IT202200001265A1 - THIN-WALLED VERTEBRAL STRUCTURE IN AUXETIC METAMATERIAL - Google Patents

Description

Description of the Industrial invention with title:

?THIN-WALLED VERTEBRAL STRUCTURE IN AUXETIC METAMATERIAL?

DESCRIPTION

The present invention refers to an innovative biocompatible shell structure in auxetic metamaterial, created by 3D printing. This structure, what? composed of auxetic hybrid elements (in titanium alloy or polymeric material) and has morphological characteristics similar to those of human vertebral bones, it is useful, in particular, for the creation of vertebral prostheses designed to suit the patient in the oncology field.

Vertebral fractures, spinal tumors and deformities? Post-traumatic injuries very often involve the removal of the damaged vertebral body. Spinal reconstruction consists of a spinal arthrodesis, where a spinal implant, called a "cage", is inserted after a total corpectomy in combination with a secondary fixation system. Recently developed additive manufacturing techniques allow the production of different trabecular structures in highly porous titanium, capable of ensuring high levels of bone fusion and vascularization.

Patent EP1803420 (Annuloplasty prosthesis with an auxetic structure) describes a tubular structure made up of auxetic chiral elements for annuloplasty operations.

Patent CN109567980 (Novel vascular stent based on chiral and anti-chiral structures) describes a vascular stent based on auxetic anti-chiral elements.

Patent CN109033486 (A two-dimensional periodic negative poisson ratio controllable expansion material) describes a planar periodic structure of the auxetic type which has the aim of reducing voltage concentrations along the connection points between the elementary cells.

The structures described in the cited patents constitute at least part of the state of the art, even if they refer to geometric and implementation models different from those developed for the present invention, as well as? to different application areas. The similarity? found ? referring only to the prismatic and/or chiral based geometry that makes up the units? elementary.

Prior art structures have several drawbacks.

The first place show localized contacts between the prosthetic endplates and the vertebral endplates.

Furthermore, the values of the elastic modulus of the structure are very high, consequently increasing the risk of bone hypo-stress (stress shielding) and the phenomena of subsidence of the structure.

? also known is an innovative auxetic structure in highly porous reticular metamaterial, useful, in particular, for the creation of the nucleus of vertebral prostheses. Said auxetic structure? of the type obtained by replicating, in three spatial dimensions, a unit cell composed of eight rotating elements connected to each other in such a way as to have auxetic behavior and? characterized by the fact that said rotating elements are cuboids comprising a central body and six ligaments suitable for connecting said cuboids with the adjacent cuboids.

Purpose of the present invention? that of proposing an innovative porous shell structure for cortical bone implants in auxetic metamaterial, created by 3D printing.

The one described and other purposes, how will it come? explained below, are achieved with a bone prosthesis element, compliant with claim 1, having a particular geometry which gives it auxeticit? and an elastic modulus low enough to guarantee perfect osseointegration.

The bone prosthesis element according to the invention is, in particular, a vertebral shell having morphological characteristics similar to those of human vertebral bones, said shell being suitable for the creation of vertebral prostheses designed to measure the patient in the oncology field. Said element of bone prosthesis? characterized by the fact of being obtained by shaping a thin-walled flat structure, in auxetic metamaterial, with a substantially two-dimensional development, said structure being obtained by replicating in the two planar dimensions (x, y) a unit cell composed of four rotating elements, each of which includes:

- a central body;

- four ligaments suitable for connecting each of said rotating elements with adjacent rotating elements;

in which each of said ligaments identifies, with said central body, a deep hollow of width (?), in such a way that, by compressing said central body in its plane (x, y), a rotation of said central body is caused around an axis (z) perpendicular to the plane (x,y), with consequent reduction of the angles (?) and retraction of said ligaments, thus manifesting? auxetic behavior.

Preferred embodiments and non-trivial variants of the present invention form the subject of the dependent claims.

It is understood that all the attached claims form an integral part of this description.

The structure according to the invention, if made of titanium using the SLM (Selective Laser Melting) 3D printing technique, allows in particular:

- obtain a stiffness very similar to that of the human vertebra, thanks to the auxetic behavior and the specific conformation of the ligaments that connect the central elements;

- obtain greater static mechanical resistance and properties? elastic similar to those of human cortical bone thanks to its peculiar geometry;

- avoid "stress-shielding"/hypo-stress phenomena and therefore the consequent bone mineral degeneration along the healthy contiguous vertebrae, thanks to a uniform transfer of loads between the bone and the prosthesis;

- reduce the risk of possible collapse or sudden failure of the adjacent vertebrae, by virtue of of the reduced elastic modulus of the system;

- reproduce the characteristic configuration of the vertebral cortical bone in terms of stiffness and elastic modulus;

- reduce any phenomena of subsidence of the system (prosthesis) with respect to its natural implant site;

- reduce the risk of interference of the prosthesis with the spinal cord thanks to the non-conventional auxetic behavior.

The bone prosthesis element according to the invention will be able to be advantageously used for:

- the design and optimization of a new hybrid auxetic structure in biocompatible titanium for the creation of bone prostheses (hybrid means that said auxetic structure simultaneously has the characteristics of a rotating square and anti-tetrachiral);

- the introduction of a new typology of auxetic metamaterial which presents a geometric shape and properties? morphological not found in the state of the art;

- introduction of bio-inspired ligaments between said rotating elements capable of determining the auxeticit? of the system and its properties? mechanical;

- the introduction of this geometry in a concept of vertebral prosthesis having elastic characteristics very similar to those of human bones, in particular, to those of the vertebral cortical bone.

Will it turn out? It is immediately obvious that countless variations and modifications can be made to what has been described (for example relating to shape, dimensions, arrangements and parts with equivalent functionality) without departing from the scope of protection of the invention, as appears from the attached claims.

Will this invention come? better described by a preferred form of embodiment, provided by way of example and not by way of limitation, with reference to the attached drawings, in which:

- FIGS. 1 (a, b) show respectively the elementary cell representative of an auxetic structure and a?unit? elementary, according to the present invention;

- FIGS. 2 and 3 show the cross section of a vertebral shell;

- FIG. 4 shows a vertebral shell obtained by repeating hybrid auxetic structures in space;

- FIG. 5 shows a prototype of an auxetic vertebral shell, made of titanium alloy, made in 3D printing.

With reference to FIG. 1a, with (1) ? indicated is an elementary cell representative of an auxetic structure according to the invention, indicated with (10) in the following FIGS. 2 and 4.

Said elementary cell (1) develops in the (x, y) plane and has a small thickness in the (z) direction, for example 1?5 mm. ? composed of an original geometry that includes four auxetic elements (2) of optimized shape and size, and has a substantially constant thickness. In the case of a vertebral shell, called thickness ? preferably between 1 and 2 mm.

With reference to FIG. 1b, called auxetic elements (2) of the elementary cell (1), defined as units? elementary, include a square-shaped central body (3) and four ligaments (4a, 4b, 4c, 4d) designed to connect each of said auxetic elements (2) with contiguous auxetic elements (2). In this way, an auxetic structure is formed, of a hybrid type, i.e. anti-tetrachiral with rotating elements.

Said ligaments (4a, 4b, 4c, 4d) have deep recesses, of width (?), created through the combination of parametric spline curves with variable arc.

According to a preferred embodiment said angle (?) ? between 5? and 90?, preferably between 5? and 60?.

The union of a plurality? of elementary cells (1) allows to obtain auxetic structures (10), as shown in the following FIGS. 2 and 4.

The FIGS. 2 and 3 show, according to a reference system (x?, y?, z?) a section of a vertebral shell (10) in which said elementary cells (1) are arranged along specific circular sectors of variable length and known radius. In this way ? created a structure with a profile similar to those of human vertebral bodies, capable of obtaining macro-properties? mechanics similar to those of cortical bone.

FIG. 3 shows the cross section, according to the (x?, z?) plane of said vertebral shell (10). Said cross section? obtained from the composition of four circular sectors (10a, 10b, 10c, 10d) tangent to each other, each with a specific angular width, defined as:

- a central sector (10a), of radius rc;

- two peduncular sectors (10b, 10c), of radius rp;

- a spinal sector (10d), of radius rs.

The dimensions of said circular sectors (10a, 10b, 10c, 10d) are comparable to those of the human vertebral cortical bone. The morphology described is obtained on the basis of the geometric dimensions of human vertebrae, extrapolated from computerized tomographic images, in such a way as to reproduce the same shape of the vertebral cortical bone, which is defined as body.

Replicating the section of FIG. 2 along the vertical direction (y? in FIGS. 2 and 3), ? It is possible to create a vertebral shell (10), of variable height, as shown in FIG 4.

The particular geometry of said vertebral shell (10) gives it the property of auxeticit? and a reduced apparent elastic modulus, capable of guaranteeing better osteointegration, reducing bone hypostress phenomena.

By stressing said elements to compression, through action along the vertical ligaments of the structure (direction (y?) which is orthogonal to the sheet in FIG. 3), a rotation of the central bodies (3) of the units is caused. elementary elements (2) of the structure, with consequent reduction of the angle (?). This behavior produces a flexion of the four ligaments (4a, 4b, 4c, 4d), thus manifesting? auxetic behavior, therefore a negative Poisson's ratio.

This deformation mechanism, combined with the specific morphology of the vertebral shell (10), shown in FIGS 2 and 3, involves an auxeticit phenomenon? overall of the shell (10) itself which translates into a reduction of the characteristic radius (rc, rp, rs) of each single portion of the section of said shell (10), with consequent auxetic behavior of the entire shell (10), that is, in a transverse contraction of the shell (10) itself with respect to the direction (y?) of application of the load with a reduction in the volume occupied by said shell (10).

Industrial application

In order to experimentally validate the structural response of the proposed solution, ? a prototype vertebral shell (11) was created in titanium alloy Ti6Al4V ELI (FIG. 5) using the SLM laser fusion 3D printing technique.

The experimental results confirm the numerical predictions in terms of Poisson's ratio and mechanical characteristics.

The vertebral shell (11) is made of titanium alloy? capable of withstanding very high compressive loads and remaining elastic up to a deformation in the (y?) direction of approximately 3%, demonstrating a stiffness similar to that of human cortical vertebral bones.

Said vertebral shell (11) allows you to:

- decrease the stress concentration factors along the shell itself;

- increase the fatigue life of the prosthetic implant;

- reduce the stiffness of the entire prosthesis;

- avoid contact with adjacent healthy tissues (for example the spinal cord) thanks to its auxetic behavior;

Compared to the structures of the prior art, the auxetic vertebral shell (11) highlights:

- excellent properties? elastic;

- a good facility? of realization;

- a significantly reduced mass.

The complexity? geometry of the vertebral shell (10) means that it can only be manufactured by 3D printing and in particular, in order to minimize the characteristic dimensions of the ligaments while maximizing the properties? elastics of the system, using laser melting technology (SLM).

Conclusions

Preferred embodiments of the invention have been described, but naturally they are susceptible to further modifications and variations within the same inventive idea. In particular, numerous variations and modifications, functionally equivalent to the previous ones, which fall within the scope of protection of the invention, will be immediately evident to experts in the art, as highlighted in the attached claims in which the reference signs placed in brackets cannot be interpreted in the sense to limit the claims themselves. Furthermore, the word "comprising" does not exclude the presence of elements and/or phases other than those listed in the claims. The article ?un?, ?one? or ?one? preceding an element does not exclude the presence of a plurality? of these elements. The mere fact that some features are cited in dependent claims different from each other does not indicate that a combination of these features cannot be advantageously used.

Claims (10)

1. Bone prosthesis element, in particular a vertebral shell (10, 11) having morphological characteristics similar to those of human vertebral bones, said shell being suitable for the creation of vertebral prostheses designed to suit the patient in the oncology field, characterized by the fact of be obtained by shaping a thin-walled flat structure, in auxetic metamaterial, with a substantially two-dimensional development, said structure being obtained by replicating in the two planar dimensions (x, y) a unit cell (1) composed of four rotating elements (2) each of which comprehends: - a central body (3); - four ligaments (4a, 4b, 4c, 4d) suitable for connecting each of said rotating elements (2) with contiguous rotating elements (2); in which each of said ligaments (4a, 4b, 4c, 4d) identifies with said central body (3) a deep hollow of width (?), in such a way that, by compressing said central body (3) in its plane ( x, y), a rotation of said central body (3) is caused around an axis (z) perpendicular to the plane (x, y), with consequent reduction of the angles (?) and retraction of said ligaments (4a, 4b, 4c, 4d), thus manifesting? auxetic behavior. 2. Element of bone prosthesis, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 1, characterized in that said angle (?) ? between 5? and 90?. 3. Element of bone prosthesis, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 2, characterized in that said angle (?) ? between 5? and 60?. 4. Bone prosthesis element, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 1, characterized in that said ligaments (4a, 4b, 4c, 4d) are created through the combination of parametric spline curves with variable arc. 5. Bone prosthesis element, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 1, characterized by having a thickness between 1 and 5 mm. 6. Element of bone prosthesis, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 5, characterized in that said thickness is ? between 1 and 2 mm. 7. Bone prosthesis element, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 1, characterized by having a cross section obtained from the composition of four circular sectors (10a, 10b, 10c, 10d) tangent to each other, each with a specific angular width, defined as: - a central sector (10a), of radius rc; - two peduncular sectors (10b, 10c), of radius rp; - a spinal sector (10d), of radius rs; in which said rc, rp, rs rays are extrapolated from computerized tomographic images, in such a way as to reproduce the same shape of the vertebral cortical bone. 8. Bone prosthesis element, in particular a vertebral shell (11) in auxetic metamaterial, according to at least one of claims 1 to 7, characterized in that it is made by 3D printing. 9. Bone prosthesis element, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 8, characterized by being made of Ti6Al4V ELI titanium alloy using the SLM (Selective Laser Melting) 3D printing technique. 10. Bone prosthesis element, in particular a vertebral shell (10, 11) in auxetic metamaterial, according to claim 8, characterized in that it is made of polymeric material.