Total hip replacement is one of the most successful orthopaedic procedures in history. Hip replacement is still growing due to an aging population. Another factor seems to be the increasing amount of hip replacements in the younger, more active population. This obviously will lead to a larger revision burden. From literature we know that cup failure is still the case with vast amount of hip revisions. The stem of the hip replacement has improved and cemented or uncemented has excellent long terms survival ship. Bearing couples have also improved.
Polyethylene has become less susceptible to wear, although it still remains to be seen if brittleness is the price in future. Ceramic bearing has become more reliable in the fourth generation, with wear reduction and more importantly the risk of fracture dramatically decreased. An advantage is also the higher versatility of the ceramic heads. Metal bearing couples have improved when compared to the older metals, however the major concern with metal on metal is the release of metal ions though it is still not proven to lead to adverse situations.
At the resurfacing congress in Ghent 2009, De Smet stated to revise all metal on metal resurfacings with high blood level metal ions. Another even more important issue is the correct placement of the components. I already stressed this in my last article in the February/March issue of OPN, on cup components.
In resurfacing hip replacements, the abduction angle of the cup is very important for reducing wear caused by edge loading. Insert fixation improves and will also lead to less wear and hopefully less revision surgery, plus the tendency to use bigger heads will also lead to less dislocation. The down side though will be more wear in every bearing couple. Interestingly, a recent thesis by Moojen states that revision due to low grade infection is probably underestimated. He states that 4-13% of hip revisions are caused by infection. All hips were revised for aseptic loosening, 4% appeared infected and in 9% a low grade infection was found. Diagnosis was made with cultures and DNA techniques, so this is also big issue which revision surgeons need to be aware of when managing their revision operations. Another reason for revision surgery is recurrent dislocation with malposition of the components.
Although materials are improving we still have a large burden of revision hip surgery. We cannot avoid biomaterials from degrading and causing wear particles. These particles induce osteolysis and lead to bone loss. Paprosky suggested a classification based on location and the amount of bone loss. There is either a segmental or cavitary defect or contained or uncontained defects. With very large defects a reconstruction cage or ring with screw fixation has to be used with a cemented all poly cup. Large reconstructions with bone can also be done or pedestal cups can be used. The use of cementless cups depends on good initial stability. If around 50% of the acetabulum is present I use a trabecular metal tantalum cup.
We started using the TM cup (Zimmer, Warsaw) in 2006 for cup revisions and I use the modular cup. Excellent results have been reported for the monoblock TM cup. Tantalum has excellent biocompatibility for it has high porosity of around 80%. The pores are small and create a cage structure for ingrowth of bone. The elasticity of trabecular metal is lower than titanium and mimics that of cancellous bone. Because of this stress shielding seems to be less of a problem. The friction coefficient is high against the cancellous bone of the prepared acetabulum. The combination of these characteristics are promising for rapid and solid ingrowth.
I am currently investigating our own series. We have up till now 37 cups in follow up. All revisions were for aseptic loosening with osteolysis of the acetabulum. All were Paprosky 1 or 2a-c defects. The range is from 3 years to 2 months. I always use the multihole cup with screw fixation and modular inserts. Hooded inserts 10 and 20 degrees and neutral inserts are available. If recurrent dislocation is a problem constrained liners can be used with this cup. Preliminary results show no major complications as such as fracture, dislocation, migration of the cup or infection. No re-operations are reported and X rays do not show any signs of loosening or migration of the cup. The short term results of several authors also seem to be promising. Obviously a longer follow up needs to be done and I hope to publish our data in the near future.
Another big issue is dislocation of total hip replacement, as from 1 to up to 7% has been reported in literature. The cause can be patient characteristics, neuromuscular disorders or recurrent falls. The approach seems to play a role also for the posterior approach used to have higher risk for dislocation. Now posterior approach with meticioulous capsular repair has almost the same risk as an antero-lateral approach. The anterior approach for this complication seems promising. Loss of abduction moment due to decreased off-set can cause dislocation. More often the problem lies with the replacement itself and malposition of the components. When the problem lies with the cup component I use two possible systems. I do not favour a constrained liner. One of my options is a big femoral head with metal on metal articulation. However, recent discussion exists concerning metal ions and ideal cup position. My other option is a double mobility cup. In France mainly, some orthopaedic surgeons use it as their primary hip replacement. Bousquet and co-workers developed the system in 1976. We see that modern devices are being marketed recently as the ADM/X3, of Stryker. I use the polar cup (ortho plus/Smith and Nephew).
The first articulation consists of a 22 or 28mm head and the polyethylene shell. The head is relatively fixed in the liner, but mobile. The second articulation is between the shell and the metal cup. Double articulation creates large freedom of motion without rapid impingement of the femoral neck. The cup has to be used with a slim femoral neck to optimize the advantage of the system. The jump distance is even greater than with a metal on metal articulation. Flexion even up to 180 degrees has been tested in laboratories. Due to the mobility of the polyethylene shell in the cup reduced wear is expected. The lower wear has now been proven in laboratory model testing. The system has promising midterm survival ship of 97.4% at seven years. The dislocation rate reported by Fiquet is 0.22%.
Another indication where I use the double mobility cup is the patient with acceptable neuromuscular disorders or mentally disabled patients with arthritis of the hip joint. One of the possible disadvantages of the Polar cup is groin pain due to impingement of the illiopsoas tendon on the anterior sharp edge of the cup. Newer designs like the ADM cup try to address this problem by a more anatomical shape.
Finally, I would like to highlight that subspecialty is of eminent importance. Experience seems to be the biggest factor to prevent malposition of a hip replacement. With resurfacing hip replacement twenty-five seems the lowest number of procedures to have good results. This was the outcome on the 2010 symposium on hip resurfacing in Ghent. Maybe we should quote a number, based on research, for hip replacements, this to prevent or minimalise malposition. Close follow up of new research results is mandatory. New products should be monitored critically and with suspicion, but they can be an asset in the war against revision hip surgery.
References
- The Dual-Mobility POLARCUP:. First Results From a Multicenter Study, Philippe Bauchu, MD; Olivier Bonnard, MD; Alain Cypr
