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Soft Toric Lenses — Australian design
Soft Toric Lenses--- Australian Design J. W. Farmer
A n Ophthalmic Optician in private practice in Victoria, John Farmer is also a clinical instructor in the Dept. Optometry, University of Melbourne, and a member of the Contact Lens Society in Australia.
Eighteen months ago four different Toric Soft Lenses were available to Victorian Optometrists in Australia. (Fig. 1.) Fig. 1 Soft Toric Contact Lenses
(Availablein Australiabefore 1979.) Name
Material
Lens Design
Location by
HYDRON
Hema
Front Surface Toric Asphefic Back Surface
Truncation
CONrI'AVUE
Hema
NISSEL
Hema
WOHLK
Hema
Front Surface Toric Spherical Back Surface Front Surface Tofic Spherical Back Sudace Hydroflex/TS Back Surface Toric Hydroflex/m-T Back Surface Toric
Prism Ballast and
Available in England and Europe Yes
Prism Ballast
No
Prism Ballast and/or Truncation Prism Ballast and Truncation Prism Ballast
Yes
No
Yes
In our practice we were mainly fitting Hydron lenses, with which we were obtaining a high degree of Success.
In our opinion a successful lens will be comfortable, physiologically acceptable and give vision equal to or better than spectacles. In general any patient we were intending to fit with soft lenses who had astigmatism greater than 0.75 D was corrected with a toric lens. There were two reasons for this: Masking higher degrees of astigmatism with spherical lenses tended to be temporary, and secondly, since toric lenses are successful we expected to give our patients the best possible vision not a spherical equivalent substitute. In February 1979, Smith & Nephew introduced the Snoflex 50 material in Australia. This material had been in use in England for some time and its higher water content and accompanying greater oxygen permeability made it attractive. This was particularly so for toric lenses which need to be thicker than
8
spherical lenses, causing a slightly greater number of oedema problems in astigmatic patients. Smith & Nephew marketed both spherical and toric lenses in Snoflex 50 and due to some service problems we were experiencing with Hydron at that time, we began using the Smith & Nephew toric lenses.
Lens Design (Fig. 2) The lens is an Australian design. It is a front surface toric with a spherical back surface. Location qs achieved with prism ballast. Normally 11/2° of prism is used depending on the axis and power of the cylinder. The lenses can have a single truncation if required to further improve stability. Lens diameters are normally 13.5mm or 14.00mm. Fig. 2 Smith & Nephew Soft Toric Lenses Australian Design Material: Snoflex 50 Design: Front Surface Toric Spherical Back Surface Prism ballast 11/2° Truncation if required All lenses are given a final wet check of the parameters before leaving the laboratory and a tight tolerance is applied. The BCOR is accurate to the Australian Standard of _+0.05ram and the power to _+ 0.12 D (+ 0.25 in higher powers). Tolerance and reproducability are important for the successful fitting of any soft toric contact lens. Smith & Nephew guarantee a replacement lens to fit as well as the original.
J o u r n a l of t h e B r i t i s h C o n t a c t L e n s A s s o c i a t i o n
Fitting Procedure The lenses are fitted using a spherical trial set. A fitting factor of 1.08 x K is used which is approximately 0.9mm flatter than K for a 13.5mm diameter lens. A vertex corrected over-refraction with the optimum fitting spherical lens, added to the trial lens power gives the required power to be ordered. Experience has shown that the toric lens should be ordered 0.1mm flatter than the optimum spherical lens to give equivalent fit. Truncation is not done as a routine and no allowance for rotation is made when ordering. When truncation is required the diameter is increased 0.5mm and the BCOR left unchanged as the truncation will loosen the lenses sufficiently. The lid angle is taken into account when ordering the cylinder axis. Truncation is necessary for cylinders greater than about 4.00 D although lower amounts of oblique astigmatism may also require truncation. We have not found the added edge feeling of truncated lenses a problem with our patients. Nor have we found the truncation to be a site of excessive deposit build up.
Case Histories: T . D : (Fig. 3.) A fairly straight forward example. This patient has almost spherical corneas and hence the astigmatism is largely lenticular. A front surface toric is desirable in such cases. The lenses were ordered about l m m flatter than K.
Patient T.D.
Lenses ordered R 8 . 6 / 1 4 . 0 / - 2 . 2 5 / - 1 . 0 0 x 95 L 8.6/14.0/ 2 . 0 0 / - 0 . 7 5 x 110 Visionwith Contact Lenses R 6 / 6
L -2.(X1/-0.75 x 110 L 6/6 L 7.74 H 7.73 V
L6/6
J.B. (Fig. 4.) A patient with oblique corneal astigmatism. The first pair of lenses ordered were not satisfactory. A small amount of rotation reduced the acuity. The centration was poor causing mislocation and an equivalently fitting larger diameter lens gave a successful result. Fig. 4
Patient J.B. Spectacle Refraction R 3 . 2 5 / - l . 2 5 x 47 Vision with glasses R 6/6 Keratometer readings R 7.38 x 45 7.22 x 135 Lenses ordered R 8 . 6 / 1 3 . 5 / - - 3 . 5 0 / - 1 . 0 0 x 45 L 8.6/13.5/-4.00/-0.50 x 1611 Vision with Contact Lenses R 6/6 L Refitted with 8.8/14.0/ . . . . . . . . . R and L Fit and vision better R 6/6 L6/6
Journal of the British Contact Lens Association
Fig. 5
Patient M.H. Spectacle Refraction R + 2 . 5 0 / - 4 . 7 5 × 178 L + 3 . 5 0 / - 7 . 2 5 x 173 Readingwith glasses R 6/9 L6/9 Keratometer readings R 7.89 H L 8.[X) H 7.14V 7.14V Lenses ordered R 8 . 4 / + 2 . 5 0 / - 4 . 7 5 x 178 L 8 . 6 / + 3 . 5 0 / - 7 . 0 0 x 173 Vision with Contact Lenses R 6 / 9 L 6/9 following truncation
A n 8.4 was ordered. The fit with this tighter lens was better but the vision worse. As can be expected with such a high cylinder power a small error in lens axis or truncation position would significantly affect the acuity. The patient returned to using the original left lens and was asymptomatic despite the slightly loose fit. She has been wearing them happily for twelve months and her vision is still good. In our practice we found 60% of our patients were successfully fitted with the first lens and the majority with the second. Only two patients were unsuccessful. One had oedema problems (his prescription was +8.00/-3.00), the second gave up due to poor motivation (vision and physiologicial response were good).
The Future
Fig. 3 Spectacle Refraction R - 2 . 2 5 / - 1 . 0 0 x 95 Vision with glasses R 6/6 Keratometer readings R 7.63 H 7.64 V
M.H. (Fig. 5.) This patient was rather spectacular. The lenses were ordered without truncation at first and location was poor. These same lenses were then truncated making them slightly loose on the eye although vision was stable and good. It was attempted to fit a tighter truncated left lens than the one shown.
L 4.0tl/ 0.50 x 160 L 6/6 L 7.38 x 160 7.22 x 70
6/6-
Smith & Nephew's laboratory in Melbourne has begun developing a new design in their toric lenses. The new lenses are of a bicurve design with the secondary curve 0.7mm flatter than the base on the last 0.5mm of the diameter, giving an edge lift effect. The lenses are being thinned to 0.06mm centre thickness and the amount of prism also being reduced. Instead of a larger amount of base down prism, computer analysis is aiding in location of the prism to one side of vertical in order to compensate for the effect of thickening on certain parts of the lens related to the cylinder. The overall effect of the design modifications is a well fitting thin toric lens with a compensated prism.
In S u m m a r y In all respects we have found the Smith & Nephew Australian design soft toric lens to be satisfactory. It is our lens of first choice for astigmatic patients. Acknowledgements to S & N Australia for their assistance. Address for further correspondence: Box 122, 57 Victoria Street, Warragul, Victoria 3820, Australia
9
A n Ophthalmic Optician in private practice in Victoria, John Farmer is also a clinical instructor in the Dept. Optometry, University of Melbourne, and a member of the Contact Lens Society in Australia.
Eighteen months ago four different Toric Soft Lenses were available to Victorian Optometrists in Australia. (Fig. 1.) Fig. 1 Soft Toric Contact Lenses
(Availablein Australiabefore 1979.) Name
Material
Lens Design
Location by
HYDRON
Hema
Front Surface Toric Asphefic Back Surface
Truncation
CONrI'AVUE
Hema
NISSEL
Hema
WOHLK
Hema
Front Surface Toric Spherical Back Surface Front Surface Tofic Spherical Back Sudace Hydroflex/TS Back Surface Toric Hydroflex/m-T Back Surface Toric
Prism Ballast and
Available in England and Europe Yes
Prism Ballast
No
Prism Ballast and/or Truncation Prism Ballast and Truncation Prism Ballast
Yes
No
Yes
In our practice we were mainly fitting Hydron lenses, with which we were obtaining a high degree of Success.
In our opinion a successful lens will be comfortable, physiologically acceptable and give vision equal to or better than spectacles. In general any patient we were intending to fit with soft lenses who had astigmatism greater than 0.75 D was corrected with a toric lens. There were two reasons for this: Masking higher degrees of astigmatism with spherical lenses tended to be temporary, and secondly, since toric lenses are successful we expected to give our patients the best possible vision not a spherical equivalent substitute. In February 1979, Smith & Nephew introduced the Snoflex 50 material in Australia. This material had been in use in England for some time and its higher water content and accompanying greater oxygen permeability made it attractive. This was particularly so for toric lenses which need to be thicker than
8
spherical lenses, causing a slightly greater number of oedema problems in astigmatic patients. Smith & Nephew marketed both spherical and toric lenses in Snoflex 50 and due to some service problems we were experiencing with Hydron at that time, we began using the Smith & Nephew toric lenses.
Lens Design (Fig. 2) The lens is an Australian design. It is a front surface toric with a spherical back surface. Location qs achieved with prism ballast. Normally 11/2° of prism is used depending on the axis and power of the cylinder. The lenses can have a single truncation if required to further improve stability. Lens diameters are normally 13.5mm or 14.00mm. Fig. 2 Smith & Nephew Soft Toric Lenses Australian Design Material: Snoflex 50 Design: Front Surface Toric Spherical Back Surface Prism ballast 11/2° Truncation if required All lenses are given a final wet check of the parameters before leaving the laboratory and a tight tolerance is applied. The BCOR is accurate to the Australian Standard of _+0.05ram and the power to _+ 0.12 D (+ 0.25 in higher powers). Tolerance and reproducability are important for the successful fitting of any soft toric contact lens. Smith & Nephew guarantee a replacement lens to fit as well as the original.
J o u r n a l of t h e B r i t i s h C o n t a c t L e n s A s s o c i a t i o n
Fitting Procedure The lenses are fitted using a spherical trial set. A fitting factor of 1.08 x K is used which is approximately 0.9mm flatter than K for a 13.5mm diameter lens. A vertex corrected over-refraction with the optimum fitting spherical lens, added to the trial lens power gives the required power to be ordered. Experience has shown that the toric lens should be ordered 0.1mm flatter than the optimum spherical lens to give equivalent fit. Truncation is not done as a routine and no allowance for rotation is made when ordering. When truncation is required the diameter is increased 0.5mm and the BCOR left unchanged as the truncation will loosen the lenses sufficiently. The lid angle is taken into account when ordering the cylinder axis. Truncation is necessary for cylinders greater than about 4.00 D although lower amounts of oblique astigmatism may also require truncation. We have not found the added edge feeling of truncated lenses a problem with our patients. Nor have we found the truncation to be a site of excessive deposit build up.
Case Histories: T . D : (Fig. 3.) A fairly straight forward example. This patient has almost spherical corneas and hence the astigmatism is largely lenticular. A front surface toric is desirable in such cases. The lenses were ordered about l m m flatter than K.
Patient T.D.
Lenses ordered R 8 . 6 / 1 4 . 0 / - 2 . 2 5 / - 1 . 0 0 x 95 L 8.6/14.0/ 2 . 0 0 / - 0 . 7 5 x 110 Visionwith Contact Lenses R 6 / 6
L -2.(X1/-0.75 x 110 L 6/6 L 7.74 H 7.73 V
L6/6
J.B. (Fig. 4.) A patient with oblique corneal astigmatism. The first pair of lenses ordered were not satisfactory. A small amount of rotation reduced the acuity. The centration was poor causing mislocation and an equivalently fitting larger diameter lens gave a successful result. Fig. 4
Patient J.B. Spectacle Refraction R 3 . 2 5 / - l . 2 5 x 47 Vision with glasses R 6/6 Keratometer readings R 7.38 x 45 7.22 x 135 Lenses ordered R 8 . 6 / 1 3 . 5 / - - 3 . 5 0 / - 1 . 0 0 x 45 L 8.6/13.5/-4.00/-0.50 x 1611 Vision with Contact Lenses R 6/6 L Refitted with 8.8/14.0/ . . . . . . . . . R and L Fit and vision better R 6/6 L6/6
Journal of the British Contact Lens Association
Fig. 5
Patient M.H. Spectacle Refraction R + 2 . 5 0 / - 4 . 7 5 × 178 L + 3 . 5 0 / - 7 . 2 5 x 173 Readingwith glasses R 6/9 L6/9 Keratometer readings R 7.89 H L 8.[X) H 7.14V 7.14V Lenses ordered R 8 . 4 / + 2 . 5 0 / - 4 . 7 5 x 178 L 8 . 6 / + 3 . 5 0 / - 7 . 0 0 x 173 Vision with Contact Lenses R 6 / 9 L 6/9 following truncation
A n 8.4 was ordered. The fit with this tighter lens was better but the vision worse. As can be expected with such a high cylinder power a small error in lens axis or truncation position would significantly affect the acuity. The patient returned to using the original left lens and was asymptomatic despite the slightly loose fit. She has been wearing them happily for twelve months and her vision is still good. In our practice we found 60% of our patients were successfully fitted with the first lens and the majority with the second. Only two patients were unsuccessful. One had oedema problems (his prescription was +8.00/-3.00), the second gave up due to poor motivation (vision and physiologicial response were good).
The Future
Fig. 3 Spectacle Refraction R - 2 . 2 5 / - 1 . 0 0 x 95 Vision with glasses R 6/6 Keratometer readings R 7.63 H 7.64 V
M.H. (Fig. 5.) This patient was rather spectacular. The lenses were ordered without truncation at first and location was poor. These same lenses were then truncated making them slightly loose on the eye although vision was stable and good. It was attempted to fit a tighter truncated left lens than the one shown.
L 4.0tl/ 0.50 x 160 L 6/6 L 7.38 x 160 7.22 x 70
6/6-
Smith & Nephew's laboratory in Melbourne has begun developing a new design in their toric lenses. The new lenses are of a bicurve design with the secondary curve 0.7mm flatter than the base on the last 0.5mm of the diameter, giving an edge lift effect. The lenses are being thinned to 0.06mm centre thickness and the amount of prism also being reduced. Instead of a larger amount of base down prism, computer analysis is aiding in location of the prism to one side of vertical in order to compensate for the effect of thickening on certain parts of the lens related to the cylinder. The overall effect of the design modifications is a well fitting thin toric lens with a compensated prism.
In S u m m a r y In all respects we have found the Smith & Nephew Australian design soft toric lens to be satisfactory. It is our lens of first choice for astigmatic patients. Acknowledgements to S & N Australia for their assistance. Address for further correspondence: Box 122, 57 Victoria Street, Warragul, Victoria 3820, Australia
9