This invention relates to cement-less type artificial joints and in particular relates to artificial joint stems that comprise composite material.
It has long been known that an artificial joint made to imitate a joint is inserted when a damaged joint is removed due to a broken bone. As one example of this artificial joint,
As shown in the figure, the socket 102 and the head 104 make a pair and have a function of a spherical bearing. This socket 102 consists of synthetic resins such as high-density polyethylene, and the spherical head 104 comprises ceramics like zirconia or cobalt alloy. Such socket 102 and the head 104 have been improved in durability with many modifications in recent years so that they can maintain the functions longer than life expectancy of many patients who undergo total hip arthroplasty, and the focus has been shifted from the socket 102 and the head 104 to the stem 105 to prolong the life of the total hip prosthesis 100.
The stem is often made of metal, and titanium alloy such as cobalt alloy and Ti6A1—4V is mainly used, considering the strength and effect on the human body.
As a method of fixing the stem to the femur, adhesive called cement-type has been used so far, and a cement-type total hip prosthesis stem using the method will be described below based on
The method of fixing the cement-type total hip prosthesis stem to the femur 103 will be described below based on
In the epiphysis of the femur 103 where the stem is fixed, as shown in
Further, describing the bone structure based on
Therefore, regarding the strength characteristic of bone, as shown in
In this way, regarding the cement-type total hip prosthesis stem, the stem 105 is fixed to the femur 103 by hardening the cement 109, so the stem 105 can be fixed to the femur 103 for a fairly short time, which has an advantage in rehabilitating early for patients who undergo replacement operation with the total hip prosthesis 100. Therefore, it is particularly effective for elderly patients who are confined to bed for a long time and concerned with negative effects on other functions including motor function.
However, the cement-type uses two kinds of resin, base resin and hardener as the cement 109, and if they are not mixed enough, or the mixture ratio is inaccurate, unreacted monomer resin components which are not polymerized would remain and have harmful effects on the human body through the melt-out, and it is a source of causing various damages to the human body. Therefore, there is hesitation in using the cement-type to the youth with a long life expectancy.
Also, as to the cement-type, the stem 105 is fixed to the cancellous bone 110 of the femur 103 through the cement 109, and since the stiffness and strength of the cancellous bone 110 are not enough, the adhesive property to the stem 105 gets worse due to the weight of the stem 105, and the stem 105 gets loose or moves downward, called a sinking-down phenomenon. Especially when the sinking-down phenomenon occurs, the spherical stem 105 creates circumferential hoop stress like severing bone. Then, when the bone is cracked, patients suffer from the pain over a long period of time since there is no way to treat it so far.
As for the total hip prosthesis, the cement-type requires re-operation at a rate of five to twenty percent within ten years, but it is difficult to pull the stem 105 with the cement-type out of bone, and the re-operation itself is not easy.
Now, the cement-less type, fixing the stem 105 to the femur 103 without the use of cement, has been developed, and the following explains the conventional cement-less total hip prosthesis stem with the use of the cement-less type, based on
As shown in
The cement-less type stem 105 is fixed to the femur 103, using growth of bone within the femur 103, and the gap between the internal surface of the insertion hole 107 and the external surface of the stem 105 narrows as the stem 105 is driven into the insertion hole 107 and bone grows from the internal surface of the insertion hole 107 toward the external surface of the stem 105, and thereby fixing the stem 105 to the femur 103.
As to this cement-less type stem 105, there is no adverse affect on the human body through the melt-out of the unreacted monomer in the cement 109 since the cement 109 is not used. Therefore, the cement-less type stem 105 can be also used to young patients. Moreover, in a re-operation because the stem 105 can be pulled out of bone with relative ease, it helps save trouble in re-operation.
However, the cement-less type fixes the stem 105 as bone grows, narrowing the gap between the bone and the stem 105, and it takes several months until the bone fills the gap, and the stem 105 is firmly fixed, and then patients need a rehabilitation period, which prolonged a period of patients' hospitalization, imposing a burden on patients. Moreover, due to a long period of hospitalization it was difficult to adopt the method to elderly people who were concerned with negative effects on other functions such as motor function.
Given this situation, in order for patients to rehabilitate, the convex portion 116 (concavity and convexity portion) is set up on the surface of the stem 105 so that the stem 105 can be fixed to the extent that patients do not have trouble living in the early stage of the postoperative period, and the stem 105 is mechanically connected to bone with the anchoring effect of the convex portion 116.
Moreover, in addition to the mechanical joint, a chemical joint method is also suggested as the convex portion 116, and for instance, crystal of hydroxyapatite, the main component of bone, is attached to the surface of the stem 105 with adhesive or the like, and the stem 105 is fixed to the femur 103 by chemically binding hydroxyapatite of the stem 105 and by growing bone. The one with either a mechanical joint or chemical joint, or both has been suggested.
In this way, by setting up the convex portion 116 on the cement-less stem 105, the initial fixation can be achieved to some extent in the early stage of postoperative period, which could relieve some of the burden from patients who were hospitalized for a long time.
However, in the case of the stem 105 also, it was hard to say the initial fixation was perfect. Also, in the case of these cement-less type stems 105f-105j, the joint between the stem 105 and bone is only partially connected to the compact bone 111 with high bone strength and mostly connected to the cancellous bone 110 with low bone strength, and thereby the joint strength between the stem 105 and bone being weak, and the stem 105 got loose by repetitive loads from the stem 105.
Also, the conventional stem 105 is made of metal such as cobalt alloy and titanium alloy, and because these alloys are difficult to cut, it is very hard to process the convex portion 116 with microscopic convexo-concave on the surface of the stem 105, which made the stem 105 very expensive.
Moreover, these alloys are excellent in corrosion resistance, and because it is difficult to apply adhesive surface treatment to the surface to form electrically neutral and stable oxide coating for adhesion of hydroxyapatite's crystal, the bonding strength of the hydroxyapatite is not stable and the hydroxyapatite exfoliates, which, as a result, creates a problem that the stem 105 gets loose.
Also, because the external form of the stem 105 is simple, it does not fit the internal form of the medullary canal, the load to the femur 103 is concentrated, and thereby becoming a source of pain and breakdown of bone through forcibly driving the stem 105 into the medullary canal. Regarding the elderly with weak bone strength and patients with osteoporosis, because they cannot bear such operation in which the stem 105 is driven into the femur 103 with a hammer, the cement-less stems 105f-105j could not be adopted.
In order to solve these drawbacks, a new cement-less type stem has been suggested.
The custom-made stem 105k is taken pictures of each section in the two-dot chain line shown in
As shown in
However, as to the custom-made stem 105k, as shown in the section perpendicular to the axis in
Although it tries to make the external form of the stem 105k fit the internal form of the medullary canal 117 as much as possible, the workability of the machine work for the external form of the stem 105k and the subsequent finish processing is required. To be more precise, generally when a three dimensional form is machined, the cutting tool for cutting the form uses a hemispheric-tipped ball-end mill, and with the ball-end mill, it cannot get a flat face only by the machine work, which leaves a trail like a furrow called sculpheight.
Therefore, it is necessary to smooth the surface by undercutting the sculpheight by hand after the machine work, but the metal used for the stem 105 such as titanium alloy is difficult to cut, and the finishing requires very hard work. Therefore, the cement-less type stem made of titanium alloy became very expensive. Moreover, when convexo-concave is formed on the stem 105 to fit the internal form of the medullary canal 117, the finishing work would become more difficult, and it was too costly to adopt the kind.
When designing the external form of the stem 105, one tries not to form convexo-concave on the surface, ensuring that the stem 105 does not get caught when inserting the stem in the medullary canal 117. Therefore, as shown in
There is a term, Fit and Fill, to describe the relationship between the stem and the medullary canal. Fit means the contact ratio to the cortical bone, which is the ratio of the length of the cortical bone touching the stem to the entire circumference of the medullary canal in a section perpendicular to the axis of bone. Fill means the filling ratio in the medullary canal of the stem, which is the ratio of the section area of the stem to the area of the medullary canal in a section perpendicular to the axis of bone.
The higher Fit and Fill is, the better the accessibility of the stem and bone and the stronger force is transmitted from the stem to the bone. Therefore, as shown in
As shown in
From the above, for the diaphysis 113, that is the distal side, there was a risk of a bone being destroyed when a large amount of force is transferred from the stem 105 since bone in this region is weak against the sidling force. Therefore, it is desirable to stabilize stem in the epiphysis (proximal side). That is, the best relationship between the stem and the medullary canal is expected in such ways that the fit and fill is high in the epiphysis section (proximal side) and the fit and fill is low in the diaphysis (distal side).
As such, it is known in the traditional system 105 that porous coating of titanium alloy is applied on the proximal side surface of the stem 105 in order to increase the conjugation of bone in the proximal side, and that fixing is not to be done in the distal side by reducing the conjugation with bone through mirror finishing the tip part of stem 105 locating in the distal side. Hereafter, the fixing in the proximal side and the fixing in the distal side are called the proximal fixing and the distal fixing respectively.
However, as shown in
Also, as shown in
Moreover, stainless alloy such as high corrosive resistant cobalt alloy and titanium alloy is used in the above-mentioned stem 105k. If the high corrosive resistant oxide film is removed through abrasion of the surface of the stem 105 by micro motion in the contacting area with bone resulting from the stainless alloy being embedded in the body for a long period of time, the micro opening called corrosion pit is generated from the body fluid because the salinity in the body is the same as that of seawater. There has been a case reported, in which the metal fatigue is caused from the corrosion pit, fracturing the stem.
As such, various materials are suggested as the stem's raw material to replace metals. Some composite materials are among the suggestions.
For example, it has been suggested to make the center of the stem metallic and its outer side wrapped around by the composite materials such as FRP (fiber reinforced plastic). In U.S. Pat. No. 4,892,552, Japanese unexamined patent publication bulletin 5-92019, and published Japanese translations of PCT international publication for patent applications 6-500945, it is suggested to manifacture the stem using the carbon fiber reinforced plastic. The stems in these proposals attain the same stiffness as metal by using the carbon fiber reinforced plastic, and unlike metal, harmful substances do not melt out in the body by making the plastic that is macerating into fiber harmless to human body.
However, none of the above inventions have been in practical use in the current status. That is to say, it has failed to make the center of the stem metallic and its external side wrapped around with FRP since the stem becomes loose in the early postoperative period, resulting from micro motion between the FRP and bone or between the FRP and the center of the metallic section. The cause of this failure is thought to be the stem's bending stiffness only applies to the center of the metallic section, making the overall bending stiffness low, and the distribution of stress in the contacting area with bone is concentrated in the both ends, leading to the occurrence of the micro motion since the stem cannot resist to the stress.
Also, U.S. Pat. No. 4,892,552 claims that from the sheet-shaped laminate made from carbon fiber impregnated with resin, coupons are cut out in a way that the carbon fiber's direction is parallel to the external form and other coupons are cut out in a way that the carbon fiber's direction is 45°, and these two types of coupons are piled up alternately and heat and pressure are applied to it to form a bloc, and the stem is manifactured by machining in which the bloc is scraped off. It is merely substituting metal with the composite material. While avoiding the harmful substance to melt out, it does not solve any other problems.
Furthermore, the unexamined patent publication bulletin 5-92019 claims the system having the first-direction strength support with reinforcing fiber in the longitudinal direction of the stem outside of the intermediate part that is hollow and the second-direction strength support with reinforcing fiber in the 45° from the longitudinal direction of the stem further outside. In this stem, the first-direction strength support deals with bending stiffness and the second-direction strength support deals with torsional stiffness with a structure utilizing the characteristics of composite material. However, the second-direction strength support located outside the stem is manifactured by wrapping the strip-shaped reinforcing fiber. With this method it is difficult to attain the external shape that fits the internal shape of the medullary canal, necessitating the coating layer further outside of the second-direction strength support, and the stem may get loose since the stress is concentrated in the both ends of the coating layer.
Moreover, the published Japanese translations of the PCT international publication for the patent applications 6-500945 claims the system having the core in the center with fiber located in the same direction as the longitudinal direction of the stem, and the filling material that is not fiber-reinforced outside the core, and the sheath with fiber arranged spirally outside the filling material. This system also cannot prevent the stem from getting loose, similar to the above-mentioned unexamined patent publication bulletin 5-92019.
The conventional systems cited above had common problems. It was the problem of concentration of stress caused by connecting the stem and bone.
Also
Furthermore,
As shown in
Given the above factors, the method in
As a result, in the example of
That is, making the relationship between the stem and bone like
However, the conventional system is manifactured from materials that are difficult to cut such as titanium alloy, and it was impossible to process in the hollow section, and thus the method in
In the example in
As such, considering the above situation, the invention can provide a cement-less type artificial joint stem with the use of composite material connecting to bone without using cement, not becoming loose over a long period of time, excellent in durability, and having appropriate external form and stiffness to each patient.
In order to solve the above issue, the cement-less type artificial joint stem with the use of composite material in the invention is structured such that “the cement-less type artificial joint stem is inserted in an insertion hole which is penetrated into bone and fixed to the bone without using cement, comprising: a main part which has an external form of an epiphysis approximately fitting an internal form of the insertion hole, wherein stiffness around a boundary between epiphysis and diaphysis of the said main part varies so as to lower the stiffness as approaching the diaphysis; and a neck to place a spherical head in an artificial joint provided at a proximal end of the main part.”
Although a specific composition of the composite material for the stem in the invention does not need to be limited, the fiber-reinforced plastic can be used. As for the fiber, carbon fiber, ceramic fiber, glass fiber, aramid fiber can be exemplified. Turning the fiber into the continuous fiber, one can use it as filaments, blind shape, woven fabrics, and nonwoven fabrics, or turning into the short fiber, one can use it as chop shape. The carbon fibers are preferable and the high modulus carbon fiber is the most preferable among them. As for the resin, polyether ether ketone, polyetherimide, polyether ketone, polyacryl ether ketone, polyphenylene sulfide, polysulfone can be exemplified. The most preferable is the thermoplastic resin that is harmless to the human body and does not melt out.
In terms of the method of matching the stem's external shape with the internal shape of the insertion hole, although it is not limited to a specific composition, one can, for example, take pictures of several cross-sections of a patient's bone to which the stem is fixed, by using a nondestructive tomography scanner such as CT and MRI, and generate a numerical data after converting the said cross-sectional images to three-dimensional using three-dimensional CAD, and penetrated insertion hole with the prescribed internal shape into the patient's bone by the computer controlled surgical robot using the said numerical data. On the other hand, one can match the stem's external shape with the internal shape of the insertion hole by formulating the molding tool using the same numerical data and form the external shape of the stem based on the said molding tool.
Furthermore, as for the method of changing the stiffness of the stem's main part, although it is not limited to a specific composition, the stiffness can be changed, for example, by formulating the stem with the composite material with the prescribed thickness and making the thickness thinner as approaching from the epiphysis area to the diaphysis area. Or the stiffness can be changed by changing the fibrous direction of the reinforced fiber included in the composite material. Also, the stiffness can be changed by reducing the reinforced fiber's portion in the composite material, volume, or quantity as approaching from the epiphysis area to the diaphysis area. Moreover, the stiffness can be changed by reducing the elastic coefficient of the reinforced fiber in the composite material as approaching from the epiphysis area to the diaphysis area. These methods can be used separately or in combination, and it is not limited to these examples so long as the stiffness can be changed.
According to the invention, the gap between the stem and bone can be reduced as much as possible since the stem's external shape fits the internal shape of the insertion hole that is penetrated into bone. As a result, the stem can be well connected to bone with the use of cement, and there is no adverse affect on the human body through the melt-out of the unreacted monomer from not being mixed enough or the mixture ratio is inaccurate, as in a case of cement-type stem.
Also, because the stem's external shape fits the internal shape of the insertion hole that is penetrated into bone, the initial fixation adequate for a normal life style can be attained in the early postoperative period, and because the rotational anchorage is high, an early discharge from hospital is possible through shortening the hospitalization period and an early social rehabilitation is possible, which could relieve some of the burden from patients. Also, this method can be utilized to the elderly, who have concerns about adverse effects to the motor functions and other functions resulting from a long-term hospitalization.
Furthermore, because the stem's external shape fits the internal shape of the insertion hole that is penetrated into bone, fit and fill can be high, and the loading from the stem can be transferred to bone without deviation, and the stress shielding can be controlled, and the stem not getting loose, through weakening of the connection between the stem and bone as a result of stress shielding that makes bone skinnier, can be prevented and the stem's durability increases.
Moreover, because the stem's external shape fits the internal shape of the insertion hole that is penetrated into bone, the stem can be fixed without slamming the stem into the insertion hole with a hammer, and the stem can be utilized for osteoporosis patients and elderly people whose bone's strength is weak.
Also, because the stem's external shape in the epiphysis area fits the internal shape of the insertion hole that is penetrated into bone, fit and fill can be high, and the stem can be fixed in the epiphysis area. That is, using an example of the femur, as the epiphysis area, the stem can be fixed near the femur, which means the proximal fixing is possible, transferring the loading well from the stem to bone.
Also, in the proximity of the boundary between the epiphysis area and the diaphysis area, the stiffness of the stem's main part varies in such a way that the stiffness becomes low as approaching toward the diaphysis. As a result, the stress concentration at the ends of the connecting section between the stem's main part and bone can be controlled, and the stem getting loose because of the stress concentration that breaks away the connecting section can be prevented. Also, since the stiffness in the diaphysis area is made low, the stem's loading is mainly transferred to the epiphysis area. If applied to the femur, for example, the proximal fixing, in which the force is transferred in the epiphysis area that is the proximal side, can be done.
Furthermore, the composite material is used as the stem's material, in particular, by using the composite material that is harmless to the human body, there is no adverse affect to the human body unlike the conventional metallic stem in which the harmful substance to the human body melts out from the stem to the inside of human body. Also, the composite material is excellent in formability and workability compared to the titanium alloy, and the desirable shape can easily be attained, which reduces the cost of producing the stems.
The cement-less type artificial joint stem with the use of composite material further comprising “a guide section, provided at the tip of the main part and placed at the disphysis, the guide section has a lower bending and stretching/tensile stiffness than the main part.”
According to the invention, the guide section is provided in the forefront of the stem, and as a result, the stem can be easily inserted in the insertion hole during the operation when inserting the stem into the insertion hole penetrated into bone because the stem's insertion is guided by the guide section.
Also, since the bending and tensile stiffness of the guide section is made lower than the main part, the stress applied to the connecting section between the guide section and bone can be less than the main part. To elaborate, the stem in the invention has the same composition as the example shown in
The cement-less type artificial joint stem with the use of composite material in the invention can also have a composition that “clearance of a predetermined quantity between an external surface of the guide section and an internal surface of the insertion hole is reserved.”
According to the invention, the loading is not transferred through the guide section since the clearance is formed between the insertion hole and the guide section and the guide section does not contact bone. That is, the fit and fill of the stem is low in the diaphysis area where the guide section is, and the stem is not fixed in this area but is fixed in the epiphysis area where the main part is, and thus the loading from the stem can be transferred to bone as a good condition.
Also, due to bone's growth after the surgery, even if the clearance between bone and the guide section is filled, it is filled with the cancellous bone that has low strength, making the stress applied at the connecting point with the guide section small. The loading from the stem is significantly applied in the epiphysis area where the main part is, and the anchorage in the epiphysis area is continuously maintained, and the loading from the stem can be transferred to bone in a good condition.
The cement-less type artificial joint stem with the use of either one of composite material, wherein “an external surface corresponding to the epiphysis has a convexo-concave surface treatment section thereon.” The surface finishing part can be a continuous convexo-concave shape, or can have the intaglio and convexity in several places on the flat surface, or can be provided with the adhesive line that includes the hydroxyapatite. These can be used separately or in combination, and the surface finishing part is not limited to these mentioned above.
According to the invention, the convexo-concave surface treatment section is provided in the external surface of the stem, and the mechanical bonding strength between the internal surface of the insertion hole and bone can be attained, and the anchorage strength adequate for a normal life style can be attained in the early postoperative period. As a result, it can relieve some of the burdens from patients who are hospitalized for a long time, and the stem in the invention can be utilized to elderly people.
Also, because the composite material is used for the stem in the invention, the surface finishing part can be provided more easily than the conventional stem, which used titanium alloy, a material that is difficult to be broken/cut. As a result, the stem's cost can be reduced even with the surface finishing part.
The cement-less type artificial joint stem with the use of composite material, wherein “the convexo-concave external surface treatment section has an adhesive layer containing hydroxyapatite on the most external surface, and fiber of composite material is positioned along with the convexo-concave external surface without breakage.” As for the hydroxyapatite, its crystal is preferable to use in order to increase the bonding strength.
According to the invention, since the hydroxyapatite crystal is included on the surface of the surface treatment section and the hydroxyapatite crystal and bone are chemically bonded, the stem and bone can be glued together more strongly in addition to the mechanical bonding by the convexo-concave of the surface finishing part.
Also, since the fiber form of the composite material are continuously provided inside along with the convexo-concave of the said surface treatment section, the fiber form of the composite material are continuous fibers and the strength of the composite material does not become low, and thus a high strength can be maintained.
Furthermore, since the composite material is used for the stem, the adhesiveness with the adhesive line, which includes hydroxyapatite, is better compared to the conventional stem of titanium alloy, and it is difficult for the stem to separate from the hydroxyapatite. Also, by using the resin for the adhesive line same as the resin used for the composite material, the adhesiveness becomes better between the adhesive line and the stem.
The cement-less type artificial joint stem with the use of either one of composite material, wherein “the main part comprises: a first external layer which contacts an internal surface of the insertion hole and has increased torsional stiffness; a main structure layer which is positioned inside the first external layer, continuing from the neck, and has increased bending stiffness; a core layer which is positioned inside the main structure layer and has lower stiffness than the main structure layer and the first external layer; and a most internal layer which is positioned between the core layer and the main structure layer.”
As for the method of increasing the torsional stiffness, the torsional stiffness can be increased by turning the direction of the composite material's fiber opposite of the torsional direction, for example, ±45° direction against the torsional direction. Also, as for the method of increasing the bending strength, the bending strength can be increased by turning the direction of the composite material's fiber perpendicular to the bending direction.
Also, in terms of the core layer with low stiffness, resin with non-reinforced fiber and plastic foam, or the composite material that uses short fiber can be used, and it is not limited to a specific material so long as its stiffness is lower than that of the main structure layer and the first external layer.
According to the invention, the main structure layer with strong bending stiffness is provided inside the stem and the first external layer with strong torsional stiffness is provided outside the stem. As a result, the stem's bending and torsional stiffness can be optimized.
The conventional stem was metallic such as titanium alloy, and its stiffness was unable to change in accordance with patients' condition, and thus the stem could not be used for the patients with weak bone as well as osteoporosis patients. However, according to the invention, the bending and torsional stiffness can be appropriately set up, and the stem can be adjusted to the characteristics of patients' bone in which the stem is to be filled. For example, for elderly people with weak bones and osteoporosis, the stem can be made in accordance with the stiffness of their bones. As a result, one can restrain a case in which bone is broken due to a significant difference in the stiffness of the stem and bone, and thus the stem can be applied to patients who had been unable to use the artificial joint.
As mentioned above, according to the invention, one can provide the cement-less type artificial joint stem with the use of composite material, which connects bones without using cement, not getting loose for a long period of time, excellent in the durability, and is provided with the stiffness and the external shape appropriate for each patient.
The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by the following detailed description of the preferred embodiments, when considered in connection with the accompanying drawings, in which:
Below, the preferred embodiments are illustrated in details based on the
As shown in
The surface finishing part 5 is formed at the main part 3 of the stem 1, provided with concave-convex on the part of its surface. Further, as shown in the enlarged view of
As shown in
The composite material used for the stem 1 is the carbon fiber reinforced plastic. As for the carbon fiber, the high modulus, high strength carbon fiber with its elasticity of 200-650 GPa, for example, is used. Also, as for the matrix, the thermoplastic resin, such as polyether ether ketone and polyetherimide which are harmless to the human body, is used. The sizing can be applied to the carbon fiber in order to increase the bonding strength to the matrix. Incidentally, as for the stem 1 in the example, if the carbon fiber with its elasticity of 630 GPa used and the layer with its fiber direction ±45° is formed, the layer's transverse modulus G is about 49 GPa, which has enough strength when comparing to the conventional titanium stem of 43.3 GPa.
For the first external layer 9 of the stem 1, the fiber form of the composite material are woven fabric, and the direction of the fibers is directed ±45° to the axis of the main part 3 of the stem 1. As a result, the torsional stiffness increases and the shear loading and the torsional loading that are applied to the stem 1 can be supported at the first external layer 9.
Also, for the main structure layer 10 of the stem 1, the fiber form of the composite material are woven fabric, and the direction of the fibers is directed toward the axis of the main part 3 of the stem 1. As a result, the bending stiffness increases and the bending loading that is applied to the stem 1 can be supported at the main structure layer 10.
As shown in
Furthermore, the taper part 14 is formed in the internal edge of the main structure layer 10, as a result of the core layer 11 going into the main structure layer 10. The thickness of the main structure layer 10 is varied on the taper part 14, and the stiffness of the main structure layer 10 is changed at the taper part 14. In this case, the stiffness in the main structure layer 10 gets lower toward the forefront side.
The core layer 11 of the stem 1 is formed with the low-stiffness material such as plastic foam, and both the inner most layer 12 and the second external layer 13 are made with the low-stiffness material or the layers with its fibers directed at ±45°. The stiffness of the core layer 11 and the second external layer is the minimum required stiffness necessary to insert the stem 1 into the insertion hole 8 in the operation.
As for the stem 1, as shown in the section views B1-B6 in
The production method of the stem 1 in the example is explained next. First of all, several cross-sectional images of the patients' bone, on which the stem 1 is fixed, are captured, by using a nondestructive tomography scanner such as CT and MRI, and a numerical data after converting the said cross-sectional images to three-dimensional is generated by using three-dimensional CAD. Then the insertion with the prescribed internal shape (the internal shape of the medullary canal is preferable) is penetrated into the patient's bone by the computer controlled surgical robot using the said numerical data. On the other hand, the molding tool is manifactured using the same numerical data, and manifacture the stem 1 using the molding tool (not shown in a figure).
In formulating the stem 1, the plastic film that is impregnated with hydroxyapatite crystal is placed at the surface treatment section 5 of the molding tool, and the woven fabric, which is made with the carbon fiber that forms the primary surface layer 9 and the fiber made by thermoplastic resin, is laid up therein. At this time, the direction of the woven fabric's fibers is ±45° to the axis of the stem 1.
Furthermore, the woven fabrics, which is made with the carbon fiber and the fiber made by the thermoplastic resin those forms the main structure layer 10, are laid up in such a way that the fiber's direction faces toward the axis of the stem 1. The woven fabric that forms the inner most layer 12 and the second external layer 13 is laid up, and the plastic foam to be the core layer 11 is placed in the space formed by the inner most layer 12 and the second external layer 13.
Next, close the forming die, and give heat and pressure it using the autoclave or hot plate. When doing so, the pressurization can be done from the inside the stem 1 as a result of the plastic foam that forms the core layer 11 being expanded by the heat.
The convexo-concave that forms the surface treatment section 5 is engraved in the surface of the forming die of the stem 1, and the surface finishing of the part 5 is created while the stem is made. As shown in
As shown in
However, as shown in
This is also illustrated in
The load transfer concept between the stem 1 and the bone 7 is the same as the one shown in
As such, according to the figure in this operation, because the external shape of the stem 1 fits the internal shape of the insertion hole 8 penetrated into the bone 7, the gap between the stem 1 and the bone 7 can be reduced as much as possible. As a result, despite the cement-less type, the initial fixation adequate for a normal life style can be attained in the early postoperative period, and because the rotational fixation is high, an early discharge from hospital is possible through shortening the hospitalization period, and an early social rehabilitation is possible and thus relieving the burden on the patient. Also, this method can be utilized to senior people, who have concerns about adverse effect of motor functions and other functions resulting from a long-term hospitalization.
Also, because the external shape of the stem 1 fits the internal shape of the insertion hole 8 penetrated into bone 7, the fit and fill can be high and the loading from the stem 1 can be transferred to the bone without deviation, and therefore the stress shielding can be controlled and loosening the stem 1, due to weakening of the connection between the stem 1 and the bone 7 as a result of stress shielding that makes the bone 7 thinner, can be prevented, thereby increasing the artificial joint's durability.
Furthermore, by providing the taper part 14 in the main structure layer 10 of the main part 3 of the stem 1, the stiffness changes in such a way that the stiffness becomes low as getting toward the forefront side of the stem 1. As a result, the stress concentration can be controlled at the end of the contact layer between the bone 7 and the main part 3 of the stem 1, preventing the stem 1 from getting loose through separating the contact layer by the stress concentration. Also, since the stiffness in the diaphysis area is made low, the loading from the stem 1 is mainly transferred to the epiphysis area. That is, the proximal fixing can be done.
Also, since the guide section 4 is provided in the forefront side of the stem 1, and the insertion of the stem 1 is guided by the guide section 4 when the stem 1 is inserted into the insertion hole 8 penetrated into the bone 7 during the operation, the stem 1 can be easily inserted into the insertion hole 8.
Furthermore, since the surface treatment section 5 with convexo-concave is provided on the surface of the main part 3 and the chemically joint layer 6 having the hydroxyapatite crystal further above it, the mechanical and chemical bonding between the stem 1 and the bone 7 are possible, making it a stronger bonding, and thus the stem 1 can be prevented from getting loose.
Also, the composite material is used for the stem 1, which has better shape formability and the workability compared to the one made of the metals, and thus the production cost of the stem 1 can be reduced. Furthermore, the surface treatment section 5 and the stem 1 are formed simultaneously, and no additional process for providing the surface finishing part 5 is necessary, and thus enabling to control a rising cost even if the surface finishing part 5 is provided with the stem 1.
Next, we will describe the artificial joint stem with the use of composite material with an embodiment different from the ones mentioned above using
The stem 20 in this embodiment has a high fit and fill at the main part 3, that is, in the epiphysis area, and a low fit and fill at the guide section 4, that is, in the diaphysis area, making a perfect anchorage between the stem 20 and the bone 7 in the epiphysis area, that is, the proximal fixing.
As shown in
From this, as shown in
As such, according to this embodiment, since the appropriate amount of clearance is formed between the external surface of the guide section of the stem 20 and the internal surface of the insertion hole 8, the guide section 4 does not contact with bone 7 in the early postoperative period, thereby the loading is not transferred to bone 7 through the guide section 4.
Also, after the surgery, even if the clearance with the guide section 4 is filled due to the growth of the bone 7, this part is filled with the low density cancellous bone, and the stress applied to the joint section with the guide section 4 is small, and the loading from the stem 20 is largely applied in the epiphysis area where the main part 3 is located. The anchorage in the epiphysis area is continuously maintained, and thus the loading from the stem 20 can be transferred to the bone 7 in a good condition.
Furthermore, as for the stem 20 in this example, since the guide section 4 is thin, the friction of the guiding 4 is low when the stem 20 is inserted into the insertion hole 8 during the surgery, and thus the insertion can be done more easily than the stem 1 in
Another embodiment of the invention using
For the stem 30, similar to the system mentioned above, the stem 30 can be well fixed in the epiphysis area and provide the same effects as the one mentioned above. In this example, the core layer 11 and the tertiary outer layer 31 can be deleted and the main part 3 can be hollow shape.
So far, we have illustrated the various embodiments of the invention, yet the invention is not limited to these embodiments, and various improvements as well as changes of design are possible to the extent it does not deviate from the scope of the invention, as indicated below.
That is, in this embodiment, the carbon fiber reinforced thermoplastic such as PEEK and PEI are shown as the composite materials, yet it is not limited to these materials. For example, as for the fiber, the ceramic fiber, glass fiber, and aramid fiber can be used, and as for the ceramic fiber, the ceramic fiber having the titanium component with the silicon carbide as a main body, such as the product name “tirano fiber” can be exemplified. Also, as for the plastic, one may use polyether ether ketone, polyacryl ether ketone, polyphenylene sulfide, polysulfone, and these raw materials can be used appropriately in combination.
Also, in this embodiment, the carbon fiber of composite material used for the stem 1 and the stem 20 that are same as the fibers for the main part 3 and the guide section 4 are shown, yet it is not limited to these materials. One may use the high modulus fiber for the main part 3 and the low modulus fiber for the guide section 4, or may use the carbon fiber for the main part 3 and the low modulus glass fiber for the guide section 4, thus these materials are not restricted so long as the stiffness of the guide section 4 is lower than that of the main part 3.
Furthermore, in this embodiment, the inner most layer 12 is provided with the stem 1, 20 and 30, yet it is not limited to such, and the stem can be without the inner most layer 12. As a result, one may reduce the cost of the stem since the manufacturing process of the stem is reduced.
Also, in this embodiment, in
The invention can provide cement-less type artificial joint stems with the use of composite material which can be connected to bone without using cement, does not become loose over a long period of time, has excellent durability, and has appropriate external form and stiffness to each patient.
Also, the invention can be used not only for the total hip prosthesis of the femur illustrated in the embodiment, but for the implant to connect joints such as knee joint, shoulder joint and fractured bone or for the substitute of damaged bone by accidents or diseases.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP03/13009 | 10/9/2003 | WO | 8/8/2005 |