(0.2 %)

This is the single biggest problem with hip replacements. This is when the ball comes completely out of the socket. A normal hip is held in place by ligaments as well as muscles around the hip. The normal femoral head is quite large and difficult to dislocate. It requires a fall from 2 stories or a blow of knee onto the dashboard of a car at 30 mph to do this. Usually a bone will break first.

Most total hip replacements (THR) have a smaller head than the normal hip. The smaller the artificial head, the higher the risk of dislocation. Also in THA a metal stem replaces the top of the femur. The stem imperfectly reproduces the natural anatomy of this bone. This is also a factor in instability. Also, during the operation, the major stabilizing hip ligaments (hip capsule) are cut. If the patient bends the hip too far, the hip may dislocate. It requires no significant force as with a normal hip.

The risk of dislocation is approximately 5% within the first year after surgery if a standard 28mm bearing is used. If a 36mm bearing is used, the risk is 1%, if anatomic sized metal bearings are used the risk is < 0.2%. When the risk of dislocation over 10 years is considered, it doubles. When a hip dislocates, you can’t walk and you need to go under anesthesia to have it manipulated back into place by your surgeon.

The Problem

About half of all people who dislocate a hip end up having repeated dislocations and require surgery to try to correct this. This is the most common cause for revision hip surgery in the US. It accounts for over 22% of all revisions. Patients with the following conditions are at higher risk: dysplasia, obese, neuromuscular conditions such as MS or Parkinson’s disease. The risk is somewhat influenced by the type of surgical approach (anterior vs. posterior) implant design features other than the head size (lateral offset, neck diameter) and implant position (primarily of the acetabular component). But bearing size is by far the most critical factor. Although surgeons often speak of ideal component positions, attempts to define a safe zone for implant positioning has so far been unsuccessful with these smaller bearings.

When larger bearing sizes are used with a plastic socket liner, the liner has to be made thinner. When this was tried 20 years ago, plastic wear increased dramatically causing extensive bone destruction (osteolysis). We had to abandon this idea and return to smaller bearings and accept a higher dislocation risk. Metal bearings became available 10-15 years ago. In the laboratory they produced very low wear rates even with larger bearing sizes. Because large metal bearings are obviously more stable, these became very popular a few years ago.

However we have learned that they do have a different set of problems. When implanted into people, large metal bearings sometimes produced higher wear rates than expected and this metal debris caused swelling, pain, and eventually soft tissue destruction that I have named an adverse wear failure (AWF). In addition, all larger bearings (including plastic and ceramic ones) may put excess stress on the stem trunion (where the head attaches to the neck of the metal stem) resulting in corrosion and also AWF.

Therefore the downside of more stable larger bearings is AWF produced by excess metallic debris. But we have found that this problem is related to specific implant design (brand) and acetabular component position. I will explain this in more detail in the section on AWF. In addition there may be very rare patients with allergy to metal, but we have no reliable test for this and the rate appears to be less than 0.025%.

The Solution

My solution to the problem is to use a proven well-designed metal bearing with precise positioning of the acetabular component. There is only one design that has been recalled for bearing wear problems, the DePuy ASR total hip and resurfacing system. For hip resurfacing I use either the Biomet or Wright system because their bearings are sound and they are the only companies to offer an uncemented femoral component (different discussion). For total hip systems we must not only consider the bearing design, but also the trunion design.

It is still not entirely clear what trunion design is optimal for large heads. But I am strongly suspicious that the Biomet design is superior for several reasons. Again, it appears that DePuy had serious flaws with their trunion and had the worst problems. Biomet has a completely different trunion than DePuy and most other companies. The stem is titanium and the neck adapter is titanium. The attachment of the neck adaptor to the cobalt-chrome head is at a massive trunion. One comparative study has shown that this unique design releases significantly lower amounts of metal ions than other trunion designs.

In addition, many companies added grooves to their stem trunions over the last 10 years to better accommodate ceramic heads. They also made them smaller to reduce the risk of impingement and dislocation. Biomet did not do this. I have not seen any trunion failures with the Biomet implants over 7 years of use. We are in the process of compiling a formal study with metal ion levels to assess this more carefully.

For hip resurfacing and large bearing THR, my risk of dislocation has historically been less than 0.2 % and AWF rate has been 1% at 10 years. Both of these are falling further now that we have finally discovered a “Safe Zone” for acetabular component positioning for large metal bearings. In a study of 761 cases we found that the dislocation rate and the AWF rate was ZERO if we placed the acetabular component within a certain range that we named RAIL (Relative acetabular inclination Limit).

In the last 2 years we have been able to place 100% of all implants within RAIL. I believe the risk of dislocation and AWF is now negligible for HSR and THR. For rare patients who cannot tolerate cobalt chrome, I think the best solution is a 36mm Biolox ceramic head on a Biomet titanium stem against vitamin E doped cross-linked polyethelene. I only rarely use this implant and can therefore not give you my personal stats. But this carries a 1% theoretical risk of dislocation, and probably little risk of trunion corrosion. The plastic liner is thin and therefore carries some as of yet unknown risk of breakage by 10 years. Impact activities should be avoided.

To prevent dislocation, patients with THR using smaller bearings need to follow lifelong restrictions. They should never bend their hip into extreme positions as required in ballet, palates, yoga and kayaking and squats. The risk is less with a 36mm bearing than the standard 28mm bearing. Patients with large metal bearings with HRA or THA have to limit extreme positions for 6 months until the hip capsule has healed, them they no longer have position restrictions.