Dual mobility hip arthroplasty: iatrogenic intraprosthetic dislocation of a 22 mm head

Abstract

We describe the first intraprosthetic dislocations of a 22 mm head in a dual mobility hip replacement in the literature. This case emphasises the importance of information gathering and planning when dealing with arthroplasty complications. Furthermore, it highlights the necessity of adequate muscle relaxation and analgesia when dealing with hip dislocations. A 78-year-old patient presented to the emergency department (ED) with a spontaneous dislocation of her left dual-mobility total hip replacement 3 months after first-stage revision surgery for infection. Reduction via manipulation under analgesia was then attempted in the ED but was ultimately unsuccessful; an iatrogenic intraprosthetic dislocation of the polyethylene liner was sustained. Several factors may have contributed to liner dislocation: a failure to appreciate the implant type, multiple manipulations in ED were attempted under analgesia, but no sedation or muscle relaxation was administered to the patient. The patient subsequently underwent revision surgery.

Background

Dual-mobility total hip arthroplasty was initially developed by the French Surgeon Gilles Bousquet in the mid 1970s. It comprises of a mobile polyethylene liner that articulates with both a ‘captured’ femoral head (size range 22 mm, 28 mm, 32 mm) and an acetabular shell, which has a highly polished inner surface (figure 1). This construct allows the majority of motion to occur at the head/liner interface. However, at extremes of motion, the prosthesis neck impinges on the liner, which starts to move and so acts as a large diameter head, thus increasing the jump distance and range of motion, with consequent benefits in terms of reduced dislocation rates and improved joint stability.^1–6

Figure 1.

Photo of the dual mobility construct.

Intraprosthetic dislocation (IPD) is a complication specific to dual mobility implants. IPD occurs when the smaller head dissociates from the large polyethylene liner. The most common cause of IPD occurs as a result of polyethylene wear between the head and liner. As such, it is a late complication and has a reported incidence of between 2% and 5% in long-term follow-up studies (range 3–16 years).^{7 8}

Early IPDs have also been reported in the literature, but their incidence is extremely rare. The proposed mechanism is mechanical leverage of the metal head out of the polyethylene head by a ‘bottle-opener’ effect. This may occur when the polyethylene head engages on either the metal rim of the shell or a bony prominence.⁹ In their Systematic review, De Martino et al⁸ identified only 19 early IPD’s within the literature. Fifteen of these were iatrogenic in nature and secondary to attempted closed reduction. While acknowledging the rarity of early IPD, De Martino et al commented on the fact that they identified no cases that occurred in femoral head sizes of less than 28 mm. This may suggest that 22 mm heads are protective with respect to early IPD.

Despite the rarity of early IPD, prompt treatment is needed to prevent damage of the metal acetabular shell by the metal or ceramic head. The resultant metallosis of a neglected IPD would necessitate potentially complex revision arthroplasty.^{4 5} Given these challenges and the consequences of delayed treatment, it is imperative to raise awareness of such difficulties and identify potential pitfalls.

Hence, we present a case of an iatrogenic IPD of a 22 mm head, resulting from a closed reduction of a dislocated dual mobility implant in the emergency department (ED) setting. We aim to evaluate our clinical approach towards this patient and identify recommendations with which to optimise the management of future such presentations.

Case presentation

An elderly woman presented to the Emergency department with a painful left hip. Anteroposterior and lateral radiographs of the hip showed a posterior dislocation of a dual mobility left total hip replacement (figures 2 and 3).

Figure 2.

Pelvic radiograph of initial presentation to Emergency department with dislocation, lateral.

Figure 3.

Hip radiograph of initial presentation to A&E with dislocation. AP, anteroposterior; GT, greater trochanter.

Relevant pastprevious surgical history included a first-stage revision of an infected left total hip replacement 3 months previously (figure 4). Having made satisfactory progress, she was awaiting a second-stage revision in the coming months. She woke with hip pain rather than incurring a traumatic dislocation. The primary left total hip replacement was performed 2.6 years prior for a fractured neck of femur, after which she was admitted for 5 weeks postoperatively because of Staph aureus sepsis.

Figure 4.

Pelvic radiograph of post first stage revision for infection, anteroposterior. The head and liner are both in socket. GT, greater trochanter; LT, lesser trochanter; L, left.

Her comorbidities included hypertension, breast malignancy treated 20 years previously with lumpectomy, radiotherapy, chemotherapy and Tamoxifen.

Clinically, at presentation to ED, her left hip wound was healed with no signs of infection. Her national early warning score was 0, she was comfortable in herself with no sciatic nerve dysfunction. As standard in our hospital, the ED team contacted the orthopaedic team to perform the relocation. In normal circumstances, the ED team would assist with analgesia and sedation in the resuscitation area; however, due to COVID-19 operational pressures, appropriate expertise to administer the sedation was unavailable.

The dual mobility nature of the replacement was considered, and a decision was made to proceed with manipulation under analgesia, but no sedation. The reduction was attempted in ED under penthrox analgesia two times by both the orthopaedic middle grade and consultant—an audible clunk was heard but the hip ‘redislocated’. Following these reduction attempts, it was noted on the radiograph that the femoral centre of rotation was sitting eccentrically within the socket. In addition, a ‘bubble’ sign was present (figure 5). CT confirmed polyethylene liner dissociation from the femoral head (figures 6 and 7). As the patient was comfortable with no neurovascular compromise, a decision was made to admit and await a further management plan following hip multidisciplinary team discussion.

Figure 5.

Pelvic radiograph of hip post manipulation showing the ‘crescent’ or ‘bubble’ sign: wide semilunar shaped lucency near to implant. Note that although the stem is relocated into the acetabular cup, it is sitting eccentrically suggesting the liner is missing, AP, anteroposterior; GT, greater trochanter; LT, lesser trochanter.

Figure 6.

Slice of CT confirming posterior dislocation of polyethylene liner: semilunar hyperdensity in posterolateral direction from hip.

Figure 7.

Slice of CT confirming posterior dislocation of polyethylene liner.

Investigations

The patient initially had AP and lateral hip radiographs taken. These demonstrated a posteriorly dislocated hip replacement. Subsequent radiographs showed an IPD. This was indicated by the presence of a semilunar lucency close to the implant—The ‘bubble sign’—and an eccentricly located head within the acetabular socket (figure 5).

On CT, the polyethylene liner was visualised outside the shell posteriorly (figures 6–7). While the diagnosis can be made on radiographs alone, the rarity of early IPD means this diagnosis may not be readily considered by the attending team. As such, CT may prove a useful imaging adjunct to confirm the diagnosis.

Infection should always be considered in the presence of a hip dislocation. This consideration was particularly pertinent given the patient’s history of a recent first-stage revision for infection. All observations were within normal parameters and, with a CRP of 1 mg/L and a White blood cells of 9.1×10⁹/L, there was no suggestion of a resurgence of infection as a cause.

Differential diagnosis

This case underlines the importance of checking previous surgical history. In our hospital, electronic operation notes are easily accessible for patients who have had surgery locally.

Both ED and orthopaedic teams need to consider the specific complication of IPD in dual mobility hip dislocations.

Treatment

Initially, closed reduction was attempted by both an orthopaedic middle grade and consultant surgeon under penthrox analgesia. However, during the reduction manoeuvres, the hip was felt to be ‘relocating and dislocating’ in a clunky manner. While an eccentric head was noted on the post manipulation radiographs, the bubble sign and diagnosis of IPD were not initially recognised. A CT scan was, thus, organised and the diagnosis made.

After discussion with the hip team, a decision was made to leave the hip dislocated until the second-stage revision could be performed during the same admission. The patient was comfortable throughout. While blood tests and observations were normal, a hip aspiration was performed the next day to more confidently rule out persisting infection.

As the patient was awaiting a second-stage revision, this was brought forward to happen 3 days after presentation to hospital. Both femoral and acetabular components were revised without incident (figures 8–12).

Figure 8.

Intraoperative photo of revision showing dislocated head.

Figure 9.

Intraoperative photo of revision showing dislocated head.

Figure 10.

Intraoperative photo of revision showing stem. GT: greater trochanter

Figure 11.

Intraoperative photo of revision showing stem.

Figure 12.

Radiograph of post revision, anteroposterior. The head and liner are both in socket. GT: greater trochanter, LT: lesser trochanter

Outcome and follow-up

The patient made an unremarkable recovery. Samples taken at time of revision surgery indicated no persistent infection. Subsequent follow-up (now 10 months) reveals ongoing normal blood markers and no further episodes of instability. Her current Oxford hip score stands at 42/48.

Discussion

IPD of a dual mobility construct is a unique occurrence specific to this implant design. Traditionally, especially with earlier designs, IPD was considered a late complication secondary to polyethylene wear at the liners retentive rim as a result of neck-liner impingement at extremes of motion. Long-term follow-up studies indicate an incidence of late IPD of somewhere between 2% and 5% and occurring at a mean of 8–11 years following the index procedure.^10–13

While more modern implant designs have sort to address this mode of failure with a degree of success,^{14 15} recently reports are emerging of a subset of IPD, which occur early and are predominantly iatrogenic in nature.5 8 11 16 17 Our report highlights such a case although is unique in detailing an early IPD of a 22 mm head.

Our patient’s journey shows similarity to other reports. The patient presented to the ED with a posterior dislocation of a total hip replacement. While the dual mobility implant was recognised, appreciation of the added challenge of reducing a large mobile head was not. An attempt at closed reduction using analgesia, but not sedation, was made. Subsequent IPD resulted. Recognition of this complication was made in a timely manner by the orthopaedic team with the help of a CT scan and revision surgery undertaken during the same admission.

The use of dual mobility implants is on the rise. While they reduce the incidence of dislocation, they do not eliminate it all together. As such, presentation of dislocated dual mobility implants in the ED will occur. Distinguishing a dual mobility from a standard hip replacement may be difficult to the non-specialist eye. It is, thus, paramount that, when available, operative records are checked or the orthopaedic team contacted for confirmation of implant type. Due to the large head size and mobile nature of the construct, dual mobility implants are not as easily reduced as standard heads. As in our case, analgesia alone is likely to be inadequate at achieving reduction and put the patient at risk of an IPD. Even with sedation reduction may be a challenge and, thus, it is now our departmental policy that all dual mobility dislocations be undertaken in theatre with fluoroscopy and the ability to give full muscle relaxation if necessary. As in this case, suspicion of an IPD should occur if the joint feels ‘clunky’ on reduction.

Post reduction films should be carefully checked for an eccentrically positioned head within the acetabulum and a ‘bubble sign’. Where there is doubt—if not already involved—the orthopaedic team should be informed and/or a CT scan organised.

Patient’s perspective.

I was puzzled as to why the dislocation has happened as I have not sustained any trauma since the implant has been put in. I experienced sudden pain, worse than childbirth in the hip spontaneously while lying in bed. I felt I could not wait for the ambulance services to turn up as I was in so much pain, and I asked my husband to drive me to the hospital instead.

Insight from multidisciplinary team

Having had multiple conversations with multiple healthcare professionals, it subsequently emerged that she might have had some trauma to the hip before the pain starting: she tripped over while walking the dog the day before the dislocation happened, however, she managed to walk home without any problems. She did not seem to recollect this event when having a discussion with the author.

Learning points.

• Iatrogenic intraprosthetic dislocation of the dual mobility cup system can be potentially avoided with appropriate pathways, vigilant history taking and a careful review of radiographs.

• Dual mobility hip dislocations should be reduced under deep sedation in emergency department or preferably in the operating theatre environment with anaesthetic and orthopaedic teams involved.

• This is the first recorded case of a 22 mm head sustaining an early intraprosthetic dislocation.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s)