Cement Fixation

In 1962, Sir John Charnley used a small (22mm) stainless steel ball on a long 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 (methyl methacrylate).

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. Stems are now made of either cobalt chrome or titanium. The socket component is usually titanium with a bone ingrowth porous surface and an exchangeable bearing liner. The liner can be made of polyethelene (plastic), cobalt-chrome (metal), or ceramic. Sockets fixed with cement have largely been abandoned in the US. Cement fixation of the stem is now used in less than 10% of hip replacements, usually in older weaker bone.

Bone Ingrowth Fixation

We are now in an era with widespread use of devices that 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 rough or porous surface. This process is called bone ingrowth or osseointegration. The bone must be prepared precisely for these devices. For successful bone ingrowth to occur the implant must achieve an initial stable press-fit when implanted and the porous coating must sit right up against live bone. In general, these devices are larger, longer, and stiffer 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 now generally accepted that these implants remain fixed to bone longer than cemented devices. The theoretical downside is occasional failure of bone ingrowth, but this is a very rare problem, except possibly for very old weak bone. There is one other problem with these implants. In 2-5% of patients activity related thigh pain may develop. Even with well-fixed implants, there can be pain due to the stiffer modulus of these implants. There is no solution to this problem.

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.

All artificial bearings create wear debris, just as the rubber wears off your tire going down the road. This wear debris is deposited in your body. If the load is small, you can usually tolerate it well for many years. Our goal is to use implants that generate the least quantity of wear debris as well as the type of debris that results in the least tissue reaction. The original Charnley bearing was stainless steel ball against plastic (polyethelene). This is no longer used. Modern alternative bearings have about 100X lower wear rates than the older cobalt chrome against standard polyethelene bearing.

Metal on Plastic

Metal on plastic are the most commonly used combinations. Plastic wear debris is deposited around the hip joint and may travel along lymph channels. But it does not leave the body. In young active people with standard plastic (polyethelene, SPE) liners, enough plastic debris is released to cause severe bone destruction in 30% of patients within 8 years of implantation. Newer cross-linked liners polyethelene (XLPE) have dramatically lowered this problem in small early studies out to 10 years. When ceramic balls are used against the plastic, wear debris seems to be further reduced.

Smaller bearing diameters produce less wear but result in higher risk of dislocation. Larger bearings were not possible with SPE for this reason. Because XLPE has better wear properties, larger heads are now being used. Although XLPE wears better, it is often more brittle. As the bearing size increases, the liner thickness decreases. More brittle XLPE may therefore be more subject to breakage rather than wear. Also, larger heads have recently been associated with trunion (where the head is attached to the stem) corrosion. Therefore, there is much disagreement about ideal bearing size because of competing problems: wear/breakage vs. instability.

Ceramic on Plastic

Ceramic on plastic has similar characteristics as above but probably cuts the wear rate in half.

For the standard 28mm bearing size dislocation risk is about 5% within 1 year, while it drops to 1% for 36mm bearings. By 10 years follow-up dislocation risk nearly doubles. About half of dislocations are recurrent and require revision surgery. The most common reason for revision hip surgery is instability.

Ceramic on Ceramic

Ceramic on ceramic bearing produces the least quantity and best tolerated wear debris of all bearings. Ceramic surfaces are brittle and can fracture, especially with impact activity. This is now exceedingly rare (less than 1:10,000). When these implants are placed in nonideal positions, they may exhibit a stipe wear pattern and emitting a loud squeak that can be head across the room. This can be very unpleasant and require revision surgery. This occurs in 1-2 % of cases, but can probably be resolved by better implant positioning and larger ceramic bearings.

The main problem with ceramic bearings is their size. The same instability problems exist with the standard 28mm bearing size. Larger sizes are now in use with a new stronger ceramic called Biolox. This will reduce the dislocation rate, but will require thinner ceramic socket liners. Time will tell if these will be equally fracture resistant as the thicker alumina ceramic liner that have 10 year data.

Metal on Metal

Metal on metal bearings made of cobalt chrome 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.

We have learned that for optimal function, there needs to be less than 5 um residual roughness and a polar bearing arrangement with a 50-100um radial clearance. Cobalt chrome is the only metal that works. Trace amounts of molybdenum and Nickel are present in this alloy. There is still controversy about ideal metallurgy (cast vs. forged, high vs. low carbon content, heat treated or not) but the most commonly used is cast, high carbon non-heat-treated. M/M devices were reintroduced in Europe in 1988. There are now U.S. manufacturers as well as European firms manufacturing all-metal bearings. The reaction of our body against excess metal debris results in more soft tissue inflammation while plastic causes more bone destruction (osteolysis).

Metal bearings are so strong that very thin (4mm) socket components can safely be built without any risk of fracture. Also a bone ingrowth layer can be directly attached to this implant. Thin, strong, one-piece sockets allow reconstructing the hip joint with a natural bearing size, virtually eliminating hip instability, the most common complication of this surgery. In combination with similar thin femoral components, hip resurfacing is made possible.

Despite laboratory studies showing minimal wear, high wear states resulting in metalosis (excess metal in the tissues) have now been reported in patients. The incidence of this adverse wear failure (AWF) problem varies; in my experience it has been a cause of failure in 1% of cases at 10 years. In my experience, revision for AWF is no more difficult than revision for other failure modes. We have now learned the proper acetabular component positions to completely avoid this problem. It turns out that the problem is caused primarily by two factors, socket components that are designed very shallow as well as steep socket component inclination position. The combination of these problems results in edge wear releasing excess metal debris. (discussed more elsewhere)

Ceramic on Metal

Ceramic on metal has shown slightly lower metal wear in laboratory studies, but this has not been confirmed in the clinical studies.

Stemmed Total Hip Replacement

Stemmed total hip replacement is by far the most commonly used. John Charnley was the first to make this a routinely successful operation in the 1960’s. Numerous modifications have occurred since his time. Several millimeters of bone is removed from inside the socket and a metal implant with a porous ingrowth surface is tightly implanted into the prepared bone bed. A bearing liner is then locked into place in the implanted metal shell. This leaves a much smaller cavity for the ball.

Therefore the head and neck of the femur must be amputated. A stem is then fixed into the hollowed marrow canal of the top of the femoral shaft using either cement or bone ingrowth technique. A smaller (than natural) ball is then attached to the trunion of the stem (morse cone taper junction). This ball fits into the smaller socket liner.

Hip Surface Replacement

Hip surface replacement accomplishes the same basic goal as sTHR with much less bone removal and preservation of normal biomechanics. Bearing size and femoral offsets remain the same as for the normal femur. This was tried in the 1950’s by Charnley with Teflon implants, by others with primitive metal on metal bearings, and in the 70’s with metal on plastic bearings. Finally Derek McMinn applied modern metal on metal bearings to resurfacing in the 1990’s. Mainly because the femoral head is preserved, it is much more difficult for the surgeon to get adequate access to accurately place the deeper socket component.

In stemmed THR, the head and neck are amputated early in the operation allowing much easier access to the deeply placed socket. This technical difficulty the primary reason why many hip surgeons are reluctant to perform this operation. It has been demonstrated in numerous scientific papers that the complication rate is much higher when surgeons are learning this operation. This learning curve extends for several hundred cases. Difficulty in placing the socket component accurately is one of the major contributing factors to recent problems with adverse wear failure.

Hemi-Surface Replacement for Osteonecrosis

One option to minimize wear debris and tissue reaction is to eliminate the artificial bearing by replacing only the diseased part of the joint. A hemi-surface replacement was sometimes recommended in the past for patients who had osteonecrosis of the femoral head (also referred to as avascular necrosis) and had intact 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 any artificial bearing wear debris. However, if only one half of the joint is replaced, the degree of pain relief is not as good as if both sides of the joint are replaced. It is not always possible to convert this to a total hip resurfacing. I do not advise the use of this operation.