Surgical prothesis

Abstract: A bone fixation device is disclosed. The device comprises an absorbable homopolymer of l-lactide or dl-lactide, or a copolymer of l-lactide, and a reinforcement material. The re­ inforcement material can be a particulate filler of hydro­ xyapatite or a plurality of alumina fibers.

Description: This invention relates to surgical structural elements consisting of bioabsorbable or semi-bioabsorbable composites, as well as to products and applications using these materials. Such surgical structural devices may include plates, screws, nails, pegs, rods, pins and any other device which would require a bioabsorbable material with relatively high rigidity and strength. The use of internal bone fixation is an established and clinically used technique. The two major types of internal fixation devices are bone plates and intramedullary rods. The particular form intended for this invention is as an internal bone fixation plate. Bone plates are applicable to a large variety of bones including load bearing as well as non-load bearing bones. Presently, more long bone fixations are done with intramedullary rods. Both of these devices are traditionally made of metal, e.g. stainless steel 316. There are two major disadvantages, however, associated with the presently used metal plates: 1) Metal plates have a modulus approximately one order of magnitude greater than cortical bone. This mismatch in stiffness is known to cause stress protection induced osteoporosis or osteopenia. The increase in porosity and decrease in cortical wall thickness results in a weakened bone which is prone to refracture once the bone plate is removed. 2) The metal plates must be removed due to their nonbiodegradable nature coupled with the eventual possibility of corrosion. A second surgical procedure is therefore required which may introduce further complications. The use of lower stiffness materials such as non-absorbable composites and tubular steel have been investigated. However, no human trials of these materials are known. Lower stiffness bone fixation devices have advantages over metal plates. For example the stiffness of a bone plate can be made essentially equal to the modulus of cortical bone. If the low stiffness bone plate is also bioabsorbable, it does not have to be surgically removed. Thus the need for a second surgical procedure is eliminated. Considerable research has therefore been devoted to the development of low stiffness bone plate materials. The properties which are most desirable in a bone plate are: 1) The bone plate should provide a firm fixation of the broken bone to promote union during the early stages of healing. 2) Once union has occurred, the load which was initially supported by the bone plate, would be gradually transferred back to the bone. This would induce the formation of stronger more dense bone at the fracture site thus accelerating the healing process. 3) After the bone heals (3 to 6 months after implantation) the bone plate would completely lose its ability to support a load. The bone would then be once again subjected to its normal stresses. Bioabsorbable materials with an initial modulus and strength at or near those of cortical bone are useful as internal bone fracture fixation devices for load bearing as well as non-load bearing bones. The completely bioabsorbable or semi-absorbable composites of this invention are superior in mechanical properties and in biological behavior to the stainless steel devices presently used. The mechanical properties of these composites can be tailored to the specific end-use application. The devices of this invention will gradually lose their mechanical properties and will ultimately fully or partially disappear. EP-A-0 011 528 discloses a bone fixation plate consisting of poly(l-lactide) reinforced with poly(glycolic-acid) fibers. FR-A-2 364 644 discloses a bone prosthetic material utilising an absorbable polymer having an adjuvent of tricalcium phosphat and/or a ceramic hydroxyapatite. EP-A-0 146 398 discloses a method of reducing ceramic fibers to small particles that can be sintered into reinforcing filaments. These filaments of fibers are then used with a bioabsorbable polymer matrix to form a bone plate or other orthopedic device. WO-A- 8600533 discloses an implant article comprising a composit of fibre material which may not be biodegradable e.g. carbon fibres and is incorporated in a porous matrix of a bio degradable organic polymer material e.g. polylactive. SUMMARY OF THE INVENTION A bioabsorbable device with an adjustable initial modulus which can be set at, above or below the modulus of bone, and which loses properties at a controllable, predictable rate after implantation has been invented. The device consist of a poly(l-lactide) matrix reinforced with α-alumina fibers (DuPont Fiber FP®). This invention uses a combination of materials which consist of a bioabsorbable polymer and a reinforcement fiber. The component materials are combined in such a way as to have bending, axial and torsional stiffness and strength suitable for the biomechanical demands placed upon it. The material will, subsequent to implantation, gradually lose both stiffness and strength according to the time frame for which useful properties are required. The material will ultimately be completely or partially absorbed by the body, any residue being both inert in the body and bereft of significant mechanical properties. No surgical procedure to remove the device would be required. The present inventions provides a semi-absorbable laminated bone fixation device comprising an impregnating agent consisting of an absorbable polymer matrix, said matrix obtained from the polymerization of l-lactide and having an inherent viscosity of 1.5 to 3.5 dl/g (o,5 g/dl in CHCl₃), and a nonabsorbable reinforcement material consisting of at least one alumina fiber said device having a flexural modulus of 6.895 x 10³ to 34.475 x 10³ MPa (1 x 10⁶ to 5 x 10⁶ psi) a loss of 25 % of its initial flexural modulus during 3 months in vivo and 45 % during 6 months in vivo. A laminated bone fixation device has been invented. The device comprises an impregnating agent consisting of an absorbable polymer matrix. The polymer is obtained from the polymerization of l-lactide. The matrix has an inherent viscosity of about 1.5 to 3.5 dl/g (0.5 g/dl in CHCl₃). The device also comprises a nonabsorbable reinforcement material. The reinforcement material consists essentially of at least one alumina fiber. The device has a flexural strength of about 68.95 to 172.38 MPa (10,000 to 25,000 psi); a flexural modulus of 6.895 x 10³ to 34.475 x 10³ (1x10⁶ to 5x10⁶ psi); a loss of 30% of initial flexural strength during 3 months in vivo and 60% during 6 months in vivo; and a loss of 25% of initial flexural modulus during 3 months in vivo and 45% during 6 months in vivo. In one embodiment, the reinforcement material is more than one alpha alumina fibers. In a specific embodiment, the reinforcement material comprises 10 to 60 volume percent of said fibers. In a more specific embodiment, the device comprises 15 to 40 percent of said fibers. In another embodiment, the device has a flexural strength of 103.4 to 172.38 MPa (15,000 to 25,000 psi). In a further embodiment, the device has a flexural modulus of up to 20.685 x 10³ MPa (3x10⁶ psi). BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the in vivo flexural modulus degradation of the device of this invention, as contrasted with the moduli of prior art bone fixation devices; Figures 2 and 3 are graphs, similar to that of Figure 1, showing in vivo flexural strength and molecular weight degradation, respectively; Figure 4 is a top view of a bone fixation device manufactured from the composite materials of this invention; Figure 5 is a partially broken side view of Figure 4; Figure 6 is a front view of Figure 5; and Figure 7 is a broken perspective view of Figures 4 to 6 showing the use of the device on a mammalian bone. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following is a description for making the composite materials used in the bone fixation device of this invention. Fiber Reinforced Systems A) Solution Impregnation and Laminations: The fiber or woven fabric is immersed in a solution of the biodegradable polymer in a low boiling point solvent (e.g. methylene chloride). The amount of polymer deposited on the fabric, chopped fiber or fiber yarn is dependent on the solution concentration, the polymer molecular weight (which effects solution viscosity), the length of immersion time and the number of immersions. The impregnated chopped fiber, yarn or fabric (prepreg) is then thoroughly dried. The prepreg is laid-up in a mold of a predetermined thickness. Vacuum is applied to the lay-up by use of a vacuum bag. Heat and compression are then applied to consolidate the laminate. B) Melt Impregnation and Lamination: Films of the biodegradable polymer are made by solvent casting or melt pressing. Alternatively, fibrous mats are made from polymer by running a solution of the polymer into a non-solvent in a thin stream to form a stringy precipitate, followed by pressing into a mat at room temperature. The films or mats are then laid between yarn or fabric layers in a mold of a predetermined thickness. Vacuum is applied to the lay-up, by vacuum-bagging the mold, and heat and compression are applied to consolidate the laminate. Figures 4 to 6 show the bone fixation device. The device can be manufactured without undue experimentation by methods known in the prior art, e.g. by compression molding. The device can be attached to a bone by any means presently known or obvious to a person having ordinary skill in the art, for example by fastening with screws, staples and the like, or by bonding, for example, by gluing the device to the bone. Figures 4 to 6 show holes 2 which are used to each accommodate a screw 5 (shown in Figure 7). To accommodate the screw head, a plurality of holes 2 are countersunk 3 in the device 1. Referring specifically to Figures 4 and 5, four holes 2 are shown. It is to be understood that any number of holes can be used, provided the device 1 is adequately attached to a bone 4 (shown in Figure 7). However, as a minimum, at least two holes 2 and screws 5 appear to be necessary. Referring to Figure 7, the preferred relationship of the device 1 to a mammalian bone 4 fracture is shown. Under many circumstances, the configuration shown in Figure 7 will allow the best possible chance for uniform healing of the fracture in the bone 4. The following examples more fully describe the above embodiments. Example 1 Poly(l-lactide)-Alumina Fiber Laminate: The laminate was formed from poly(l-lactide) of inherent viscosity 2.68 dl/g (0.5 g/dl in CHCl₃, after consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 10 cm by 10 cm (4" by 4") square films, two films of 0.127 mm (0.005") and two films of 0.584 mm (0.023") thickness. The laminate was formed by stacking the films and fabric in alternating layers. Three piles of fabric were used. The 0.0127 mm (0.005") thick films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 19% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 12.479 X 10³ MPa (1.81 X 10⁶ psi) FLEXURAL STRENGTH 1.144 X 10² MPa (16.6 X 10³ psi) This material is of particular interest due to it's superior in vivo performance in subcutaneous rabbit implant experiments. The degradation of this material in vivo is shown in Figures 1-3. Figures 1 and 2 show the physical property degradation profile and Figure 3 shows `she molecular weight degradation profile, as contrasted with materials used in prior art bone fixation devices. The initial properties of this composite system can be varied over a wide range by varying fiber loading. In addition, the degradation profile can be altered by varying the initial molecular weight of the matrix polymer. Mechanical properties declined in a roughly linear manner over a 6 month period in rabbits to a level at 39% of initial flexural strength and 53% of initial flexural modulus. Inherent viscosity data suggest that mass loss of the poly(l-lactide) matrix would begin after approximately 42 weeks. After mass loss onset, the rate of mechanical property degradation should increase and any remaining load bearing capability would quickly deteriorate. Example 2 Poly(l-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 1.64 dl/g (0.5 g/dl in CHCl₃, after consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was reprecipitated from a chloroform solution into methanol. The dried precipitate was pressed into 10 cm by 10 cm (4" by 4") square mats, two mats of 6.5 g and two mats of 1.2 g. The laminate was formed by stacking the mats and fabric in alternating layers. Three plies of fabric were used. The 1.2 g mats were used for the two outside layers. The laminate was consolidated by heating to 195°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 17% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 12.479 X 10³ MPa (1 81 X 10⁶ psi) FLEXURAL STRENGTH 1.372 X 10² MPa (19.9 X 10³ psi) Example 3 Poly(l-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.65 dl/g (0.5 g/dl in CHCl₃, after consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was reprecipitated from a chloroform solution into methanol. The dried precipitate was pressed into 10 cm by 10 cm (4" by 4") square mats, two mats of 6.5 g and two mats of 1.2 g. The laminate was formed by stacking the mats and fabric in alternating layers. Three plies of fabric were used. The 1.2 g mats were used for the two outside layers. The laminate was consolidated by heating to 195°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 17% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 10.480 X 10³ MPa (1.52 X 10⁶ psi) FLEXURAL STRENGTH 1.137 X 10² MPa (16.5 X 10³ psi) Example 4 Poly(l-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 4.14 dl/g (0.5 g/dl in CHCl₃, after consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was reprecipitated from a chloroform solution into methanol. The dried precipitate was pressed into 10 cm by 10 cm (4" by 4") square mats, two mats of 6.5 g and two mats of 1.2 g. The laminate was formed by stacking the mats and fabric in alternating layers Three plies of fabric were used. The 1,2 g mats were used for the two outside layers. The laminate was consolidated by heating to 195°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 17% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 10.066 X 10³ MPa (1.46 X 10⁶ psi) FLEXURAL STRENGTH 1.034 X 10² MPa (15.0 X 10³ psi) A summary of the flexural strength and flexural modulus data for Examples 2 to 4 is contained in the following Table. Example 5 Poly(l-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.64 dl/g (0-5 g/dl in CHCl₃, before consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 10 cm by 10 cm (4" by 4") square films, two films of 0.127 mm (0.005") and one film of 1.143 mm (0.045") thickness. The laminate was formed by stacking the films and fabric in alternating layers. Two plies of fabric were used. The 0.127 mm (0.005") films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 13% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 14.410 X 10³ MPa (2.09 X 10⁶ psi) FLEXURAL STRENGTH 1.220 X 10² MPa (17.7 X 10³ psi) Example 6 Poly(l-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.64 dl/g (0.5 g/dl in CHCl₃, before consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 10 cm by 10 cm (4" by 4") square films, two films of 0.127 mm (0.005") and two films of 0.584 mm (0.023") thickness. The laminate was formed by stacking the films and fabric alternating layers. Three plies of fabric were used. The 0.127 mm (0.005") films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 19% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 13.100 X 10³ MPa (1.90 X 10⁶ psi) FLEXURAL STRENGTH 1.220 X 10² MPa (17.7 X 10³ psi) Example 7 Poly(l-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) or inherent viscosity 2.64 dl/g (0.5 g/dl in CHCl₃, before consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 10 cm by 10 cm (4" by 4") square films, two films of 0.127 mm (0.005") and three films of 0.381 mm (0.015") thickness. The laminate was formed by stacking the films and fabric in alternating layers. Four plies of fabric were used. The 0.127 mm (0.005") films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 24% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 20.271 X 10³ MPa (2.94 X 10⁶ psi) FLEXURAL STRENGTH 1.489 X 10² MPa (21.6 X 10³ psi) Example 8 Poly(l-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.64 dl/g (0.5 g/dl in CHCl₃, before consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 10 cm by 10 cm (4" by 4") square films, two films of 0.127 mm (0.005") and four films of 0.203 mm (0.008") thickness. The laminate was formed by stacking the films and fabric in alternating layers. Five plies of fabric were used. The 0.127 mm (0.005") films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1.6 mm (1/16"). The laminate contained 30% alumina fabric by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 24.960 X 10³ MPa (3.62 X 10⁶ psi) FLEXURAL STRENGTH 1.661 X 10² MPa (24.1 X 10³ psi) A summary of the flexural strength and flexural modulus data for Examples 5 to 8 is contained in the following Table. Example 9 Poly(l-lactide)-Alumina Laminate: A laminate was formed by impregnating 12.7 mm (1/2") chopped alumina fiber with poly(l-lactide). The polymer bad an inherent viscosity of 2.64 dl/g (0-5 g/dl in CHCl₃, before consolidation). The impregnation was accomplished by dissolving the polymer in chloroform (10 g/dl) followed by stirring in the chopped fiber The mixture was then dried under vacuum to constant weight. The impregnated fiber was consolidated using vacuum and compression at 200°C, forming a laminate containing 30% alumina by volume. The laminate had the following mechanical properties: FLEXURAL MODULUS 12.204 X 10³ MPa (1.77 X 10⁶ psi) FLEXURAL STRENGTH 1.055 X 10² MPa (15.3 X 10³ psi) Example 10 In Vitro Degradation of Poly(l-lactide)-Alumina Laminates: An accelerated in vitro degradation test was used to assess the relative degradation rates of laminates made with poly(l-lactide)s of different molecular weights reinforced with alumina fabric. The in vitro procedure involved immersing the sample in a pH 6.09 phosphate buffered aqueous solution at 67°C. The samples were removed from the bath, dried and tested for mechanical properties using the ASTM D790 method. Samples from Examples 3, 4 and 5 were used in this study. The results are shown in Table I. These data indicate that the composite fabricated with the lower molecular weight polymer (Example 3) possessed higher initial mechanical properties than the composites made with higher molecular weight polymers. It also appeared to have less scatter in its degradation profile. Example 11 In Vitro Degradation of Poly(l-lactide)-Alumina Laminates: An accelerated in vitro degradation test was used to assess the relative degradation rates of laminates made with poly(l-lactide) reinforced with different loadings of alumina fabric. The in vitro procedure was identical to, that described in Example 10. Samples from Examples 5, 6, 7 and 8 were used in this study. The results are shown in Table II. These data indicate that the composites possessed higher initial mechanical properties as the fabric volume increased. This relationship allows the tailoring of a material to have the mechanical properties desirable for a specific application within a fairly broad range.

Claims: A semi-absorbable laminated bone fixation device comprising an impregnating agent consisting of an absorbable polymer matrix, said matrix obtained from the polymerization of l-lactide and having an inherent viscosity of 1.5 to 3.5 dl/g (o,5 g/dl in CHCl₃), and a nonabsorbable reinforcement material consisting of at least one alumina fiber said device having a flexural modulus of 6.895 x 10³ to 34.475 x 10³ MPa (1 x 10⁶ to 5 x 10⁶ psi) a loss of 25 % of its initial flexural modulus during 3 months in vivo and 45 % during 6 months in vivo. A device of Claim 1 wherein said reinforcement material is more than one alpha alumina fibers. A device of Claim 2 wherein the fibers comprise 10 to 60 percent of the volume of said device. A device of Claim 2 wherein the fibers comprise 15 to 40 percent of the volume of said device. A device of Claim 4 having a flexural strength of 1.034 x 10² to 1.724 x 10² MPa (15.000 to 25.000 psi). A device of Claim 5 having a flexural modulus of up to 20.685 x 10³ MPa (3 x 10⁶ psi).

Inventors: CASEY DONALD JAMES, JARRETT PETER KENDRICK, WANG DAVID WEI, CASEY, DONALD JAMES, JARRETT, PETER KENDRICK, WANG, DAVID WEI
Applicants: AMERICAN CYANAMID CO, AMERICAN CYANAMID COMPANY
Application year: 1987
Main classification: A61L31
IPC codes: A61L31