DEVICES

One topical issue is what happens to implanted devices after they have been in the body for a number of years. Recovery studies are not the norm, partly because of the difficulties of organising these systematically, also because of the ethical problems of dealing with cadaveric recovery. Last year, 3M withdrew a hip implant from the UK market because of unacceptable failure. Some commentators put this down to either the cement in use or to faulty techniques. A study recently carried out at the University of California San Francisco might vindicate the view that the cement was at fault, although the numbers involved are rather low. The PMMA cement from 22 surgically retrieved artificial hip joints was analysed and the chemical stability determined, judged by molecular weight compared with unimplanted control samples. Two brands of cement were involved, and both showed signs of breakdown, since there were 10-30% reductions in molecular weight over the maximum implantation periods, 16 and 23 years.

A new delivery system has been developed at the Georgia Institute of Technology, USA. The device is a patch only a few millimetres square, on which an array of very fine needles is mounted. Liquid drug can be pumped through the needles into the outermost layer of skin without pain. Reverse pumping allows tissue fluid to be extracted. One possible development is an insulin-delivery micro-needle patch allied to a blood glucose sensor for automatic detection of need for insulin. The invention has not yet been licensed.

A new implantable hearing aid is under clinical study at the University of Tübingen, Germany. A sound receiver is implanted in the wall of the ear canal, the battery and processor are subcutaneously implanted behind the ear and an electro-mechanical transducer is implanted in the mastoid and laser-welded to the incus bone of the middle ear. The different elements of the hearing aid are housed in titanium capsules. The battery is rechargeable and the hearing aid external elements are expected to last for up to 5 years. All 5 patients receiving the hearing aid have so far experienced improved hearing of 20-60%. The US company Symphonix Devices has also developed hearing aids that use internally-implanted sound processors, with or without externally-implanted processors to boost reception of higher-frequency sounds.

A device weighing less than 3.5 oz (95g) has successfully been used to support and improve prospective heart transplant patients in Europe, for 75 days in the longest case so far, until suitable donor hearts were found. The implantable heart pump, the DeBakey ventricular assist device, is positioned in the left ventricle and boosts blood flow to 10 litres/minute in cases with congestive heart failure. The manufacturer, MicroMed Technology of Texas USA, plans to start US trials later in 1999.

Rohm & Haas and the University of California, Berkeley have been collaborating on development of contact lenses based on a new type of hydrogel polymer, containing sulfoxides. The hydrogel improves fluid flow over the cornea, without encouraging protein deposition.

In the UK, ophthalmic surgeons have been investigating the long-term tolerance of implanted lenses (intraocular lenses or IOLs) made from PMMA, silicone and polyacrylate. Use of polyacrylate appears to reduce considerably the post-operative adhesions and other problems that subsequently require laser removal. According to the study report, 10-15% of cataract patients in USA require removal of the posterior capsule after initial corrective surgery. In the study of 90 eyes, this further laser surgery was required for 26% of the PMMA lenses, 14% of the silicone lenses and 0% of the PA lenses.

Polymers are also being used in implants in the surface of the cornea and correct mild to moderate long and short sight. The US company KeraVision has started studies of its Intacs implants in Germany. The curvature of the polymer implant can be altered to correct defects in the +1 to +3.5 and –1 to –5 dioptre range. More severe short sight can be corrected using intraocular lenses implanted just in front of the existing lens (anterior chamber phakic IOLs), fixed to the iris, or in the posterior chamber. The anterior PIOLs, developed by Morchir of Stuttgart and Bausch & Lomb, can be used to correct short sight of –15 to –24 dioptres. Clinical studies suggest that 90% of patients will experience complete correction of sight defects and the remaining 10% will be corrected to a level where simple spectacle lenses of –1 to –2 dioptre correction can be used. These lenses promise to be a very strong competitor to corneal refractive surgery. Cataracts accompanied by astigmatism can also be tackled using intraocular lenses developed by Staar Surgical Co of Monrovia, California USA. The Staar Toric lenses require precise alignment with the abnormal corneal curvature.

The UK-based technology firm Cambridge Consultants has developed a new hand-held arthroscope for minimally-invasive surgery in the knee and shoulder joints, for its client Gyrus Medical. The components were designed using CAD and prototypes were made using stereolithography and vacuum casting. Computerisation of the design and manufacturing process cut the usual development time drastically to about 3 months to toolmaking for final product.