What is a hip replacement system?
Hip joints deteriorate for various reasons. When this happens, hip replacement is one option for patients who still want to lead an active lifestyle.
How does a hip replacement work?
The hip is a simple ball-and-socket joint. All total hip replacement systems consist of a ball (femoral head), stem (femoral stem), and cup (acetabular component). The device works by securing the stem inside the femur bone. The ball connects to the stem which is then aligned and sized to fit inside a cup on the hip bone, acting as the artificial hip joint. All components must be durable and sturdy, therefore metal, plastic (polyethylene), and ceramic are commonly used to make artificial hip components.
The Beginning of Metal-on-Metal Hip Replacements
Metal-on-metal hip systems have been in use since the 1950s when English surgeon Sir John Charnley modified the existing design to mimic what we see today. Beginning in the early 1960s metal-on-plastic (metal ball and plastic cup) became a popular choice amongst U.S. hip replacement surgeons and their patients until 1999 when the FDA approved the metal-on-metal design for use in total hip replacement surgeries – or, total hip arthroplasties (THAs).
Types of Metal-on-Metal Hip Replacement Systems
Resurfacing Hip Systems
In a hip resurfacing system, the surgeon trims down the existing femoral head, inserts a small metal stem, and caps it with a metal cover. Next, damaged bone and cartilage is removed from the hip socket which is then resurfaced with a metal acetabular cup.
Currently the FDA has approved only three metal-on-metal hip resurfacing systems:
- Corin USA Limited Cormet Hip Resurfacing System
- Smith & Nephew, Inc. Birmingham Hip Resurfacing (BHR) System
- Wright Medical Technology, Inc. CONSERVE® Plus Total Resurfacing Hip System
Total Hip Replacement Systems
As previously mentioned, all total hip replacement systems consist of three components: a ball, a stem, and a cup. The metal stem is placed deep inside the femur bone, coming to a taper on one end where the metal ball attaches. Next, instead of resurfacing the hip socket with a metal liner, an entirely new acetabular component made of metal is attached to the pelvis. The ball is sized to appropriately fit inside the cup, giving the patient ability to move their leg from the hip socket.
Today there are six main manufacturers of total metal-on-metal hip replacement systems:
- Smith & Nephew
What is the approval process for hip replacement systems?
Premarket Approval (PMA) Program
All three hip resurfacing systems were given FDA approval through the premarket approval (PMA) program. Premarket approval is a process of scientific and regulatory review used to evaluate the effectiveness and safety of Class III devices. The FDA defines Class III devices as, “those that support or sustain human life, are of substantial importance in preventing impairment of human health, or which present a potential, unreasonable risk of illness or injury.” The premarket approval requirement for such devices is intended to be a safeguard for consumers against products that could cause serious harm if not properly evaluated.
510(k) FDA Approval Program
Traditional metal-on-metal total hip replacement systems became available in the U.S. through the FDA’s 510(k) program. Through this program, the FDA reviews the device and issues a letter stating that the device is found to be “substantially equivalent”, clearing the device for commercial distribution and marketing. To be substantially equivalent means to be deemed safe and effective in comparison to similar devices on the market having received 510(k) clearance.
On February 18, 2016, the FDA issued a final order requiring manufacturers to submit a premarket approval application for hip joint metal acetabular components after realizing that defective or improperly designed metal-on-metal hip components could lead to painful and costly injuries.