Anterior Chamber Depth
The anterior chamber depth is defined differently by the two instruments. The IOLMaster defines the distance from the tear film to the anterior surface of the lens as the anterior chamber depth. This definition also includes the corneal thickness. In the LENSTAR, the corneal thickness is measured separately and the distance between the posterior corneal (endothelial) surface is measured to the anterior lens surface. In order for a comparison to be possible, we added the value of the corneal thickness to the endothelial-lens surface distance to create an equivalent or comparable value.
There were 107 eyes out of 124 where both instruments had measurements. The IOLMaster has a mean of 3.06 mm (range 2.10-5.06, sd 0.527) and the LENSTAR has a mean on 3.26 mm (range 2.25-5.94, sd, 0.686). As can be seen the graph below 87 out of 107 eye ((81.3%) were within 0.3 mm of each other. The remaining 20 out of 107 (18.7%) eyes were measured with deeper anterior chambers by the LENSTAR.
Discussion of Anterior Chamber Depth Results:
Anterior chamber depth is the third most important factor in IOL calculation and source of post-operative refractive error. Six sample eyes with true measurements were chosen to represent the range of IOL powers based on the IOLMaster data. Different elements were examined to see how much impact they have on the predicted refractive outcome. It is well known that the greatest impact is related to A’s then K’s followed by the ACD. The two instruments had good correlation in ACD results with 81.3% of the ACD’s within 0.3mm. IOLMaster was slightly shorter vs. LENSTAR, (3.06 vs. 3.26mm), but had a tighter Standard Deviation (0.527 vs. 0.686).
The IOL powers were calculated using the Holladay II formula in the Holladay IOL Consultant Surgical Outcomes Assessment Program, Professional Edition. We arbitrarily chose 3.00 mm and 4.00 mm as our ACD’s to be entered into the Holladay Consultant. When looking at the sample eyes, even with a 1.0 mm difference in the ACD resulted in the maximal impact at the 29.5D implant, it resulted in about a 0.44 D predicted refractive error, and 0.22 D in the 14.0 D implant.
While still an important element, Anterior Chamber Depth has modest impact on the final IOL power calculation, and there was minimal difference between the two instruments.
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