Patent Application
20060009857
The present invention relates to orthopedic implants and particularly to a method of surface hardening of bearing surfaces of the implant.
It is generally known in the art to use prosthetic devices to replace portions of the human anatomy that have been damaged due to injury or age. Often these prosthetic devices are formed of materials that are inherently strong yet easily formable. Many modular prosthetic devices are formed of at least metal stem portions that are inserted into long bones to provide a base for an external portion that extends from the boney portion. A taper or neck often interconnects the portion that extends from the bone, such as a head of a humerus or a femur, and the stem that is inserted in the bone. A taper may also be used to interconnect modular positions that are disposed within the bone after implantation. It is also known to provide bearing surfaces that must interact with one another while not wearing quickly or producing much wear debris.
The taper or neck that interconnects the two portions of the prosthesis, sometimes referred to as a Morse taper, must be strong enough to withstand cyclic loads that will be seen in a wide variety of anatomies, patient activity levels, and compromised boney constructs. The neck must also allow a range of movement that closely simulates the natural human anatomy. Other types of prosthetic devices are also modular and are formed from multiple interconnecting components. These components may also be interconnected by way of a Morse taper.
While materials generally used in these devices are inherently strong and have high tensile strengths, they require a particular thickness or mass to provide enough support for the portion of the anatomy that is being replaced. Due to this, the Morse taper is often larger and does not provide a full or natural range of motion. If the taper is for internal bone connection, a strong enough connection may produce a taper that is too big to fit into smaller bones. Due to this, it is desirable to produce prosthetic devices that include neck or interconnection portions that are small enough to fit into smaller bones and allow a full range of motion while being strong enough to support the stresses which the prosthetic will encounter.
One solution has been to provide new metal alloys that are particularly strong. These metal alloys may be formed into a myriad of shapes while still providing much of the support necessary for the prosthetic device. These new metal alloys, however, are still required to have large enough interconnection portions to provide the necessary strength to the materials.
Other known methods include the cold working or work hardening of prosthetics, such as that disclosed in the commonly assigned U.S. Pat. No. 6,067,701 entitled “Method for Forming a Work Hardened Modular Component Connector,” which is hereby incorporated by reference.
Cold working induces residual stresses within the component and may also produce the required or different residual stresses within the component. Particular residual stresses can be either compressive or tensile depending upon their nature. Compressive residual stresses are particularly desired. In particular, compressive residual stresses inhibit or stop cracks which may form in the prosthetic device. Furthermore, compressive stresses inhibit the initiation of a crack within the area which is loaded by external forces.
Compressive stress, especially near the surface of the component, also provides additional benefits. In particular, compressive stress near the surface can decrease fatigue and stress corrosion failures. In particular, these fatigue and stress corrosion failures originate at the surface and the compressive stress help inhibit such failures. In addition, the compressive residual stresses increase resistance to other undesired events such as fatigue failures, corrosion failure, stress corrosion cracking, hydrogen assisted cracking, fretting, galling, and corrosion caused by cavitation. Additionally, work hardening, which produces the compressive stresses, increases intergranular corrosion resistance, surface texturing, and closing of surface porosity.
Although compressive stresses, or other particular residual stresses, provide these many benefits, it is more beneficial to precisely create the desired residual stresses within the prosthetic device. Although exploratory cold working a component may produce the desired residual stresses, predetermining and work hardening components to produce predetermined residual stresses is preferable. Therefore, it is desired to provide a known process to produce within the prosthetic device, known and predetermined residual stresses that will provide compressive and tensile stresses, that are desired in a component.
Thus, it is desirable to have a method of producing prosthetic devices that leave no uncertainty to the strength being introduced into the prosthetic device. This would allow for more efficient manufacturing and an increase in prosthetic strength that survive the testing phase. That is, it is desirable to produce a prosthetic device that needs not be tested as often while still assuring that the prosthetic device will be able to handle the loads after being implanted.
A method for producing a prosthetic having metal articulating surfaces is disclosed. The method includes impinging at least a portion of the articulating surface with high energy laser electromagnetic radiation, and allowing the surface to cool to form a layer having a predetermined hardness. A hardened portion of the surface is then post-processed to form a layer having a predetermined finish. In another embodiment of the present invention, a method for producing an acetabular cup is disclosed. The process subjects the hemispherical articulating surface of the acetabular cup to lasershot peening to form a surface having a predetermined hardness. Honing is performed on the surface to provide an articulating surface having a predetermined hardness and surface finish.
In another embodiment to the present invention, a implantable femoral prosthetic is provided having an articulating head portion. The surface of the head portion is subjected to lasershot peening to form a hardened surface. As with the previously described bearing, the head surface is machined to provide a articulating surface having a suitable surface finish.
In another embodiment to the present invention, a method of producing a Morse taper joint for a prosthetic is disclosed. At least one surface of the Morse taper is subjected to pulses of high intensity laser electromagnetic radiation to cause surface hardening of the Morse taper joint. Post-processing is conducted on the treated surface to produce the proper surface finish.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the embodiments are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses.
Referring to
The outer surface 26 of the acetabular prosthesis 20 defines a plurality of locking projections 42 for coupling the acetabular prosthesis 20 to the prepared acetabulum. It is envisioned that the outer surface 26 can be surface treated to facilitate bone ingrowth or fixation to bone cement, such as by porous coating.
Although there are many ways to couple the constraining ring 36 or insert bearing 38 into the bearing cavity 40 defined by the inner surface 22 such as fasteners or tabs, shown is the locking mechanism 34 is formed by a locking flange 35 which defines a coupling groove 46. The coupling groove 46 is designed to accept the locking ring 48. The locking flange 35 has a plurality of alignment notches 50 disposed therein to facilitate the acceptance of the locking ring 48 and alignment of the acetabular prosthesis 20, further discussed herein.
FIGS. 2 illustrates a cross-sectional view of head 67 of a femoral prosthesis 68 into the acetabular prosthesis 20, along with the use of the optional constraining ring 36. It should again be noted that as the inner surface 22 of the acetabular prosthesis 20 is a highly polished hardened metal bearing surface formed from a bio-compatible material such as titanium, cobalt chrome, stainless steel, etc. The first head 67 of the femoral prosthesis 68 will articulate within the bearing cavity 40 defined by the inner surface 22 of the acetabular prosthesis 20. As can be seen, the first head 67 is inserted into the bearing cavity 40. Next, the constraining ring 36, which was previously disposed about the neck 102 of the femoral implant 68, is positioned adjacent to the-peripheral surface 30 of the acetabular prosthesis 20.
The locking ring 48 is inserted into the coupling groove 46 defined by the locking flange 35 and the constraining ring 36 is released to affix the constraining ring 36 onto the acetabular prosthesis 20. In this way, a metal-metal articulating bearing surface is formed between the inner surface 22 and the femoral head 67. As is shown, the first femoral head 67 engages the metal bearing surface 24. The locking ring 48 is positioned within the constraining ring groove 56 to fix the constraining ring 36 to the acetabular prosthesis 20, thus locking the first head 67 into its proper orientation.
As shown in
The laser beam shock induced deep compressive residual stresses in the compressive pre-stressed regions are generally about 50-150 KPSI (Kilo Pounds per Square Inch). These pre-stressed regions extend from the laser shock surfaces to a depth of about 20-50 mils into laser shock induced compressive residually pre-stressed regions. The laser beam shock induced deep compressive residual stresses are produced by repetitively firing a high energy laser beam from the laser 70 that is focused on surface which is covered with paint to create peak power densities having an order of magnitude of a gigawatt/cm2. As is known, the laser beam is fired through a curtain of flowing water that is flowed over the surface and the surface is ablated generating plasma which results in shock waves on the surface of the material. These shock waves are re-directed towards the surface by the curtain of flowing water to generate traveling shock waves (pressure waves) in the material below the surface. The amplitude and quantity of these shock waves determine the depth and intensity of the compressive stresses. The surface is used to protect the target surface and also to generate plasma. Ablated surface material is washed out by the curtain of flowing water.
a and 4b show the hardening of the acetabular component 20. The laser peened surface 23 has a surface roughness which would prevent its use as an orthopedic implant bearing. As such, further surface honing and polishing of the bearing surface using a honing instrument 73 provides a prosthetic having a bearing surface with a predetermined hardness and surface finish.
In general, a method for implanting a medical device, for example a femoral prosthetic 68, into a patient is described. Prior to implantation, at least one surface of a femoral prosthetic 68 is subjected to laser shock peening. The surface can be an internal coupling surface such as a Morse taper joint or can be the bearing surface of the femoral head. The femoral prosthetic 68, which can be a single piece or a modular component, is then implanted into a prepared joint. In this regard, the stem of the femoral prosthetic 68 is inserted into a resected femur. In the case of a modular femoral prosthetic 68, the surface hardened head 67 is coupled to the stem portion 102 of the femoral component 68.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, the teachings of the present invention are disclosed in a description of an acetabular and femoral prosthetic. It is however envisioned that the teaching are equally applicable to other joints, for example knee, ankle, or shoulder joints. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
