Special Types of Contact Lenses and Their Uses

Contact lenses for corneal observation

When carrying out specular reflection to examine the corneal endothelium using the slit-lamp biomicroscope, the bright reflection from the tear film is obtrusive. Haag-Streit's Eisner Contact Glass is a thick aplanatic plano-convex lens used for this purpose ( ). The plane face contacts the cornea using a solution such as Celluvisc (Allergan) and transfers the annoying reflex to the front of the lens, out of the field of view of the microscope ( Fig. 27.1a and b).

Gonioscopy contact lenses

Observation of the anterior chamber angle is made possible by the use of a lens incorporating mirrors that either partially or completely neutralise the power of the cornea. Most modern gonioscopy lenses are based on the designs introduced by , and . The Goldmann lens incorporates a mirror and is used in conjunction with a slit-lamp biomicroscope ( Fig. 27.2 ). The angle of the anterior chamber is seen by reflection.

To see the entire angle, the lens must be rotated on the eye and the slit beam rotated with it but perpendicular to it, a horizontal slit being used when the mirror is vertical and vice versa. Magnification is given by the biomicroscope. The need for all but small amounts of rotation of the lens has been overcome by the use of multiple mirrors, as in the Zeiss pyramid or four-mirror gonioscope ( Fig. 27.3 ); the fundus can also be viewed through one of these lenses (see below).

Contact lenses for examination of the fundus

Slit-lamp examination of the fundus can be carried out using a Volk lens, even if the cornea is irregular. A contact lens facilitates ophthalmoscopic examination of the media and fundus by removing the irregularities. Viewing the fundus binocularly with a slit-lamp allows a three-dimensional magnified view of the fundus. A contact lens with a flat front surface is used with a Volk lens to eliminate corneal refraction ( Fig. 27.4 ). The macula region of the fundus is observed directly with such a high negative lens, and mirrors are used to observe the mid-periphery and extreme periphery of the fundus. The fundus of a high myope is also more easily observed through a high-minus contact lens, which reduces the high magnification obtained in direct ophthalmoscopy.

Haag-Streit makes a lens for binocular slit-lamp observation of the pupil, the macula, its surroundings out to 30° as well as the central vitreous body sections. For examination of the anatomical conditions in the area at the front of the iris, the Haag-Streit 902 one-mirror glaucoma contact glass ( Fig. 27.5 ) with a narrow slit beam utilises the swivelling and adjustable illumination on the slit-lamp to assess the entire angle of the iris.

The Haag-Streit 905 two-mirror contact glass inclines at an angle of 62° making it possible to examine the opposite iridocorneal angle without turning the contact glass.

Other models of the Haag-Streit contact glasses allow examination of the ora serrata, the pars plana and the extra-peripheral parts of the vitreous body and the fundus.

Haag-Streit's original Goldmann paediatric three-mirror lens is available in two sizes: newborn, 906, and younger than 4 years of age, 907.

Contact lenses for laser surgery

Argon laser photocoagulation used for iridectomy and trabeculotomy, and neodymium:yttrium–aluminium–garnet (Nd:YAG) lasers used in capsulotomy and vitrectomy, must avoid laser damage to areas of the eye not being treated. Special contact lenses, which are antireflective coated to give maximum transmission of the low-energy beam, are used. These:

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  magnify the image seen through the slit-lamp biomicroscope, enabling accurate location of the laser beam focus

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  increase the cone angle of the beam ( Fig. 27.6 ), dissipating its energy, thereby avoiding tissue damage

  

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  form a pinpoint focus rather than a comatic blur.

Radiation treatment

For malignant tumours of the orbital region, a plastic-covered lead scleral shell can be used to protect the eye. Where the tumour is ocular, a partial lead shell can be specially constructed to cover the entire anterior eye except the part to be irradiated.

Biometry measurement

Axial lengths needed for biometry prior to cataract surgery can be carried out using an immersion method instead of an applanation method, in order to avoid indenting the cornea and affecting the axial length of the eye. A scleral shell is inserted into the eye of a supine patient; the most common varieties are the Prager shell or the Hansen shell. The shell is attached to a chamber which is filled with balanced salt solution (BSS), and the ultrasound probe is positioned within the chamber at the correct orientation (90°) to the eye but without coming into contact with the cornea.

Solar power

Google has filed a patent for an Iris Reading contact lens which will be operated by solar cells. The circuitry scans the eye to produce an iris ‘fingerprint’ which could allow access to their owner's privileges.

Medical Sensor Lenses

Self-monitoring, analysis and reporting technology (SMART) biometric data collection has been incorporated into contact lenses which are used in managing various eye conditions.

IOP measurement

The Triggerfish ^® lens from Sensimed measures intraocular pressure (IOP) over a 24-hour period ( Fig. 27.7 ). As IOP is highest before waking, it can only really be measured by a contact lens. The lens is made of silicone and is embedded with a micro-sensor that picks up any changes in lens curvature. When the curve changes, an electric signal is sent to a wireless antenna that is placed around the eye, and the antenna sends this information to a portable recorder worn by the patient ( ). This data can then be sent to the patient's doctor via Bluetooth. Researchers found that patients who had the highest peaks in lens curvature at night and who had an overall greater number of peaks in signal transfer experienced faster glaucoma progression. The Sensimed Triggerfish has already been approved for use in many European countries, including the UK, Switzerland, Italy and Austria as well as having FDA approval in the USA.

Another option undergoing development is the use of capacitive contact lens sensors which measure IOP, again from the curvature of the cornea, and are suited to low-force applications. These sensors consist of two independent layers: an outer reference layer and an inner sensing layer. The sensing layer can detect changes in the curvature of the lens (relative to the reference layer), which are related to changes in IOP. As the curvature of the lens changes, the resonance frequency of the inductor-capacitor circuit also changes, which can be measured and correlated to IOP ( ). The sensor is fabricated from medical-grade silicone using transfer moulding, and the electrodes and inductive coil are etched from copper foil.

The smart lens may be an effective tool in helping to identify which glaucoma patients are at greatest risk for disease progression, but in both the above systems, the thickness of the contact lens and the method used to collect the data can make the lens uncomfortable to wear. Other concerns are changes to the lens shape due to tightening of the lens during wear ( ).

Glucose-monitoring lenses

It has been known for many years that blood sugar levels could be measured from the tears, but it was extremely difficult to obtain the volume of tears required. The Google smart diabetic sensor lens, trialled in conjunction with Novartis, needs only a microscopic drop from the tiny aperture in the lens ( Fig. 27.8 ). The approach involves an enzymatic electrochemical reaction to generate an electric current whose magnitude is proportional to glucose concentration ( ).

Another group working on similar technology is Medella Health from the University of Waterloo (Canada) led by Harry Ghandi. The data are sent to a mobile device to alert both patient and caregiver to changes in glucose levels.

Slow-release drug lenses

Hydrophilic lenses have been used for many years in the application of several drugs ( ; see also Section 8 , History. available at: https://expertconsult.inkling.com/ ). Lenses were saturated in the drug for about 2 hours. Once the lens was inserted onto the eye, the drug was released into the conjunctival sac over the following 2 hours. This fast release was a problem, and newer slow-release molecular imprinted contact lenses can release the drug over several days. To improve comfort, a mixture of diverse comfort agents are included in the newer materials. Silicone hydrogel contact lenses have been engineered to simultaneously release up to four template molecules including hydroxypropyl methylcellulose (HPMC), trehalose, ibuprofen and prednisolone ( ). By adjusting the ratio of functional monomer to comfort molecule, a high level of control has been demonstrated over the release rate of the drugs. Daily disposable lenses can be used for single-day release or extended-wear lenses for multiple-day release. The molecular imprinted lenses are therefore expected to lead to higher efficacy for patients, compared with topical eye drops, by improving compliance and mitigating different concentration peaks from multiple drop use. However, the controlled delivery of ophthalmic drugs will need further exploration of the impact of different lens materials on the release characteristics ( ).

considered the feasibility of slow release of atropine from silicone hydrogel lenses to control myopia progression and found that adequate drug concentration could be maintained in the tear film.