It is possible to wear contact lenses that correct both the need for bifocals (presbyopia) and the presence of astigmatism, though this requires a specialized lens design and a precise fitting process. Astigmatism is a common condition where the cornea is shaped more like a football, causing light to focus unevenly and resulting in blurry or distorted vision. Presbyopia is the age-related loss of the eye’s ability to change focus for close-up tasks, making reading and near work difficult and necessitating bifocals.
Correcting Vision: Toric Lenses for Astigmatism
Standard spherical contact lenses provide uniform corrective power and cannot compensate for the eye’s irregular, non-spherical curve caused by astigmatism. Toric contact lenses are specifically engineered with a varying power profile to match the unique curvature of the astigmatic eye. This design requires different corrective powers along different axes, or meridians. The lens has a specific cylinder power at a precise angle, known as the axis, to properly focus light onto the retina.
To function correctly, the toric lens must remain stable and resist rotation on the eye, since even a small misalignment can compromise visual clarity. Manufacturers use sophisticated stabilization methods to ensure the lens maintains its alignment. Common designs include prism ballasting, where the lens is slightly thicker or weighted at the bottom to utilize gravity and eyelid interaction, or dual-thin zones. Other advanced stabilization technologies, such as blink-stabilized designs, use four distinct zones that interact with the eyelids to quickly realign the lens with every blink.
The rotational stability of the lens is paramount, as a misalignment of even 10 degrees can lead to a 30% loss in visual clarity. The lens must not only settle into the correct orientation initially but also quickly return to that position after a blink or eye movement. This on-eye stability allows the precise corrective power to remain aligned with the eye’s astigmatic axis, providing consistently clear vision. A precise fit is necessary to avoid issues like excessive lens movement or rotation, which would result in fluctuating blur.
Correcting Vision: Multifocal Lenses for Presbyopia
Presbyopia correction in contact lenses focuses on providing clear vision across multiple distances, including near, intermediate, and far, without the distinct line found in traditional bifocal glasses. This is achieved using multifocal lenses, which incorporate different prescription powers across various zones of the lens. The most common approach is simultaneous vision, where the lens presents the eye with multiple focal points at once. The brain then learns to instinctively select the appropriate image, filtering out the blurred images from the other focal zones.
These simultaneous vision designs primarily use two optical structures: concentric and aspheric. Concentric designs feature a central zone for either distance or near vision, surrounded by alternating, ring-shaped zones of the other power. Aspheric designs use a gradual change in curvature across the lens surface to create a progressive power profile, offering a smooth transition between distance and near correction. Most modern soft multifocal lenses use a center-near design, placing the reading power directly over the pupil, especially for patients with a higher need for near correction.
The goal of multifocal contact lenses is to provide a continuous range of vision, which is a key advantage over traditional bifocals. Unlike glasses, which remain stationary, the contact lens moves slightly on the eye, meaning the brain must constantly adapt to the different power zones presented over the pupil. This requires a short period of neuro-adaptation, where the brain adjusts to the new visual information. While translating designs, which mimic traditional bifocals by shifting position when the wearer looks down, exist, they are less common in soft contact lenses.
The Advanced Solution: Toric Multifocal Contact Lenses
Toric multifocal contact lenses combine the precise rotational stability needed for astigmatism correction with the complex, multiple focal zones necessary for presbyopia. The challenge lies in ensuring the multifocal design’s optics remain centered over the pupil while the toric component maintains its critical alignment axis, both of which are influenced by the lens’s on-eye movement.
The design of a toric multifocal lens uses a multifaceted approach, such as a balanced progressive technology for the presbyopia correction, which is then overlaid onto a stable toric platform. One successful design, for example, utilizes an optimized toric lens geometry, which includes a wide ballast band and uniform thickness to quickly and consistently orient the lens. This design ensures that the cylinder axis for astigmatism correction remains fixed, regardless of the multiple powers contained within the lens.
The complexity of these lenses results in a wide range of parameters available to the eye care professional. Some manufacturers offer over 200,000 unique prescription combinations to correct nearly all astigmatic presbyopes. This level of customization is necessary because the lens must specify a spherical power, a cylinder power, a cylinder axis, and a near-vision add power, often with a choice of center-near or center-distance designs for each eye. Studies have shown high rates of success, with initial fitting success rates reaching 93% when using manufacturer-specified fitting guides.
The Necessary Fitting and Trial Process
Fitting toric multifocal lenses requires a systematic, multi-step process that is more involved than fitting standard contact lenses. It is important to work with an experienced eye care professional. The process often begins by determining the patient’s ocular dominance and calculating the necessary spherical and astigmatism corrections. Trial lenses are then used to assess the on-eye performance.
During the fitting appointment, the professional assesses how quickly and consistently the lens settles into the correct rotational position. The patient is instructed to wear the trial lenses for a period, often 10 to 15 minutes, allowing the lens to fully equilibrate on the eye before vision is checked. If the initial vision is not optimal, the professional performs an over-refraction using loose trial lenses, making small adjustments to the power of the distance or near correction in each eye.
This process often requires multiple appointments and potentially several different trial lenses from various brands or designs. Patients should have realistic expectations, understanding that the goal is to comfortably perform 80 to 90 percent of daily visual tasks without glasses. While the lenses aim for functional clarity at all distances, they may not achieve the perfect 20/20 clarity of single-vision correction for every distance. Due to the highly customized nature and the need for multiple trial lenses, the overall cost associated with the fitting and the lenses themselves tends to be higher than with standard contact lenses.