Cataract SURGERY

Understanding How HOAs Affect Quality of Vision

The more you know about the relationship between aberrations and vision quality, the better your outcomes and patient satisfaction will be.

By Steven C. Schallhorn, M.D.

In this era of high-technology IOLs and wavefront-guided LASIK, understanding how higher-order aberrations (HOAs) affect vision is a key factor in providing the best patient care. While the surgical tools and techniques are available to ophthalmic surgery training programs, this clinical perspective is sometimes lost in residency and real-world private practice.
The real issue is determining how much HOA we should target to give our patients the best possible vision. It isn’t an easy question to answer. For instance, the Stiles-Crawford effect in the retina may mitigate against the deleterious visual effects of aberrations in the peripheral cornea. In addition, there can be coupling effects on vision between lower- and higher-order aberrations.
My colleagues and I have studied how HOAs are related to visual quality for many years. We have analyzed visual models, evaluated the effect of aberrations on vision in LASIK patients and correlated aberrations in Navy pilots to their “supernormal” vision. Based on this research, I have concluded that the fewer the aberrations, the better the visual quality. Understanding this phenomenon will help you achieve better outcomes, which will lead to happier, more satisfied patients.

Investigating Aberrations
Optical modeling simulates the visual effect of aberrations. These models clearly demonstrate that the best image quality is obtained with the least amount of aberrations (Figure 1). So to achieve best vision using optical modeling, the target is zero aberrations.
Studies of aberrations in LASIK patients also reveal a relationship between the degree of HOAs and vision quality. My colleagues and I chose conventional LASIK patients for our analysis because it’s well known that standard LASIK can significantly induce HOAs. By looking at LASIK patients who had an increase in HOAs postoperatively, we were able to gather information about how those aberrations affected vision quality.
We analyzed 150 subjects who underwent conventional LASIK. At 1 month post-op, 90% achieved uncorrected visual acuity (UCVA) of 20/20 or better. How-ever, analysis of their HOAs showed that the lower the level of spherical aberration, or the lower the amount of surgically induced spherical aberration, the less likely they were to complain about halos around lights at night (Figure 2). Again, this supports the theory that lower levels of aberrations translate into better vision quality.
Cmdr. David Tanzer, M.D., and I also evaluated the vision of 140 Navy pilots. The pilots have what is typically considered ‘supernormal’ vision. We compared the results of various vision tests to those of a sample of 228 normal clinic patients who sought refractive surgery. The tests included standard visual acuity as well as contrast
sensitivity using a contrast acuity chart in room light (photopic) and low light (mesopic) conditions. A
6-mm WaveScan was obtained on all eyes. The pilots had average UCVA of 20/12, while the average best-corrected visual acuity of the clinic patients was 20/16. The pilots also had better average photopic and mesopic contrast sensitivity than the clinic patients.
Next, we addressed how the pilots’ HOAs were related to their supernormal vision. Careful analysis showed no relationship between high contrast visual acuity and HOAs. However, pilots with lower levels of HOAs — the average amount of spherical aberration was 0.08 µm — were more likely to have better photopic and mesopic contrast sensitivity. So even in pilots with supernormal vision, the lower the HOAs, the better the quality of vision.

Making the IOL Connection
To apply this information to everyday clinical practice, consider that several aspheric IOLs are de-signed to negate the natural spherical aberration of the cornea. Further extrapolation illustrates that the type of IOL can impact the quality of vision. Three aspheric IOLs are currently available in the United States: Tecnis (AMO), AcrySof IQ (Alcon) and SofPort (Bausch & Lomb).
The Tecnis provides –0.27 µm of spherical aberration. The AcrySof IQ provides –0.17 µm of spherical aberration and the SofPort is spherical aberration neutral (in contrast to conventional spherical lenses, which have positive spherical aberration).
Although aspheric IOLs are relatively new, we use aspheric lenses for other applications that require the best optical quality — the indirect ophthalmoscope and high-quality imaging equipment, for example. It only makes sense to consider aspheric IOLs to minimize spherical aberration and provide our patients with the best vision possible.
These newer generation IOLs are akin to wavefront-guided LASIK. Both can result in significantly less postoperative spherical aberrations than a standard treatment. Once you understand the correlation with aspheric IOLs, you can begin to use them effectively in your practice.

Aspheric IOLs in Practice
In my opinion, a conventional spherical IOL is the best choice for eyes that have a significant amount of negative spherical aberration on the cornea, but that represents just 2% of the population. As it turns out, most patients are best suited for the Tecnis because most human corneas have a significant amount of positive spherical aberration, which can be counteracted by an IOL with the appropriate amount of negative spherical aberration. About 25% of the population, or those with +0.08 µm to +0.22 µm of corneal spherical aberration, would benefit most from the AcrySof IQ aspheric IOL. Approximately 7% of the population have no corneal spherical aberration and would benefit from a spherical-neutral IOL.
Taking all of this into account, when aiming for zero spherical aberration, we have two options. We can use a special corneal topography device to measure the corneal spherical aberration in every cataract patient and then choose the IOL that most closely counters the corneal spherical aberration. Or we can implant the aspheric lens that negates spherical aberration in most patients. Obviously, given that we do not commonly measure corneal HOAs in clinical practice, option two is easier and more practical. The average cataract patient has
+0.27 µm of corneal spherical aberration, and the Tecnis provides –0.27 µm of spherical aberration, which provides an appropriate balance. It makes sense to use the IOL that will help the most people.

Hit Your Target
It’s up to the newest generation of ophthalmic surgeons to recognize the potential of the new generation of IOLs. By using aspheric IOLs to target zero aberrations, I believe you can achieve better visual outcomes and more satisfied patients. qnMD