Friday, February 24, 2017

Benefits and Barriers of Accommodating Intraocular Lenses

In Chapter 14 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss vision. In particular, we analyze the refraction of light by the lens of the eye, and examine different disorders such as hyperopia, and myopia. We then write
This ability of the lens to change shape and provide additional converging power is called accommodation…. As we age, the accommodation of the eye decreases... A normal viewing distance of 25 cm or less requires 4 diopters or more of accommodation... This limit is usually reached in the early 40s. To make up for the lack of accommodation, one can place a converging lens in front of the eye when viewing nearby objects (reading glasses).
When a patient has a cataract, their lens becomes cloudy. A common surgical procedure is to remove the opaque lens and replace it with an artificial intraocular lens. A conventional IOL is designed to supply the correct power to provide clear distance vision, but it cannot accommodate. Reading glasses provide one option for close vision, but many patients find them to be an inconvenient nuisance.

Researchers are now racing to create accommodating IOLs. A recent review by Jay Pepose, Joshua Burke, and Mujtaba Qazi discusses the “Benefits and Barriers of Accommodating Intraocular Lenses” (Current Opinion in Ophthalmology, Volume 28, Pages 3–8, 2017).
Presbyopia [the loss of accommodation] and cataract development are changes that ubiquitously affect the aging population. Considerable effort has been made in the development of intraocular lenses (IOLs) that allow correction of presbyopia postoperatively. The purpose of this review is to examine the benefits and barriers of accommodating IOLs, with a focus on emerging technologies.
Apparently the current accommodating intraocular lenses don’t function by changing their focal length, but rather by being pushed forward when the eye muscles responsible for accommodation contract. They only provide about 1 diopter of accommodation, which is not enough to avoid reading glasses. The review concludes
Such limitations [of the presently available accommodating IOLs] may be circumvented in the future by accommodative design strategies that rely more on shape-related changes in the surfaces of the IOLs or in refractive index than by forward translation alone. Fibrosis and contraction of the capsular bag, which can alter the position of the IOL optic or the performance of an accommodating IOL represent other challenges, and at least one accommodating IOL … has been designed for implantation in the ciliary sulcus. Approval of accommodating IOLs capable of delivering three or more diopters of accommodation would allow a full range of intermediate and near vision without the compromise of photic phenomenon or loss of contrast inherent to other optical strategies, and perhaps also allow refractive targeting that could minimize hyperopic surprises by taking advantage of this expanded amplitude of accommodation.
Some “accommodating” IOLs are multifocal, providing two focal lengths and therefore two images simultaneously, one for distance vision and one for reading. The brain then sorts out the mess. Apparently this is not as difficult as is sounds.

I predict that accommodating intraocular lenses will soon become very sophisticated. Cataract surgery is performed on millions of people each year; it is common in the elderly population, which is growing dramatically as baby boomers age; in principle the problem is not complex, you just need to make a lens that can adjust its focal length by about ten percent; compared to other medical devices like pacemakers and defibrillators, accommodating IOLs should be cheap; and new nanotechnologies plus knowledge gained from the miniaturization of other medical devices may pave the way to rapid advances. Accommodating intraocular lenses may soon become an example of how to successfully apply physics to solve a problem in medicine.

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