WO2005092236A1

WO2005092236A1 - Biomechanical implant

Info

Publication number: WO2005092236A1
Application number: PCT/GB2005/001096
Authority: WO
Inventors: David T. Downes-Powell
Original assignee: Downes-Powell David T
Priority date: 2004-03-25
Filing date: 2005-03-23
Publication date: 2005-10-06
Also published as: GB0406741D0

Abstract

An implant (10) comprises at least one extendible deformable member (13) that aids anchorage of the implant against bone (35). The implant (10) allows for bone growth, and hence osseointegration of the implant (10). The implant includes a core (11) having a socket (12) for receiving a prosthetic element, and may be particularly useful in dentistry. This implant (10) can be used in patients having low bone volume.

Description

BIOMECHANICAL IMPLANT

This invention relates to biomechanical implants such as sub- periosteal implants which are a type of implant placed beneath the layer of soft tissue immediately covering bone.

For example, a dental implant is a device placed into the hard or soft tissues of the mouth to support a prosthesis intended to restore dental function. Dental implants have been generally used to create a stable, fixed anchorage for some form of dental restoration or prosthesis, for example where there is a need to replace lost teeth. To place pins or screws into the jaw bone is generally unsuccessful because, under load, the pin or screw moves within the bone and the bone is pliable enough to allow such movement to occur.

Movement enlarges the hole and allows the ingress of bacteria between the implant and the bone. The combination of physical loss of contact with, and infection of, the surrounding bone, rapidly causes the implant to fail.

Early attempts to use artificial connection to the jawbone to stabilize dentures consisted of a frame that was custom made for the patient. The model on which it was made was cast from an impression taken directly from the patient's jawbone, which had to be widely exposed surgically for the purpose. The wound was then re-opened to fit the framework, which left, typically, four extensions protruding through the gum into the mouth. These were used to stabilize the denture. This type, being fitted below the periosteum, is called a sub-periosteal framework. This type of implant has a sub-periosteal frame that rests only on the surface of the bone, beneath the periosteum. This type is prone to failure as an implant because the necessary emergence of the implant through the overlying soft tissue leaves a space into which bacteria can enter, with invasion of the^" b^'acfefia over t e surface of the frame w thϊh die body- tissues. This causes chronic infection and rejection and loss of the frame .

A further problem was that as each implant had to be custom made for each patient, there was a wide variation in the quality of work from case to case. Furthermore, the procedure was cumbersome and surgically very invasive, with attendant fear of treatment and post-operative pain for the patient .

Other types of implant, known as endosseous implants, are placed within the bone. This type of implant is of a cylindrical, or modified cylindrical form, fitted tightly into a prepared hole in the jawbone. Healthy endosseous implants do not suffer from this infection problem because they are surrounded by, and emerge from, bone to which soft tissues are attached. Drilling a hole into the bone requires a certain volume of bone to be present. When teeth are lost, the bone that was supporting them becomes redundant and the natural reaction of the body is to remove it. When dentures are worn as a prosthesis to replace the missing teeth, the pressure that the dentures exert on the redundant bone speeds up the process of removal. This bone is necessary to stabilise the denture but the denture causes it to be lost. When the bone has been resorbed, the denture becomes so unstable that eating and talking can become difficult. Insufficient bone volume is thus common in areas of the jawbone where the teeth have been absent for a long period (a common situation in those who present wanting this treatment) , and^" can make the use of this^" type^' of imprant, impractical . The current solution to this problem is to fit two or more endosseous implants in order to use them to stabilise the denture. However, more and more frequently patients who need to have implants to stabilise a denture that has become loose because of chronic bone loss, are found, on CT scan, to have too. little bone volume remaining to accept an endosseous implant. Many patients who have been wearing dentures for some time have inadequate bone volume for the placement of endosseous implants.

This is being addressed by placing implants earlier, but this only helps newly edentulous patients. The only answer for those already bone-depleted is to graft new bone to the areas where the implants are required, or to expand the bone that is there. Both are highly specialised treatments with few practitioners experienced in their practice, and both are highly prone to problems, which can and often do lead to failure of the procedure. Such problems can be serious, sometimes even life-threatening. Sinus lift procedures which are used to create new bone volume in the maxilla to accept conventional endosseous implants, create the bone in low volumes and in a position too far away from the point of application of load from the prosthesis, leading to a substantial mechanical disadvantage for the necessarily small implant under load.

This invention seeks to provide an improved biomechanical implant which may be used, for example, in situations where there is too little bone to accommodate an endosseous implant .

According to an aspect of the present invention there is provided an implant including a core having a socket for receiving a prosthetic element, at least one deformable anchorage member extendible laterally from the core, wherein the deformable member includes passages extending through the deformable member, the passages allowing the proliferation of blood vessels through the deformable member and bone growth over the deformable anchorage member.

In use, the implant is preferably surrounded by osseointegrated bone and allows for such integration and growth. The implant of the present invention allows for growth of bone around the emergence of the implant so that the implant is able to mimic an endosseous implant without the need to drill into the bone. This helps prevent the bacterial ingress that is prevalent in prior art sub- periosteal implants. Moreover, the bone growth holds the implant securely so it can be made much smaller than a sub- periosteal frame. Preferably the implant allows for bone growth around the socket. This helps to reduce infection further.

The deformable member is preferably perforated or of an open mesh construction. It is preferably in the form of a plate extendible from substantially the entire circumference of the core. This provides greater stability for the integrated implant . Preferably more than one deformable member is provided, the deformable members being spaced apart one from the other and with bone or bone substitute material being provided in the space between the deformable members. The preferred implant is fitted to the surface of the bone as a sub-periosteal implant. The bone or bone substitute material enables the core to become surrounded by and supported by new bone. Preferably, at least portions of the implant in the region of the core are of a material having osseointegrating properties such that the new bone growth becomes osseointegrated to the implant in the region of the core. Preferably, substantially all of the implant, including the core, is made of titanium, allowing both original site bone and new bone growth to osseointegrate with the implant over all implant-bone contact areas.. Any other suitable material could be used in place of titanium.

With preferred embodiments, the implant takes its support from the bone surface and requires little depth or volume of bone. After the period of integration, the point where the core emerges from the gum is surrounded by healthy bone osseointegrated to the plates, core and/or to the socket that carries the prosthetic element. The soft tissues attached to this bone prevent the ingress of infection because there is no space for the potential entry of bacteria. Since the implant plates are made of an osseo-integrating material, such as titanium, it integrates fully with^" the^" underlying bone and provides additional support. The prosthetic element that emerges through the gum can be fitted separately after integration and can be a standard transmucosal or prosthetic element relating to an existing endosseus implant system.

In the preferred embodiment a membrane cover is provided. This is preferably resorbable so that it does not need to be surgically removed. Preferably the membrane is coated on its lower surface with a layer of bone or bone substitute.

Practical embodiments may be fabricated by a number of suitable methods. Preferred methods include milling from the solid, swaging, fabricating from pre-made components, and casting.

Preferred implants are made outside the placement site, either by freehand methods or to an analogue or model produced by suitable means.

Other preferred embodiments need not be constructed wholly or even partially in titanium since other materials with suitable surface morphology or finishes may be preferred in certain applications. Advantageously, there is no requirement for a separate surgical intervention to obtain a model on which the implant is to be based.

According to another aspect of the present invention there is provided an implant^' incorporating^" one of more magnetic indication means. Preferably, the or each magnetic indication means comprises an element having magnetic properties which may be attached to the core or to another part of the implant. For example, it may be incorporated in the head of a cover screw which engages the core before or during placement. The indication means remains in position during the healing phase which follows placement. Preferably, a user employs sensor means operable to detect magnetic properties of a type known in the art to scan the area containing the implant.

Once the or each indication means has been located, surgical intervention can be made precisely at the location of the implant core. The or each indication means may generate a magnetic field with predetermined directional properties, allowing the orientation of the implant to be determined. Alternatively, orientation of the implant may be determined by a predetermined spatial arrangement comprising two or more indication means .

Implants fitted with preferred magnetic indication means are particularly useful where implants are difficult to locate by conventional means. For example, to identify accurately the position of the implant core using metal detection can be difficult owing to the abundance of metal in the vicinity of the core.

According to a further aspect of the present invention there is provided a method^" of^' fitting ah implant^' to the^" surface of the bone, comprising: exposing the bone at the placement site; placing the implant on the bone such that the underside of the implant and the surface of the bone conform with each other; pressing the deformable member so as to conform with the topology of the surface of the bone; replacing the soft tissues and closing the wound; and allowing bone growth and osseointegration of the implant.

Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a top plan view of an embodiment of an implant;

Figure 2 is a cross-section through the implant of Figure 1;

Figure 3 is a cross-section through the implant of Figures 1 and 2 immediately after placement;

Figure 4 is a perspective view of a membrane cover for the implant of Figures 1 and 2; Figure 5 is a cross-section of the implant of Figures 1 and 2 with the membrane cover in place;

Figure 6 is a cross-section of the implant of Figures 1 and 2 after wound closure;

Figure 7 is^" a cross-sectϊόnal view of the implant of Figures 1 and 2 showing the implant after osseointegration with the core exposed; and

Figure 8 shows a cross-sectional view of the implant of Figures 1 and 2 in place and showing a standard abutment fitted.

Referring to Figures 1 and 2, an example of a preferred implant 10 is shown.

This implant comprises a core 11. The core 11 terminates at its lower portion in a point. Extending from the central core 11 are two thin plates 13 of titanium. The plates 13 include perforations 14 or are of an open mesh construction. The perforations 14 allow new bone growth. Typically, perforations of about 1.75mm in diameter are suitable for the establishment of a blood supply. The plates 13 are separated by a gap 21 of about two millimetres. Between the plates is a soft pack of bone substitute material 22.

The centre of the core 11 can be designed as required for the particular application. As shown here, the centre of the core 11 has been machined to replicate an endosseous implant head and comprises a socket 12 for the attachment of a standard abutment .

Referring to Figure 3, an example of a preferred implant is provided in the form of a dental implant shown sited on a portion of bone 35. The implant is placed following surgical expo¥ure of the^" pracem^'eήt site^". The^" site^" is prepa e^"d^" by^" rotary instrumentation to create a reasonably flat site for the implant core. The preparation is more or less limited by the available bone depth. If the bone is very thin, then the core can be gently tapped into the thin bone thereby deforming it to conform to the underside of_. the implant.

With the core as- stable as it can be made, the plates are pressed down into as close an apposition to the bone contours as possible, and the edges of the plates are tacked into place using small titanium retaining pins (not shown) . The deformable nature of the plates ensures that the implant extends over the bone surface conforming with the topology of the bone such that as far as possible the underside of the lower plate 13 is in direct contact with the bone. This close spatial relationship between the plates and the bone provides reliable anchorage for the stresses placed upon the implant after placement.

In this embodiment, the implant further includes a resorbable membrane 44. A typical membrane is shown in Figure 4, along with a retaining screw 46. In this embodiment the membrane is coated on its lower surface with a thin layer of a soft pack of bone substitute. Referring to Figures 3 and 5, the implant is fitted to the bone 35 and the membrane 44 is placed substantially to cover the implant and is held by fastening the cover screw 46 during a placement operation. Although not shown, the mucosa (soft tissues) are preserved in areas substantially removed from the placement site.

Portions of plates 13 may comprise means for anchoring the extremities of the membrane 44. In addition, portions of plates 13 are optionally provided with several small countersunk holes for the placement of, for example, a few short countersunk head titanium tacks which can be driven into and adjusted through the cortex of the underlying bone to give extra security to the implant 10 during integration or to help anchor the extremities of the membrane 44. The extremities of membrane 44 can also be anchored directly to the bone surface by well known means. It will be apparent that perforations 14 (Figure 1) provide access means via which the new bone growth will enter the space and receive a continuous blood supply.

Once the implant has been sited and the membrane 44 fixed in place, the mucosa 61 can be replaced to cover the membrane 44 (and the implant 10 below it) and the wound closed using, for example sutures 62, as shown in Figure 6.

The wound is closed by releasing the periosteum at the upper limit of the flaps, to allow closure without tension over the implant. No prosthesis can be used directly over the integrating implant.

During the healing phase that follows, the bone grows through the perforations 14 to occupy the space 21 between the plates 13. The bone substitute 22 may be autograft bone, allograft bone or any of the many types of bone substitute or combinations thereof. This provides a framework for bone growth and improves stability throughout the osseointegration phase. The perforations 14 ensure an adequate blood supply reaches the area. New bone envelopes and fuses with the bone substitute 22, eventually leading to osseointegration with the implant 10 components. Where autogenous or donor bone is used in this way, all of the space 21 is eventually filled with live bone. Where a bone substitute is used, live bone fuses with it, penetrates it and envelops it, providing a similar result.

The open mesh or perforated construction of the plates allows the proliferation of blood vessels from the surface of the underlying bone and from the periosteum, through the plates, and allows the conversion of the bone substitute to natural bone. This locks the implant into the newly formed bone, anchoring it firmly. It also creates new bone around the top of the implant at the point of its emergence into the mouth, giving it the same resistance to the ingress of infection as the conventional cylindrical endosseous implant.

In this embodiment, the selected membrane 14 is made of a resorbable material that dissolves after a predetermined time, typically six weeks or so, leaving only new bone growth 71 as illustrated in Figure 7. When the membrane 14 dissolves (or is removed) , new bone growth 71 surrounds and is osseointegrated to the core 11 of the implant and at all other points of bone-titanium contact, including on both sides of the titanium plates 13. As the wound heals and new bone becomes available, the mucosa attaches itself to the underlying bone in the area surrounding the core 11. The new bone created around the implant is produced above the level of the original bone, and brings the top of the implant nearer to the point of application of load on the attached prosthesis, giving a mechanical advantage over sinus-lift techniques of bone creation.

Referring to Figure 7, the socket 12 of core 11 is uncovered by surgery or, if suitable, a tissue punch at the end of the osseointegration phase. The implant is uncovered surgically in the same way as a conventional endosseous implant, and loaded in the same way. However, the plates 13 and the area surrounding the core 11 remain enveloped with and osseointegrated to live bone. Soft tissues 61 attached to the new bone form a seal which prevents the ingress of bacteria and maintains the integrity of the implant 10. Customised and/or standard transmucosal prosthetic elements can be supported by the socket 12 of core 11. Figure 8 shows how a standard abutment 81 can be attached to the frame by means of socket 12 and releasable screw 82. It will be apparent that the number and connectivity of members such as plates 13 that make up the implant may vary. Similarly, the size, shape, and material of the various implant components will depend, for example, upon the application and the placement site. In the case of dental implants, the dimensions preferably provide for at least about 1.5 mm of new bone growth. The size and shape of the perforations 14 provided in the plates 13 is arbitrary. The membrane 44 may be of resorbable material such as vicryl or a non-resorbable material such as Gortex. Selection of membranes 44 is not limited to those disclosed herein.

In one modified version of the implant, core 11 can be made deeper at the bone side to increase the depth available for screw 82 and/or to increase the area available for osseointegration. This modification also creates the effect of increasing lateral stability at the socket 12 by resisting twisting and bending relative to the implant 10.

In another modified version, the plates 13 or other elements that make up the implant and are in contact with the site bone surface may be of a substantially reduced cross-section on the bone side. This "apex" shape improves performance of implants because it allows new bone growth right into the areas 63 where the soft tissues are lifted away from the bone surface immediately adjacent the deformable members. Once the bone has grown into this space and has integrated with the implant surface, the area of plate-bone contact is greater than would otherwise have been achieved resulting in greater lateral stability and more effective soft tissue attachment in the region of the plate. The greater pressure generated as a result of the reduced cross-section allows it to settle into the bone surface and thereby assists integration.

When a suitable site has been produced for the central core II^" of fϊTe^" implant^" 10, a pilot hole^" can 5^"e^" drilled^' centrally with a 1.5 mm parallel twist drill. During placement of the implant 10, an incision is made in the soft tissues over the site of placement and to one side of the area where the core 11 will emerge. This ensures that the wound-line is not over the emergence area.

Closure of the wound can be by 3/0 resorbable sutures 62 through the periosteum and superficial closure by suitable sutures .

Three or so months after the fitting of the implant 10 the abutment may be fitted. The first stage is to locate the position of the core 11. There may be too much metal located below the gum for a conventional locator to work. A preferred alternative is to incorporate magnetic or other location indication means. For example, within a Teflon (RTM) cover screw at the time of placement followed by use of magnetic sensor means to locate the core 11.

Preferably, the or each magnetic indication means comprises an element having magnetic properties which may be attached to the abutment socket 12 or another part of the implant 10. For example, it may be incorporated in the head of the cover screw 46 which engages the abutment socket before or during placement. The indication means remains in position during the healing phase which follows placement. Preferably, a user employs sensor means operable to detect magnetic properties to scan the area containing the implant.

Once the or each indication means has been located, surgical intervention can be made precisely at the location of the abutment socket 12. The or each indication means may generate a magnetic field, allowing the location of the implant to be determined. Alternatively, location of the implant may be determined by a predetermined spatial arrangement comprising two or more indication means.

Implants 10 fitted with preferred magnetic indication means are particularly useful where implants are difficult to locate by conventional means. For example, to identify accurately abutment sockets 12 using metal detection can be difficult owing to the abundance of metal in the vicinity of socket 12.

Once located, the cover screw can be uncovered by using a machine driven tissue punch as is conventional. An abutment can then be fitted to socket 12 in a conventional manner and prosthetic reconstruction can proceed as with a conventional endosseous implant.

Only a single surgery is needed to place the implant 10 and a further minor surgical procedure is required after three months to uncover the socket 12 of core 11 so that abutments 81 can be fitted. In some cases more than one intervention may be appropriate. In a modified version of the implant 10 the plates 13, core 11, sockets 12 or coverscrews 46 can be adapted to facilitate location of the emergence point by different means, such as electronic locator means or by probing. A computer could be used to design and create the frame. Using different layouts framework connectivity or varying diameter holes allows the strength of the metal and its swagability to be optimised, while keeping it as open as possible .

The preferred embodiment of the present invention provides an implant which can be pre-manufactured, sold as a standard unit and then fitted to the patient .

This embodiment provides a subperiosteal implant 10 that is placed into close apposition to the vital bone of a patient's jaw 35, the site being prepared to improve the adaptive fit of the implant to the bone surface, and to produce a certain amount of frictional stability between the prepared bone and the implant.

The implant is placed by reflecting the mucosa 61 and periosteum to expose the bone. The surface of the bone 35 is modified if necessary using a form tool, the implant 10 is placed on the prepared site, and the periosteum is replaced over the implant. To mature, and for the newly formed bone to integrate to the implant subsequently, it is uncovered and loaded like a conventional implant.

That part of the implant 10 to which the core 11 is attached should be surrounded by newly formed bone 71. The attachment of the overlying soft^" tissues 61 fo Oil^'s ew bόhe 71^' prevents^" the ingress of pathological organisms that could otherwise colonise the metal surface of the implant.

The form of the implant is of a central core 11 with its axis at right angles to the bone 35 surface. Circular plates 13 radiate at an angle to the vertical axis of the core 11. These plates are perforated 14 to allow blood vessel proliferation into the areas between the fins without impediment .

The areas between the plates 13 are packed with a bone graft or substitute, or a material that actively or passively promotes osteogenesis .

The bottom of the implant 10, that end in contact with the patient' s vital bone, is shaped and surface textured to increase its area of fit to the prepared bone 35, and to encourage integration of that surface of the implant to this existing vital bone 35.

The top of the implant 10, that end in contact with the periosteum, is shaped, if desired, to replicate the top of a suitable implant from an endosseous system, for attachment of transmucosal elements.

After an appropriate time, the top of the implant 10 is exposed and the implant 10 put to use exactly like a conventional endosseous implant.

Even where the bone volume is low, bone surface is available, and this lends itself to the placement of a surface fitted implant such as the original sub-periosteal frame. If such a non invasive implant could be made as infection resistant as the endosseous type, it would have an immediate and wide application.

This application describes an implant which is minimally invasive, allowing it to be placed onto the surface of low- volume bone, but which osseointegrates both to increase its stability and load-carrying ability, and to prevent the ingress of infection. These features make it unique in its type and usefulness.

One of the great advantages of this implant is that it would be produced as a saleable item as part of the product range of an existing manufacturer of endosseous implants. This creates excellent standardisation and quality control, and allows placement using simple and standard techniques that can be easily taught.

Claims

1. A sub-periosteal implant (10) including a core (11) having a socket (12) for receiving a prosthetic element, at least one deformable anchorage member (13) extendible laterally from the core (11), wherein the deformable member (13) includes passages extending through the deformable member, the passages allowing the proliferation of blood vessels through the deformable member and bone growth over the deformable anchorage member.

2. An implant as claimed in claim 1, wherein the implant allows for bone growth around the socket.

3. An implant as claimed in claim 1 or 2, wherein the deformable member (13) is perforated or is of an open mesh construction.

4. An implant as claimed in claim 1, 2 or 3, wherein the deformable member (13) is in the form of a plate extendible from substantially the entire circumference of the core (11).

5. An implant as claimed in any preceding claim, wherein more than one deformable member (13) is provided, the deformable members (13) being spaced apart one from the other .

6. An implant as claimed in claim 5, wherein bone or bone substitute material (22) is provided in the space between the deformable members (13) .

7. An implant as claimed in any preceding claim, wherein at least portions of the implant (10) in the region of the core (11) are of a material having osseointegrating properties such that new bone growth (71) is able to become osseointegrated to the implant (10) in the region of the core ll)-.

8. An implant as claimed in claim 7, wherein substantially all of the implant (10), including the core (11), is made of titanium.

9. An implant as claimed in any preceding claim, further comprising a membrane cover (44).

10. An implant as claimed in claim 9, wherein the membrane cover (44) is resorbable.

11. An implant as claimed in claim 9 or 10, wherein the membrane cover (44) is coated on its lower surface with a layer of bone or bone substitute (22) .

12. An implant as claimed in any preceding claim, wherein the implant (10) incorporates one or more magnetic indication means .

13. An implant as claimed in claim 12, wherein the or each magnetic indication means comprises an element having magnetic properties that may be attached to the core (11) or to another part of the implant (10) .

14. A method of fitting an implant as claimed in claim 1 to the surface of bone, comprising: (a) exposing the bone at the placement site; (b) placing the implant on the bone such that the underside of the implant and the surface of the bone conform with each other; (c) pressing the deformable member so as to conform with the topology of the surface of the bone; (d) replacing the soft tissues and closing the wound; and (e) allowing bone growth and osseointegration of the implant .

15. A method as claimed in claim 14, wherein prior to step (b) , the placement site is prepared by creating a substantially flat site for the implant core on the surface of the bone.

16. A method as claimed in claim 14 or 15, wherein prior to step (d) , a membrane is placed substantially to cover the implant .

17. A method as claimed in any of claims 14, 15 or 16, further comprising locating the position of the core and uncovering the socket of core so that the socket is available to receive an abutment.

18. A method as claimed in claim 17, wherein the implant incorporates one or more magnetic indication means, and a sensor means operable to detect magnetic properties is employed to scan the areas containing the implant, so as to locate the position of the core.