Bipolar versus fixed-head hip arthroplasty for femoral neck fractures in elderly patients

A bipolar hemiarthroplasty design has been used for the treatment of femoral neck fractures for more than 30 years. The proposed advantages of using the bipolar design rather than the conventional fixed-head designs for femoral neck fracture in elderly patients are still controversial [8, 9].

In a study of forty cases of Austin Moore replacement done for femoral neck fractures over an average follow-up period of 26 months, Jadhav et al. [10] reported a high incidence of early postoperative pain of noninfective origin, which correlates well with osteolysis. Shortening was seen in 75% cases ranging from 1 to 7 cm. A limp was seen in 35 cases (87.5%) due to pain, shortening, or abductor muscle weakness. Radiological evidence of complications like sinking, protrusion, and calcar resorption, etc. was seen in majority of the cases. In Andersson’s series [11], only 6% cases walked without a limp. Sarmiento [12] in his post-mortems of 24 cases stated that there was “noticeable or excessive motion of stem in the canal and failure of cancellous bone to fill the entire fenestrations in the stem”. Also, in his study of 160 Moore and Thompson prostheses, Whittaker et al. [13] reported that 5% of the acetabula had protrusion and 25% had narrowing after one to 4 years; 24% had protrusion and 64% had narrowing after more than 5 years. Gingras et al. [6] studied cemented Thompson endoprostheses for femoral neck fracture over an average follow-up period of 17 months. Ninety-two percent had no or slight pain, but 8% had evidence of protrusion. Wetherell and Hinves [14] reported a rate of erosion of 11% with the cemented Thompson prosthesis.

Efthekar [15] stated “pressure brought by the femoral prosthesis upon the acetabular cartilage makes subsequent migration of the prosthesis inevitable.”

The bipolar prosthesis has two bearing surfaces; load and frictional torque can theoretically be absorbed in part by the metal on polyethylene inner bearing reducing the magnitude of forces between the implant and acetabulam thus decreasing acetabular erosion [16]. Drinker and Murray [17] in a retrospective series compared the bipolar prosthesis with the Thompson prosthesis and could not show a significant advantage to the bipolar prosthesis. Calder et al. [18] in his study concluded that there is no justification for the use of the expensive bipolar hip prosthesis in femoral neck fracture. On the other hand, La Belle et al. [19] reported that bipolar prosthesis resulted in less pain and decreased protrusio in comparison with the conventional fixed-head prosthesis. Lestrange [20] reviewed 496 patients with bipolar replacements for displaced femoral neck fractures and compared them with patients having fixed-head prosthesis. He found that the bipolar prosthesis offered advantages over one piece designs in terms of stability, decreased acetabular erosion, and improved function.

In the current study, at the final follow-up, the overall results showed better results that were statistically significant for the bipolar group over the fixed-head group in Harris hip scores, acetabular erosion and protrusion. Our results disagree with the randomized prospective study of Van Thiel et al. [21] and Calder et al. [18], who did not find any differences between the Moore unipolar and bipolar prostheses concerning acetabular erosion. Our results are consistent with Yamagata et al. [22] and D’Arcy and Devas [23] who found more erosion with unipolar prosthesis, and Wetherell and Hinves [14] who reported a rate of erosion of 5.6% with the bipolar implant compared with 11% for the cemented Thompson prosthesis. Whittaker et al. [13] reported 5% of the acetabula protrusion after 1–4 year follow-up with Moore and Thompson prostheses. It was demonstrated in our series a lower incidence of acetabular protrusion in bipolar group in comparison with the fixed-head prosthesis. Micheal et al. [16] reported that the bipolar prosthesis reduces the acetabular shear forces through the use of an outer free acetabular cup that also articulates with a prosthetic femoral head. Sikorski [24] reported dislocation rates of 10% in the Thompson prostheses. LaBelle [19] in his study of bipolar hip arthroplasty for femoral neck fractures reported incidence of 0.8% dislocation. In our study, there was no dislocation of prosthesis in the bipolar group, while the incidence of dislocation with fixed-head prosthesis was 4%. This is consistent with Attarian et al. [25] who reported that bipolar prosthesis has a self-aligning acetabular component, which finds a correct orientation on its own (a self-centering mechanism), and the incidence of subluxation and dislocation is low.

The theoretical advantages of inner bearing motion of bipolar prosthesis have not been supported in the majority of the motion studies [1]. Verberne [26] reported that intraprosthetic motion was absent 3 months after surgery. Tsukamoto et al. [27] suggested that motion during walking occurred mainly at the outer bearing. Brueton et al. [28] showed that the size of the inner head was an important determination in allowing inner bearing motion. Small heads (22 mm) allowed bipolar motion, whereas large heads (32 mm) hindered inner bearing motion. The prostheses studied by Verberne [26] had a 32-mm head. Calder [18] reported that movement within the prosthesis may also reduce the pain caused by the prosthesis moving against the acetabulum. In our study, the bipolar group had a better range of motion and 90% of the patients had no limping and 10% had a slight limping, while in the fixed-head group, 22% of the patients had a moderate to severe limping. Favorable results of bipolar may be contributed by the fact that the modularity of the bipolar prosthesis allows for greater flexibility in “customizing” prosthetic sizing so that soft tissue tension and limb length equalization can be improved by ability to use variable neck lengths intra-operatively. This coincides with Cornell et al. [8] who reported that patients with bipolar prosthesis did better on walk tests and had better range of motion at 6 months.

From our results, the bipolar hemiarthroplasty seems to offer a better range of movement with less pain and more stability than the fixed-head hemiarthroplasty in elderly patients with displaced femoral neck fractures in spite of the increased cost factor.