The use of a dual mobility cup in the management of recurrent dislocations of hip hemiarthroplasty
© The Author(s) 2015
Received: 29 December 2014
Accepted: 15 June 2015
Published: 18 July 2015
Dislocation is one of the most frequent causes of failure of hemiarthroplasties of the hip, which is the most common treatment for femoral neck fractures in elderly patients. A revision with conversion to total hip arthroplasty is the gold standard in case of failure of closed reduction: however, the use of standard or modular components shows variable outcomes. The use of a dual mobility cup has been evaluated in patients with unstable implants, given the good outcomes obtained in primary and revision surgery. The aim of this study was to assess the results of revisions by dual mobility cups in unstable hemiarthroplasties.
Materials and methods
Thirty-one patients (mean age 75.4 years) were retrospectively evaluated between 2006 and 2010 after conversion to total hip arthroplasty with dual mobility cups for recurrent dislocations. The mean number of dislocations was 2.6 (range 2–5). The evaluation was performed by the American Society of Anesthesiologists physical function score (ASA) and the Harris hip score, and several radiologic criteria.
The mean follow-up was 3.8 years. No recurrence of dislocation was recorded. The ASA score remained unchanged, and the mean Harris hip score improved from 62.2 before dislocation to 76.0 points postoperatively.
Dual mobility cups may be a useful option in the treatment of a hemiarthroplasty dislocation. No risk of a new revision due to instability after insertion of dual mobility cups resulted in our experience, and this option may be strongly considered in cases of revisions of unstable hemiarthroplasties.
Level of evidence IV.
Dislocation is one of the major causes of failure of a hemiarthroplasty of the hip (HAH). Its incidence is rated at 6–10 % with respect to 2–3 % for total hip arthroplasty (THA) [1, 2]. Dislocations occur typically within 6 months after surgery , particularly in the first 2–6 weeks. Several factors have been advocated, such as sex, cognitive status, anatomy of the acetabulum (related to patients); femoral head diameter, femoral stem rotation and off-set, surgical approach and excessive removal of joint capsule (related to surgeons) [4, 5]. It is crucial to understand the causes of dislocation before facing surgery with an adequate strategy, in order to limit the recurrence of the instability. Several procedures have been proposed depending on the cause of the dislocation: repositioning of femoral stem , conversion to THA [6, 7], revision with traditional or modular neck components [7–10], use of constrained components [11, 12], trochanteric advancement , removal of acetabular or femoral osteophytes , and repair of the abductor muscles and of the joint capsule [14, 15]. However, all these procedures showed rates of success ranging from 60 to 80 %, independently by the cause leading to instability [6, 10, 13, 16–19]. Particularly, the conversion of HAH to THA demonstrated discouraging results with reports of even worse failure rates than a full revision [6, 7]. The implant of constrained acetabular inserts also showed variable results, with a high risk of increased wear, osteolysis, and instability in THA [11, 12]. Revisions of unstable THAs are generally considered technically demanding procedures [20–22]. Recently, good results have been obtained by the use of “dual mobility” cups for revisions of unstable THAs [23–31] and primary implants after femoral neck fractures , in terms of limitation of dislocation recurrence and preservation of a wide range of motion (ROM): low wear is also expected. To date, no report addresses similar outcomes for the management of unstable HAHs treated by revisions with dual mobility cups.
The purpose of this study was to assess the short-term results of a series of patients affected by unstable HAHs managed by a conversion to THA with dual mobility cups.
Materials and methods
Considering the small size of the study population, only the Wilcoxon signed rank test was used to compare pre- and postoperative HHS scores.
All patients were followed at least for 2 years, with a mean follow-up of 3.8 years (range 2–7 years). The average blood loss was 210 cc (range 100–400), and the mean surgical time was 57.8 min (range 45–120). Seven patients were assisted after surgery in an intensive care unit for 24–48 h. No intraoperative complication was recorded. Postoperative complications were present in six cases (19.3 %): three deep vein thromboses (one unilateral, one bilateral) managed by a mechanical compression and therapeutic doses of low-molecular-weight heparin; one case of urinary tract infection, treated by antibiotics; one case of superficial wound infection, managed by an advanced wound care treatment and oral antibiotics; and one case of an acute imbalance in diabetes mellitus, managed by tailored insulin therapy.
No case of dislocation was recorded during the mentioned follow-up. Radiographic studies revealed radiolucent lines in zone 2 according to DeLee and Charnley in three patients (all with cementless cups). However, these were not progressive and were less than 2 mm in width: these cups were correctly implanted. In three additional cases radiolucent lines of about 1 mm without progression around the femoral component were found in zone 1 (the only patient with the stem revision) and zone 5 (two patients) according to Gruen et al. The mean cup inclination was 45.4° (range 42–49°). An adequate hip centre restoration was achieved in 23 cases. A suboptimal hip centre was achieved in the remaining subjects; however, due to good stability, the patients accepted well the residual length discrepancy (in all cases <1.5 cm). No osteolysis, significant subsidence, or cement mantle fractures were noted, according to the criteria of Loudon and Charnley. No implant was found to be unstable or poorly stable according to Engh’s classification. We recorded three cases (9.6 %) of heterotopic ossifications grade 1 and one grade 2 (the patient with the revised stem), without, however, referred symptoms or functional impairments: two of them did not undergo prophylaxis due to clinical contraindications.
The pillow was maintained for an average interval of 2.8 days (range 2–4). The mean HHS improved from 62.2 points (range 34–75) before the dislocation to 76.0 points (range 71–80) postoperatively with a significant difference (p = 0.002). The ASA score remained basically stable after surgery in all the patients. Symptoms and functional disability progressively decreased over the follow-up period, allowing all patients without neurologic impairments to return to their common daily activities. Poorly or uncollaborative patients were not substantially able to complete a full functional recovery, however, without further episodes of dislocation.
Dislocations of HAHs are generally associated with an insufficient restoration of the centre of rotation or other mechanical problems due to a wrong primary implantation. The conversion of an unstable HAH to a standard THA is a procedure with a high risk of further dislocations, with an incidence often higher than revision THA itself [2, 20–22, 41, 42]. Several reasons have been advocated: the reduction of the diameter and offset of the femoral head, which may produce an inadequate soft tissues tension; the inappropriate positioning of a retained femoral stem, frequently maintained to avoid long surgical procedures in critical patients; and the insufficient retaining properties of the acetabular cup/liner complex. Several other options such as the use of a cemented cup with a structural bone graft fixed with screws, threaded cups with or without bone grafting, constrained cups, reinforcement rings, or “anti-protrusio” cages have been proposed over the decades. Variable results have been obtained in cases of acetabular discontinuity or severe bone loss, poor acetabular rim coverage, and substantial alterations of shape of the acetabulum [43, 44]. In the remaining cases, outcomes were not satisfactory.
Figved et al.  reported a lower risk of complications, including instability, based on the Norwegian Arthroplasty Register, in cases of conversion of HAH to THA with stem revisions, compared to stem retaining procedures. Moreover, in the same series, modular implants for revision presented more advantages related to head size, neck length, and worn head replacement. However, no mention of dual mobility cups has been described.
Only a few studies showed no relationships or even higher rates of dislocation between large diameter heads and the risk of instability in primary and revision implants [41, 42]. Llinas et al.  reported the long-term outcomes of a series of failed HAHs treated with THA with traditional components: higher rates of earlier radiologically detected loosening of acetabular components inserted following HAH failure were found with respect to primary THAs. No mention of dual mobility cups was made in this series.
Constrained cups and liners have been proposed over the years with variable results [11, 12]. Reduction of ROM related to component impingement, increased wear related to high local stresses, and higher risk of loosening were considered the reasons related to significant rates of failure of these implants [23–25].
Dual mobility cups and large femoral heads have their rationale in limiting instability, ensuring a wide ROM with respect to traditional implants, and maintaining low wear in primary and revision hip arthroplasties. Satisfactory long-term outcomes have been reported in several series in primary and revision hip arthroplasty [23–31, 45]. A single multicentre study reported the use of this type of implant for the primary replacement in patients affected by a femoral fracture: a dislocation occurred in three cases out of 214 patients (1.4 %) within the first 3 months . The authors found no recurrence of the dislocation in these patients treated by closed reduction under general anaesthesia, even if they used a posterior approach, generally associated with a higher risk of dislocation with respect to the direct lateral approach [47, 48]. However, to date there has been no significant experience regarding series of HAHs failed for instability and managed by revision with dual mobility cups. Bouchet et al. reported a statistically lower risk of dislocation for the dual mobility cup compared to a conventional 28-mm head and polyethylene inserts implanted through a posterior approach. The instability rate was 0 % compared with 4.63 % for the conventional prostheses . In our series, we recorded improvements in the HHS, and complication rates were comparable to other reports in the literature. Nonetheless, we had no recurrence of dislocation, and no specific failure related to choice of implants. A specific mechanism of failure of dual mobility cups is effectively represented by the intraprosthetic dislocation [49–51]. It consists of the loss of the polyethylene retentive rim, with escape of the femoral head from the liner that may manifest particularly in younger, high-demand patients undergoing a primary THA with this implant [28, 51]. No similar complication was recorded in our series.
The present study has some limitations. It is a retrospective analysis with a small number of patients, and without a control group. However, we do not usually perform revisions with standard or constrained cups for unstable HAHs, using in most cases a dual mobility component: related costs are similar to other choices of treatments. Nevertheless, at short-term follow-up we had no recurrence of instability, with both versions (cemented and cementless) of the dual mobility cup.
We feel that dual mobility cups may be a useful and effective option worth considering in the treatment of HAH dislocations.
Compliance with Ethical Standards
The authors state that the study conforms to the 1964 Helsinki declaration and its later amendments; the study was approved by the local or institutional Ethical Review Board; all the patients provided informed consent before being enrolled.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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