The invention relates to a bone plug, for blocking of a canal in a bone, comprising a radially expandable blocking element which comprises an at least nearly solid body, which body under axial compression expands into radial direction and is provided with a continuous axial opening, which body has a height and an average wall thickness along said height, which bone plug comprises means to keep the body in expanded condition locked in the axial direction, which means comprise two locking elements which can be fixed on various distances to each other, in between which the body with its axial boundaries is positioned, of which a first locking element comprises a stem which is provided with a number of radial protrusions which are positioned next to each other in axial direction on the stem, which stem protrudes through the axial opening of the body, whereby the second locking element is positioned around the stem and can hook behind the protrusions. With the height of the body is meant the size of the body in the longitudinal direction of the stem. The termxe2x80x94xe2x80x9caverage wall thickness along the heightxe2x80x9dxe2x80x94is used, to be able to bename the wall thickness in case of non cylindrical shapes of the body like for example a cambered shape.
Bone plugs are used in the medical field for permanent or temporarily blocking of a canal in a long bone. For fixating of an endoprosthesis or artificial joint, for example an artificial hip prosthesis, in a bone, a stem of the prosthesis is inserted in the intramedullary canal of a long bone which is filled with bone cement. In order to prevent the bone cement to protrude in the canal any further than necessary for fixating of the stem and to assure that the bone cement is only present between the stem and the endosteal wall of the bone and to prevent leaking of the bone cement any further into the intramedullary canal, the canal beneath the stem is blocked with a bone plug.
A bone plug of the abovementioned type is known from the French patent publication FR-A-2708193. The body of the known bone plug is made out of a flexible, resorbable, biocompatible material which under axial compression expands into radial direction. The body of the blocking element of the known bone plug is slim and has in relation to the height a minor wall thickness. In practice it has shown that such ratio of the sizes of the body are unsuitable in order to obtain a sufficient large radial expansion of the body. At such large axial compression the body will fold and crumple instead of expanding in radial direction, which is adversable for obtaining a good blocking of the canal in a bone. Hence the use of the known bone plug for blocking of canals is of limited value.
Part of the invention is to provide a bone plug of the kind described in the introduction, which is better than the known bone plug. Therefor the bone plug according to the invention is characterized in that the height of the body is at most 8 times the average wall thickness of the body. With this ratio between the height and the wall thickness, the body will also evenly expand in radial direction in case of large axial compression and will not crumple up in an irregular manner. Hereby the range of application is considerably larger than of the known bone plug. Preferred embodiments of the bone plug according to the invention are characterized in that the height of the body is at most 7, 6 or 5 times the average wall thickness, and in practice a suitable value has shown to be nearby 5.
A bone plug of the abovementioned type is known from the French patent publication FR-A-2708192. The body of the known bone plug is made out of a flexible, resorbable, biocompatible material which under axial compression expands into radial direction. The body of the blocking element of the known bone plug is slim and has in relation to the height a minor wall thickness. In practice it has shown that such ratio of the sizes of the body are unsuitable in order to obtain a sufficient large radial expansion of the body. At such large axial compression the body will fold and crumple instead of expanding in radial direction, which is endosteal wall.
An embodiment of the bone plug according to the invention is characterized in that the radial protrusions are located at least nearly to each other in the longitudinal direction of the stem. This makes compression of the body possible in smaller steps than with the known bone plug, where the protrusions are positioned on a relatively large distance to each other. By this possibility of accurate control of the amount of compression of the body, the bone plug can better be installed in a canal and a canal can better be blocked than with the known bone plug. Preferably the radial protrusions have an at least nearly saw-tooth shape in a longitudinal section of the stem.
It is noted that a bone plug with a stem with protrusions, which are positioned near to each other in the axial direction of the stem, is known from patent publication DE-A-92 13120.4. However a fully different shape of the body has been applied here. Experience showed that in particular by a combination of a solid body with a stem provided with protrusions which are positioned near to each other, a well functioning bone plug is obtained.
It is noted that the beneficial effect of the fact that the protrusions are positioned near to each other also occurs when the ratio between the height of the body and the average wall thickness does not fulfil the abovementioned condition, by which explicitly the possibility is preserved to claim this feature separately from the feature that the height of the body is at most 6 times the average wall thickness of the body.
A further embodiment of the bone plug according to the invention is characterized in that the material of the body of the blocking element has at least nearly elastomeric properties. A body of an elastomeric material is nearly incompressible, whereby the axial compression is nearly fully converted in a radial expansion without loss of volume. In case of the known bone plug an axial compression will in contrary not only result in a radial expansion, but the body itself is also compressed, which means that the volume decreases. Hereby, the bone plug according to the invention will, at equal axial compression, achieve a larger radial expansion than the known bone plug and the bone plug according to the invention will better be fixed in the canal than the known bone plug.
It is noted that the beneficial effect of a body of an elastomeric material also occurs when the ratio between the height and the average wall thickness does not fulfil the aforementioned condition by which explicitly the possibility is preserved to claim this feature separately from the feature that the height of the body is at most 6 times the average wall thickness of the body.
A preferred embodiment of the bone plug according to the invention is characterized in that the material of the body of the blocking element is composed mainly from a mixture of gelatine, glycerine and water. It is noted that a material composed from a mixture of gelatine, glycerine and water is known on itself and that it is known to use such a material for the production of bone plugs. This known application for bone plugs however is only used in order to obtain a flexible material which is biologically degradable. Hereby no advantage is made of the specifically for the type of bone plug according to the invention, beneficial incompressible properties of the material.
It is also noted that the beneficial effect of the fact that the material of the body of the blocking element is composed mainly of a mixture of gelatine, glycerine and water also occurs when the ratio between the height and the average wall thickness of the body does not fulfil the aforementioned condition, by which explicitly the possibility is preserved to claim this feature separately from the feature that the height of the body is at most 6 times the average wall thickness of the body.
A further embodiment is characterized in that the material of the locking elements is mainly made out of polymethylmethacrylate. Also the bone cement which is applied in the intramedullary canal of a bone is mainly made out of polymethylmethacrylate. Hereby the bone cement becomes during its polymerization process chemically bonded to the locking elements and will form one solid piece. This will facilitate any future revision procedures when the bone cement and the bone plug have to be removed.
The second locking element of the known bone plug is formed by an elastic ring which expands in radial direction when it is pushed over a protrusion and which contracts again behind the protrusion. The known ring therefor has to be sufficient stiff to remain locked behind the protrusion while being pressed upon by the body. Through this a relatively large force is needed to push the ring over the protrusions during installation of the known bone plug. This is not beneficial for an accurate and proper installation of the known bone plug. In order to eliminate this disadvantage, a further embodiment of the bone plug according to the invention is characterized in that the second locking element comprises an interrupted ring. Hereby the ring can be pushed with moderate force over the protrusions and it is better secured that the ring will remain locked behind the protrusions.
It is noted that the beneficial effect of the fact that the second locking element comprises an interrupted ring also occurs when the ratio between the height and the average wall thickness does not fulfil the aforementioned condition, by which explicitly the possibility is preserved to claim this feature separately from the feature that the height of the bone plug is at most 6 times the average wall thickness of the body.
In order to keep the body of the blocking element better locked between the locking elements, a further embodiment of the bone plug according to the invention is characterized in that the second locking element also comprises an annular shaped plate which is positioned between the body and the interrupted ring. In this manner the interrupted ring can optimally be designed to lock behind the protrusions on the stem while the annular shaped plate fulfils the function of locking the body.
In order to prevent loosening of the interrupted ring an embodiment is characterized in that the annular shaped plate contains a recession in which the interrupted ring is positioned. By shaping the recession such that the interrupted ring is locked in place, the interrupted ring no longer can bend open and therefor can not come loose.
A practical beneficial embodiment of this is characterized in that the interface between the outer diameter of the interrupted ring and the wall of the recession is bevelled. Preferably the interrupted ring has a triangular cross-section.
A further embodiment of the bone plug according to the invention is characterized in that the stem is hollow and at least at the free end is open. This hole can be used to position a prosthetic component. A prosthetic component which is provided with a tip which fits accurately in the hole of the stem can be reproducibly positioned relative to the intramedullary canal by inserting the tip in the hole of the stem.
A further embodiment which is useful for temporarily fixating an instrument in the intramedullary canal, is characterized in that the stem of the first locking element is provided at its free end with a screw thread. The stem for example is provided with an internal screw thread into which the instrument can be screwed.
These last applications can also be realised with an embodiment with a solid stem provided with an external screw thread or providing the prosthetic component with a hole hich fits accurately around the stem. However in practice it has shown advantageous when an instrument or prosthetic component is respectively fixed or positioned inside the stem instead of around the stem.
Finally a further embodiment is characterized in that the stem of the first locking element has a weakened spot near to its free end. This weak spot can for example be formed by construction of the stem or by a groove in the stem. During installation of the bone plug the second locking element is kept immobile by an insertion instrument while simultaneously the insertion instruments pulls at the stem of the first locking element. Hereby the body is compressed in the axial direction and the stem will break when the instrument pulls too hard. This will cause that the maximum compression force on the body is in all cases nearly the same and sufficient to obtain a strong fixation of the expanded body inside the canal. In order to prevent the stem to fracture near a protrusion, the stem is deliberately weakened near the free end which is pulled upon by the insertion instrument.