Totally Biodegradable Implants for Bone Fixation and Ligament Repair
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in energy and protein metabolism. Implanted biodegradable devices do not require a removal Operation, which is a financial and psychological benefit to the patient. This article mainly deals with the use of totally biodegradable implants in the fixation of bones and ligaments and, to some extent, with the properties of some absorbable synthetic polymeric materials. It is based on the present literature and on our own investigations and experience.
Strength and Loss of Strength of Biodegradable Devices The devices used in bone fixation must be strong enough to keep the fragments securely together during healing. Meltmolded absorbable polymers are either too brittle or too flexible for such a safe fixation. In contrast, self-reinforced (SR) biodegradable composites fulfill the strength demand of secure fixation ma terials and devices because of their ductile deformation mode, ultrahigh initial strength, and appropriate modulus. They lose their strength gradually and are to tally absorbable. The loss of strength in vivo oecurs during the first weeks. Several microstructural, macrostructural, and environmental factors significantly contribute to the absorption behavior of SR biodegradable composites.
Experimental Studies Before (and in parallel with) the clini
cal introduction of biodegradable orthopedic implants, we studied their use for fraction fixation in more than 6000 animals. We investigated parameters such as strength, strength retention, biodegradation, bone changes, healing of osteoto mies (cutting of the bone) and fractures, and the fixation properties of the im plants. We did macroscopic, radiographic, microradiographic, histologic, histomorphometric, and oxytetracyclinelabeling investigations. Strength measurements of implants and bone fixations were also completed. As an example of our results, the fixa tion of an experimental osteotomy of the distal femur in a rabbit model with ab sorbable thread alone resulted in successful healing in 24 out of 34 cases. With absorbable thread and rod used to gether, successful healing oecurred in all 19 cases. A first-generation poly(glycolic aeid) (PGA) rod biodegraded to a great extent in cancellous (porous) bone in 12 weeks. A self-reinforced poly(L-lactic aeid) (SR-PLLA) rod biodegraded only in part in the cancellous bone in 48 weeks. A 2-mm hole drilled through the distal femoral growth plate of a five-week-old rabbit, destroying 3% of bone tissue, did not cause any permanent growth disturbance or shortening of the femur. Fixa tion of experimental osteotomies with two SR-PGA pins of 1 m m in diameter resulted in uneventful healing without any complications. It was also possible to fix experimental osteotomies with SR-PGA and SR-PLLA rods in rabbits and dogs. It has been proved that as the strength of the healing osteotomy gradually increased, the strength of the biodegrad able SR-PLLA rod used gradually decreased (Figure 1). Rods, screws, plates,
20 30 Weeks Figure 1. As the strength of an experimental osteotomy gradually increases with healing, the st
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