Bitoric GP Contact Lenses: Design and Power Effect

What is a bitoric GP lens?

A bitoric lens is one in which both the anterior and posterior surfaces carry two curvatures rather than one.

According to Ento Key, Rigid Contact Lenses: Basics (Stein, Slatt, Stein and Freeman, Fitting Guide for Rigid and Soft Contact Lenses), a toric lens has different radii of curvature in each meridian, the meridians of the shortest and longest radii are the principal meridians and lie 90 degrees apart, and in a bitoric lens the axes of the anterior and posterior toroidal surfaces may coincide or be oblique but usually coincide.

The same chapter separates the three toric forms that a GP order can take:

• Front surface toric — two anterior radii, spherical back surface. Usually requires prism ballast for orientation, because nothing about the back surface holds an axis.
• Back surface toric — two posterior radii, spherical front surface. The toricity is there to make the lens sit correctly.
• Bitoric — both surfaces toric.

The clinical distinction is which problem is being solved. A toric back surface is a fitting device: it contours the cornea so the lens centres and stays put. Front-surface cylinder is an optical device: it corrects cylinder the tear lens could not reach. A bitoric lens can be doing either or both, and the vocabulary for that difference — spherical power effect and cylindrical power effect — is below.

When does corneal cylinder cross the bitoric threshold?

Published thresholds start at about 2.00 D of corneal toricity, and rise from there depending on the author and on what the spherical lens is actually doing on the eye.

According to Contact Lens Spectrum, A Toric GP Primer (Annie Chang, OD, FAAO and Dawn Lam, BSc, MSc, OD, FAAO, December 2013), ideal toric GP candidates have regular corneal toricity of at least 2.00 D; below that a spherical GP masks most of the corneal astigmatism while still holding an adequate fitting relationship. Patients above 2.00 D are generally fitted with back-surface toric designs, and surveyed practitioners in that article placed the initiation point between 2.00 D and 2.75 D (Blackmore et al, 2006).

According to Contact Lens Spectrum, Putting a Bitoric Fitting Guide to the Test (Kirby Pitts, OD and colleagues, October 2001), opinions differ on the same question: Mandell recommends a toric base curve to maintain lens stability when corneal toricity reaches 2.00 D, while Bennett notes a bitoric lens is essential at regular astigmatism of 3.00 D or more. That article also records the stability rule of thumb from Edrington, Stewart and Woodfield — rotation is minimised when the ratio of lens toricity to corneal toricity in with-the-rule astigmatism is between two-thirds and one, so a 3.00 D cornea is most stable with 2.00 D to 3.00 D of base curve toricity.

A threshold is not the decision. The Pitts article records Mandell’s clinical trigger as the behaviour of the spherical lens itself: use a bitoric when a spherical lens shows decentration, excessive movement, or an unacceptable fluorescein pattern. The Toric GP Primer describes the same evidence from the other end — spherical GPs on corneas above about 2.50 D of toricity tend not to centre, positioning superiorly or inferiorly on a with-the-rule cornea and nasally or temporally on an against-the-rule one, with peripheral corneal desiccation and, from a high-riding lens, corneal moulding and end-of-day spectacle blur. Reading that pattern is on Fluorescein Patterns and Fit Troubleshooting of GP Lenses.

What is the difference between SPE and CPE?

A spherical power effect bitoric has the same difference in diopters between its base curves as between its powers. A cylindrical power effect bitoric does not.

That arithmetic has a clinical consequence. The Toric GP Primer states that an SPE design enhances the fit on a toric cornea while maintaining an overall power equivalent to a spherical GP lens, so the lens can rotate on the eye without inducing residual astigmatism. A CPE design has an unequal difference between base curve radii and powers because it is also correcting residual lenticular astigmatism, and therefore requires good rotational stability — any rotation could blur vision, although toric back surfaces typically align with the corresponding toric corneal meridians.

The GPLI, GP Lens Management Guide — Diagnostic Toric Fitting arrives at the same two designs from the chair rather than from the order form: where there is little to no physiological residual astigmatism, a spherical over-refraction through the diagnostic lens gives good acuity and the resulting lens is an SPE bitoric; where significant residual astigmatism is present, a sphero-cylindrical over-refraction is added to the diagnostic lens power per meridian and the resulting lens is a CPE bitoric.

The Toric GP Primer names a third form, the base curve toric, in which the difference in powers is approximately 1.5 times the difference in base curves. It has a toric back surface and a spherical front surface, which allows front-surface modification such as polishing or adding minus.

One troubleshooting note from the same article: if a back-surface toric lens rotates, the usual cause is insufficient base curve toricity, and increasing it lets the lens lock into the corneal meridians.

How are bitoric parameters written?

A bitoric order states two base curves and two powers, flat meridian first, in the same order.

The GPLI diagnostic toric worked example shows the notation end to end: keratometry 42.50 @ 180 and 45.50 @ 090, a diagnostic lens of 42.00/45.00 D with powers plano/−3.00 D, a spherical over-refraction of −3.25 D, and a final order of 42.00/45.00 D; −3.25/−6.25 D. The base curve pair and the power pair here differ by the same 3.00 D, which is what makes it a spherical power effect lens. In its cylindrical power effect example the sphero-cylindrical over-refraction is added meridian by meridian instead, and the two differences no longer match.

For a diagnostic-fitted bitoric, the Toric GP Primer gives the back-surface step as adding half the observed toricity to the steep meridian and subtracting the other half from the flat meridian, then placing the over-refraction on an optical cross to reach the final powers per meridian. Meridian arithmetic of that kind is what the Optical Cross Calculator is for, and the over-refraction itself belongs on the Over-Refraction Calculator for Contact Lens Parameters.

Empirical bitoric design is done on published fitting guides rather than by hand — the Mandell-Moore Bitoric Lens Guide is the one both articles above name, it treats each meridian as a separate lens, carries its own fit factor and vertex table, and was validated against 61 eyes of successful wearers with differences under 0.37 D. This site names those guides and links to them; it does not reproduce or clone their worksheets, and no calculator on this site outputs a bitoric design.

Why the tear lens changes what cylinder is left to correct

A spherical rigid back surface on a toric cornea leaves a toric fluid layer that corrects most with-the-rule corneal cylinder before the lens power does anything.

That is why refractive cylinder and corneal cylinder are not interchangeable inputs to this decision: the design threshold is read from keratometry, while the residual cylinder that decides SPE against CPE is read from the over-refraction. A patient with significant refractive astigmatism and very little corneal toricity is not a back-surface toric candidate at all — the Toric GP Primer routes that astigmatism, which is primarily lenticular, to a front-surface toric GP or a soft toric lens. The optics of the fluid layer are on Tear Lens and Lacrimal Lens Power of GP Contact Lenses.

Aspheric back surfaces are the other route through moderate toricity: they flatten continuously toward the periphery and can align a mildly toric cornea without a toric back surface, at the cost of induced astigmatism when they decentre. Where a toric lens is ordered, the peripheral system usually follows it — the Toric GP Primer’s diagnostic sets specify the secondary curve fabricated with the same toricity as the base curves so the optical zone stays round. Peripheral curve widths and radii as ordered fields are on Peripheral Curves and Edge Lift of GP Contact Lenses.

Where soft toric rotation and LARS live

Not here. A soft toric lens has no tear lens to mask corneal cylinder, so it carries the cylinder itself and depends on rotational stabilisation.

Orientation markings, measured rotation at the slit lamp, and the LARS compensation rule are on the Toric Contact Lens Calculator, which converts spectacle cylinder and axis into starting toric parameters. Sensitivity to rotation is real in both modalities — the Toric GP Primer cites Snyder (1989) that every 10 degrees of misalignment manifests about one-third of the correcting cylinder in the sphero-cylindrical over-refraction, so a −2.25 D cylinder rotated 10 degrees induces about −0.75 D of unwanted cylinder — but the mechanism that fixes it differs by lens type. A GP is stabilised by its back surface; a soft toric is stabilised by ballast or thin zones and compensated by axis.

Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site, and GP Contact Lens Design: Zones, Curves, and Edge Lift for the parameter set a bitoric order sits inside. Plus- and minus-cylinder notation is converted on the Plus and Minus Cylinder Transposition Calculator.

Sources

Clinical claims on this page are attributed to the publications below.

• Contact Lens Spectrum, A Toric GP Primer(Annie Chang, OD, FAAO and Dawn Lam, BSc, MSc, OD, FAAO, December 2013) — corneal toricity of at least 2.00 D as the toric GP candidate; the 2.00–2.75 D initiation range (Blackmore et al, 2006); decentration behaviour above 2.50 D; SPE, CPE and base curve toric definitions; the 1.5× power-to-base-curve relationship; half the toricity to the steep meridian and half from the flat; insufficient base curve toricity as the cause of rotation; Snyder (1989) on 10 degrees of misalignment; lenticular astigmatism routed to front-surface toric or soft toric.
• GPLI, GP Lens Management Guide — Diagnostic Toric Fitting— diagnostic toric lenses fitted like spherical lenses, often 0.12–0.50 D flatter than K; the spherical over-refraction route to an SPE bitoric and the sphero-cylindrical route to a CPE bitoric; the worked 42.00/45.00 D order notation.
• Contact Lens Spectrum, Putting a Bitoric Fitting Guide to the Test(Kirby Pitts, OD, Latricia Pack, OD, William Edmondson, OD, FAAO and Charles E. Pack II, MS, October 2001) — Mandell at 2.00 D and Bennett at 3.00 D; decentration, excessive movement or an unacceptable fluorescein pattern as the clinical trigger; the Edrington, Stewart and Woodfield two-thirds-to-one toricity ratio; the Mandell-Moore guide validated on 61 eyes with differences within 0.37 D.
• Ento Key, Rigid Contact Lenses: Basics(Stein, Slatt, Stein and Freeman, Fitting Guide for Rigid and Soft Contact Lenses) — principal meridians 90 degrees apart; front surface toric with prism ballast; back surface toric; bitoric with usually coinciding axes.