Uncemented threaded cups for total hip arthroplasty: Are they warranted?

Issue: Issue 4 2005

ByHans Jürgen Refior, MD

Hans Jürgen Refior, MD [photo] --- Hans Jürgen Refior, Editor

Orthopaedic surgeons first attempted uncemented screw fixation of hip implants in the mid-1900s. In Europe, this approach was linked with Wilis (1938) and McKee (1951). In the 1960s, Ring, a British orthopaedic surgeon, followed with an acetabular component that was fixed to bone with one screw.

However, the principle of uncemented screw fixation of hip sockets saw a revival only after the bone cement introduced by Charnley did not fulfill all expectations of stable long-term anchoring. Acetabular component loosening in young patients led to the further development of screw cups. Using a cobalt-chromium-molybdenum alloy, Lord, from France, used a screw cup in the 1970s, which for the first time included a thread on the implant body.

Turning to threaded cups

Threaded cups made from ceramics and polyethylene followed. However, these implants required a pre-cut thread, and the result was disappointing. The use of uncemented polyethylene cups led to high loosening rates.

At the same time, the development of self-cutting threaded cups first made from cobalt-chromium-molybdenum and their introduction to the European market led to good initial results and a rapid propagation on this fixation principle. The various models had a design that was either spherical or conical, and the preferred models were titanium alloys. One example was the Mec-ring, a spherical shell with a polyethylene inlay that was introduced in Germany and the United States.

It comes as no surprise then that following good initial results in the 1980s, an increasing number of U.S. and European publications reported failures after the implantation of uncemented threaded cups.

In particular, Engh and others reported conspicuously high loosening rates at midterm follow-up with the Mecron threaded ring. Since his loosening rates were higher than those with cemented or press-fit cups, Amstutz and his colleagues recommended against using the smooth-surfaced Mecron threaded ring. However, after analyzing problems related to shape, surface structure and thread design of these early threaded rings, European surgeons made some refinements to the implants. Their new designs began with the understanding that titanium alloys have superior biocompatibility and that porous surfaces allow for the desired osteointegration.

Focusing on design aspects

With the goal of safe, stable primary anchoring, surgeons directed greater attention to thread design. For the second generation of uncemented threaded cups, orthopaedic surgeons recommended deep, thin threads with a large distance between turns, allowing for generous contact with the bone bed.

Theoretical analyses of the physiological situation led developers to produce spherical designs for the outer shape of the cup. This shape led to a physiological load transmission into the periacetabular bone and to reduced loss of subchondral bone stock as a result of limited reaming.

Extensive studies by Pellengahr at the Munich University department of orthopaedics found that compared to pressfit cups, threaded cups demonstrate a significantly reduced micromotion of less than 200 µm. This is seen as an important precondition for successful implant osteointegration. The same studies also showed that the maximum tilting momentum of threaded cups was superior to that of micro-structured pressfit cups.

Casting aside doubts

Now that the problem of osteolysis induced by polyethylene bone contact has been recognized and solved, excellent threaded cups are now available that satisfy the demands for primary and secondary stability. For example, the good long-term results published in Europe with the Zweymueller threaded cup as well as the analysis of the Norwegian implant register with similarly good results for hydroxyapatite-coated threaded cups at six years clearly justifies, in my opinion, a recommendation for the use of cementless threaded cups.

The reservations some U.S. surgeons still have toward the uncemented system no longer seem warranted.

Hans Jürgen Refior, MD, of Munich, is an editor of Orthopaedics Today.