Technical Note Rigid lens fitting made easier using a modified keratometer Andrew K. C. Lam Optometry Section, Hong Kong Polytechnic, Hong Kong (Received 13 July 1992, in revised form 12 October 1992) After simple modification, a keratometer cati measure both the central and peripheral regions of the cornea. The apical radius and /j-valuc of the cornea can be determined with this information using a computer program. This computer program suggests lens parameters similar to those chosen by eonven- tiotial methods in rigid lensfitting.The computerized fitting method is also applicable to Chinese subjects. The keratometer is an ophthalmic instrument which measures corneal curvature by calculating the image size of an object after reflection at the corneal surface. However, a conventional keralometer utilizes an an- nular region of the anterior corneal surface 3 mm in diameter to measure the radius of curvature. It does not give any information to the contact lens practitioner about the corneal curvature at the apex nor in the periphery of the cornea. Stone' and Smith- have provided detailed reviews of different methods for measuring the corneal curvature. Clinically, Douthwaite and Sheridan-^ have described an inexpensive modification of a Bausch and Lomb keratometer to measure the corneal apical radius and /j-value of the corneal ellipse. The p-\a\ue is a mathematical term describing the form of an ellipse. A general equation to describe the family of conies was given by Baker^ and later by Bennett\ >'- = 2r,,.v -px- where r,, is the apical radius and p <0 hyperbola p =0 parabola 0 < p < \ prolate ellipse p = 1 circular arc p > 1 oblate ellipse The modification of the keratometer is simply the addition of larger targets to the keratometer to enable it to meaure the radius of curvature of the peripheral corneal surface for an annular zone ^ 6 mm diameter. A supplementary prism is added to the existing prism inside the keratometer to allow the alignment of the auxiliary targets. The corneal radii measured by the standard and auxiliary targets are used to calculate the apical radius and /7-valuc of the corneal ellipse. This instrument was compared with the Wesley Jessen PEK' and it was shown to give a similar indication of corneal topography. Douthwaite'' has applied this modified keratometer in rigid contact lens fitting. An optimum fitting contact lens was obtained by a computer program using the apical radius and p-vd\ue information from the modified keratometer. The computer program provided what he termed the equivalent spherical radius (ESR). Figure I illustrates the ESR concept. The ESR is the radius of a circular arc which matches an ellipse with the sag value ,v with a tear layer thickness of zero over a chord length 2v. Douthwaite stated that the result from the comput- erized fitting method was similar to the conventional fitting method using fluorescein pattern assessment. The present study investigates any difference in the back optic zone radius (BOZR) determination in Chinese subjects from the computerized method de- scribed above and from the conventional fitting method using fluorescein pattern assessment. Rigid lens fitting is not popular in Hong Kong** because the contact lens wearers in Hong Kong appreciate the comfort of soft lenses. The conventional fitting method, involving sev- eral trials of rigid lenses with different BOZRs. may dis- courage the patients. If the same result can be obtained as in Douthwaite's study\ those contact lens prac- titioners who do not frequently prescribe rigid lenses can then get the optimum fit after one or two trials. Thus. the method would be able to reduce the 'chair time". Method Twenty-five Chinese subjects with no contact lens wear- ing history nor any corneal disease were recruited. The age range was 19-26 years. There were 10 women and 15 men. All were myopes with refractive errors < - 8 . 0 0 D , They had corneal astigmatism <2.00D with principal meridians lying within 3" of the horizontal and vertical meridians. After their right eyes were measured using the modified keratometer. a practitioner used the central K reading to select a contact lens using the 'near alignment' of the flatter corneal meridian for each subject from two rigid lens trial sets {total diameter of 8.80 and 9.50 mm) using the conventional fitting method. This lens was applied to the eye. the fiuorescein pattern evaluated and the lens altered until an 'opti- mum' fit was judged to be achieved. The apical radius 100 Ophthal, Physiol. Opt.. 1993. Vol. 13. January © 1993 Bulterworth-Heinemann for British College of Optometrists 0275 5408/93/00100-02