Method for manufacturing an implantable bone augment

The present invention relates to a method for manufacturing an implantable bone augment. The invention further relates to an augment obtainable by the method according to the invention.

More specifically, the present invention relates to support structures and methods for making support structures or components which can be used to fill defects in bone. These methods are based on imaging information of the bone defect and its mechanical environment. The support structures can then be tailored such that they have optimal biological and mechanical properties that may enhance bone ingrowth and have the desired shape to repair the defective object.

In bone and joint reconstructive surgery, often bone defects need to be filled. One option is the use of alloplastic (e.g. metal) bone augments. These augments typically have a standardized shape and are designed as an assembly of unit cells resulting in a porous homogeneous structure. To adapt these augments to the bone defect, these unit cells are typically cut off to form a bone contacting surface which is as complementary as possible to a corresponding surface of the bone, in particular a surface of the bone defect.

These cut-off elements, i.e. the partial unit cells, at the boundary of the augment form an irregular surface which may compromise the augment structural integration in the surrounding bone. This is due to geometrical irregularities, for instance resulting in a pore size at the boundary which is less optimal for bone ingrowth, and mechanical irregularities, for instance leading to loading conditions less optimal for bone ingrowth. It is therefore a goal, among other goals, of the present invention to provide an improved, efficient and/or reliable method for manufacturing an implantable bone augment.

This goal, among other goals, is met by a method for manufacturing an implantable bone augment according to appended claims 1. More specifically, this goal, among other goals, is met by a method for manufacturing an implantable bone augment arranged to at least partially fit in a bone defect in a bone of a patient, wherein the method comprises the steps of:

providing a three-dimensional bone model of at least a part of the bone of the patient comprising the bone defect;

generating a three-dimensional augment model based on the bone model such that the augment model at least partially fits in said defect, wherein said augment model is a beam network model formed by a plurality of beams interconnected at interconnection points, wherein a plurality of interconnection points on an outer surface of the beam network model define a bone contacting surface arranged for contacting a corresponding outer surface of the bone, in particular a surface of the bone defect, and wherein the interconnection points on said bone contacting surface are positioned such that the bone contacting surface is formed complementary to the corresponding outer surface of the bone;

generating a combined numerical model of the bone model and the augment model for numerically analyzing the loading conditions in said combined model;

iteratively numerically analyzing said combined model and optimizing the mechanical properties of said beam network model for minimizing local peaks in the loading conditions; and manufacturing the optimized augment model.

According to the invention, the implant augment is designed as a beam network and is at least at the bone contacting surface formed of interconnected beams. The bone contact surface therefore does not contain any cut-off elements, i.e. partial lengths of beams which are not interconnected at an interconnection point or node at the bone contacting surface. The beam network, at least at the bone contacting surface, follows the defect boundary. In other words, a plurality of interconnection points on the outside of said network define the bone contacting surface.

The formation of the beam network according to the invention is preferably done using meshing techniques, for instance using mesh elements having a predetermined size. A meshing technique then creates a network of beams within the outer surfaces, at least a part of which is based on the bone model for forming the bone contacting surface. The bone contacting surface of the augment is arranged to be in contact with a corresponding surface of the bone, in particular a surface of the defect. The bone contacting surface is hereto formed complementary to the corresponding surface. Complementary in this respect means that the two surfaces in inserted state extend adjacently without any substantial play, or at least with minimal play.

Using a model based on a beam network is advantageously since such a model allows relatively easy analysis of the mechanical conditions in use, for instance using the Finite Elements Method. Preferably, the beam theory is applied to numerically analyze the mechanical characteristics.

Modelling, for instance by numerical analysis, of the mechanical environment with a bone contacting surface having cut-off elements is difficult, such that it is hard to predict the mechanical conditions in implanted state. The numerical analysis of the beam network model according to the invention is therefore more accurate, such that the mechanical environment of the augment according to the invention and the surrounding bone can be reliably predicted. By providing a model of the augment of which the bone contacting surface is defined by interconnection points of the beams, the loading conditions in use can be more accurately determined.

The combined model of the augment and the bone is therefore numerically analyzed to characterize and optimize the structure by adjusting the beam network to achieve a mechanical environment with as little as possible peaks, for instance in terms of local peaks of stresses and strains throughout the beam network. Local peaks in the loading conditions may result in poor bone ingrowth or even in bone resorption due to stress shielding. It is to be understood that a variation in stresses and strains throughout the augment in use cannot be prevented. However, the optimization process is preferably arranged to optimize the beam network to achieve an as natural distribution of the loading conditions throughout the beam network as possible.

According to a preferred embodiment, the step of providing the three-dimensional model of the bone comprises the step of obtaining an image of the bone and defect therein. Digital patient- specific image information can be provided by any suitable means known in the art, such as for example a computer tomography (CT) scanner, a magnetic resonance imaging (MRI) scanner, an ultrasound scanner, or a combination of Roentgenograms. A summary of medical imaging has been described in "Fundamentals of Medical imaging", by P. Suetens, Cambridge University Press, 2002.

For example, the step of obtaining an image of the bone and the defect therein may for example comprise the steps of obtaining 2D datasets of the bone and reconstructing a 3D virtual bone model from said 2D datasets. Indeed, the first step in a planning is the construction of a 3D virtual model of the bone. This reconstruction starts with sending a patient to a radiologist for scanning, e.g. for a scan that generates medical volumetric data, such as a CT, MRI scan or the like. The output of the scan can be a stack of two-dimensional (2D) slices forming a 3D data set. The output of the scan can be digitally imported into a computer program and may be converted using algorithms known in the field of image processing technology to produce a 3D computer model of a relevant bone. Preferably, a virtual 3D model is constructed from the dataset using a computer program such as Mimics(TM) as supplied by Materialise N.V., Leuven, Belgium. Computer algorithm parameters are based on accuracy studies, as for instance described by Gelaude at al. (2008; Accuracy assessment of CT-based outer surface femur meshes Comput. Aided Surg. 13(4): 188- 199). A more detailed description for making a perfected model is disclosed in U.S. Patent No. 5,768, 134 entitled 'Method for making a perfected medical model on the basis of digital image information of a part of the body'. Once the three-dimensional model of the bone is reconstructed for instance as disclosed in Gelaude et al. (2007; Computer-aided planning of reconstructive surgery of the innominate bone: automated correction proposals Comput. Aided Surg. 12(5): 286-94), the size and the shape of the augment can be designed based thereon as described above. Based on this bone model, at least the contours of the bone contacting surface of the augment is preferably determined. This surface is formed complementarily to the corresponding surface in the bone model.

Although it is possible that the beam network has a varying coarseness, the beam network can for instance be more coarse at the surface and dense inside, it is preferred according to a preferred embodiment, that each of the beams has a substantially equal predetermined length. This results in a regular geometry and ensures that an optimal pore size for bone ingrowth can be achieved at the bone contacting surface. According to a further preferred embodiment, the interconnection points on said bone contacting surface are positioned at substantially equal mutual distances. As the pore size of the porous beam network model is particularly important at the bone-augment interface, it is preferred if these interconnection points are placed at predetermined optimal mutual distances for ensuring an optimal bone ingrowth.

Although various parameters, such as the number or the lengths of beams of the beam network can be adapted during optimization, it is preferred if each of the beams has a substantially equal predetermined initial diameter and wherein the step of optimizing the mechanical properties comprises adapting the diameters of the beams forming the beam network model. This results in a regular geometry and ensures that an optimal pore size for bone ingrowth can be achieved at the bone contacting surface.

To further improve the bone ingrowth of the manufactured augment, it is preferred if the step of optimizing comprises optimizing the mechanical properties of said beam network model for minimizing the strains in the outer surface of the bone in contact with the bone contacting surface of the augment model. Peak strains at the bone-augment interface compromise the bone ingrowth at this interface. Using the combined model, the stresses and strains in the augment-bone interface are calculated and the geometry of the beam network is optimized, for instance by varying the diameter of the beams as mentioned above, for obtaining as little strains as possible in the bone contacting surface.

It is further preferred if the bone contacting surface is defined by said interconnection points and the beams interconnecting said interconnecting points. The beams interconnecting the

interconnection points or nodes on the bone contacting surface hereby extend substantially parallel to said surface and therefore form a smooth surface. This further reduces any peaks in local loading conditions, such that the bone ingrowth is even further enhanced. The beams defining the bone contacting surface further improves the optimization process. Also the beams are hereby taken into account in the optimization process, such that an implantable augment can be designed and manufactured with as little strains in the bone-augment boundary as possible.

To also provide a smooth outer surface for the surfaces other than the bone contacting surface, it is preferred if the interconnection points on the outer surface of the beam network model define the complete outer surface of the model. Cut-off elements at these outer surfaces are then also prevented, which further allows easier numerically analyzing the model. It is again preferred if the outer surface is further defined by the beams interconnecting said interconnecting points on the outer surface.

According to a further preferred embodiment of the method according to the invention, the step of generating the three-dimensional augment model comprises:

generating an outer surface of the three-dimensional augment model based on the bone model, wherein at least a part of the outer surface forms the bone contacting surface which is formed complementary to the corresponding outer surface of the bone;

generating the beam network model by:

i. arranging a plurality of interconnection points on the bone contacting surface at substantially equal mutual distances;

ii. arranging a plurality of interconnection points for forming the remainder of the

augment model such that the interconnection points are at substantially equal mutual distances;

iii. interconnecting said interconnection points with beams for forming said three- dimensional beam network model, each of the beams having substantially the same length.

According to this embodiment, the bone contacting surface is first defined by the plurality of interconnection points or nodes of the beam model to be able to form a surface being as complementary to the bone surface as possible. When this surface has been defined by these points, which are preferably arranged at substantially equal mutual distances for enhancing bone ingrowth as mentioned above, the remainder of the model is formed using a meshing technique. Forming the remainder of the model hereby means that the a three-dimensional beam network is formed having an outer contour corresponding to the earlier generated outer surface of the three- dimensional augment model. The remainder of the interconnection points are hereto provided at preferably substantially equal distances for creating a beam network having a regular geometry. This beam network can then subsequently be optimized as described above.

According to a preferred embodiment, the beam network consists of a plurality of polyhedrons, such as tetrahedrons and/or hexahedrons. Meshing techniques using for instance tetrahedrons as such are known and allow an efficient formation of a model with a predetermined outer shape, in particular the formation of a bone contacting surface which is accurately formed in

correspondence, i.e. complementary, to the bone surface. Preferably, the beam network model contains only whole polyhedrons, such as tetrahedrons or hexahedrons to prevent the formation of cut-off elements.

According to a specific embodiment, the invention comprises the following steps:

- Generating a three dimensional (3D) model of the defective region based on one or more medical images thereof

- Generating a finite element tetrahedron mesh based on the 3D model with controlled edge length

- Convert the tetrahedron mesh to a beam network and perform a structural analysis and optimization of the diameter of the beam network by finite element method and engineering optimization technology

- Generate the augment by converting the beam network to a 3D model utilizing the beam network diameter values calculated by the optimization tool.

To be able to reliably and accurately manufacture the augment according to the design of the model, the step of manufacturing preferably comprises using a three-dimensional printing technique, also referred to as rapid manufacturing technique, layered manufacturing technique, additive manufacturing technique or material deposition manufacturing technique.

Rapid manufacturing includes all techniques whereby an object is built layer by layer or point per point by adding or hardening material (also called free-form manufacturing). The best known techniques of this type are stereolithography and related techniques, whereby for example a basin with liquid synthetic material is selectively cured layer by layer by means of a computer-controlled electromagnetic beam; selective laser sintering, whereby powder particles are sintered by means of an electromagnetic beam or are welded together according to a specific pattern; fused deposition modelling, whereby a synthetic material is fused and is stacked according to a line pattern;

laminated object manufacturing, whereby layers of adhesive -coated paper, plastic, or metal laminates are successively glued together and cut to shape with a knife or laser cutter; or electron beam melting, whereby metal powder is melted layer per layer with an electron beam in a high vacuum. In particular embodiments, Rapid Prototyping and Manufacturing (RP&M) techniques, are used for manufacturing the augment of the invention. Rapid Prototyping and Manufacturing (RP&M) can be defined as a group of techniques used to quickly fabricate a physical model of an object typically using three-dimensional (3-D) computer aided design (CAD) data of the object.

Currently, a multitude of Rapid Prototyping techniques is available, including stereo lithography (SLA), Selective Laser Sintering (SLS), Fused Deposition Modeling (FDM), foil-based techniques, etc. A common feature of these techniques is that objects are typically built layer by layer.

Stereo lithography (SLA), presently the most common RP&M technique, utilizes a vat of liquid photopolymer "resin" to build an object a layer at a time. On each layer, an electromagnetic ray, e.g. one or several laser beams which are computer-controlled, traces a specific pattern on the surface of the liquid resin that is defined by the two- dimensional cross-sections of the object to be formed. Exposure to the electromagnetic ray cures, or, solidifies the pattern traced on the resin and adheres it to the layer below. After a coat had been polymerized, the platform descends by a single layer thickness and a subsequent layer pattern is traced, adhering to the previous layer. A complete 3-D object is formed by this process.

Selective laser sintering (SLS) uses a high power laser or another focused heat source to sinter or weld small particles of plastic, metal, or ceramic powders into a mass representing the 3- dimensional object to be formed.

Fused deposition modeling (FDM) and related techniques make use of a temporary transition from a solid material to a liquid state, usually due to heating. The material is driven through an extrusion nozzle in a controlled way and deposited in the required place as described among others in U.S. Pat. No. 5.141.680.

Foil-based techniques fix coats to one another by means of gluing or photo polymerization or other techniques and cut the object from these coats or polymerize the object. Such a technique is described in U.S. Pat. No. 5.192.539. Typically RP&M techniques start from a digital representation of the 3-D object to be formed, in this case the design of the augment. Generally, the digital representation is sliced into a series of cross-sectional layers which can be overlaid to form the object as a whole. The RP&M apparatus uses this data for building the object on a layer-by-layer basis. The cross-sectional data representing the layer data of the 3-D object may be generated using a computer system and computer aided design and manufacturing (CAD/CAM) software.

The implantable augment of the invention may be manufactured in different materials. Typically, only materials that are biocompatible (e.g. USP class VI compatible) with the human body are taken into account. Preferably the augment is formed from a heat-tolerable material allowing it to tolerate high-temperature sterilization. In the case SLS is used as a RP&M technique, the surgical template may be fabricated from a polyamide such as PA 2200 as supplied by EOS, Munich, Germany or any other material known by those skilled in the art may also be used.

The invention further relates to an implantable bone augment arranged to at least partially fit in a bone defect in a bone of a patient, wherein the augment comprises a body formed of a beam network which is formed by a plurality of beams interconnected at interconnection points, wherein a plurality of interconnection points on an outer surface of the beam network define a bone contacting surface arranged for contacting a corresponding outer surface of the bone, in particular a surface of the bone defect, and wherein the interconnection points on said bone contacting surface are positioned at substantially equal mutual distances and such that the bone contacting surface is formed complementary to the corresponding outer surface of the bone, wherein the diameters of said beams in the network are adapted for minimizing the strains in the bone contacting surface of the augment model. As already described above, each of the beams has a substantially equal length and the beam network preferably consists of a plurality of polyhedrons, such as tetrahedrons and/or hexahedrons.

The present invention is further illustrated by the following Figures, which show a preferred embodiment of an augment and method according to the invention, and are not intended to limit the scope of the invention in any way, wherein:

Figure la schematically shows a boundary of an augment according to the prior art;

Figure lb schematically shows a boundary of an augment according to the invention;

Figure 2 schematically shows the process of creating the initial beam network model of the augment; and

Figure 3 schematically shows the result of the optimization process. In figure la, an augment 1 according to the prior art is shown. The body of the augment 2 comprises a porous structure having beams 10 which are interconnected at interconnection points 11. The outer surface 2, which is arranged to be in contact with the bone, is either defined by the cut off beams 10a extending at thus outer surface or by interconnection points 11a which happen to be at the outer surface 2. The surface 2 is formed by simply cutting through the structure to create an outer surface which corresponds to the surface of the bone for which the augment 1 is intended. This cutting results in the cut-off elements 10a which compromise bone ingrowth. According to the invention as shown in figure lb, an outer surface 2 of the augment 1 is defined by outer interconnection points 11a which are interconnected with outer beams 10a. The inner structure of the augment is formed by corresponding beams 10 connected at interconnection points 11 to form a beam network. The outer surface 2 is therefore smooth and does not contain cut-off elements such as in the augment according to the prior art. The beams 10, 10a and interconnection points 11, 11a form tetrahedrons.

It is preferred to form such a beam network by first determining the outer shape or outer surface 2 of the augment 1 , which is schematically shown in the left of figure 2. A plurality of

interconnection points 11a are subsequently placed on this outer surface 2, wherein the mutual distances between the points 11a are substantially the same. This ensures an optimal pore size at the boundary for bone ingrowth. When the surface 2 is defined by the points, the remainder of the model, for instance the inner structure thereof, can be defined using a meshing technique. The resulting beam network is shown in the middle figure of figure 2. The design of the model can then be manufactured using a three-dimensional printing, as shown in the right.

Before printing, the structure of the beam network is however optimized to minimize peak loads in the structure, in particular at the surface 2 intended to contact the bone. A combined Finite Element Method model of the augment, as seen in the middle of figure 2, in combination with the surrounding bone is made. By applying boundary conditions which mimic typical loading conditions of the bone, the mechanical environment in the augment in implanted state can be calculated.

On the left in figure 3, it can be seen that at the location indicated with the arrows, local increased stresses are calculated. By optimizing the geometry of the beam network to reduce these peak loads, a more or less even distribution of the stresses can be obtained, see the right of figure 3. The geometry is optimized by locally increasing or decreasing the diameters of the beams in dependence of the local mechanical environment. In case higher stresses are calculated, the beam diameter is for instance increased and vice versa. Increased beam diameters can therefore be seen at the location indicated with the arrows on the right of figure 3. The present invention is not limited to the embodiment shown, but extends also to other embodiments falling within the scope of the appended claims.