Patent Application
20210361830
The present invention relates to medical materials and devices, and particularly, although not exclusively, to materials and devices for use in orthopaedic applications.
A medical implant is a device typically manufactured to replace, support or enhance a missing, or damaged biological structure within the human body. In the field of medical implants for orthopaedic applications, such implants include e.g. hip prostheses, knee replacement implants, intervertebral prostheses, cranial prostheses etc.
Typically, such implants must be made out of a material which is biocompatible for the intended application. Biocompatible is generally understood to mean the ability of a material to perform with an appropriate host response in a specific application [The Williams' Dictionary of Biomaterials, D.F. Williams, 1999]. For articulating implants having bearing surfaces, e.g. hip prostheses, knee replacement implants, spinal implants, spinal disc prostheses etc., the tribological properties of the implant are also important, because excessive wear of the implant material and/or production of particulate matter within a joint may shorten the lifespan of the implant, and are generally undesirable.
A wide range of materials are presently used for such medical implants, as will be known by the skilled person. For orthopaedic, spinal and craniomaxillofacial applications, a selection of known materials includes titanium and its alloys, acrylic, ultra-high-molecular-weight polyethylene (UHMWPE), polyetheretherketone (PEEK), hydroxyapatite (HAP) and others. However, each of these materials has a number of associated problems or limitations. For example, use of titanium may result in postoperative CT & MRI scan artefacts, which can render them impractical or unsuitable for use in situations where post-operative imaging may be required. UHMWPE and acrylic are non-steam sterilisable due to the risk of material degradation at the required temperatures, whilst acrylic has both a relatively low toughness and has been indicated to present a relatively high post-surgical infection rate when used in cranial prostheses. PEEK is relatively expensive, and furthermore has a relatively low toughness compared to similar materials. HAP, whilst being known to promote osseointegration in some situations, is expensive and brittle with poor malleability, and has relatively low strength resulting in risk of implant collapse in-vivo.
Whilst the above materials are a selection of common matrix materials, it is also known to use additional filler materials to improve various properties of the matrix materials for use as a medical implant material. For example, TiO2 is known to be a photocatalytic material that is nontoxic, and generally chemically stable. There has been some work done on incorporating TiO2 into materials for orthopaedic applications. E.g. Visai et al (2011) have reported on use of TiO2 for use in surface coatings for biomedical devices. However, providing a surface coating to a device requires one or more additional manufacturing stages after formation of the device. Furthermore, care must be taken to ensure adequate attachment of the coating to the device, due to the risk of production of particular matter from the coating in-vivo which may be undesirable.
Diez-Pascual et al (2014) have reported on the properties of nanocomposites of biocompatible polyphenylsulphone and titanium dioxide nanoparticles prepared via ultrasonication and solution casting. Here, it was shown that the nanocomposites exhibited inhibition against Gram-positive and Gram-negative bacteria. However, here it was shown that loading of TiO2 was not always effective for bacterial growth inhibition without UV-irradiation, particularly at low loading amounts. Accordingly, the materials proposed therein may not provide suitable antibacterial effect on implantation in-vivo, where UV irradiation of the implants will not occur.
Accordingly, it would be desirable to provide a material which provides superior antibacterial, mechanical and/or tribological properties compared to known alternatives.
One procedure in which medical implants are commonly used is cranioplasty. Cranioplasty is a medical procedure used to repair cranial defects, and may be required for a number of indications including brain haemorrhage requiring decompressive craniectomy for reducing raised intracranial pressure, or for intracranial disorders caused by trauma, tumours or infection. Whilst it is possible to use autologous bone for cranioplasty procedures, this has a number of disadvantages. The procedure may be complex and present a higher risk of infection due to multiple surgical sites. Only a limited quantity of bone can be taken from a donor site, which may be insufficient for large and/or complex defects. Furthermore, there is a possibility of donor site morbidity, and the risk of reabsorption of the donor bone, particularly in children. So, a cranial prosthesis is commonly used instead, generally being made out of materials as discussed above. While cranial prostheses made from the above materials will clearly suffer from the associated problems, there are a number of additional problems which must be considered which are specific to craniomaxillofacial applications. Firstly, the cosmesis of craniomaxillofacial surgeries is often considered to be a highly important factor. It is desirable that the final post-surgery result looks as ‘natural’ as possible. For some present cranial prostheses, e.g. most present titanium sheet and mesh prostheses, it is necessary for the prosthesis to overlap the edges of the cranial defect to ensure satisfactory attachment of the prosthesis to the skull. This can reduce cosmesis.
Furthermore, another issue which sometimes occurs after cranioplasty surgery is the need for subsequent revision surgery. However, because current prostheses fix directly into, and are intended to form a bond with the existing skull, such revision surgery generally necessitates major surgical intervention and total removal of the pre-existing prosthesis, which can increase the risk of such surgery.
The present invention has been devised in light of the above considerations.
Proposed herein are materials and devices for use in orthopaedic applications. The term “orthopaedic applications” is used herein to refer generally to applications associated with the musculoskeletal structures and systems of the body, for example the correction or prevention of deformities, disorders, or injuries of the musculoskeletal structures and systems of the body due to degenerative, tumour, trauma, infection, developmental, congenital, metabolic, arthritic, surgical or other causes. Orthopaedic applications includes such applications as manufacture of orthopaedic, craniomaxillofacial and spinal implants or prostheses.
Accordingly, in a first aspect, there is provided a composite material for use in orthopaedic applications comprising a polymeric matrix material and filler material comprising TiO2 and a material selected from graphene, graphene oxide or reduced graphene oxide.
As discussed above, TiO2 exhibits photocatalytic activity under suitable irradiation, such as by irradiation with ultraviolet (UV) radiation. In other words, TiO2 is a photoactive material. When irradiated with radiation such as UV light, gamma rays, or any other suitable form of radiation, TiO2 undergoes charge separation according to the following mechanism:
TiO2+hν→TiO2(h+e)
This charge separation can be considered as ‘photoactivation’ of the TiO2. The charge carriers which are formed promote formation of reactive species and in particular reactive oxygen species such as hydroxyl and peroxide radicals and superoxide anions at the material surface. Such reactive species are theorised to cause damage to bacteria cell membranes. Provision of a filler material comprising TiO2 therefore allows for a photoactive antibacterial effect which may be beneficial in orthopaedic applications.
Advantageously, by providing a filler material which further comprises a suitable electron-accepting or charge-scavenging material in addition to TiO2, electrons formed upon irradiation such as UV-, gamma-, or electron beam irradiation of TiO2 particles can be transferred to the electron accepting material. This charge transfer may prevent or reduce subsequent recombination of the electrons and holes produced upon irradiation. It is theorised that this reduced recombination of charge carries promotes the formation of reactive species at the surface of the material, thus improving antibacterial properties. Furthermore, it is also suggested that the reduced recombination may allow for continued formation of reactive species even in the absence of suitable illumination, making the material particularly suitable for use in applications where UV illumination, or irradiation by other suitable means (e.g. gamma irradiation, or electron beam irradiation) is not possible or readily practicable, such as in in vivo implantation of the material. Finally, another advantage of providing this ‘active’ material as a filler material in a composite rather than e.g. as a surface coating includes improved ease of manufacture and reduced risk of particulate formation in use, particularly when used for making e.g. articulating orthopaedic implants.
Whilst it is hypothesised that any suitable electron-accepting or charge-scavenging material may be suitable for achieving such an effect, the present inventors propose that graphene (GR), graphene oxide (GO) and/or reduced graphene oxide (rGO) may be particularly suitable for use in materials for orthopaedic applications, because of their known biocompatibility and other desirable properties that will be discussed further below, including tribological properties. For example such materials, in particular graphene, may act as solid lubricants within the polymeric matrix material to improve the tribological properties of the composite material, and in particular may improve the wear resistance of the composite material. The material selected from graphene (GR), graphene oxide (GO) and/or reduced graphene oxide (rGO) may be in the form of nanoplatelets. It is also theorised that other carbon nanostructures such as carbon nanotubes and fullerene structures may be suitable electron-accepting or charge-scavenging materials.
A further property of TiO2 includes radiation activated (such as UV-activated, gamma-activated or electron beam-activated) superhydrophilicity. This effect is a result of a similar mechanism as discussed above in relation to antibacterial effect of TiO2, i.e. due to photogeneration of electron hole pairs upon irradiation of TiO2 with e.g. UV light or gamma rays (photoactivation). A superhydrophilic surface is generally defined as a surface having a very low contact angle, for example a zero- or almost-zero degree contact angle for water droplets. The exact mechanism by which TiO2 becomes a superhydrophilic surface as a result of irradiation is not precisely known, but it is theorised that photogenerated holes in TiO2 migrate to the surface and weaken the bond between titanium and oxygen atoms, the oxygen atoms being liberated to create oxygen vacancies with the conversion of Ti4+ to Ti3+. It may then be possible for water molecules to become chemisorbed at these Ti3+ sites, thus creating a hydroxylated surface with increased hydrogen bonding activity. Increased hydrogen bonding activity may accordingly provide a superhydrophilic effect. In a similar manner as that discussed above, by providing a filler material comprising a combination of TiO2 and a suitable electron-accepting or charge-scavenging material such as graphene, graphene oxide, or reduced graphene oxide, the superhydrophilic effect may be synergistically and advantageously enhanced due to reduced recombination of electron-hole pairs. Furthermore, it is also suggested that this reduced recombination may allow for continued superhydrophilicity even in the absence of suitable illumination, making the material particularly suitable for use in applications where UV illumination, or irradiation by other suitable radiation (e.g. gamma rays) is not possible or readily practicable, such as in in vivo implantation of the material.
A filler material comprising TiO2 and a material selected from graphene, graphene oxide or reduced graphene oxide could in theory be used to form a composite material by combination with a number of different matrix materials. However, in the field of materials for use in orthopaedic applications it may be particularly advantageous to use a polymeric matrix material, because such materials may allow for improved ease of manufacture compared to other known alternatives (e.g. the ability to manufacture articles from the material using 3D printing methods such as fusion deposition modelling). Furthermore, use of a polymeric matrix material can reduce or avoid CT & MRI scan artefacts.
The term ‘filler material’ is intended to refer generally to materials (particulate, fibrous or other) added to the polymeric matrix material for a range of purposes including lowering consumption of the matrix material and altering the properties of the matrix material. Filler material may herein refer to both a single material for use as a filler material, and also more generally to refer to a mixture of materials used as filler materials.
TiO2 as referred to herein may include all phases of TiO2 (anastase, rutile, brookite etc.) and combinations thereof. For example, the TiO2 may be entirely in the anastase phase. Alternatively the TiO2 may be e,g, 80% anastase and 20% rutile. The exact phase composition of the TiO2 is not particularly limited and may be selected as appropriate for the particular application.
The average particle diameter of TiO2 may be from 5 nm to 50 nm, preferably from 10 nm to 30 nm.
The material selected from graphene, graphene oxide and reduced graphene oxide may comprise sheets or nanoplatelets. The thickness of the nanoplatelets or sheets may be from 0.25 nm to 10 nm, preferably from 0.4 nm to 2 nm. The average diameter of the nanoplatelets or sheets may be from 1 μm to 20 μm, preferably from 1.5 μm to 5.5 μm.
The amount of filler material may be from 0.05 wt % to 80 wt % based on total weight of the composite material. That is, the amount of filler material based on total weight of the composite may be 0.05 wt %, 1 wt %, 2 wt %, 3 wt %, 5 wt % or 10 wt % or more, up to 80 wt %, 70 wt %, 60 wt %, 50 wt % or less. It will be understood that the total amount of filler material is not particularly limited other than to the extent that the composite material remains suitable for use in orthopaedic applications.
The amount of TiO2 as a filler material in the composite material may be from 0.05 wt %, 0.5 wt %, 1 wt %, 1.5 wt %, 2 wt %, 2.5 wt %, 3 wt %, 5 wt % or 10 wt % or more, up to 80 wt %, 70 wt %, 60 wt %, 50 wt % or less, based on total weight of the composite material. The precise amount of TiO2 may be selected as appropriate for the particular application. Because of the synergistic effect achieved by the combination of TiO2 and a material selected from graphene, graphene oxide or reduced graphene oxide, it is theorised that a suitable antibacterial effect can still be achieved even at low concentrations. Accordingly, the amount of TiO2 as a filler material in the composite material may be less than 3 wt %, less than 2 wt % or less than 1 wt % based on total weight of the composite material.
The amount of a material selected from graphene, graphene oxide or reduced graphene oxide may be from 0.05 wt %, 0.5 wt %, 1 wt %, 1.5 wt %, 2 wt % or 2.5 wt % or more, up to 30 wt %, 20 wt %, 10 wt % or less, based on total weight of the composite material. The precise amount of this electron-accepting or charge-scavenging material may be selected as appropriate for the particular application. Adding greater amount of these materials may improve the mechanical properties of the composite material. However, it may also be desirable to reduce or minimise the amount used due to the cost of these materials.
The combined amount of TiO2 and the material selected from graphene, graphene oxide or reduced graphene oxide may be less than 3 wt %, less than 2 wt % or less than 1 wt % based on total weight of the composite material. Again, it is theorised that the synergistic effect of providing a combination of these materials can reduce the need for inclusion of a large amount of these materials.
The ratio of TiO2 to material selected from graphene, graphene oxide or reduced graphene oxide (GR/GO/rGO) in the filler material may be 50:50, 60:40, 70:30, 80:20, 90:10, 95:5, or 99:1 by weight. In other words, the amount of TiO2 may be from 50 wt % to 99 wt % based on combined weight of TiO2 and GR/GO/rGO. Preferably, the ratio of TiO2 to GR/GO/rGO is 80:20 by weight.
The filler material may further comprise hydroxyapatite (also known in the art as hydroxylapatite). Hydroxyapatite is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH). Hydroxyapatite in the form of bone mineral makes up approximately up to 70 wt % of human bone. Accordingly, by including hydroxyapatite as a filler material in the composite material, bone ingrowth may be promoted.
Where the composite material comprises hydroxyapatite as a filler material, the amount of hydroxyapatite may be from 1 wt % to 70 wt % based on total weight of the composite material. That is, the amount of hydroxyapatite based on total weight of the composite may be from 1 wt % or more, 2 wt % or more, 3 wt % or more, 5 wt % or more or 10 wt % or more, up to 70 wt % or less, 60 wt % or less, 50 wt % or less, 40 wt % or less, or 30% or less. Providing a higher amount of HAP may assist in promoting bone integration. However, it may be desirable for the amount of HAP to be below a threshold value so that the composite material has adequate strength.
Preferably the polymeric matrix material is selected from polyphenylsulphone, polycarbonate urethane, a PAEK (polyaryletherketone), polycaprolactone, poly-L-lactic acid, polyvinyl acetate or poly(lactic-co-glycolic acid). Where the polymeric matrix material is a PAEK, preferably it is PEEK (polyetheretherketone) or PEKK (polyetherketoneketone). The skilled person will understand that a wide range of other polymeric matrix materials may be suitable, however these polymers are particularly preferred due to their known biocompatibility and regulatory approval status. More preferably, the polymeric matrix material is a polyphenylsulphone. Polyphenylsulphone (PPSU) has superior toughness compared to some alternatives, and furthermore may offer a close property match to cranial bone in terms of features such as density, tensile strength, compressive strength and impact resistance
The filler material may comprise a nanocomposite of TiO2 and a material selected from graphene, graphene oxide or reduced graphene oxide. Preferably, the filler material comprises a TiO2-reduced graphene oxide nanocomposite. A TiO2-reduced graphene oxide nanocomposite may be produced by any suitable method. However, methods such as UV-assisted photocatalytic reduction of graphene oxide in the presence of TiO2, or by hydrothermal methods such as that proposed by Fan et al. (2011) may be particularly convenient. Such nanocomposite materials can have ‘hybrid’ material properties. For example, a TiO2-reduced graphene oxide nanocomposite material may comprise heterojunctions on the nanoscale. It is theorised that the formation of such heterojunctions can promote the efficient separation of electron-hole pairs under irradiation, discussed above, thus leading to improved photocatalytic activity.
As discussed above, such nanocomposite materials are photoactive, and can be photoactivated by irradiation with suitable radiation, for example by irradiation with UV- or Gamma-radiation, or by electron-beam irradiation (EBI). This irradiation may take place either during or after formation of the nanocomposite. If the irradiation takes places during formation of the nanocomposite material (e.g. during UV-assisted photocatalytic reduction of graphene oxide in the presence of TiO2), the irradiation time may be a few hours or more. Preferably the irradiation time is 5 hours or more, although irradiation of up to 24 hours or more, or 48 hours or more is contemplated.
The method of photoactivation is not particularly limited. One particularly convenient method of UV-irradiating such materials may be to use a low pressure mercury lamp—in particular, lamps having narrow band UV emission at a wavelength of 254 nm wavelength, and a power of e.g. 16-18 W. Such lamps are generally energy efficient, and relatively easy to obtain.
Another possible method of photoactivation of photoactive materials is by use of gamma irradiation, such as the gamma irradiation used in some known commercial sterilisation methods. Kralchevska et al (2012) suggests that TiO2 photoactivated using gamma irradiation may continue to remain photoactive even when not subject to further illumination for relatively long periods of time. In this particular study, photocatalytic properties of the photoactivated TiO2 were found to be effectively unchanged after 45 days of storage. Without wishing to be bound by theory, the present inventors contemplate that gamma irradiation may have a significant influence on the photocatalytic activity of TiO2, through the creation of induced defects in the crystal lattice of the TiO2 (mostly Ti3+ sites) influencing the recombination rate of electron-hole pairs. Additionally, the present inventors contemplate that gamma irradiation may create a higher density of e-h pairs compared to lower energy irradiations such as UV or visible light. Accordingly, photoactivation by gamma irradiation may offer particular advantages for photoactivation of materials intended for in-vivo use (for example, in orthopaedic implants), as it may provide an increased photoactive lifetime in the absence of illumination/irradiation. Additionally, gamma-irradiation may be further advantageous for photoactivation of materials intended for use in in-vivo applications because both sterilisation and photoactivation of the material can occur at the same time.
Where gamma irradiation is performed, the material may be irradiated at a dose of 8 kGy-40 kGy (8,000-40,000 JKg−1). A preferred dosage is about 25 kGy (25,000 JKg−1). The rate of dosage may be around 10 kGy/hour (10,000 JKg−1hour−1) although this will depend on the precise irradiation setup.
It may in some cases be advantageous to perform ‘top-up’ irradiation/photoactivation of the material to ensure maintenance of the photoactive properties of the material. One example of a situation where it may be desirable to perform a ‘top-up’ irradiation step is where such a material is used as a filler material e.g. in an orthopaedic implant. Then, it may be advantageous to perform ‘top-up’ irradiation immediately before implantation of the implant.
In a second aspect, there is provided an orthopaedic implant made from the material of the first aspect. The orthopaedic implant may be an orthopaedic prosthesis. Preferably, the orthopaedic implant is a cranial prosthesis, or an articulating implant having one or more bearing surfaces. However, the skilled person will readily understand that the material of the first aspect may be employed in manufacture of a wide range of orthopaedic implants as appropriate.
For non-articulating implants such as cranial implants, as discussed above, use of such material can offer superior antibacterial properties and may according reduce infecting risk on implantation. For articulating implants intended for use in predominantly water based lubricated environments (e.g. articulating knee and spinal implants), in addition to superior antibacterial properties, the superhydrophilicity of the material as discussed above may provide superior tribological properties and reduce wear and particulate formation within the joint.
In a third aspect, there is provided a cranial prosthesis comprising a peripheral frame portion defining an aperture, and a removable insert portion for closing the aperture.
Preferably the peripheral frame portion comprises the entire periphery of the prosthesis. The frame portion may be a regular polygonal shape. However, given the nature of cranial prostheses, preferably the frame portion is shaped to match a cranial defect of an intended recipient of the prosthesis. By providing a prosthesis with a shape matching the cranial defect, cosmesis of cranioplasty surgery using the prosthesis may be improved due to reduced or zero overlap of the prosthesis with the edges of the cranial defect. The precise dimensions of the prosthesis are not particularly limited, and may be selected as appropriate for the specific application.
The peripheral frame portion may be provided with one or more integrated fixation lugs for attachment to the cranial bone. Provision of integrated lugs may provide intraoperative ease of use by removing the requirement for provision and attachment of separate fixation plates.
The aperture defined by the frame portion is not particularly limited in size or shape, however preferably should be sized to allow suitable access through the aperture for subsequent revision surgery after an initial cranioplasty procedure. Accordingly, the aperture may span 10% or more of the total area defined by the prosthesis. Preferably, the aperture spans 20%, 30%, 40% 50%, 60%, 70%, 80% or 90% or more of the total area defined by the prosthesis. Providing a larger aperture may allow easier access through the aperture for subsequent revision surgery. However, by providing a larger aperture, the mechanical strength of the prosthesis may be reduced. So, a suitable aperture size may be selected which balances these two factors. The skilled person will appreciate that this parameter may also be dependent on the overall size and shape of the prosthesis, e.g. a smaller prosthesis will likely require a relatively larger aperture to give suitable access for revision surgery, whilst a larger prosthesis will likely require a relatively smaller aperture to provide suitable mechanical strength.
Preferably the peripheral frame portion defines a single aperture for closing with a single removable insert portion. In other words, preferably the cranial prosthesis is a two-part prosthesis. Minimising the number of parts may reduce manufacturing complexity and improve intraoperative ease of attachment of the prosthesis.
The insert is provided for closing the aperture, and it is anticipated that in general use the aperture will be closed by the insert. However, the insert is removable as discussed above, and may be removed for the purpose of performing revision surgery, or for e.g. decompressive purposes to assist in removing pressure on the brain post-implantation. The insert may therefore be removably fixed to the peripheral frame portion using e.g. screws or other suitable attachment means.
Preferably the insert portion lies flush with the peripheral frame portion when closing the aperture. Accordingly, the peripheral frame portion may have recessed fixation points for the removable insert portion such that in use, the insert portion sits flush with the frame portion. The recessed fixation points may be e.g. lugs or a recessed ledge.
The prosthesis may be formed of any material suitable for use as an orthopaedic prosthesis. As such, the prosthesis may comprise materials including but not limited to: titanium, acrylic, ultra-high-molecular-weight polyethylene (UHMWPE), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), hydroxyapatite (HAP), polyphenylsulphone (PPSU), polycarbonate urethane (PCU), polycaprolactone (PCL), poly-L-lactic acid (PLLA), polyvinyl acetate (PVA) or poly(lactic-co-glycolic acid) (PLGA). Preferably the prosthesis is formed from a composite material comprising a polymeric matrix material and a filler material. More preferably, the prosthesis is formed from a composite material according to the first aspect disclosed herein.
Preferably, the peripheral frame portion of the prosthesis is porous. Preferably, the frame portion has an average porosity from 5% and up to 95%. Different regions of the frame portion may have different porosities, in a manner discussed in more detail below in relation to the fourth and fifth aspects of the invention, below. Preferably the average pore size of the peripheral frame region is from 100 μm to 600 μm.
In particular, the peripheral frame portion may have laterally graduated porosity. That is, the porosity of the peripheral frame portion may vary in an in-plane direction laterally across the implant. In this way the porosity of different regions of the frame portion can be selected to both encourage bone integration whilst providing suitable mechanical properties. The peripheral frame portion may have through-thickness graduated porosity. That is, the porosity of the peripheral frame portion may vary between an inner and an outer surface of the implant. In this way, the porosity of the peripheral frame portion can be selected to more closely match that found in natural cranial bone and encourage bony integration whilst providing suitable mechanical properties.
Preferably the removable insert portion has a lower porosity than the peripheral frame portion. Preferably, the average porosity of the removable insert portion is less than 5%, or less than 1%. By providing such a low porosity, bone integration into this portion of the prosthesis may be discouraged. Whilst this is against the typical teaching that bony integration should be promoted in orthopaedic implants, it is advantageous in the present invention because it may allow for ease of removal of the removable insert portion for e.g. subsequent revision surgery. Therefore more preferably, the removable insert portion is non-porous.
Where the peripheral frame portion provides fixation points (e.g. fixation lugs or a fixation ledge) for the removable insert portion, the fixation points may have a lower porosity than the rest of the peripheral frame portion. Preferably the fixation points are non-porous. In this way the fixation points may have a more suitable mechanical strength and provide for more secure fixation of the removable insert portion.
The prosthesis may be manufactured by any suitable known process including e.g. moulding, machining, or additive manufacturing. Preferably the prosthesis is manufactured using an additive manufacturing process such as e.g. fusion deposition modelling. Such process can provide a cost-effective method of producing ‘custom’ prostheses e.g. those which are shaped to match a cranial defect of an intended recipient of the prosthesis.
In a fourth aspect, there is provided a cranial prosthesis comprising a core layer and a first skin layer, the first skin layer having a lower porosity than the core layer. In other words, the prosthesis has a through-thickness porosity graduation thereby defining a core layer and a skin layer. A porosity graduation here also includes step-changes in porosity.
By providing a cranial prosthesis comprising multiple layers with differing porosities, it is possible to provide a prosthesis which more closely matches the morphology of cranial bone. Cranial bone typically has two main types of bone: inner trabecular bone (‘spongy’ or cancellous bone) having a typical porosity of around 75-85% and a typical pore size of around 300-600 μm, and outer cortical bone having a typical porosity of around 5-10% and a typical pore size of around 10-50 μm. Accordingly, a higher porosity core layer of the prosthesis may be used to match or mimic trabecular bone, whilst a lower porosity skin layer of the prosthesis may be used to match or mimic cortical bone.
Preferably, the prosthesis comprises a second skin layer disposed on the opposite side of the core layer from the first skin layer. This second skin layer may have a lower porosity than the core layer. The porosity of the second skin layer may be equal or approximately equal to the porosity of the first skin layer. In this way, the through-thickness structure may be symmetrical about the lower-density core layer.
The average porosity of the core layer may be from 50% up to 95%. Preferably, the average porosity is from 70% up to 90%. More preferably, the average porosity is from 75% up to 85%. In this way, the average porosity of the core layer may more closely match or mimic trabecular bone.
The average porosity of the skin layer(s) may be from 50% or less, 30% or less, 20% or less, 10% or less or 5% or less. Preferably the average porosity of the skin layer(s) is from 5% up to 10%. In this way, the average porosity of the core layer may more closely match or mimic cortical bone. However, the skin layer(s) may be non-porous.
The average pore size may also vary across the prosthesis. Accordingly, the average pore size of the core layer may be from 250 μm to 650 μm, more preferably from 300 μm to 600 μm. In this way, the pore size of the core layer can more closely mimic typical pore sizes found in trabecular bone. The average pore size of the skin layers may be from 50 μm to 250 μm, more preferably from 100 μm to 200 μm. In this way, the pore size of the skin layer(s) can more closely mimic typical pore sizes found in cortical bone. By more closely matching pore sizes to those found in natural cranial bone, it may be possible to provide a prosthesis which both encourages improved bone integration whilst providing suitable mechanical properties.
In typical cranial bone, the thickness of trabecular bone ranges from approximately 1.5 mm to 4 mm. The thickness of cortical bone varies from approximately 0.5 mm to 3.5 mm. Accordingly, the core layer of the prosthesis may have a thickness of 1 mm to 5 mm. The skin layer(s) may (each) have a thickness from 0.25 mm to 4 mm. The thickness may be selected as appropriate to more closely match that of cranial bone, whilst providing suitable mechanical strength for the prosthesis.
Preferably the cranial prosthesis is manufactured using an additive manufacturing technique or process. For example, the prosthesis may be manufactured using fusion deposition modelling. By using an additive manufacturing process it is possibly to provide a controlled pore distribution and morphology, to more closely match or mimic cranial bone than is possible using other manufacturing methods. Fusion deposition modelling may be particularly suitable due to the resolution of such processes offering the opportunity to produce parts having finer detail than in some other additive manufacture processes. E.g. the skilled person will be aware of commercially available fusion deposition modelling machines having resolutions as low as 50 μm or less, which would be suitable for manufacturing a prosthesis to the above specifications. Such fusion deposition modelling machines may have print temperatures of e.g. up to around 450° C. to allow processing of a wide range of materials. Preferably the working chamber is a closed, heated working chamber of a suitable size.
In a fifth aspect, there is provided a cranial prosthesis having laterally graduated porosity such that the porosity of a peripheral region of the implant is greater than the porosity of a central region of the implant. Where the prosthesis is a planar or plate-like prosthesis (as is typically the case for cranial prostheses), the porosity of the prosthesis may therefore vary in an in-plane direction across the prosthesis.
By having laterally graduated porosity that is greater at a peripheral region and lower at a central region, the prosthesis may encourage bone integration at the peripheral region, whilst maintaining suitable mechanical strength in the central region of the prosthesis.
A peripheral region may be a bone-contacting region of the prosthesis, or including a bone-contacting edge of the prosthesis. The peripheral region may be a region which extends inwardly from a bone-contacting edge of the prosthesis for a distance of 5 cm or less, 4 cm or less, 3 cm or less, 2 cm or less or 1 cm or less.
The average porosity of the peripheral region may be from 5% up to 95%. Preferably, the average porosity of the peripheral region is from 50% to 90%, more preferably 70% to 90%, more preferably 75% to 85%. In this way, the average porosity of the peripheral region may more closely match or mimic trabecular bone and accordingly encourage bone integration.
The average porosity of the central region may be from 10% or less, 5% or less or 1% or less. Preferably the central region is non-porous. By limiting from the porosity of the central region, or providing a non-porous central region, the mechanical strength of the central region may be improved.
The average pore size of the peripheral region may be from 50 μm to 650 μm, preferably from 100 μm to 600 μm. The average pore size of the central region may be 100 μm or less, preferably 50 μm or less. More preferably, the central region is non-porous.
The peripheral region may be defined by a frame portion of the prosthesis. The cranial prosthesis may be a cranial prosthesis having a frame portion defining an aperture and a removable insert portion for closing the aperture. Accordingly the peripheral region having higher porosity may be a frame portion defining an aperture, and the central region having lower porosity may be a removable insert portion for closing the aperture.
The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. In particular, as discussed above, it is contemplated that cranial prostheses according to the third, fourth and fifth aspects of the invention may be made from the material according to the first aspect of the invention.
Whilst the composite material according to the first aspect of the present invention disclosed herein is principally disclosed in relation to use in orthopaedic applications (another aspect of the invention being use of the material of the first aspect in orthopaedic applications), the composition material may also be suitable for more general use in any in-vivo application (e.g. use in any in-vivo structure) insofar as one or more of the advantageous properties of the composite material derivable from the present disclosure would be beneficial in such application.
Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:
Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.
The cranial prosthesis comprises a single peripheral frame portion 101 defining a single central aperture 103. In other words, the prosthesis is a two-part prosthesis. The peripheral frame portion here comprises the entire periphery of the prosthesis 100. The frame portion is an irregular shape selected to match a cranial defect of an intended recipient of the prosthesis, and accordingly improve cosmesis of a cranioplasty using the implant. The frame portion has four integrated fixation lugs 105 which are approximately evenly distributed about the perimeter of the peripheral frame portion 101. Each lug has a hole 107 for receiving a screw (not shown) for fixing the lug to the skull of a recipient of the prosthesis in a manner well known to the skilled person. Providing integrated fixation lugs gives improved intraoperative ease, as it removes the requirement for provision and attachment of separate fixation plates: the prosthesis can be attached directly to the skull of the recipient.
The prosthesis comprises a removable insert portion 109 which is of a shape suitable for closing the aperture 103. Here, the removable insert portion entirely closes the aperture 103 when it is located within the aperture, such as after implantation of the prosthesis. The removable insert portion also has four integrated fixation lugs 111 for removably attaching the insert portion 109 to the peripheral frame portion 101. The integrated fixation lugs 111 and the shape of the peripheral frame portion 101 are selected to provide an appropriate degree of keying between the lugs and the frame which can assist in correct location of the insert within the aperture. In a similar manner to the lugs 105 of the peripheral frame portion, each fixation lug 111 of the insert portion has a hole 115 receiving a screw (not shown) for fixing the lug to the peripheral frame portion. Here, the (countersunk) holes 107, 115 have chamfered edges to allow for recessing of a screw head into the hole. The holes 107, 115 could alternatively be counterbored.
The fixation lugs 111 of the insert portion attach to the peripheral frame portion 101 at recessed fixation points comprising suitably shaped ledges 113 by which the fixation lugs of the insert portion can be supported. Because the ledges 113 are recessed from the surface of the peripheral frame portion, the outer surface of the insert lies flush with the outer surface of the peripheral frame portion when it is closing the aperture 103. This can be seen most clearly in
The peripheral frame portion and the insert portion have different porosities represented by the different density of hashing in
The cranial prosthesis shown in
Here, the core layer is thicker than each of the skin layers, being approximately twice as thick as each skin layer. The skin layers are of approximately equal thickness. This arrangement also mimics that of cranial bone, where the central trabecula bone layer is typically thicker than the cortical bone layers.
Each of the first members 305, 307 has a bearing surface 321, 323 (respectively) as shown in
Because the prosthesis is an articulating prosthesis having multiple bearing surfaces, it is advantageously made from a material according to Example 2, below. That is, the prosthesis is formed from 98-99 wt % PPSU reinforced composite containing 1-2 wt % reduced graphene oxide/titanium dioxide nanocomposite based on total weight of the composite material, the reduced graphene oxide/titanium dioxide nanocomposite having a TiO2:rGO ratio of 80:20 by weight. In this way, the prosthesis can provide improved tribological performance in combination with antibacterial capability.
Porosity Measurement
Porosity as discussed in the above description refers to bulk porosity defined as 1−Vs, where Vs=solid volume fraction of the material. The solid volume fraction for a sample can be calculated by calculating the ratio between the density of the sample and the theoretical density of a non-porous sample of the same material. However, the skilled person will be well aware of a wide range of suitable methods for measuring bulk porosity of a material.
Where the above description contains detail of porosities and pore sizes of natural bone, these estimates are calculated from methods including e.g. magnetic resonance imaging (MRI), micro computer tomography (mCT) or microradiography.
Whilst the above description generally refers to bulk porosity, the skilled person will also understand that it is possible to calculate the interconnected porosity or pore volume of a material using e.g. mercury intrusion porosimetry (MIP). The materials and devices disclosed herein preferably have an interconnected porosity/pore volume which promotes osseointegration.
Pore Size Measurement
Pore size as discussed in the above description refers to average diameter of pores within a selected region. The skilled person will be well aware of a wide range of suitable methods for measuring average pore diameter, including e.g. optical microscopy (OM), scanning electron microscopy (SEM), X-Ray scanning, and CT scanning.
One composite material according to the present invention is as follows:
29.5-34.5 wt % PPSU reinforced composite containing 65-70 wt % hydroxyapatite (HAP) nanoparticle and 0.5 wt % reduced graphene oxide/titanium dioxide nanocomposite based on total weight of the composite material, the reduced graphene oxide/titanium dioxide nanocomposite having a TiO2:rGO ratio of 80:20 by weight.
Such material may be particularly suitable for promoting osseointegration in combination with antibacterial capability.
Another composite material according to the present invention is as follows:
98-99 wt % PPSU reinforced composite containing 1-2 wt % reduced graphene oxide/titanium dioxide nanocomposite based on total weight of the composite material, the reduced graphene oxide/titanium dioxide nanocomposite having a TiO2:rGO ratio of 80:20 by weight.
Such material may be particularly suitable for providing enhanced tribological performance in combination with antibacterial capability.
Example Material Specifications
The initial materials used in manufacture of the above composite materials above are standard, commercially-available materials such as the following:
PPSU: Solvay Specialty Polymers VERIVA™ VR-500 Polyphenylsulfone having the following typical properties:
TiO2: 99.9% purity, anastase phase, 18 nm (US Research Nanomaterials) having the following typical properties:
Graphene oxide: Research Grade Single Layer Graphene Oxide Dry Nanopowder (US Research Nanomaterials) having the following typical properties:
Hydroxyapatite: nanoXIM HAp200 (Fluidinova) having the following typical properties:
The composite materials of Example 1 and Example 2 may be made from these initial materials using any suitable method known in the state of the art, including but not limited to acoustic mixing, acoustic milling ultrasonification, mechanical paddle mixing, melt flow mixing, static melt blending or spiral flow jet milling to disperse or distribute the filler material components throughout the polymeric matrix material in a known manner. The filler material or filler material components may initially be provided as a suspension or dispersion in a suitable aqueous or polar solvent e.g. ethanol.
In some cases it may be advantageous to first dissolve the polymeric matrix material in a suitable solvent before addition of the filler material, however this may not be necessary where the polymeric matrix material is hot melt processable e.g. in powder, or granule form.
In some cases it may be preferred to first form a TiO2-Graphene nanocomposite from the initial materials before combining this nanocomposite filler material with the matrix material. One method for preparation of TiO2-reduced graphene oxide (rGO) nanocomposites using UV-assisted photocatalytic reduction is described in a paper by Williams et al: “TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide,” ACS Nano, (2008), 2 (7), 1487-1491. Another method is described in Yang et al. “Synthesis of r-GO/TiO2composites via the UV-assisted photocatalytic reduction of graphene oxide” (2016). In this method, irradiation is performed for 5 hours using a low pressure mercury lamp having a narrow band UV emission at a wavelength of 254 nm and a power of 18 W.
An alternative method for preparation of TiO2-reduced graphene oxide (rGO) nanocomposites is via a hydrothermal method, such as that described in a paper by Fan et al.: “Nanocomposites of TiO2 and Reduced Graphene Oxide as Efficient Photocatalysts for Hydrogen Evolution”: J. Phys. Chem. C 2011, 115, 10694-10701.
The photoactive nanocomposite may then be dispersed through the polymeric matrix material using one of the methods discussed above.
Photoactivation (or top-up photoactivation) of the nanocomposite material may be performed, before or after dispersion of the nanocomposite in the polymeric matrix material.
The resulting powder/polymer aggregate may then be pelletised, extruded, compression or injection moulded, thermoformed or 3D printed. E.g., the powder/polymer aggregate may be extruded via a hot melt process to form filament for use in fusion deposition modelling.
Example Method for Production of Composite Material Filament
One suitable method for production of a TiO2/rGO PPSU nanocomposite filament having a composition of e.g. Example Material 1 is as follows. The quantities of each material are not specified below as this method may be used to manufacture TiO2/rGO PPSU nanocomposite filaments having a range of compositions. Accordingly, the quantities of each material may be selected as appropriate for the desired composition of the composite material.
Fusion Deposition Modelling
As mentioned above, materials and devices according to the present invention may be processed or manufactured using fusion deposition modelling techniques. The skilled person will be aware of commercially available fusion deposition modelling machines having resolutions as low as 50 μm or less, which would be suitable for use in processing and/or manufacturing materials and devices as discussed above.
To generate a prosthesis by fusion deposition modelling, an STL or OBJ file of the desired product should first be generated (e.g. for a cranial plate by converting patient specific high resolution 3D CT, MRI or surface scan data to an STL or OBJ file using software in a manner well known to the skilled person). The model can then be imported into software associated with the FDM machine, which computes the required tool paths for the extrusion nozzle of the FDM machine. The extrusion nozzle then draws cross-sectional layers one at a time in the X, Y, Z coordinates using a heated material extrusion process to form the FDM printed prosthesis.
Example FDM processing parameters for production of an orthopaedic, spinal or craniomaxillofacial implant from the material of the first aspect including PPSU as the polymeric matrix material are as follows:
A) Filament air drying prior to printing at about 80° C. for 12 hrs
B) Extruder nozzle diameter: 0.2 mm
C) Printer extruder nozzle temperature:
D) Post-print surface finishing followed by annealing
The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.
While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.
For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.
Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.
Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” or “around” one particular value, and/or to “about” or “around” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedents “about” or “around”, it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example+/−10%.
A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1800143.8 | Jan 2018 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/085952 | 12/19/2018 | WO | 00 |
