The hip joint is called a ball-and-socket joint because the spherical head of the thighbone (femur) moves inside the cup-shaped hollow socket (acetabulum) of the pelvis.
To duplicate this action, a total hip replacement implant has three parts: the stem, which fits into the femur; the ball, which replaces the spherical head of the femur; and the cup, which replaces the worn out hip socket. Each part comes in various sizes to accommodate various body sizes and types.
In some designs, the stem and ball are one piece; other designs are modular, allowing for additional customization in fit.
Several manufacturers make hip implants. The brand used by your doctor or hospital depends on many factors, including your needs (based on your age, weight, bone quality, activity level, and health), the doctor’s experience and familiarity with the device, and the cost and performance record of the implant. These are issues you may wish to discuss with your doctor.
Many people credit Sir John Charnley, a British orthopaedist, with performing the first modern total hip replacement. His innovations included combining a metal stem and ball with a plastic shell and using a methacrylate cement to hold the devices in place.
Today, the stem portions of most hip implants are made of titanium (or cobalt/chromium) based alloys. They come in different shapes and some have porous surfaces to allow for bone ingrowth.
Cobalt/chromium-based alloys or ceramic materials (aluminum oxide or zirconium oxide) are used in making the ball portions, which are polished smooth to allow easy rotation within the prosthetic socket.
The acetabular socket can be made of metal, ultra-high molecular-weight polyethylene, or a combination of polyethylene backed by metal.
All together, these components weigh between 14 and 18 ounces, depending on the size needed.
All the materials used in a total hip replacement have four characteristics in common:
- They are biocompatible. They can function in the body without creating either a local or a systemic rejection response.
- They are resistant to corrosion, degradation, and wear. Therefore, they will retain their strength and shape for a long time. Resistance to wear is particularly significant in maintaining proper joint function and preventing the further destruction of bone caused by particulate debris generated as the implant parts move against each other.
- They have mechanical properties that duplicate the structures they are intended to replace. For example, they are strong enough to withstand weight-bearing loads, flexible enough to bear stress without breaking, and able to move smoothly against each other as required.
- They meet the highest standards. These high standards extend to fabrication and quality control at a reasonable cost.
During a total hip replacement surgery, the orthopaedic surgeon will take a number of measurements to ensure proper prosthesis selection, limb length, and hip rotation. After making the incision, the surgeon works between the large hip muscles to gain access to the joint.
The femur is pushed out of the socket, exposing the joint cavity. The deteriorated femoral head is removed.
The acetabulum is prepared by cleaning and enlarging it with circular reamers of gradually increasing size. The new acetabular shell is implanted securely within the prepared hemispherical socket. The plastic inner portion of the implant is placed within the metal shell and fixed into place.
Next, the femur is prepared to receive the stem. The hollow center portion of the bone is cleaned and enlarged, creating a cavity that matches the shape of the implant stem. The top end of the femur is planed and smoothed so the stem can be inserted flush with the bone surface. If the ball is a separate piece, the proper size is selected and attached. Finally, the ball is seated within the cup so the joint is properly aligned and the incision is closed.
Hip replacements may be cemented, cementless, or hybrid (a combination of cemented and cementless components), depending on the type of fixation used to hold the implant in place. Although there are certain general guidelines, your surgeon will evaluate your particular situation carefully before making any decisions. Do not hesitate to ask which type of implant will be used in your situation and why that choice is appropriate for you.