Considering Positive Spherical Aberration

The ongoing evolution of IOLs that extend depth of focus offers a myriad of opportunities to enhance patient outcomes after cataract surgery. Navigating the nuances of lenses designed for this purpose, however, can spark healthy debates about the best strategy. While some believe IOLs with positive spherical aberration (SA) offer the most benefit without significantly compromising visual performance, others argue that IOLs with negative SA yield better quality vision for some patients (Table 1) (1-4). Ongoing dialogue aside, the modulation of rotationally symmetric wavefront aberrations – such as SA – has been shown to provide a wider range of vision than a single focal point (4-6). Additionally, evidence demonstrates that both positive and negative SA may increase depth of focus and produce favorable clinical outcomes (7-12). In my clinic, I may choose an IOL with positive SA for one patient and an IOL with negative or zero SA for another. My preference depends on which lens I feel will benefit a patient the most.

Understanding spherical aberration

Lower-order aberrations such as myopia, hyperopia, and regular astigmatism typically impact vision to a greater extent than higher-order aberrations such as SA. In patients who are candidates for premium IOLs, however, SA can play an important role in visual quality after surgery (13).

Negative SA occurs when peripheral rays of light cross the center of the cornea and converge on a point anterior to the peripheral rays. Conversely, positive SA occurs when peripheral rays of light focus in front of the central rays on the retina.

Unlike an aspheric IOL with negative SA optimized for a single focal point, an IOL with positive SA creates a broader focal zone. In my clinical experience, IOLs designed with positive SA strike a delicate balance between extending depth of focus and avoiding unwanted visual side effects. They provide a modest enhancement in the range of vision and maintain distance vision without creating the glare, halos, and contrast sensitivity loss often seen with some multifocal IOL designs (14, 15). While a positive SA IOL may not achieve the same range of vision as an extended depth of focus (EDOF) technology, it is an excellent choice for a large number of patients who desire a broadened focal point without significant blur and other visual side effects.

Potential candidates and patient satisfaction

IOLs with controlled positive SA are particularly well-suited for many patients, including those outlined here.

• Highly aberrated eyes. Some patients with a history of refractive surgery, especially myopic LASIK, may have a certain degree of induced positive spherical aberration, which may benefit from a lens with negative or zero spherical aberration. Patients with a history of hyperopic LASIK or PRK typically have induced negative spherical aberration, which may be balanced by a lens with positive spherical aberration or with no aberrations. These patients often have a significant degree of higher-order aberrations associated with loss of contrast sensitivity, so they may be particularly sensitive to photopic phenomena associated with multifocal or EDOF lenses.

• Small pupils. Positive SA IOLs are appropriate for a broad range of patients. Unlike some lenses with a small central optical zone, the RayOne EMV’s design does not rely on pupil size to optimize its performance (Figure 1). With that said, in my experience eyes with a small pupil usually achieve greater depth of focus with positive SA. Additionally, age-related miosis enhances depth of focus naturally, making positive SA IOLs even more effective in this group to increase the depth of field.

• A compromised ocular surface. Unlike some premium IOL models, lenses with positive SA are more forgiving of mild corneal irregularities. Patients may therefore maintain high-quality vision even in the presence of residual higher-order aberrations.

Patient satisfaction often hinges on managing their expectations. When explaining the benefits of an IOL with positive SA, I emphasize its ability to provide excellent distance vision with a practical improvement in intermediate vision, aiding in tasks like computer use, reading menus, and engaging in face-to-face conversations. I also explain that the IOL may avoid disruptive visual artifacts common with other premium lens designs (14, 15). I have found positive SA delivers a high level of predictability and safety, broadening the patient candidate pool compared to EDOF and multifocal lenses.

Addressing common misconceptions

Critics of IOLs with positive SA cite data suggesting that positive SA decreases near vision quality and contrast sensitivity (1). While laboratory studies using adaptive optics simulations may demonstrate these effects in tightly controlled settings, clinical outcomes paint a more detailed picture. In real-world conditions, many patients achieve a broader range of functional vision with positive SA without perceiving the slight reductions in contrast sensitivity that may appear in optical bench testing.

My clinical observations align with studies highlighting the functional advantages of positive SA. Research by Rocha and colleagues demonstrated that both positive and negative SA can extend depth of focus, though their effects differ (2). While some studies question the benefits of positive SA (1), I’ve found these lenses not only provide excellent distance vision but may also improve intermediate vision to a greater extent than negative SA and aspheric IOLs.

Overall, patient satisfaction has been high with positive SA IOL like the RayOne EMV and the SensAR40e (Johnson & Johnson Vision) (Table 1). Most of my patients report gaining additional range of vision, and the extension of the focal point is minimal enough that they don’t typically notice any blur or contrast loss like they may with an EDOF or multifocal IOL. With both multifocal lenses and full monovision technology, as the range of vision increases, so does the potential for optical side effects. I generally advise patients that this compromise is inherent to gaining additional freedom from glasses. If a patient is not willing to tolerate any degree of dysphotopsia, I find that advanced monofocal technology is preferable to extended range of focus lenses (EDOF and multifocal technology).

Results with other enhanced monofocal IOLs such as AcrySof Clareon (Alcon) and Eyhance (Johnson & Johnson Vision) are also promising for reducing the potential for dysphotopsias through the use of negative spherical aberration (16, 17). With all of these lenses, the depth of focus is increased in patients with smaller pupils, which is a well-known trend with increasing age.

Future refinements

The refinement of SA technology continues. Advances such as the Light Adjustable Lens (LAL; RxSight), which allows us to fine-tune the visual outcome postoperatively to meet individual patient needs, incorporate SA to increase depth of focus. Such innovations demonstrate the flexibility of SA in achieving tailored outcomes without compromising optical quality.

Compared to monovision, less myopic offset in the nondominant eye is needed to gain an increased range of vision with the LAL as compared with standard monofocal monovision. This helps patients achieve adequate binocular distance and near vision. The less anisometropia, the better it is for the summation of vision and maintenance of depth perception.

Conclusion

Positive SA is a patient-friendly approach to extending depth of focus. While not a panacea, lenses like the RayOne EMV offer meaningful advantages for a broad range of patients. As clinicians, we must continue to weigh the evidence and integrate our experiences to guide patients toward the best options for their unique needs.