Bausch+Lomb held two roadshows recently to teach practitioners how to fit their new acquisition, the Maxim CV lens. The first workshop was in Bradford and the second at City University, the one I attended. The Maxim CV lens, a semi-scleral in size, is from a US-based company called ACCU Lens and their CEO and president, William Masler OD, arrived to lecture and run a workshop ably helped by B+L sponsored personnel.
Maxim CV
The lens is a custom scleral with a multi-curve/aspheric posterior surface produced in plasma-treated Boston XO2 material (Dk 141). The design is targeted at distorted corneas, in particular those caused by keratoconus. Due to lack of movement and the resting of the lens upon the conjunctiva, the main benefit to a patient is comfort.
The buzz words relating to fitting seem to be ‘coverage and clearance’; the coverage being the coverage of the limbus and beyond by about 2mm, while the clearance is the vaulting of the cornea. The lenses are available in diameters of 15.40mm to 18.70mm. Interestingly, pre-fitting measurements such as keratometry are not necessary for the initial lens selection. If you have not got access to a topographer, then a standard starting point of 7.70/16.50 is suggested for normal corneas, and 7.50/16.50 for the mildly keratoconic eye. It was suggested you can look for Munson’s sign (the forward extension of the lower lid on down gaze caused by a forward bulging cornea) to decide to go steeper. The fitting is on sag rather than back optic zone radius (BOZR), so the parameters of particular importance are the sag value and optic zone diameter. In fact the BOZRs are unusual figures, obviously based on the conversion from dioptres in US to millimetres in UK.
Evaluation of fit
The goal is to find the minimum sag value that vaults the cornea with little (or preferably no) apical bearing. On insertion one ideally wants 250 microns of clearance, as then about 100 microns once the lens has settled back is assured. Apparently the 250µm value can be judged by the optic section of the 0.25mm lens thickness compared with the fluorescein tear lens thickness seen. Always err on the thick side. The idea is to get the corneal clearance first and then look at the peripheral curve. Both the optic zone and the total diameter can also be altered, the latter to make sure the limbus is covered; if there is a need to go smaller, then the periphery needs flattening. It is best to allow 30 minutes settling time and estimate whether there is any touch. If so, increase the sag. In the case of limbal touch, increase the diameter. If vessels are compressed, increase the edge clearance, and if there is stand-off, then reduce that edge clearance.
Concepts of acceptability are interesting. Nasal bearing is acceptable if it disappears when the patient looks down. One ‘rule of thumb’ states that 1mm of bearing area needs an increase of 0.1 in sag. I think that judging bearing and then correcting it is much easier than assessing if there is too much clearance and then how much to drop the sag. Also a change in the periphery changes the sag, so the diameter is altered to keep the ideal sag – clinical equivalents come to mind.
Collection and handling
The lenses arrive dry. The chosen lens needs to be cleaned, rinsed with saline and rubbed with a multipurpose solution (MPS). It is then filled with saline to just near overflowing, with an added drop of fluorescein. The insertion is either with a large suction cup or with the lens balanced between two fingers. The patient’s head is horizontal and the eye central to the lids. On contact with the cornea there is an extra push needed to make sure no bubbles are produced, as otherwise the process will have to be repeated. For removal, a thin suction holder is used which is wetted with MPS. The practitioner should never attach it to the centre of the lens and pull to remove, but should ask the person to look temporally and attach the suction to the nasal edge to lift it out.
Not surprisingly, one would assume that there is minimum tear exchange, but apparently that is not a problem. However, if debris does collect, for example related to meibomian gland dysfunction, then the person may need to refresh the lens during the day. However, removal by the patient after a day of wearing could be difficult, as the lens will have settled snugly back on to the cornea. Lubrication will be needed, with some physical loosening of the lens before attempting to either lift it off with the two lids or attach an MPS-wetted thin suction to the inferior portion of the lens and lift it away from that point.
Interesting additional design features include a centre-near multifocal, a front-surface toric, a quadrant-specific design for high astigmatism and a notched version to fit around a pingueculum. The design is obviously good for sporting pursuits. There will be a comprehensive practitioner fitting guide and patient leaflet produced in the UK (which will definitely be needed).
Judith Morris is senior lecturer in Contact Lens Education, City University and senior practitioner at the Institute of Optometry