Stem Type without Cement Fixation
Stem Type with Hybrid Fixation
Bearing Materials Used in Joint Replacement
Metal-on-Metal Bearings
Ceramic-on-Ceramic Bearings
Total Hip Replacement
Stem Type with Acrylic Cement Fixation
In 1962, Sir John Charnley used a small (22 mm) stainless steel ball on a stem which was inserted into the bone to replace the femoral (ball) side of the joint and a high density plastic socket to replace the acetabular (socket) side. Both of these components were secured to bone with a self-curing acrylic polymer commonly referred to as bone cement. Several generations of designs have evolved from this original Charnley prosthesis. The ball is now modular thereby allowing balls of different sizes, materials and neck lengths to be placed onto the stem. Most balls are now made of either a cobalt chrome metal alloy or a ceramic material (Figures 2A and 2B). Results include consistent pain relief due to immediate fixation and rapid recovery with early weight bearing. It has been the general experience, however, that the long term results of cemented total hip replacements in young, active and/or heavy patients are not as consistently durable as desired. The loosening rate of cemented acetabular components increases with time leading to many failures after 10 or 15 years. For these reasons, cementless fixation has been advocated by some for younger or more active patients.
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Figure 2A |
Figure 2B |
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Total Hip Replacement
Stem Type without Cement Fixation
We are now in an era with widespread use of devices which are designed to attach to bone without the use of cement. Bone will attach to a metal implant if the surface of the metal has a certain "topography". This process is called porous ingrowth or osseointegration. The bone must be prepared precisely for these devices because close apposition to bone is necessary for bone to grow up to the smooth surface (osteointegration) or into the pores of the porous surfaces (porous ingrowth). In general, these devices are larger and longer than those used with cement but are proportional to the size of the individual bone. Surface coatings, such as hydroxyapatite, are also being utilized in an effort to hasten and/or enhance bone fixation. An example of this type of device is shown in Figures 3A and 3B.
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Figure 3a |
Figure 3b |
Many different devices using cementless fixation have been utilized since their introduction in the U.S. in 1977. It is hoped that these devices will maintain their attachment to bone longer, but some caution is advised in their application. Complete pain relief after surgery is not as predictable as with cemented stems. This is related to the type of cementless hip prosthesis and the patient’s anatomy, although most improve with time as fixation becomes more rigid. Candidates for these devices are generally younger and more active than those for cemented application.
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Total Hip Replacement
Stem Type with Hybrid Fixation
Hybrid fixation is when one component is inserted without cement, usually the socket, and one component is inserted with cement, usually the stem. (Figures 4A and 4B)
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Figure 4a |
Figure 4b |
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Bearing Materials Used in Joint Replacement
Durability is dependent on the components used (materials, type and preparation of the surfaces, as well as the design of the components), technique and the quality of fixation, activity level of the patient, and the biological tissue reactivity which varies among individuals. The most commonly used bearing combinations in joint replacement today are metal or ceramic against ultra high molecular weight polyethylene. These combinations have functioned well for most patients. The durability is less in younger patients because of higher activity levels. The fine particulate debris that is produced causes tissue reaction. This process can undermine fixation and result in loosening. While there is undoubtedly variability in individual tissue reactivity to debris, there is no known methodology to evaluate and determine in advance which patients will react more severely. Since polyethylene wear is proportional to the ball size of the femoral head, it is recommended that the ball size should be reduced to 22 mm (roughly one-half to one-third that of the normal hip) to minimize wear for young and active individuals. However, the use of the small ball can produce instability problems in some individuals who have a greater amount of flexibility in their joints especially if the components are not optimally positioned.
Because of the known deleterious effects of wear debris, research has begun in an effort to minimize the wear of ultra high molecular weight polyethylene. However, it will be many years before we can determine the success of these developments.
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Metal-on-Metal Bearings
Metal/Metal (M/M) bearings were first used in the United States when joint replacement began in the late 1960s. The component design and fixation techniques were primitive by today’s standards. Further, the bearing manufacture was inconsistent and these devices were discontinued in the 1970s. Now with modern technology, bearing surfaces can be made optimally smooth and round and thus the wear is minimized. Volumetric wear, compared to polyethylene, can be reduced between 20 and 100 times depending on ball size. It is also possible that the wear will be reduced even further as research into this aspect intensifies. M/M devices were reintroduced in Europe in 1988. There are now U.S. manufacturers as well as European firms manufacturing all-metal devices.
In addition to reduction in volumetric wear, the biological tissue reaction locally, based on observation periods of up to 30 years, is less inflammatory, and therefore, less likely to undermine the component’s fixation. With metal/metal bearings, unlike metal/polyethylene bearings, there is no penalty for increasing the ball size. Therefore, it is possible to safely improve the stability to minimize the risk of dislocation.
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Ceramic-on-Ceramic Bearings
All alumina-ceramic bearings have been utilized in Europe since the early 1970s. A problem with the early ceramic materials was its large grain structure which led to fractures. Manufacturing of ceramics is now much improved with small grain size creating a much stronger material. These bearings also produce low wear similar to that of metal-on-metal bearings with substantial reductions over plastic bearings. Because of concerns related to the strength of the material, the shells must be made thicker in order to minimize fracture and, therefore, surface replacements are not feasible. The new generation components are much improved for stem-type devices. The all-alumina bearings are another option in the effort to minimize wear and tissue reaction and to provide longer term durability. However, the components must be optimally manufactured to minimize the risk of fracture and inserted precisely to minimize wear.
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Total Hip Replacement
Orthes, spol. s r.o., Palackého 481, 756 61 Rožnov p./R., tel.: +420 651 757940-4, Fax: +420 651 620923, e-mail: orthes@orthes.cz
Copyright
ORTHES, spol. s.r.o. 2001