Prevention of Dislocation of the Hip :
Posterior Approach and Trochanter Bone Block

I was most interested to read J.P. Meyrueis' article on dislocations following THR, and his answers to Cazeneuve's comments. I, too, have sought to abolish this complication, and have used a variety of approaches. In a homogeneous series of 863 total hip replacements, presented at the 1992 SOFCOT meeting, we had 7.3% dislocations after a posterior Moore approach; 4.1% after a Hardinge; and 2.6% after using a posterior incision and a trochanter bone block technique.

Meyrueis rightly makes the point that posterior dislocations are not always caused by faulty positioning of the implant components. Pascarel had identified three causes: faulty positioning; soft tissue relaxation; and accidental dislocation as a result of excessive flexion-adduction-internal rotation movements during the first postoperative weeks. We routinely practise CT after dislocation, in order to get exact information on the anteversion of the cup and the anteversion of the femur in relation to the bicondylar axis. In the ten cases examined, we did not find any instances of major malpositioning. Correct placement of the implant components is, of course, necessary for a good stress transmission pattern: It does not, however, protect against dislocation.

The dislocation of a THR is a complication that is pre-programmed by the arthroplasty procedure. The short muscles around the hip joint are external rotators, which oppose internal rotation. They are the posterior stabilizers of the hip. If they are divided and left unrepaired, the stage will be set for subsequent dislocation. We must admit that, for 20 years, we had a policy of division and non-repair of these crucial muscles.

When dislocation did occur, we were surprised. With hindsight, we know that there was nothing to be astonished about, since we had, ourselves, destroyed the structures that could have kept the hip stable.

The preservation - or, if they must be divided, the eventual sound reinsertion - of the short external rotators is both mandatory and sufficient for getting rid of dislocations. Lavigne has described a technique involving a posterior approach and a trochanter bone block. With this technique, the stabilizers can be soundly reinserted at the end of the procedure. However, the technique must be properly executed; and it requires a certain learning curve. In my series of 266 cases managed with a trochanter bone block that was reattached using two stout nylon sutures, the dislocation rate went down (to 2.6%); however, the fixation of the block was insufficiently sound to withstand the considerable tensile stresses acting at this site. Only since the routine use of a specially designed anchoring system has dislocation ceased to be a complication (no dislocations in 277 THR patients since May, 1994).

This anchoring system ensures such sound reinsertion of the external rotators that anti-dislocation tricks such as excessive cup anteversion, or limb lengthening to "retension" the muscles, are now well and truly a matter of the past.

The posterior augment described by Meyrueis is very useful in cases where the posterior stabilizers have been destroyed. However, I feel that it is preferable to preserve the natural hip stabilizers in the first place, rather than having to resort to implant design features that will guard against dislocation.

Once the short muscles around the hip joint have been soundly reinserted, forced internal rotation will not be possible. The risk of dislocation will, thus, have been banished. In our series of 227 implants, we did not have a single dislocation.

POSTERIOR APPROACH AND TROCHANTER BONE BLOCK ATTACHED WITH AN ANGKOR® ANCHOR

fig.1	fig.2	fig.3	fig.4
fig.5	fig.6	fig.7	fig.8
fig.9	fig.10

Fig. 1
The short external rotators stabilize the hip joint. If they are divided and left unrepaired, the stage will be set for subsequent dislocation of the THR.
Posterior view of right hip:
1 = gluteus medius; 2 = piriformis; 3 = quadratus femoris; 4 = vastus lateralis

Fig. 2
We detach a trochanter bone block with the insertions of the short external rotators and the posterior tendon of gluteus medius. For sound fixation, the block must be sufficiently thick. The recommended dimensions are 1.5 - 2 cm wide x 3 - 4 cm high.
Lateral view

Fig. 3
The long cut is made with an oscillating saw.

Fig. 4
The transverse cut is made with a chisel.

Fig. 5
The bone block must not extend beyond 4 cm from the tip of the greater trochanter, to guard against the risk of weakening the posterior cortex of the femoral neck.

Fig. 6
For reinsertion, a tunnel is drilled in the bone block, using a 3.2 mm drill bit. Through this tunnel, the Angkor® anchor will be inserted.

Fig. 7
The Angkor® anchor is passed through the tunnel. Another tunnel is drilled across the metaphysis, after incising the vastus lateralis. For optimum reapproximation, it is recommended that the drill should be brought out in the centre of the cut bone surface.

Fig. 8
The Angkor® is railroaded through the tunnel using a crochet-hook.

Fig. 9
The washer and sleeve are inserted, and the anchor is tensioned using a tensioning device.

Fig. 10
After tensioning, the sleeve is crimped using special pliers.

Fig. 11
After the sleeve has been crimped, the tensioning device is removed, and the Angkor® anchor is cut flush with the sleeve, using a wire cutter.

Fig. 12
The gluteus medius tendon is repaired with three absorbable sutures. The sleeve is buried under the vastus lateralis.