WO2014015262A1

WO2014015262A1 - Metal composite hybrid orthopaedic implants

Info

Publication number: WO2014015262A1
Application number: PCT/US2013/051281
Authority: WO
Inventors: Darren J Wilson; Henry B. Faber; Joseph M. Ferrante; Gene E. Austin; David L. Evans
Original assignee: Smith & Nephew, Inc.
Priority date: 2012-07-19
Filing date: 2013-07-19
Publication date: 2014-01-23

Abstract

A method of manufacturing an intramedullary nail is disclosed. The method includes: providing a mold having a proximal section, a distal section, and a bend section in between the proximal section and the distal section; inserting at least one core into the mold; heating melt between 205 degrees and 400 degrees Celsius; and injecting the heated melt in at least two gate locations adjacent the bend section. In the preferred embodiment, the melt is made of PEEK material.

Description

METAL COMPOSITE HYBRID ORTHOPAEDIC IMPLANTS

Cross-Reference to Related Applications

[0001] This application is a PCT International Application of United States Patent Application No. 61/673,643 filed on July 19, 2012, and United States Patent Application No. 61/674,092 filed on July 20, 2012. The disclosure of each application is incorporated by reference in its entirety.

Background of the Invention

1. Field of the Invention

[0002] In current orthopaedic practice, metal orthopaedic implants are typically used to either repair broken bones or reconstruct diseased joints in a patient. These metal orthopaedic implants are commonly made from cobalt chrome, titanium-64 and stainless steel. High strength composites provide an alternative material for manufacturing orthopaedic devices and offer advantages in terms of improved visualization and interpretation of the healing site, accelerated healing, improved patient comfort, and reduced volume of metal. However, the commercial motivation to switch to a fully compositized structure has been tempered due to higher manufacturing and raw material costs, coupled with the risks associated with the introduction of a new technology into the market place at a cost premium. Given there are significant patient and surgeon advantages for including composite materials in a particular format within the device, a more cost effective approach would be to develop metal-composite hybrid devices, which has the potential for reduced manufacturing costs. Hybrid orthopedic implants made of plastic and metal present advantages by combining the benefits of each material and avoiding their disadvantages. In particular, the material that is strongest or is easiest to manufacture or shape into complex or thin sections, can be selectively used for different portions of the implant. [0003] In the case of trauma fixation plates, metal implants present the advantage of malleability, i.e. the surgeon can permanently change the shape of the implant to suit his needs by bending or twisting during application. However, because of their hardness, it is difficult for the surgeon to cut, or to shave, a metallic implant intra-operatively. In the case of injection molded metal polymer composite hybrid plates, these devices offer the opportunity of being shaped intra-operatively by either bending or twisting operations due to the malleability of the metal core component while their size and shape can be modified by either cutting with scissors or shaving with a scalpel. Furthermore, plastic is more elastic and therefore contours to the unique shape of a patient's bone, if made thin enough and pressed or molded onto the bone surface.

[0004] A key issue in manufacturing metal-polymer composite hybrid systems is the transitive region that joins the two dissimilar materials. There are a variety of methods to secure the metal portion to the composite portion. For example, certain physical or chemical etching or anodization pretreatment processes, which produce oxide films on the metal surfaces which, because of their porosity and microscopic roughness, mechanically interlock with the polymer forming much stronger bonds than if the surface were smooth. Alternatively, machined features are added to the metal core component, e.g. grooves, dovetails and pins to interlock the over-mold component onto the metal surface.

Summary of the Invention

[0005] It is in view of the above problems that the present invention was developed.

The various embodiments of the present invention described below and shown in the Figures provide a hybrid orthopaedic implant, wherein the outer-layer can be manufactured using a plurality of different methods depending upon the end application. The over-mold component allows the stresses in the surrounding bone to be closer to their normal physiological level, which cannot be achieved with an all metal system. [0006] Further areas of applicability of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the particular embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

Brief Description of the Drawings

[0007] The accompanying drawings which is this case is a group of sketches prepared by the inventor and, which are incorporated in and form a part of the specification, illustrate the embodiments of the invention and together with the written description serve to explain the principles, characteristics, and features of the invention. In the drawings:

[0008] Figures 1-20 illustrate various embodiments of the invention.

Detailed Description of the Embodiments

[0009] The following description of the depicted embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

Intramedullary nail

(a) Metal core component

[0010] The metal core component can be tapered or non-tapered and solid or cannulated with or without the inclusion of design features, e.g. screw holes, slots, key- ways, Figure 2. The inner diameter of the core is typically 4.8 mm whereas the outer diameter of the core ranges from 7.3 mm up to 10.5 mm. This part is fabricated by conventional metal working techniques and may consist of any of a wide variety of metals, the most preferred being stainless steel, cobalt-chrome alloy, and titanium alloy. [0011] The dimensions of the non-tapered, stainless steel core component designed specifically to clone the distal and proximal sections of a standard commercially available "all metal" tibial intramedullary nail for a 10 mm, 11.5 and 13 mm are illustrated in Figure 3(a-c) respectively. In all cases, the minimum outer diameter of the core is fixed at 8.9 mm, which allows the proximal section to be threaded to receive a standard drill guide used with the tibial intramedullary nail instrumentation set.

[0012] The dimensions of the tapered, stainless steel core component designed specifically to clone the distal and proximal sections of a standard commercially available "all metal" tibial intramedullary nail for an 8.5 mm, 10 mm, 11.5 and 13 mm are illustrated in Figure 4(a-d) respectively. The tapered core component can be formed by turning down the distal section to a specified outer diameter with a known bending stiffness.

[0013] In both designs, the over-mold component can be a polymer (e.g. injection mold grade PEEK) or a composite (e.g. 30 % w/v short carbon fiber reinforced PEEK). The theoretical bending stiffness of the metal hybrid composite intramedullary nail for both the tapered and non-tapered design can be determined from the product of the Young's modulus and the second moment of inertia. For comparative purposes, the bending stiffness's of the "all metal" tibial intramedullary nail in the distal and proximal section are indicated in Table 1. For the tapered 10 mm hybrid core design, the bending stiffness can reach up to 70% of the theoretical bending stiffness measured for an "all metal" tibial intramedullary nail of equivalent outer diameter, Table 1. This assumes that the core and over-mold components selected are cobalt chromium alloy and PEEK respectively. For the non-tapered hybrid nail design, the bending stiffness mismatch is lower.

Standard TIBIAL 8.5 OD/4.8 ID 12.0 OD/4.8 26.3 113.2

NAIL (8.5 mm) ID

Standard TIBIAL 10.0 OD/5.4 12.0 OD/5.4 51.2 111.3

NAIL ID ID

(10.0 mm)

Standard TIBIAL 11.5 OD/5.4 12.0 OD/5.4 93.1 111.3

NAIL ID ID

(11.5 mm)

Standard TIBIAL 13.0 OD/8.4 13.0 OD/8.4 131.6 131.6

NAIL (13.0 mm) ID ID

Tapered core design - 10 mm TRIGEN META TIBIAL NAIL clone

Titanium-64 8.5 OD/4.8 ID 8.9 OD/4.8 ID 26.3 32.2

Core

Over-mold PEEK 10 OD/8.5 ID 12 OD/8.9 ID 0.7 2.1

Over-mold PEEK 10 OD/8.5 ID 12 OD/8.9 ID 4.7 14.2

(carbon fiber

reinforced)

Stainless steel 7.6 OD/4.8 ID 8.9 OD/4.8 ID 26.7 54.5

Core

Over-mold PEEK 10 OD/7.6 ID 12 OD/8.9 ID 1.0 2.1

Over-mold PEEK 10 OD/7.6 ID 12 OD/8.9 ID 6.5 14.2

(carbon fiber

reinforced)

Cobalt 7.3 OD/4.8 ID 8.9 OD/4.8 ID 26.8 66.4

Chromium Core

Over-mold PEEK 10 OD/7.3 ID 12 OD/8.9 ID 1.1 2.2

Over-mold PEEK 10 OD/7.3 ID 12 OD/8.9 ID 7.0 14.2

(carbon fiber

reinforced)

Non-Tapered core design - 10 mm TRIGEN META TIBIAL NAIL clone

Titanium-64 core 8.9 OD/4.8 ID 8.9 OD/4.8 ID 32.2 32.2

Over-mold PEEK 10.0 OD/8.9 12.0 OD/8.9 0.5 2.1

ID ID

Over-mold PEEK 10.0 OD/8.9 12.0 OD/8.9 3.7 14.2

ID ID

(carbon fiber reinforced)

Stainless steel 8.9 OD/4.8 ID 8.9 OD/4.8 ID 54.5 54.5

core

Over-mold PEEK 10.0 OD/8.9 12.0 OD/8.9 0.5 2.1

ID ID

Over-mold PEEK 10.0 OD/8.9 12.0 OD/8.9 3.7 14.2

ID ID

(carbon fiber

reinforced)

Cobalt 8.9 OD/4.8 ID 8.9 OD/4.8 ID 66.4 66.4

Chromium Core

Over-mold PEEK 10.0 OD/8.9 12.0 OD/8.9 0.5 2.1

ID ID

Over-mold PEEK 10.0 OD/8.9 12.0 OD/8.9 3.7 14.2

ID

(carbon fiber ID

reinforced)

Table 1 Theoretical bending stiffness measurements calculated from "all metal" tibial nails and metal composite hybrid designs as a function of diameter. E (stainless steel) = 193 GPa, E (Cobalt chromium) = 235 GPa, E (titanium-64) = 114 GPa, E (CF reinforced PEEK) = 20 GPa, E (PEEK) = 4GPa.

[0014] The surface of the metal core is either physically or chemically textured or grooved to help key-in the over-mold material. Alternatively, the core consists of a plurality of pins or ridges extending axially or radially or a combination of both from the surface or is equipped with dovetails located at the distal and proximal ends of the core, as indicated in Figures 5(a) and (b) respectively.

(b) Over-mold component

[0015] The over-mold component is typically 0.5 mm - 3.0 mm thick. The matrix is preferably selected from polysulfone, polyaryletherketone including but not limited to polyether-ether-ketone, polyether-ketone-ketone, polyimide, polyacetal, epoxy, polyethylene and polycyanate. The optional composite portion for increased stiffness comprises of one or more filaments disposed about a longitudinal axis and within a polymer matrix. The reinforcement agent can be either a tow or a fiber, e.g. glass, carbon or aramid fibers or high strength fiber drawn polyester.

[0016] The two parts may also be connected via an adhesive joint, or a shrink fit joint. Alternatively, in the orthopaedic device, the metal portion may be received within the composite portion and secured thereto. Pre-heating the metal core might be necessary to bring its surface temperature closer to the melt temperature of the over-mold (~350°C) so at to reach optimum bond strength.

[0017] Interference fit. CF reinforced PEEK tube made using ram extrusion. The steel inner tube is cooled using liquid nitrogen to contract it. The steel is inserted into the composite tube and allowed to warm up, thus expanding inside the composite tube.

Manufacturing

(a) Filament winding

[0018] One particular method of making the device is filament winding, which is an automated process of wrapping filaments in a helical pattern over the metal core. The advantages of this method include precise fiber orientations, high fibre-to-resin ratios, straight uncrimped fibre paths, high consistency and reproducibility. This method comprises the steps of first filament winding a tow circumferentially around a metal core at various angles with respect to the longitudinal axis to form an outer-layer comprising one or more layers. Each layer may contain fibers oriented at a constant angle along the longitudinal axis or fibers oriented at a changing angle with the longitudinal axis to provide additional resistance to torsional forces. The angles used are selected to give desired mechanical properties both globally and locally in the structure. Consequently, the compositized over-mold layer can be engineered to provide a variable modulus along the length of the core due to the use of either filament winding or braiding techniques.

(b) Injection molding

[0019] Another method of making the device is using a hybrid injection molding process outlined in Figure 6. A thin wall tapered metal tube is placed in the injection- molding tool/mold. The design features located in the metal core are shut off using a series of metal inserts (1-8). The tool closes and is then filled with a polymer resin at the open end. The molten polymers flows into the empty cavity left between the core and the mold, referred to hereafter as the "gate" using a standard injection molding process. During the fill cycle, polymer flows through the openings left by the inserts and surrounds the edges of the metal frame profile filling the gate, Figure 6. Solidification of the polymer creates a mechanical, interlocked connection between both materials producing a single unified component. Once cooled, the composite structure ejects from the tool as a hybrid product, Figure 7. Secondary operations include laser etching the over mold component, cleaning, inspection, sterilisation and packaging. Alternatively, the polymer can be molded separately and can then be pressed with the metal core in a secondary operation. Different additives can be added to the polymer to provide benefits such as conductivity, radiopacity, therapeutic effects, toughness, crystallinity, etc.

[0020] A 3D model acquired from the non-tapered core and molding tool, which also requires minimal inserts to seal off the design features on the implant is given in Figure 8. A 3D model acquired form the final over-molded product is given in Figure 9.

[0021] Alternatively, the design features required on the core (e.g. screw holes, keyway, slot) can be introduced after the injection molding process eliminating the need for extensive metal inserts within the molding process, Figure 10.

[0022] During heating in the injection molding machine at temperatures above

150°C, the core is surrounded by nitrogen/argon or subjected to vacuum to minimize oxidation of the surface of the core to enhance adhesive bonding between the metal core and the polymer. At these temperatures, the malleability of a hybrid device is suitable for the thermoplastic component minimising the risk of delamination and flaking.

3D filling studies

[0023] 3D filling analyses can be used to determine the mold filling characteristics of the short (18 cm) and long (44 cm) straight and tapered core designs in terms of (a) optimum process conditions, (b) gate positions, (c) fill patterns, (d) weld lines, (e) clamp forces, (f) pressure and (g) temperature distribution across the mold, Figures 11- 14. These analyses assume that the over-mold component is non-reinforced PEEK, and the mold and melt temperature are 205 °C and 400°C respectively.

[0024] For the short cores (tapered and non-tapered), two gates were positioned adjacent to the proximal bend near to where the supporting side cores are located in the hope that the opposing gates and good mechanical support will cancel outside forces on the core, Figure 11 and Figure 13.

[0025] For the long cores (tapered and non-tapered), two gates were positioned adjacent to the proximal bend near to where the supporting side cores are located, and two additional gates in the distal section located mid- way along the flow path, Figure 12 and Figure 14.

[0026] In all four cases, the wall thickness profile indicates the thickness range over the simulation model, Figure 11(a), 12(a), 13(a) and 14(a).

[0027] The fill time result shows the position of the flow front at regular intervals as the cavity fills. Figure 11(b), 12(b), 13(b) and 14(b). The shading represents the parts of the mold, which were being filled at the same time. At the start of injection, the result is full cross hatch, and the last places to fill are light hatching. If the part is a short shot, the section which did not fill has no shading. The temperature at flow front is a mid-stream nodal result generated from a flow analysis, and shows the temperature of the polymer when the flow front reached a specified node, Figure 11(c), 12(c), 13(c), 14(c). The pressure at V/P switchover is generated from a flow analysis, and shows the pressure distribution through the flow path inside the mold at the switchover point from velocity to pressure control, Figure 11(d), 12(d), 13(d) and 14(d).

[0028] For the straight core design (where the over-mold thickness varies between 0.5 mm distally and 1.5 mm proximally), the fill pattern results indicate a short shot condition due to extreme fill pressure and eventual freeze of the melt when the maximum pressure was reached at about 90% of fill, Figure 11(c), 12(c). Fill pressure is extremely high (-180 MPa), which is the default maximum pressure of most high capacity machines. The suggested maximum design pressure is 100 Mpa. Consequently, the tubular core may suffer structural damage under pressures of this magnitude.

[0029] For the tapered core variant (where the over-mold thickness varies between 1.2 mm distally and 1.5 mm proximally), the melt flows all the way to the opposite end with a reasonable fill pressure and minimal drop in flow front temperature, Figure 13(c), 14(c).

[0030] The most practical option is the tapered core given that it incorporates a reasonable wall section profile allowing the melt to flow the full length of the molding within a reasonable pressure of around 60 MPa. An area of concern is the ability of the tubular core to withstand injection molding pressures within the cavity in the region of 60 MPa (8700 Psi). Core shift is also a concern as well as potential for the tube to collapse under the extreme pressure applied during molding.

Tooling requirements

[0031] Shutoff design, gating, venting, and texture are key considerations when designing tooling for overmolded parts. The design of the shutoff between the substrate and overmold is critical to the success of the resulting adhesion between the two components.

[0032] Injection molding potential offers the following advantages:

• Increased device functionality and added value using multimaterial molding, which takes economical advantage of two materials with uniquely different properties by incorporating them into a single molded component. • Volume production process.

• A process route designed to exploit the beneficial properties of metal and composites.

• An end product which is more readily adapted into a conservative market. · A process route which is easier to produce a tightly toleranced device with an injection molded over layer compared to other technique such as tape winding.

• A process which is more suitable for high volume manufacture.

• A process which offers increased design freedom through availability of different process routes.

· An over-mold process which negates the need for expensive grinding process, and obviates prejudices associated with the surface finishes of tape wound nails, which can potentially increase the incidence of nail incarceration.

• A reduction in metal volume content by up to 50% (assuming an 18 cm long standard metal tibial nail conforms to a hollow tube with two straight sections :- Distal (8.8 cm long x 1 cm OD x 0.54 cm ID), Proximal (6.3 cm x 1.2 cm x 0.54 cm) and a tapered mid-section:- 3.1 cm long; V = 12.6 cm^A3. The volume of the tapered cannulated metal core (OD 8.9 mm / ID 4.8 mm - proximal section) and (OD 7.6 mm / ID 4.8 mm - distal section) is 6.2 cm^A3. The combination of an over-mold and a reduced metal content will reduce the risk of metal ion toxicology in a patient through reduced incidence of wear at the bone-implant interface.

• The over-mold component produces a radiolucent layer at the bone-implant interface making diagnosis of fracture healing easier for the surgeon.

[0033] However, injection molding potential offers the following disadvantages:

• Reduced fatigue resistance compared to a fully tape wound carbon fiber reinforced PEEK structure.

• Increased risk of de-bonding between the metal core and over-mold placing more emphasis on the surface finishes of the mating components.

• The hybrid device only offers partial radiolucency compared to fully tape wound composite structures.

· Higher tooling costs and more complex supply chain due to procurement requirements of metals and polymers.

(c) Shrink tubing

[0034] A still further method of securing the metal portion to the composite portion is by a shrink fit joint exemplified below with a spinal rod, Figure 15. The shrink fit joint takes advantage of the orientation of the polymer chains during the production process of the shrink fit tubing. By heating above the glass transition point in the tubing, the orientated chains relax allowing the polymer to shrink onto the core component. The components are assembled at room temperature, and then the assembly is heated above 343°C to allow shrinkage to occur for at least 5 minutes. There is clearance between the metal portion and the composite device used; the dimensional characteristics of the metal portion and the composite portion will change relative to one another, causing a dimensional interference to secure the portions together. Consistent shrink ratios of up to 1.2 to 1.0 and above, and 10-20% longitudinal shrinkage

( d) Plating

[0035] With this metallizing process, the core component would be made from a high strength polymer composite core, which would then be coated with a high strength nanocrystalline coating, e.g. cobalt chromium or nickel alloy using techniques such as electro-deposition. This coating technique would increase the mechanical properties of the polymer e.g. strength and stiffness. However, for an orthopaedic device subject to cyclical torsional and flexural strains, the coating could be expected to suffer fatigue and flake off over time.

(e) Resin transfer/ compression molding

[0036] This process involves heating the core and forcing molten polymer either within the cavity or a separate vessel into the tool cavity. In another embodiment, the core is pre-heated to help receive the over-mold component.

(f) Extrusion

[0037] In one particular process, the polymer is extruded from the profile die covering the metal core. Subsequently, the polymer melt flows through a channel of a die producing the final over-molded extruded part. The melt temperature for PEEK is in excess of 380°C. In a second process, the over-molding component is produced separately, and secured to the core using methods described above.

[0038] Osteosysnthesis devices - In most cases, the material of choice for the bone plate remains a metallic alloy, e.g. stainless steel. However, the use of use of stainless steel plates for internal fixation has disadvantages such as stress shielding wherein the stresses are exerted primarily on the plate rather than bone in the region of the fracture causing a weakening of the cortical bone (at about 15 GPa) under the plate. This stress shielding has been found to be the cause of significant bone resorption and consequent reduction of strength of the bone in the region of the healed fracture. The use of a bone plate has not satisfactorily solved the problem of stress shielding, because the initial strength and rigidity of steel plates is desirable is desirable for most fractures. Consequently, it is advantageous to have an internal metal core in the bone plate to provide optimal strength, load-bearing ability and the ability to be shaped by either bending or twisting operations conducted by the surgeon intra-operatively, Figure 16. The non-metallic over-mold component attached to the metal core allows the forming of complex shapes and thin sections to best adapt to and support the bone whilst minimizing damage to the periosteum and soft tissue. The over-mold component also may cover the screw holes in the metal core providing additional stability for self-tapping screws. The surface of metal endoskeleton can be roughened by either spraying of molten metal droplets, mechanical methods, chemical or electrochemical methods, or by direct casting, by direct forging.

[0039] The overmold component described in this IDR for trauma applications is non-resorbable to avoid direct coupling between the surface of the bone and the spacer unit via bone-ingrowth. When a resorbable spacer or overmold unit is absorbed as in prior art inventions, the gap between the bone and the plate can permit excessive motion of the plate relative to the screws, which can promote corrosion of the stainless steel plate. Given that the bone plate over-mold retains its structural integrity due to the non-absorbable nature of the material, this excessive motion does not occur.

[0040] Conventional metal implants - Effective stress transfer between an orthopaedic implant and the surrounding bone is impeded by the high stiffness of the metal. If the stress applied to the bone is not controlled, excessive stresses may fracture the remaining bone stock while on the other hand stress shielding may result in bone resorption. The advantage of the present invention is that the subject orthopaedic implants may possess a lower modulus outer layer that is more closely matched to the biomechanical properties of the neighboring bone offering improvements in stiffness and strength in bending, compression, axial and torsional loading. Therapeutic agents may also be incorporated into the microstructure of the over-mold material, which is not possible with "all metal" solutions. The processing costs associated with an injection molded hybrid orthopaedic device are also slightly lower than those associated with an "all metals" system.

[0041] Conventional PEEK composite implants - The hybrid design negates the need for internal land marking for screw holes, key-ways etc. given the opacity of the inner metal core, which is required for a fully compositized structure. The processing costs associated with an injection molded hybrid orthopaedic device are significantly lower than those associated with an "all polymer composite" system manufactured using a filament winding process.

[0042] The mass production aspect of the rapid prototyping technology:

• Rapid prototyping to make nails with variable cross sections, especially in ways that current metal cutting machines are not capable of - i.e a nail cannulation that tapers from large to small then back to large again

• Making a nail with different materials along its length to vary its stiffness

• Build your own polymeric nail in surgery by selecting a polymer from an offering to meet the fracture treatment need

• Over-molding locking screw features that would function like our current polyethylene bushing in the META nail.

• Over-molding stiffness reducing features such as a distal clothespin slot Customized patient-matched metal hybrid implants

[0043] Technical goal: The goal is to develop a nailing system with an optimum bending and torsional stiffness, which offers the potential for accelerated healing through improved stability of the bone fragments and the resultant quality of the regenerated bone compared to standard IM nails.

[0044] Background: There are a number of commercially available implants, which are used for simple and complex fractures. However, it is unknown what is the optimum stiffness (in terms of bending and torsional) opening up the possibility of developing customized implants matched to the patients anatomy and physiology. It is important that the biomechanical properties of the external Ilizarov fixator are taken into consideration, i.e. transversal and torsional rigidity of the individual structures during the design process.

[0045] During healing the nail behaves as a load sharing device. Initially, the majority of the load must be supported by the nail, but as the fracture heals and gains a greater mechanical stability, the load will be shared by the bone. It has been shown that on fracture healing, there is a reduction of up to 60% in loading of the interlocking nail. This shows that the nail still contributes significantly to the load-carrying ability of the construct when the fracture is fully healed. This residual load, transferred to the bone after nail removal, might lead to refracture unless patient activity was limited for a period.

[0046] Solution: This type of implant system is possible using a combination of computer modeling, additive manufacturing and over-molding techniques.

[0047] The process for creating such an implant optimized to the patient is likely to involve the following steps :-

[0048] Step 1: Patient images are captured from digital X-rays/CT imaging providing information relating to (but not limited to) the pattern of fracture, canal dimensions to ascertain the bend an bow in the nail and attachment points for the screws, bone density, cortical wall thickness, second moment of area, kinematics and anthropometric data. Other important variables which need considering include patient's age, ambulatory status, condition of the soft tissue envelope, and associated injuries.

[0049] Step 2: The patient images are converted into CAD files, e.g. stl files.

[0050] Step 3 : FEA simulation is carried out to verify the number and placement of screw holes, wall thickness, material properties, e.g. Ti-6A1-4V; elastic modulus 110 GPa, Ti-24Nb-4Zr-7.9Sn; elastic modulus 33 GPa, and cross-sectional geometry for the custom implant (closed vs. open sections). It is also used to determine the stiffness range and degree of micromotion required for optimum healing.

[0051] Step 4: A CAD library of patient-specific implants designs is built up, which can be uploaded on the SLS rapid prototyping machine "Creation of device custom record."

[0052] Step 5: The exported STL- file of the custom implant is sent to the AM- machine and prepared for manufacturing in a preparation software package and STL editor. The part is oriented for building and a support structure is made for the downfacing surfaces of the part. Nb Support structures are not required for an intramedullary nail. Afterwards, cross-sections of a given thickness, known as 'slices', are generated virtually from 3D CAD descriptions of the part and support structures.

[0053] Step 6: Additive fabrication is performed directly from a 3D CAD file in which a geometrical model of part is stored, Figure 2.

[0054] Step 7: Finally, the parts are post processed to meet the demands of the specific implant.

[0055] Advantages over prior art: This process for making "made to order" implants overcomes significant inventory issues associated with conventional off-the-shelf implants [0056] 1. Molding a metal insert titanium plate to the patient bone

[0057] 2. Stamping out ultra-thin bone plates with variable angle locking holes and coating them with an over-molded layer of polymer or composite material.

[0058] 3. Bioactive overmolded metal core for patient at risk of infection or with compromised bone healing. For example, antibiotic- impregnated over-molded biodegradable layer where the antibiotic coating would be released over a period of time to help in the prevention and treatment of any infection that might occur.

[0059] 4. Composite core with metallic overmold to reduce the cost of the tape winding process for composite-based materials.

[0060] 5. In situ curable polymers added into to the core just before surgery to allow the Surgeon to tailor the inner or outer stiffness according to a specification provided by the manufacturer.

[0061] 6. Over-molded locking screws to mimic the performances of standard bushings used in osteosynthesis plates.

[0062] 7. Injecting a bioresorbable cement down the intramedullary nail which allows temporary fixation. It removes the need for removal of interlocking screws to achieve dynamization, as the cement will gradually degrade.

[0063] 8. Variable stiffness across the diameter and length of the part to allow for higher stiffer sections for screw fixation and low stiffness sections intersecting the fracture site.

[0064] 9. Eccentrically cannulated nails produced by additive manufacturing, i.e. a nail cannulation that tapers from large to small at large (proximal to distal).

[0065] 10. Intramedullary nails with snap off sections

Assumptions:

· The ideal bending and torsional stiffness may be significantly less than that achieved with current products.

• Overly stiff constructs may lead to impaired fracture healing and stress concentration at the fractures site.

• Optimal micro-motion at the fracture site may results in an abundant amount of fracture callus distributed away from the neutral axis of the bone gives a large second and polar second moment of area and hence increased stiffness. This results in reduced stress shielding If the implant stiffness is closer to the bone.

• The implant stiffness needs to be at its maximum during the early phases of fracture healing and then taper off as the fracture heals and consolidates minimizes the risk of stress shielding issues after the bone has healed. The stiffness requirements between these two phases are unknown, however, the graph outlined in Figure 2 is representative of an idealized healing curve.* The low-rigidity nail needs to be strong enough to maintain alignment of the fracture without delayed union and neutralizing shear forces at the fracture.

[0066] As various modifications could be made to the exemplary embodiments, as described above with reference to the corresponding illustrations, without departing from the scope of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents.

[0067] In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained.

Claims

What is Chimed Is:

1. A method of manufacturing an intramedullary nail comprising:

a. providing a mold having a proximal section, a distal section, and a bend section in between the proximal section and the distal section; b. inserting at least one core into the mold;

c. heating melt between 205 degrees and 400 degrees Celsius; and

d. injecting the heated melt in at least two gate locations adjacent the bend section.

2. The method of claim 1, wherein the melt comprises PEEK.

3. The method of claim 1, wherein the at least one core has a taper.

4. The method of claim 1, wherein the at least one core is surrounded by nitrogen and argon.

5. The method of claim 1, wherein the at least one core is subjected to a vacuum.

6. The method of claim 1, further comprising the step of injecting the heated melt in at least two gate locations adjacent the distal section.

7. The method of claim 1, wherein a resulting wall thickness is between 0.5 mm and 1.5 mm.

8. The method of claim 1, wherein a resulting wall thickness is between 1.2 mm and 1.5 mm.