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The main problem with dual mobility cups in the medium term is intraprosthetic dislocation. The head comes out of the polyethylene through wear of the retentive rim. The head then lodges itself in the metalback shell which can be seen by a characteristic view on x.ray. (figure 1). Alas this type of incident is reported in all the literature on series of dual mobility cups, which is an indicator of this sad reality.

Figure 1 bilateral intraprosthetic dislocation

The departments’ retrospective series

We were able to conduct an exhaustive retrospective analysis of all the cases of intraprosthetic dislocation in our departments care between 1991 and 2002. 63 cases were analysed. For each of these 63 cases we had complete medical, surgical and radiological files. Average age at implantation was 51 years.

In 30 cases the stem implanted opposite was a PF. This was a screw in stem with a modular monoblock polished stainless steel neck with a fixed 22.2mm head. The ratio between the head and neck was 1,38. (ie the diameter of the neck below the head was 16mm)
After 1986 the stems implanted were PRO II, a screw in stem with a modular titanium neck with a morse cone to accept 22.2mm, 26mm or 28mm heads. For 22mm necks the head to neck ratio was 1.7. (ie the diameter of the neck below the head was 13mm). 33 patients had this type of stem. In 30 cases the head diameter was 22.2mm.

Time before occurrence (of the dislocation)

In the global series the average time to dislocation was 91 months, 7 and half years. The characteristics of the PF and PRO stems appeared to have an effect on the time before occurrence.

For the PF stems the time was 112 months (SD = 51 months), ie nearly 10 years.
For the PRO stems the time was 77 months (SD = 23.6 months), ie only 6 and a half years.

There is a statistically significant difference between both groups.

Incidence

First we must insist on the rare occurrence of this intraprosthetic dislocation. We found only 63 cases in 12 years, which is equivalent to 5 cases per year in the department, which under Gilles Bousquet, consistently only used dual mobility cups. The only figure truly reported in the literature on the risk of intraprosthetic dislocation, was from within the department on a PRO stem Tripod cup configuration, by Rémi Phillipot. The risk reported was 2% at 10 years for this configuration. It is interesting to note that this report demonstrated the absence of intraprosthetic dislocation in patients implanted after the age of 70.

François Lecuire who reported 7 such cases, estimates the risk at 4%. J.H Aubriot and S.Leclerc report only one case in their study on the dual mobility cup used with a Charnley stem (100 THR seen at 5 years)

Fréderic Farizon declared one case in his series.

It would appear that this failure conducts itself much like a mechanical fatigue phenomenon, and so it is possible to define the cumulative risk with time by a statistical model.

With the PRO stem the cumulative risk at 15 years would be 7% for a young population with an average age at time of implantation of 51. This risk must be considered with the survivorship curves of the implants as well as the consideration that the system ensures the absence of prosthetic instability in the first years.

The decisive influence of age at implantation

The age at implantation is an essential factor in influencing when intraprosthetic dislocation occurs, because three quarters of the observations occurred in patients implanted before the age of 60. In this series of 63 cases, 3 cases, 5%, occurred in patients implanted over the age of 70. The risk of intraprosthetic dislocation in patients over the age of 70 is therefore exceptional (figure 2), and so is not a contra indication for this elderly population which is confronted with an increased risk of post operative prosthetic instability.

Figure 2 : influence de l’âge

The ambiguous role of the head – neck ratio

Impingement of the neck on the retentive zone has been cited as the principal cause of intraprosthetic dislocation. However in the PRO stem series, even though the groups were small, it was in the worst situation of a small head and thick neck that the incidence of intraprosthetic dislocation was longest (74 months for 28mm metal heads, 74 months for 26mm metal heads and 78 months for 22.2mm metal heads). Also when we compare 22.2mm heads for the PRO series to the PF series, the onset of the intraprosthetic dislocation for the PRO group is 78 months compared to 112 months for the PF group despite a more favourable head to neck ratio. The difference is highly statistically significant. Under these conditions the head to neck ratio is not enough to explain the onset of such incidents.

Two wear mechanisms in intraprosthetic dislocation

Analysis of explants demonstrates that there are in fact two types of very different mechanisms which can explain intraprosthetic dislocation.

Wear can be homogenous and symmetrical, the wear being regular around the entire retention rim (figure 3). This wear is a consequence of impingement between the neck and the retention rim. It is the type of wear resulting in dislocation seen in the PF series. In these cases the head can come out of the polyethylene in any position. Clinically this type of dislocation manifests itself most often quite brutally (dislocation, shortening, abnormal noise) which makes the patient come in for consultation quite quickly.

There is a second type of mechanism of wear of the retentive zone. This type of wear is due to a tilting of the polyethylene through gravity, and so responsible for asymmetrical wear. There is a superior polar depression, which reproduces exactly the shape of the head, situated at the junction between of the concavity and the flat equatorial edge of the polyethylene, associated with an inferior polar angular imprint on the flat equatorial edge of the polyethylene as a consequence of an angular sweeping motion of the neck (figure 4). The superior mark is polished by the head and the inferior mark is irregular with raised polyethylene shavings due to an abrasion mechanism. This is the type of mechanism responsible for intraprosthetic dislocation in the PRO series. In this series there is always homogenous and symmetrical wear of the polyethylene in association with this asymmetric wear. However in this PRO series the escape of the head from the polyethylene retentive rim is only possible in one relative position of the components. In this type of wear the dislocation is more insidious and often goes un- noticed; and often revealed during a standard follow up.

We have been able to explain a model for these mechanisms of failure. Bearing in mind our clinical findings and this model, it would appear that asymmetric wear is 5 times faster than homogenous and symmetrical wear. When implanted both mechanisms evolve their own way. Depending on the configuration of the implants implanted, one mechanism or the other occurs. It is the neck design which contributes most to the mechanism of wear of the retentive zone.

Figure 3: Homogenous and symmetrical wear of the retentive zone

Figure 4: Asymmetric wear -Asymmetric wear of the retentive zone due to tilting of the polyethylene -Degradation by progressive subluxation of the head -Degradation through sweeping contact with the neck.

Neck design and head diameter

Obviously the ratio of the head and neck explains homogenous wear by impingement. The best head to neck ratio will retard this homogenous wear.

But it is also the neck which more or less prevents the polyethylene tilting through gravity. The smaller the neck diameter the greater the tilting and the quicker the asymmetric wear leading to dislocation. On the other hand a thicker neck prevents tilting and so retards the onset of dislocation.

Given the same neck diameter, a greater head diameter will reduce impingement and therefore homogenous wear; but on the other hand the greater diameter of the head will favour tilting of the polyethylene and accelerate the onset of intraprosthetic dislocation by asymmetric wear.

It must be remembered that asymmetric wear evolves 5 times faster than homogenous wear. In these cases, rather than preventing homogenous wear through optimisation of the head to neck ratio, it is better to prevent polyethylene tilting.

Some teams propose making the centres of rotation eccentric, of the cup, and the polyethylene, in order to avoid this unwanted tilting; this is an illusion. This might work if there was no rubbing between the polyethylene and metal back which is far from the truth.

Confronted with what we now know, it is probably best to favour 22.2mm heads and especially a specific neck configuration which puts off impingement and opposes polyethylene tilting. The Charnley neck with its cone shape is probably a good solution but which seems perfectible for use with dual mobility; a circular trapezoidal neck would also appear to be a good approach. But is must be remembered that use of a big head, or a small diameter neck are not good solutions in avoiding intraprosthetic dislocation. These are new and totally original elements;

Polyethylene design

Some teams insist on the importance of a chamfer on the free edge of the polyethylene in order to retard collision with the neck. This modification does allow for increasing the cone of range of motion of the first mobility; but the amplitudes remain below functional needs, and so second mobility is always caused by the impact of the neck on the polyethylene. The principle of removing material to create a chamfer does not prevent impingement, but decreases the amount of polyethylene which is eroded before the head can escape. Incorporating a chamfer accelerates the onset of intraprosthetic dislocation. The difficulty lies in finding the right compromise.

Influence of the diameter of the cup

It would therefore appear that the morphology of the neck is the most important element in explaining intra prosthetic dislocation. However, Philippe Adam has raised the issue of the diameter of the cup as an influencing factor. In the PRO series, cup diameter had no influence, but if the PF series is compared to a control series then there is a statistically significant influence of large size cups. In the PRO series it is tilting which favours intraprosthetic dislocation independently of the sizes of the components. In the PF series it is impingement which results in intraprosthetic dislocation; the greater cup diameter, the greater the momentum which mobilises the polyethylene, therefore probably leading to more abrasion.

Influence of calcifications

François Lecuire has quite rightly insisted on the role of calcifications in favouring this incident. Calcifications will prevent mobilisation of the polyethylene and all of the resultant movement constraints will be absorbed by the retentive rim which will wear more rapidly.

Macrogeometry and surface of the neck

Danier Noyer has demonstrated the wear of the retentive rim is in part dependent on macro geometry of the neck (extraction hole) and also the surface finish of the neck; to use the expression, “the rugosity of the neck rubs down the free edge of the polyethylene like sandpaper”. It is best to favour highly polished necks.

Questions

Intraprosthetic dislocation is influenced by a number of factors. Not all patients present this complication, why? On the other hand some patients present with this complication on both sides, why? Other patients, operated on for intraprosthetic dislocation will re-present with the same complication on the same side, why? Some young patients will never present this complication, why?

Conclusions

Though the risk of intraprosthetic dislocation exists, it must be remembered that it is a rare complication, to be balanced with the fact that the prime interest of dual mobility in preventing instability.

Intraprosthetic dislocation is favoured by the young age of the patient at the time of implantation of the prosthesis.

The neck of the stem by its shape, diameter, surface and position will above all else determine how quickly intraprosthetic dislocation will occur.

Questions still need answering, it would seem, on intraprosthetic dislocation and dual mobility.