Dentistry – Prosthodontics – Holding or positioning denture in mouth
Reexamination Certificate
1998-05-21
2002-11-05
Lewis, Ralph A. (Department: 3732)
Dentistry
Prosthodontics
Holding or positioning denture in mouth
C433S174000
Reexamination Certificate
active
06474991
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an implant system, specifically a dental implant system for a partially or fully edentulous patient which comprises a fixture for implantation into the bone tissue of the maxilla or mandible of the patient and a superstructure for mounting to the fixture which in turn supports or presents a dental restoration comprising one or more artificial replacement teeth. As an example, the superstructure may be an abutment which is adapted in use to be connected to the implanted fixture to bridge the soft tissue layer (gingiva) overlying the maxilla or mandible for a restoration structure such as a crown or bridge to be secured thereto.
In a typical dental implant system the superstructure is screw retained to the fixture. To this end, the fixture has an internally threaded socket which opens in a rear end of the fixture. The internal threads may be present throughout the socket or alternatively just in a is section of the socket. The superstructure is then either (i) integrally formed with an externally threaded projection for screwing into the internally threaded socket to secure the superstructure to the fixture, or (ii) comprises a sleeve part having a lumen in which a transverse bearing surface is provided, and a screw part for passage through the lumen of the sleeve into the internally threaded socket, the screw having an enlarged head which is adapted in use to bear against the transverse bearing surface in the lumen of the sleeve thereby ensuring that the superstructure is secured to the fixture on screwing of the screw into the internally threaded socket.
In other dental implant systems the superstructure is secured to the fixture through non-screw thread connections, e.g. by cementing.
Whatever the form of securement of the superstructure to the fixture, the superstructure transmits a load to the fixture in the implantation direction through mutually interfacing surfaces.
Fixation of a fixture of a dental implant system in the bone tissue of the maxilla or mandible primarily relies upon mechanical interlocking of bone tissue to the outer surface of the fixture. Substantial interlocking between the outer surface of a fixture and the bone tissue of the maxilla or mandible occurs when the outer surface is roughened, for example by providing the fixture outer surface with screw threads, recesses, holes etc. (macroroughening), forming pits in the fixture outer surface by blasting, plasma spraying, etching, hydroxyapatite coating, the provision of beads on the outer surface etc. (microroughening) or machining the fixture outer surface. Interlocking between the outer surface of a fixture and bone tissue of the maxilla or mandible into which the fixture is implanted enables the fixture to greater withstand shear stresses. One can consider this to be due to bone tissue growing into the recesses in the fixture outer surface created by the roughening.
A macroroughened fixture outer surface produces more effective interlocking than a microroughened fixture outer surface which in turn produces more effective interlocking than a machined fixture outer surface. All, however, produce more effective interlocking than fixtures having an outer surface which is smooth, e.g. a polished outer surface. More than one form of surface roughening can be applied to a fixture to promote interlocking. For example, it is known to provide a fixture with both a macro- and a microroughened outer surface.
In 1892 it was suggested by J. Wolff (
Das Gesetz der Transformation der Knochen. Berlin: A. Hirschald
, 1892) that bone tissue remodels itself in response to the mechanical loading history of the bone and in accordance with mathematical laws. This has since proved to be correct and is therefore now known as Wolff's law. Some consequences of Wolff's law are that
1. If the stresses or strains to which bone tissue is exposed are lowered a net loss of bone tissue (bone resorption) will result until a new steady state is obtained where the stresses or strains are normal.
2. If the stresses or strains to which bone tissue is exposed are normal then no net change to the bone tissue will result.
3. If the stresses or strains to which bone tissue is exposed are increased within physiological limits bone tissue build up will result until a new steady state is obtained where the stresses or strains are normal.
4. If the stresses or strains to which bone tissue is exposed are extremely high then bone resorption will result.
Wolff's law thus indicates the need for stress conditions in the bone tissue adjacent an implant fixture which are favourable for the maintenance of a steady state in the bone tissue. Otherwise, marginal bone resorption will result which will cause destabilisation of the osseointegration of the fixture with the bone tissue and a reduction in the aesthetic appeal of the dental implant system when implanted, an important commercial consideration.
With this in mind, the Applicant's have identified an interdependence between the stress conditions in the bone tissue adjacent an implant fixture and the position of the interfacing surfaces through which a fixture is loaded in the implantation direction by a superstructure vis-à-vis the level on the fixture outer surface where interlocking starts. The present invention proposes to provide a dental implant system which takes account of this interdependence to improve the stress conditions in the bone tissue adjacent the outer surface of an implanted fixture.
SUMMARY OF THE INVENTION
According to the present invention there is provided a dental implant system comprising:
a fixture for implantation in bone tissue of the maxilla or mandible by displacement thereof in a forward direction, the fixture having:
a forward end,
a rearward end, and
an outer surface which extends between the forward and rearward ends and which forwardly of a predetermined position on the outer surface is adapted for interlocking with bone tissue of the maxilla or mandible; and
a superstructure for mounting on the fixture, the superstructure having:
a forward end for interfacing with the rearward end of the fixture, the forward end of the superstructure and the rearward end of the fixture presenting interfacing surfaces for interfacing with one another;
characterised in that the rearward end of the fixture and the forward end of the superstructure are so constructed and dimensioned that a predetermined interfacing surface of the superstructure is able to interface in the forward direction with a predetermined interfacing surface of the fixture which is at a level which is disposed forwardly of the predetermined position on the outer surface.
The result of this arrangement is that when the fixture is implanted the superstructure applies a load to the fixture in the forward direction through interfacing surfaces at a level which is disposed forwardly of the predetermined position on the fixture outer surface which is the attachment level at which bone tissue starts to interlock with the fixture when implanted. Finite element analysis indicates that loading of a fixture in the forward direction forwardly of the attachment level results in an improved stress distribution in the adjacent bone tissue of the maxilla or mandible. For example, application of a forwardly directed load through interfacing surfaces forward of the attachment level shifts the peak interfacial shear stress forwardly where the risk of triggering off stress induced bone resorption is less. In addition, the longitudinal compressive stresses (relative to the maxilla or mandible ridge) in the bone tissue and the stresses caused by the horizontal loads or bending moments on the fixture are less as one moves more forwardly into the bone tissue and therefore the peak interfacial shear stress generated in the bone tissue can be more easily accommodated there. The peak interfacial shear stress value is also generally reduced in value.
In an embodiment of the invention the predetermined interfacing surfaces of the fixture and the superstructure are t
Astra Aktiebolag
Lewis Ralph A.
White & Case LLP
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