect the vaulting of the haptics as the bag contracts.

A stable A-Constant is assumed when converting across IOL platforms for IOL Power formula predictions. If bag contraction causes the vault of the IOL to change with capsular bag contraction, the A-Constant is no longer constant. The purpose of this study is to compare the amount of change in A-Constant that occurs over time with 3-piece IOLs vs 1-piece IOLs, providing clinically relevant suggestions.

This study is based on the schematic ray trace-based eye model at Alcon Research, Fort Worth, Texas, USA. This study also utilizes clinical data from both Great Lakes Eye care Siant Joseph, MI, USA and Alcon Research, Fort Worth, Texas, USA. 

AcrySof multi-piece MA60AC and MA60BM IOLs (Alcon Vision) have the same 10 degrees of angulation or vaulting. The optical design features are different between these two IOLs. MA60AC is the multi-piece version of the original single-piece SA60AT IOL. A ray trace-based model was set up with a 21 D SA60AT IOL in an average schematic model eye to obtain the best focus on the retina. First MA60BM and then MA60AC were evaluated with planar (no vault), 5°, 7.5° and 10° of vaulting to determine the SRK/T based A-Constant by obtaining the respective best focus on the retina. Both MA60BM clinical data for 6 and 12-months post-op and MA60AC clinical data (Great Lakes Eye Care) were analyzed for determining the clinical A-Constant values.

Six months post-op (N=153) MA60BM clinical data show an average IOL power of 20.931 D, Corneal mean power of 43.628 D, axial length of 23.711 mm with MRSE of -0.069 D. Based on that the A-Constant is 119.27. The twelve months post-op (N=153) MA60BM clinical data with an MRSE of -0.044 D resulted in A-Constant of 119.3. MA60AC clinical data (Great Lakes Eye Care) were analyzed for the A-Constant for different post-op periods. The average of the optimized A-Constants of Barrett, K6, SRK/T and RBF 3.0 ranges from 118.93 to 119.08. The model eye-based A-Constant values for MA60BM are: 118.8 (no vault), 119.03 (5°), 119.2 (7.5°) and 119.37 (10°). The A-Constant values for MA60AC are: 118.6 (no vault), 118.9 (5°), 119.06 (7.5°) and 119.24 (10°).

The major challenge is obtaining the right IOL power for the multi-piece IOL because of the angulation or vaulting of the haptic in the eye. Once the angulation is known, it is relatively standard to get the right power in the bag for the normal eye and then lens power can be adjusted for sulcus. Based on our analysis with clinical data and ray trace-based eye model, both the clinical and theoretical A-Constant for MA60AC are in agreement with a vault of about 7.5°.