The visual axis of the human eye lies nasal to the geometric center of the cornea due to the temporal displacement of the fovea. Coincidentally, the pupil is also not centered and is most often displaced nasally, centering itself over the line of sight. Unfortunately, the distance, intermediate, and near optics of our current soft multifocal lenses are in the geometric center of the lens.
The optical malalignment is made even worse by the fact that when a soft contact lens is placed onto the eye, it has a tendency to be displaced temporally (Figure 1). Clinical experience has shown that this temporal lens decentration is significantly greater in right eyes than in left eyes. Together, the nasal position of the line of sight and the frequent temporal position of soft contact lenses create the “perfect storm” for suboptimal optics with multifocal lenses.
Put to the Test
In her 2017 thesis, Pacific University Masters Student Jeanine Doung compared the visual performance and patient preference of presbyopes wearing both multifocal lenses with geometrically centered optics and lenses with offset optics designed to position nasally over patients’ line of sight (Figure 2). The visual optics of the two designs were similar when each pair was worn for one month; however, eight out of the nine subjects strongly preferred the visual performance of the offset optic lenses.
This work was expanded upon in a study from the Michigan College of Optometry presented at this year’s Global Specialty Lens Symposium in Las Vegas. Ramdass et al (2018) reported that the 1.0mm nasally offset lens design provided both a statistically and clinically significant difference in subjective performance when viewing various types of near targets. And, just as we had found, distance viewing was similar regardless of lens pair worn.
Perhaps the most important “takeaway” of the Michigan study was that 19 out of 20 subjects favored the nasally offset optic design. If we combine the results of the two universities (which used similar study protocols and the same lens designs), after one month of lens wear, 27 of 29 subjects preferred the nasally offset optic design.
We believe that the mismatch between the geometrically centered lens optics and the nasal position of the eyes’ optics can lead to poor visual clarity (especially at near) and to patient dissatisfaction with multifocal soft lenses. Our presbyopic patients deserve the best possible optics; in most patients, this will require offsetting the optics 0.5mm to 1.0mm nasally (Figure 3). CLS
For references, please visit www.clspectrum.com/references and click on document #271.