1. Field of the Invention
The field of the invention relates to a method and apparatus for making fibers and for providing fibers useful as haptics for intraocular lenses.
2. Description of the Related Technology
The replacement of a natural lens with an artificial intraocular lens implant in the human eye has become a well known procedure to physicians specializing in ophthalmology. In the implant procedure, a corneo-scleral incision is made in the eye through which the natural lens is removed and the artificial intraocular lens is inserted. The intraocular lens may be designed to be positioned within either the anterior or posterior chamber of the eye.
Intraocular lenses typically include a central lens section, referred to as the optic, which focuses light onto the retina, and one or more supporting structures, called haptics which extend outward from the optic to align and stabilize the optic on the pupillary axis. Typically haptics comprise one or more filamentous arms or loops extending radially outwardly from the periphery of the optic. The haptics locate within the eye by engagement with predetermined ocular tissues.
Intraocular lenses are typically of two types: a three-piece lens where two haptics are mechanically fixed to the lens optic, and a one-piece lens where the haptics and optic are made as a single unit.
For both one-piece and three-piece lenses conventional practice is to construct the optic of a hard biocompatible polymer, such as polymethylmethacrylate (PMMA). More recently, innovations in optically clear elastomeric materials have allowed construction of lens which can be folded or rolled and inserted through very small incisions. Small incision surgery is thought to reduce trauma and the likelihood of complications.
Installation of an intraocular lens should be permanent so that subsequent surgical adjustments are not required. Accordingly, the reliability of the haptic is of great importance. The materials of construction and the design parameters must be selected such that the haptic can endure significant stresses with minimum risk of breakage. Moreover, the haptic must be capable of functioning safely in the presence of small stress risers, such as notches or nicks, which may be inflicted during manufacture and surgical handling and manipulation.
Unfortunately, haptics often develop clusters of fractures, referred to as craze, when subjected to impact and bending forces during handling. If the individual craze fractures are large enough to extend across a significant portion of the shaft diameter, a broken haptic results.
Therefore, it is critical that the haptic of the lens exhibits significant resistance to breakage during use. Although certain haptic materials such as polypropylene used in construction of a three-piece lens offer acceptable resistance to breakage, other conventionally used haptic materials such as PMMA are brittle and more frequently prone to breakage. This problem becomes especially acute when the haptics are lathe cut from a single lens blank to prepare a one-piece lens with integrally attached haptics. The problem of haptic breakage is a serious one, and efforts have been made to provide the haptics with increased breakage resistance.
Several thermoplastic forming techniques have been used to induce orientation in the material in order to impart enhanced mechanical properties in PMMA intraocular lens haptics. One such effort is described in U.S. Pat. No. 5,169,569. This patent describes preparing intraocular lenses with haptics exhibiting greater ductility and fatigue resistance. The PMMA sheet plastic is modified by means of blowing the sheet into a hemispherical bubble followed by forming into a flattened circular portion.
Another effort is disclosed in European Patent Application 0 438 043 A2. The PMMA material is modified by multi-axis stretching, to produce haptics with increased tensile strength, flexibility and resilience. A further example is described in U.S. Pat. No. 5,674,284 where a one-piece intraocular lens with fracture resistant haptics is disclosed.
Nevertheless, these and other conventional methods have not ideally solved the problem of creating a flexible yet durable intraocular lens. In general, the conventional one-piece lens is often too brittle and can break upon insertion. The three-piece lens, while in some cases offering more durability and flexibility, is generally comprised of materials that are prohibitive to efficient sterilization techniques.
It is necessary to sterilize each lens before insertion. Ethylene oxide is currently the most commonly used method of IOL sterilization. Environmental concerns with ethylene oxide release and concerns regarding removal of trace amounts of ethylene oxide and its ethylene glycol byproducts from the sterilized IOL and its package have caused the industry to seek other sterilization methods.
However, the conventional three-piece lens relies on materials which can be damaged during standard steam sterilization and dry heat sterilization techniques. The problem with both steam sterilization and dry heat sterilization methods has been inadequate thermomechanical stability of the formed haptic parts of the sterilized lens assembly. The haptics made of conventional materials relax and lose their heat set configuration during heat sterilization. This problem is particularly acute with elastomeric lenses. Lens manufacturers have resorted to so called xe2x80x9cdry transferxe2x80x9d sterilization methods for elastomerics wherein they sterilize the lens components separately, assemble the lens in a sterile field and transfer the assembly to a sterile saline solution-containing package. This method is expensive, potentially unsafe and is in general deplored by health authorities.
Accordingly, there is a need for an intraocular lens having haptics which are tough and flexible allowing for proper insertion and retention. Moreover, there is a need for an intraocular lens haptic filament which is not only tough and flexible but also thermomechanically stable to allow for safer and less costly sterilization techniques.
One aspect of the invention is directed to a method for manufacturing a fiber, the method comprising forming a fiber having a first level of orientation from an amorphous thermoplastic with a glass transition temperature above approximately 160xc2x0 C. and inducing a second level of orientation on the fiber, wherein the second level of orientation is of a higher degree of orientation than the first level of orientation. Forming of the fiber can occur in a controlled environment at a certain temperature and inducing the second level of orientation on the fiber can occur in a controlled environment at a second certain temperature. Specifically, the fiber can be formed from a fiber from a polymer selected from the group consisting of polyether sulfone (PES), polysulfone (PSU), polyetherimide (PEI), and polyphenylene sulfone (PPSU). The high glass transition temperature permits use of standard sterilization techniques, steam sterilization and dry heat sterilization, for the fiber. Consequently, the fiber is well suited for use as a haptic for an intraocular lens (IOL) which requires sterilization but also requires tough materials.
Forming of the fiber can include preparing a preform comprising the amorphous thermoplastic and melt drawing the preform to form the fiber in a controlled environment at a certain temperature. Alternatively, forming the fiber can comprise extruding the amorphous thermoplastic to form the fiber.
The preform can be prepared by casting the amorphous thermoplastic into a form of a cylindrical rod and machining the cylindrical rod to smooth the surface of the cylindrical rod. The casting can include casting the thermoplastic into a colored cylindrical rod and casting the thermoplastic into an outer tube, wherein the cylindrical rod is inserted into the outer tube to form the preform.
Inducing the second level of orientation on the fiber can comprise heating the fiber in a controlled environment at a certain temperature and tensioning the fiber. Once a second level of orientation has been induced on the fiber, the fiber can be cut into lengths suitable for use as a haptic and then the lengths can be formed into shapes suitable for use as a haptic.
Another aspect of the invention is directed to an intraocular lens which includes a lens body and a haptic attached to the lens body, wherein the haptic is formed from a fiber comprising the thermoplastic described above. The lens body of the intraocular lens can be foldable, hydrophilic, or hydrophobic.
Another aspect of the invention is directed to a system for manufacturing the fiber which can be used as a haptic in an IOL. The system can comprise a preform drawing machine configured to form the fiber from a preform comprising the thermoplastic and a tensioning device configured to induce a greater level of orientation on the fiber.