Introduction
Dislocation of the hip is a common complication following Total Hip Arthroplasty (THA), with an incidence of 2-3%1, but can be as high as 10%. However, following revision, the rate can more than double2 with rates as high as 20% reported 1,3. To compound this problem up to 74% of these dislocations may become recurrent 4-6.
Studies at the Mayo Clinic have estimated that the cost of an open revision for the treatment of dislocation is 150% of the index procedure 7, with outcomes for dislocated hips significantly worse than for those which do not dislocate, with survivorship studies revealing a two year survival rate of 51% 8.
A dislocation occurs when the femoral head slips out of the acetabular socket, and can be split into three distinct modes of dislocation: neck/ lip impingement, boney (trochanteric) impingement and spontaneous separation (see Fig. 1).
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| Figure 1. Modes of dislocation: A.) Neck /Liner impingement. B.) Trochanteric Impingement. C.) Spontaneous Separation. (Scifert et al) |
Dislocations are commonly associated with falls (which are themselves common), but often the dislocations are not in these challenging situations, but during activities such as rolling over in bed, or high flexion of the leg (such as standing from a chair), with the majority of dislocations (77%) being posterior 9.
There are a number of different factors affecting dislocation of the hip, amongst these are clinical (such as the surgical technique, and postoperative treatment), anatomic (often related to the soft tissue) and prosthetic variables, some of which are interdependent. These can also be divided into preoperative, perioperative and postoperative, some of which are beyond the control of the surgeon.
Along with careful repair of the soft tissue, a careful choice of implant may reduce the risk of dislocation. This is especially important in any area of surgery where there is an increased risk.
Patient Risk Factors
The risk factors for revision surgery are slightly different to those of primary surgery, with soft tissue dissection being far greater. However, many factors contribute to this heightened risk.
It is well known that age is a contributory factor to dislocation 10. Morrey et al. showed that THA in patients older than 80 years had a twofold to threefold increase in the rate of dislocation compared with younger patients, and it is no surprise that revision patients are older on average than primary THA patients. More significantly, the number of previous operations also increases the risk 11. So it may be logical that revisions carry an increased risk of dislocation, especially when approaching the second or third revision.
The other important preoperative factor affecting the risk of dislocation is the indication for revision, with often more than one indication present. Patients who have revisions for recurrent dislocation are inherently at risk of suffering from further dislocations, with some studies into dislocation often excluding these patients due to the effect they have on the rate.
It is well known that fractured neck of femurs have an increased risk of dislocation, over THA for osteoarthritis. Compared with their 'normal' revisions (of a THA), Woo et al., and Sah et al. found the rate of dislocation doubled, with both studies giving a figure of 22% dislocation for revision hemiarthroplasty (conversion to THA) 11,12. Sah postulated that this was due to the anatomic mismatch pre and post surgery, with the reduction in the size of the femoral head, and considerable soft tissue compromise due to revision. Sah concluded that increased head size increased stability in revision surgery and recommended its use along with an emphasis on soft tissue tensioning to minimise dislocation risk. Patients who have had their primary surgery for the reason of rheumatoid arthritis are also known to have an increased risk of dislocation 13.
Following periprosthetic fractures, dislocation has been noted as a leading complication by a number of authors 14,15. with complication rates around 10% 6 for dislocation and as high as 27% reported.
Woo et al also gives a figure of 12% for dislocations for patients who have had a history of infection of a total hip, which often damages soft tissues or may have been immobilised as part of the treatment.
Clinical factors associated with late dislocation included previous subluxations without dislocation, a substantial episode of trauma and onset of marked cognitive or motor neurologic impairment, which was also noted by Ali Khan 16.
Soft Tissue
Most authors appreciate that the soft tissues play the most significant part in the stability of the hip joint and the restoration of the integrity and function of the capsule is increasingly recognised as a major goal in the treatment of recurrent dislocation after THA 17. Procedures to restore posterior soft tissue integrity include; capsular plication, fascia lata transfer, synthetic mesh reconstruction, staged Girdleston reconstruction and Achilles Tendon allograft (which is intended to restore the posterior soft tissue stock, and limit the range of motion), especially for the treatment of recurrent posterior dislocations.
It has been known that the posterior approach may decrease blood loss and increase range of motion; however, it is thought to be more susceptible to dislocation than the anterior or lateral approach. This is partly due to the dissection of the short external rotators and sometimes a partial posterior capsulectomy 11. However, a number of studies have concluded that with good soft tissue repair, there is no significant increase in dislocation rate 18,19.
Exposure by trochanteric osteotomy is often used for difficult revision surgery, even by surgeons for whom this is not routine. Complications associated with this are common and have been noted elsewhere in the literature 20. Alberton et al. found that trochanteric non union was a dominant risk factor for dislocation 6. This was also found to be an important complicating factor in other operations 21,22, with Kaplan advocating trochanteric advancement for reducing dislocations. Either way, it is evident that this is an area that needs careful consideration for surgery.
Importantly to primary and revision surgery, the soft tissues appear to weaken with time. Berry et al. investigated the cumulative risk of dislocation in primary THA and found a rate of 1% at one month postoperatively, 1.9% at one year postoperatively, a constant 1% increase every 5 years, and a 7% increase at 25 years, 23 showing that late dislocation is more common than often thought. Coventry 24 postulated that stretching of the supporting soft tissue structures (pseudocapsule) over time and extremes of range of motion (ROM) may lessen soft tissue constraints and allow for late dislocation which may account for the results. Clearly dissection of the soft tissues and scar tissues will weaken the supporting soft tissues.
Prosthetic Factors
As with primary surgery, the implant and its positioning may also play an important part in the biomechanics of the hip.
The Head/Cup
One of the largest controversies concerning dislocation surrounds the head size of the implant and how this affects the stability of the implant. Some early results showed a significant increase in dislocation with small heads, though this was probably due to neck length discrepancies as shown by Eftekhar et al. 25 (which was due to surgeons placing small heads in standard necks without taking into account the effect that this would have on the effective neck diameter). As a result, most studies have shown that head size is not a significant risk 26. However they do show that the likelihood of recurrent dislocations is increased with a smaller head: other authors have investigated why this might be.
In cadaveric and FEA studies Scifert et al. 27 calculated that neck on lip impingement was the predominant factor in dislocation; however, boney impingement became the dominant mode of dislocation as the head size increased. Scifert et al. also found that for every 1mm increase in head diameter, there was a 0.27
