Patent Application
20050137711
The present invention relates to orthopaedic implants or prostheses, and particularly to implants subjected to high tensile loads. The invention has particular application to implants or prosthesis that form part of a joint of the human body, such as the hip, knee or shoulder.
Implants or joint prostheses have improved significantly over the last few decades, largely due to improvements in the bio-compatibility, strength and durability of the implant materials. New machining processes and material coatings have been developed that enhance the fixation of the implant within the natural bone of a patient. Alloys and ceramics have been developed that emulate the strength of natural bone, while still preserving the biomechanical attributes of the joint being repaired.
In a typical implant or prosthesis, a stem is inserted into the medullary canal of a long bone, such as the femur or humerus. Bone cement or a bone ingrowth coating can be introduced to fix the implant within the bone. The proximal end of the implant can be configured to replace the damaged portion of the patient's natural bone or joint. For instance, in a hip implant, the head of the femur can be removed and an implant utilized that fills the space left by the removed bone. The implant can include a ball to mate with the articulating socket of the hip joint.
The head of the femur is the strongest bone of the human body. It endures significant loads through millions of cycles and a variety of movements during the normal lifespan. Any implant used in the hip must be capable of enduring the same loads without fracture. Strength and durability issues become more acute with smaller implants. Stress concentrations in the region between the stem and head portions of smaller implants can become problematic. While stronger materials have been developed to extend the life of all implants, including hip prostheses, there is always a desire to improve the strength and durability of the implants even further.
Smaller implants are particularly susceptible to mid-stem fractures. The smaller implants are necessary to meet the anatomic constraints of smaller patients. Consequently, there is no room to increase the cross-section of the implants to add strength. The need for increasing resistance to flexure loads is particularly critical with these smaller implants.
In order to meet the need for stronger and more durable implants, the present invention contemplates a pre-stressed implant that is especially suited to endure high cyclic tensile loading. In one aspect of the invention, a tension member is fixed within a bore of the implant. This tension member places the implant in compression. When the implant is subjected to loads, the resulting tensile forces acting on the implant act, at least initially, to reduce the compressive load that is generated on the implant by the tension member. In other words, the applied tensile forces de-compress the implant before the implant experiences any meaningful tensile loads. The implant can readily withstand the compressive loads exerted on it by the tension member without any significant risk of failure or fatigue. Moreover, pre-stressing the implant opens up the universe of acceptable materials for the construction of the implant. For instance, the implant can be formed of a high strength ceramic in lieu of the typical metal alloy.
In a preferred embodiment of the invention, the tension member comprises a bolt that extends through a bore in the implant. Where the implant is a hip prosthesis, the bore can extend through the stem or neck/head of the prosthesis. The bore is preferably open at one end of the prosthesis and terminates in internal threads at a closed end of the bore. The head of the bolt bears against the prosthesis at the open end of the bore so that the bolt is put in tension as it is tightened into the internal threads. The bolt tension compresses the prosthesis along the axis of the bore. It is contemplated that the bolt will be threaded into the prosthesis prior to implantation within the bone. The bolt can be tightened to a pre-determined torque that corresponds to an appropriate amount of tension in the bolt, and consequently compression in the implant.
In one specific embodiment, the open end of the bore is disposed at the proximal face of the implant. In another embodiment, the bore opens at the distal end of the implant—i.e., the part of the implant that is buried within the bone. With this embodiment, the head of the bolt can be covered at the distal end of the implant, such as by an end cap. In an alternative configuration, the end cap itself serves as the head of the bolt. The end cap can be configured to be independently coupled to the remainder of the implant, although the bolt tension may be sufficient to hold the end cap in position throughout the life of the prosthesis.
The bolt is sized relative to the bore in the implant to leave a pre-determined gap between the bolt and the inner wall of the implant bore. This gap is calibrated to allow a certain amount of flexure in the implant without causing a commensurate flexure in the tension member.
In a further aspect of the invention, the tension member can be a tension cable. The cable can be a wound multi-filament, pre-stretched cable. The ends of the cable are engaged to caps that are configured to mate with the prosthesis at opposite ends of the bore through the prosthesis. In one embodiment, the bore extends through the entire dimension of the implant so that it is open at its opposite ends. One cap is configured to engage one open end of the bore and to provide an anchorage for the cable. The other cap is configured to engage the opposite end of the bore and to permit tightening of the cable when the cable extends through the bore.
It is an important object of the invention to improve the strength and fatigue resistance of an orthopaedic implant or prosthesis. One benefit achieved by the present invention is that it reduces the tensile stress experienced by the implant. This benefit manifests itself in longer life for the implant.
A further benefit of the invention is that it allows smaller implant dimensions, thereby improving the range of motion for a prosthetic joint. Yet another benefit resides in the ability to use different materials for the implant that might not otherwise be available for traditional designs of the implant. These and other objects and benefits of the invention will become apparent upon consideration of the following written description taken together with the accompanying figures.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains.
In one embodiment of the invention, an implant or prosthesis 10 is disposed within a bone, such as the femur F, as shown in
In accordance with one aspect of the present invention, the implant 10 defines a bore 18 passing through the implant. In the embodiment shown in
The implant 10 includes a tension member 25 that engages the implant within the bore 18. In the embodiment shown in
As shown in the cross-sectional view of
In certain embodiments, the gap G can also be filled with a material having a high compressive strength. Alternatively, only portions of the gap are filed with an adjunct material. For instance, some parts of the length of the bore 18 can include a series of washers that are formed, for instance, of ceramic, metal or silicone. The material can also be a hardenable material or an incompressible gel.
In the preferred embodiment, the bore 18 extends substantially along the entire length of the stem 12. In addition, the internal threads 20 are situated at the end of the bore. In alternative configurations, the bore can terminate nearer the middle of the implant 10. It is believed that the greatest stress concentration for a hip implant of the type shown in
In the embodiment of the invention shown in
Since the bolt is introduced from the distal end 12b′ of the implant 10′ of the embodiment in
In another embodiment of a distally inserted tension member, a modified end cap 37 can be provided with the tension member 38 attached, as shown in
In order to enhance the fixation of the end cap 37 and the tension member 38 to the stem 12′, the tension member can be provided with a tapered portion 39. This tapered portion can mate with a complementary tapered portion at the distal end 12b′ of the bore 18′ (not shown). Preferably, the taper is a self-tightening Morse taper. The provision of a Morse taper interface at the distal end of the tension member 38 can operate as a mechanism to prevent over-tensioning of the element. The orientation of the taper interface is calibrated so that the taper fixation occurs at or after the point at which the tension member is at its pre-determined tension and the implant is at its pre-determined compression.
As depicted in
In order to improve the overall strength of the implant, an implant 60 can be provided as shown in
In the previous figures, the tension member has been shown engaged within the portion of the prosthesis that is implanted in the patient's bone. The tension member can also be used at other locations of the prosthesis that are susceptible to bending or tensile loads. For instance, as shown in
The bore 44 is provided with internal threads 46 at its closed distal end and is open at a platform 48 at the proximal end of the neck 42. A tension member 50 is mounted within the bore 44 that is in the form of a bolt 51. The head 53 of the bolt contacts the platform 48 as the threaded portion 54 engages the internal threads 46 of the bore. In this respect, the bore 44 and the tension member 50 can be configured similar to the like components described above. With this embodiment, the neck is placed in compression so that imposition of a load L (
In the embodiments described above, the tension member includes a bolt. The present invention contemplates other elements that are capable of being placed in tension and ultimately capable of compressing at least a portion of an orthopaedic implant or prosthesis. Thus, the tension member can include a cable or a spring system, as depicted in
The implant 70 can include a tension member 76 passing through bore 73 and a tension member 77 passing through bore 75, if it is present. These tension members can be springs or cables. The opposite ends of these tension members 76, 77 are fastened to caps that close the ends of the bores and anchor the tension members within the implant. For instance, the tension member 76 can be a cable that is fastened to an end cap 80 using an anchor 81 mounted within the cap. The tension cable passes through the bore 73 from the distal end to the proximal end and is fastened to a proximal cap 83 by an anchor 84.
In one embodiment, the tension cable 76 can include a multiple strand pre-stretched cable that is looped around the anchor 81 in the distal cap 80. The two ends of the cable pass through the bore 73 and are twisted around the anchor 84 in the proximal cap 83, as depicted in
A similar approach can be followed for the cable 77 passing through the bore 75 in the implant neck 74. In this instance, one cap 86 and anchor 87 are configured to permit attachment of an articulating component, such as the ball B depicted in
In certain embodiments of the invention, the tension member can be completely sealed within the implant, allowing the use of non-traditional materials to form the tension member that might be less resistant to the joint environment that more traditional materials. For instance, the tension member can be formed of a “liquid metal”—i.e., a metal created using nanotechnology to have an amorphous structure that resists crack propagation and that provides fatigue strength, yield strength and elastic limit properties superior to traditional crystalline metals. Sealing the tension member within the implant will mitigate potential biocompatibility concerns for such non-traditional materials.
The tension members of the present invention put critical portions of an orthopaedic implant or prosthesis in compression. By so doing, when the critical portion of the implant is exposed to a tensile load, the load must first reduce the implant compression before the critical portion experiences any tensile stress. A proper combination of material compressive strength and tension member tension can significantly reduce the amount of tensile stress experienced at these critical portions of the implant. As a consequence, the implant can be formed of non-traditional materials. These non-traditional materials may be very strong in compression but weaker than the traditional implant metal allows when subject to tensile loads.
Another consequence of the pre-stressed implant of the present invention is that the critical portions of the implant can be constructed with reduced cross-sections. For instance, the neck 74 of the implant 70 shown in
Other tension members and tension protocols are contemplated by the present invention. For instance, the bolt tension member 25 shown in
In one embodiment, an implant 70, depicted in
As an alternative approach, the rod 72 is longer than the implant so that a portion 78 is accessible beyond the end of the implant. This portion 78 can be engaged by a distraction tool operable to pull the proximal end of the rod while the distal end is held fixed with the implant by pin 74. Once the desired tension has been reached, the proximal end can be fastened using the pin 76 or other similar mechanical fastener. With this approach, the opening in the proximal end of the implant 70 for receiving the pin 76 is located to be aligned with the corresponding pin-receiving opening 79 through the rod 72 only when the rod has been appropriately tensioned or stretched. In lieu of passing the pin 76 through an opening at the proximal end of the implant, the pin 76 can bear against a washer 80 that contacts the end of the implant.
As a further alternative to the tension member shown in
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.
