The information provided below is meant to educate the patient about the options available to hip replacement candidates. Your orthopedic surgeon should explain to you which hip replacement is best for you and why.
In a total hip replacement, both the thigh bone (femur) and the socket are replaced with implant materials and prostheses. Specifically, a metal stem is inserted into your thighbone. Attached to the neck of the stem is a hip ball, just over an inch in diameter. The hip ball fits into a liner. Together, the ball and liner create the new joint. The liner is inserted into a metal shell that in turn is anchored to your pelvis. But there are a number of different approaches a surgeon can take, depending on her analysis of your particular case.
Because of the advances in the last thirty years, several different types of implant materials have also been found useful in hip replacement procedures. Each material has its own advantages and disadvantages, but it is ultimately up to your surgeon which materials to use. Beyond the different types of materials, a surgeon must also decide whether to anchor the hip implant to the bone using cement fixation or something called "bone ingrowth."
Fixation
For bone ingrowth, the surface of the prostheses simulates the bone with a special granular surface into which the bone grows, thereby locking the implant into place. Cement fixation holds the implant in the bone by acting as a filler between the bone and the implant. It is made of a substance that is mixed at the operation and hardens into a durable, long lasting polymer.
Modularity
You may also want to consult with your doctor about the design of the implant. Modular systems offer the ability to interface different sizes of femoral heads to fit over the stem; angles, sizes, and lengths of the implant are made to fit your anatomy. The advantage of a modular system is that it gives the patient more flexibility.
Choosing Hip Replacement Implant Material
The main issue that doctors and patients confront when choosing implant material revolves around the wear debris that is released into your body from any of the implant materials. Even the materials with the most wear debris (metal ball and polyethylene liners) show up only after many years (10-15). Nevertheless, it is important to educate yourself about the various options for hip replacement surgery.
Metal-on-Polyethylene
Because of its durability and performance, Metal-on-Polyethylene has been the leading artificial hip component material chosen by surgeons since FDA approval 30 years ago. The metal ball is cobalt chrome molybdenum alloy and the liner is polyethylene.
Metal-on-Polyethylene is the most understood and practiced of all the procedures. Using these materials, a surgeon has a range of options to obtain stability in the body while the operation is underway. This ability to adapt and customize during the surgical procedure is an important attribute of polyethylene. It is also the least expensive bearing.
The disadvantage of Metal-on-Polyethylene is shedding over time. After 10 to 15 years, patients who are extremely active may shed debris that results in joint inflammation and bone loss. However, new wear resistant polyethylene liners have been introduced, called "highly crosslinked polyethylene." If you are a very active individual or a relatively young patient, your surgeon may prescribe an all-ceramic hip joint or all-metal hip joint.
Ceramic-on-Polyethylene
Your next option is a ceramic ball and polyethylene liner. Ceramic heads are harder than metal and are the most scratch resistant implant material. The hard, scratch resistant, ultra-smooth surface can greatly reduce the wear rate on the polyethylene bearing. The wear rate for this type of implant is less than Metal-on-Polyethylene.
One disadvantage found in Ceramic-on-Polyethylene are the incidents of fractures, but newer, stronger ceramics have resulted in fewer of these. Overall, the wear rate for Ceramic-on-Polyethylene is 50% less than that of Metal-on-Polyethylene.
In terms of cost, Ceramic-on-Polyethylene is more expensive than Metal-on-Polyethylene, but less than Ceramic-on-Ceramic.
Metal Ball and Metal Liner
Metal-on-Metal bearings were approved by the FDA in 1999, offering the potential for greatly reduced wear, with less inflammation and less bone loss.
One clear advantage of metal-on-metal implants is the variety of femoral head sizes and neck lengths available (modularity). Metal-on-metal components allow the largest heads throughout the entire range of implant sizes. Large ball heads provide increased range of motion and greater stability, which can significantly reduce the risk of hip dislocation, a crucial factor in the long-term success of an implant.
Although wear is reduced, the wear products (sub-microscopic particulates, soluble metal ions) are distributed throughout the body. This has raised concerns about long-term bio-compatibility. At present these are only concerns, for there have been no definitive clinical findings that these wear products are harmful.
Anatomic Size Metal-on-Metal Ball Heads
Because the human femoral (ball) head is naturally large, it makes sense to implant a large, anatomic replacement. This was not possible in the past because traditional polyethylene liners made smaller femoral heads necessary. However, with the introduction of metal-on-metal implant components, liners may be eliminated, allowing surgeons to use large femoral heads. Use of a larger ball head has been shown to increase the range of motion an individual may experience as well as decrease the possibility of dislocation.
Ceramic Ball and Ceramic Liner
Ceramic is the hardest implant material used in the body, and has the lowest wear rate of all, to almost immeasurable amounts (1000 times less than Metal-on-polyethylene). These implants have been used in Europe for more than 30 years, and since 2003 in the United States.
Consequently, there is usually no inflammation or bone loss, nor systemic distribution of wear products in the body. New ceramics offer improved strength and more versatile sizing options. However, there has been a very few incidents reported of noises from such implants.
Hip-Resurfacing Option
For younger patients, a total hip replacement may not be the best solution for their hip pain because it can mean difficult and numerous revisions later in life. Hip resurfacing, however, leaves more of the bone in place, giving these patients more time before a total hip replacement becomes necessary.
Conclusion
If you are a candidate for hip replacement surgery, fixation, modularity, and implant materials are all topics that factor into making the right decision. Your doctor should explain to you which procedure and materials are best for you.
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