Hip Replacement — the engineer as a patient

artificial_hipPeople from all walks of life undergo hip replacement surgery.  Some of those people are engineers — people who design things, build things, and worry about the materials for manufacturing.  Those pesky engineers want information about hip surgery the average person might find boring but the engineer finds comforting.  So this guide is for the engineer who needs to have a hip replaced, not the engineer who actually makes the parts.

Original Equipment Failure.  The original specifications for the hip joint were not provided.  It seems there is a lot of variation from person to person.  Some hips last a lifetime, others do not.  The cause of hip failure mostly seems to be the result of gradual wear of the cartilage bearing surfaces within the large ball and socket joint that is our hip.  When the cartilage is so worn the underlying bone is exposed the joint fails with pain, inflammation and restriction of motion.  The time-course of joint failure can be sudden, but usually it is prolonged over many years with increasing symptoms of pain and loss of function.

Timing of Replacement:  The joint should be replaced when the joint function is causing unacceptable limitation of activity.  The time from initial symptoms until replacement is variable ranging from a couple of years to a decade.  There can be complications of surgery (including death) so jumping into surgery before it’s needed is a bad idea.

Expert Advice:  Orthopedic surgeons who specialize in large joint replacement are the experts.  They use a combination of examination and x-rays to give an assessment of the joint.  A good surgeon will never say you must have surgery — they always leave the decision to the patient.  Sometimes that attitude is disconcerting but actually is very reasonable.  After surgery, neither you or the surgeon want to say “I wish I had waited.”

Method of Replacement:  Special tools and jigs are provided by each manufacturer of  artificial hip systems.  Often, a hospital will limit the options to one manufacturer for obvious cost and storage reasons.  A sterile four-part hip replacement is selected:  1) the acetabular cup  2) the liner for the cup  2) the ball and 3) the femoral stem.  When the parts are ready the surgeon must cut into the thigh, visualize the hip joint, remove (with a saw) the ball from the top of the femur bone, ream out the inside of the femur and press fit a stem into the femur.  Then, the natural socket is removed (with a grinder) and the metal cup is inserted (often fastened with screws).  The liner is snapped into the cup (the socket of the joint).  The ball is pressed on top of the stem. Finally the ball is eased into the liner.  The surgeon sutures the various layers of the incision closed.  The final layer, the skin, is closed with staples.  A sterile dressing is applied over the incision.


Surgical Incision: Each surgeon has their favorite: anterior (in front of the joint), lateral (the side of the joint) or the posterior-lateral (the side of the joint toward the back).   The lateral incision technique was developed first so many surgeons use that method.  Surgeons trained within the past 10 years may have experience with the anterior approach.  The anterior approach seems to result in a quicker recovery and less discomfort.

Metal parts:  Stainless steel was used in the past but fell out of favor because tiny amounts the cobalt and chromium in the steel could be absorbed by the body and cause undesirable side-effects.  Now, metal parts are constructed with either titanium or zirconium.  Also, the newer materials are slightly more flexible than steel, spreading stress to the bones more evenly.

Acetabular cup:  A metal part that fits into the pelvic bone where the natural joint socket was located.  The side that touches the bone is rough.  There are holes through the cup for screws to attach it to the bone.  The other side is where the liner snaps into position.  

Liner for the cup:  This is one of the bearing surfaces in the joint.  Older liners were made of simple polyethylene plastic.  Excessive wear was a problem for some patients.  Now, the most popular liner is a special polyethylene treated with radiation to result in a highly cross-linked material that is much more durable.  Polyethylene also provides some shock resistance.  Some companies make a ceramic liner, but because of the concave geometry (fracture risk), the brittle nature of the material and the tendency to squeak it is not requested as often, but it is very wear resistant.  People in their 40’s and 50’s might want the extreme wear resistance of the ceramic liner just to avoid a repeat surgery 20 or 30 years later.

Ball: The most wear-resistant material is ceramic.  Because of the spherical shape of the ball a fracture of this ceramic part is less likely than for a ceramic liner.  Ceramic balls are becoming a standard.  Nitride coated zirconium is also an option for the ball but it not as wear resistant as the ceramic.

Stem:  This is the part that is wedge-fit into the thigh bone called the femur.  It is tapered and has a rough finish.  The tool kit for the system includes several test-fit rods to make sure the reaming of the femur is exactly the size needed.  Once the fit is adequate the stem is seated in the bone with a tap from a malet.  The ball is press fit on the other end of the stem.  The ball may be an integral part of the stem in some systems.

Attachment:  Metal parts pressed against bone need to stay in position for a long time.  If a patient has healthy bone the most desirable solution is a special coating on the metal that provides a rough surface for the bone to grow into.  Titanium parts can be plasma sprayed with pure titanium as one method to obtain a rough surface.  Another method is to bind a bony material called hydroxyapatite to the metal surface.  The alternative method, for people with weak bones, is polymethyl methacrylate glue.

Anesthesia:  A short stay in the hospital is desirable to reduce exposure to germs that are resistant to antibiotics.  Effective anesthesia with good pain control after surgery make shorter stays possible.  A favorite is spinal anesthesia (makes the body numb below the waist for a couple of hours) along with a sedative, like propofol which has few side-effects.  Of course, a few injections of pain medication and pain pills are needed.  For reasonably healthy people just an overnight stay is all that is needed.

Prevention of Complications:  The surgical technique, the anesthesia, and the materials in the artificial hip all are designed to minimize complications.  Two feared complications are infection and blood clots — both of which can be fatal in extreme cases.  An infusion of an antibiotic within two hours of surgery helps to reduce infection.  Pneumatic compression stockings or “boots” help to reduce blood clots during surgery.  After surgery, maintaining activity with frequent walking reduces the risk of clots.  Some surgeons add a low-dose aspirin to reduce the chances of heart attack and stroke.  Constipation is a common complication easily prevented with prunes, stool softener or a mild laxative.

Comment:  If you are an engineer or just someone who wanted additional detail hopefully this summary is helpful.  There is a lot of information available on the Internet but it is always difficult to understand medical jargon.  Out-of-date information can be really confusing.  Of course, your surgeon is always happy to answer questions — write down the questions and take them with you to your appointment.

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