TOTAL JOINT REPLACEMENT TECHNICAL BRIEF

Total Joint replacement is a surgical procedure in which injured or damaged parts of the joint are replaced with artificial parts.
The following discussions are the sole opinions of Thomas D. Petersen MD, a practicing Orthopedic Surgeon who has specialized in Total Joint Replacement for over 35 years.

TOTAL HIP REPLACEMENT

What are the significant differences in total hip design?
Which total hip design is right for me?

Opinion:

There are many design differences to consider. The reason that there is not a clear industry preference after all these years is because all total hip designs work reasonably well for 10-15 years. After that, design features and materials play a big part in the longevity of the implant.

Design Features:

Femoral stems come with or without a collar at the top of the femoral neck. I prefer the collar because it prevents settling of the prosthesis, which averages almost 1-2 cm over time. This might mean you could have to wear a lift on your shoe. The hip components can be fixed to bone by two methods. Either by press fitting into very precisely cut bone or using "bone cement", each method has its proponents. I prefer the press-fit if excellent stability can be obtained at surgery. Many older people have "stove pipe" bone with deficient internal bone structure. In these folks it is difficult to get good stability, therefore bone cement should be used. A press-fit prosthesis uses porous surfaces that bone grows into, to provide fixation of the component.

Materials:

Femoral stems are made from two basic metal materials, cobalt-chromium or titanium. In non-cemented hips I prefer the titanium stems because they do not stress shield the bone as much as cobalt-chrome. Stress shielding of bone causes atrophy and eventual failure of the femoral stem.  With cemented stems polished cobalt-chrome stems are preferred because of less surface wear at the cement–prosthesis interface.

Femoral cups are made out of polyethylene (plastic) by itself, or in a combination of polyethylene with cobalt-chromium or titanium metal backing. Over the past few years the FDA has released two new wear couplings that hold great promise because of the marked decrease in wear debris.  The first is metal on metal and the second is ceramic on ceramic. These are combination cups using a cobalt-chromium bearing surface or a ceramic bearing surface with a polyethylene liner (shock absorber) with titanium metal backing. I prefer this approach because many studies have confirmed a metal on metal bearing or a ceramic on ceramic bearing has 60-80 times less wear than the standard metal on polyethylene cup combination. A build-up of wear particles within the joint tissues is one of the major reasons for failure. Femoral heads are presently made out of cobalt-chromium or ceramic. The ceramic head demonstrates about 50% less wear against polyethylene than cobalt-chrome but this is minuscule compared to 7000% less wear with the cobalt chrome head bearing against a cobalt-chrome (metal on metal) or ceramic on ceramic bearing surfaces.

Minimally Invasive Surgery:

MIS is all the rage these days and highly debated by prominent Orthopedic Surgeons. The advantages are less tissue disruption, blood loss and shorter hospital stays. The major disadvantage is your surgeon cannot see what he is doing most of the time and fluoroscopy during surgery along with the accompanying radiation is frequently necessary and recommended. Many patients with severe osteoarthritis need cysts removed from the acetabulum (hip socket) that cannot be visualized or removed in the minimal invasive techniques. My advice, trust your surgeon to use the incision and technique that he is comfortable with to get the job done. What I would not do is self-refer yourself to an “advertised” MIS surgeon just because your trusted surgeon does not use that technique.

Post-Operative Care:

Usually, minimal therapy is necessary after total hip surgery, unless you have residual tightness of the ligaments and soft tissues about the hip that need to be stretched out. You need to minimize your activity for at least two months until your normal joint lubrication system re-establishes itself. Otherwise you are creating a lot of wear particles by over-using a non-lubricated joint. There are many factors to consider in determining which total hip is best for you. You need to discuss these factors with your orthopedic surgeon. It is a wonderful operation with a history of excellent results.

TOTAL KNEE REPLACEMENT

What are the significant differences in total knee design?
Which total knee is right for me?

Opinion:

The design that is best for you depend on your knee ligament integrity and the severity of the deformity. If you have poor ligaments, especially the posterior cruciate (internal) ligament and/or severe deformity causing over-stretching of the collateral ligaments, you will need a Posterior-Stabilized prosthesis with inherent stability. Technically, this is the easiest prosthesis to install because your surgeon removes the posterior cruciate ligament when replacing your knee with this prosthesis. If your doctor decides to retain the posterior cruciate ligament then a Posterior-Cruciate Retaining prosthesis is required. Installation of this prosthesis is more difficult because the bone resection needs to be more exacting in order to maintain the balance of the retained posterior cruciate ligament which can be scarred down and require a surgical release to function properly. An unbalanced or poorly functioning posterior cruciate ligament can severely compromise the end result.

Materials:

The femoral component needs to be made out of cobalt-chromium. The tibial component base tray can be made out of cobalt-chromium or titanium. I prefer titanium because it causes less stress shielding of bone. Presently the only bearing surface available is polyethylene. However several different types of polyethylene have been released recently. The highly cross-linked poly is one example that may stand the test of time. These newer types offer theoretical advantages in wear longevity when simulated in wear machines.

Design Features:

Fortunately, the poor designs of the past have disappeared. Most of the total knees look alike and function very well. There are some subtle differences in the patella groove designs that should be mentioned. The groove between the kneecap (patella) and the femoral component needs to be deep enough so the patellar resurfacing prosthesis tracts well throughout a full range of motion. I always recommend resurfacing the under-surface of the patella to avoid the 20-30% residual pain in those that have not been resurfaced. The prosthesis can be fixed to bone by two accepted methods: Either by press-fitting onto very accurately cut bone or using bone cement. I always recommend cement fixation for all the components in total knee arthroplasties. There is a well-documented 20-30% failure rate for un-cemented tibial tray components. When in-growth of bone is studied in long-term retrievals, there is only an average of 30% of the porous material filled with bone. This can allow the wear products to migrate under the components and eventually cause failure.

Post-Operative Care:

Rehabilitation of total knees is very important. I use the continuous passive motion machine (CPM) to decrease swelling and pain the first week. But studies have shown there is no difference in the long-term results of those patients that used the machines and those who did not. I believe ice compresses are very beneficial in controlling pain and swelling for the first three weeks. Strenuous physical therapy on a daily basis can actually cause bleeding, excessive swelling and retard your progress. I recommend therapy only three times a week for about six weeks. Your objective should be to get as much motion as possible before restrictive scarring starts at six weeks. If your doctor has recommended a total joint for your arthritic condition, do not despair. Total Joint Replacement is one of the most successful operations that we do. The relief of pain and improvement in function is truly a miracle.

Thomas D. Petersen, MD, Assistant Clinical Professor of Orthopedics, UCSD
Copyright © 2005; Alvarado Orthopedic Research; All Rights reserved

What are the differences in Total Knee design? This brief will cover issues including the UCSD Orthopedic Department- Prosthetic Materials - Preferred total joint materials- Optimal total joint materials- To cement or not to cement total joints - Total Joint metals - Differences in Prosthetic Design- Total Joint failures. Total hip failures - Causes of total joint failures - Total Joint Rehabilitation - Total hip Rehabilitation - Total knee Rehabilitation - Continuous Passive Motion Machine - Cross-linked Polyethylene