Bone fusion dental implant with hybrid anchor

Bone fusion dental implant of the type able to form an artificial dental root as a support for a fixed prosthesis composed of one or more dental crowns, said implant being inserted into the bone of the upper or lower jaw by drilling a hole at a suitable location and to which implant the surrounding bone tissue fuses permanently as it grows, said implant also having a substantially cylindrical shape, with a crown end (3) provided with means (11, 4) for connecting with a prosthetic reconstruction and an apical end (2) designed for insertion into the bone, said implant having an externally threaded section (6) extending from crown end (3) over no more than one-third of the entire length of the implant, and a s section (1) without an external thread extending from apical end (2) over no less than twothirds of said length. In the section (1) not threaded externally, the implant is coated with a bioactive material, preferably hydroxyapatite, and is surface-threaded to make it rough and/or porous.

BACKGROUND OF THE INVENTION 
1. Field Of The Invention 
The present invention relates to a bone fusion dental implant with a hybrid 
anchor. More specifically, the invention relates to an implant able to 
constitute an artificial dental root, supported by a fixed prosthesis 
comprised of one or more dental crowns, which is inserted into the bone of 
the upper or lower jaw by drilling a hole at a suitable location and to 
which the surrounding bone tissue fuses permanently as it grows. 
Bone fusion dental implants are composed of generally cylindrical elements 
of dimensions such that they can be accommodated firmly in the dental 
crest in the toothless area concerned and can withstand the mechanical 
forces transferred to them through the connected dental crowns by chewing. 
This connection is normally made by providing around the implant a coaxial 
cavity, with internal threads and open at the crown end of the implant, 
into which, after a suitable period of time during which the implant is 
left unloaded to allow proper fusing of the bone, the prosthesis element 
is screwed, possibly with interposition of intermediate elements. 
The most widespread bone fusion implants in current use are divided into 
two major categories according to the method of anchoring to the bone in 
which the implant is inserted. One type is comprised of an external thread 
by which the implant is firmly screwed into the bone, while a second type 
consists of a substantially cylindrical body accommodated in the surgical 
cavity by inserting it axially then completing its positioning by 
percussion. 
In both cases, the application procedure begins with making a first hole in 
the bone in the appropriate position and with the appropriate orientation, 
then enlarging the pilot hole to increasing diameters until the desired 
diameter is reached. At this point, insertion of a cylindrical implant by 
percussion requires only a few simple steps which consist of introducing 
the implant into the hole and striking it with a suitable hammer, with 
interposition of a suitable instrument to cause it to penetrate for the 
final distance. With a screwed implant, on the other hand, it is necessary 
to tap the bone unless implants of the self-threading type are used, and 
either the tapping or insertion of the implant must be done very slowly 
because overheating of the bone tissue breaks down its protein component 
leading to necrosis with consequent loss of the ability of the bone to 
fuse to the implant. 
For both types of implant, the material of choice in terms of mechanical 
properties and compatibility with bone tissue is titanium, possibly 
alloyed with other elements. To speed up the process of bone fusion, 
particularly in the case of cylindrical percussion implants, coatings of 
bioactive material are provided, primarily hydroxyapatite, which has the 
same chemical composition as human bone and a porous surface structure, 
and is highly suitable for favoring growth of bone tissue in intimate 
contact with it. Surface treatments of the titanium element rendering it 
rough and porous, such as grit blasting and plasma spraying, have a 
similar effect. 
In the case of cylindrical percussion implants, to favor anchoring to the 
bone and to cause the implant to withstand the twisting forces generated 
during the prosthetic procedures, axial blind holes, slits, or grooves are 
provided to interrupt the regularity of the cylindrical surface and later 
encourage occupation by newly grown bone tissue. 
One example of the screw-type implant is described in European Patent 
Publication No. 0 343 135 (The Institute For Applied Biotechnology) which 
represents a further development of the original Branemark implant, the 
first known version of the bone fusion implant. Like the preceding 
implants, it has a cylindrical element with external threads for the 
greater part of its length, with a head portion at the crown end from 
which extends an internally threaded blind coaxial hole for connection 
with the prosthesis element. The head is slightly widened and constitutes 
a flange above which is provided with a portion in the shape of a 
hexagonal nut designed to engage a corresponding cavity with a hexagonal 
section in the prosthesis element to prevent relative rotation of the two 
elements. 
Examples of cylindrical percussion implants are illustrated in European 
publication No. 0 370 590 (IMZ-Fertigungs und Vertriebsgesellschaft fur 
dentlale Technologie mbH) and in U.S. Pat. No. 5,316,476 (J. T. Krauser). 
The former represents a further development of the original IMZ implant 
which relates in particular to an intermediate structure made of plastic 
to be mounted between the implant and the crown installed thereon, but 
which illustrates the fundamental characteristics of the basic implant. 
The latter is comprised of a cylindrical element in which the apical 
hemispherical end and the crown end are provided with an internally 
threaded blind coaxial hole and having, at the apical end, two parallel 
slits passing through the cylindrical body, properly rounded at the edges. 
The implant described in U.S. Pat. No. 5,316,476 is also cylindrical, with 
the frustroconical apical end and four axial grooves with rounded edges 
instead of through slits. Indeed, it has recently been demonstrated (see 
for example M. S. Block et al., Loaded Hydroxyapatite and Grit-Blasted 
Titanium Implants on Dogs, The International Journal of Oral and 
Maxillofacial Implants, Vol.4, pages 219-225) that the newly formed bone 
rarely occupies a through hole fully so that this function can be assumed 
by recesses rather than slits in the cylindrical surface. The crown end of 
the implant can be enlarged in a flange and surmounted by a hexagonal 
head, or, if sufficient bone thickness is available, the diameter of the 
implant can be increased until the flange is incorporated and accordingly 
disappears, leaving only the hexagonal head connected directly with the 
cylindrical body. According to other variants, the hexagonal head can be 
replaced by a widened cavity created at the entrance to the threaded hole. 
In order to promote rapid bone fusion, the surface of the implant below 
the flange, when present, or below a thin cylindrical crown strip when the 
flange is absent, is coated with a bioactive material or treated with 
titanium plasma or by other procedures rendering it sufficiently rough or 
porous. 
It has been experimentally demonstrated (A. B. Carr etal., Reverse Torque 
Failure of Screw-Shaped Implants in Baboons: Baseline Data For Abutment 
Torque Application, The International Journal Of Oral and Maxillofacial 
Implants, Vol. 10, pages 176 sic!-173) that surfaces coated with 
bioactive material or treated as already described achieve faster bone 
fusion which peaks as early as the end of the first three months while the 
untreated or uncoated titanium surfaces achieve maxium bone fusion in the 
fourth three-month period. 
On the other hand, it has also been found that a reabsorption cone is ofter 
created around the implant due to initial bone retraction so that it is 
possible for a small crown area to remain uncovered. If the surfaces are 
exposed to the environment of the mouth as a consequence of initial bone 
retraction, the rough surfaces such as those coated with bioactive 
material or treated in some other way have distinctly inferior behavior 
with respect to the untreated surfaces, retaining more bacterial plaque 
and thus more easily giving rise to inflammatory phenomena with consequent 
eventual bone loss. 
It can be seen from the above that both the macroscopic configuration and 
the surface configuration of the implants cited have both advantageous 
features and typical drawbacks, which offset each other. In fact, 
cylindrical percussion implants are inserted by a simpler and faster 
surgical procedure, while screwed implants take longer to insert, thus 
causing greater trauma to the bone tissue and the patient; on the other 
hand, the latter implants, because of the flights of the screw, offer 
primary (and hence immediate) stability which is far superior to that 
offered by cylindrical implants, and many studies demonstrate that this is 
crucial for achieving bone fusion. As far as the surfaces are concerned, 
coating with bioactive materials and roughening speed up the bone fusion 
process considerably but render any exposed parts more vulnerable to 
bacterial attack. 
Hence, the goal of the present invention is to furnish a bone fusion dental 
implant that combines the best features of cylindrical implants and 
screwed implants as well as of coated and uncoated surfaces, allowing both 
easy insertion and firm anchoring to the bone from the outset, and 
favoring both rapid bone fusion and good resistance to attack by bacterial 
plaque. 
For this purpose, the invention proposes a dental implant of the hybrid 
type which, in the portion at the crown end, has the characteristics of a 
screwed implant, being provided with an external thread extending for 
several flights, and toward the apex has the characteristics of a 
cylindrical percussion implant, being unthreaded and coated with a 
suitable bioactive material, or roughened or rendered porous on the 
surface. By means of this configuration, the implant can be easily 
installed by axial insertion as in the case of a cylindrical implant for 
part of its length, with insertion being completed by a screwing motion of 
only the final part, so that only a few turns are made. Since the densest 
region of the bone is precisely in the surface area (bone cortex) screw 
insertion through this layer only will provide as firm a connection as 
that of a normal screwed implant, and hence optimum primary stability, 
with far less trauma to the bone tissue, which is traversed by the thread 
for only a short distance at the surface. On the other hand, the rough or 
bioactive coating provided in the cylindrical part of the implant will 
ensure rapid bone fusion, shortening waiting times by comparison to those 
for uncoated or untreated implants. As used herein the term "bone fusion" 
means osseo integration. 
Hence a specific object of the present invention is a bone fusion dental 
implant, substantially cylindrical in shape and having a crown end 
provided with means for connection with a prosthetic reconstruction and an 
apical end intended for insertion into the bone of the upper or lower jaw, 
and made of and/or coated with material compatible with the bone tissue, 
characterized in that it comprises an externally threaded section 
extending from said crown end over no more than one-third of the entire 
length of the implant, and a section without external threads extending 
from said apical end over no less than two-thirds of the entire length of 
the implant, said section being coated externally with a bioactive 
material or surface-treated so that it is rough and/or porous. 
Preferably, the externally threaded section has no more than three or four 
flights of the self-threading type and the connection means include, as 
usual, a blind hole coaxial with the implant, open at the crown end and 
threaded internally, into which the prosthesis element is screwed, 
possibly by means of one or more intermediate elements. 
To improve the resistance to twisting forces, two or more lengthwise 
grooves are preferably provided in the cylindrical part of the implant, 
said grooves having a round cross section and two ends which are also 
round. According to a specific embodiment of the invention, the grooves 
are two in number, diametrically opposite each other, and terminate at 
their apical ends in a hole passing through the implant, which hole is 
circular or at least has rounded ends. 
As in most known cases, the material from which the implant is made is 
titanium or one of its alloys, and the cylindrical section extending from 
the apical end is preferably coated with hydroxyapatite or titanium plasma 
.

The dental implant shown in FIGS. 1 to 4 comprises a substantially 
cylindrical portion 1 which extends over about two-thirds of the entire 
length of the implant (apical third and middle third) and ends in 
hemispherical apical end 2 and a crown portion 3 extending over no more 
than one-third of the length of the implant, said portion having a 
hexagonal head 4, a flange 5, and an externally threaded portion 6. 
Cylindrical portion 1 has on its surface two opposite lengthwise grooves 7 
which are fairly wide and shallow and terminate at apical end 2 in a 
through hole 8 and extend over much of the middle third and apical third 
of the implant. 
Flange 5 also covers the surface of the cylindrical body of the implant and 
connects to it by means of a frustroconical transitional area 9. Flange 5 
has a flat surface 10 which surrounds hexagonal head 4, said surface 
providing adequate support for the prosthesis element (not shown) that 
will be supported by the implant. Immediately below frustroconical area 9 
several self-threading flights of a screw thread begin that constitute 
externally threaded portion 6. This portion does not extend beyond the 
boundary between the crown third and the middle third of the implant. 
The connection with the prosthesis element is accomplished by means of 
internally threaded blind hole 11 which receives a connecting screw and 
ends with said hexagonal head 4. As already noted, the hexagonal shape of 
head 4 is such as to prevent relative rotation between the implant and the 
prosthesis element, which is in turn provided with a recess able to engage 
hexagonal head 4 rotatably. 
The implant illustrated in FIGS. 1 to 4 is made of titanium or one of its 
alloys; the crown third, including head 4, flange 5, and externally 
threaded portion 6, is not coated or treated in any particular way so that 
its surface is substantially smooth, while the middle third and the apical 
third, including substantially cylindrical portion 1 and apical end 2, are 
coated with hydroxyapatite. Consequently, while bone growth is favored by 
the presence of the hydroxyapatite coating, in the event of partial 
exposure to the environment of the mouth due to initial bone retraction, 
the implant will not offer a surface subject to attack by bacterial 
plaque. 
The implant is inserted after preparation of a cylindrical hole in the bone 
Cf the upper or lower jaw by known drilling techniques. The implant is 
then inserted by an extremely simple and nonaggressive procedure, 
inserting it by the apical end 2 and pushing it to the bottom until the 
entire cylindrical portion 1 penetrates into the bone. At this point a 
screwing movement is executed until the entire implant including the crown 
third, penetrates the bone structure. The presence of an extremely small 
number of flights means that only a small number of rotations is necessary 
for complete insertion of the implant, with a consequent considerable 
reduction in surgical trauma. As already noted, there are enough flights 
to guarantee excellent primary stability without which bone fusion would 
not take place, since at the end of the insertion said flights are within 
the bone cortex. 
During a period of time that allows the newly formed bone to reach the 
surface of the implant (this is known to be approximately three months due 
to the fact that the middle and apical thirds of the implant are 
surface-coated with hydroxyapatite), cap screw 12 is held on the implant 
as shown in FIG. 2 and has on its upper surface a cavity 13, preferably 
hexagonal and suitable for insertion of an Allen wrench, which both holds 
and turns screw 12. Cavity 13 tapers slightly downward such that the Allen 
wrench can fit flush, affording a firmer grip and more secure movement. 
Cap screw 12 has on its lower surface a cavity 14 in the form of a 
circular crown able to receive hexagonal head 4 of the implant in order 
completely to close blind hole 11. 
At the end of the bone fusion period, when the dental implant is able to 
support the forces transmitted by chewing, cap screw 12 is removed after 
incision of the overlying oral mucosa, and a dental prosthesis is applied 
by connecting it to hexagonal head 4 in a manner already indicated. 
Once bone fusion has occurred, lengthwise grooves 7 and through hole 8 have 
the dual function of stabilizing the dental implant against the rotary 
forces applied when the prosthetic connection is made and the extractive 
forces created by chewing sticky foods. In addition, the presence of 
grooves 7 and through hole 8 increases the surface of the bone/implant 
interface and furnishes an optimum geometry for transferring forces to the 
adjacent bone. 
In the two dental implants shown in FIGS. 5-6 and 7-8, similar to the 
foregoing, similar structural elements are indicated by the same reference 
numerals. The first of the two variants illustrated shows an embodiment in 
which the entire implant has a constant diameter and flange 5 is absent, 
while hexagonal head 4 continues to be present. Between surface 10 to 
which the prosthesis is applied and the beginning of externally threaded 
portion 6 is a smooth cylindrical section 15 whose axial length is no more 
than one-eighth of the entire length of the implant, thus avoiding 
exposure of the threaded portion in the event of possible bone retraction. 
This version of the implant according to the invention later simplifies 
surgery by eliminating the necessity of creating in the bone a recess for 
flange 5, present in the first version. 
The variant illustrated in FIGS. 7 and 8 is of constant diameter like the 
previous variant and in addition has at the crown end a recess 16 of 
hexagonal shape in addition to hexagonal head 4 (shown in FIGS. 1-6). This 
variant lends itself better to the solution of esthetic problems where, 
because of a considerable angle between the implant and the prosthetic 
restoration, the external hexagonal block would invade the restoration 
itself, altering its emergence profile. In the version illustrated, cap 
screw 12, which is not intended to cover a projecting hexagonal head, is 
not provided with a recess in the shape of a circular crown but: can have 
a step 17 on its lower surface in the form of a circular crown which will 
fit over hexagonal recess 16. In this case, as in the preceding case, a 
smooth cylindrical section 15 is provided at the crown end of the implant 
to allow for possible bone retraction. 
The present invention has been described with particular reference to some 
of its specific embodiments, but it must be understood that variations and 
modifications may be made thereto by individuals skilled in the art 
without thereby departing from its scope of protection.