Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study
Condition: Myopia
MeSH terms: Myopia
Study type: Interventional

Summary: 
      This project aims to study the effects of treatment characteristics on myopia control in
      orthokeratology by investigating the effects of modified treatment zone by reducing the lens
      back optic zone diameter on the effect of myopia control. The project will be a two-year
      randomized, double-masked (examiner - mask axial length only) study conducted at The Hong
      Kong Polytechnic University recruiting sixty Chinese subjects aged between 6-11. Subjects
      will be randomly assigned to two ortho-k groups. The Control group will wear KATT BeFree lens
      with a BOZD of 6mm while the Test group will wear KATT MC lens with a modified BOZD of 5mm.
    

Detailed description: 
      Orthokeratology (Ortho-k) treatment reduces the degree of myopia of the eye by flattening the
      central cornea. This central flattened zone is referred to as the treatment zone (TZ).
      Munnerlyn's formula describes the negative correlation between refractive correction and
      treatment zone size given a maximum ablation depth limited by the patient's corneal thickness
      in refractive surgery. This implies smaller TZ for higher myopic reduction and, indeed, a
      study found marginally smaller TZ of 0.3mm in higher myopic group. In ortho-k, it has been
      speculated that TZ may be associated with the effect of the treatment. The TZ is defined as
      the central flattened area enclosed by points with zero power/curvature changes comparing to
      pre-treatment condition on various topography maps.

      Previous studies determine the TZ characteristics based on different types of topographical
      subtractive maps including tangential, axial and refractive maps. A study compared TZ
      parameters derived from the three different maps. The study defined the TZ as the area
      enclosed by zero power change in different subtractive maps; 12 such points equally spaced 30
      degree apart to construct a best-fit ellipse. Geometrical center of this ellipse was used to
      determine the TZ decentration and TZ diameter along vertical and horizontal axes. The study
      found significantly less TZ decentration and diameter in tangential map comparing to axial
      and refractive maps and no significant difference between the latter two. Based on these
      findings, the investigators suggested that either refractive or axial map would be of high
      validity and accuracy in determining TZ characteristics.

      The current study would investigate the effect of TZ on myopic control in ortho-k. TZ,
      defined as the area enclosed by zero power change in subjective maps will be employed. The
      tangential and refractive subtractive maps will be used to determine the TZ. Characteristics
      of the TZ, including diameter, slope, depth, volume, would be analyzed based on topographical
      subtractive maps and the effect of each of these characteristics on myopia control will be
      evaluated. The role of pupil size during normal viewing condition (distance and near) will
      also be investigated.
    

Intervention name(s): Modified ortho-k lenses; Ortho-k lenses
Intervention type(s): Device; Device
Intervention description(s): KETT MC lens; KATT BeFree lens
Allocation: Randomized
Intervention model: Parallel Assignment
Masking: Double (Participant, Outcomes Assessor)
Primary purpose: Treatment
Keywords: Myopia control; Orthokeratology; Children; Treatment zone