Troubleshooting Toric Soft Lenses
By Richard G. Lindsay, BScOptom, MBA,
Become a pro at determining when to use toric soft lenses and achieving a comfortable and stable toric soft lens fit.
Toric soft lens use in contact lens practice has become well established over the last 10 years due to improvements in design and reproducibility and innovations such as frequent replacement (disposable) toric lens forms.
While prescribing toric soft lenses is a relatively straightforward process, a successful toric soft lens fitting for the patient may require a certain degree of problem solving by the practitioner.
Choosing Between Spherical and Toric Soft Lenses
Whether to use toric or spherical soft lenses depends on a number of factors. The most important is the degree of ocular astigmatism. In fitting soft lenses, carefully consider how much astigmatism can be left uncorrected without significantly compromising patient acuity. As a generalization, correct 1.00D or more of astigmatism. About 25 percent of the general population requires a cylindrical correction of this magnitude. However, the need for cylinder correction in soft lens fitting will vary among patients depending on other factors such as ocular dominance, cylinder axis and patient visual needs.
Ocular dominance is important because higher degrees of uncorrected astigmatism are more likely to be tolerated in a non-dominant eye or, in patients with unequal visual acuities, in the eye with the poorer acuity. The cylinder axis is also important. An uncorrected cylinder with an oblique axis will cause a greater degradation of the visual image compared to an equivalent amount of uncorrected with-the-rule or against-the-rule astigmatism. Also carefully assess the visual needs of the patient, as the more critical the visual task, the smaller the amount of astigmatism that can be left uncorrected .
The availability of both toric and spherical disposable lenses has simplified this decision-making process, as practitioners may fit patients with such lenses for a trial period to help determine which form of correction works for the patient.
A well-fitting lens is comfortable in all directions of gaze, gives complete corneal coverage and demonstrates good centration. On blinking there should be about 0.5mm of vertical movement when the eye is in the primary position to maintain post-lens lubrication and, in turn, ensure a complete post-lens tear film. On upward gaze or lateral movements of the eye, the lens should lag by no more than 0.5mm. Excess lens movement can cause discomfort and may decrease vision by reducing rotational stability.
Choose the back optic zone radius (BOZR) so that the lens has a stable and consistent orientation on the eye. A well-fitting lens reveals a stable lens orientation with a quick return to axis if mislocated. A tight-fitting lens shows a stable lens orientation but a slow return to axis if mislocated. A loose-fitting lens demonstrates an unstable and inconsistent lens orientation.
Total lens diameter influences both lens centration and lens stability. Generally, err on the large side when specifying lens diameter, as a larger diameter means more area available for the stabilization zones in the periphery of the lens. However, do not make the diameter so large that it causes undesirable factors such as limbal indentation and conjunctival hyperemia.
Some practitioners advocate fitting toric soft lenses very steep (tight) with minimal lens movement, on the assumption that this aids stability and reduces lens rotation. This is not necessary with the designs available today, and, if a lens adheres too tightly to the eye, the locating forces designed to stabilize lens orientation will have no effect. Consequently, a steeply-fitted lens may actually decrease stability and lead to limbal indentation and fluctuating vision, the latter caused when the soft lens vaults the corneal apex.
Allow for Lens Rotation
To appropriately correct a patient with a toric soft contact lens, the back vertex power of the lens on the eye (BVP in situ) should equal the patient's refraction at the ocular plane. Where you observe lens rotation, the cylinder axis of the BVP in situ will differ from the prescription axis by an amount equal to the degree of lens rotation. In this case, the practitioner must ensure that the specified cylinder axis incorporates an allowance for this rotation, so the cylinder axis of the lens on the eye coincides with the cylinder axis of the ocular refraction.
Practitioners can induce significant cylindrical error when they do not make due allowance for lens rotation. For example, 10 degrees axis mislocation of a contact lens with a cylindrical correction of -2.00D x 180 results in a cylindrical error of -0.69D x 40. The higher the cylinder power, the greater the induced cylindrical error (and subsequent reduction in visual acuity) for a given degree of rotation. Practitioners must consider lens rotation in the final lens order when they expect a toric soft lens to demonstrate some degree of rotation on the eye.
How much allowance for lens rotation should be incorporated into the prescription? On average, toric soft lenses tend to rotate nasally by about five to 10 degrees (where nasal lens rotation is defined as rotation towards the nose with respect to the inferior aspect of the lens). A significant variability, however, will exist among toric soft lens wearers in the actual amount and direction of lens rotation, due to such factors as lid anatomy, the fitting relationship between the lens and the eye, and lens thickness.
Toric soft lenses usually have markings at a specific reference point so practitioners can assess the degree of rotation when the lens is on the eye. The markings may be in the form of ink, laser trace, scribe lines or engraved dots. Note that these markings do not represent the cylinder axis they are simply a point of reference from which you can assess the rotation of the lens. Figure 1 shows an ink dot lens marking (reference point 6 o'clock) on a loosely fitting toric soft lens exhibiting significant (approximately 60 degrees) nasal rotation on a right eye.
Figure 1: An ink dot marking on a toric soft lens helps determine lens rotation.
Many practitioners think toric soft lenses must position at the reference point to consider them a success. A small degree of lens rotation is acceptable, provided it is consistent and the lens is stable at this point. The movement of the lens marking away from its reference point permits us to assess the amount and direction of lens rotation so we can make appropriate allowance in the final lens order.
A common problem in prescribing toric soft contact lenses is determining the extent of lens misalignment with unexpected lens rotation. Lens misalignment occurs when the cylinder axis of the toric soft lens on the eye does not coincide with the cylinder axis of the ocular refraction, and is caused by the lens rotating by a different amount than the practitioner allowed for in the final lens prescription. Lens misalignment results in lower visual acuity due to induced cylindrical error.
Determine lens misalignment by observing the location of the toric soft lens on the eye to estimate the degree of lens rotation. Compare this value with the expected lens rotation which was incorporated into the back vertex power of the contact lens. The difference between the actual and expected values represents the degree of lens misalignment.
You can also calculate the effective back vertex power of the contact lens on the eye (BVP in situ) by utilizing the patient's ocular refraction (Oc Rx) as well as the spherocylindrical refraction obtained over the mislocating lens (SCO) to calculate the degree of lens misalignment.
When a toric soft lens axis mislocates on the eye, the BVP in situ will differ from the ocular refraction and induce significant cylindrical error. The combination of the BVP in situ and the SCO should equal the patient's ocular refraction.
BVPin situ + SCO = Oc Rx
This formula can be rearranged to solve for BVP in situ:
BVPin situ = Oc Rx SCO
Given the overrefraction with the lens and the patient's ocular refraction, we can resolve these obliquely-crossed cylinders and calculate BVP in situ using matrix optics and the following method:
1.Express both the spherocylindrical ocular refraction and the spherocylindrical overrefraction in dioptric power matrix form.
|F =||S + C sin2q-C sinq cosq|
|-C sinq cosq S + C cos2q|
S is the sphere power, C is the cylinder power and q is the axis (in radians) of the cylinder.
2. Subtract the dioptric power matrix for the over-refraction from the dioptric power matrix for the ocular refraction to obtain the dioptric power matrix, Fr, for the BVP in situ.
|Fr =||Sr + Cr sin2qr -Cr sinqr cosqr|
|-Cr sinqr cosqr Sr + Cr cos2qr|
3. Convert the matrix form of the BVP in situ back to spherocylindrical notation using the following formula:
If the lens power matrix is
trace (t) = a11 + a22 and
determinant (d) = (a11a22) - (a12a21)
To convert the matrix form of the BVP in situ back to spherocylindrical notation, determine Sr, Cr and qr (the sphere power, cylinder power and cylinder axis respectively of the BVP in situ) as follows:
|Sr =||(t - Cr)|
|atan||(Sr - a11)||x 180||
(where qr is in degrees)
Cr = - �t2 - 4d
(The minus sign prior to the radical symbol simply means that the final solution will be in minus cylinder form.)
A Java applet for this purpose is accessible on the Internet at www.optometry.unimelb.edu.au/java/ophthalmicalc.html.
For example, consider a toric soft lens being fitted on a patient's right eye. The patient's ocular refraction is 3.00 2.00 x 180. The specified BVP of the contact lens is 3.00 2.00 x 170, hence this prescription incorporates an allowance for 10 degrees of nasal rotation. An SCO with this lens yields +0.50 1.00 x 37.5. Solving for BVP in situ gives 3.00 2.00 x 165. The specified cylinder axis was 170; however, the effective cylinder axis on the eye is 165. Therefore, the lens is exhibiting five degrees of temporal rotation on the eye (instead of the expected 10 degrees of nasal rotation). To allow for this five degrees of temporal rotation, reorder the contact lens with a cylinder axis of 5 to achieve the target cylinder axis on the eye of 180.
If an unexpected degree of lens rotation leads to lens misalignment, but the lens demonstrates good rotational stability at this location, then reorder the lens with the revised allowance for lens rotation. Note that changing the cylinder axis to allow for the observed amount of lens rotation will alter the lens thickness profile and may lead to a slightly different degree of rotation.
A patient may experience a drop in visual acuity for many reasons when wearing a toric soft contact lens. In this situation, a spherocylindrical overrefraction can help ascertain why vision with a toric soft lens is reduced.
If the SCO improves visual acuity to the expected level, either the contact lens back vertex power is incorrect and/or lens misalignment has occurred. The BVP in situ calculated using the previous formula will not only indicate if the lens is misaligning but also if the lens has been made to the correct specifications. A toric soft lens mislocating on the eye but incorporating the appropriate spherical and cylindrical powers will produce an overrefraction with zero spherical equivalent. When the sphere or cylinder power is incorrect, the spherical equivalent of the overrefraction will not equal zero. This arises because the equivalent spherical powers of the ocular refraction and toric contact lens are equal and opposite regardless of lens axis. In the previous example, the spherical equivalent of the SCO (+0.50 1.00 x 37.5) was zero, indicating that the misaligned lens did have the correct spherical and cylindrical powers.
If visual acuity is not improved by the SCO, the clinician must consider three possible causes. First, it may be a poorly fitting lens. A lens with a BOZR that is too flat may demonstrate excessive movement and an unstable and inconsistent lens orientation. A lens with a BOZR that is too steep may result in fluctuating vision as a result of the soft lens vaulting the cornea. Another possible reason may be the condition of the lens. Significant deposits or poor lens optics will invariably cause a decrease in vision. Finally, the practitioner must contemplate the possibility of pathology.
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Dr. Lindsay, a diplomate and fellow of the American Academy of Optometry, is in private practice in Melbourne, Australia, and is also a Senior Fellow of the Department of Optometry and Vision Sciences at the University of Melbourne.