7.1 Orthokeratology Reverse-Geometry Principles, Jessen Formula & Topographic Patterns

7.1 Orthokeratology Principles, Selection, and Map Patterns

Orthokeratology uses specially designed reverse-geometry rigid lenses during sleep to produce temporary corneal shape change and temporary reduction of refractive error. It is a prescription medical-device treatment. Use the approved labeling, manufacturer fitting system, and prescriber's protocol for the specific product.

Mechanism

The response occurs primarily through redistribution of epithelial thickness across the treatment and surrounding zones. It is not stromal ablation, permanent cross-linking, or simple mechanical crushing. Changes are reversible after wear stops, although the regression time varies.

Evaluate response with unaided and corrected acuity, manifest refraction, slit-lamp examination, and repeatable topography acquired under comparable conditions. Axial maps show the global refractive effect, while tangential maps can localize decentration and transition zones. Neither display should be interpreted without raw-map quality and the clinical examination.

Candidate Assessment

Record age, refractive error, pupil size, corneal diameter, keratometry, topography, pachymetry when indicated, tear film, lids, allergy, eye rubbing, occupation, hygiene, water exposure, and ability to attend follow-up. Exclude or refer active inflammation, infection, significant epithelial disease, unexplained irregularity, and suspected ectasia.

Treatable refractive ranges, cylinder limits, corneal-shape requirements, and age language vary by device labeling and design. Do not convert one platform's nominal limits into a universal NCLE cutoff. Pediatric use also requires careful discussion of goals, evidence, caregiver supervision, and the exact product indication.

Jessen and Compression-Factor Concept

A historical Jessen-style calculation subtracts the intended myopic treatment plus an overcorrection or compression factor from flat keratometry to estimate a treatment curve. The calculation is useful for understanding direction: more intended myopia reduction generally requires a flatter treatment relationship.

Modern designs implement sagittal depth, return zones, alignment zones, diameters, and toricity differently. Therefore, a manual formula is a starting concept, not a final order. Follow the design nomogram and verify the result on eye.

The keratometric conversion between radius and diopters is also an approximation based on the keratometric index. It does not describe the full reverse-geometry lens or predict epithelial response by itself.

Baseline and Difference Maps

Acquire a high-quality baseline map before treatment. At follow-up, compare like with like: same device, map type, scale when possible, fixation, and adequate tear film. A subtractive difference map helps show the treatment-zone location and surrounding steepening.

Common descriptive patterns include:

• Centered bullseye: a central flattened treatment zone with a surrounding steep ring that is reasonably centered over the pupil.
• Smiley face: superior treatment-zone decentration with a steeper crescent inferiorly.
• Frowny face: inferior treatment-zone decentration with a steeper crescent superiorly.
• Central island: relative central steepening within the treatment zone.

These names describe the image; they do not identify one guaranteed parameter correction. Decentration can reflect corneal elevation, lid forces, diameter, sagittal relationship, landing alignment, application, or lens position during sleep. A central island can have more than one design-related cause. Use the specific fitting guide and actual lens behavior.

On-Eye Evaluation

Inspect centration, movement, fluorescein pattern where appropriate, edge relationship, bubbles, debris, and epithelial response. After removal, inspect for staining, indentation, edema, infiltrates, or infection. Review unaided vision throughout the day because regression and visual demands differ.

Verify the actual lens against the order and preserve right-left identification. Teach cleaning, disinfection, case replacement, hand hygiene, and strict avoidance of tap water. A fresh sterile solution and correct care system are essential.

Follow-Up and Red Flags

Follow the product and prescriber's schedule, with closer review during initiation or parameter change. Document hours worn, sleep quality, removal time, visual pattern through the day, refraction, maps, staining, lens condition, and adherence.

Pain, increasing redness, photophobia, discharge, infiltrate, epithelial defect, or reduced vision is not a routine adaptation finding. Stop lens wear and arrange urgent clinical evaluation. Do not recommend leftover antibiotic or steroid medication.

Exam Traps

• Orthokeratology is temporary and primarily epithelial; it is not permanent stromal reshaping.
• Device ranges and compression factors are not universal.
• Smiley and frowny patterns describe decentration but do not prescribe one curve change.
• Compare repeatable maps and examine the eye.
• Overnight wear increases the importance of hygiene, water avoidance, and immediate red-eye triage.

Measuring Daytime Performance

Record removal time and measure vision at times that match the patient's complaints. Good acuity immediately after removal can coexist with afternoon regression, glare, or fluctuating quality. A diary of removal time, unaided acuity, symptoms, and sleep duration helps separate inadequate treatment from normal regression or inconsistent wear. Parameter changes should respond to repeatable patterns, not one unusually short or long night.