Total Hip Replacement

VII. TYPES OF HIP REPLACEMENT
Total hip replacement is an operation designed to replace the damaged joint. Many types of total hip replacements are currently utilized and can be considered in several different categories.

TABLE	Types of Hip Joint Replacement
Cemented	Type of Fixation
Uncemented	(implant to bone)
Total Hip
Hemiarthroplasty	How Much Bone/Joint
Surface replacement	is Replaced
Metal – polyethylene
Ceramic – polyethylene	Bearing Surface
Ceramic – ceramic
Metal – metal

There are many brands available of each category and there are hundreds of factors (e.g., type of metal, shape of implant, sterilization method, tools for insertion, etc.) that must be considered when choosing the appropriate implant in each case.



Total Hip Replacement-Stem Type with Cement Fixation
In 1962, Sir John Charnley used a small (22mm) 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. 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.



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 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.

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.



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.

BEARING MATERIALS USED IN HIP REPLACEMENT
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 28 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.



Metal-On-Metal Bearings
Metal/Metal (MM) bearings were first used in the U.S. 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 approximately 100 times. 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 (by increasing the ball size) to minimize the risk of dislocation.



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.



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 sometimes 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. 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.

However, if only one half of the joint is replaced, the degree of pain relief is not as good as for a total joint replacement. Also, if later complete joint replacement is elected, it is more difficult to perform a total surface replacement and sometimes a stem type implant is required.



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.

There are five potential advantages to these implants that make them worth considering in younger patients. First, less wear and no polyethylene debris generated at the joint surface may lead to a much longer lasting implant that could tolerate more stressful work or recreational activities. Second, less bone is removed at the original procedure, preserving normal anatomy and allowing simpler and more successful revision surgery if needed at a later date. Third, because a large ball size can be used, the stability is much improved over standard total hip replacement which means the patient will have fewer restrictions on their range of motion postop and will have a lower chance of dislocation of the hip. Fourth, thigh pain is not seen (6% in uncemented stem type total hips) because there is no stem that protrudes into the femoral canal.
Fifth, there is less blood loss during surgery and therefore, a lower chance of requiring transfusion.

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.

One possible undesirable problem with the metal on metal devices is the generation of metallic debris. We believe that the metallic wear debris, based on histological observations to date, appears to be well tolerated in the tissues. However, some people fear that this debris may be toxic to the body. However, at this point, there is no evidence to support this concern. In fact, in a recent study of over 400 patients with metal on metal implants who were observed for 15 years, showed no higher rate of cancer than the general (aged matched) population.