Prosthetic devices having sintered thermoplastic coatings with a porosity gradient

The invention relates to a method for producing an implant, consisting of a load-transferring member and a porous coating of thermoplastic material from the group of polysulfone, polyphenylene sulfide, polyacetals, thermoplastic polyester, polycarbonate, aromatic polyamide, aromatic polyamideimide, thermoplastic polyamide, polyaryletherketone, polyarylether nitrile and aromatic polyhydroxyether, in which a core is placed in a mold and in the space between the core and the walls of the mold for the preparation of the coating sintered powder having an average particle diameter between about 50 μηι and about 600 μίτι filled and sintered and the coating thus obtained at the load-transmitting part is attached.

Such a process is described in DE-OS 28 16 072. There, a sintered powder consisting of a mixture of different particle sizes is used, in which the ratio of the particle diameters is approximately between 7: 1 and 5: 1. Preferably, particle sizes between about 300 μίτι to 50 μΐη are used. The particle size distribution ranges from about 40% to about 60% by weight. The sintered powder is then fed into a mold which is then heated to a prescribed sintering temperature which is higher than the glass transition temperature and lower than the melting temperature of the sintered powder. Only the pressure caused by thermal expansion during sintering is used.

With this method, a substantially homogeneous coating is formed, but does not meet all the requirements placed on such an implant. It has been found to be favorable if, on the outside of the coating, this is as porous as possible in order to promote the ingrowth of bone material into the cavities, while on the inside of the coating a low porosity leads to an improved adhesion.

In the case of a homogeneous coating, it is therefore impossible to combine these two contradictory requirements.

DE-OS 26 14 170 describes the preparation of an implant, wherein one applies to a carrier a coating of a porous, polymeric material of high density, which has a total volume enforcing network and an average pore diameter of about 50 μπι to about 300 jxm The coating has a pore volume of at least 30% and a melt index between 0.005 and 5. This document also teaches that the physical properties of porous, polymar material vary greatly depending on the pore size and internal pore geometry.

DE-OS 26 58 716 describes the production of implants of porous plastic, in each case the optimal choice of the right pore size for the desired type of tissue attachment is out. For this purpose, a mixture of acrylic polymer and monomer is prepared with water-soluble crystals having a particle size corresponding to a desired pore size in a certain surface area of ​​the implant, in proportions corresponding to the degree of desired porosity. Subsequently, the mixture is introduced into the mold and then the material is cured. After removing the hardened material from the mold, the water-soluble crystals are leached out and then the finished implant results

DE-OS 25 46 824 finally describes the idea of ​​varying the penetration depth of bioactive substances in a carrier layer in order to create a layer with a porosity gradient for the growing tissue. This is achieved there by changing the viscosity of an enamel-like carrier layer by changing the pressure and / or temperature and then causing the carrier layer to solidify. On the other hand, if the coating is produced by sintering sintered powder, this process can not be used.

The invention is therefore based on the object to perform the method of the type mentioned so that a defined porosity gradient is formed in the sintered porous coating with porosity higher on the outside than on the inside.

This object is achieved in that the sintering powder is heated more strongly during sintering on the inner surface in the region of the core than on the outer surface and that the sintering powder is heated in the region of the inner surface to a temperature up to 40 ° C above the sintering temperature of the sintered powder and in the area of ​​the outer surface to a temperature which is up to 40 ° C below the sintering temperature.

Experiments have shown that this process gives a coating which has a lower porosity in the region of its inner surface, good adhesion of the coating to the load-transferring

Achieved part of the implant. The porosity of the coating in the area of ​​the outside, however, is higher and so

high, that there the bone material can easily grow into the coating.

A coating made of a biotechnical thermoplastic produced by the process according to the invention is illustrated by the attached drawing. Fig. 1 is the microphotograph of the cross section

a porous polysulfone coating, wherein the inner surface of the top coat is shown including a part of the attached to the top of the iastübertragenden part of an implant

F i g. Fig. 2 is a microphotograph of the cross section of a porous coating showing its outer surface

F i g. Figure 1 shows at 75X the inner surface of a porous, thermoplastic coating. On an inner surface 10 of a porous upper layer 12, which at the same time the intermediate surface to a Lastüber_tprojecting part i4 of an endoprosthesis, the density of the sintered part is greatest. These sintered surface particles in effect form a skin or continuous coating which provides a maximum surface area in contact with the part 14. Thus, improved adhesion to the part 14 is achieved with the requisite stiffness and strength where needed

Particles 16 which are more distant from the inner surface 10 have a sintered configuration characterized by larger pores and interconnections, and thus the greatest porosity

Also, Fig. 2 is a microphotograph of the cross-section of a porous polysulfone coating at 75X magnification, showing that the porosity of the coating 12 is greatest at an outer surface 18, providing good access for bone ingrowth

An implant is made by a sintering process in which particles of biotechnological thermoplastic material are heated in particular order for a sufficient time to a sufficient temperature to effect sintering, ie, around the particles at one or more points of contact to form a porous, continuous composite material melt the thermoplastic together with a desired porosity gradient and mechanical properties.

With respect to the desired mechanical properties, it has been found that the modulus of elasticity of a porous material can be predicted by known relationships. To obtain a material with a porosity of z. B. 55% and a modulus of elasticity over 2 800 kg / cm² the modulus of elasticity of the starting polymer must exceed 14,000 kg / cm² lie. Thus, most polypropylenes and all high density polyethylenes can not form a material of 55% porosity with a modulus of 2,800 kg / cm² are processed. On the other hand, since the modulus of elasticity of solid polysulfone exceeds 23,800 kg / cm² is a material of 55% porosity and a modulus of elasticity over 4 900 kg / cm² receive.

The coatings have a porosity of over 20%, especially from about 30% to about 70%.

One can achieve the desired level of porosity without compromising mechanical properties by appropriate choice of particle size, molecular weight distribution and sintering conditions. All three factors are related and required to provide a coating with the necessary properties. So z. B. the sintering time and sintering temperature resulting in a desired pore size distribution do not give the desired modulus of elasticity and / or tensile strength. Based on the particle size distribution sintering time and temperature must be adjusted so that one reaches the desired balance between pore size, porosity and mechanical properties

In terms of particle size distribution, it has been found that sintered materials having the porosity and mechanical properties requirements for coating an implant can be made by single or multiple particle size sintering powder. In practice, for porous coatings of thickness greater than 2 mm, a mixture of particle sizes in which the particle gauge ratio is between about 1: 7 to about 5: 1 has been found to be useful. Particle sizes of from about 300 to about 50 μm are particularly preferred. A mixture of particles which are retained on a test sieve of the clear mesh width of 0.29 mm according to DIN I 171 and pass through it of 0.054 mm yields coatings with the desired particle size Porosity and biomechanical features. It has further been found that optimum results are achieved when the ratio of fine to coarse particles is between about 40 to about 60% by weight. Also ratios of fine to coarse particles of 40: 60 wt .-% to 60: 40 wt .-% can be used.

For porous, sintered coatings of a thickness of 0.5 to about 2.0 mm, particles pass through a test mesh of 0.42 mm in accordance with DIN 1171 and are retained on a sieve of 0.21 mm (average particle size). βε = 320μΐτι), the most advantageous mechanical and biotechnical properties.

As already mentioned, the sintering conditions are particularly important not only for the desired overall porosity but also for achieving the desired porosity gradient in the same operation. As described in DE-OS 28 16 072 sintering is carried out by charging a metal mold with powder and heating the mold to a prescribed sintering temperature Ti, which is above the glass transition temperature Tg, and below the melting temperature T._n, lies (ie T_q<T_s<Tl_n). The sintering temperature is kept constant for the given time. In practice, no pressure other than the pressure induced by the differential thermal expansion is applied. Applying a pressure at T₅ leads to melting of the material. This shows that lower pressures and shorter time cycles must be applied when pressure is applied to maintain porosity in the sintered parts.

In contrast to this teaching, high heating values ​​with controlled heating direction flow are now used during sintering to create a well-defined porosity gradient. By applying high calorific values, a temperature gradient exists inside the sintered powder, which leads to a lower porosity in the region of the higher temperature. Thus, an ordered porous structure is achieved by directional heating, ie by driving heat from inside to outside during sintering.

Depending on the particular material used for the load-transferring member, one or more methods may be used for differential heating. So you can z. B. produce the coating of the sintered porous material separated from the load-bearing member and attach by means of adhesive and / or shrinkage on the load-bearing part. As shown in example 2, the coating of sintered material can be made in a mold in which

a core is first heated to form a skin on its inner surface which corresponds exactly to the shape of the core and which has a low porosity at the interface. The heating can be carried out by various methods, for. By circulating a heated silicone oil through the interior of the core.

After completion of internal heating, the mold is heated either internally and externally simultaneously or only externally. This is continued until sintering is complete and the coating has the desired pore structure and mechanical properties. After sufficient cooling, the coating is then removed from the core. Then, the free-standing sintered coating can be applied to a metal load-transferring member by dipping it with an adhesive film which is then baked at 270 ° for 10 to 15 minutes⁰C is networked. Then the pre-made coating also receives by dipping a film of the adhesive, which is then dried. After drying the adhesive film, the load-transferring member is again immersed in an adhesive and the porous coating is placed while the adhesive is still tacky. Then the finished implant is dried, cleaned and sterilized.

Other methods may be used if the implant is not uniform or too small for internal heating. Such as As indicated in Example 3, it has been found that certain hip prostheses have dimensional deviations in humans. Instead of producing a new mold for each implant, induction heating can be used.

The main difference between this method and the previous one is that the implant itself is used as a core and the powder is heated first from the outside and then from the inside. Internal sintering is achieved by induction heating the implant.

The load-transferring portion of the implant is prepared for attachment by applying and curing an adhesive film thereon and then applying and drying a second adhesive film. Then, the implant is inserted into the mold and fixed, whereupon the gap (of 1 mm or more width) between the implant and the walls of the mold is filled with adhesive powder of the desired particle size distribution. Subsequently, the outside of the mold by heating, for. B. by band heater, heating tape by means of electrical resistance or induction coil, if the mold is made of the appropriate material, brought to the sintering temperature The external F.rhitzen is maintained only a short time. An incompletely sintered preform of the oversized coating is obtained because the polymer powder sinters most at the surface of the cavity and shrinks away from the load transferring portion. If the preform has sufficient integrity to retain its shape, the mold cavity is slashed and removed, leaving an oversized coating on the load transfer member. Then load transfer member and coating are placed in an induction coil, The part is then inductively heated to the appropriate sintering temperature and held at this temperature until the sintering and shrinkage of the coating is complete, so that the coating conforms to and joins the surface of the iast-transferring member the implant is cooled cooled and sterilized.

The sintered porosity gradient coatings are also suitable as root cores of dental implants. Due to the small size of these implants, however, it is not appropriate to heat only from the inside. Accordingly, a mold was designed to achieve a temperature gradient of sufficient magnitude. The mold consisted of an upper plate, a molded article and a bottom plate. The bottom plate had one or more cavities for receiving the part to be coated in the reverse position. That is, the part has been set up so that the pillar part to be covered faces upward. Then, the molded article is placed on the bottom plate whose openings are aligned with the upstanding pillars, whereupon the space between the core or pillar parts and the cavity walls of the molded article is filled with a polymer powder of the desired particle size distribution. The powder is introduced through holes in the top of the molding and pulped so that the entire gap is thoroughly filled. Then the top plate is placed on top and fastened with cover screws. The mold is placed in a press whose plates have been previously heated. The upper heating plate is up to 40⁰C below the desired sintering temperature and the lower heating plate heated to 40 ° C above the desired sintering temperature. This temperature adjustment causes the titanium seed cores to first heat up and then maintain a higher temperature than the cavity walls of the shaped body. This results in a very good conformity of the inner surface of the porous coating with the outer surface of the part to which it must be bonded. After the mold has reached the desired temperature and held it for the necessary duration, the mold is removed from the press and cooled. The finished implants are removed from the mold, cleaned and sterilized.

The load bearing part of the implant may be made of different metals or alloys. As examples may be mentioned titanium and tantalum, as pure metals and stainless steels and alloys based on cobalt and titanium as alloys. They are corrosion resistant and can be shaped to the desired shape.

It can be good for some purposes. Incorporate additives to the material of the coating, which increase the wear and tear resistance of the implant. Carbon fibers, graphite fibers, teflon, molybdenum disulphide are suitable additives that give tough coatings. Carbon fiber preparations are preferred for injection molding or machine fabrication of joint prostheses, such as femoral cups, tibial and glenoid portions, for knee and shoulder replacement

The sintered coating can be connected to the load bearing member by various methods. So z. For example, reactive silyl polymers, such as reactive silyl polysulfone, can be used to attach the porous polymeric coatings to metal load-transferring members. Reactive silyl polysulfone resins have three important properties: first, the presence of hydrolyzable silane end groups provides inherent coupling ability to metallic surfaces. Second, the PSF-SR resins have a low melt (or solution) viscosity, which significantly increases wetting during the formation of the adhesive bonds.

lightened. And third, they are polymeric adhesives without solubility in physiological fluids and thus without biological / toxilogical effects at implantation. Other methods, such as heat shrinking a partially sintered sleeve, may also be used.

Example 1 Effect of sintering conditions on pore size

This example demonstrates the effect of sintering conditions (ie, particle size, time, and bath temperature) without attempting to achieve a porosity gradient. For this simple shapes were made of steel tubes with 9.5 mm outer diameter. The tubes were cut to 15 cm in length and provided with screw plugs. The wall thickness of the tube was about 0.97 mm. A sintered plastic part obtained after sintering had a diameter of 7.6 mm and a length of 15 cm. This proved to be a convenient sample size for the presentation of tensile properties.

A powder with the particle size distribution listed in Table 1 was used. This material was sintered according to the following scheme: filling the powder into the mold; Immerse the mold in an oil bath of 220 ° C for different times from 10 to 30 minutes. Then, the obtained rod of 7.6 mm in diameter was cut to sample lengths of 6.4 cm.

The size distribution of the connecting pores was then determined by mercury intrusion porosimetry, the data being listed in Table 1. The characteristic pore size is given as a percentage of pores equal to or greater than 132 μιτι. If the time is increased from 10 to 30 minutes at the respective temperature, then the number of pores of> 132μΐη diameter increases. However, if the material reaches 220 for more than 30 minutes⁰C, then the obtained sample is no longer porous. On the other hand, if the material is held under temperature for less than 10 minutes, little or no sintering occurs. Thus, there is an optimum temperature and time for a given particle size and molecular weight distribution to achieve a desired pore size.

Table 1

Percentage distribution of particle sizes

<p><tgroup cols="2"><tbody><row><entry>Test sieve in clear mesh size</entry><entry>distribution</entry></row><row><entry>in mm (DIN 1171)</entry><entry></entry></row><row><entry>to 0.50</entry><entry></entry></row><row><entry>to 0.42</entry><entry>track</entry></row><row><entry>on 03</entry><entry>14,0</entry></row><row><entry>to 0.25</entry><entry>50,0</entry></row><row><entry>to 0.15</entry><entry>18,0</entry></row><row><entry>by 0.15</entry><entry>—</entry></row><row><entry>to 0.10</entry><entry>10,0</entry></row><row><entry>to 0.062</entry><entry>4,0</entry></row><row><entry>by 0.062</entry><entry>4,0</entry></row></tbody></tgroup></p>

<p><tgroup cols="3"><tbody><row><entry>Sintering time at 220 ° (min)</entry><entry>at</entry><entry>% Pore volume</entry></row><row><entry></entry><entry>(Greater than or equal to 132 μΐη)</entry></row><row><entry>10</entry><entry>49,4</entry></row><row><entry>12</entry><entry>52,6</entry></row><row><entry>14</entry><entry>56,5</entry></row><row><entry>16</entry><entry>58,1</entry></row><row><entry>18</entry><entry>61,8</entry></row><row><entry>20</entry><entry>69,5</entry></row><row><entry>30</entry><entry>75,4</entry></row><row><entry>game 2</entry></row></tbody></tgroup></p>

Effect of heating sequence on porosity

In this experiment, a male and female mold was made from steel tubes of different internal diameters. The core part had an outer diameter of 6.4 mm and fit into the larger shell part with an inner diameter of 12.7 mm. The mold was mounted so that heated oil could flow through the inner, hollow portion of the male, and the entire mold was externally heated by separate means.

The space between the core and cavity portions of the mold was filled with the polymer powder of the desired particle size distribution, and the powder was not compacted as in the conventional powder metallurgy sintering process. Silicone oil with a temperature of 250⁰C was passed through the core for 15 minutes. Thereafter, the outside was heated by means of electric heating tape until reaching an outside temperature of 240 ° C. This temperature was maintained for about 10 minutes. After cooling, a sintered sleeve was removed from the mold whose porosity gradient was similar to that in FIG. 1 and 2 was shown.

B ice ρ ie 1 3

Sintering a conformal, porous coating by heating an implant

Due to dimensional deviations (up to 1.25 and 1.5 mm) found in different human hip implants.1 it would be necessary to create a new shape for each hip. The present example demonstrates a method of induction heating the implant itself which results in simultaneous sintering and attachment of the coating. Thus, it was possible to coat overcasting parts whose geometry did not permit the application of a compliant, pre-sintered coating in one piece. The main difference between this method and the previous method is that the implant itself is used as the core part of the mold and the powder is heated first from the outside and then from the inside. Internal sintering is achieved by inductive heating of the implant.

The jaw-transferring member is prepared for fixing by applying and curing an adhesive film and then applying and drying another adhesive film. Then, the load-transferring member is inserted and fixed in the die.

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The space (of 1 mm or more width) between the load-transferring member and the walls of the mold cavity is filled with a plastic powder of the desired particle size distribution. Then the outside of the mold is brought to sintering temperature and held only a short time on this. A sub-sintered preform of the oversize coating is obtained because the polymer powder sinters most at the surface of the cavity and shrinks away from the load transferring portion. After the preform has sufficient strength to retain its shape, the cavity portion of the mold is cut open and removed, leaving an oversized coating on the load transfer member. Then load-transferring member and coating are placed in an induction coil designed to achieve uniform heating of the implant surface of tissue. The implant is inductively heated to and maintained at the appropriate sintering temperature until sintering is complete and the coating is shrunk to conform and bond with the surface of the load transferring member. Then the implant is cooled, cleaned and sterilized.

The sintering, shrinking and fastening was also carried out by placing a load transferring member in a hot air oven at 255⁰C was heated. Then, the porous coating preform was allowed to slide on the hot load-transferring member, completing its sintering and shrinking until it was properly fitted. Even without adhesive bonding, the porous coatings were difficult to remove.

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