INTRODUCTION
When we insert one of our implants, what we are all seeking to achieve is a biomechanically optimal restoration of the hip joint. But currently there appear to be three major problems to be overcome:
- the long-wise symmertry of the two lower components;
- ensuring hip stability and eliminating the risk of dislocation: finding the right relationship between the position of the cup, the femoral stem and the implant neck is the number one factor for good hip stability;
- restoring the correct tension of the soft tissues, with a view to restoring the offset, by combining the orientation and length of the neck and the length of the head.
Modular hip implants are a useful tool for dealing with such challenges.
We have tried to adapt implants to the hip anatomy of which the extra- and intra-osseous variability are well known.
A modular approach offers surgeons a wide range of options when selecting implants. At the femoral level, it may be necessary to vary the length and orientation of the implant neck to reproduce the same conditions as the natural joint.
The growing trend towards modularity goes hand in hand with a higher risk of a mismatch between the prosthetic components and a possible limitation to the range of joint motion. The most important risk, however, remains an increase in the amount of metal debris and greater polyethylene wear over the long term. This risk can be reduced by adopting important elements such as the design of the H MAX implant, the choice of a double mobility 2M cup (for patients over 75 years of age), and the planning of arthroplasty.
Double mobility joint systems are unquestionably interesting. In both joints, the aims are similar:
* reduce wear
* minimise the risk of loosening
* restore the original physiology and anatomy as much as possible
* enhance intra-prosthetic stability
The rationale for the double mobility cup was to merge the benefits of two quite different systems, i.e. combine lower PE wear, according to the “low friction” principle described by Charnley, with intrinsic joint stability by re-implanting a “femoral head” of a similar size to that of the patient’s original anatomy, according to the McKee-Farrar principle.
In point of fact, each articulation level of the double mobility cup incorporates the advantage of one of these two systems, combining reduced intra-joint constraints with mechanical hip stability.
Thus the 2M cup appears to meet the initial aims of the double mobility concept: low friction helps to reduce constraints, and thus a more durable bond subject to less stress, and to reduce polyethylene liner wear, whilst the greater diameter of the mobile insert ensures great
er intra-joint stability, approaching the patient’s natural physiology.
TECHNICAL CHARACTERISTICS
The double mobility system is comprised of a cementless LIMA 2M cup lined with a mobile polyethylene insert. Thus there are two concentric articulations:
- the femoral head, articulating within the concave polyethylene liner: the “small” articulation, and
- the convex articulation of the liner within the metal cup: the “large” articulation.
The metal cup is cylindrical-spherical. This complex shape further enhances the stability of the double mobility system, The “grooved” cylindrical-spherical shape permits a wide range of femoral neck excursion, and also enhances coverage in the upper quadrant.
Depending on the implant size, the polyethylene liner represents about five-eighths of the sphere. It always features a femoral head retention device, so the prosthetic head should be pressed firmly into the liner.
Of course, if both articulations were fully and independently mobile, the risk of wear would be very high.
However, in practice, the two articulations are independent only in the static phase (hip joint unloaded), and are highly dependent on one another in the dynamic phase (loading and moving)
The double mobility concept considerably reduces the shear forces induced by the friction coupling, since the main friction coupling is that of the “small” articulation, whilst the other coupling is partly absorbed by the "large" articulation; ultimately, the friction coefficient at the bone-cup interface is minimal.
Numerous factors may cause wear:
- the quality of the polyethylene;
- the shape of the collar. (the use of a grooved collar permits friction-free prosthetic neck excursion);
- the size and shape of the neck; the neck must be small and long enough to avoid impingement between the polyethylene and the base of the implant;
- the surface texture of the neck. In this type of implant there is an articulation between the neck and the PE liner. Therefore the neck should be smooth and polished, preferably in stainless steel or cobalt-chromium
The use of retroverted or medialised necks avoids any residual cam effect, and permits the offset and length to be well regulated without constraining the position of the cup or stem.
Joint stability
The main complication of a primary hip replacement is early dislocation. For the surgeon it is also the most disturbing complication because it often translates into the short term failure of the procedure.
Moreover, though the actual figures may vary from author to author, the rate of recurrence after an initial dislocation remains high.
Therefore, even if a technical error may be an aggravating cause of early dislocation, it is usually a combination of factors that is to blame: improper implant orientation, incorrect component length or lateralisation, muscle failure, trochanter non-union, neurological disorders, alcoholism, cam effect, etc..
CONCLUSIONS
If implant conditions are respected, this combination of modularity and double mobility can achieve excellent functional results, especially the avoidance of dislocation.
The confidence that this approach generates helps the surgeon in the post-operative period: with few exceptions, most are spared the anxiety of having to deal with dislocations; patients also gain benefits, insofar as their hospital stay is likely to be shorter, their rehabilitation faster and their return to everyday life smoother.
