Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2000-01-20
2002-07-16
Hirsch, Paul J. (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S016110
Reexamination Certificate
active
06419708
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an implant element for permanent anchorage in bone tissue in which at least the surface intended to face the tissue in the implantation region is made of a biocompatible material such as titanium and having a clinically well documented surface.
It is previously well established to use medical implant elements for a variety of purposes. Specifically, in the dental field Brånemark System® implants for replacement of lost dental roots have been successfully used for 30 years. The treatment comprises three stages: (1) One or more titanium screws are installed in the jawbone and are left to integrate with the bone for between three and six months, (2) Special abutments are connected to the fixtures, (3) When the gums have healed, the dental prosthesis is fitted, for permanent use and providing a similar feeling as with natural teeth.
Titanium is a lightweight metal with high strength, low thermal conductivity and fine corrosion resistance. The most important property in this context is the unique bio-compatibility of titanium which might be related to the spontaneous formation of titanium oxide on its surface. Brånemark System® is based on the ability of titanium to integrate permanently with bone tissue, which is a medical phenomenon named osseointegration.
The implant screws are machined from commercially pure titanium. The surface topography of the machined surface shows features in the micron range. According to U.S. Pat. No. 4,330,891 the interaction processes between the titanium oxide surface and the surrounding tissue which results in implant-bone integration are improved if the implant surface is micro-pitted with pits having a diameter in the range of from 10 nm up to about 1000 nm, i.e. the size of the micro-pitting approaches the order of magnitude of the cell diameter in the surrounding tissue or a few multiples thereof.
In addition to a well-defined implant surface topography special care also has to be taken with respect to the surgical technique to assure that the prerequisites for achieving osseointegration are fulfilled. The implant screws are normally installed in the bone tissue at a first operation. Thereafter they are left unloaded for a period of three to six months covered by the soft tissue. At a second surgical session the soft tissue covering the implant screws is removed and the screws are connected to a superstructure and loading can be permitted.
It is assumed that such a two-stage surgical procedure with an early post-operative period without loading is important for the implant stability during the early healing phase. However, the two-stage surgical technique is a disadvantage for the patient and makes the installation time-consuming and therefore expensive. It has also been demonstrated that for specific indications a correct clinical bone anchorage can be achieved using a non-submerged approach, i.e. a one-stage surgical procedure. Also in case of such a one-stage surgical procedure it is assumed that a critical healing period, approximately three months long, during which unfavourable loading should be avoided, is important in order not to jeopardize the process of osseointegration.
Specifically in the mandibular bone the success rate for this type of dental implants is very high. However, in the maxilla and the posterior mandible the success rate very much depend on the quality of the bone.
An object of the present invention is to provide an implant element which allows for a possible reduction of the healing period but which still guarantees a long term stability during clinical loading conditions.
A further object of this invention is to increase the possibilities to use the implants more successfully also in low bone qualities, which is often the case in the maxilla and the posterior mandible.
A review of the literature shows that implant mobility and radiographic bone loss are associated with failures; either to early (primary) failures or late (secondary) failures. The early failures are the consequence of biological processes which interfere with the healing process of bone and the establishment of osseointegration. The majority of these failures are host-related, whereas late failures are the consequence of mainly overload and host-related factors. Clinical retrieval studies indicate that a high degree of bone-implant contact is a consistent finding in functioning, successful clinical osseointegrated titanium implant systems (Sennerby et al, 1991). On the other hand, ongoing studies show that clinically mobile implants with radiolucency are characterized by absence of bone and the presence of fibrous capsule formation and inflammatory cells.
On the basis of available literature and knowledge it may therefore be concluded that the integration of titanium implants and bone and the maintenance of this integration are prerequisites for the clinically documented long-term function and relatively high success rate with this implant. However, experimental studies have shown that bone is not formed directly on the surface of the titanium implant. Instead the process of bone formation originates from existing bone surfaces and solitary islands of bone either in the bone marrow distant from the implant surface or in the distal threads. The bone formation is directed towards the titanium surface and the immediate interface zone is the last to be mineralized (Sennerby, Thomsen, Ericson, 1993a; 1993b). The early phase of bone healing is therefore of particular importance for the establishment of osseointegration. If the healing process is jeopardized, for instance by insufficient implant stability in bone of poor quality or other negative factors, such as previous irradiation of tissues or local inflammation, osteopenia and rheumatoid arthritis the implant-bone connection may be inadequate and the implant-bone structure may not withstand loading (Sennerby & Thomsen, 1993; Öhrnell et al, 1997, Brånemark et al, 1997). It is therefore an urgent need to improve the treatment of patients with the osseointegrated implant technique in case of non-optimal conditions. Such improvements may allow patients with a non-optimal, deranged and/or damaged tissue structure to benefit from treatment with osseointegrated implants.
It is previously known that coatings of biocompatible material of controlled chemical composition and crystalline structure may be deposited onto a substrate to provide articles to be used as medical, dental or orthopedic implants. An objective of such deposition or coating is to develop an implant with a surface which provides for a stable bone-implant connection.
An example of a suitable biocompatible material in this context is hydroxylapatite, i.e. Ca
10
(PO
4
)
6
(OH)
2
, and mixtures of calcium-phosphates, CaP, which resemble the primary inorganic chemical constituent of bone. Various attempts have been made to deposit CaP coatings onto metal substrates in which case the CaP mixture is acting as a biocompatible coating.
According to several scientific investigators, see for example Dhert 1992, CaP coatings may stimulate bone growth during an initial stage. However, the long term results are not convincing, see for instance Johnson B W et al (1992), Lemons J E et al (1988) and Gottlander & Albrektsson 1991.
A possible explanation for the poor long term results may be that the coatings which have been used so far normally have been plasma-sprayed to a thickness of 50-100&mgr;m. Such coatings may have an inherent significant probability for fractures, which may be due to in vivo dissolution and the insufficient mechanical strength of the relatively thick coatings.
A review of the literature reveals that, in addition to the plasma spraying, there are several methods to prepare surface coatings. One way to classify these methods is to divide them into (1) those methods where a coating is prepared by “dry deposition” and (2) those methods where “wet” coating techniques are used.
The former methods include (i) plasma spraying, (ii) laser ablation, (iii) ion assisted sputtering and (iv) radio
Hall Jan
Krozer Anatol
Thomsen Peter
Connolly Bove Lodge & Hutz
Hirsch Paul J.
Nobel Biocare AB
Priddy Michael B.
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