Introduction
Total hip replacement is a standard treatment for hip arthritis, and due to the aging of our population and higher standards required for the quality of life in younger patients with osteoarthritis, it is being performed more frequently. The expectation is that there will be a growth of 150% for total hip replacements in the next decade. There is however not yet consensus on the total hip system with the best long term survival. A lot of debate exists on bearing surfaces, cement, type of stem or cup component. In this article I would like to describe the evolution and possible future in cup components.
One of the first acetabular systems was the all poly cup, of highly molecular weight polyethylene, introduced in 1962 by Sir J Charnley. The results were promising at first, however failure soon became apparent with “cement disease” thought to be the cause. It was later understood that the failure mechanism was caused by the wearing of polyethylene particles. Through time polyethylene quality has improved with ultra high molecular weight polyethylene and crosslinking. The sterilization process changed and oxidation has been reduced by adding vitamine E. This process has dramatically reduced polyethylene particles and thus reduced failure of cemented cup prosthesis. Long term results of the modern cemented all poly cups are very good with low failure rates. They are still widely used, more in Europe than in the United States. However, as more surgeons choose an uncemented cup placement, the technique of cementing is getting lost to more and more new trainees.
The failure of the first cemented cups led to the introduction of uncemented cups. These were dependent on mechanical fixation as they were threaded and screwed into the acetabular bone. The firm mechanical fixation was supposed to lead to ingrowth. The results of the screw cups were and still are very good. The difficulty though is the low versatility and sometimes challenging revisions with bone loss. The cups depended not only on mechanic but also biologic fixation. The purpose was to get bone ingrowth, but initial stability was needed. The cups were first placed in the prepared acetabulum and reamed size to size. Fixation came mainly from stabilizing screws, which was a risk for neurovascular damage. G?E?Hill introduced the press fit technique in 1985 where stabilization comes from underreaming the acetabulum. The surface of fixation is larger and fretting is reduced because screws are not used for fixation, so mechanical stability is not the sole importance.
Biological fixation meanwhile helps osseointegration, as the surface has been roughened with porous coating or grit blasting. This could facilitate bone ingrowth. Hydroxy apatite and Tricalcium fosfate are added to the surface area to get additional osteoconduction. There were a number of failures with this where the layer came loose from the surface.
Coating the cups led to a tremendous increase in bony ingrowth of the cup systems. One of the newest materials is trabecular metal. The TMr cup has good and promising midterm results. Most papers are on revision surgery of cup components, classified according to Paprosky. Tantalum has a greater porosity and better elastic characteristics. The downside it seems is mainly economic because of the cost price. In Belgium, for example, it is not covered for by the health insurance for primary use, only for revision surgery.
The most frequently used bearing surface is polyethylene, where the fixation of the insert is very important in cup survival. The first generation locking mechanisms had incongruency of the surface and the shell, this led to wear and to dissociation of the liner. Often a thin rim was used to get extra fixation of the insert. This was a weak link and could progress to a fracture of the rim with dissociation of the insert. The second-generation shells have a greater congruency with less to no screw holes reducing backside wear. The head size is gradually increasing in time too, from 28mm to 36mm or more. This increases hip stability and excursion is greater while the thickness of the polyethylene decreases and is thinner at the edges of the shell. The head is lateralized and has a greater excursion which can cause impingement with edge wear or rim fracture. In the third generation, the locking mechanism has sunk in the shell without any rim on the edge, here the lower wear characteristics come to full benefit. Impingement risk is decreased dramatically. The lower mechanical properties of highly cross linked polyethylene are bypassed by this locking mechanism.
There are now fourth generation acetabular cups available (see Fig 1) as surgeons wanted to have a greater intraoperative freedom of bearing choice. The enhanced locking mechanism provides the surgeon with the possibility to use a polyethylene, metal or ceramic insert. The surface porosity of this generation is even more enhanced to improve ingrowth. The short term results look promising, but long term results are not yet available.
The most important factor in survival and patient satisfaction is the position of the cup, something which seems to be forgotten in the discussion on bearing couples and wear issues. A less than optimal placement of the cup leads inevitably to postoperative problems in the short or long term. Forty-five degrees of abduction and ten to twenty degrees of anteversion seems ideal. This is not entirely true because we need to know what type of cup is being placed and what the properties of the cup are. For example, the resurfacing cups need to be placed in less abduction to avoid edge loading. There are all sorts of devices to help optimize cup position. Computer assisted navigation of cup placement is promising. The jury on the use of this is still out there.
Conclusion
Decision making for orthopaedic surgeons on what total hip system to use has to be based on literature. There are however numerous types of prosthesis on the market and many different bearing couples available. National joint registries show different “best options”. Development of acetabular systems since the 1970s has grown rapidly and with the fourth generation available now new research needs conducting. For the orthopaedic surgeon it seems to be a system with great freedom of choice. The same insertion technique can be used for all ages and different bearing options suitable for any age group can be used. There are ceramics or metal for the younger patients and polyethylene for the elderly. Improving the ingrowth of the cup combined with better locking mechanisms will probably lead to less wear and greater survival of total hip replacements. In the end, optimal placement plays a key role in survivorship of the total hip prosthesis.
References
- The Next Generation of Acetabular Shell Design and Bearing Surfaces Robert B. Bourne, MD, FRCSC; Richard W. McCalden, MD, MPhil (Edin) FRCSC; Douglas Naudie, MD, FRCSC; Kory D.J. Charron, Dipl MET; Xunhua Yuan, PhD; David W. Holdsworth, PhD. Orthopaedics 2009
- Review of the Evolution of the Cementless Acetabular Cup. Michael D. Ries, MD. Orthopaedics
- The Adult Hip. John J. Callaghan. Second edition, Lippencott William and Wilkins. 2007 Philadelphia
- Tantalum Metal in Acetabular Revisions, H. Miettinen; J. Kettunen; S. Miettinen; M. H
