The replacement of destroyed or damaged human joints is one of the great achievements of twentieth century orthopaedic surgery. However, total joint prostheses, composed of various combinations of metal, ceramic, and polymeric components, continue to suffer from distressingly limited service lives. For example, the current generation of high-load bearing prostheses for the hip and knee have a typical lifetime on the order of 6-12 years. Generally, failed implants can be replaced once or twice, which means that current technology provides a solution for--at most--about 25 years.
With human life expectancies steadily increasing, there is a driving need to increase significantly the effective lifetime of a single implant. One of the problems encountered in designing such prostheses is the difficulty of finding materials which are both biocompatible and also durable enough to replace a human joint. In use, a human joint is exposed to substantial, repetitive loads and frictional stresses.
Although the geometric details may vary, a natural human hip, knee, or shoulder joint generally includes: (a) a more-or-less spherical ball; (b) an attachment to a long bone; and, (c) a hemispherical socket (the "acetabular cup") in a contiguous bony structure which retains the spherical ball so that the long bone may pivot and articulate. In a healthy joint, nature minimizes the friction between the joint components and prevents bone-on-bone wear and destruction by using mating porous cartilaginous layers that provide "squeeze-film" synovial fluid lubrication. This lubrication results in a low coefficient of friction on the order of 0.02.
When a human joint has been destroyed or damaged by disease or injury, surgical replacement (arthroplasty) normally is required. A total joint replacement includes components that simulate a natural human joint, typically: (a) a more-or-less spherical ceramic or metal ball, often made of cobalt-chromium alloy; (b) attached to a "stem," which generally is implanted into the core of the adjacent long bone; and (c) a hemispherical socket which takes the place of the acetabular cup and retains the spherical ball. This hemispherical socket typically is a metal cup affixed into the joint socket by mechanical attachments and "lined" with UHMWPE so that the ball can rotate within the socket, and so that the stem, via the ball, can pivot and articulate.
One of the difficulties in constructing any device for implantation into the human body is the need to avoid an adverse immune response. The possibility of an adverse immune response is reduced when certain synthetic materials are used. Cobalt-chromium alloy, titanium, and UHMWPE are examples of such synthetic materials. Unfortunately, the use of UHMWPE in the bearing of a total joint replacement may be the cause of at least one type of failure of such devices.
Three basic problems may cause a total joint replacement to fail or to have a limited service life. The first problem, which manifests itself at the bone-stem interface, is not the focus of the present application. Because the elastic modulus of the stem greatly exceeds that of the bone, flexural loading caused by walking creates local cyclic stress concentrations due to the non-compliance of the stem. These stresses can be intense and even severe enough to cause death of local bone cells. If this occurs, pockets of non-support are created, and the stem may loosen or fail.
The two other basic problems, which are coupled, are the subject of the present application. One of these problems, known as ball-cup friction and wear, results from frictional wear between the hemispherical bearing (which is "lined" with UHMWPE) and the polished spherical ceramic or metal ball attached to the stem. The other problem, known as sub-surface fatigue, results from brittleness of the UHMWPE bearing and the resulting tendency of the UHMWPE bearing to fail under reciprocating applied loads.
For many years, the acetabular cup in joint implants has been "lined" with UHMWPE, or other materials, in order to decrease the coefficient of friction of the socket or bearing. Unfortunately, clinical experience has shown that, at least when UHMWPE is used to line the bearing, either the surface of the UHMWPE "bearing" and/or the surface of the metal/ceramic ball ultimately is destroyed by friction-induced wear. Alternately, the acetabular cup loosens after a period of use, greatly increasing ball-cup friction and wear.
Some insight into the cause of failure due to ball-cup friction and wear has been gleaned from histological studies of the surrounding tissue. These histological studies show that the surrounding distressed tissue typically contains extremely small particles of UHMWPE which range from sub-micrometers to a few micrometers in size. Larger particles of UHMWPE appear to be tolerated by the body, as is the solid bulk of the UHMWPE bearing. However, the body apparently does not tolerate smaller particles of UHMWPE. In fact, these small particles of UHMWPE cause powerful histiocytic reactions by which the body unsuccessfully attempts to eliminate the foreign material. Agents released in this process attack the neighboring bone to cause "wear debris-induced osteolysis" which, in turn, leads to a loss of fixation and loosening of the prosthesis due to "remodeling" of the bone.
The first step in the generation of the small particles of UHMWPE appears to be the formation of a very thin layer of polyethylene between the spherical ball and the UHMWPE lining of the bearing. This thin film of polyethylene adheres to the "ball" and serves as a soft, shearable, solid lubricant composed of millions of submicrometer particles. Adhesive wear between the ball and the bearing produces strong, adhesive junctions on the ball. When exposed to further friction, fibrils of the polymer shear off of these adhesive junctions and are drawn into slender connecting ligaments, eventually producing ligament rupture.
This ligament rupture apparently produces the lubricous, extremely small particles of UHMWPE which eventually migrate to the bone-acetabular cup bond line. The reason for migration of these particles into the "crevice" between the ball and the cup are the microcurrents that are generated in the synovial fluid by joint motion. Once a sufficient number of small particles enter the bone-cup crevice, the bone tissue begins to degrade and the joint replacement eventually loosens and fails.
One way to reduce friction between the metal and UHMWPE components would be to coat one or both of the components with diamond-like carbon (DLC), which is chemically inert, biocompatible, and is known to have a low coefficient of friction. Unfortunately, the very properties of DLC that make it a desirable coating for parts that will be frictionally engaged make it difficult to achieve strong adhesion of the DLC coating to the substrate, particularly where deposition temperatures must be low. This limited adhesion problem can be exacerbated by very high compressive stress, such as that found in a plasma-deposited DLC (up to 8 GPa). Therefore, some have concluded that DLC--or at least plasma-deposited DLC--cannot be used in orthopaedic applications.
Energetic ion beam-associated DLC has a far lower residual stress than plasma-deposited DLC, and is a better candidate for a high integrity DLC. The substrate material to which all forms of carbon adhere most successfully is silicon. This is because strong covalent Si--C bonds are easily formed between the coating and the silicon substrate. Some have attempted to improve the adhesion of DLC to other materials, such as metal alloys, by forming an interposed silicon bond-coat to which the DLC will adhere more strongly.
Unfortunately, this simple approach does not result in adhesion that survives in applications, such as orthopaedic applications, where the DLC coating is subjected to substantial friction and stress. The simple formation of a silicon bond-coat on a metal alloy appears to create another relatively weak interface between the silicon and the metal or alloy.
Therefore a method is needed by which a DLC coating can be strongly adhered to a metal surface, and by which the shearing of polymer fibrils from an UHMWPE component can be prevented. The method would be most efficient if it rendered the UHMWPE compound less brittle so that sub-surface fatigue failure was reduced.