Unforeseen Complexity in an Average Fitting

Greg Nel,

Greg graduated from the School of Optometry at the Technikon Witwatersrand in Johannesburg, South Africa, in 1988, and has practiced in South Africa and New Zealand. He worked Ophthalmology Department at Baragwanath Hospital in Soweto during his National Service. This experience was formative as it illustrated the role for optometry in primary vision care and the value of multidisciplinary care. Greg obtained membership of the British College in 1991 and immigrated to New Zealand from Durban in 1997. He settled in Lower Hutt and is a partner at Black Gates Meek & Dong Optometrists, a large multi-practitioner optometry practice. 

Greg's professional interests are varied. He is interested in the perceptual and behavioral aspects of the visual process, but enjoys contact lens practice and the challenge of fitting custom contact lens fitting to complex corneas in particular. He has special a special interest in Keratoconus and contact lens strategies for presbyopia. He is President of the Cornea and Contact Lens Society of New Zealand (CCLS) and edits its email newsletter, KeratoSITE.  He completed the his Ocular Pharmacology qualification in 2005 and is an enthusiastic member of the Wellington Diabetic Retinal Photo screening Program which he currently serves as the co-ordinator.

This routine fitting of a low myope highlights some aspects of silicon hydrogel lenses (SiHy) which sometimes makes them a little unpredictable. Like a lot of practitioners, I have found SiHy lenses to be invaluable in high ametropia, astigmatism and problem solving. I feed off patient’s enthusiasm when patients report better wearing times and whiter eyes. Not surpringly, SiHy lenses have become my lenses of first choice; but they are stiffer and have novel surfaces, and consequently do not always behave in a predictable way.  This recent fitting of a neophyte illustrates this.

Case Study:
Sam is 14 and feels socially awkward enough not to wear his spectacles unless he is watching TV at home, and then not all the time. He has stopped playing sport. His parents have agreed to let him wear contact lenses. A familiar enough scenario in any contact lens practice…
There were no special considerations and his baselines were within normal limits; although he has a heavy superior lid anatomy and consequently a slightly narrower inter-palpebral aperture.

Relevant Measurements:

Figure 1 Corneal Topography Plots (Placido):

click to enlarge

Contact lens fitting:
Fit Strategy and lens choice: I decided to minimise design variance by initially fitting a nett sphere in the right and avoid fitting a toric lens.  The lenses below were trialed in sequence and the results summarized below.

Lens Trials:

4.	Proclear Compatible (Omafilcon A)	Dia:  14.2		BC: 8.6	Modulus: 0.49 Mpa		Power: R:-1.25 [6/6-1] L: -1.75 [6/6]
	Initial evaluation:	(9/10) As above – good comfort and fit
	Evaluation at 7 hours:	(9/10) As above – good comfort and fit: Instruct in Care and Handling. On lens Removal: No ocular surface staining
	Evaluation at 1 week:	(9/10) As above – good comfort and fit
	click to enlarge	Conclusion: ACCEPTABLE

Discussion:
I regard this prescription as sufficiently low that anoxia/hypercapnia are factors of secondary concern. I attribute the fitting difficulty in this case to lens modulus and I attempted to reduce this sequentially. A smaller lens may also have reduced the saggital tightening effects, although this was probably not significant in this sequence.

Sam was comfortable enough and certainly sufficiently motivated to wear all but the first lenses trialled, but I’d assert that we fit need to protect ocular surface integrity above more “theoretical” considerations. Acuvue Advance (Galyfilcon A, modulus 0.4) has the lowest modulus of the currently available SiHy lenses and would have been interesting to trial, but I had started to test Sam’s parents' patience. The very different performance between the right and left lenses in all but the last lens trialled highlights the pitfalls of contralatteral trialling of different designs at the same time.

The low modulus of hydrogel lenses allows them to stretch and mould themselves to a very wide range of topographies, taking a lot of the thinking out of the process. SiHy lenses have many well documented advantages, but are not always the ideal lens at present.  Minimizing oclular surface trauma is essential principle in contact lenses fitting. I look forward to the continued development of Silicone Hydrogel materials and the expanding range of lens properties and parameters, but at present they need to be fitted with precision.