Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1997-02-18
2003-08-26
McDermott, Corrine (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023270, C623S023110
Reexamination Certificate
active
06610099
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fixture for anchorage in human skeletal or bone tissue. More specifically, the invention concerns a screw-threaded, self-tapping fixture which is especially suitable for permanent anchorage of a hip-joint prosthesis in a cavity formed in the femoral collum. A fixture according to the preamble to claim
1
is known from the applicant's WO 93/16663. The invention also relates to the use of such a fixture, to a prosthesis including such a fixture, and to a method for anchoring a fixture.
BACKGROUND ART
GB 1 291 470 discloses a prosthesis fixture for anchorage in bone tissue, specifically for the mounting of a dental prosthesis in the jawbone. The fixture comprises a substantially hollow bolt which is externally threaded. The threads are intended to be screwed into a bored and pre-threaded bone cavity for permanent implantation. Adjacent to its distal end or insertion end, the bolt is provided with a number of radially-directed holes, each hole leading from the threaded outer surface into a central well. The radial holes are intended to promote an integrating growth of new-formed bone tissue during the healing process. The final ingrowth forms a “rotational lock”, which prevents undesirable unscrewing and loosening of the fixture. In this document, it is also envisaged that not only the new-formed bone material, but also loose bone tissue separated as a result of the insertion of the fixture into the cavity, may participate in the integrating growth in the radial holes. The central well, in which the scraped-off bone tissue is collected, opens downwards into the distal end surface of the bolt.
EP-A1-0 237 505, which also discloses a fixture for permanent anchorage in bone tissue, specifically permanent anchorage of a dental prosthesis, teaches a modification of the above fixture according to GB 1 291 470, the purpose being to avoid an alleged drawback of scraped-off bone tissue falling down from the central well of the bolt onto the bottom of the bone cavity, thereby disturbing the osseointegration of the base of the fixture. To this end, the central well is dispensed with in the fixture of EP-A1-0 237 505, and the radial through-holes are replaced with one or more peripheral cavities communicating only with the outer cylindrical surface of the fixture. As a result, the distal end surface of the fixture is unbroken, and loose bone tissue is prevented from falling down onto the bottom of the bone cavity. The edge of each peripheral cavity in the cylindrical surface forms a sharp cutting edge for self tapping when the fixture is screwed into the bone tissue. An essential feature of this prior-art fixture is that the total tissue-collecting volume of the peripheral cavities is selected so as to contain and store all the bone tissue scraped off by the cutting edges.
U.S. Pat. No. 5,129,901 discloses a self-tapping, self-drilling, cannulated hip screw for impermanent anchorage in orthopaedic bone surgery. The screw is designed to minimise thermal necrosis and to enable efficient removal of scraped-off bone tissue (referred to as “chips”) during both insertion and removal of the screw, thereby to facilitate the insertion of the screw and the subsequent removal thereof. The screw has a central bore which opens in the cutting end surface of the screw and is intended to receive a drill guide pin. The outer peripheral surface of the screw presents a relatively short threaded portion adjacent to the drilling end of the screw, and a relatively long non-threaded shank portion at its rear end. The diameter of the shank portion is smaller than the diameter of the threaded portion. A first pair of relatively short, peripheral and longitudinal cavities or “flutes” are formed and circumferentially spaced in the screw-threaded portion adjacent to the cutting end. These flutes form self-tapping cutting edges along the threads and open into the forward end surface of the screw. When the screw is drilled into the bone, the flutes direct the cut bone chips rearwards into an annular space formed between the inner wall of the drilled bone channel and the periphery of the shank portion of reduced diameter. Also, a pair of reverse cutting flutes is formed in the screw-threaded portion at an axial location rearward-spaced from the drilling end, and is arranged to cut through the bone upon unscrewing of the screw from the bone. Similar flutes are disclosed also in U.S. Pat. No. 4,537,185, cited in U.S. Pat. No. 5,129,901 mentioned above. A self-tapping, self-drilling fixation screw in U.S. Pat. No. 4,537,185 is provided with external threads intersected by flutes or grooves. The grooves form self-tapping cutting edges along the fixture, and drilling teeth are provided in a forward, non-threaded end portion of the fixture.
In WO 93/16663, which was mentioned in the introduction to this specification, a hip-joint prosthesis for permanent anchorage in the human femoral collum is disclosed. The prosthesis comprises a step-formed fixture, including a forward cylindrical first fixture member and a rear plug-like, cylindrical second fixture member rigidly connected or integrated with the first fixture member, and a prosthesis caput supported by the rear end of the fixture. Each of the two fixture members presents a screw-threaded outer surface provided with short sharp-edged cutting recesses at their insertion ends. The first fixture member is self-tapped into a relatively narrow hole drilled through the femoral collum in the cancerous bone along a predetermined axis, whereas the second member is self-tapped into a wider cylindrical cavity cut in the cancellous bone of the collum and being co-axial with the narrow hole. The axial extent of the cutting recesses is substantially less than the overall axial extent of the screw-threaded portion. More specifically, the cutting recesses of the second member are confined to a relatively short forward end portion, which tapers slightly inwards to facilitate the insertion and centering of the fixture when inserted and screwed into the wider cavity. Thus, the major part of the screw-threaded outer surface of the second member is unbroken, i.e. presents no recesses, and serves to establish an aimed-at mechanical screw connection with the inner wall of the pre-formed cavity. By contrast with the above fixture according to EP-A1-0 237 505, the cutting recesses of the second fixture member extend forwards all the way to, and open into, the forward end surface of that fixture member. By this arrangement, the bone tissue cut or scraped off by the cutting recesses will exit in the forward direction, i.e. in the insertion direction, out of the recesses and into the cavity.
As discussed in WO 93/16663, the fixture is preferably brought into engagement with the inside of the hard cortical bone of the collum in order to reduce the risk of mechanical loosening of the fixture. However, the mechanical contact between the second fixture member and the cortical bone is limited to a circumferential minor part of the screw-threaded surface. The remaining, major part of the screw-threaded surface of the second fixture member does not engage the cortical bone, but only the substantially softer cancellous bone. As a result, only a minor part of the second fixture member will be in mechanical engagement with the “hard” bone and participate in transferring the load from the caput to the femur collum.
The present invention aims at overcoming, or at least reducing, this problem. Thus, a specific purpose of the present invention is to improve a fixture of the kind disclosed in WO 93/16663, and more generally to enhance the mechanical connection between the bone and a prosthesis fixture, thereby to accomplish a stronger permanent anchorage.
DISCLOSURE OF THE INVENTION
In accordance with the present invention, there is provided a fixture for insertion into and permanent anchorage in a cavity formed in bone tissue, comprising a fixture member, which has an insertion end surface and a cylindrical peripheral surface provided with scr
Albrektsson Björn
Carlsson Lars
Jacobsson Magnus
Röstlund Tord
Wennberg Stig
Astra Aktiebolag
McDermott Corrine
Stewart Alvin
White & Case LLP
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