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Total hip replacement is an operation designed to replace the damaged hip joint. Various prosthetic designs and types of procedures are available to the surgeon. Our surgeons carefully evaluate the patient to: 1) determine if surgery is indicated; 2) determine the most appropriate type of procedure; and 3) develop a plan of treatment. The types of replacement, methods of fixation and new alternate bearing materials are discussed below.
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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| Top 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.
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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Figure 3A |
Figure 3B |
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Top 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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Top 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.
Top 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.
Top 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. More: Ceramic-on-Metal IDE Study
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Hemi-Surface Replacement for Osteonecrosis
One option to minimize wear debris and tissue reaction is to eliminate the bearing by replacing only the diseased part of the joint. A hemi-surface replacement is often recommended for patients who have osteonecrosis of the femoral head (also referred to as avascular necrosis) and have some remaining articular cartilage on the acetabulum or pelvic side (Figure 5). The hemi-surface replacement preserves and maintains bone by providing physiological stress transfer to the femoral neck and proximal femur. It avoids inflammatory reaction and loosening due to polyethylene wear debris.
Beginning in 1981, custom hemi-surface devices were inserted utilizing a titanium alloy which is a relatively soft metal and scratches easily. These devices have been surprisingly successful with many still functioning over 16 years even in young patients whose average age was 32 years.
In 1996, newly designed components and instruments became available and are now being used in many centers in the United States and internationally. Although the durability depends on the quality of the cartilage at the time of surgery, it is possible that even longer durability may be achieved with the new, harder surface cobalt chrome components which do not scratch easily. The technique is exacting and does require precision fitting of the hemi-arthroplasty to the articular cartilage of the pelvis. Patients who have had fractures of the neck of the femur require a stemmed hemi-arthroplasty. Surface hemi-arthroplasty has definite advantages over stem-type hemi-arthroplasty for patients with osteonecrosis because of its conservative nature.
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Figure 5 |
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Top Surface Replacement of the Hip
In surface replacement, the femoral neck is preserved rather than amputated as is done in conventional stem-type total hip replacement. The femoral head is reshaped and resurfaced with a prosthetic shell. As a result, the femoral bone is loaded more like a normal hip and the bone is preserved. Since the resurfaced head is very similar in size to the normal hip (about 40-50 mm), it is more stable and dislocation risk is minimal. An example of a surface replacement is shown in Figures 6A and 6B.
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Figure 6A |
Figure 6B |
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Several different hip resurfacing systems were introduced in five countries in the early 1970s. They were implanted in young patients who were expected to require more than one replacement in their lifetime because they were thought to be more conservative devices than the conventional replacements. Some surface replacements with polyethylene have had long-term durability of up to 18 years thus far. However, because of the large diameter ball size of the surface replacement, there was more polyethylene bearing wear (the debris accumulation which undermines the fixation) which results in loosening of the prosthesis. Most surgeons abandoned surface replacements with polyethylene in the 1980s and early 1990s. Despite the fact that the durability was often less than desired, the femoral bone preserved by these systems was especially valuable for these young patients when revision surgery was required making that second surgery comparable to primary replacement.
The conservative and more physiologically compatible nature of the surface replacement has always been appealing to both surgeons and patients. There is renewed interest which has been fostered by the reintroduction of all-metal bearings which could dramatically increase the durability. The first of these metal/metal surface replacements was introduced in Germany and subsequently in England and at the JRI in the early 1990s. The instruments and design have been improved using modern techniques to further reduce wear and to facilitate the procedure. The major advantage of the surface replacement is in the preservation of the femoral head and neck. Further, unlike the acetabular reconstruction with the earlier designs which contained polyethylene bearings, very little bone is removed and the procedure is now conservative on the acetabular pelvic side of the joint as well. In short, no "bridges are burned" with the surface replacement procedure.
Surface replacement may also permit higher levels of post- surgery activity with fewer downside risks than does a stem-type device. The increased stability is particularly conducive for sports and work activities where a more normal range of motion of the hip is required.
The lessons that we have learned regarding design and technique issues during the past 25 years combined with the modern precision manufacturing of metal/metal bearing surfaces have led to a very much improved device. Although it is impossible to predict how much increased durability will be achieved, the volumetric wear reduction is substantial, and it is unlikely that these devices will "wear out". We also believe that the metallic wear debris, based on our histological observations to date, appears to be well tolerated in the tissues. Potential long term undesirable consequences of these devices are unknown. However, they have been successfully implanted for over thirty years so we believe the risk is minimal. Top
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