Surgery – Instruments – Orthopedic instrumentation
Patent
1988-10-14
1990-11-06
Hafer, Robert A.
Surgery
Instruments
Orthopedic instrumentation
606 76, A61B 1758
Patent
active
049683179
DESCRIPTION:
BRIEF SUMMARY
Surgical implants with good mechanical strength properties can be manufactured of resorbable polymeric materials (resorbable composites) which contain resorbable reinforcing elements. Resorbable or absorbable means in this connection that the material is metabolized by living tissues. Such resorbable materials and implants manufactured of them can be applied e.g. as rods, plates, screws, intramedullary nails etc. for fixation of bone fractures, osteotomies, arthorodesis or joint damages. An advantage of such implants and materials is that they are resorbed (depolymerized to cell nutrients) after the healing of the treated tissue. Therefore the resorbable implants do not need a removal operation, like metallic implants in many cases need.
Invention U.S. Pat. No. 4,279,249 describes resorbable implant materials comprising polyglycolide fibers as reinforcement and polylactide as a binding polymer (as a resorbable matrix). In the patent application FI No. 851 828 are described self-reinforced resorbable materials, where the resorbable polymer matrix has been reinforced with resorbable reinforcement elements which have the same chemical element content as the matrix. Typical reinforcement elements in this connection are fibers or structures which have been constructed of them.
The known resorbable materials reinforced with resorbable organic reinforcement elements have fairly high mechanical strength values. Therefore, such materials can be applied in orthopedics and traumatology in treatment of cancellous bone fractures, osteotomies, arthrodesis or joint damages. For example the self-reinforced resorbable materials of FI No. 851 828 describe bending strengths over 300 MPa (S. Vainionpaa, Thesis, Helsinki 1987), which values clearly are higher than even the average strength values of cortical bone. Also the elastic moduli of known self-reinforced resorbable composites are quite high, typically of the order of magnitude of 10 GPa. So the strength values of these materials are clearly better than those of resorbable materials which have been manufactured by melt molding techniques.
When one manufactures resorbable polymers, copolymers or polymer alloy rods, profiles, plates, etc. implants by melt molding techniques like by injection molding or by extrusion, the mechanical properties of the products remain on the level which is typical for thermoplastic polymers. The strength values (like tensile, shear and bending strength) typically do not exceed the value 150 MPa showing, typically, strength values between 40 and 80 MPa and moduli between 1 and 6 GPa. The reason for this behavior is the fact that the flow orientation which exists in the flowing polymer melt is relaxed as a consequence of molecular thermal movements when the melt molded sample is cooled. When it is a question of a crystallizable polymer, the sample is crystallized to a partially crystalline, spherulitic structure. So the polymeric material manufactured by melt molding typically consists of folded crystalline lamellae (the thickness 100-300 .ANG., the breath about 1 .mu.m), which are surrounded by the amorphous polymer. On the other hand, the lamellae can be thought to consist of mosaic-like folded blocks (the breadth of some hundreds of .ANG.). The lamellae, as a rule, form ribbon-like structures which grow from crystallization centers, so-called nuclei, to three-dimensional spherical spherulitic structures. Because the polymer material which has been crystallized with the spherulitic mechanism does not show, as a rule, significant orientation of polymer molecules with strong covalence bonds, its mechanical strength values remain on the above mentioned level. Only on the surface of the sample molecular orientation can remain because of rapid cooling in the mold (as in the case of injection molding).
Although the reinforced resorbable composites show considerably better strength properties than melt molded resorbable composites, it is often necessary to manufacture of resorbable reinforced composites quite big implants, like rods, intramedullary
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Heponen Veli-Pekka
Laiho Juha
Pohjonen Timo
Rokkanen Pentti
Tormala Pertti
Hafer Robert A.
Rooney Kevin G.
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