Resurfacing total hip replacement is a variation of hip replacement that conserves the femoral neck (the area just below the ball of this ball and socket joint) as well as a portion of the femoral head (ball) itself. It differs from traditional total hip replacement in that the worn out or arthritic surfaces of the hip joint are the only portions that are replaced.

Traditional THR

Resurfacing THR

To duplicate the action of a ball and socket hip joint, a resurfacing implant has 2 parts. The femoral component, which fits on top of the femur (thigh) bone, is made from metal and is cemented in place. The shell or cup is also made from metal; it replaces the worn out acetabulum. Bone grows onto the outside of this component; the inner side is highly polished and articulates with the ball.

	Radiograph (x-ray) of a resurfacing total hip replacement. Both parts of the joint are made from wear resistant metal.
Photograph of a Birmingham Resurfacing Hip Replacement. These joints are “polar” bearing and depend on fluid lubrication.

The traditional bearing surfaces used in hip replacement surgery were metal and plastic. Although the very first hip replacement surgeries used varied bearing materials, a polished metal ball and high molecular weight polyethylene cup became the standard for the first three decades that this surgery has been offered. Over the last five years alternative bearings have been used in younger, active patients. Alternative bearings are those that are considered highly wear-resistant and are an “alternative” to conventional polyethylene (plastic). The exact “best” option is unknown, and the choice of bearing surface is controversial. The main reason for this uncertainty is that the results of laboratory testing of the various surfaces do not always translate to similar findings when the new materials are used in people. The newest technologies and techniques do not have enough follow-up to know which, if any, will be better than the current gold standard. At Orthopaedic Associates, clinical follow-up studies in ceramics, metals, and crosslinked polyethylene are ongoing.

Surface choices can be broken down into hard-on-soft and hard-on-hard bearings. In general, hard-on-soft bearings demonstrate more linear wear for the same time period as hard-on-hard bearings, but are likely less susceptible to catastrophic failure. Hard-on-soft couples include not only cobalt-chrome on polyethylene, but also ceramic or ceramic like materials on polyethylene. Hard-on-hard surfaces demonstrate less linear wear in most cases but are likely more sensitive to failure due to surgical technique (e.g., component positioning, third body debris, etc). Hard on hard bearings include metal-on-metal and ceramic-on-ceramic. Transition forces between prosthesis and bone may be elevated in hard-on-hard couples, as there is no dampening effect of a soft material. Micro-separation, a newly discussed concept where the ball and socket separate slightly during the swing phase of gait, may change the expected wear profile of hard-on-hard surfaces. Resurfacing hip replacement is a hard-on-hard articulation made from metal-on-metal..
 

Metal-on-Metal. Although metal-on-metal couples were originally introduced in the 1960s, a poor understanding of optimal design characteristics and limitations of the manufacturing process caused poor results. Second-generation metal-on-metal products were developed to address problems such as loosening of the hip replacement, high frictional torques and seizing of the articulation. Important design characteristics appear to be use of a hard, high-carbon alloy; the goals of polar contact, optimal clearance, and maximal spericity; and extremely low surface roughness. Despite a lower volume of wear associated with metal-on-metal implants, the particles that are produced are very small, possibly resulting in a larger number of particles. This may be of some concern because the full biological response to metal particles or ions is currently unknown. Rare reports of a lymphocyte dominated immunological response requiring early revision have been published and there is a chance of metal hypersensitivity. Metal toxicity and carcinogenesis have been explored and elevated urine and blood levels of the metals that make up the prosthesis have been measured. Ongoing research will confirm the safety of this combination, although metal on metal surface articulations are both FDA approved in the United States and popular in Europe. This issue seems to be most important in patients with poorly functioning kidneys and women of child-bearing age, as the metal ions can build up in the blood and cross the placenta.

Indications. Resurfacing hip replacement may be considered for younger, active patients requiring hip replacement for osteoarthritis or inflammatory arthritis. Osteonecrosis patients may also be considered for resurfacing, but the literature reports to date show that osteonecrosis patients do less well than other patients who undergo resurfacing surgery (90% versus 96-98% prosthesis survival at early follow-up after surgery). Although the outcomes of conventional total hip replacement are generally good for very active men less than age 55 years, there is a 10-20% failure at 10 years seen in a large European joint registry. The main causes of failure are loosening and instability. This data may not reflect recent advances in surgical techniques and biomaterials; but based on this information, this subgroup of patients seems to be the most likely to benefit from resurfacing hip replacement. If failure occurs with a resurfacing hip arthroplasty, revision to a conventional arthroplasty remains an option and may be better than a standard revision because of the amount of bone that is available to work with. To date more than 65000 resurfacing total hip replacements have been implanted worldwide.

Activity after Resurfacing Hip Replacement. Because a joint replacement is a mechanical device, and one that can’t heal or regenerate, it will wear down over time. Newer designs and materials are thought to be more robust than those used a decade ago, but they still have their limits. Impact sports like running have traditionally not been allowed after hip replacement, although many surgeons would allow doubles tennis and downhill skiing. Some of the leaders of the development of resurfacing hip replacement have had patients return to a higher level of impact activity after this type of surgery (e.g. distance running); but the long-term outcomes with this type of activity are unknown. One recent report showed the type of bone loss one sees with other joint replacements with long-term use (namely, osteolysis), in a patient who ran vigorously after his resurfacing replacement.

Last update 04/06/2007

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