Patent GB-685759-A (GB)
Title: Improvements in or relating to contact lenses and blanks therefor
IPC: G02C7/04
Filed: 1949-04-26
Abstract: 685,759. Contact lenses. ABBEY, A. (American Optical Co.). April 26, 1949, No. 11061/49. Class 97 (i). [Also in Group V] A plastic contact lens is formed from a lens blank the internal size of the scleral portion of which is larger than the exterior of a normal eye. The contact lens may have a corneal portion with concentric spherical or aspherical surfaces which merge smoothly into the corresponding surfaces of the scleral portion and may be formed of polystyrene, polycyclohexyl methacrylate or, preferably, polymethyl methacrylate. Such a lens may be formed by clamping marginally a disc of thermoplastic material of suitable diameter and thickness in a recess formed between two rings, one of which threads into the other. The clamp 27, Fig. 7, after the disc has been subjected to radiant heat for a short time to soften it, is positioned over the open end of a cylindrical. member 37 threaded into a fixed base 44. An internal shoulder 41 on the member and, if irequired, one or more shims 42, support a moulding block 38 having an aperture 40 therein communicating through a chamber 43 and a pipe 45 with a control valve 46. This valve is connected to a vacuum pump by a pipe 51, may communicate with the atmosphere through pipes 52 and 53 by operation of a handle 46<SP>1</SP> and is connected to a housing 48 by a pipe 50. Slidable in the housing 48 is a rod 47 which is co-axial with the aperture 40 in the moulding block 38 and terminates in a curved member 471 as of brass. Immediately after the clamp has been placed on member 37, the valve handle 46<SP>1</SP> is turned so that air is allowed to enter the upper part of the housing 48. The rod 47 thus descends and since it is at room temperature will cause the plastic material in that part of the disc 30 which is contacted by the end 47<SP>1</SP> to set. When such contact is made, the valve handle 46<SP>1</SP> is moved further to preclude the ingress of air into housing 48 and to connect the vacuum pump with chamber 43 so that the disc is sucked downwards, and the material surrounding the central part thereof stretched and takes up the shape of the inner surface of the block 38. When the plastic material has set, the clamp 27 is removed from the device of Fig. 7, and positioned between centring pins 82, Fig. 9, carried by a ring 80 supported by spacing rings 79 surrounding columns 75 on a base 76 having, in a central recess thereof, a block 78. This block is formed in its upper surface with a recess 78<SP>1</SP> of spherical or aspheric shape and the central portion of the plastic preform 54 carried by clamp 27 contacts this recess so that clamp 27 rests above the ring 80. When the clamp 27 has been correctly positioned, a plate 74 is placed on the columns 75 and is clamped thereon. The plate 74 has threaded therein a cylindrical member 67. Carried in the member 67 is a post 68 to which is secured by a set screw 73 a member 69 as of brass having a spherical surface 70 and held frictionally in a bore in member 69 is a button 71 as of glass having a spherical or aspherical surface 72 which projects slightly from the surface 70 of member 69. The member 67 is now rotated until the opposite surfaces of the central area of the plastic preform 54 are contacted respectively by the surface 72 of the button 70 and the recess 78<SP>1</SP> of the block 78. The assembly is thereafter placed in an oven and as the material of the preform 54 softens, it tends to return to its initial shape. The outer and inner surfaces of the central area take up the shape of the recess 78<SP>1</SP> and surface 72 respectively and the inner surface of the rest of the preform takes the shape of the spherical surface 71. Thereafter, the assembly is removed from the oven and is cooled rapidly to set the plastic material in its new shape. The clamp 27, when it is removed from the device, carries a lens blank having a corneal portion, the surfaces of which are such as to correct for a defect in the eye of a particular patient, and a scleral portion, the inner surface of which is suited to fit the largest sclera, there being no surface discontinuity between the corneal and scleral portions. It is now necessary to modify the shape of the scleral portion of the lens blank so that it will fit the eyeball of a patient for the eye of whom the corneal portion of the blank is suited. In order to do this, an impression of the patient's eye is taken and a model thereof has co-axial holes 118 and 120 drilled therethrough. This cast is positioned on a pin 122, Fig. 16, shaped to fit the holes 118 and 120. This pin also holds in place a thin metallic cap 128 which completely covers the corneal portion of the cast and which, if desired, may be surrounded by a small quantity of wax 131 filling the space between the edge of the cap and the surface of the cast. The pin 122 is carried by a plate 124, which is slidably mounted in a frame 126 by a threaded knob 132. Frame 126 may be connected at 136 to a vacuum pump and has set screws 134 for holding the clamp 27 carrying the lens blank 30, over the corneal portion of which is placed a heat insulating cap filled with wax. ' When lamp 27 has been positioned in the frame, the knob 132 is rotated to displace the plate 124 and the cast until the metal cap 128 contacts the corneal portion of the lens blank. The assembly is then positioned in an electrical heating device and is heated to soften the lens blank, the inner scleral surface of which tends to take the shape of the scleral surface of the cast 102. The assembly is cooled to room temperature and the lens blank is removed therefrom and from the clamp 27. The lens blank is thereafter cut along predetermined lines and the edge thereof is polished to provide a contact lens having a corneal portion suited to correct the defect in the eye of a particular patient and a scleral portion which will fit the eyeball of that patient. If, however, the power of the corneal portion is only a close approximation to that required, this lens is used as a trial lens and a plaster cast thereof is made. This cast is placed in an oven with the corneal portion thereof in contact with the corneal portion of a lens blank. The corneal portion of this lens blank is of the correct power and has a heat insulating cap filled with wax stuck thereto. The cast and blank are then heated for a time sufficient to allow the scleral part of the lens blank to soften and take up the shape of the scleral part of the cast. When the blank has cooled, it is removed and is then cut marginally and polished to provide the finished contact lens. Instead of forming the lens blank by stretching in the device of Fig. 7, a liquid monomer, a powdered polymer or a mixture of a liquid monomer and powdered polymer as of methyl methacrylate is placed or injected into a mould and a complementary mould is pressed into the cavity thereof, the assembly being heated to form the lens blank. The subsequent operations necessary to form the finished lens are then carried out as substantially before.