Development of slotted scleral lenses

Abstract

Brief Historical Introduction Advances in contact lens designs have largely provided tolerance levels greater than those achieved with the use of scleral lenses. However, scleral lenses remain the primary choice for a significant group of contact lens wearers. As most of this group consists of patients who are among the most dependent on contact lens wear, it is regrettable that so little energy has been directed to,yards the development of scleral lenses in recent years. Production of scleral lenses, with ensuing modifications and after-care has always been a time consuming and costly affair. The great majority of contact lens practitioners do not wish to burden themselves and their patients with the production and fitting of such appliances. As a result, the task of continuing scleral lens practice has fallen largely to the hospital ophthalmic departments and a few specialist practices which are fully equipped to cater for the particular difficulties. There is little doubt that this is the correct direction to move, particularly in view of the fact that scleral contact lens wearers nearly always have some underlying ocular pathology. In order to look at possible improvements to scleral lens design, it is necessary to consider how designs have already developed. Sabell (1981) I gave a comprehensive discussion of the history of scleral lenses. For the purposes of this article, it is relevant to mention the contribution of a few individuals. Eugene Fick (1888) 2 is accredited with the first attempts to put into practice the theoretical corneal neutralisation with fluid expounded upon by Leonardo da Vinci and Rene Descartes centuries earlier. Fick realised the potential of contact lenses in aphakia and keratoconus, two conditions for which correction with scleral lenses is sometimes indicated. Fick also recognised one of the principal complications of contact lens wear, namely, corneal clouding due to hypoxia. August Mfiller (1889) 3 also produced lenses principally for the correction of myopia. The first thirty years of the 20th Century saw little real progress made, with glass lenses being produced by Miiller of Weisbaden and Zeiss of Jena, but few contact lens wearers were able to achieve any satisfactory long term use. At the start of the 1920's W. Stock (1920) 4 attempted to use a four lens set to correct keratoconus. To cover the variations of the eye more comprehensively, such preformed fitting sets were extended, and inevitably became more expensive and cumbersome. The improvement in the understanding of corneal topography in the 1930's led to the realisation fhat reduction of pressure on the cornea would greatly improve ocular tolerance. The knowledge of optical principles dictated that the space between the front surface of the cornea and the back surface of the lens should be continually fluid filled to prevent impairment of the visual function. Josef Dallos emerged as one of the principal contributors in this time. Working on the assumption that the lacrimal fluid offered a good chance of achieving these ends he set about to conserve tears and allow for flesh tear interchange. He recognised that the success of scleral tens wear depended not so much on the accuracy of the scleral fit but on the adequacy of the fluid movement between the lens and cornea. From this, the technique of fitting with minimum corneal clearance was developed. Over the years he accumulated thousands of casts, and latterly favoured the use of 'type shells', selecting a near fit from an existing cast rather than carrying out an impression procedure every time. PMMA was developed in the late 1930's and was preferred by many contact lens workers to glass (Ruben 1975) 5 . Orbrig (Jenkin and Tyler-Jones 1969) 6 produced the first PMMA contact lens in the mid 1930's, and