An Ophthalmic Success Story: The History of IOL Materials
Around 2000 BC, the ancient Greeks manufactured glass hemispheres that had properties to magnify text. Recent knowledge assumes, though, that the spheres were used as ornaments or jewelry. The first reports on treating cataract were found in Asia in 500 BC, where a needle pressed the cloudy lens into the vitreous to provide patients with some visual function. Surgical techniques for extracting the human lens continued to improve over time by means of incision size, use of phacoemulsification for lens extraction and hygienic standards. Now, cataract procedures are performed worldwide, and remarkably, two-thirds are performed in only six countries (ie, India, the United States, Japan, Germany, China and France).
The invention of an implantable, tolerable and technically feasible IOL was a revolution in cataract surgery. In 1795, the first lenses, made from glass Casaamata — named for the inventor — were used, but the IOL dropped in the eye due to its weight. The real beginning of usable IOL technology occurred during World War II. Sir Harold Ridley was an on-duty eye surgeon who cared for crashed pilots. He observed that small pieces of cupolas (ie, PMMA from crashed planes) that penetrated the eye during the crash did not cause an inflammatory reaction. The material was not rejected by the body and stayed inert in the eye.
Ridley recognized that this material (Figure 1) could be ideal for artificial lens implantation. After detailed research, Sir Harold Ridley implanted the first IOL on November 27, 1947, at Saint Thomas Hospital, in London. The IOL, Transpex I (Rayner, Brighton Hove, UK), was successfully implanted into a 45-year-old female's eye.2
Dr. Ridley's colleagues were highly distrustful and ignored his achievements, however, he continued to implant IOLs and presented good outcomes. Finally, in the 1970s, IOL implantation after cataract surgery was considered a standard procedure.
As a material for IOLs, PMMA (Figure 2) is the biomaterial with the longest scientific history. It is a reasonable material for rigid IOLs because of its (1) high-quality hydrophobic surface, (2) excellent transmissibility of light and (3) possibility to add an UV absorber to the monomers. Ridley implanted the original PMMA lens in 750 patients by 1959.3
PMMA is a light polymer with an approximate specific weight of 1.19 and a refractive index of approximately 1.49. It is a rigid material at room temperature (ie, temperatures <100ºC). Originally, the incision size for IOL implantation had to be as large as the IOL. Surgeons also had to sew the incision, which could have caused undesired postoperative astigmatism. Using smaller IOLs is now possible because of flexible/foldable IOLs made of silicone or acrylic material.
PMMA is still a suitable material for single-piece and three-piece IOLs as well as IOL haptics. Due to scientific data, excellent biocompatibility and economical reasons, IOLs made from PMMA are still implanted most frequently in developing countries. In industrialized countries, the implantation rate is between 10% and 20%. The advantages of PMMA are (1) decades of experience and (2) inert material and surface, and disadvantages include (1) large incision size and (2) induced astigmatism.
The next milestone in IOL technology, the development of flexible/foldable IOLs, avoided the disadvantages associated with PMMA. On October 31, 1989, the first silicone IOL, which was suitable for mass production (Phacoflex SI-18; Advanced Medical Optics [AMO], Santa Ana, California) (Figure 3), made of polydimethylsiloxane, was implanted. Newer silicone materials include the addition of phenyl groups, which leads to a higher refractive index.
As a stabilizing agent, silicone material may as well contain silica. Silicone is a synthetic polymer constructed as an organic polysiloxane molecule. These molecules consist of periodically repeated silicone-oxygen groups. This structure is the backbone for a polymer, which is identical for all silicone IOLs. Bound to the silicone atom are side chains that have essential influence on the properties of the material. First-generation silicone materials (eg, polydimethylsiloxane) were characterized by methyl side chains. In modern silicone materials of second-generation (eg, SLM2, from polydimethyldiphenylsiloxan; AMO), the methyl side chains have been replaced by vinyl groups.
The advantage of silicone IOLs compared with rigid PMMA IOLs is the flexibility and foldability, allowing for small incisions. Extremely helpful tools are implantation devices like the AMO Unfolder. The sterile cartridge reduces the risk of an intraocular infection and does not enlarge the incision.
Besides using PMMA as an inflexible acrylic material, substituting side chains in the molecular structure of MMA leads to flexible materials that are also used for IOLs. Two material properties must be differentiated: Flexible acrylic material can be hydrophilic as well as hydrophobic (eg, Sensar or Tecnis, both AMO). In hydrophilic IOL materials, hydroxyethylethacrylate (HEMA) is used. This material is flexible due to its 38% water content.
While developing modern acrylic IOLs, the challenge was to transfer the positive properties of PMMA to a flexible material. Studies found that the two groups of acrylic material show a significant difference in the posterior capsule opacification (PCO) rate. For example, Duncker et al1 showed evidence of a higher PCO rate with hydrophilic IOLs versus hydrophobic or PMMA lenses. Opacification, caused by calcification of hydrophilic lenses, was also discussed in several scientific articles.2,3
A critical property of an acrylic material is the alteration of the polymer structure when it is folded at low temperatures. For describing this property, the glass transition temperature (Tg) is relevant. A high value for Tg means that the IOL must be warmed up to avoid damage to the IOL in an air-conditioned, cold operating room.
Today, millions of people benefit from Ridley's genius. IOLs are not only used in cataract surgery, but they have also become a routine method of refractive surgery. Despite the technological progress of IOLs, however, the technique of treating cataract by couching the lens and leaving the eye without an IOL is still daily practice in developing countries.
Sibylle Scholtz, PhD, is employed by AMO in an administrative function. She states that she is a paid consultant for AMO. Dr. Scholtz may be reached at sibylle.scholtz@gmx.de.
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