Breathing in the fresh ocean air of Little Bay, on Western Australia’s Coral Coast, inspired thoughts of gas permeable contact lens materials…
As a child I used to help my Dad polish and adjust polymethyl methacrylate (PMMA) hard lenses as well as help him to mould, cast and press PMMA haptic lenses.
Years later, I started my contact lens career at the beginning of the Rigid Gas Permeable (RGP) revolution. That was back in the late 1970s and early 80s, and until then, PMMA, also known as Perspex, had been the only option. To overcome the lack of oxygen permeability of PMMA, the ‘Hard Lens Pioneers’ used small diameter hard lenses, small optic zones and the tear pump to try and prevent hypoxia and oedema. In some cases amazing longterm results were achieved, while others developed refractive changes, gross corneal oedema and central corneal clouding. Ultimately, endothelial damage and corneal exhaustion syndrome resulted.
Fortunately, modern practitioners fitting the latest generation RGP materials and designs don’t experience these complications. Modern scleral lenses, on the other hand, are still limited by oxygen supply as there’s little tear mixing, so the tear pump is no help. To try and address these issues, some 200Dk (and greater) RGP lens materials are in development, which may help.
…the best material I’ve used over the past two decades, has been Boston XO (100Dk)
I’ve watched the evolution of gas permeable materials and polymer science and technology with fascination and I’ve been fortunate to do some FDA protocol studies on materials, solutions and innovations for both daily and extended wear designs. There have been some interesting ones along the way; diffraction multifocals, early hybrids, extended wear and even photochromic materials.
There were also some challenges with unstable materials, deposits, crazing, wetting and ‘allergy’ issues.
Early materials like Polycon had a Dk of around 10 units, which along with the tear pump led to much better outcomes. It was a breath of fresh air for suffering corneas.
After the early materials, with Dk of 8–12 units, we moved onto things like Boston II and XL40 (Paraperm O2) at around 16 units, Boston IV (Dk 24) and many others.
Fluorosilicone acrylates were the ultimate winners, while the unstable cellulose acetate butyrate materials, fell by the wayside.
FluoroPerm 30 was a great success and it’s still used as a problem solver. Many other materials followed, which tested the 40–60Dk range. FluoroPerm HDS 50 was an interesting material, which we also still use. Hyperpurified delivery systems were the result of studying polymerisation in very low gravity, which Paragon achieved with a mini lab in the space shuttle. I thought that was a pretty cool innovation.
Contamac in the UK provide us with many successful polymers like Optimum Extra and Extreme in the 100–120Dk range, as well as good problem solvers in their Classic and Comfort materials. Other material manufacturers like GT Labs, Vista, Lambda and Lagado spring to mind.
By far my favourite, and what I regard as the best material I’ve used over the past two decades, has been Boston XO (100Dk). It has stood the test of time lasting, in a number of cases, 10–15 years, with great stability, in situ wetting and optics.
Boston XO came from Polymer Technology, which is now part of the B+L stable. XO evolved from a material which has a strong Australian connection – Quantum II – developed for research in Sydney at the CCLRU for Brien Holden’s team.
IF IT AIN’T BROKE…
For some patients, switching materials will be at your peril – it’s a case of ‘if it ain’t broke, don’t fix it’. Some patients with older lenses had, over the years, tweaked the edges, fit and optics by repeated cleaning with abrasive cleaners. As a consequence, when fitted with a new, ‘perfect’ lens, they’d often complain that their near vision had ‘gone to pot’.
During my practising career, I left many successful patients in the same designs and materials and simply serviced them. I’d usually only switch to ‘newer, better’ materials if I noted any issues.
I’ll delve more into this topic in coming months, as there have been some recent developments.
