The skeletal system includes many long bones that extend from the human torso. These long bones include the femur, fibula, tibia, humerus, radius and ulna.
A joint within the human body forms a juncture between two or more bones or other skeletal parts. The ankle, hip, knee, shoulder, elbow and wrist are just a few examples of the multitude of joints found within the body. As should be apparent from the above list of examples of joints, many of the joints permit relative motion between the bones. For example, the motion of sliding, gliding, and hinge or ball and socket movements may be had by a joint. For example, the ankle permits a hinge movement, the knee allows for a combination of gliding and hinge movements and the shoulder and hip permit movement through a ball and socket arrangement.
The joints in the body are stressed or can be damaged in a variety of ways. For example, gradual wear and tear is imposed on the joints through the continuous use of a joint over the years. The joints that permit motion have cartilage positioned between the bones providing lubrication to the motion and also absorbing some of the forces direct to the joint. Over time, the normal use of a joint may wear down the cartilage and bring the moving bones in a direct contact with each other. In contrast, in normal use, a trauma to a joint, such as the delivery of a large force, from an accident, for example, an automobile accident, may cause considerable damage to the bones, the cartilage or to other connective tissue such as tendons or ligaments.
Arthropathy, a term referring to a disease of the joint, is another way in which a joint may become damaged. Perhaps the best known joint disease is arthritis, which is generally referred to a disease or inflammation of a joint that results in pain, swelling, stiffness, instability, and often deformity.
There are many different forms of arthritis, with osteoarthritis being the most common and resulting from the wear and tear of a cartilage within a joint. Another type of arthritis is osteonecrosis, which is caused by the death of a part of the bone due to loss of blood supply. Other types of arthritis are caused by trauma to the joint while others, such as rheumatoid arthritis, Lupus, and psoriatic arthritis destroy cartilage and are associated with the inflammation of the joint lining.
The hip joint is one of the joints that is commonly afflicted with arthropathy. The hip joint is a ball and socket joint that joins the femur or thighbone with the pelvis. The pelvis has a semispherical socket called the acetabulum for receiving a ball socket head in the femur. Both the head of the femur and the acetabulum are coated with cartilage for allowing the femur to move easily within the pelvis. Other joints commonly afflicted with arthropathy include the spine, knee, shoulder, carpals, metacarpals, and phalanges of the hand.
Arthroplasty as opposed to arthropathy commonly refers to the making of an artificial joint. In severe cases of arthritis or other forms of arthropathy, such as when pain is overwhelming or when a joint has a limited range of mobility, a partial or total replacement of the joint within an artificial joint may be justified. The procedure for replacing the joint varies, of course, with the particular joint in question, but in general involves replacing a terminal portion of an afflicted bone with a prosthetic implant and inserting a member to serve as a substitute for the cartilage.
The prosthetic implant is formed of a rigid material that becomes bonded with the bone and provides strength and rigidity to the joint and the cartilage substitute members chosen to provide lubrication to the joint and to absorb some of the compressive forces. Suitable materials for the implant include metals and composite materials such as titanium, cobalt chromium, stainless steel, ceramic and suitable materials for cartilage substitutes include polyethylene, ceramics, and metals. A cement may also be used to secure the prosthetic implant to the host bone.
A total hip replacement, for example, involves removing the ball shaped head of the femur and inserting a stem implant into the center of the bone, which is referred to as the medullary canal, or marrow of the bone. The stem implant may be cemented into the medullary canal or may have a porous coated surface for allowing the bone to heal directly to the implant. The stem implant has a neck and a ball shaped head, which are intended to perform the same functions as a healthy femur's neck and a ball shaped head. The polyethylene cup is inserted into the acetabulum and has a socket for receiving the head on the stem implant.
The polyethylene cup may be positioned directly into the acetabulum. Preferably, the polyethylene cup is secured to a metal member, which is in turn secured to the acetabulum. This metal member is typically called a cup or a shell. The cup or shell may include a porous coating for promoting bony in-growth to secure the shell to the acetabulum. Alternatively or in addition the shell may include an opening or a plurality of openings for receiving bone screws to assist in the attachment of the shell to the acetabulum. As an alternative to the polyethylene cup, a cup of a different material may be inserted into the shell. For example, the cup may be made of a metal, for example, cobalt chromium, stainless steel, or titanium. Alternatively, the cup may be made of a ceramic.
More recently, the polyethylene cup as a hip cup prosthesis has been replaced with a more rigid component. For example, in more recent hip cup prostheses, the cup is made of, for example, a metal or a ceramic. The head may be made of a metal or a ceramic. For example, the cup may be made of a ceramic and the head may likewise be made of a ceramic. Alternatively, the cup may be made of a metal and the head may likewise be made of that similar metal. It should be appreciated that a ceramic cup may be utilized with a metal head and a metal cup may be utilized with a ceramic head.
Metal on Metal (MoM) hip joint prosthesis achieve very low wear rates. Steady state wear rates are negligible. The vast majority of wear debris generation occurs during the so called break-in phase. During this phase, the metal surfaces are thought to “run-in” producing a highly conformal joint that is efficiently lubricated by synovial fluid components. It would be of great benefit to eliminate this break-in wear period.
Wear rates of articulating surfaces are in large part determined by the lubrication regime that may be established between the two surfaces. Three lubrication regimes are often described in the literature: Partial or Boundary, Elastohydrodynamic, and full Hydrodynamic. The friction and wear behavior characteristic of these lubrication regimes is illustrated in FIGS. 22 and 23, respectively, taken from “Fundamentals of Fluid Film Lubrication” by Bernard Hamrock (NASA publication 1255, dated 1991).
To maximize the life of the prosthesis, the accuracy of the dimensional characteristics of the components of the prosthesis as well as the surface condition, for example the surface finish, is extremely critical in the life of the prosthesis. Dimensional errors and surface finish imperfections may cause the prosthesis to prematurely wear. The components that wear on the prosthesis, particularly those that wear rapidly, may lead to reactions with the tissues of the body. Such reaction to foreign objects is called osteolysis. Osteolysis can damage soft tissue and further complicate the replacement of the prosthesis.
Attempts have been made to provide for improved finishes and geometries of the articulating surface of a prosthesis. For example, the surfaces may be polished by hand by, for example, a rubbing compound or by a metal or cloth buffing wheel. Alternatively, the surfaces may be smoothed by robotic manipulators using similar tools as are used by hand. Alternatively, the components have the articulating surface of the prosthesis may be polished by a finishing device, for example a RotoFinish® tumbling machine. These prior art attempts at providing improved geometry and finish to the articulating surface of a prosthetic component are slow and inaccurate. Further, attempts to improve the finish on the part may affect its geometry or shape. Imperfections in shape and or finish may greatly reduce the operating life of the prosthesis and may lead to osteolysis.
Processes have been developed for improving surfaces of optical components. For example a Magnetorheological Polishing fluid (hereinafter referred to as “MP-fluid”) may be used in a computer controlled machine to polish optical components. The fluids are mixtures of abrasive particles and magnetic particles. The abrasive particles are in suspension and magnetic particles are in suspension in a fluid. The magnetic particles are coated with Teflon®, a trademark of E.I. DuPont de Nemours and Company, to protect them from degradation. These particles could be suspended in solutions of glycerin, glycol, water, oil, alcohol, or mixtures thereof. When a magnetic field is applied, the magnetic particles create a plastic zone, and the abrasive particle provide for polishing action. The fluids are used in manufacturing equipment that utilizes the MP-fluid finishing process is commercially available from QED Technology, Inc., Rochester, N.Y. and sold as the Q-22MRF System.
Another process has been developed by University College, London WC1E6BT, England and Zeeko Ltd., Precise Group, The Stables, East Lockinge, Oxfordshire, OX12 8QJ, England. These efforts are more fully described in an article entitled “The ‘Precessions” Tooling for Polishing and Figuring Flat, spherical and Aspheric Surfaces” published in the Apr. 21, 2003 Optics Express, Vol. 11, No. 8, hereby incorporated herein in its entirety by reference.
The process is known as the Precessions™ process. Machines incorporating the Precessions process may be acquired from Satisloh North America Inc., N116W18111 Morse Drive, Germantown, Wis. 53022 USA. Machines are available from Zeeko Ltd, The Stables, East Lockinge, Wantage, Oxfordshire, OX12 8QJ, United Kingdom.
The baseline of the Precessions process is a physical sub-diameter tool operating in the presence of a polishing slurry, The tooling is hosted by a 7-axis CNC polishing machine available from Satisloh North America Inc. The tool comprises an inflated, bulged rubber membrane of spherical form, covered with one of the usual proprietary non-pitch flexible polishing surfaces familiar to opticians. The tool is rotated. The rotation axis of the tool is inclined to the surface to be polished at an angle of typically 10 to 25 degrees.
Partial or boundary lubrication is characterized by extremely thin lubrication films and considerable metal to metal contact. At the other end of the spectrum, in hydrodynamic lubrication, the load is fully supported by a layer of lubricant that prevents the surfaces from contacting. Wear rates are negligible in this latter regime. Lubrication fluid film thickness is proportional to velocity and inversely proportional to load. Full hydrodynamic lubrication is considered to be achieved when the ratio of film thickness to surface roughness exceeds a value of about 3. (Johnson K L, Greenwood J A, Poon S Y: A simple theory of asperity contact in elastohydrodynamic lubrication. Wear 19:91-108, 1972), incorporated herein in its entireties by reference.
This ratio is defined as lamda=λ=H/Rq where Rq=(Rqa2+Rqb2)1/2 where Rqa and Rqb are the rms surface roughness of the two articulating surfaces. This equation shows that smoother surfaces will achieve full hydrodynamic lubrication at lower velocities and higher loads than rougher surfaces. For example, if two surfaces have a λ=1 with Rq's of 0.01 um, we can achieve a λ of 10 by reducing the rms surface roughness to 0.001 um.
The rougher surface would be operating in a clear boundary layer lubrication regime while the smoother surface would be completely in the hydrodynamic regime at the same speed and load. This benefit has generally been appreciated in the orthopaedic industry and considerable attention is paid to maintaining low errors of form and low surface roughness in articulating metal on metal hip components. The subject has been documented particularly well by Chan (F. W. Chan, D. Bobyn, J. B. Medley, J. J. Krygier, M Tanzer, “Wear and lubrication of metal on metal hip implants” Clinical Orthopaedics and related research number 369, pp 10-24, 1991)), incorporated herein in its entireties by reference.
FIG. 2, taken from this reference shows the experimental results they obtained on the wear of metal on metal hip implants in a hip wear simulator. From the data presented in this paper, one can predict a 53% reduction in wear of Metal on Metal Hip implants if Ra surface roughness can be reduced from present industry values of about 10 nm to 2 nm.
On the other hand, little progress has been made in reducing surface finish values below 10 nm in a production environment. Finishing procedures generally consist of polishing the finished femoral and acetabular components with a fabric impregnated with fine abrasive materials that remove material by strictly mechanical means. The polishing process causes some plastic working of the surface as metal is removed. A mechanically polished surface yields an abundance of scratches, strains, metal debris and embedded abrasives, and always distorts the metal surface. Burnishing metal by lapping or buffing decreases the rms roughness of a surface, but it never completely removes the debris and damaged metal caused by mechanical polishing.
Additionally, the microstructure of typical CoCrMoC alloys used in orthopaedics is generally two phase consisting of the matrix plus metal carbides. The later are significantly harder than the matrix. The result is that mechanical polishing generally leaves the harder carbide phases sticking out somewhat from the metal matrix. This contributes to a higher rms surface roughness than otherwise could be obtained.
The present invention is adapted to solve at least some of the aforementioned problems with the prior art.
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