GB2475679A

GB2475679A - Bone implant screw

Info

Publication number: GB2475679A
Application number: GB0920579A
Authority: GB
Inventors: Andrew Dawood
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-11-25
Filing date: 2009-11-25
Publication date: 2011-06-01
Also published as: GB0920579D0; WO2011063958A1

Abstract

An implant screw for attachment into bone, wherein the shank of the screw is characterized by having a threaded portion (TB) closest to the head of the screw (HD), and a non-threaded portion (a) closest to the tip. The non-threaded portion (a) may have features to promote osseointegration, such as micro-grooves.

Description

A dental implant with high precision directional characteristics for guided surgery or other high precision applications Dental implants are typically screw shape structures which when screwed in into the jawbone provide anchorage for dental prostheses.

These endosseous implants later osseointegrate as bone cells grow onto their surface. Typical materials used for their fabrication include titanium, zirconium, titanium-aluminium-vanadium and titanium-zirconium, alloys but other possibilities will exist.

When preparing a site in the jawbone with rotary drills the operator would anticipate that the implant would precisely follow the prepared site. However this is not always the case for several reasons: one important reason is that the tip of many implant varieties is provided with sharp threads or cutting flutes or other sharp or acute' geometrical apical attributes, which may be provided with the intention of assisting in the process of cutting through the bony site, or providing retention in the bone.

Other fixtures have rounded tapered, threaded ends which are blunt but do not impose a specific directionality on the fixture. The direction in which the fixture travels will be much affected by the inclination of the fixture at the point of entry, as the fixture may be inserted at a slightly different angle to the intended site preparation, and then forced off-course by the tapered and thus aggressively engaging design.

The ability to precisely follow the site prepared by a drill or sequence of drills is particularly important in narrow bone stock and in guided surgery situations.

In guided implant surgery, a drill guide or surgical guide is provided with constraining guiding sleeves, which restrict and constrain drill position so as to prepare the bony osteotomy exactly along a prescribed path. Typically this sort of surgery is planned using CAD software, in a virtual environment. The intention is that by carefully following an incremental drilling protocol, implants may be precisely inserted along the planned trajectory with such precision that prefabricated dental prostheses may be fitted immediately upon completion of the surgery.

Because this is the most exacting circumstance for a high precision insertion for a dental implant, this implant is described with this exacting role in mind, although this sort of fixture might be used in any conventional or non-conventional implant situation, where precise fixture positioning is required.

In the course of implant site preparation, an initial osteotomy is generally performed with a narrow drill, which might be described as preparing the way for further incremental preparation with further, wider drills.

If a straight-sided cylindrical type fixture is planned, site enlargement will typically take place from a narrow starting' drill e.g 2mm, up to a larger diameter drill, eg 3.2mm for a standard 4mm diameter fixture, to create a generally cylindrical osteotomy. Because the constraining drill guides are parallel walled the drill is guided and constrained along its entire length.

If a widely tapered implant is selected then the site will be opened up with tapered drills or reamers, which will enlarge the site with a typically narrower but similar geometry to the shape of the planned fixture. Because the tapered drill is tapered, it is able to drop further into the site, which may improve guidance offered by the guiding sleeve, by virtue of the fact that the guiding surface at the hilt of the tapered drill will be quicker to contact the guiding sleeve. Accurate preparation of the site is also enhanced by the fact that the drills are side-cutting' reamers, so with the drill firmly seated in the constraining sleeve, the walls of the osteotomy may be definitively shaped.

In general, the operator will prepare soft bone less than dense bone so as to allow the implant to firmly engage the bony site as it is inserted, such that insertion torque is sufficient to stabilise the fixture adequately, but does not exceed the level prescribed for the particular implant system selected -typically 30-70 Ncm.

Fixtures with sharp cutting tips will progress more easily through a narrow osteotomy site, whereas fixtures with blunter tips require more preparation of the site, or may not be able to force their way along the site.

In guided implant insertion, implant insertion may be constrained by a close fitting guiding sleeve which contacts the fixture mount, which is typically parallel sided, such that the walls of the mount constrain insertion as they are guided through the guided sleeve. However the height of this sleeve may be such that only a fixture which has a shorter length than the extent of the guiding sleeve above the site, may be fully guided by the sleeve. Alternatively, a cylindrical fixture might itself be directly guided by a sleeve, and the affixed mount might similarly guide the fixture, if it has the same width as the fixture. However, unless the sleeve tightly constrains the fixture there may be a tendency for the fixture to proceed off-course.

All this means that in most contemporary implant systems, fixtures that are longer than this are not guided along the first few millimetres of their passage through the osteotomy as they are inserted through the guided sleeve. These parameters will change with the length of the guiding sleeve, the distance off the tissues and the length of the fixture.

Once a fixture proceeds off-course there will be a tendency for the fixture to develop friction against the guiding sleeve, causing stress and strain on the drill guide, which may lose its intended relationship with the bony substrate, causing deviation of all the fixtures from the intended computer plan.

Against this background, a fixture having a superior ability to follow a prepared osteotomy site will now be described in the following specific embodiments, with reference to figures 1-5.

Figure 1 a is a representation of a fixture having a generally cylindrical form, said fixture having a threaded body TB. Figure lb depicts a similar, but slightly tapered version.

The head (HD) of the fixture is designed for the connection of a prosthesis, and is unremarkable.

The key aspect of this innovation relates to the tip of the fixture (a) and the initial threaded part (b).

The tip of the fixture macroscopically takes the form of a blunt cylinder having a diameter that corresponds to, or is close to that of the last drill size for the intended implant system. This blunt non-cutting tip has little option but to follow a prepared site as the fixture is rotated into place. Although there may be surface characteristics present to enhance osseointegration, it is not macroscopically threaded in a way that will determine a path through the bone.

Where bone is of intermediate quality a fixture may be selected which incorporates self-tapping cutting threads (i) into the base of the threaded portion (b), as in Figuresla and lb. These threads will simultaneously further prepare the walls of the implant site as the fixture is advanced along the osteotomy site, but as there are no threads in the blunt tip, this further preparation will not result in a deviation of the fixture. However where the bone is soft, a cylindrical or slightly tapered, or distinctly tapered version of the fixture may be available without cutting threads eg, figure Ic.

The fixture in Figure 2 is blunt ended, but in this case lacks self-tapping threads. It is being placed with the aid of a Drill Guide (DG), which is seated in position over the Jaw Bone (JB). When placing a cylindrical or slightly tapered implant, the implant site is often prepared with a sequence of drills to its full depth. This leaves a cylindrical osteotomy (c) of a diameter, which is generally smaller than that of the threaded body of the fixture, such that the threads of the fixture are able to engage the freshly prepared bony walls of the site.

The core of the fixture (d) may be cylindrical or tapered. It is being inserted through the guiding sleeve of the drill guide (e) by a surgical handpiece, and is connected to the handpiece by means of a fixture mount. The mount (f) having a guiding surface (g), which contacts the walls of the guiding sleeve.

The fixture and mount combination thus has a guiding surface at its base (on the mount) and at its tip, (on the fixture), such that the fixture is guided through the osteotomy by its blunt tip and the cylindrical mount. If it is a short fixture, both guiding surfaces will act together. If it is a longer fixture, the tip will guide the fixture on its own until the mount contacts the sleeve.

Figure 3 shows the tip of the fixture. In order to enhance and expedite healing of bone onto the implant surface (osseointegration), the surface of the fixture is prepared with micro grooves (g) and or other forms of surface treatment, as typically found on the surface of dental implants.

The threaded body of the fixture may be cylindrical or parallel walled overall, but may have a tapering core', or alternatively may have a slightly tapered central core and a tapered threaded form overall. If intended for soft bone, no self-tapping element is necessary.

In dense bone situations, opening the osteotomy site to the width of the blunt tip may be inadequate to allow the threads to cut through the bony walls without further preparation. This situation may be dealt with by the use of a dedicated bone tap ( figure 4 and 5), having a similar blunt end (a), and a core of diameter equivalent to that of the last drill used so that it too follows the course of the prepared osteotomy, and tapping threads (i) which remove material from the walls of the osteotomy site, thus reducing friction between the implant and the walls of the site, and perhaps providing a threaded preparation for the threads of the fixture to follow.

Said tap would also have a guiding surface (g) at its base, such that the tap is guided through the osteotomy by its blunt tip and its cylindrical base if it is a short tap, and by its tip first, if it is a long tap, until the base comes into play.

In practice there may be more than one tap produced for the system, such that the width of the cutting threads of the tap are available in different sizes or with different pitches, so that the bulk of bone that is removed by the tap may be varied in accordance with the operators perception of the bone quality.

Thus when a fixture is placed it is guided into position by its blunt tip initially, and as the fixture mount comes into contact with the guiding surface it is also guided by the fixture mount.

However, in some situations, the fixture may be placed using conventional rather than guided' instrumentation, without the guided mount as guidance from the tip is considered to be adequate.

In a guided' surgical environment in which Cone beam CT or CT scan data are used as the basis of the planning, the software may suggest an appropriate bone tap based on the Hounsfield value or the grey levels as measured by the software.

Claims

Claims 1. An implant system for guided surgery or other situations where there is a demand for increased accuracy, which uses matched bone taps and fixtures having apical elements that are specifically intended to follow the osteotomy site as prepared by cylindrical drills.
2. An implant system for guided surgery or other situations where there is a demand for increased accuracy, which uses matched bone taps and fixtures have apical elements that are specifically intended to follow the osteotomy site as prepared by tapered drills.
3. An implant system for guided surgery or other situations where there is a demand for increased accuracy which uses matched bone taps and fixtures have apical elements that are specifically intended to follow the osteotomy site as prepared by slightly tapered drills.
4. An implant in which the apical portion of the fixture macroscopically appears generally as a smooth cylinder having no bone engaging threads or projecting features of a type which would generally be used to stabilise the body at the time of insertion, e.g. macroscopic screw threads.
5. An implant, in which the apical portion of the fixture macroscopically appears as a smooth cylinder having no bone engaging threads or projecting features of a type which would generally be used to cut threads, or engage the bony walls of the osteotomy, e.g. macroscopic screw threads or self-tapping threads; instead, microscopic non-spiral or spiral screw threads or grooves' are provided, intended to encourage osseointegration, and biological stabilisation during the subsequent post insertion healing process.
6. A tap in which the apical portion is a smooth cylinder having no bone engaging or cutting threads.
7. An implant in which the apical portion of the fixture takes the form of a macroscopically smooth cylinder having no bone engaging threads or retentive features of a type which would generally be used to stabilise the fixture at the time of insertion, e.g. macroscopic screw threads. Having surface treatments, grooves, or other retentive features designed to encourage Osseointegration.
8. An implant and matched tap in which the apical portion of the tap is a macroscopically smooth cylinder having no bone engaging threads or retentive features of a type which would generally be used to stabilise the fixture at the time of insertion, e.g. macroscopic screw threads. The implant having surface treatments, grooves, or other retentive features designed to encourage Osseointegration, said microscopic or retentive features may be of a type that is currently found at the coronal aspect of contemporary implant systems, such as micro grooves, and other surface treatments intended to encourage osseointegration.
9. A fixture having a blunt cylindrical tip as described in previous claims, said blunt cylinder may have a slight taper so as to develop a press-fit' into the prepared site.
10. A fixture and matched screw tap as described in previous claims in which there is provided in addition to the blunt apical element a threaded body, said threaded body having a central core diameter equivalent to the diameter of the apical part, and a screw thread extending outwards from the central core to engage the bony walls of the implant site.
11. A fixture and matching screw tap as described in previous claims in which there is provided in addition to the blunt apical element, a threaded body, said threaded body having a central core diameter equivalent to the diameter of the apical part, and a screw thread that extends outwards from the central core to engage the bony walls of the implant site, said screw thread having a cylindrical outer configuration.
12. A fixture and matching screw tap as described in previous claims in which, in addition to the apical element, there is provided a threaded body, said threaded body having a central core diameter equivalent to the diameter of the apical part, and a screw thread that extends outwards from the central core to engage the bony walls of the implant site, said screw thread having a tapering configuration, that is, wider at the neck of the fixture than apically.
13. A fixture and matched screw tap as described in previous claims in which in addition to the apical element there is provided a threaded body, said threaded body having a central core diameter which tapers slightly to more firmly engage the bony walls of the implant site. The associated screw thread also having a tapering or cylindrical configuration.
14. A bone tap as described in previous claims in which the tapping elements intended to tap threads into the walls of the osteotomy, are selected according to the perceived density of the bone in the site, such that larger amounts of bone are removed from the bony site in dense bone environments and lesser amounts of bone are removed in soft bone environments, thus allowing a fixture to be inserted in bone of differing density without needing to widen the osteotomy with drills of a differing diameter.
15. A threaded fixture having threads for its insertion that begin above a blunt tip that is intended to follow the pre-prepared osteotomy.
16. A fixture as described in the previous claim where some of the threads have self tapping properties
17. A bone tap for the ongoing preparation of an osteotomy for a dental implant, having cutting / tapping threads for its insertion that begin above a blunt tip that is intended to follow the pre-prepared osteotomy
18. An implant system described in previous claims in which a variety of taps are available having different arrangement of thread form, such that a different tap is selected for use with bone of different perceived density.
19. An implant system as described in previous claims in which software associated with implant planning software, or Cone beam computed tomography systems make an assessment of implant site density and suggest an appropriate tap from the system so as to optimise preparation of the implant site.
20. An implant substantially as described in the accompanyingdescription and diagrams.
21. A bone tap substantially as described in the accompanyingdescription and diagrams.
22. An implant system substantially as described in theaccompanying description and diagrams.