Tuesday, March 20, 2012

Glossary of Medical Terms Related to the DePuy ASR Acetabular System

When you receive a hip implant, your doctor does his or her best to give you all the information you need to make an informed decision. However, it's unlikely that you will ever be as informed as your doctor is on any medical issue, no matter how relevant to your health. All those years of medical school amount to a wealth of knowledge that simply can't be communicated in a few hours.

That said, there are a number of medical terms to which we frequently refer in these articles, and we'd like to make sure you have them available in an easy-to-access location if you need to describe your concerns to your doctor or your legal counsel.

Acetabulum

The acetabulum is more commonly called the hip socket, and it is simply the cup-shaped impression in your pelvis where the top of your femur bone fits. The acetabulum and the head of the femur connect to form the hip joint.  

Acetabular cup

When inserting a hip implant, the acetabulum is kept intact, but is hollowed out to make room for a cup that is inserted into the hip socket to make the other half of the hip joint fit smoothly. This cup is called the acetabular cup.

Articular Surface Replacement (ASR) Hip Resurfacing System

The ASR Hip Resurfacing System is a hip resurfacing system that uses the trademarked ASR 1-piece metal bearing acetabular cup. Unlike the ASR Acetabular System, the ASR Hip Resurfacing System does not replace the head of the patient's femur. Instead, the femoral head is merely "resurfaced" with metal prosthesis that fits into the acetabular cup. It is secured to the patient's femur with a short stem inserted into the top of the femur bone.

This hip resurfacing system was recalled in August of 2010 along with the ASR XL Acetabular System, but it is a separate and unique medical device. The ASR Hip Resurfacing System was never approved for use in the United States, though it was released worldwide. The National Joint Registry of England and Waves reported a 5-year revision rate of approximately 12% for the ASR Hip Resurfacing System.

For more information on the difference between a total hip replacement and hip resurfacing, please scroll down to Total hip replacement and Hip resurfacing.

Articular Surface Replacement (ASR) XL Acetabular System

The ASR XL Acetabular System is a total hip replacement system that uses the trademarked ASR 1-piece metal bearing acetabular cup, which is placed in the hip socket. This hip implant uses a traditional femoral ball and stem (see conventional hip replacement). The "XL" in the device's name refers to the fact that the sockets are available in larger sizes.

This total hip replacement system was recalled in August of 2010 along with the ASR Hip Resurfacing System, but it is a separate and unique medical device. The ASR XL Acetabular System was sent to market in 2004 and made available worldwide. Currently, it is believed that approximately 93,000 have an ASR XL Acetabular System. The National Joint Registry of England and Wales reported a 5-year revision rate of an estimated 13% of recipients.

Criticisms of the ASR XL Acetabular System include insufficient testing, a lack of clinical testing, a high rate of failure, a delayed recall after problems were reported, poor design and engineering, and medical complications including metallosis, heavy metal poisoning, metal sensitivity, bone deterioration, tissue damage, and hip implant failure. Currently, DePuy has only recommended that recipients of the ASR XL Acetabular System who are experiencing pain or system failure have hip revision surgery.

Femoral component

In a hip implant, there are two parts to the hip joint that need replacing: the acetabular cup and the femoral component. The femoral component is the part of the implant that connects to the patient's femur bone, which runs the length of the thigh.

There are several different types of femoral components: it may be attached with cement or without, and the femoral component may replace part of the patient's femur or simply cap the top of the femur bone with a prosthetic surface and a stem inserted into the bone. 

Femoral head

The top of your femur bone forms a smooth, rounded ball that fits into your acetabulum (also called your hip socket). The part of your femur that fits into the acetabulum is called the femoral head.

In a hip implant, the term for the corresponding replacement is the same. The rounded top of the femoral component which connects to the acetabulum is also called a femoral head.

Hip implant failure

Hip implant failure occurs when a hip implant no longer performs its function properly. Often, hip implant failure requires hip revision surgery (see hip revision, below). The most common causes of hip implant failure are loosening of the hip replacement device, infection, breakage or wear, and damage to the surrounding bone. Hip implant failure may also occur due to poor design and engineering.

Heavy metal poisoning (see metal poisoning, below)

Hip replacement (total and partial)

A hip replacement is any surgical procedure in which the hip joint is replaced by a prosthetic hip implant. There are two parts to the hip joint and therefore two parts to a hip replacement: the acetabular cup and the femoral component.  

Hip replacement (total)

A total hip replacement, also called a total hip arthroplasty, replaces both the acetabulum and the femoral head with prosthetics.

Hip replacement (partial)

A partial hip replacement, commonly referred to as a hemiarthroplasty, generally only replaces the femoral head, though it may in some cases only replace the acetabular cup.

Hip resurfacing

A hip resurfacing system is a total hip replacement system that does not remove a part of the patient's femur to accommodate the femoral component. Instead, the top of the femur is simply capped with a prosthetic replacement and a stem is inserted into the bone.

Hip revision

A hip revision surgery is any surgery after the original hip replacement surgery.

Metallosis

Metallosis is a reaction to the immune system attacking metal objects within the body. In the case of a metal hip implant, the immune system reacts to the amount of metal being released into the bloodstream and concludes that the implant is a foreign body that needs to be attacked and destroyed.

As the immune system attempts to attack the foreign substance, it can over-compensate and kill healthy bone and soft tissue around the area. The surrounding area may also become infected, cutting off blood flow and leading to necrosis.

Metallosis has been linked directly to devices that release metal into the bloodstream via friction. Women, people of small stature, and the obese are more susceptible to metallosis.

Metal poisoning

Also referred to as heavy metal poisoning, metal poisoning is the build-up of metals in the soft tissues of the body. The most common heavy metals associated with poisoning are lead, mercury, arsenic, and cadmium; the heavy metals associated with the ASR XL Acetabular System are chromium and cobalt.

Metal poisoning causes nausea, vomiting, diarrhea, stomach pain, headaches, metallic taste in the mouth, and in extreme cases the loss of cognitive, motor, or language function. Chromium and cobalt have also been linked to increased cancer risk.

Metal sensitivity

Metal sensitivity can be viewed as an allergic reaction to metals. Metal sensitivity affects approximately 10-15% of the population, and as with other allergies, it can develop over time if the patient is over-exposed to certain metals. The most common reactions include hives, eczema, redness and itching; however, in patients who have received metal implants, the metal sensitivity may also cause pseudotumors (see pseudotumors).

Necrosis

Necrosis is the premature death of living cells and tissue. Usually, necrosis occurs as a result of infection, toxins, or trauma to the area.

Pseudotumors

A pseudotumor is essentially an enlargement that resembles a mass of abnormal cells, but is in fact an inflammation. In the case of the DePuy ASR Acetabular System, doctors who have found pseudotumors in their patients have found that they are filled with fluid. 

Recommended Readings:

Hip Implant Risks May Include Premature Failure

How the ASR XL Acetabular System is Constructed

DePuy Asks Hip Implant Patients to Sign Away Their Legal Rights

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Author: Andrew Sullo
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