In order to understand hip replacement it helps to first understand the hip joint itself, the types of hip replacement surgery, and a little about the hip replacement implants for each type of hip replacement.
The Parts of Your Hip Joint
The hip joint is a common ball joint comprised of just two bones, the head of the femur (thigh bone) and the socket (acetabulum). The femur has a head, a neck and the trochanters, large on the outer side and lesser on the inner side below which is the shaft. The bones themselves have a hard outer shell, the cortex, with a cavity inside. Surrounding the joint are other tissues including articular cartilage, different ligaments, synovial membrane and a fibrous capsule.
Hip Replacement Processes
There are two parts to a hip replacement – acetabular preparation and femoral preparation.
In the acetabular preparation, special mushroom-shaped reamers are used to remove the diseased and damaged cartilage and bone. The area is power washed to remove all blood, fat and debris and thoroughly dried. Once the bone is dry, cement (if used) is put in and pressure applied to force the cement into the honeycomb of the bone. Implants comprised of a metal shell and a liner are then inserted. Sometimes the shell is fixed in place with two or three screws through into the hip bone.
For the femoral preparation, the femoral head is removed and ream the shaft out with specially shaped rasps. The area is power washed to remove all blood, fat and debris and thoroughly dried. Once the bone is dry, cement (if used) is put in and pressure applied to force the cement into the honeycomb of the bone. Implants comprised of a metal stem and a ball for the head are inserted.
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Hip Replacement Implants
In a total hip replacement surgery, some damaged parts of the hip joint are replaced with artificial hip parts called an implant or prosthesis, a device that substitutes or supplements a joint. Your surgeon will select the design of the hip replacement and size of “femoral ball” at the top to give you the range of motion and stability that you need to function. There are several different choices of hip implants to consider, each using varying materials and having different pros and cons:
- Metal Ball and Polyethylene Liners
- Ceramic Ball and Polyethylene Liner
- Metal Ball and Metal Liner
- Ceramic Ball and Ceramic Liner
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Mechanism of a Hip Implant: During a total hip replacement procedure the socket is cleared of the worn cartilage and bone and a metal cup shaped shell put in its place. A plastic liner is inserted into this metal shell. A metal stem is inserted into your thighbone (femur). Attached to the neck of the stem is a hip ball (sizes vary from 28-58mm) which fits into the socket’s liner. Together, the ball and liner create the new joint. In terms of fixation, your surgeon has the choice of anchoring the hip implants securely to bone using either cement fixation or fixation by ‘bone ingrowth’.
For some patients, a total hip replacement may not be the best solution for their hip pain because it can mean the possibility of one or more revisions being needed later in life. Hip resurfacing, however, leaves more of the bone in place making revision easier.
Partial hip resurfacing, or hemi-resurfacing, is the most bone-conserving approach to hip surgery. During this procedure, only the femoral head (where the leg joins the hip) is reshaped and resurfaced. The hip socket (acetabulum) is left completely intact. The benefit with resurfacing is that the patient keeps most of his or her own bone, which allows for easier revisions in the future (if one becomes necessary).
Should there come a time that revision to total hip replacement becomes necessary, less bone is excised during hip resurfacing which should make the procedure to change to a THR simpler. This procedure is, however, most commonly used to repair broken hips, as opposed to osteoarthritis.
Robotics, Computer Assisted Surgery (CAS), and Minimally Invasive Surgery (MIS)
Increasingly, orthopedic surgeons are looking to computer-assisted robotic surgical techniques to enhance the hip replacement process – the theory being that computers can enhance the precision and accuracy of hip and knee replacement surgeries. Through computer-assisted surgery, it is possible for your surgeon to obtain a 3-D visualization of the joint which can make it possible to achieve a better alignment of the implant joint.
Minimally Invasive Surgery is when very small incisions are made to carry out the surgery. In hips, MIS is keyhole surgery but in knees what most surgeons actually practice is minimal incision surgery. This simply means that they use a shorter incision. Incisions merely have to be big enough the get the cutting guides and the implants in.
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