Patent Grant
9061090
The present invention relates to a stem structure for composite prosthetic hip and a method for manufacturing the same, more particularly, to a femur prosthesis formed by stacking FRP (fiberglass reinforced plastics) materials which are much more appropriate than metal materials for a stem to be custom-made, and a method for manufacturing the same.
A composite material formed by impregnating carbon fiber with resin as matrix is lighter and higher both in longitudinal elastic modulus and in fatigue strength than those of a metallic material, therefore, it is quite possible that the composite material is appropriate to a prosthesis for hip joint broken due to a traffic accident or the like. However, the composite material has unavoidable anisotropy caused by woven cloth used as reinforcement, thus making it difficult to have the same isotropy as a metallic stem has. Yet, there is a possibility of obtaining pseudo-isotropy by alternately changing the direction of fiber of woven cloth, so that the method has been very well researched of introducing the composite material into artificial hip prosthesis stem.
A hip joint consists of a caput as a spherical joint which enables a personal body to rotate and bend by engaging with an acetabulum of pelvis and a neck connecting the caput with a femur. If the spherical joint is damaged, the hip joint loses its proper function. In order to recover it, the acetabulum is in need of repair, or as shown in
The stem 8, when made of FRP, has a composite structure of, for example, woven cloth of carbon and a resin impregnated in the woven cloth, such as PEEK which is harmless to the human. The stem is formed by stacking the FRP materials in the cross section thereof with the materials concavely curved so as to fit the outer shape of the stem. More particularly, the stem is divided in the anterior and posterior direction of human body into two halves which mate together to form the stem. The divided surface 32 is selected so as to contain a longitudinal reference line 8a linking a center 1a on the end of the neck and a center 2a in the diaphysis of the distal portion of the stem. Each of the two halves is placed in a flat state to have an upper half 8U and a lower half 8L respectively. The FRP sheets are stacked in each of the molds where these upper and lower halves are formed. The upper half corresponds to the anterior portion of the stem and the lower half corresponds to the posterior portion of the stem (in the figure the stem is for a left leg). The stem 8 in the center of the figure is illustrated by using a set of lines of the edge of the prepreg sheets on the widest cross section thereof.
The prepreg sheets, i.e., thin sheets formed by impregnating woven cloth such as carbon with a thermoplastic resin, are heated to be deformable, for being overlaid in contact with the contour 33 of the cavity of the mold D, as shown in
With the stem 8 formed by curvedly stacking sheets like growth rings of a tree, air voids 34 are frequently produced on the cross section of the stem 8, as shown in
In the field of optical molding, as disclosed in JP2001-347572A1, e.g., an desired shape of an object is obtained in the following method; contour lines of an object are calculated by level-cutting the three-dimensional data of the object, uncured photo-setting resin is sliced into some layers in uniform thickness on the basis of the contour lines, successively the layers are stacked to be integrated into one piece by curing.
When a three dimensional object is obtained by the optical molding, the primary purpose is to reproduce the outer shape of the object, not to obtain the structure so as to withstand complex loads, although it may be possible depending on the mechanical characteristics of cured resin. Even though a stem might be formed by using the method of optical molding, there is few possibility of obtaining a stem such that it can withstand tension, bending moment and shearing load caused by the influence of his weight or the like and has resistance to the load such as a hoop stress, what is more, these loads mentioned above will act irregularly. From this point of view, it is obvious that the FRP stacked product formed by applying the principle of optical molding thereto will not have the mechanical structure to be required as a stem.
[Patent Document 1] U.S. Pat. No. 3,901,717
[Patent Document 2] JP2001347572A1
As mentioned above, an artificial hip prosthesis composite material stem ought to be pseudo-isotropic in spite of the fact that originally anisotropic materials are applied thereto and be a molded product having few air voids by closely stacking FRP materials as well. In order to obtain such a stem, much more advanced technology has to be applied to the current technology, on structure of the FRP elements to be determined for the stem, on designing the structure to withstand such peculiar load acting on the stem, on devices for the molds to form the structural elements, and on careful handling required during the curing operation. Thus, it will be possible in the near future to provide a low cost stem which fits patients perfectly in shape, whereas as to casting metal stems, patients have to select an acceptable one among ready-made metal stems of some typical shapes.
The object of the present invention is to solve the problems mentioned above in custom-making a stem which varies in shape from patient to patient; the first is to propose a stem which enables to prevent air voids from occurring by applying the technology of evenly stacking prepreg sheets to the process for forming the stem. The second object is to propose a structure of an artificial hip prosthesis composite material stem and a method of manufacturing thereof by means of evenly stacking sheets, which enables the stem to have desired strength, proof stress, and rigidity and to obtain a shape with high accuracy through the solution of mechanical and structural defects of the stem caused by curvedly stacking sheets.
The present invention is applied to an artificial hip prosthesis stem formed by stacking FRP (fiberglass reinforced plastics). Referring to
With the prepreg sheets forming each of the halves 3U and 3L of the main structure, the sheets 7M having carbon fibers arranged at angle of 0/90 degrees and the prepreg sheets 7N having carbon fibers arranged at angle of ±45 degrees are alternately stacked as shown in
As shown in
Each of flat surfaces of the prepreg sheets evenly stacked in the main structure upper and lower halves 3U and 3L, as shown in
In producing any type of the artificial hip prosthesis stems described above, after stacking the FRP structural elements in the mold, the whole mold containing the FRP structural elements is entirely wrapped in a bag 12 made of heat resisting resin film (see
According to the present invention, the stem comprises the following structural elements made from FRP materials: an upper outer shell, a main structure upper half, a main structure lower half, and a lower outer shell, and by stacking these FRP structural elements and applying heat and pressure thereto to melt resins impregnated in each of the elements, the FRP structural elements are integrated into one piece as a stem. Each of the upper and outer shells is a thin prepreg sheet formed by impregnating carbon fiber with a thermoplastic resin, enabling the air voids occurring during the process of forming to be easily removed in spite of the fact that the sheet is curved in shape. Since the carbon fibers are arranged at angle of ±45 degrees, the shearing load, which acts larger on the stem as it is closer to the surface of the stem, may be received on the fibers arranged to be equivalent for the direction of the load and consequently the outer shells are very useful for reducing the shearing load reaching the resin sheets evenly stacked.
Furthermore, the shells, functioning as a hoop, have thick parts on the left and right sides of the main structure, which is described below, enabling the stem to maintain strength, proof stress and rigidity which an FRP-made stem formed merely by stacking FRP materials may not obtain, and accordingly the problems may be resolved which are caused when the internal structure covered with the shells is an evenly stacked part. The thin outer shells will fit the mold perfectly, as mentioned above, so that the stem will have accuracy in shape and size.
The main structure upper and lower halves are evenly stacked parts in which prepreg sheets formed by impregnating carbon fiber with thermoplastic resin are stacked, consequently as many air voids as possible left in the main structures may be removed. The overlapping sections of the upper and lower outer shells are to be formed on the left and right portions of the integrated main structure so as to reinforce the portion where the heaviest shearing load acts. The shearing load is transmitted at the thick overlapping sections and consequently a hoop effect will be increased in spite of the fact that the rest of the shells are thin. The inside structure of the stem is tightly held by the shells, enabling the property of keeping the shape of the stem close to the shape of a metallic stem.
The overlapping sections of the upper and the lower outer shells are formed so as to have no stepped outer surface of the stem, and consequently undesired unevenness will not be formed on the surface of the stem. With a cement stem, the advantage exists that an adhesive treatment for cement may be given uniformly on the surface of the stem. And with a cement-less stem, the advantage exists that a treatment may be effectively given of medicament for spongiosa bone growth, such as hydroxyapatite, on the surface of the stem. The stem may fit in the medullary cavity with little inclination, resulting in the improvement in fit and fill of the stem in the medullary cavity.
The structure of the main structure upper and lower halves in which the prepreg sheets having carbon fibers arranged at angle of 0/90 degrees and the prepreg sheets having carbon fibers arranged at angle of ±45-degrees are alternately stacked, promotes the pseudo-isotropy of each main structure upper and lower halves. The direction of the fibers at the cut ends meet to the tangential lines of the periphery of the layers at right angles, acute angles and obtuse angles, so that the layers may have neither cracks of cured resin nor the separation of resin from fibers. The fibers of the layers are engaged with the fibers of the outer shells to promote integrating the internal and external structures of the stem.
Covering the whole surface of the outer shell with a thin film made of melted PEEK resin compound may prevent the carbon fibers in the outer shell from appearing, so that the carbon fibers will not contact with the wall of the medullary cavity. Even if the carbon fibers are cut into small pieces in the shell, the thin compound film prevents the pieces from moving into and floating in the medullary cavity.
Each of the flat surfaces of the prepreg sheets evenly stacked in the main structure upper and lower halves is parallel to a plane which is on a line toward the direction giving the least loss of shearing load, i.e., a plane to resist the moment at the maximum, enabling the stem to have the maximum resistance against the moment caused by the load acting on stem. The adhesiveness among the sheets is improved owing to the flat surfaces of stacked sheets, which promotes the durability of the neck that receives heaviest load in the stem.
Each of the flat surface of the prepreg sheets evenly stacked in the main structure upper and halves is parallel to a plane which is on a line toward the direction minimizing the number of sheets to be stacked, as a matter of course, the sheets to be used can be decreased in number, resulting in reducing the labor required for forming.
In producing any type of the artificial hip prosthesis stems mentioned above, after stacking the FRP structural elements in a mold, the mold is entirely wrapped in a bag made of heat resisting resin film. Thus, the FRP structural elements in the mold may adhere to each other by keeping a vacuum in the bag made of heat resisting resin film while heat and pressure are applied to the mold. Even if the elements do not contact the contour of the cavity well or slightly extrude from the cavity before closing the mold, the elements are contained in the mold spontaneously during the curing operation, thereby the stem can be formed which is identical in outer shape to the cavity of the mold.
a)-1(g) are exploded views of a structure of composite prosthetic hip according to the present invention.
a)-3(c) are diagrammatic illustrations of the prepreg sheets evenly stacked in the stem.
a)-4(b) are diagrammatic illustrations of the prepreg cutting sheets evenly stacked in the mold.
a)-5(c) are views of the surface of the prepreg cutting sheets evenly stacked for forming the main structure, which shows the modification of arrangement of the carbon fiber.
a)-6(d) are views for explaining the direction to extend the stacked prepreg sheets for the main structure upper and lower halves.
a)-7(f) show a flow diagram illustrating the first half of the process for forming the main structure lower half.
a) and 8(b) are views for explaining a way to illustrate the cross sectional structure of the stem.
a)-10(f) show a flow diagram illustrating the latter half of the process for forming the main structure lower half.
a)-11(f) show a flow diagram of forming the upper outer shell.
a)-12(g) show a flow diagram of forming the main structure upper half.
a)-13(d) show a flow diagram of the first half of the process for the final assembling of the stem.
a)-14(d) show a flow diagram of the latter half of the process for the final assembling of the stem.
a) and 15(b) show a schematic view of the upper and lower blocks as a set of mold with their cavities being opened.
a) and 16(b) show a schematic view of the upper left, upper right, lower left and lower right blocks as a set of mold with their cavities being opened.
a)-19(d) are views for explaining another direction to extend the stacked prepreg sheets for the main structure upper and lower halves.
a)-20(d) are views for explaining furthermore another direction to extend the stacked prepreg sheets for the main structure upper and lower halves.
a)-21(d) are schematic views of each of the FRP structural elements for forming a stem provided with a fin sustaining itself in the hollow.
a)-22(d) are schematic views of a stem provided with a fin sustaining itself in the hollow and of the prepreg cutting sheets applicable thereto.
a)-24(c) are views of the surface of the prepreg cutting sheets evenly stacked for forming the main structure, showing other modifications of arrangement of the carbon fiber.
a)-26(c) are views for illustrating a cross section of a stem in which sheets are stacked by applying a conventional technology thereto and for explaining the occurrence of air voids in the stem.
Referring to the drawings showing some embodiments, a structure of an artificial hip prosthesis stem and a method for producing the stem according to the present invention are disclosed as follows: The present invention is applied to a stem used as a prosthesis so as to restore the function of femur whose caput is broken due to a traffic accident or the like and the function of walking by reinforcing the femur which declines in rigidity due to osteoporosis. An artificial hip prosthesis stem is installed in the femur by inserting the stem into a hollow previously made from the epiphysis to the diaphysis of the femur. Two major methods are currently known to secure the stem in the femur; a method of using cement milk previously filled in the hollow, and a biomaterial method of using bone growth that spongiosa gradually fills up the space between the hollow and the stem to unite the stem and the femur without using cement. The main object of the present invention is to provide FRP stem without air voids and is applicable to both the methods as well.
More particularly, the stem may be obtained by stacking the structural elements: the upper outer shell, the main structure upper half, the main structure lower half and the lower outer shell, and applying heat and pressure to the structural elements by means of autoclave to melt the resin impregnated in each structural elements, and cooling them to be integrated into one piece. As a fiber woven in the structures, the carbon fiber is used which is tough and harmless to the human, thermoplastic resin such as PEEK resin or poly ether imid, which is also harmless to the human, is used as the matrix in order to keep the preferable shape of the stem. In order to melt the resin, a circumference temperature of approximately four hundred degree Celsius is required, even to soften it, heating it up to a temperature of more than three hundred degree Celsius is indispensable.
Each of the outer shells 4 is a curved prepreg sheet 15 formed by impregnating carbon fibers arranged at angle of ±45 degrees with a thermoplastic resin, which is mentioned below. Each of the main structure upper and lower halves 3U and 3L is an evenly stacked prepreg cutting sheets 7 formed by impregnating carbon fiber with a thermoplastic resin, as mentioned below in
Each of the upper outer shell 4U and the lower outer shell 4L mentioned above is a curved thin layer to wrap up the main structure 3U and 3L, as the upper and lower halves of the main structure shown in
Due to the carbon fibers arranged at angle of ±45 degrees, the shearing load, the closer to the surface of the stem the larger the shearing load acts on the stem, may be received on the fibers arranged to be equivalent to the direction of the shearing load, and consequently the outer shells contributes to the reduction of the shearing load reaching the resin sheets evenly stacked. The upper outer shell 4U is secured to the lower outer shell 4L to form the overlapping section 5 shown in
It is preferable for the outer shell to have a film to cover the surface thereof which is made from the resin compound 6, as shown in
Prepreg cutting sheets 7 are used for forming the main structure upper and lower halves 3U and 3L. Not only sheets fully impregnated with resin, but semi-prepreg sheets (or semi-preg sheets) insufficiently impregnated with resin may be usable, for curing enables any sheet to have the same quality of products. It is sufficient to select the sheet in consideration of either the relative difficulty in cutting the sheets into the prepreg cutting sheets 7 according to the contour lines or the financial costs for producing materials including the sheets, which vary with a degree of the curing.
As shown in
In the left and right portions of the main structure, which is the vicinity of the upper end of the lower outer shell 4L overlapping the lower end of the upper outer shell 4U, i.e., the margin for overlapping sections of the upper and lower outer shells, as clear in
An artificial hip prosthesis stem made of composite material is composed as described above and formed in a method mentioned hereinafter, so that the stem can be obtained as a molded product whose main structure upper and lower halves 3U and 3L are evenly stacked parts in which prepreg cutting sheets 7 formed by impregnating carbon fiber with a thermoplastic resin are stacked. Air voids remaining in the main structure may be eliminated as many as possible. The overlapping sections 5 of the upper and lower outer shells 4U, 4L are formed on the left and right portions of the integrated main structure 3, meaning that the section on which the heaviest shearing load acts is effectively reinforced by the two-ply shell.
The process for forming the stem which is shown in
The method for outlining the prepreg cutting sheets 7 stacked to form the main structure is mentioned hereinafter. From the shape of the stem to be applied to the patient, the outline is obtained of the main structure placed in a flat state every contour line by calculating. The prepreg sheets are cut in accordance with the contour lines by means of NC cutting machine. At the same time, a hole 10 for inserting a guide bar (see
As shown in
While the compound sheet 6 and the initial prepreg sheet 15 are kept to be soft, a guide bar 11 made of PEEK resin is disposed in a hole 11a of the bottom of the cavity of the lower mold A, as shown in
The guide bar disposed at the point so as to pass through the first stacked sheet 7a, inevitably passes through every hole 10 of all the prepreg cutting sheets 7 stacked in the mold. If the prepreg cutting sheets should be preferably in position, the periphery thereof ought to be held by the cavity of the mold. Although thin plates, the prepreg cutting sheets are stiff at room temperature and often have flash on the periphery thereof when being cut out, therefore the sheets inevitably slip when they are stacked. The guide bar is usable to prevent the sheets from slipping mentioned above.
As shown in
As shown in
What should be mentioned previously, is that a middle mold C is used in
The process of forming the upper outer shell 4U and the main structure upper half 3U will be disclosed as follows:
As shown in
Each of the FRP structural elements: the upper outer shell 4U, the main structure upper half 3U and the main structure lower half 3La having the lower outer shell mentioned above, is formed by the method explained above. After the process as shown from (a) to (d) of
In the case that the stem comprises the four FRP structural elements: the upper outer shell 4U, the main structure upper half 3U, the main structure lower half 3L and the lower outer shell 4L, the lower mold A is usable for forming the lower outer shell as mentioned above, but in order to form the main structure lower half, the mold A′ (not shown) is necessary whose cavity is lessened by a thickness of the shell, which is the same as forming the main structure upper half. In this case, not only the lower mold A′ but the middle mold C′ (not shown) will be indispensable, this is clear from
In the case that the stem comprises the four FRP structural elements: the upper outer shell 4U, the main structure upper half 3U, the main structure lower half 3L and the lower outer shell 4L, in addition to the four molds, the lower mold A′ and the middle mold C′ described above, six molds in total, have to be prepared, which is not shown in the figures. As far as the number of the molds concerned, in custom made stem, of course, decreasing the total number of the mold means that the stems can be manufactured at a lower cost than usual.
A and B with their cavities 9 being opened.
As it is clear from the above-mentioned, according to the structure of the hip-prosthesis stem, since the overlapping sections of the upper and lower outer shells are formed on the right and left portions of the integrated main structure, the shearing load is transmitted at these thick overlapping sections, meaning that the shearing load acting on the evenly stacked sheets can be reduced owing to the overlapping sections. These overlapping sections reinforce the evenly stacked FRP materials to give the stem much strength, durability and rigidity, and help the rest sections to promote the hoop effect in spite of the fact that they are thin films without overlapping sections. The inner structure having evenly stacked sheets is tightly hold by the overlapping sections, enabling the property of keeping the original shape of the stem to be close to that of a metallic stem.
Since the overlapping sections of the upper and lower outer shells are formed such that the left and right portions of the main structure formed by integrating the both halves have no stepped outer surface, undesirable unevenness is not formed on the surface of the stem. With a cement stem, it is advantageous to give an adhesive treatment uniformly on the surface of the stem. With a cement-less stem, it is advantageous to give a treatment of medicament for spongiosa bone growth, such as hydroxyapatite, effectively on the surface of the stem. Furthermore, it allows the stem to fit into the medullary cavity without inclination, which contributes to the improvement in Fit and Fill of the stem in the medullary cavity.
Since each of the main structure upper and lower halves has the structure such that the prepreg sheet having carbon fibers arranged at angle of 0/90 degrees and the prepreg sheet having carbon fibers arranged at angle of ±45 degrees are laminated alternately, semi-isotropy is promoted of each of the main structure upper and lower halves. The direction of the fibers at the cut end makes an angle of right, acute and obtuse relative to the tangential lines of the periphery of the sheet, which prevents the main structure from occurring cracks and separating resin from fibers. The fibers around the cut ends are closely intertwined with the fibers in the shells so as to promote the integration with the internal and external structures.
The outer shell is concavely curved to tightly cover the cavity of the mold, however the air void occurrence can be eliminated, because the outer shell is a thin prepreg sheet, as mentioned above. And the sheet is flexible to be made to fit the surface of the mold, the outer shell is precise in shape and size, thus the outer shell may be applicable to the above-mentioned cement-less stem which is required to be accurate both in shape and size. With the main structure, evenly stacking the sheets facilitates pressing out air bubbles therefrom, meaning that few air voids occur to raise the adhesiveness between layers, resulting in the improvement of durability of neck which receives the maximum load in the stem.
Whichever process mentioned above is applied to manufacturing the artificial hip prosthesis stem, it is necessary to wrap the mold containing the FRP structural elements in a bag made of heat resisting resin film. While heat and pressure are applied to the mold, the FRP structural elements stacked in the mold may tightly contact with each other by keeping a vacuum in the bag made of heat resisting resin film. Even if there is a slight slip caused by partial contact or extrusion of the elements before autoclaving, the mold is spontaneously closed during the curing operation.
Before evenly stacking the sheets for forming the main structure, a guide bar made of thermoplastic resin is disposed at the point where it certainly passes through the sheet to be initially stacked in each of the main structures, enabling the prepreg sheets not to slip while the sheets are stacked. The guide bar will melt during the curing operation to soak into the evenly stacked sheets, promoting the integration of the stacked sheets. Incidentally, as shown in
The main structure lower half and the lower outer shell are previously integrated into one piece by applying heat before integrating the main structure upper half and the main structure lower half, enabling the process to be omitted of individually forming the main structure lower half, that is, it will not necessary to prepare the mold for forming only the main structure lower half whose cavity has to be lessened by a thickness of the outer shell. Decreasing the number of the mold required for forming the stem contributes to lowering of the manufacturing costs as a whole.
It has been explained so far that each of the flat surfaces of the prepreg sheets stacked for forming each of the main structure upper and lower halves is parallel to the plane containing the line 7S toward the direction giving the least loss of shearing load acting on the stem, alternately may be parallel to the plane being on a line toward the direction minimizing the number of the sheets to be stacked, as shown in
In selecting the directional line 7T, the line is best on the cross sectional plane which contains the highest points 18 and 19 of the largest part of the stem in width, shown in
The following is a list of symbols: 1: a neck, 1a: a center on the surface of upper end of the neck, 2: a body, 2a: a center in the end of diaphysis of the stem, 3: main structure, 3U: a main structure upper half, 3L: a main structure lower half, 3La: a main structure lower half integrated with a lower outer shell, 4: an outer shell, 4U: an upper outer shell, 4L: a lower outer shell, 5: overlapping sections, 6: a resin compound, 7: a prepreg cutting sheet, 7M: a sheet having carbon fibers arranged at angle of 0/90 degrees, 7N: a sheet having carbon fibers arranged at angle of ±45 degrees, 7S: a line toward the direction giving the least loss of shearing load, 7T: a line toward the direction minimizing the number of sheets, 8: a stem, 8a: a longitudinal reference line, 12: a bag made from heat resisting resin film, Ws: a component in the direction of shear, Ms: a moment caused by a load, A: a lower mold, B: an upper mold, C: a middle mold, B′: an upper mold, A′: a lower mold, C′.
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| 2010-061067 | Mar 2010 | JP | national |
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| PCT/JP2011/056476 | 3/17/2011 | WO | 00 | 10/15/2012 |
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| WO2011/115229 | 9/22/2011 | WO | A |
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