Composition of orthopedic knee implant and the method for manufacture thereof

Abstract

The present invention discloses a composition of a knee implant comprising biomaterials such as combination of TiNbZr alloy and tantalum to support osseointegration. The present invention further discloses a method of manufacturing customized patient-specific knee implant using 3D printing technology to suit the patient. The method involves the use of high energy source such as fiber laser or electron-beam. The base plate is mounted on the CNC. The energy source creates a melt pool on the base plate and the energy source is fed with a biomaterial in the form of wire or powder. The biomaterial is deposited on the base plate layer by layer, which solidifies in the melt pool of the base plate. The knee implant thus fabricated suits the elastic modulus of the bone and is useful as customized implant in patient undergoing replacement surgery.

Claims

1. A composition of a knee implant for a patient undergoing replacement surgery, the knee implant comprising: a. tantalum at a concentration of 30% to 60%; and b. a TiNbZr alloy at the concentration of 40% to 70%.

2. The composition as claimed in claim 1, wherein the surface of the implant in contact with a bone is made of tantalum and the surface of the implant not in contact with the bone is made of TiNbZr alloy.

3. A method for manufacture of a knee implant, wherein the method uses an additive manufacturing technology or 3-Dimentional (3D) printing technology, the method comprises the steps of: a. mounting a base plate on a Computer Numerical Control (CNC) table in a vacuum chamber; b. mounting a high energy source such as a micro-plasma torch or a laser beam on a vertical Z axis of the CNC table such that the energy source creates a melt pool on the base plate; c. feeding the energy source with a biomaterial in the form of a wire or a powder or a combination of both; d. allowing the biomaterial to melt from the energy source and depositing the melted biomaterial on the base plate layer by layer and allowing the layers to solidify in the melt pool of the base plate; e. continuing the movement of melt pool and deposition by simultaneously moving the vertical energy source and the CNC table to obtain an optimum configuration of the knee implant; f. stopping the deposition of the biomaterial when about 500 to 750 microns of the bearing surface is yet to be deposited in case of femoral component; and g. in-situ surface hardening by initiating the deposition again after triggering the flow of a gas or ion source such as oxygen and/or nitrogen into the melt pool and also after applying a suitable bias, where necessary, to an implant being deposited.

4. The method as claimed in claim 3, wherein the vacuum chamber is evacuated to ensure that the oxygen content is less than 1 ppm (parts per million) so as to eliminate oxidation of the biomaterial.

5. The method as claimed in claim 3, wherein the energy source is flexible in movement within the axis and the capacity of the energy source is maintained at 1 to 2 kW (Kilo Watt) for plasma torch or 500 W for cw fiber laser or 20 kW peak for pulsed laser.

6. The method as claimed in claim 3, wherein in-situ surface hardening of the implant is achieved by introducing nanopowder selected from a group comprising titanium dioxide (TiO2), titanium nitride (TiN), titanium carbide (TiC), tantalum pentoxide (Ta2O5), or tantalum nitride (TaN) into the melt pool.

7. The method as claimed in claim 3, wherein the biomaterial is a combination of TiNbZr alloy and tantalum.

8. The method as claimed in claim 3, wherein the rate of deposition of the biomaterial varies between 50 gms per hour to 500 gms per hour.

9. The method as claimed in claim 3, wherein the surface of the implant in contact with a bone is electrochemically oxidized to create multiple oxide nanotubes such that increasing the porosity and promoting osseointegration by traversing or scanning of laser beam on one or more porous parts of the implant prior to electrochemical oxidation.

10. The method as claimed in claim 3, wherein the porosity of the implant is altered through space-holder-technique by adding sodium chloride or ceramic oxide to the melt pool of the base plate such that porosity of the implant achieved is 100 to 700 microns.

11. The method as claimed in claim 3, wherein the elastic modulus of the implant is matched with that of the bone by varying the porosity of the implant and the biomaterial.

12. A method for selection of customized and patient-specific knee implant, the method comprises the steps of: a. subjecting a damaged knee of a patient to a Computer Tomography (CT) scanning; b. converting the CT scan data to a Standard Template Library (STL) file of the knee and designing a knee implant based on this data to fit the damaged knee with minimal bone chipping; c. checking the physical model of the knee implant for approval by a surgeon with respect to size and configuration of the knee implant; d. manufacturing a metal knee implant for the approved physical model by using additive manufacturing technology; and e. fixing the metal knee implant into the patient along with one or more patient-specific instruments manufactured along with the implant.

13. The method as claimed in claim 12, wherein the physical model of the knee implant is prepared using acrylonitrile butadiene styrene plastic.

14. The method as claimed in claim 12, wherein the knee implant is patient specific resulting in accurate fixation with minimal bone chipping.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The features of the embodiments will become clearer from the detailed descriptions that follow, particularly when read together with the accompanying figures.

[0032] FIG. 1 illustrates a flow chart depicting the manufacturing method for the knee implant.

[0033] FIG. 2 illustrates a flow chart illustrating the design and selection of the customized and patient-specific knee implant.

DETAILED DESCRIPTION OF THE INVENTION

[0034] In order to clearly and concisely describe the subject matter of the claimed invention, we provide definitions for specific terms that are subsequently used in the written description.

[0035] The term Implant refers to a substitute for the damaged bone, tissue etc., which is fabricated artificially to match the damaged portion of the bone, and which is attached to a patient undergoing knee replacement surgery.

[0036] The term Alloy refers to a metal made by combining two or more metallic elements, particularly to provide greater strength or resistance to corrosion.

[0037] The present invention discloses a novel composition of a knee implant biomaterials such as the combination of a TiNbZr alloy and tantalum to promote osseointegration. The present invention further discloses a method of manufacturing a customized and patient-specific knee implant using 3D printing technology.

[0038] The biomaterials are combined in different proportions depending on the requirements of the patient (i.e., the biomaterial combinations are patient-specific). The first composition of the biomaterials consists of 100% tantalum, since tantalum exhibits an improved rate of osseointegration. The second composition consists of 100% TiNbZr alloys, such as Ti-10Nb-10Zr, Ti-13Nb-13Zr and Ti-23Nb-13Zr, which are also preferred for improved osseointegration. The third composition consists of combinations of tantalum (between 30% to 60%) and TiNbZr alloy (between 40% to 70%) depending on the implant design. In this type of implant, the parts or surfaces of the implant that are in contact with the bone are made of tantalum. The other parts of the implant, which are not in direct contact with the bone, are made of TiNbZr alloy. This combination of tantalum and TiNbZr alloy represents the ideal implant composition to achieve optimum osseointegration, and it is also cost effective.

[0039] The knee implant is manufactured using combination of biomaterials such as TiNbZr alloy and tantalum. The combination of these biomaterials promotes osseointegration of the knee implant with high wear resistance. Tantalum is used in areas of the knee implant that are in contact with the bone due to its better osseointegration, as compared to the TiNbZr alloy. At the same time, the surface of the implant, where there is no bone contact, is made of TiNbZr alloy since it is less expensive compared to tantalum. The use of wire and powdered forms of the biomaterials facilitate specific biomaterial configurations for the patient-specific knee implant, in a cost effective manner.

[0040] The present invention also discloses the method of manufacturing the customized and patient-specific knee implant. The present invention further overcomes the problems associated with the use of readily available knee implants, through patient-specific knee implants.

[0041] FIG. 1 illustrates a flow chart depicting the manufacturing method for the knee implant. The present invention discloses the use of additive manufacturing technology, also termed as 3D printing technology, to manufacture accurate and patient-specific knee implants. The additive manufacturing technology involves the use of a high-energy source such as a fiber, pulsed laser, electron-beam, or micro-plasma torch. Further, the method involves the use of an inexpensive wire made of a suitable biomaterial. The method 100 of using additive manufacturing technology for the manufacture of a knee implant begins at step 101, where a base plate is mounted on a Computer Numerical Control (CNC) table in a vacuum chamber. The vacuum chamber is evacuated to ensure that the oxygen content is less than 1 ppm within the vacuum chamber, so that oxidation of the biomaterial is eliminated inside the vacuum chamber. At step 102, the high-energy source such as a plasma torch or laser beam is mounted on the vertical Z-axis of the CNC table such that the energy source creates a melt pool on the base plate for the deposition of the biomaterial. The energy source is flexible in movement within the axis and the capacity of the energy source is maintained at 1 to 2 kW (Kilo Watt) for the plasma torch, or between 200 to 500 W for continuous wave (cw) fiber laser, or a 20 kW peak in the case of a pulsed laser. At step 103, the energy source is fed with a suitable biomaterial in the form of a wire or powder or a combination of both. In addition, the process allows for a change in biomaterial used depending on the specific requirement. The biomaterial used in the present invention is a combination of a TiNbZr alloy and tantalum. At step 104, the biomaterial is melted by the energy source and is deposited on the base plate, layer by layer. The deposited biomaterial solidifies in the melt pool of the base plate. At step 105, the movement of the melt pool and deposition is continued in order to obtain the optimum configuration of the implant. The rate of deposition of the biomaterial usually varies between 50 gms per hour and 500 gms per hour. The implant is created as per the required configuration through the simultaneous movement of the vertical energy source and the CNC table. The process parameters such as laser beam and/or plasma torch power, feed rate of wire and/or powder, speed of the CNC system and pitch or distance between deposition tracks are varied accordingly such that the porosity the deposition is varied from 1% to 80% as required. The porous parts of the implants are integral parts of the implant and are deposited at the same time and they are not welded or coated. The porosity is very low, i.e., 1% to 5% in the bearing surface of the femoral component and can be as high as 80% in the parts in contact with the bone so as to allow the bone to grow into the pores. The size of the pores is typically 100 to 700 microns. The elastic modulus of the implant is matched with that of the bone by varying the porosity and/or biomaterial or both. The melting of the wire and/or the powder, along with the movements of the CNC table and vertical energy source, creates a customized, patient-specific knee implant. At step 106, in the case of the femoral component, the deposition of the biomaterial is stopped when about 500 to 750 microns of the bearing surface is yet to be deposited. At step 107, the deposition is initiated again, after triggering the flow of an ion and/or gas source such as oxygen or nitrogen, and also after applying a suitable bias to the implant being deposited (where necessary). The process results in the fabrication of one implant in 1 to 4 hours depending on the feed rate, the number of biomaterials used, etc.

[0042] The porosity of the implant plays an important role in promoting the growth of the bone, and for osseointegration. The porosity of the knee implant is altered by varying the deposition current of the energy source, the feed rate of the wire or powder and also by changing the pitch or distance between the tracks. The process further allows the porosity of the implant, composed of TiNbZr alloy and tantalum, to be altered so that the elastic modulus of the implant matches that of the bone.

[0043] Where it is in contact with the bone, the implant's porosity promotes osseointegration by enabling the bone to grow into the pores. In order to achieve load transfer from the implant to the adjoining bone, the elastic moduli have to match. Porosity helps reduce the elastic modulus of the implant so as to match that of the bone. This helps avoiding of stress shielding and also enables load transfer to the bone, ensuring a healthy bone.

[0044] The use of inorganic compounds through the space-holder-technique can also alter the porosity of the knee implant. Sodium chloride or ceramic oxide is added to the melt pool of the base plate, which does not mix with the metal or alloy. The process also enables substantial variation of porosity. Low porosity in the range of 1% to 5% is maintained on the surface of the implant, where wear resistance is required. In contrast, high porosity of 75% to 80% is maintained on the inner surface of the implant, which is in contact with the bone. Osseointegration, or cement-less bonding of the bone to the implant, requires the bone to grow into the pores of the implant, thereby creating the bond. Hence, for the bone to grow into the pores of the implant, pore sizes must be in a specific rangetypically between 100 to 700 micronswhich is achieved by the present invention. The present invention enables the creation of porosity as an integral feature of the implant, without resorting to subsequent joining, welding or coating.

[0045] Since the knee implant is manufactured by the simultaneous deposition of biomaterials, it exhibits functionally gradient properties without the risk of failure.

[0046] The method disclosed in the present invention produces a knee implant surface that is continuous and dense. The implant surface acts as a bearing surface, i.e., one where there is movement during the regular functioning of the knee. At the same time, the surface in contact with the bone is porous so as to promote osseointegration and to help match the elastic modulus of the bone. In other words, the same material can be deposited as dense with very low porosity, or as highly porous, by changing the deposition conditions. This minimizes stress shielding of the bone, and improves the reliability of the joint replacement surgery.

[0047] Titanium and tantalum exhibit poor wear resistance. In order to overcome this limitation, the present invention utilizes a method of in-situ hardening of the bearing surface of the knee implant. During deposition of the biomaterial, the bearing surface of the femoral component of the knee implant is subjected to hardening.

[0048] Hardening the surface of the femoral component is achieved in one of the following ways:

[0049] The first method includes nitriding or oxidation, which is done in-situ during the deposition of the last few layers, usually for a thickness of 0.5 to 0.75 mm (500 to 750 microns). During the deposition, oxygen and/or nitrogen is introduced into the melt pool, along laser beam or a plasma torch, which results in the formation of oxides or nitrides. The extent of oxide or nitride formation is controlled by the flow rate of oxygen and/or nitrogen. It may be necessary to apply a negative bias on the work piece to improve the hardening process. The objective is to obtain a functionally gradient surface, such that the softest portion is in contact with the interior of the implant and the hardest portion is on the exterior portion of the implant. The depth of hardening should include a machining allowance, so that the depth of hardening is at least 0.3 mm in the final implant. Further, it may also be necessary to introduce oxygen and/or nitrogen as ions from an ion source rather than only as gas molecules.

[0050] Another method is to introduce nanopowders of titanium dioxide (TiO.sub.2), titanium nitride (TiN), titanium carbide (TiC), tantalum pentoxide (Ta.sub.2O.sub.5), or tantalum nitride (TaN) into the melt pool during the oxidation and/or nitriding steps. The titanium nitride (or tantalum nitride), titanium carbide (or tantalum carbide), and titanium dioxide (or tantalum pentoxide) thus formed are biocompatible.

[0051] The hardness (including scratch hardness) of the in-situ hardened layers of the knee implant is higher than that of the CoCrMo implant, which is presently used. Moreover the co-efficient of the friction is less than that of the CoCrMo implant. These in-situ hardened layers of the implant also exhibit better wettability of the body fluids compared to the traditional CoCrMo implant. These factors result in a higher wear resistance of the femoral component, but at the same time they result in a reduction in the wear of the polyethylene insert. This method of in-situ hardening improves the overall wear resistance of the knee implant.

[0052] This method of in-situ hardening improves the wear resistance of tantalum and titanium alloys and improves the surface hardness of the femoral component to reduce wear during its movement. This surface hardening is being done in-situ during deposition and not as an additional step, to achieve the requisite hardening depth and to minimize the steps involved in the manufacture of the knee implant.

[0053] The porous parts of the implant are electrochemically oxidized so as to form multiple oxide nanotubes on the surface. The nanoporous structure on the porous surface of the implant promotes osseointegration after the implant is installed in the patient. Prior to electrochemical oxidation, it is preferable to laser texture the surface of the porous parts of the implant by traversing or scanning the laser beam on the surface of the implant. Both the in-situ hardened parts and the porous parts are produced in the same manufacturing process, and therefore represent integral parts of the implant. There are no welding or coating requirements.

[0054] Regulatory authorities require consistency, repeatability and reliability in the implant manufacturing process. As a result, process control is critical. The 3D printing process disclosed in the present invention is associated with a continuous monitoring of the melt pool temperature and dimensions and also bead dimensions through three cameras, one 2-wavelength pyrometer, and suitable image processing algorithms. These enable the generation of a microstructure map between process conditions and implant microstructure (and hence the metallurgical properties) at a given location.

[0055] The elastic modulus of the TiNbZr alloy chosen is substantially less than the Ti-6Al-V alloy. The elastic modulus is further reduced through the porous structure, so as to match the elastic modulus of the bone. This ensures load transfer from the implant to the bone, improving the density and life of the bone. In other words, the present invention overcomes the stress shielding limitation exhibited by current implants.

[0056] Usually, the 3D printing process does not offer surfaces with high surface finish. The present invention minimizes this problem by using wire in portions or areas that require high surface finish. The use of wire offers a better finish compared to using powder. However, final machining and polishing are still necessary to achieve the requisite dimensional tolerance and surface finish.

[0057] Laser machining with nano and picosecond lasers to 30-micron accuracy is possible. However, it is not feasible to use this as a finishing step immediately after 3D printing, due to possible dimensional variation from the annealing step. Therefore, to avoid the high cost of conventional 5-axis CNC finish milling, laser machining is performed in the present invention after annealing in the same 3D printing setup to achieve the requisite final dimensions and surface finish.

[0058] The surface structure of the porous part of the implant plays an important role in the osseointegration. In order to improve the rate of osseointegration, the surface of the implant, which is in contact with the bone, is electrochemically oxidized to create multiple oxide nanotubes on the surface inducing porosity. The nanoporous structure on the surface of the implant promotes improved osseointegration allowing the growth of the osteoblasts after the implant is installed in the patient.

[0059] In order to improve the osseointegration further, the porous parts are laser textured by traversing or scanning a laser beam, prior to electrochemical oxidation.

[0060] FIG. 2 illustrates a flow chart illustrating the design and selection of the customized and patient-specific knee implant. The method 200 starts with step 201 of Computer Tomography (CT) scanning of the damaged knee of the patient. At step 202, the CT scan data is converted to a Standard Template Library (STL) file of the knee. A physical model of the knee implant is designed to fit the damaged knee in a patient. The physical model of the knee implant is prepared using acrylonitrile butadiene styrene plastic or any suitable polymer. The physical plastic model is patient-specific and is produced in order to check the accuracy of the size and configuration of the knee implant. The knee implant is designed to fit the damaged knee with minimal bone chipping. At step 203, the physical plastic model of the knee implant is checked for approval by the surgeon with respect to size and configuration of the knee implant. At step 204, the metal knee implant is manufactured as per the approved physical plastic model by using additive manufacturing technology or 3D printing technology. The metal knee implant is the actual knee implant, which is customized to each patient. The knee implant is manufactured using the combination of biomaterials such as titanium alloy and tantalum using additive manufacture technology. At step 205, the surgeon fixes the knee implant to the patient with the help of patient-specific instruments manufactured along with the implant. As the knee implant is patient specific, and is based on the dimensions of the damaged knee, installation is convenient for the surgeon with minimum chipping of the bone in order to fix the knee implant.

[0061] The present invention discloses the use of wire and powder forms of the biomaterial to obtain the biocompatible knee implant. The present invention also discloses the creation of porosity as an integral feature of the implant.

[0062] The present invention further discloses the in-situ hardening of the bearing surface as an integral part of the process.

[0063] The biomaterials used in the present invention such as TiNbZr alloy and tantalum are superior in terms of biocompatibility and osseointegration, compared to Cobalt-Chromium (CoCr) alloy usually used in current implants. The TiNbZr alloy comprises titanium, niobium and zirconium, all of which are biocompatible and do not result in toxicity or incompatibility in the patient.

[0064] The process disclosed in the present invention is also useful for the manufacture of suitable patient-specific jigs, fixtures, and gauges that aid the surgeon during surgery using 3D printing technology. These jigs, fixtures, and gauges may be produced in any suitable biocompatible and sterilizable polymer. This ensures better fit between the bone and implant.

[0065] The knee implant produced by the present invention matches the elastic modulus of the bone, which is achieved by using a low modulus Ti alloy and by creating porosity in the implant.

[0066] Another advantage of the present invention is the use of the Android operating system, which enables easy communication and transfer of models and images to the surgeon through smart phones and tablets.

[0067] The present invention is not only specific or restricted to knee implants. The similar composition and the method of preparation are also applicable for other implants such as the hip, etc.