The human hip is a joint that connects the leg to the body. The top of the thigh bone (femur) is round and is called the femoral head. The femoral head rotates in the socket of the pelvis. This socket is called the acetabulum. The femoral head and acetabulum are separated by cartilage and fluid that lubricates the joint for ease of motion.
Resulting from factors including arthritis and inflammation from general wear and tear, previous injury or disease, rough surfaces may be created between the femoral head and acetabulum. This may lead to pain and make it difficult to walk and carry out normal daily activities, leading many hip implant patients to undergo surgery to repair or replace the damaged hip.
Hip replacement surgery (arthroplasty) is very common in the United States. During surgery, damaged cartilage and surrounding bone is removed from the acetabulum. The acetabulum is replaced with an artificial cup kept in place with cement or screws, though the design of certain hip implants allows new bone to grow into the implant to keep it stable. The femoral head is replaced with an artificial ball, and an artificial stem is placed in hollow space in the femur to support the artificial ball. The artificial ball fits into the lined artificial cup to yield a reconstructed hip joint.
Hip Implant Material Combinations
Most hip implants are generally designed the same, and they replicate the ball-and-socket structure of the human hip to allow for natural movement of the hip joint. An implant consists of an artificial femoral stem, an artificial ball replacing the femoral head, and an artificial cup replacing the acetabulum. A liner is placed in the cup to reduce friction and enable smooth movement. Hip implants are intended to last between 10 – 20 years, with durability based upon each patient’s unique condition, body composition and lifestyle. The largest hip implant manufacturers are global medical device companies such as Stryker, DePuy / Johnson & Johnson, Smith & Nephew and Zimmer Biomet.
Hip implants were traditionally made out of ceramic and other non-metal materials. In an effort to make hip implants more durable, manufacturers started designing and producing metal-on-metal joints. However, it was subsequently discovered that metal-on-metal hip joints may release toxic metal debris into a patient’s body due to friction caused by the artificial ball and cup rubbing together. This type of metal poisoning is called metallosis, and can lead to hip implant failure and many serious and painful side effects throughout the body.
Researchers from the Bristol Medical School have found that there is no evidence that any of the newer hip implant combinations, such as ceramic or uncemented, are better than the widely used small head metal-on-plastic cemented hip combination, which has been commonly used since the 1960s. The National Institute for Health Research (NIHR)-funded research also confirms previous knowledge from observational evidence that resurfacing hip replacements and metal-on-metal implants fail more than the metal-on-plastic small head cemented implants.
The research team reviewed 77 randomised controlled trials and analysed data from 3,177 hip replacements to estimate which implants fail less in total hip replacement surgery. Hip replacement implants can have different materials such as metal-on-plastic, ceramic-on-plastic, ceramic-on-ceramic, or metal-on-metal, with large or small head sizes, and can be fixed to the bone with or without cement.
The American Joint Replacement Registry released its 2017 Annual Report, outlining data on hip and knee replacements. The report, released at the 2017 American Association of Hip and Knee Surgeons Annual Meeting in Dallas, TX, includes data from 654 institutions and 4,755 surgeons who performed 860,080 hip and knee replacements.
“The AJRR has seen significant growth since the previous report; this year’s report has a 101 percent increase in procedures, a 57 percent increase in reporting institutions and a 50 percent increase in surgeons,” said AJRR Board of Directors Chairman Daniel Berry, MD. The data includes primarily procedures performed in the hospital, although eight ASCs reported data for 1,020 procedures.
Key highlights from the report include:
- Hip revision surgery burden: 8.6 percent (down from 14.6 percent in 2012)
- Knee revision burden: 5.1 percent (down from 6.2 percent in 2012)
- Primary knee implant design: Posterior stabilized – 48.5%, Cruciate retaining – 35.3%, Ultracongruent – 5.6%, Other – 8%
- Unicompartmental knee replacements: 3.2 percent of all primary knee replacements
- Surgeons performing unicompartmental knee replacement: 24.3 percent
- Surgeons performing patellofemoral arthroplasty: 4.7 percent
- Average length of stay for primary hip replacements: 3.5 days
- Average length of stay for primary knee replacements: 2.9 days