Total hip arthroplasty versus hemiarthroplasty
Total hip arthroplasty in the elderly is a safe and effective procedure with a survival rate as high as 90% after 15 and 75% after 25 years . Whereas, in older patients, the majority of patients are treated with a THA because of primary osteoarthritis, the indication for an arthroplasty in young patients varies and includes secondary osteoarthritis (most commonly secondary to developmental dysplasia of the hip or trauma), osteonecrosis, ankylosing spondylitis, juvenile idiopathic arthritis, epiphyseal dysplasia, sequelae of Perthes disease, chondrodystrophica and fractures. Although RCT’s show superior results of THA compared to hemiarthroplasties in the short term and have the tendency to be superior after 7–10 years of follow up; these studies regard fractures in elderly patients with a mean age of >70 years [13–15, 25, 26].
Young patients (under 50 years of age) have a much higher implant failure of THA, especially the acetabular component [1–5]. Several studies showed the revision rate, after a THA due to aseptic loosening of the acetabular component to be between 20 and 63% after 10 to 22 years [7–11]. the revision rate for femoral stem loosening however, was between 0 and 23% after 10 to 22 years. Publications from the same institute showed a better 10 year survival of 88% with impaction bone grafting when an acetabular defect was present in combination with cemented cups [5, 28, 29].
As the femoral component is less likely to fail in young patients, it can support the hypothesis that endoprosthetic survival in the younger patients is longer with hemiarthroplasties. The reported revision rate of bipolar hemiarthroplasties in young patients is between 7-21% after 6–14 years [24, 30, 31]. This concurs with our series, in which the femoral component had to be revised in one case because of pseudo-arthrosis at the femoral-allograft junction. Dislocation is a serious complication more often seen after THA compared to hemiarthroplasties, especially in the elderly group after a femoral neck fracture . In our group 3 arthroplasties dislocated.
Our results indicate that a bipolar hip replacement, with 96% survival after 15 and 60% after 20 years independent of age or underlying disease, can be superior to a THA in young patients. In the unipolar group, however, the survival rate was only 50% after 2 years.
One of the major mechanisms of failure after hemiarthroplasty is protrusion of the metal head as the acetabular articular cartilage degenerates [6, 21–24]. In rare cases even osteolysis of the acetabulum is seen .
The degeneration of the articular cartilage is believed to be influenced by, mostly, activity level . The histological process of this degeneration begins with abnormal stress to the articular cartilage due to the hard bipolar cup. This facilitates the secretion of degenerative enzymes which induces the loss of initial glucosaminoglycan. The articular cartilage softens and loses elasticity. Collagen fibres are destroyed and the surface integrity changes. This process is correlated with activity (repetitive stress) levels and the duration of articulation of the implant with the acetabulum . In the end the head will migrate through acetabular cartilage, which is a major cause of the failure of (bipolar) hemiarthroplasty.
Several studies regarding the improvement in functional status after a conversion to a THA after a failed hemiarthroplasty found excellent results [34–36]. When these conversions to a THA were followed in time, they showed a better survival rate of the acetabular component as compared to the femoral component [34, 37]. One author reported a reoperation rate of only 4.5% for aseptic loosening of the acetabular component after10 years .
We hypothesize these results are explained by the repetitive stress caused by the hard bipolar head. This stress not only causes degeneration of articular cartilage, but also causes the subchondral bone to harden. This process, well known in osteoarthritis, might make the acetabulum component less vulnerable to loosening when conversion to a THA is necessary [38, 39]. Especially in the young and active patients in our series, the acetabulum is almost always without damage. Subsequently the subchondral bone should be softer than in patients who have suffered abnormal stress levels and this, we hypothesize, might be the cause for the high rate of aseptic loosening of the acetabulum in THA’s in young patients. We could not confirm this in the current literature and further study should be conducted to explore this hypothesis. In our series we did not encounter problems of protruding bipolar heads.
Hemiarthroplasties produce abnormal stress levels on the acetabulum which in turn causes degeneration [21, 40]. However, when this degeneration leads to pain in the patients or complications, or even failure, of the implant; differs from patient to patient. Extensive follow up, both clinical and radiographical, should be advised.
Two authors studied the clinical outcome after bipolar hemiarthroplasties. In both a correlation was shown between a lower Harris Hip Score and articular degeneration and the incidence of buttock, groin or thigh pain could be used as a marker for failure of the implant. Groin or buttock pain was reported for articular degeneration, whereas thigh pain was believed to be a symptom of loosening of the femoral component or an impending fracture. Both authors suggest early revision or conversion in patients with one of these symptoms [23, 36]. The patients in our series with a failed implant reported the same complaints (Table 5).
Unipolar versus bipolar hemiarthroplasty
Bipolar hemiarthroplasties articulates at two different levels and, due to this dual bearing, is thought to have less acetabular wear. Another advantage of this design would be increased range of motion compared to unipolar implants . A potential disadvantage of the bipolar implants is the risk of polyethylene wear, causing synovitis and loosening of the stem. Several RCT’s have failed to present convincing data on differences in clinical outcome between unipolar or bipolar designs and a Cochrane review in 2010 concluded there is currently not enough evidence to support the use of either unipolar or bipolar prosthesis when performing hemiarthroplasty . Acetabular erosion is thought to be the major factor influencing clinical outcome and reason of revision or conversion. Studies regarding acetabular erosion in patients with hemiarthroplasties show ranges from 2% to 36% for unipolar, and 0% to 26% for bipolar implants [13, 14, 18, 22, 41, 42]. Baker et al . introduced a grading system for acetabular erosion and reported 66% erosion, mostly grade I, after only 3 years of follow up. A recent study found a much lower percentage in bipolar hemiarthroplasties; with only 14% acetabular erosion (all grade I) after four year of follow up . The same author performed a RCT which concluded equivalent clinical outcome between unipolar or bipolar hemiarthroplasties, but a significantly higher incidence of acetabular erosion in the unipolar group . Again, it should be mentioned that these studies are based on elderly patients. In our study we saw acetabular erosion in 35.7%, mostly grade I, after a mean follow up of 7.1 years, which is longer than the studies mentioned above. Because of the small number of unipolar hemiarthroplasties performed in our study, we could not analyze differences between the two types of hemiarthroplasties.