The correction of both astigmatism and presbyopia with a single contact lens represents a significant achievement in optical engineering. Astigmatism is a common vision condition where the cornea has an irregular curvature, causing light to focus unevenly on the retina. Presbyopia, often beginning around age 40, is the natural, age-related loss of the eye’s ability to focus on nearby objects due to the stiffening of the lens. Combining the stabilization features of a toric lens with the multi-power optics of a multifocal design presents a complex technical challenge. Lenses that successfully manage this dual correction demand high precision in both material science and physical design to deliver clear vision at all distances.
The Mechanics of Dual Correction
These specialized contact lenses must solve two distinct optical problems simultaneously. The astigmatism correction necessitates a toric design that maintains a precise orientation on the eye to align with the irregular corneal shape. This essential rotational stability is often achieved through mechanisms like prism ballast, which incorporates a thicker zone at the bottom of the lens to interact with the lower eyelid and gravity, or by utilizing thin zones that allow the eyelids to actively center the lens during a blink. If the lens were to rotate even slightly, the astigmatic correction would be misaligned, resulting in blurry vision.
The presbyopia correction is managed through a multifocal design, typically a simultaneous vision system, which allows the eye to receive multiple focal points at once. The two primary simultaneous vision designs are concentric and aspheric.
Concentric Designs
Concentric designs feature alternating rings of distance and near power, which can be center-distance or center-near, depending on the patient’s needs.
Aspheric Designs
Aspheric designs use a gradual, smooth change in power across the lens surface, often transitioning from distance power in the center to near power in the periphery, or vice versa. Both designs present the brain with simultaneous images for distance and near. The visual system learns to subconsciously filter out the slightly blurred image, selecting the clearest focus for the task at hand.
The combination of a highly stable toric mechanism and a multi-zone optical design allows the lens to correct both conditions without compromising vision.
Available Wear Schedules and Replacement Options
Available wear schedules impact hygiene, comfort, and cost. Daily disposable multifocal toric lenses offer the highest convenience and hygiene because a fresh, sterile lens is used every day, eliminating the need for cleaning solutions. While daily disposables typically carry the highest per-lens cost, they can reduce issues related to deposit buildup and long-term lens care discomfort.
Two-week or monthly replacement lenses provide a more cost-effective option for many wearers. These reusable lenses require diligent daily cleaning and disinfection. Over the extended wear period, there is a greater potential for protein and lipid deposits to accumulate on the lens surface, which can lead to reduced comfort or compromised vision over time. The choice between these schedules balances the desire for lower maintenance and superior hygiene against the overall annual cost of the lenses.
Material Choices for Comfort and Stability
The material from which these complex lenses are made influences their performance, concerning oxygen transmission, comfort, and on-eye stability. The two main material categories are hydrogel and silicone hydrogel (SiHy).
Hydrogel lenses, the older technology, are soft, pliable, and rely on water content to transmit oxygen to the cornea. While hydrogels offer good surface wettability, their oxygen permeability is tied to their water content, which can be a limitation for extended wear or for higher prescriptions where the lens is thicker.
Silicone hydrogel materials incorporate silicone to allow much higher oxygen permeability, irrespective of the lens’s water content. This high oxygen transmission is beneficial for long-term eye health and extended wearing times. SiHy lenses also tend to be slightly more rigid, which is advantageous for maintaining the complex toric axis alignment. This increased stiffness enhances the lens’s on-eye stability, ensuring consistently clear vision.
Critical Steps in the Fitting Process
Achieving a successful outcome with multifocal toric lenses requires a meticulous fitting process guided by an eye care professional. Fitting these dual-correction designs requires multiple precise measurements. The process begins with a trial of diagnostic lenses, as the rotational stability needed for the toric component must be assessed directly on the eye. The practitioner observes the lens’s settling and rotation after a blink, making adjustments to the prescribed axis based on how the lens naturally positions itself on the cornea.
Determining the patient’s ocular dominance—the eye the brain prefers for distance vision—is important. The fitting strategy often involves a subtle bias, prioritizing distance vision in the dominant eye and slightly favoring near vision in the non-dominant eye. This customized approach, sometimes referred to as modified monovision, helps the brain adapt to the simultaneous vision design.
The final refinement involves a precise over-refraction performed with the trial lenses on the eye, using handheld lenses rather than a traditional phoropter. The practitioner makes minor power adjustments, typically in 0.25 diopter increments, to optimize distance or near vision in each eye until the patient achieves the best possible clarity binocularly. This systematic trial-and-error process, combined with a follow-up appointment, ensures the complex optics of the lens translate into satisfactory real-world vision.