Wear processes in polymer implants - Durability and Reliability of Medical Polymers - page 154

from highly crosslinked polyethylene and conventional UhmWpE. The
penetration rates were found to be less than 6 mm for the highly crosslinked
polyethylene and 72 mm for the conventional UhmWpE.
With single-piece silicone implants for the finger and wrist, the stems of
the implant are free to move and therefore silicone wear debris is generated
as the silicone abrades against the bone. These silicone wear particles can
cause an inflammatory response that results in pain, joint stiffness, loss of joint
motion and soft tissue swelling (Shepherd, 2002; Shepherd and Johnstone,
2002).

7.6 Future trends
The use of polymers in joint replacement implants is unlikely to change in
the near future. There are many concerns over the generation of polymer
wear debris in the body, such as osteolysis, leading to alternative bearing
combinations being sought. however, even 40 years after the take off of hip
replacement, polymers are still used in the majority of designs. Crosslinked
polyethylene was used as a replacement for UhmWpE in many designs. The
addition of vitamin E to the crosslinked polyethylene was investigated as a
way of preventing oxidation and this technique is likely to increase in use
(Oral et al. , 2008). many companies and researchers are currently looking
at the potential of pEEK for use in joint replacement implants. Soft layer
bearings for joint replacement that involve the use of a polyurethan bearing
surface is likely to develop further in the future (Dowson, 2001). although
the design has not been implanted into patients yet, animal studies have
shown positive results. hip arthroplasty with a compliant bearing surface
was implanted into sheep (Khan et al. , 2005; Carbone et al. , 2006). The
retrieved implants after up to three years of implantation show no significant
macroscopic damage to the compliant layer. The future use of polymers in
the body will include tissue engineering, where polymer scaffolds are used to
support cells as they lay down extracellular matrix (Shepherd and azangwe,
2007). These polymers (such as polycaprolactone) are bioresorbable, meaning
that over a period of time they break down in the body.

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7.7 Sources of further information and advice
There are a number of peer-reviewed journals that specialise in biomaterials,
biomedical engineering, joint replacement and wear. The main ones are:
Biomaterials ; Journal of Arthroplasty ; Journal of Biomechanics ; Journal
of Bone and Joint Surgery ; Journal of Engineering in Medicine ; Medical
Engineering and Physics ; Wear .
professional bodies that engage in polymers for use in implants include:
European Society of Biomechanics; Institution of mechanical Engineers;