A moderate amount of treatment zone decentration during orthokeratology (ortho-k) may provide a better balance between slowing myopia progression and maintaining ocular safety than striving for perfect lens centration, a new study suggests.
The two-year retrospective study found that children whose ortho-k treatment zones were moderately or severely decentred experienced significantly less axial elongation – a key marker of myopia progression – than those with only mild decentration.
The findings were published in the Eye & Contact Lens: Science & Clinical Practice.
They add to a growing body of evidence challenging the long-held assumption that the ideal ortho-k fit is always one with a perfectly centred treatment zone.
Researchers analysed data from 60 myopic children undergoing overnight ortho-k treatment, categorising eyes into three groups according to treatment zone decentration: mild (less than 0.5 mm), moderate (0.5-1.0 mm), and severe (greater than 1.0 mm).
After two years, children in the mild decentration group showed an average axial length increase of 0.55 mm. By comparison, axial elongation was significantly lower in the moderate decentration group (0.36 mm) and the severe decentration group (0.28 mm).
Multivariate analysis confirmed that greater treatment zone decentration was independently associated with reduced axial length growth.
However, while severe decentration produced the slowest axial elongation numerically, researchers concluded there was little additional benefit over moderate decentration. Instead, they said moderate decentration represents the optimal compromise between efficacy and safety.
“Moderate decentration of the treatment zone may be the optimal choice in view of both myopia control and corneal safety,” they said.
“Greater treatment zone decentration was associated with less axial length elongation. Moderate ortho-k treatment zone decentration should be optimal choice for safety and efficiency of myopia control in myopic children.”
They said excessive decentration had been linked to increased risks of corneal staining, epithelial indentation, higher-order aberrations, glare, halos and reduced contrast sensitivity, all of which may compromise patient comfort and long-term treatment adherence.
Treatment zone decentration was relatively common during ortho-k wear, they said, and could result from factors including lens fit, corneal shape, eyelid pressure and sleeping position.
The researchers suggest that a slightly decentred treatment zone may enhance peripheral retinal defocus or alter higher-order optical aberrations in ways that strengthen the treatment’s myopia control effect.
They also found that the direction of decentration did not influence axial elongation. Most treatment zones were displaced inferotemporally, followed by the superotemporal quadrant, consistent with previous studies and thought to reflect natural differences in corneal shape.
The study also reaffirmed findings from earlier research that older children and those with higher baseline myopia experienced slower axial elongation during ortho-k treatment.
While the authors acknowledge limitations including the relatively small sample size, retrospective design and evaluation of only one lens design, they believe the findings have important clinical implications.
Rather than attempting to achieve perfect centration in every patient, practitioners may be able to accept – or eventually even predict and optimise – a moderate degree of decentration during lens fitting, they said.
Looking ahead, the researchers suggest advances in artificial intelligence, corneal topography analysis and customised lens manufacturing could allow practitioners to predict treatment zone decentration before fitting and develop personalised, asymmetrical lens designs that maximise myopia control while minimising adverse effects.
If confirmed in larger prospective studies, the findings could influence future ortho-k fitting strategies, shifting the focus from achieving perfect centration to achieving the optimal balance between visual performance, safety and long-term control of childhood myopia.
The researchers were from the Department of Ophthalmology, The Third Hospital of Hebei Medical University, China.
