Surgery – Instruments – Orthopedic instrumentation
Patent
1990-11-09
1992-12-08
Hafer, Robert A.
Surgery
Instruments
Orthopedic instrumentation
606 72, 606 77, A61B 1758
Patent
active
051694000
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a bone screw made of resorbing plastic material and comprising an externally threaded shaft in which an insertion channel open at the top, arranged concentrically with the shaft and extending along most of the length of the shaft is located for a screwing-in tool, the cross-section of the insertion channel being non-circular and corresponding to the cross-section of the screwing-in tool.
Hitherto, bone screws for joining implants to bone substance or for joining adjacent bone fragments have mainly been made of stainless steel or titanium. Owing to the mechanical properties of this material, these constructions exhibit a high strength which enables the construction to be based on screws for wood and sheet metal processing. However, a disadvantage of such screws is that after completion of the healing process, they have to be removed from the body again, i.e., a further operation is necessary.
Recently, bone screws made of plastic material, in particular of resorbing plastic material, for example, of polylactide or poly-L-lactide have become known. These have the great advantage that after completion of the healing process, they are resorbed by the body, thereby making an additional operation for removal of such implants superfluous. With such plastic screws it has, however, proven disadvantageous that the plastic material exhibits substantially lower strength than conventional steel or titanium screws. There is, therefore, the danger that on screwing such bone screws into pre-drilled and pre-cut threaded holes, the plastic screw will be twisted off by the torsional tension exerted by the screwdriver on the plastic screw and thereby destroyed. This can be counteracted to a certain extent by using altered core diameters, flank angles and thread pitches, however, in spite of these measures, it is not always possible to avoid destruction of the screws when these are screwed in with hexagon socket, slotted or Phillips screwdrivers which engage the head of the bone screw in the conventional manner.
The danger of such a bone screw made of resorbing material shearing off is prevented in a known bone screw by the latter comprising an insertion channel extending over the entire length of the screw shaft for a non-circular screwing-in tool which thus comes to rest in a positively connected manner against the wall of the insertion channel along the entire length of the shaft. The torque introduced into the screw is thereby distributed over the entire length of the shaft (DP-A-0260222). However, a disadvantage is that this results in a weakening of the firmness of the bone screw after insertion in the bone. The resorption of the plastic material starts at the surface and with such a bone screw, therefore, quickly leads to disintegration of the plastic material of large molecular weight.
The object underlying the invention is to so design such a bone screw in spite of a continuous insertion channel in the shaft that the firmness of the bone screw is maintained as long as possible, even when resorption of the screw material starts to take place.
This object is accomplished in accordance with the invention with a bone screw of the kind described at the beginning in that a closure pin made of resorbing material is inserted in the insertion channel, thereby filling it out completely, the dimensions of the cross-section of the closure pin being such that it does not expand the screw.
The closure pin not only prevents undesired entry of body fluid and other body substances into the insertion channel but also increases the strength of the bone screw and so a weakening of the bone screw which might possibly result from the formation of an insertion channel is substantially eliminated again.
The insertion channel may be closed at the screw-in end of the bone screw. The screw-in depth of the screwing-in tool is thereby fixed, i.e., in this case the position of the screwing-in tool in the insertion channel is precisely defined.
In another embodiment, the insertion channel may penetrate the bone screw comple
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Hurst Achim
Lutze Theodor
Muhling Joachim
Pistner Hans
Aesculap AG
Hafer Robert A.
Lipsitz Barry R.
Rimell Sam
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