The prosthetic ball in the socket can dislocate because the components wear down or break, causing instability. The good news is that advances in design and materials are leading to more stable, durable components. For example, the femoral head in the newer implants is typically larger, and thus more stable, than it was just a few years ago. Also, the quality of the plastic bearing surface has improved, making it much more durable and resistant to wear. Alternate bearing surfaces have also been developed including those made of ceramics and metal. More refined surgical approaches and tissue closure have also markedly lessened the chance of a dislocation after surgery.
Leg Length Inequality
Creating equal leg lengths after surgery is a priority after THR and very much an “art”. If a patient’s legs are not equal in length after surgery due to improper implant positioning, instability and pain can result. Critical to reconstructing a stable hip with equal leg lengths is implanting the cup and stem in an optimal position in the bone and optimizing the length and tension in the surrounding soft tissues. At the Leone Center, we use the pelvic alignment level (PAL) to optimize positioning and leg length. This device, which Dr. Leone invented, has been a critical success factor in his ability to achieve consistent outcomes.
Osteolysis (Bone Loss)
Osteolysis refers to the condition when the bone that is supporting the prosthesis is being destroyed, which leads to prosthesis becoming loose and makes further revision surgery more difficult. This occurs when bones cells are stimulated to remove the bone matrix. After millions of cycles and years of use, the prosthetic ball can wear down the socket, releasing tiny particles of debris that accumulate in the tissue. The “wear” debris causes inflammation which, in turn, stimulates these cells to reabsorb the bone.
Better quality plastics and alternate bearing surfaces are reducing the incidence of osteolysis. Most modern prostheses are made of highly cross-linked polyethylene which is also helping to decrease the incidence and severity of osteolysis and resulting in improved prosthetic longevity.
Metal on Metal Implants
THR implants include a ball and socket. Various materials and combinations have been used over the years to produce these components, among them: balls made out of various metals and ceramics and cup liners made out of metals, plastics and ceramics. Today, most consider a ceramic ball articulating against a modern “highly cross linked polyethylene” socket to be the gold standard combination.
Many patients who have had metal on metal implants are now experiencing problems that require revision. We are also seeing issues with patients who have undergone metal on metal hip resurfacing. Hip resurfacing involves implanting a metal cup directly into the pelvis and covering the arthritic femoral head with a metal cap.
Theoretically, a metal on metal articulation has the potential to last longer and be more durable than a metal or ceramic ball on a plastic liner. However, these implants can fail if the ball and sock are not perfectly manufactured and positioned in the body, or if the person exceeds the mechanical limits of the THR. If lubricating fluid between the surfaces breaks down, the metal surfaces can rub together (edge loading), creating metal debris and other problems. High concentrations of metal ions can destroy tissues locally and be absorbed systemically, which can cause a host of problems.
Some patients remain asymptomatic until the destruction becomes extensive. For this reason, it is important for us to be proactive. For patients with metal on metal implants, we look for potential problems with baseline studies and have more regular follow ups than standard metal or ceramic on plastic articulations. If problems are developing, it is better to revise the hip sooner rather than later to minimize soft tissue destruction.
Modular Stem Implants
Historically, the metal ball attached to the femoral stem was made from a single piece of metal. Today, nearly all stems implanted worldwide are “modular,” that is, the ball is separate from the stem and is impacted on the taper of the stem. The ability to mix and match ball sizes and neck lengths means that the surgeon can fine tune the components for more precise hip mechanics and leg length.
Because cup plastic quality is so much better than it was years ago, larger diameter balls are being implanted on the tapers of these modular stems, which has the advantage of improving stability and reducing the risk of dislocation.
However, we are now seeing corrosion developing between the taper and the ball (taper corrosion). Debris from the corrosion can leach into the tissue, causing local tissue destruction and problems systematically (around the body). Many patients with taper corrosion require revision.
There is no clear consensus about why this is occurring, but the cause is likely multifactorial. We know it is creating very serious problems for some patients and is being widely studied. It could become a significant clinical problem worldwide because so many individuals have modular head neck junctions.
Modular Neck Implants
Modularity was taken a step farther with the advent of modular necks. The neck is the upper part of the stem with the taper that the ball is impacted on. Rather than make the neck with the taper out of one piece of metal, manufacturers made the neck and body of the stem two separate parts, again with the hope of giving the surgeon more intra-operative flexibility to optimize hip mechanics, leg length and stability. Unfortunately, this class of primary hip stems have not performed well and a large percentage of patients with these stems have required revision. In some cases, the stems have been recalled by their manufacturer. Like hips with metal on metal articulation, failure can be asymptomatic until very advanced and can result is irreparable soft tissue destruction and systemic symptoms. It for that reason more frequent follow is indicated and if revision indicated it is preferable to perform sooner rather than later.
The best treatment for infection associated with a total joint is prevention. For this reason, every effort is made to prevent a total joint from getting infected. If infection occurs, the only way to treat it is with surgery. Antibiotics alone will not cure an infection.
If the infection is acute, that is, it occurs in the first several weeks after surgery and the bacteria that is infecting the prosthetic joint is sensitive to antibiotics, then we typically re-open the hip, thoroughly and meticulously debride and irrigate the area while removing and then replacing the modular ball and plastic liner. The patient is treated with IV antibiotics during and then after surgery (typically for 6 weeks) and then PO antibiotics. This may salvage the artificial joint and clear up the infection. It may also fail.
If the infection is chronic, that is, has been present for more than several weeks, involves several organisms or is a fungus or particularly resistant to common or easily administered antibiotics), then patients typically must undergo a two stage re-operation to treat and hopefully cure the infection. The first re-operation involves removing the total hip prosthesis, extensively and meticulously debriding and irrigating the area and then implanting a “dynamic spacer,” or temporary total hip. We add large quantities of antibiotics to the bone cement, which leach out into the local tissue to treat the infection. We also give IV antibiotics (typically for 6 weeks) and then often a short course of PO antibiotics.
We perform the second operation after the antibiotic courses are complete and pre-operative studies indicate that the infection has been cured. In the second-stage surgery, we remove the temporary total hip, again irrigate and debride the joint and implant the definitive revision components. We continue antibiotics until intraoperative cultures are negative and many times longer.
While the two-stage operation is optimal for addressing chronic infection, it is not always practical or appropriate. Some patients, for example, are too frail or sick to undergo two major surgeries. For patients who have already undergone one or more revision surgeries (in which the incidence of infection is much higher than in primary THR), removing the prosthesis could preclude or compromise the ability to re-implant another. In these cases, the joint is surgically debrided and irrigated, the modular head and liner are replaced with new ones, the patient is treated with IV antibiotics and then remains on a suppressive PO antibiotic hopefully for life. Our goal in these cases is not to eradicate the infection but rather to suppress the infection so it is not expressed.