CHRONIC INSTABILITY IN TOTAL HIP ARTHROPLASTY

Abstract

This paper reviews the causes of chronic instability after total hip arthroplasty (THA).

The overall reported incidence varies from 0.5% to 9.5%. At 2% to 6%, the incidence following primary THA is higher with a posterior approach than with an anterior approach (0.5% to 3%). The incidence is reported to be as high as 22% after revision THA and 50% after extensile triradiate approach for pelvic discontinuity.

Inadequate soft tissue lengthening, damaged abductors and nonunion of trochanteric osteotomy are known to predispose patients to chronic instability after THA. Elderly women are particularly susceptible. Poor patient compliance is also a cause.

Surgical technique is also a factor. The lateral decubitus position often causes flattening of the lumbar lordosis, leading to potential cup retroversion. Over 90% of all dislocations are posterior, and disruption of external rotators and capsular damage should be repaired if possible. The optimal implant position appears to be 40° TO 45° of abduction, 15° to 20° of femoral anteversion, and 20° to 30° of cup flexion. Elevation of the hip centre weakens abductor pull, causing instability. Because a reduced femoral offset causes potential instability, this should be measured preoperatively to make sure that the stem can provide adequate offset. It may be necessary to add a thicker liner to increase the offset.

Prosthetic factors which play a role in chronic instability include the use of smaller femoral heads, thick necked stems and heads with skirts. A larger femoral head increases stability simply by increasing the radian about the hip centre, increasing the potential range of motion. Extended posterior wall-adds improve the range of motion, and consequently the stability. However, there are fears that their use may increase the incidence of impingement and/or lead to increased wear. Skirted femoral heads impinge on the liner, limiting movement, and their use should be avoided in most cases of instability.

Femoral stem offset relates to the neck shaft angle and the effective hip centre/shaft axis length or offset. It is easier to increase offset with lower neck shaft angle than to lengthen the leg. Because a bell curve is used in the design of femoral stems, many prosthetic systems lack adequate offset, especially when larger stems (48 mm to 52 mm) are used.

In earlier prosthetic designs, bulk was added to the necks to eliminate stem breakage. In certain stems, the way in which dimensions were scaled meant the neck dimensions of larger prostheses were disproportionately big. We stopped using Depuy Stability stems sizes 16 mm and 18 mm because of this. Thornberry et al have shown that a circulotrapezoidal neck design is the best shape and leads to the least impingement. They have also shown that increasing the width of the chamfer of the acetabular liner rim improves the range of motion.

In treating early instability (occurring less than 30 days postoperatively) most authors recommend bracing for six to eight weeks and warning patients severely about the long-term potential of redislocation. In cases of chronic instability (occurring more than 30 days postoperatively) all potential problems must be explored: these include soft tissue laxity, cup retroversion, inadequate offset, surgical approach, etc. In managing multiple dislocation, the use of extended immobilisation is less desirable although patients who have undergone revision have been subjected to a great deal of soft tissue dissection and potentially should be braced for up to 12 months. If the cause is correctable-malpositioning, soft tissue laxity or bony impingement – treatment is likely to be successful in 85% of cases. However, if the implants are in good position, the ‘bloodless revision’ (Fehring) has less than 50% chance of succeeding. The implication is that an extended posterior wall liner, longer modular femoral head, and soft tissue reconstruction are not going to work in the majority of cases.

Designed by Noiles, the J& J SROM constrained acetabular liner uses a polyethylene capture mechanism that is secured by two additional screws. The pullout strength of this device is 1 350 N but torque required (lever-out strength) diminishes to 17.3 N.m for a 28-mm head. With a 32 mm head, 105° of flexion was obtained (while the normal hip needs up to 113° for usual flexion). Following up 21 patients with this implant for over two years, Anderson et al found redislocation in 29%. The only causative factor identified was an abduction angle of more than 70°. However, there were no cases of implant loosening of this device. Prevention of loosening was one of the design goals in using a ‘softer’ locking mechanism. Dislodgement of the liner requires immediate re-operation.

The Osteonics constrained liner cup has a dual socket. The inner socket has a polished chrome surface manufactured fit to the outer socket. It fits a 22 mm or 28 mm head, and has a locking ring identical to the bipolar implant that holds the head in place. The implant can be snap-fitted into a 52-mm or larger Osteonics cup. This liner can also be cemented into another metal-backed liner. Goetz et al evaluated 56 cases, in 10 of which this implant had been cemented and in 46 lock-fitted in appropriately matched metal shells. In one case, the cemented constrained liner had separated from the metal shell. None of the constrained liners had separated from the metal shells, but one shell had loosened.

There are many similar constrained acetabular liners. The choice is between a ‘locked’ liner that can never separate and a ‘softer’ lock that may protect fixation of the cup.