Replacement of carpal bones with exact-fitting silicone rubber implants. An experimental study.

In a pilot study in dogs, carpal bones were removed and replaced with silicone rubber implants fashioned to fit exactly into the space left by the excision. The implants were fabricated from Dow Corning MDX-4-4210 and Silastic 399 Medical Grade Elastomers. Bone cement was used for the cast and the mould was made of alginate or plaster of Paris. It was intended to replace the bone in a one-stage procedure. MDX-4-4210 was found unsuitable because adequate cure could only be achieved in about 5 hours, even when accelerated with external heating. Silastic 399 cured rapidly and was ideal for the experiment. Implants of exact fitting were fabricated, sterilized and inserted within 1--2 hours. 6 dogs were used and the follow-up period ranged from 1 to 7 months. The results were good in 5 and unsatisfactory in 1 dog because of postoperative infection. The method presented permits exact reproduction of the excised part, which guarantees an accurate fitting and consequently reduced risk of implant rotation and migration.