Intraocular lenses

Also called an artificial lens, this intraocular lens is currently inserted in most cataract surgeries. Convex ( To ) Because the lens is removed, a replacement convex lens is needed, and before intraocular lenses were available, vision was corrected with thick glasses or contact lenses after surgery.

Since intraocular lenses are placed in the body for a long period of time, they must be made of a material that the body does not recognize as a foreign object. Aqueous Humor ( All ) It was thought that the density of the lens should be close to that of the water inside the eye. After going through various stages, foldable lenses, which are made of materials such as polymethyl methacrylate, the same material as hard contact lenses, and more recently acrylic, have become mainstream. By folding the lenses, the surgical incision size can be reduced by about half. Recently, the range of applications has expanded, and they are even inserted during cataract surgery for babies. Once inserted, intraocular lenses are not usually removed.

Intraocular lenses improve vision and are convenient because they do not need to be removed or inserted, but they cannot eliminate presbyopia. When we are young, our eyes can see clearly at various distances. This is because the lens is soft and can freely change its thickness and diameter, allowing us to adjust the focal length of the lens to the distance we want to see. As we age, a core gradually forms in the center of the lens. This core is hard, and as the core grows, the lens stops expanding. When it expands, we can see things up close, making it difficult to focus on nearby objects. This is presbyopia.

Since intraocular lenses are solid, their thickness does not change. This is the same as presbyopia. Intraocular lenses that are designed to focus at several distances have also been created, and future evolution is expected.

Source: Houken “Sixth Edition Family Medicine Encyclopedia” Information about the Sixth Edition Family Medicine Encyclopedia

　人工水晶体ともいわれ、現在、ほとんどの白内障手術でこの眼内レンズが挿入されています。手術で水晶体という凸(とつ)レンズを取ってしまうので、代わりの凸レンズが必要になり、眼内レンズができる前は、手術後は分厚い眼鏡やコンタクトレンズで視力矯正をしていました。

　眼内レンズは体のなかに長期間入れておくので、体が異物と認識しない材質で、かつ前房水(ぜんぼうすい)という眼内の水の比重に近いものが考えられてきました。いろいろな段階をへて、ハードコンタクトレンズと同じ素材のポリメチルメタクリレートや、最近ではアクリルなどの材質で、折りたたんで入れて眼内で開くフォールダブルレンズが主流となっています。折りたたむことによって、手術の傷口を約半分に縮小できました。最近は適応範囲も広がり、赤ちゃんの白内障手術時にも挿入しています。いったん入れた眼内レンズは、普通取り出すことはありません。

　眼内レンズは視力を改善し、出し入れの必要がなく便利なものですが、老眼をなくすことはできません。ヒトの眼は若いころはいろいろな距離のものが、どれもはっきり見えます。これは、水晶体が軟らかくて厚みや直径を自由に変えられるため、見たいと思う距離に水晶体の焦点距離を合わせられるからです。加齢に伴い、水晶体の中央に徐々に芯ができてきます。その芯の部分は硬く、芯が大きくなるにつれ、水晶体はふくらまなくなっていきます。ふくらむと近くが見えるので、近くにピントが合いにくくなります。これが老眼です。

　眼内レンズは固体なので、厚さは変わりません。このため老眼と同じ状態になります。いくつかの距離でピントが合うように工夫された眼内レンズもつくられており、今後の進化が期待されています。

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