A Bone Implant

Abstract

There is described a bone implant comprising at least one means for providing at least one bone stimulation dynamic interaction to at least one area of a bone-implant interface formed when the implant is inserted into bone, wherein the bone stimulation is one or more of bone growth, bone strengthening, bone densification and/or osseointegration between bone and the bone-implant interface.

Claims

1. A bone implant comprising: an implant body, an implant head, and optionally, an implant head extension for removeable attachment to the implant head, and at least one mechanical means associated with the bone implant for providing at least one bone stimulation interaction to at least one area of a bone-implant interface formed when the implant is inserted into bone, wherein bone stimulation is one or more of bone growth, bone strengthening, bone densification and osseointegration between and/or around the bone and the bone-implant interface, wherein the mechanical means comprises one or more actuatable elements arranged to provide active load stimulation to the bone-implant interface.

2. The bone implant of claim 1, wherein one or more of the actuatable elements are freely mobile within the implant, implant body, implant head and/or a removable implant head extension such that on experiencing movement, the elements translate, for example, longitudinally, transversally, and/or radially within the implant body, implant head and/or a removable implant head extension.

3. The bone implant of claim 1, wherein one or more actuatable elements comprise one or more discrete bodies that are freely mobile within the implant body, the implant head and/or the implant head extension.

4. The bone implant of claim 1, wherein one or more actuatable elements comprise grains, hollow or solid spheres, rolling or ball bearings, particles, and/or one or more translatable rods, pins, cylinders, or combinations thereof or a cannulated rod or cylinder.

5. The bone implant of claim 1, further comprising at least one chamber, region, passageway, cannulation or orifice in the implant body, implant head and/or the implant head extension, comprising the mechanical means, and optionally wherein the mechanical means is removable from the implant.

6. The bone implant of claim 1, wherein the bone implant is dissolvable in situ.

7. The bone implant of claim 1, wherein attachment of the implant head to the implant body is a moveable attachment wherein the implant head is translatable along a portion of a longitudinal axis of the implant and/or is rockable when positioned on the implant body, and optionally, the implant head is removeable from the implant body.

8. The bone implant of claim 1, wherein the mechanical means moves with the bone implant on kinetic movement of a subject having the implant, or the mechanical means move independently of subject's kinetic movement, and optionally, is controlled by a motor or other controlling device.

9. The bone implant of claim 1, wherein the implant body, implant head and/or the implant extension head is cannulated.

10. The bone implant of claim 1, comprising an implant body for insertion and retention of the implant into bone, an implant head engagable with a tool for insertion of the implant body into the bone, and optionally a neck portion for connecting the implant body with an implant head.

11. The bone implant of claim 1, further comprising an implant body cover and/or an implant head cover, and/or an implant extension cover for closing off internal parts from tissue ingress.

12. The bone implant of claim 11, wherein the cover is a temporary cover, and optionally is a pierceable cover formed from a layer of biocompatible material which can be penetrated.

13. The bone implant of claim 12, wherein the implant head extension and/or the temporary cover has a shape and/or configuration that enables location under the subject's skin, and optionally comprises a pointed or textured surface for tactile location under the skin or a radio opaque material for location via CT or radiological method.

14. The bone implant of claim 1 which is a pedicle screw and further comprises an implant set or locking screw or cap for locking a spinal rod into the implant head.

15. The bone implant of claim 14, wherein the locking screw or cap comprising the mechanical means, preferably wherein the mechanical means is removable from the locking screw or cap.

16. The bone implant of claim 1 used in the treatment and/or prevention of a bone wasting/weakening disease or disorder.

17. A surgical method for the treatment, prevention and/or correction of one or more of spinal stenosis, spondylolisthesis, spinal deformities, fracture, pseudoarthosis, tumour resection, failed previous fusion, degenerative disc disease, dislocation, scoliosis, and kyphosis, wherein the surgical method comprises the use of the bone implant of claim 1.

18. A kit of parts comprising a plurality of bone implants according to claim 1.

19. The kit of claim 18, further comprising a sterilisable housing which accommodates the plurality of the implants.

20. The kit of claim 18 which is a surgical instrumentation kit used in a spinal fixation surgical procedure.

21. The bone implant of claim 1, which when implanted promotes bone stimulation including bone growth, strengthening, densification and/or osseointegration.

22. The surgical method of claim 17 which is a two stage surgical method which includes: a first stage surgical step whereby the bone implant of claim 1 is inserted into a bone at a first timepoint, and subsequently, a second stage surgical step at a second timepoint in which a load is applied to the bone implant inserted in the first stage surgical step, whereby the interval between the first step and the second step is sufficient to allow an acceptable level of osseointegration to occur between the implant and bone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0125] A preferred embodiment of the invention will now be described by way of specific example with reference to the accompanying drawings, in which:

[0126] FIG. 1 is a schematic view of a partially sectioned vertebra including two prior art pedicle screws;

[0127] FIG. 2 illustrates a cannulated pedicle screw comprising multiple free moving metallic balls;

[0128] FIG. 3 illustrates a similar embodiment to that of FIG. 2 although the multiple free moving metallic balls have been replaced with a single load in the form of a weighted cylinder; and

[0129] FIG. 4 illustrates the use of an external magnetic field to increase the displacement of the load/loads located within the inside chamber of the screws detailed in FIGS. 2 and 3.

[0130] FIG. 5(a) to (c) illustrates further preferred embodiments of the invention; (a) is a pedicle screw and tulip head arrangement with one example of a removable implant head extension with freely mobile particles within the implant head extension of the invention; (b) is a pedicle screw shank and a mobile, slidable implant head of the invention; (c) is a cannulated pedicle screw with conical shaped removable implant head extension, the extension having a temporary anti-tissue overgrowth cap.

DESCRIPTION OF THE INVENTION

Healing Process and Bone-Implant Integration

[0131] Healing and bone integration is a complex process. Insertion of the screw causes disruption of the normal bone structure and results in the formation of a blood clot around the insertion site. The blood clot attracts blood cells to clean up the wound, initially monocytes then macrophages, as it the case for any wound. These cell, especially the macrophages, produce growth factors which promote angiogenesis and activate fibroblasts to produce an extracellular matrix. The “tissue” produced at this stage is called granulation tissue. Weak bone called woven bone or fibrous bone then results. The bone will be produced first around the new blood vessels, explaining why the local environment is so important (for example smokers have been shown to have poor bony healing because of the toxins in the cigarette affecting newly formed blood vessels). Mesenchymal progenitor cells then differentiate into osteoblasts cells producing new bone and the woven bone soon bridges between the implant or screw, for example, and the host bone. If the distance is too large this process cannot happen and this explains why it is important to have good initial contact between the implant and the host bone. The next step involves formation of stronger bone called lamellar bone and only after, bone remodelling (which is influenced by loading and stress) occurs. Wolff's law holds that bone adapts to functional loading. For example, if loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading. Likewise, if the loading on a bone decreases, the bone will become less dense and weaker due to the lack of the stimulus required for continued remodelling. However, the inventors believe that too much loading prevents optimal osseointegration and not enough (e.g., a screw left alone in the bone completely unloaded) loading negatively affects osseointegration. The devices described herein, particularly those with internal loads, produce an adequate level of stimulation of the surrounding cells to achieve optimal osseointegration within the best timeframe. After sufficient time, when optimal osseointegration is achieved as a result of utilisation of the device of the invention, the screws and osseointegrated bone can readily and better sustain the required loading produced by, for example, rod insertion, compared to the performance where conventional screws are used and are loaded immediately after surgical implantation.

Evaluation of Osseointegration

[0132] There are several ways of assess bony healing of an implant. One method involves a clinical assessment. A solid and/or secure implant should not cause any pain or instability (pain or neurological deficit during movement). Another method of confirming bony healing is radiologically, using for example, plain X-rays, dynamic X-rays (there should not be any movement at the level fused during the flexion and extension view of the spine) and/or fine cut bony sequence CT to look for bone crossing the implant (cages) or lucency around the screw which is an indirect sign of loosening. Alternatively after a few months there should be very limited residual bony activity around the implant and therefore there should not be any uptake tracer activity on the bone scan+SPECT. In some cases, the assessment must be made by exploratory surgery.

[0133] There are several other methods for the evaluation of the degree of osseointegration, broken into invasive and non-invasive categories. Invasive methods include histological section, histomorphometric, transmission electron microscopy, pull out tests, torque gauge tests. Microscopic or histologic analysis has been considered as the gold standard method to evaluate the degree of osseointegration. However, due to the invasiveness of this method and related ethical issues, various other methods of analysis have been proposed. Suitable non-invasive methods include light microscopy, microcomputed tomography, backscatter electron imaging, percussion test, radiographs, reverse torque test, periotest, and resonance frequency analysis.

[0134] In the percussion test, an osseointegrated implant will be found to make a ringing sound on percussion whereas an implant that has undergone fibrous integration produces a dull sound.

[0135] In the torque gauge test, a reverse or unscrewing torque is applied to assess implant stability at the time of abutment connection. Implants that rotate under the applied torque are considered failures and are then removed.

[0136] The periotest relates to a device which is an electrically driven and electronically monitored tapping head that percusses the implant a total of 16 times in about 4 s.

[0137] Scanning electron microscopic study of the interface typically shows the characteristic absence of connective tissue between the bone and implant surface.

[0138] Resonance frequency analysis measures implant stability and bone density at various time points using vibration and structural principle analysis. Classically, the implant stability quotient (ISQ) has been found to vary between 40 and 80, the higher the ISQ, the higher the implant stability. It is inversely proportional to the resonance frequency. Implant stability can be determined for implants with an ISQ of 47. All implants with an ISQ more than 49 osseointegrated when left to heal for 3 months. All implants with an ISQ more than 54 osseointegrated when immediately loaded.

[0139] Osseointegration may also be assessed by applying the Alberktsson Success Criteria: 1. The individual unattached implant should be immobile when tested clinically. 2. The radiographic evaluation should not show any evidence of radiolucency. 3. The vertical bone loss around the fixtures should be less than 0.2 mm per year after first year of implant loading. 4. The implant should not show any signs of pain, infection, neuropathies, parasthesia, violation of mandible canals and sinus drainage. 5. The success rate of 85% at the end of 5 year and 80% at the end of 10 years.

Staged Implantation Technique

[0140] The implant of the invention can be used in open surgery, minimally invasive surgery. Desirably, implant can be used in a staged implantation technique whereby in a first stage, the implant is positioned and given time for a desirable amount of osseointegration to occur.

[0141] After the surgical placement of implants into a desired location, the traumatized bone around the implant begins the process of wound healing which can be separated into an inflammatory phase (involving vascular and cellular events), proliferative phase (involving vascularization, cell differentiation into fibroblasts, osteoblasts and chondroblasts and eventual bone callus formation) and a maturation phase (involving ossification of the bone callus, and bone remodelling).

[0142] The staged implantation technique requires a bone implant that comprises a modular distal threaded bone anchor and a proximal screw head, wherein the bone implant comprises one or more of described loading mechanisms, for example, an internal rolling element or cylindrical member. A preferred implantation technique involves the following: [0143] i) Surgical Stage 1—a first bone implant, for example, a bone anchoring component is inserted into a bone, for example, a bone pedicle. This bone anchoring component used is suitably adapted to function in use with one or more of the loading mechanisms required to stimulate osteogenesis. The method preferably further comprises a pre-preparation step involving inserting bone substitute or osteoinductive bone morphogenic proteins (BMPs) into a passageway provided in the implant to facilitate osteoconduction. It will be understood that osteoinduction is the process by which osteogenesis is induced. It is a phenomenon regularly seen in any type of bone healing process. Osteoinduction implies the recruitment of immature cells and the stimulation of these cells to develop into preosteoblasts. [0144] ii) Surgical Stage 2—is preferably separated from Surgical Stage 1 by a period of at least 6 weeks or other period required to ensure a sufficient amount of osseointegration has taken place. Surgical Stage 2 involves removal of the aforementioned osseointegration promotion means for applying a dynamic physical force to bone at one or more bone-implant interfaces, if necessary, followed by insertion of an internal member that completes the bone anchoring component, for example, a poly-axial screw head in the case where the bone anchoring component is a threaded pedicle screw shank, allowing final construct fixation and fusion with a rod.

[0145] When used in the staged implantation method the clinician will be able to identify sufficient bony on growth or in growth and subsequently decide to insert polymethylmethacrylate or another suitable biocompatible bonding agent into the passageway.

[0146] The benefit of staged implantation allows the clinician to assess after at least a 6 week period, the quality of osseointegration with the bone anchor device. This can be assessed radiologically or by other investigative means. This is of particular utility in patients who are osteopenic, in that the level of adequate integration of the device can be measured before attempting to finalise a construct for fusion. Early fusion of construct without adequate bone anchor integration can result in poor fixation, fusion failures and other complications. The clinician can track and assess the required level of fusion and then at the appropriate time either fuse or revise surgical strategies.

System—Novel Instrumentation

[0147] In a preferred embodiment, the implant includes but not limited to: an in vivo bone density measurement tool (qualitative).

[0148] Further instrumentation can be devised, including but not limited to an electronic device that is placed into the stage 1 bone anchoring component that runs a current through the implant and bone and provide feedback and qualitative measurement of the degree of osseointegration. This adaptation provides easy feedback by way of instrumentation for the surgeon.

Novel Internally Loaded Design for Dynamic Osseointegration

[0149] The internal chamber of the threaded shank is unique in that its bone promoting mechanism can be described as follows: [0150] i) One or multiple ball bearings that are freely mobile within the screw shaft that actively load and vibrate the screw/bone interface in order to stimulate osteogenesis (for use in open screw techniques) [0151] ii) A cannulated cylinder that longitudinally translates within the cannulated screw shank that provides an active load and mechanical vibration to stimulate osteogenesis in the screw/bone interface (for use in percutaneous application) [0152] iii) The internal mechanical devices described in i) and ii) could be comprised of magnets that are translated by polarised magnets placed at both proximal and distal ends of the screw shaft. [0153] iv) An alternative to translating the mechanism in iii) is by the external induction of a current or electrical stimulation device to facilitate active internal load translation. [0154] v) An internal cylinder as described in ii) that emits a sonic vibration/frequency that stimulates osseointegration across the bone/screw interface.

Screw Head

[0155] While the implant described herein can be used as a classic pedicle screw, it is believed that using the implant in conjunction with a two-stage implantation and loading procedure would increase significantly the quality of the osseointegration. This means leaving the implants, e.g., screws, inside the host for a sufficient length of time to allow optimal osseointegration to occur as a result of the dynamic loading provided by the implant of the invention (e.g., 2-6 months), prior to applying the functional load, in a second procedure, for example, connect them to a rod in the case of spinal fixation.

[0156] In the context of a two-stage implantation, a preferred head for the implants of the invention ideally have specific characteristics to facilitate this use. For example, preferred implant heads are designed to avoid any bony or tissue ingrowth inside them which is undesirable, e.g, resulting in rod interference or prevention of installation of a set screw. A preferred implant head is easy to find. For the most preferred implant heads, re-cannulation without direct visual sight is possible. A preferred implant head has a conical shape that can be re-cannulated regardless of the angle. A preferred implant head has pierceable cap, which can be pierced by a specific instrument, and which can be located via tactile feedback or other kind of feedback to confirm the surgeon that the head was adequately cannulated. The pierceable cap, for example, a plastic cap can desirably also prevent tissue or bone ingrowth inside the head.

[0157] Preferred implant insertion involves a minimally invasive approach (e.g., stab incision) under X-ray, or more preferably, CT guidance using a technology such as the intraoperative O'Arm. A similar technique may be used to connect the implant head to the rod, however, the tulip head typically used can be quite difficult to find as the implants are usually quite deep under a thick layer of muscle and/or fat and reattaching towers or inserting a screw driver inside them can be challenging.

Dissolvable Implant

[0158] A dissolvable screw can be inserted using a minimally invasive approach. Once the implant is fully dissolved then the final procedure (e.g., a standard lumbar fusion surgery) would be performed.

[0159] A preferred implant has the internal structure described herein, but the shaft and/or internal load could have small balls that are moving freely and will also gradually dissolve. The balls or the shaft is at least partially coated in recombinant bone morphogenic protein which will enable more rapid and more efficiently differentiation of mesenchymal cells into bone forming cells. Furthermore, the implant thread is preferably designed to create enough granulation tissue to enhance bone formation rather than optimal primary fixation, as the implant of this aspect of the invention would not be loaded. The purpose of the dissolvable implant is localized bone structure enhancement. Use of the dissolvable implant in weak bone will, through application of the dynamic forces described herein, change the internal bony structures of a vertebra from normal or weak or cancellous or osteoporotic bone into a stronger bone which would offer a better structure for the subsequent insertion of a load bearing implant such as a pedicle screw. In other words, the dissolvable implant of the invention functions as a localized bone structure enhancing device.

Description of Preferred Embodiments of the Invention

[0160] Turning now to FIG. 1, which shows a top down view of a vertebra 101 into which vertebral body 102 is inserted a pedicle screw 107 through the pedicle bone 108 of the vertebra 101.

[0161] FIG. 2(a) is a cannulated pedicle screw 201a having a shank 202 and a tulip head and a dummy cap weighted for bone loading provided at one end of the shank. The shank 202 is provided with fenestrations 203 to allow egress of bone cement and the like and/or ingress of bony tissue. The cutting tip geometry 204 is also shown. FIG. 2(b) shows the side profile of the screw of FIG. 2(a), which FIG. 2(c) shows a sectional view of the screw in which the following are illustrated: the dummy cap (removable implant head extension) 205 which comprises a rod portion 206 to lock polyaxial motion during osseointegration; a section view of the cannulation 207; freely moveable elements 208, particles or spheres, inside the cannulation 207 of the screw 201. The wide diameter cannulation 209 is also shown which accommodates the moveable elements 208.

[0162] FIG. 3(a) is a cannulated pedicle screw 301a having a shank 302 and a tulip head and a standard locking cap or set screw for locking a rod in place provided at one end of the shank. The shank 302 is provided with fenestrations 303 to allow egress of bone cement and the like and/or ingress of bony tissue. The cutting tip geometry 304 is also shown. FIG. 3(b) shows the side profile of the screw of FIG. 3(a), which FIG. 3(c) shows a sectional view through line A-A of 301b of the screw in which the following are illustrated: the locking cap; a section view of a portion of narrow cannulation 307, and larger diameter cannulation 309; translatable element 308, a single cylinder, inside the implant body 302.

[0163] FIG. 4 illustrates a patient 400 having implanted pedicle screw in L4 vertebra 401 and a magnetic field generator 402 for enabling trans cutaneous magnetic stimulation of magnetised actuatable elements within the implant.

[0164] FIG. 5(a) illustrates a pedicle screw 500a having a removable implant head extension 501 which comprises internally the actuatable elements, in this case particles 501 which are freely moveable and which resultant kinematic energy or forces is translated through the implant to the bone-implant interface. FIG. 5(b) illustrates a pedicle screw 500b having a mobile implant head 530 that is adapted to moveably engage with the implant body. The mobile implant head can translate back and forth along a portion of the longitudinal axis of the shaft in the directed indicated by the arrows to thereby generate the required mechanical motion to generate the required forces. FIG. 5(c) illustrates a cannulated pedicle screw 500c having a shank 507 and a tulip head and a conically shaped removable implant head extension 504, which in this example contains freely moveable particles. The upper surface of the removable implant head extension is covered with a frangible cover material to prevent tissue ingress or ingrowth within the implant and/or around the upper portion of the implant body where the rod and/or set/lock screws are to be placed, for example, a biocompatible polymer which is pierceable by incision or puncture, with for example, a K-wire for bone cement introduction or other such operation. The conical shape of the head or the extension advantageously allows for a much wider degree of cannulation.

EXAMPLE 1

[0165] One or more rolling elements or bearings, for example, ball bearings, that are freely mobile within the implant body such that on movement of the implant, the rolling elements actively apply a dynamic load and vibration to the bond-implant interface in order to stimulate osteogenesis.

[0166] This embodiment is illustrated in FIG. 1 which shows an example of a preferred implant of the invention, that is, a cannulated pedicle screw. The tip of the screw is designed to allow bone ingrowth within the shaft. The internal chamber contains multiple free moving metallic balls. The outside shaft of the bone anchor depicted has a specific thread to ensure maximal pull-out strength or primary fixation. In this example, the pedicle screw has a poly-axial head adapted to engage rods. The poly-axial head is closed with a specific cap preventing tissue ingrowth when the pedicle screw is used as an anchor for example in Stage 1 surgery whilst waiting for osseointegration to occur, and is designed to receive a drive tool such as a hexagonal tool or another such torque transmission tool.

EXAMPLE 2

[0167] A cannulated cylinder that longitudinally translates within the implant body that provides an active load and mechanical vibration to stimulate osteogenesis in the bone-implant interface and is particularly suited for use in a percutaneous application. The internal cylinder can be adapted to emit a sonic vibration/frequency that further stimulates osseointegration across the bone/screw interface.

[0168] This embodiment is illustrated in FIG. 2 which shows a similar embodiment to FIG. 1 although the multiple free moving metallic balls have been replaced with a single load. The load has a cylinder configuration and traverses up and down within the inside chamber generating the required forces and/or vibration at the bone-implant interface.

EXAMPLE 3

[0169] The loading mechanisms described in Examples 1 and 2 comprising a magnetic material such that the elements or cylinder can be translated by polarised magnets placed at both proximal and distal ends of the implant. Alternatively, the elements or cylinder can be translated by the external induction of a current or electrical stimulation device to facilitate active internal load translation. This embodiment is illustrated in FIG. 3 which shows the use of an external magnetic field to increase the displacement of the load/loads located within the inside chamber of the screws detailed in FIGS. 1 and 2. The patient applies on his skin at the level of the implant, a magnetic field generator contained in a small ‘portable’ device. The generator delivers an electrical current across the tissue without physical contact increasing the displacement of the loads within the inside chamber of the screw.

EXAMPLE 4

[0170] The example in FIG. 5(a) is a pedicle screw and tulip head arrangement with one example of a removable implant head extension with freely mobile particles within the implant head extension of the invention which overlays a typical tulip head of a pedicle screw. The removeable head extension is placed during an initial surgical step to prevent tissue ingress into the tulip head during healing. The gentle dynamic forces resulting from movement of the actuatable elements in the removeable head extension are translated through the screw head to the implant body and to the implant/bone interface.

EXAMPLE 5

[0171] The example in FIG. 5(b) is a pedicle screw and tulip head arrangement where the actuatable element of the mechanical means is the tulip head itself. The tulip head is mounted on a leg portion which is adapted to fit within the internal circumference of the implant body but it is not locked into this position. Instead the implant head is free to move up and down the longitudinal axis of the implant body in the direction of the arrows shown in the Figure. When sufficient healing has occurred, locking of a spinal rod in the tulip head can cease the head movement.

EXAMPLE 6

[0172] The example in FIG. 5(c) is a pedicle screw and tulip head arrangement where the actuatable element of the mechanical means is provided in a conically shaped removable implant head extension with freely mobile particles provided within the implant head extension. In this example, the conically shaped removable implant head is provided on K-wire to facilitate insertion into the cannulated implant body as shown. While prevent tissue ingress, the wider diameter of the conically shaped removable implant head in the distal portion of the conically shaped removable implant head advantageous allows easier location of the implant under the skin and tissue after healing has occurred, and provides a wider guide for inserting a K-wire or syringe or the like to for example facilitate insertion of bone cement or other materials. In this example, the conical tip can comprise or more apertures to facilitate this insertion of other devices. The conically shaped removable implant head has a temporary and/or frangible membrane cover enabling later removal of the actuatable elements and/or facilitation of insertion of other tools/objects.