Dual Mobility in Total Hip Arthroplasty: Biomechanics, Indications and Complications–Current Concepts

Abstract

Dual-mobility (DM) articulations are increasingly utilized to prevent or manage hip instability after total hip arthroplasty (THA). DM cups offer enhanced stability due to the dual articulation resulting in larger jump distance and greater range of motion before impingement. Improvement in design features and biomaterials has contributed to increased interest in dual-mobility articulations due to lower risk of complications compared to their historic rates. The incidence of implant-specific complications like intra-prosthetic dislocation (IPD) and wear has reduced with newer-generation implants. DM THAs are used in primary THA in patients with high risk for dislocation, e.g. neuromuscular disorder, femoral neck fracture, spinopelvic deformity, etc. They offer an attractive alternative option to constrained liner for treatment of hip instability in revision THA. The medium- to short-term results with DM THA have been encouraging in primary and revision THA. However, there are concerns of fretting, corrosion and long-term survivorship with DM THA. Hence, longer-term studies and surveillance are required for the safe use of DM THA in clinical practice.

Introduction

Total hip arthroplasty is one of the most successful and cost-effective procedures in orthopedics [1, 2]. Despite advances in techniques and biomaterials, instability remains one of the major and common postoperative complications necessitating revision procedure. The incidence of dislocation after primary total hip arthroplasty can vary from 1 to 6% depending upon indication, risk factors and approach [3]. Instability can be multifactorial, and several strategies have been recommended to mitigate its incidence. Besides recommendations in surgical approaches and technique (e.g. capsular closure), certain prosthetic options like large femoral heads and constrained liners have been traditionally utilized to prevent or treat hip instability after total hip arthroplasty.

Recently, there has been increased interest in dual-mobility articulations for total hip arthroplasty (THA) to address the issue of hip instability. Dual-mobility (DM) cups offer increased stability by virtue of their dual articulation resulting in larger jump distance and greater range of motion before impingement. It has gained widespread acceptance and appears to be one of the major current strategies for prevention and treatment of hip instability [4, 5]. The number of DM THAs performed in the United States and Europe have increased incrementally over the last decade [6]. Dual-mobility cups have been utilized in primary THA in patients for high risk of dislocation, e.g. neuromuscular disorder, femoral neck fracture, spinopelvic deformity, etc. It is performed in revision scenarios to treat or prevent instability in anticipated high-risk patient population.

In view of growing interest on this topic, we elected to review its history, biomechanics, indications and outcomes in primary and revision THA, complications and concerns associated with DM articulations.

Historical Background

Bosquet pioneered the development of DM designs in 1970s in an effort to reduce the risk of instability following total hip arthroplasty. His initial designs incorporated low friction arthroplasty principle of John Charnley and the concept of inherent stability of larger heads proposed by Mckee [7, 8]. The first-generation DM cups composed of cementless metallic acetabular shell and a mobile polyethylene liner that captured prosthetic femoral head. Press fit fixation of those acetabular cups was achieved by pegs driven into pubis and ischium with alumina surface coating and a dome screw [9]. However, the first-generation designs were associated with risk of delamination and loosening. Additionally, there were several reports of intra-prosthetic dislocation (IPD) with older implant designs [10, 11]. This complication is unique to dual-mobility constructs secondary to failure of retentive mechanism between the mobile polyethylene liner and head leading to direct articulation of the head and shell. It was attributed to polyethylene wear as well as some design features of the construct. The combination of design and biomaterial improvements has contributed to reduction of implant-related failures with current generation of DM constructs [12, 13].

Dual-mobility implants have been available in US following food and drug administration (FDA) approval of its first design in 2009. Variations of this concept have been used clinically in Europe for last 35 years. The improvement in surface coating (e.g. Hydroxyapatite, titanium plasma spray) of the acetabular cups lends themselves to better osteointegration, thus reducing the risk of loosening. Newer designs have been modified to include provision for screw fixation which can aid to improve initial mechanical fixation of the acetabular cup. Furthermore, modifications of the femoral neck in the form of thin and polished neck have helped with reduction of impingement around the edges of the mobile polyethylene insert. The use of skirted femoral heads is discouraged to prevent impingement and hence IPD. Finally, the introduction of highly crosslinked polyethylene in early 2000s has been associated with very low wear rates in long-term clinical studies for standard bearings in total hip arthroplasty [14]. Laboratory studies of highly crosslinked polyethylene in DM constructs have shown encouraging results with low wear rates even in adverse conditions [15, 16]. This can reduce the incidence of late IPD attributed to polyethylene liner wear in the previous generation of the implants.

The newer designs utilize cobalt alloy bearing as opposed to stainless steel which was widely used historically in Europe. The stainless-steel bearings (Polar cup) offer advantage of cemented fixation for complex acetabular reconstruction in revision scenarios. Additionally, there are some subtle variations in implant design and biomaterials between different manufacturers, e.g. anatomic (right and left implants) components with cutout to prevent psoas impingement (Restoration) (Fig. 1), Vit E-infused polyethylene which has shown to reduce wear in stimulated studies (Fig. 2).

Fig. 1.

Stryker dual mobility implant A MDM and B anatomic (ADM)

Fig. 2.

Zimmer Biomet dual-mobility system (cup, metal liner, Vit E mobile poly and head)

Biomechanics of Dual Mobility

Dual-mobility cup designs comprise two main articulations: primary between the prosthetic femoral head and mobile polyethylene and secondary articulation between outer surface of mobile bearing and the metallic acetabular cup. Additional less conspicuous articulation has been described between the rim of the mobile liner and the neck. The primary articulation is engaged during majority of activities of daily livings, whereas the secondary articulation is engaged with activities that exceed normal range of motion (Fig. 3). The dual articulations permit greater range of motion, superior head-to-neck ratio and large jump distance (the magnitude of lateral femoral translation required to dislocate the prosthetic hip joint) reducing the risk of dislocation [17]. Design improvements including chamfered polyethylene and low-profile polished neck can theoretically reduce impingement concerns with DM cups. Terrier et al conducted a comparative biomechanical study between standard, constrained and DM acetabular constructs using finite element analysis [18]. They noted highest range of motion to impingement with DM construct compared to conventional and constrained designs. Furthermore, DM constructs demonstrated lower resistive torque during subluxation events and lower stress on the polyethylene and at the implant–bone interface.

Fig. 3.

Biomechanics: A primary between the prosthetic femoral head and mobile polyethylene and B secondary articulation between outer surface of mobile bearing and the metallic acetabular cup

Primary Total Hip Arthroplasty

DM hip arthroplasty appears to have gained popularity in patients with high risk for hip instability following total hip arthroplasty. Patients at increased risk for instability include those undergoing THA with spinopelvic deformity, neuromuscular disorder, femoral neck fractures and tumor resection. The results of contemporary DM articulations have been encouraging in high-risk patients [13].

(1) DM in femoral neck fractures: hip fractures are one of the most common injuries treated by orthopedic surgeons worldwide. Historically, displaced femoral neck fractures in elderly have been treated with hemiarthroplasty (HA) versus total hip arthroplasty. There have been concerns of hip instability with THA in this patient population and inferior functional outcomes with HA [19]. DM total hip arthroplasty is an attractive option for surgeons wanting to maximize functional outcome while reducing dislocation rates. Several large-scale studies and registry data have demonstrated good clinical outcome with low dislocation rates in patients undergoing DM THA for femoral neck fractures [13, 19]. Furthermore, DMTHA is a good option as a salvage procedure for failed fixation of hip fractures [20]. Long-term studies are required to validate its widespread use in this patient population.

(2) DM in fixed spinopelvic alignment: there is increasing evidence of higher rates of dislocations in patients with fixed spinopelvic alignment undergoing THA [21, 22].The alterations in spinopelvic mechanics and ante inclination with loss of compensatory mechanisms in these patients put them at higher risk for dislocation with change of posture. Recommendations to tackle this issue include patient-specific functional zone placement of acetabular component and consideration of DM implants. Nessler et al. reported encouraging results in a multicenter study on 93 patients who underwent DMTHA with prior history of instrumented spine fusion. There were no cases of instability at a mean follow-up of 2.7 years [23].

(3) DM in patients with neuromuscular disorders: patients with cerebral palsy and acquired neuromuscular disorder, e.g. parkinsonism, traumatic brain injury are at increased risk of dislocation following THA with standard bearings [24, 25]. Anterior surgical approaches and use of constrained implants are some of measures recommended to mitigate the risk of instability in these patients [26]. DM THA is also an effective option in patients with neuromuscular disorders. Harwin et al. reported results on group of patients with neuromuscular disorders at a mean follow-up of 3.3 years [27]. There were no dislocations in this patient cohort. Few other studies have demonstrated promising results in these patients with unique challenges [28, 29]. There is limited evidence on the use of DM articulations in patients with neuromuscular disorders and further long-term studies are necessary to substantiate its benefit in these patients.

Revision Hip Arthroplasty

Historically, the risk of recurrent dislocation has been greater after revision THA than primary surgery. The reported incidence of instability following revision THA varies from 2 to 11% [30]. In the past, constrained liners were frequently used implants for treatment of hip instability. However, they were associated with high failure rates [31]. DM constructs appear to be a viable option for revision procedures for the management of hip instability following primary THA with lower failure rates compared to constrained liners. Contemporary modular dual-mobility implants provide option of adjunct multiple screw fixation in compromised bone stock in complex revision cases. Furthermore, certain dual-mobility metal liners (e.g. polar cup) can be cemented into revision acetabular constructs, thus reducing the risk of potential instability in some of those challenging revision procedures (Fig. 4).

Fig. 4.

Intraoperative photograph of cemented dual-mobility liner in revision THA

Several of these factors make dual-mobility constructs an attractive option in revision scenarios. Multiple studies have demonstrated excellent outcomes with regard to low rates of aseptic loosening and dislocation in revision THA [32–35]. Chalmers et al compared the rates of dislocation and survival of large heads (36 mm and greater) with dual-mobility constructs in the conversion of THA after hip hemiarthroplasty. They reported lower dislocation rates for the dual-mobility construct group at 2-year follow-up [34]. Similarly, Wegrzyn et al. reported a dislocation rate of 1.5% at 7.3 years in a large study on 994 revision dual-mobility constructs [35]. Finally, it has proved to be highly effective strategy for treatment of recurrent hip instability [5].

Complications and Concerns

IPD and dislocation of the DMTHA are some of the common complications seen with this implant (Figs. 5, 6). Additionally, there have been recent concerns of metal liner interface corrosion and stem notching leading to raised metal ion levels.

Fig. 5.

Radiograph of dislocated DM THA

Fig. 6.

Radiograph of intra-prosthetic dislocation following closed reduction of DM THA. Note the eccentric position of the head in the cup

(A) Intra-prosthetic dislocation: intra-prosthetic dislocation is a unique and disconcerting complication associated with dual-mobility articulation THA [11]. It is characterized by disengagement of the mobile polyethylene liner from the head resulting in direct articulation of prosthetic head with the acetabular cup. It may be insidious or of sudden onset associated with hip pain and shortening in the lower extremity. Radiographic findings are subtle with eccentric positioning of the head (Fig. 6) and radiolucent bubble sign in the vicinity of the greater trochanter and acetabulum. High index of suspicion should be maintained for diagnosis of this entity.

The incidence of IPD is much lower with newer-generation of DM constructs [36]. However, there have been recent reports of IPD after closed reduction of dislocated DM THA. Addona et al reported a case series of IPDs following closed reduction of DM THA. [37] It may have been related to loss of retentive mechanism during forceful manipulation during closed reduction. They recommended closed reduction in operating room under regional or general anesthesia and the use of fluoroscopy to potentially avoid this complication. Typically, acute IPD can be managed with bearing exchange whereas chronic cases with metallosis may require component revision.

(B) Raised metal ion levels: there have been concerns of raised metal ion levels with modular DM implants. Gkiatis et al analyzed 248 DM THA performed using cobalt chromium liners [38]. They noted elevated serum cobalt ions in 5.2% of patients at a mean follow-up of 27.4 months. There was a trend towards reduction in metal ion levels between 1 and 2 years and no patients were revised for metal ion-related complications. The use of ceramic head for metal head may help to mitigate metal ion release [38, 39]. Besides malseating of the metal liner should be avoided by meticulous technique to avoid corrosion at modular metal interface [40].

There are recent case reports of femoral stem notching with DM THA [41]. It was noted to occur with titanium stems and attributed to impingement which could be multifactorial in nature. This can lead to high metal ion levels and or catastrophic stem fracture. Hence, close surveillance of DM THA implants over long term is critical with its widespread use.

Conclusion

There has been renewed interest in DM THA primarily due to its potential benefit in prevention and management of instability in high-risk patient population. The indications for DM THA have expanded to different clinical scenarios, e.g. spinopelvic deformity and femoral neck fractures in addition to management of hip instability. DM THA has demonstrated significant reduction in dislocation rates compared to conventional bearings in both primary and revision THA. There may be added theoretical advantages of reduction in wear rates and implant loosening with DM THA. It is critical to have thorough knowledge of the nuances of different implant designs available in the market so that it can be used in appropriate clinical setting. Several studies have shown lower incidence of IPD with the new-generation implants. However, there are concerns of fretting, corrosion and survivorship in younger patients undergoing DM THA. Hence, further long-term studies and surveillance are required for the safe use of DM THA in clinical practice.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical standard statement

This article does not contain any studies with human or animal subjects performed by the any of the authors.

Informed consent

For this type of study informed consent is not required.