Hydroxyapatite Formation on Implanted Hydrogel Intraocular Lenses

Three Chinese patients undergoing implantation with the same hydrogel intraocular lens (IOL)(Hydroview; Bausch & Lomb Surgical, Claremont, Calif) developed delayed IOL opacification with unusual clinical features. The IOLs were made from a 2-hydroxyethylmethacrylate/6-hydroxyhexylmethacrylate (HEMA/HOHEXMA) copolymer. Opacification was progressive, whitish, and generalized, and developed 4 to 15 months after surgery. The appearance resembled a mature cataract. All 3 patients showed a significant reduction in vision. There was no response to Nd:YAG laser treatment, and all 3 IOLs had to be explanted. The explanted lenses were analyzed with electron microscopy, elemental analysis, Fourier Transform (FT) Raman spectroscopy, and x-ray diffraction to define the nature of the opaque material. Results showed electron-dense deposits in the superficial 5 µm of the lenses. The deposits were predominantly composed of calcium and phosphorus, and x-ray diffraction identified the presence of hydroxyapatite. The polymer structure of the lens was unaltered.

A 67-year-old man underwent phacoemulsification on his left eye and IOL implantation in July 1998. He had diabetes, was taking insulin, and suffered from ischemic heart disease, hypertension, and renal dysfunction. Best-corrected visual acuity improved from 20/200 before the operation to 20/70 three months afterward. Opacification of the implant developed 4 months after surgery (Figure 1). Visual acuity dropped gradually until the patient could perceive only hand motions 6 months postoperatively. The IOL was explanted in July 1999.

A 72-year-old woman underwent phacoemulsification on her right eye and IOL implantation in June 1998. She had diabetes, was taking insulin, and had had macular grid laser treatment for diabetic macular edema. Her best-corrected visual acuity went from 20/400 before surgery to 20/100 three months afterward. Opacification of the IOL developed 9 months after surgery, and best-corrected visual acuity dropped to 20/200. An IOL exchange was done in August 1999.

A 79-year-old man underwent phacoemulsification on his right eye and IOL implantation in June 1998. He was in good overall health. His best-corrected visual acuity improved from 20/200 before surgery to 20/50 three months afterward. Intraocular lens opacification developed 15 months after surgery, and his visual acuity became perception of hand motions at 17 months. An IOL exchange was done in December 1999.

All 3 patients were given local anesthesia and underwent clear corneal phacoemulsification. Sodium hyaluronate/chondroitin sulfate sodium (Viscoat; Alcon Surgical, Fort Worth, Tex) and balanced salt solution (BSS Plus; Alcon Surgical) with added adrenaline (0.1 mL of 1:1000 adrenaline in 500 mL BSS Plus) were used intraoperatively. Acetylcholine chloride (Ciba Vision Ophthalmics, Atlanta, Ga) was used for patients 2 and 3. No surgical complications were noted. For 1 month following surgery, all patients received 1% dexamethasone sodium phosphate and 1% neomycin sulfate eyedrops. We used the same intraoperative agents to implant about 500 IOLs from 1997 to 1999. Most patients were not affected by IOL opacification.

Each explanted lens was submitted to the pathology laboratory in a dry, sterile container without fixatives. All 3 lenses underwent the same tests. They were examined under both a dissecting microscope and a light microscope. Each lens was then cut into several pieces with a razor blade and subjected to transmission electron microscopy (TEM), scanning electron microscopy (SEM), elemental analysis, FT Raman spectroscopy, and x-ray diffraction. Several unused IOLs served as controls.

For TEM analysis, the lens was cut into 1-mm³ blocks and fixed in a solution of 1.5% cacodylate-buffered glutaraldehyde, postfixed in a solution of osmium tetroxide, dehydrated in a series of ethanol, and embedded in a resin block (Polybed 812; Polysciences Inc, Warrington, Pa). Ultrathin sections were stained with uranyl acetate–lead citrate and examined with TEM (Philips CM100; Philips Electron Optics, Eindhoven, the Netherlands) at an accelerating voltage of 80 kV.

For SEM analysis, 1-mm³ blocks of lens material were sampled and fixed in a solution of glutaraldehyde, postfixed in a solution of osmium tetroxide, dehydrated in a series of ethanol, and dried with carbon dioxide in a critical point dryer (Bal-Tec CPD 030; Bal-Tec AG, Liechtenstein). They were mounted on aluminium stubs and coated with gold and palladium in a sputter coater (Bal-Tec SCD 005; Bal-Tec AG). The specimens were examined with SEM (Leica Stereoscan 360; Leica Instruments Ltd, Cambridge, England).

For elemental analysis, 1-mm³ blocks of lens were air-dried and analyzed with an energy-dispersive spectrometer (Link eXL system; Link Analytical Ltd, Bucks, England) attached to the Leica Stereoscan 360.

For FT Raman spectroscopy, the sample was studied with an FT Raman spectrometer (FT-Raman; Bio Rad, Hercules, Calif), and the Nd:YAG laser (1064 nm) was used as the excitation laser source.

For powder x-ray diffraction, the test was performed with an x-ray diffractometer (Siemens D5000; Siemens, Munich, Germany) using CuKα radiation (wavelength = 1.5406 Å), and copper was used as the generating source.

Test results were similar for all 3 lenses. The lenses were semi-opaque on gross examination. Under the dissecting and light microscopes, both the anterior and posterior surfaces were reticulated, similar to a piece of dried leather.

With TEM analysis, the explanted lenses showed an undulating surface. The distance between the top of the elevated area and the bottom of the depressed area was approximately 10 µm. Electron-dense aggregates, which extended to a depth of 5 µm, were present along the superficial part of the lenses. The size of these aggregates varied, with the largest measuring approximately 1 µm. They were larger and more numerous near the surface (Figure 2). The aggregates contained needle-shaped crystalline deposits, which contrasted with the smooth surface and absence of deposits in the control IOL.

SEM analysis revealed that the explanted lens surfaces were irregular and had a "cerebriform" appearance, with convolutes of elevated areas alternating with crevices. This 3-dimensional architecture corresponded to the 2-dimensional appearance with TEM analysis (Figure 3). The control IOL revealed a smooth surface.

Elemental analysis showed calcium, phosphorus, oxygen, carbon, and trace amounts of sodium and magnesium present on the explanted lenses (Figure 4). Only carbon and oxygen were found on the control IOL.

The explanted lenses and the control IOL showed essentially the same features with FT Raman spectroscopy, indicating that they were the same type of polymer.

With x-ray diffraction, the explanted lenses showed a diffused halo at approximately 2THETAS = 16°-24° (where 2THETAS describes the range of angle between the incident x-ray and the test sample), which was due to the amorphous polymer matrix. The peaks at 2THETAS = 25.9°, 31.9°, 32.1°, and 33.1° (Figure 5) matched the major diffraction peaks for a typical hydroxyapatite sample.¹ These peaks were absent on the control IOL.

Intraocular lens calcification has not been frequently reported. Olson et al² reported intraoperative calcification of the IOL surface. Bucher et al³ reported the formation of calcium hydroxyapatite on a polyHEMA IOL on the first postoperative day. The clinical features they reported were very different from the delayed calcification observed in our patients.

Bucher et al³ attributed IOL calcification to the use of phosphate-containing solutions during surgery. We used balanced salt solution and viscoelastics, both of which contained calcium and/or phosphate. Following surgery, patients were given eyedrops that contained the steroid dexamethasone sodium phosphate. Materials used during surgery may explain early postoperative calcification, but this does not necessarily explain calcification several months afterward.

Normal aqueous humor contains calcium and phosphate.⁴ This continuous supply of minerals is a more convincing explanation for the delayed and progressive calcification in our patients. In addition, hydrogels are permeable to aqueous humor. The small amount of sodium and magnesium detected probably came from the aqueous carried by the explanted lenses. Carbon and oxygen were detected in both the explanted lenses and the control. Oxygen probably existed together with phosphorus in the form of hydroxyapatite. The presence of carbon may not be abnormal; more investigation is necessary to clarify its presence in the deposit.

The bulk of deposits were found just beneath the IOL surfaces. The physical presence of the deposits led to expansion of the superficial layers and subsequent surface folding, which produced the cerebriform appearance. Hydrogels make up a large family of polymers, and certain hydrogels are very active in promoting calcification. Winter and Simpson⁵ reported the calcification of synthetic polyHEMA sponges implanted into young pigs. The calcified IOL reported by Bucher et al³ was also made from polyHEMA. We believe that the calcium affinity of the HEMA/HOHEXMA copolymer is comparable with that of polyHEMA and is an important factor leading to IOL calcification.

Most of the IOLs we used remained transparent after implantation. Although we believe that the calcium affinity of the polymer is responsible for calcification, this cannot explain its occurrence in only a handful of patients. Multiple factors might be involved, such as irregularities in IOL manufacturing, interaction with intraoperative materials, and patient factors such as race and diabetes mellitus. More investigation is necessary to determine the mechanism of calcification and the factors involved.

In conclusion, we observed delayed opacification of 3 IOLs that necessitated explantation 12 to 18 months after implantation. The opacification was caused by compounds containing calcium and phosphorus, specifically hydroxyapatite. Irregularly shaped deposits formed beneath the surfaces of the lenses, giving the surfaces a cerebriform irregularity. The calcium and phosphorus in the deposits were probably derived from the patients' aqueous humor. We believe that the affinity of the hydrogel copolymer to calcium was responsible for the dystrophic calcification in these 3 cases. Why this kind of opacification occurs in only a handful of patients remains unexplained, and further work is necessary to define the multiple factors that may be involved.