The association of dual-mobility total hip arthroplasty with dislocation compared to conventional hip arthroplasty for neck of femur fracture

1.   Introduction

Neck of femur fractures are one of the commonest orthopaedic injuries worldwide, with the incidence expected to rise due to the ageing population [1]. These injuries are significant, with a mortality rate of 15% at 1 year according to the National Hip Fracture Database in 2020 [2]. Treatment modality is based on both fracture and patient specific factors, which include fracture fixation or total/partial replacement of the joint. Although total hip arthroplasty (THA) has been found to provide improved functional outcomes compared to hemiarthroplasty, it comes at the cost of an increased dislocation rate [3].

Dual-mobility cups (DMCs) are designed with two mobile interfaces; a prosthetic head that moves freely within a polyethylene liner that is also mobile within a metal acetabular cup. The concept was introduced in the 1970s by Gilles Bousquet and aimed to combine Charnley's low friction principle and the theory of increased femoral head-neck ratio to improve stability and lower dislocation rate [4]–[6]. Several studies agree that lower dislocation and revision rates occur in primary hip arthroplasty and revision surgery when using DMCs [7]. However, the evidence for DMC use in neck of femur fractures is comparatively limited. The aim of this study was to compare the rate of dislocation for patients who received a conventional THA to those that received a DMC for neck of femur fracture at our institution.

2.   Materials and methods

Institutional Ethical Committee Approval was received from Queen Elizabeth University Hospital Department of Trauma and Orthopaedics. A prospectively collated database was reviewed to identify all patients undergoing THA for neck of femur fracture in which a DMC was utilised between the years 2013–2019. This enabled a minimum of 2 years follow-up. Details of their operation, follow up and outcomes were collected from historical hospital records. A comparative group of 100 patients who underwent THA with standard acetabular components was also established using the same database. Surgeon preference dictated which patients received a DMC during this period, and all operations were performed by a surgeon specialising in hip arthroplasty. Patients who were lost to follow-up prior to 2 years post-operative were excluded. Dislocation and revision rate were determined, and radiographs analysed for evidence of aseptic loosening performed. Results were analysed using a paired student T test with SPSS, with a p value of <0.05 considered significant.

3.   Results

Forty-three patients were identified who received a DMC implant (Modular Dual Mobility, Stryker, Newbury, UK) and 100 who received a standard THA. Four patients from the DMC group and five from the conventional THA group died prior to two years follow-up and so were excluded. One additional patient in the DMC group was lost to follow up due to moving to another country. This resulted in 39 patients in the DMC group and 95 in the conventional THA group included in the study. None of the excluded patients suffered a dislocation. All patients were operated on by surgeons who specialised in hip arthroplasty.

The median age for DMC cups was 64 and 69 for conventional THA. 15 patients who received a DMC were male, compared to 30 for conventional THA. All implants were manufactured by Stryker (Newbury, UK) and patient demographics are shown in Table 1. All patients were followed up for a minimum of 2 years. Mean follow up time was 43 months.

There was a non-significant trend towards a lower dislocation rate in the DMC group, with two dislocations (5.1%) in the DMC group compared to seven patients (7.3%) in the conventional THA cohort (p = 0.49). Of the two DMC patients who suffered dislocation, one had a background of a right hemiparesis and the other suffered from balance issues which resulted in a fall and dislocation. Both dislocations occurred within one month of surgery. Regarding the conventional THR patients who suffered a dislocation, three were a result of a fall and two occurred on standing from a seated position. All dislocations occurred within the first year post-operatively. One patient in the DMC group was revised for dislocation, and one from the conventional THA group was revised for aseptic loosening. No patients in the conventional THA group required revision for dislocation. No other cases in either group demonstrated radiographic features of loosening.

Table 1.  Patient demographics and implants used.

Demographic	DMC patients	Conventional THR patients	Total
Number of patients included in study	39	95	134
Male gender (%)	15 (38)	30 (32)	45 (33.6)
Median age	64	69	66.5
Stem (%)	Exeter V40 (92) Restoration stem (8)	Exeter v40 (100)	N/A
Head (%)	MDM liner cementless (100)	V40 (77) Biolox Delta (20) LFIT anatomic (3)	N/A
Cup (%)	Trident (100)	Trident (98) XLFit (2)	N/A

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4.   Discussion

DMCs have shown promising results compared to conventional THA with regard to dislocation rate due to greater inherent stability [7],[8]. This characteristic may be particularly desirable in fractured neck of femur patients, for whom dislocation remains significantly higher than elective THA. Reasons for this include older age and increased ligament laxity in patients with a neck of femur fracture [9],[10].

We have demonstrated an insignificantly reduced rate of dislocation in DMCs compared to conventional THA in patients with neck of femur fractures. These findings are comparable with other papers. The most recent study demonstrated no incidence of dislocation in 53 patients [11]. A larger study of 241 patients showed a 1.4% incidence of dislocation [8] whilst other authors reported no incidences of dislocation [4] and favourable outcomes when compared with conventional THA [12] and bipolar hemiarthroplasty [13],[14]. Nich et al. found a dislocation rate of 6.7% (n = 45) [15] and two systematic reviews demonstrated a maximum dislocation rate of 4.6% [16] and 4% [17]; lower than that of conventional THA. In our study, it is also worth noting that both patients in the DMC group who suffered a dislocation had a medical history that pre-disposed them to falling. This contrasts with the conventional THA group who had no patients with similar predisposing factors. We also found a low incidence of revision rate following DMC THA; however, a large multi-centre study found no reduction in revision risk when a DMC is used compared to THA [18].

Despite encouraging clinical results, concerns have been raised over the increase in polyethylene wear with DMC compared to conventional implants due to the additional articulation surface [19],[20]. Additionally, there is the unique complication of intraprosthetic dissociation (IPD), initially demonstrated by Lecuire et al. [19]. This occurs when the mobile insert that holds the femoral prosthetic head is worn, which leads the head to separate from the mobile insert. This complication arising from long-term wear requires revision surgery in almost all cases [21]. Although a serious complication, the incidence is low with reports of large-scale studies ranging from 0.3 to 1.3% [22]. Factors contributing to IPD are thought to be BMI, femoral stem type and the diameter of the acetabular cup used [21].

It is also important to consider the economic implications when choosing an implant, with DMCs costing almost three times more than a conventional THA cup. A cost analysis by Khoshbin et al. suggested that DMCs are cost effective, particularly in younger patients, however, for patients >75 it is not conclusive [23]. A more recent study suggested DMCs were not cost-effective in any patients for the first two years or in patients over 80 years old. However, DMCs become cost-effective for those aged under 80 years between 5 and 15 years [24]. In the most recent systematic review, the average age of patients receiving a DMC was 77.8 years [24] and was 64 in this study.

There are several limitations to this study. First, the sample size was not large. Although comparable to other similar studies in the literature [4],[8],[11],[12] this is likely responsible for the absence of statistical significance in our results. Additionally, although our database is prospectively collated the study design was retrospective and there was no implant randomisation which could lead to bias in the results. In addition, operations were chosen by each individual surgeon which introduces selection bias, however, with comparable demographics, surgical approach and femoral implants, this may have mitigated some of these potential issues.

5.   Conclusions

DMCs have demonstrated promising outcomes when utilised for degenerative disease. However, fewer studies have investigated their use in neck of femur fractures. In this study, we showed a non-significant trend towards lower dislocation rates with DMC compared to conventional THA in neck of femur fracture; however, this was statistically insignificant. Whilst there are potential issues associated with DMC implants, such as increased wear, as well as cost implications, our results suggest they may be advantageous in patients at high risk for dislocation.

Use of AI tools declaration

The authors declare that they have not used Artificial Intelligence (AI) tools in the creation of this article.