The Lenstar LS 900 uses optical principles (laser interferometry) to measure axial length, measuring distance from the corneal apex to the retinal RPE. In addition to axial length, it also measures corneal thickness, anterior chamber depth, lens thickness, radii of curvature of flat and steep meridian, axis of flat meridian, white to white distance, and pupil diameter.
In clinical studies, precision has been demonstrated with intraobserver consistency of +/- 25 um. Consistency also has been demonstrated with variability of +/- 21 um between 5 examiners. Besides improved precision and consistency compared to immersion techniques, optical biometry allows a less invasive, non-contact, measurement, and has shown to be superior for staphylomas, pseudophakic eyes, and silicone oil eyes. Unfortunately, there are disadvantages that may lead practitioners to also have immersion in their office. Optical biometry is limited by media opacities such as corneal scars, PSC cataracts and vitreous hemorrhage. In addition, it requires patient fixation.
Keratometry is incredibly important for accurate IOL power selection, as 1D K error = to 1D IOL order. Several measurements should be taken on the same machine to ensure consistency, and patients wearing contact lenses should discontinue use prior to measurements. The IOLMaster takes measures at one zone (2.3 mm) collected from 6 points. The Lenstar, on the other hand, measures at two zones (2.3 mm and 1.65 mm). In addition to its 32 points of measurements, each scan captures 128 points of measurements, such that five sets of readings captures 640 points of measurements. In clinical studies this has been proven that the keratometry on the Lenstar is equivalent to the gold standard manual keratometry.
In clinical practice, I have found the Lenstar easy to operate for both myself and my technician. Measurements are easy to take and integration with my EMR—EMA from Modernizing Medicine—is incredibly simple. Results, the most important aspect of all, are fantastic. In fact, in many cases I have found the astigmatism magnitude and axis to be more accurate than my Atlas topographer.
In summary, The Lenstar is an outstanding instrument for optical biometry that is a great addition to any cataract/refractive practice.