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What Is the Jessen Factor (Ortho-K)?

The Jessen factor is an overcorrection amount built into some orthokeratology, or Ortho-K, lens designs. Ortho-K lenses reshape the cornea overnight, then you remove the lenses in the morning and see without glasses or daytime contacts for part of the day. Because the effect can regress during the day, some designs add extra myopic correction so vision stays clearer for longer hours. You might also hear similar wording like ?compression factor,? depending on the fitting system. The exact value depends on your prescription, corneal response, and the clinic's fitting approach.

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What Is the Jessen Factor (Ortho-K)?

The Jessen factor is an overcorrection amount built into some orthokeratology, or Ortho-K, lens designs. Ortho-K lenses reshape the cornea overnight, then you remove the lenses in the morning and see without glasses or daytime contacts for part of the day. Because the effect can regress during the day, some designs add extra myopic correction so vision stays clearer for longer hours. You might also hear similar wording like ?compression factor,? depending on the fitting system. The exact value depends on your prescription, corneal response, and the clinic's fitting approach.

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Why Do Ortho-K Designs Use a Jessen Factor?

Ortho-K changes the cornea shape in a controlled way, but the eye can drift back toward its starting shape as the day goes on. That small regression is why some fitters plan a bit of extra correction at the start of the day. The Jessen factor is one way to plan for that, so the wearer is less likely to notice blur by late afternoon or evening. The amount chosen depends on how strong the prescription is and how stable the cornea response has been during follow-ups. If the fit is too aggressive, the wearer can wake up overcorrected, so clinics watch early results closely and adjust if needed.

What Numbers Are Often Used for Jessen Factor?

Clinics often talk about the Jessen factor in diopters, but the ?right? number is personal and depends on the lens design system. Your fitter chooses the value based on how your eyes respond over the first weeks.

  • 0.50 D is a common starting point in many fitting systems
  • 0.75 D is also used when regression tends to be more noticeable
  • Higher values can be used in some designs, especially when daytime regression is consistent
  • The value can change after early follow-ups, based on measured vision and corneal shape
  • Different brands use different naming, like ?compression factor,? for similar planning

How Does the Jessen Formula Affect Lens Base Curve Choice?

In many Ortho-K systems, the planned overcorrection is built into the lens selection step, which changes the target flattening compared with the patient's starting prescription. That can affect the chosen base curve and the overall sagittal depth of the lens, since the lens needs to center well while creating the intended treatment zone. The fit is then checked with topography, fluorescein patterns, and early vision results. If vision drops off too early in the day, the fitter can adjust lens parameters, including how much overcorrection is planned. The goal is stable day vision with a cornea response that stays healthy and predictable under monitoring.

What Risks Should You Discuss Before Starting Ortho-K?

Ortho-K is an overnight contact lens modality, so hygiene and follow-up matter a lot. Wearing lenses overnight is linked with a higher infection risk than many daytime-only lens routines, and microbial keratitis is the complication clinicians watch most closely. Risk goes up if lenses contact tap water, if cleaning steps are skipped, or if you keep wearing lenses with pain or light sensitivity. Clinics usually set clear rules like no sleeping in lenses outside the Ortho-K plan, no water exposure, and same-day care for warning signs. If Ortho-K is for a child or teen, parent supervision is part of safe wear habits.

Frequently Asked Questions About the Jessen Factor (Ortho-K)

Is Jessen Factor the Same as Compression Factor?

In many clinics, people use the two terms in a very similar way. Both describe planned overcorrection built into an Ortho-K design to help vision stay clear as the effect regresses through the day. The exact meaning can differ by brand, so your fitter might use one label or the other based on the system used. If you are unsure, ask what number was planned and how it is checked at follow-ups.

Does a Higher Jessen Factor Always Give Clearer Day Vision?

No, more overcorrection is not always better. If the plan is too strong, you can wake up overcorrected, which can feel strange and can reduce visual comfort. Too much change can also lead to more staining or lens intolerance in some wearers. Fitters usually adjust based on real results, not only on the starting prescription.

Can Jessen Factor Affect Myopia Control Results?

Myopia control with Ortho-K depends on more than one setting, including treatment zone size, lens centration, and the overall wearing routine. The Jessen factor is mainly about day vision stability rather than myopia control by itself. Still, if the planned correction changes centration or the treatment zone in a way that affects wear time, it can influence the overall outcome indirectly. Your clinic will track refraction, axial length when available, and topography to guide changes over time.

Is Ortho-K Safe for Children and Teens?

Ortho-K can be used in kids and teens under close clinical supervision, but it carries infection risk because the lenses are worn overnight. Eye doctors stress strict cleaning routines, no water contact, and stopping lens wear right away if there is pain, redness, discharge, or light sensitivity. Large reviews and clinical guidance discuss microbial keratitis as a serious complication, even though the overall rate is low with proper habits and follow-up. Parents usually play a big role in monitoring hygiene and spotting early warning signs.

References

1. What Is Orthokeratology? American Academy of Ophthalmology. https://www.aao.org/eye-health/glasses-contacts/what-is-orthokeratology. Accessed February 5, 2026.

2. The Safety of Orthokeratology, A Systematic Review. National Library of Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC4697954/. Accessed February 5, 2026.

3. Benefits and Risks of Orthokeratology Treatment: A Systematic Review and Meta-Analysis. National Library of Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC11192849/. Accessed February 5, 2026.

4. Incidence of Microbial Keratitis Associated With Overnight Orthokeratology. National Library of Medicine. https://pubmed.ncbi.nlm.nih.gov/39549214/. Accessed February 5, 2026.

5. The Relationship Between Compression Factor and Overnight Corneal Shape Change in Orthokeratology. National Library of Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC7268711/. Accessed February 5, 2026.