Dentistry – Prosthodontics – Holding or positioning denture in mouth
Reexamination Certificate
1999-10-13
2003-04-15
McDermott, Corrine (Department: 3738)
Dentistry
Prosthodontics
Holding or positioning denture in mouth
C606S060000, C606S065000, C606S075000, C606S075000, C623S011110, C623S023500
Reexamination Certificate
active
06547564
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an implant having a shaft which is adapted in use to be embedded in bone tissue and which has an outer surface provided with a circumferentially-oriented roughness. This will hereinafter be referred to as an “implant of the type defined”.
BACKGROUND OF THE INVENTION
Implants of the type defined are known for use as the anchoring members of dental and orthopaedic prostheses. To this end, the implant is inserted into a bore-hole drilled into the bone tissue of a bone tissue structure at a site where a prosthesis is required, ordinarily by screwing of the implant into the bore-hole. The convention in the art is for the circumferentially-oriented roughness to take the form of a screw thread and in this case the bore-hole will ordinarily be (i) provided with internal threads in advance, or (ii) left untapped with the implant being provided with a self-tapping capacity, e.g. by the provision of one or more axially-extending cutting recesses or notches in the screw thread.
A superstructure having the prosthetic part of the prosthesis is then secured to the implant. In the case of a dental prosthesis, the superstructure will typically consist of a spacer or transmucosal component which engages to the implant to bridge the gingiva overlying the maxilla or mandible at the implant site and the prosthetic part, e.g. a crown, bridge or denture, is then secured to the spacer. There are various other forms that the superstructure can take as is known in the art. For instance, the prosthetic part may be secured directly to the implant.
The long-term integrity of the prosthesis is highly dependent on the successful osseointegration of the implant with the bone tissue structure, that is to say, the remodelling of the bone tissue in the bone tissue structure into direct apposition with the implant. A study on the factors which affect the osseointegration of implants was undertaken by Professor Per-Ingvar Br{dot over (a)}nemark and co-workers and the results were published in a book entitled “
Osseointegrated Implants in the Treatment of the Edentulous law: Experience from a
10-
Year Period”, Almqvist & Wiskell International. Stockholm, Sweden
, 1977. It was found by Br{dot over (a)}nemark et al that successful osseointegration depends upon inter alia the use of biocompatible materials for the implant, for example titanium and alloys thereof, and the surgical procedure adopted, for example leaving the implant unloaded for several months before adding the superstructure. Implants of the type defined are not necessarily always used as part of a prosthesis, in some instances they can be a “stand alone” structure. As an example, implants of the type defined are known for use as bone fixation screws. The success of these “stand alone” implants is also highly dependent on their successful osseointegration.
Implants of the type defined have some notable advantages in promoting successful osseointegration with the adjacent bone tissue, a major one being as a result of the fact that the main loads on the implant in the clinical situation are axial loads. These implants are very well suited to support axial loads and this may be particularly important in the initial stages of the osseointegration process in which it is important that the implant is fully stable and as immovable as possible in the bore-hole (primary fixation). One can consider this to be due to the bone tissue growing into the troughs between adjacent peaks of the circumferentially-oriented roughness on the implant.
The Applicant has also identified that it is advantageous for an implant of the type defined to transmit the axial loading thereon evenly to the adjacent bone tissue to prevent high stress concentrations occurring in the adjacent bone tissue and concomitantly marginal bone tissue resorption. If marginal bone tissue resorption occurs this will reduce the anchorage of the implant and may undermine the long term stability of the implant resulting in due course in failure of the prosthesis. In the particular case of dental prostheses, the aesthetic appeal is also undermined by marginal bone tissue resorption, an important drawback since dental prosthetics forms part of the field of cosmetic surgery.
The present invention proposes to provide an implant of the type defined having features which promote its maintenance in a bone tissue structure whilst at the same time facilitating its insertion into the bone tissue structure in the first place.
SUMMARY OF THE INVENTION
According to the present invention there is provided an implant of the type defined in which the circumferentially-oriented roughness has first and second axial sections each comprising a series of circumferentially-oriented peaks which have a crest and which are axially spaced apart by troughs, the axial spacing between the crests of adjacent peaks in the first axial section is less than the axial spacing between the crests of adjacent peaks in the second axial section and the first and second axial sections of circumferentially-oriented roughness are adapted in use to provide the same or substantially the same pitch.
The larger inter-peak spacing in the second axial section of circumferentially-oriented roughness acts to promote primary fixation of the implant in the bone tissue during the early phases of osseointegration since each trough between adjacent peaks can capture a relatively large volume of bone tissue to interlock the implant with the bone tissue. The smaller inter-peak spacing in the first axial section, on the other hand, enables the stiffness of the implant to be increased thereby improving the ability of the implant to transmit loads more evenly to the bone tissue to inhibit marginal bone resorption. Adapting the first and second axial sections to have the same or substantially the same pitch means that both axial sections produce the same or substantially the same axial displacement into the bone tissue on one rotation thereof thus ensuring that the provision of the two different axial sections of circumferentially-oriented roughness does not complicate insertion of the implant into the bone tissue. If the first and second axial sections of circumferentially-oriented roughness did not have the same or substantially the same pitch then a greater force would need to be applied to insert the implant resulting in fractures being formed in the bone tissue.
In an embodiment of the invention such as the one hereinafter to be described the pitch is a predetermined distance, the ratio of the predetermined distance to the axial spacing between the crests of adjacent peaks in the first axial section is a first multiple integer and the ratio of the predetermined distance to the axial spacing between the crests of adjacent peaks in the second axial section is a second multiple integer which is less than the first multiple integer. The first multiple integer may be a multiple integer of the second multiple integer.
In an embodiment of the invention such as the one hereinafter to be described the peaks in the first and second axial sections are circumferentially-oriented at a common inclined angle to the main axis of the implant.
In an embodiment of the invention such as the one hereinafter to be described the shaft has a coronal end and an apical end and the first axial section is located coronally of the second axial section.
In an embodiment of the invention such as the one hereinafter to be described the first and second axial sections are contiguous.
In an embodiment of the invention such as the one hereinafter to be described the first axial section extends from the coronal end of the shaft to a position coronally of the apical end and the second axial section extends from the first axial section towards the apical end of the shaft. The implant may have a coronal end which is spaced coronally from the coronal end of the shaft by a smooth coronal portion of the implant, as in the embodiment of the invention hereinafter to be described, in which case the smooth coronal portion is preferably no more than
Astra Aktiebolag
Chattopadhyay Urmi
McDermott Corrine
White & Case LLP
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