This article was originally published in a sponsored newsletter.
In the 1950s, with the birth of corneal contact lenses fabricated from polymethylmethacrylate (PMMA) material, there was expanding interest in this new modality. PMMA provided excellent optics and machining capabilities, although its non-permeable nature caused corneal edema, resulting in steepening of the corneal curvature. This steepening resulted in what was termed spectacle blur where the patient’s vision was blurred until the cornea rebounded to its normal shape.
Additionally, if these corneal lenses were inadvertently fitted too flat, the cornea was also reshaped; but, instead of spectacle blur, patients noted that they were seeing better without their eyeglasses than ever before. This post-wear visual improvement caught the attention of some researchers who wondered whether there could be planned reshaping of the cornea to decrease the dependence on corneal lenses for visual correction.
George Jessen described a process he called “orthofocus” at a 1962 conference in Chicago.1 He described a lens where the base curve (or back optic zone radius [BOZR]) of his design would be fitted flatter than the flat keratometry (K) reading by an amount equal to the amount of myopia targeted for the temporary reduction. So, for instance, a –3.00D myope with K readings of 43.00 @ 180 and 43.25 @ 90 would have a base curve of 40.00D. Ultimately, this became known as the Jessen Factor.
At the same conference, it was suggested that “orthokeratology” (ortho-k) was a better term,2 a term that remains in place today. These designs were standard spherical lenses worn during daytime hours and were fit either incorporating the Jessen Factor or by prescribing a series of progressively flatter lenses to alter the corneal shape. Patients would wear their lenses for a few hours and then after removal they were able to see well the rest of the day.
Near the end of the 20th century, four events altered the course of ortho-k: 1) Reverse geometry ortho-k designs were introduced; 2) Sophisticated manufacturing with computer numeric controlled (CNC) lathes changed fabrication techniques; 3) Overnight lens wear was being investigated and 4) Topography began to play an important role in both baseline and follow-up documentation of the corneal surface curvatures.
At this point, a new term was introduced—the “compression factor.”3 Now, patients wore their ortho-k lenses overnight yet maintained quality vision while being device-free throughout the day. This compression factor (typically an additional flattening of the BOZR by 0.75D) compensated for regression of the ortho-k effect during the non-wearing daytime hours.3 Now the –3.00D myope with K readings of 43.00 @ 180 and 43.25 @ 90 would have a base curve of 39.25D to achieve the desired effect.
BOZR = flattest K - (target reduction + 0.75D)
The ideal compression factor has been debatable over the years. Some have argued that 0.75D is not adequate and underestimates the result.4 Instead, it’s described that the optimal compression factor should be 1.75D or greater flatter than flat K.4
There’s also growing interest regarding the impact of increasing compression factor on inducing higher-order aberrations (HOAs) such as spherical aberrations. Based on a recent investigation, Lau and colleagues hypothesize that increasing the compression factor of a specific lens design may improve the myopia control efficacy of ortho-k without compromising visual performance.5
Although there’s been a significant increase in our understanding of the impact of the Jessen and compression factors, there is still much to be learned regarding what the proper formula is. For now, this needs to be evaluated on a patient-by-patient basis to achieve maximum outcomes.
References:
1. Jessen G. Orthofocus techniques. Contacto. 1962;6:200-204.
2. Nolan JA. Flashback: the first ortho-k meeting. Contacto. 1995;38:9-1
3. Mountford, J. Retention and regression of orthokeratology with time. Int Contact Lens Clin. 1998 Mar-Apr;25:59-64.
4. Chan B, Cho P, Mountford J. The validity of the Jessen formula in overnight orthokeratology: a retrospective study. Ophthalmic Physiol Opt. 2008 May;28:265-268.
5. Lau JK, Vincent SJ, Cheung SW, Cho P. The influence of orthokeratology compression factor on ocular higher-order aberrations. Clin Exp Optom. 2020 Jan;103:123-128.
