Dual Mobility Total Hip Arthroplasty

Dual mobility earns its place in primary THA in patients whose baseline dislocation risk is elevated.

Neuromuscular disorders. Parkinson's disease (impaired proprioception, rigidity), cerebral palsy (spasticity, muscle imbalance), multiple sclerosis (weakness, spasticity), prior stroke with residual weakness or spasticity, and muscular dystrophy.

Cognitive impairment. Dementia or Alzheimer's disease, post-stroke cognitive deficit, inability to follow hip precautions, and psychiatric disorders affecting compliance.

Anatomical risk factors. Abductor muscle deficiency or tears, prior hip surgery with soft tissue damage, spinopelvic imbalance (flat lumbar spine, fixed sagittal imbalance), high BMI (over 35) with increased soft tissue laxity, and developmental dysplasia of the hip (DDH) with abnormal anatomy.

Medical and patient factors. High fall risk (multiple comorbidities, frailty), anticipated poor compliance, substance abuse affecting adherence, and a patient anxious about instability.

Femoral neck fracture in the elderly. A displaced intracapsular fracture in a frail elderly patient with a high dislocation risk (cognitive impairment, prior falls) can be treated with either a hemiarthroplasty or a THA with dual mobility.

Recurrent instability is the gold standard indication and the strongest evidence-based one, with excellent outcomes: a previous THA with two or more dislocations, an atraumatic dislocation pattern, or failed prior stability measures (constrained liner, head size change).

Complex revision. Massive bone loss requiring structural grafts, abductor deficiency not amenable to repair, trochanteric nonunion or loss, and a radiation-treated pelvis with soft tissue compromise.

Infection. In two-stage revision for chronic infection, soft tissue damage from the infection raises the dislocation risk, and dual mobility is used at the reimplantation stage.

Oncological reconstruction. Proximal femoral replacement for tumour and acetabular reconstruction after tumour resection carry a significantly increased instability risk.

Absolute contraindications

• Active infection - it must be treated first
• Inadequate bone stock - stable shell fixation cannot be achieved
• Severe acetabular deficiency - it may require alternative reconstruction

Relative contraindications

• Young, high-demand patients, including high-activity athletes - wear concerns with higher activity
• Severe obesity preventing safe surgery, BMI over 50 - a surgical challenge, not an implant issue
• Insufficient acetabular bone - it may require augments or custom components

Technical requirements. A minimum of acetabular bone for shell fixation, adequate soft tissue coverage and the ability to achieve a stable component position.

The debate is the young, active patient. Very young patients (under 50) with a low dislocation risk may prefer a standard THA, and some patients prefer proven long-term designs. Here the long-term wear and revision burden must be weighed against the stability benefit.

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The acetabular side. A metal shell, typically titanium with a porous coating for bone ingrowth, in sizes typically from 44mm to 66mm. Inside it sits the mobile liner of highly cross-linked polyethylene (HXLPE), free to move within the shell, with a liner retention mechanism whose enhanced design features are there to prevent dissociation.

The femoral side. A standard femoral stem, the same as in conventional THA and cemented or uncemented, carrying a small femoral head.

Two articulations. The primary, intraprosthetic articulation is the small head rotating within the mobile liner. The secondary, extraprosthetic articulation is the liner rotating within the metal shell. A standard THA has one articulation, and its effective head is simply its actual head diameter, 28-40mm; in dual mobility the effective head is the outer diameter of the liner, which improves the head-to-neck ratio.

Jump distance is the distance the femoral head centre must travel perpendicular to the acetabular opening to dislocate: jump distance = (effective head radius) - (neck radius) × sin(inclination angle). The larger it is, the greater the force and displacement needed to dislocate. With dual mobility it is typically 12-18mm, against 5-8mm for a standard THA with a 32mm head.

Wear. With two bearing surfaces, polyethylene wears at both articulations, and the greater total polyethylene surface area means more volumetric wear, and more polyethylene debris, than a standard metal-on-HXLPE THA. That is the long-term concern. HXLPE reduces the wear rate, and the modern evidence shows acceptable wear rates with current-generation designs; the figures are under Outcomes.

What changed. Liner retention mechanisms brought IPD down from 5-10% to under 1%, HXLPE significantly reduced volumetric wear, uncemented options improved long-term fixation, and modular designs allow flexibility at revision. Together these took dual mobility from experimental to mainstream.

Standard dual mobility is the most common configuration, the most cost-effective and the workhorse for most applications: a cobalt-chrome or stainless steel head, an HXLPE mobile liner and a titanium shell with porous coating.

Ceramic head dual mobility puts an alumina or delta ceramic femoral head in an HXLPE mobile liner. It may reduce wear at the primary articulation in theory, but it is less common, costs more and has limited long-term data in dual mobility.

Modular dual mobility cements a dual mobility liner into a retained, well-fixed shell. It converts an existing shell without removing it, which makes it useful in revision and saves bone stock.

Cemented. A metal-backed polyethylene cup cemented to bone. Historical designs were predominantly cemented, and it is still used in elderly patients with poor bone quality, where it may be appropriate for osteoporotic bone. Its advantage is immediate fixation, its price a higher long-term loosening risk.

Uncemented. A porous-coated titanium shell that achieves biological fixation by bone ingrowth. It requires adequate bone quality, gives superior long-term fixation in good bone and is preferred in young patients with good bone. It is the modern standard for most patients, and current practice favours it for most primary and revision cases.

Hybrid. An uncemented shell with a cemented liner insert, a revision-specific technique that allows a stable shell to be retained. It is the modular option described under Bearing Classification.

1. Assess dislocation risk with the stratification table under Clinical Assessment.
2. Counsel the patient. Explain the dislocation benefit and the unique complication, IPD, with the figures in the complications table. Review recovery and precautions, which may be relaxed with dual mobility, and make it a shared decision for the moderate-risk patient.
3. Select components. Choose the dual mobility system, size the acetabular shell on the native acetabulum, select the femoral stem on canal geometry, and plan the combined anteversion (targets under Surgical Technique).
4. Choose the approach on surgeon experience. Posterior is the most common for dual mobility; anterior or anterolateral is also suitable, with standard soft tissue protection whichever is used.

1. Confirm the instability pattern: the number of dislocations (two or more is recurrent instability), direction (posterior, anterior, multidirectional), timing (early, under 90 days, or late) and whether it was traumatic or atraumatic.
2. Assess the components on X-ray and CT: cup inclination and anteversion against the Lewinnek safe zone, combined anteversion against Ranawat's (both under Surgical Technique), wear or loosening, and the sites of impingement.
3. Identify the causes: component malposition (the most common correctable cause), abductor deficiency (examination, MRI), spinopelvic imbalance (sitting and standing lateral radiographs), patient factors (cognitive, neuromuscular, falls), and bone or soft tissue deficiency.
4. Revise to dual mobility, the first choice and gold standard. A well-fixed shell within 5 degrees of ideal is retained and a dual mobility liner cemented into it; a malpositioned or loose shell is revised to a new dual mobility shell; a malpositioned or loose stem is revised with appropriate version. Address abductor deficiency if present (repair, trochanteric advancement).

The alternatives are inferior. The constrained liner has a higher failure rate, 10-15% against 2-5% for dual mobility, for the reasons set out in the next section. A large head revision requires stem revision and is less effective, although the only randomised trial to date found no significant difference in dislocation (see Outcomes). Repositioning components alone may not address all the causes. The evidence favours dual mobility as first-line treatment for recurrent instability.

1. Confirm the indication for arthroplasty: a displaced intracapsular femoral neck fracture (Garden III/IV) in an elderly patient (typically over 65-70 years), ambulatory before injury, with a life expectancy that allows benefit from arthroplasty.
2. Choose the arthroplasty. Hemiarthroplasty is for the less active patient, limited life expectancy and significant comorbidity; THA is for the more active patient, longer life expectancy and acetabular arthritis. Dual mobility is considered for patients at high dislocation risk in either group.
3. Assess dislocation risk. High-risk indicators are cognitive impairment, prior falls, Parkinson's and institutionalised living; standard risk is the cognitively intact, community-dwelling patient without neuromuscular disease.
4. Decide. High risk is a strong dual mobility indication, hemiarthroplasty or THA; at standard risk, weigh dual mobility against standard on activity level and preference. The trend is towards dual mobility for all femoral neck fracture arthroplasty.
5. Operate within 24-48 hours, through any standard approach, with a cemented femoral stem often preferred in fragile elderly bone, and follow standard postoperative care with early mobilisation.

Dual mobility can be used with any standard THA approach. The choice rests on surgeon experience and patient anatomy, not on the implant.

Posterior. The most commonly used approach for dual mobility, with excellent acetabular exposure for shell positioning. Capsular repair matters for stability, as with all THA, using standard posterior soft tissue repair techniques.

Anterolateral. Good visualisation of the acetabulum and preservation of the posterior capsule, through an abductor split or detachment.

Direct anterior. Increasingly popular for dual mobility: an intermuscular interval with no muscle cutting and good component positioning, though it may be more challenging for larger shells.

Reaming. Standard spherical reaming to bleeding native bone, 1-2mm undersize relative to the final shell. Assess bone quality to ensure adequate fixation for an uncemented shell, and control bleeding so the shell seats well.

Sizing. The final reamer size guides shell selection, remembering that dual mobility shells are slightly larger in outer diameter than standard. Ensure peripheral rim contact for stability, and confirm fit and stability with a trial shell before the final implant.

Position. Target inclination 35-45 degrees and anteversion 15-25 degrees, inside the Lewinnek safe zone of inclination 30-50 degrees and anteversion 5-25 degrees, and consider cup and femoral anteversion together (Ranawat safe zone, under Femoral Preparation). Excessive inclination increases the risk of wear and edge loading, and malposition of any kind increases impingement and IPD risk.

Fixation. A 1-2mm press-fit for initial stability, supplemental screws through the peripheral holes as needed, and no micromotion before the liner goes in.

Femoral preparation is identical to standard THA, because the dual mobility design is on the acetabular side. Broach or ream to the standard technique for the chosen stem, cemented or uncemented as the surgeon prefers, with appropriate size and fill.

Stem position. Aim for 10-15 degrees of anteversion, with a combined cup and stem anteversion of 25-50 degrees (Ranawat). Avoid retroversion, which increases the risk of posterior impingement, and restore leg length and offset.

Trial reduction. Trial femoral component with trial head and liner. Check range of motion without impingement, combined version in functional positions, and leg length and offset, restoring them or adjusting appropriately.

Head size is typically 22mm or 28mm, depending on the stem taper and system design. A smaller head reduces intraprosthetic friction and may allow more motion; a larger head increases constraint, and some systems offer the choice.

Liner insertion is the critical step in preventing early IPD.

1. Liner selection - size on the metal shell inner diameter, confirm the manufacturer-specific liner-shell match, and confirm the orientation markings
2. Liner insertion - clean and dry the shell, removing all blood and debris from the shell taper; inspect the liner for damage or debris; align the orientation marks if present; press the liner in until fully seated, with the audible or tactile click that modern locking mechanisms give; confirm visually that there is no gap between liner rim and shell
3. Head-liner assembly - on the back table or in situ; the head must be concentrically and fully seated, with the liner fully encompassing it
4. Trial reduction, if not already done - reduce the assembled construct, test stability through a full range, look for neck-on-liner and liner-on-shell impingement, and confirm no subluxation or maltracking
5. Final implant - disassemble any trial, insert the final components, reduce the final head-liner construct and assess stability

Errors to avoid

• Incomplete liner seating, which may lead to early IPD
• Blood or debris in the shell taper, which prevents proper locking
• Excessive force during reduction, which may damage the liner
• Malpositioned components, which increase impingement

Wear and osteolysis. Modern HXLPE makes this rare. It is watched for on surveillance radiographs (schedule under Postoperative Care), and progressive osteolysis is treated by revision with bone grafting and new components.

Standard THA complications still apply. Infection, periprosthetic fracture, neurovascular injury and leg length discrepancy carry the same risk as in standard THA; loosening may carry a potentially higher polyethylene wear load.

What it is. IPD is the complication unique to dual mobility: the femoral head escapes the retentive rim of the polyethylene liner, the liner dissociating from the head. By definition the liner remains in the metal shell, but a freed liner can migrate: in one reported case it was found lying in gluteus medius.

Why it cannot be watched. With the liner gone, the head articulates directly against the metal shell. That generates metallosis, raises cobalt and chromium levels and damages the shell. The retention mechanism has failed and cannot repair itself.

Early and late. The timing points to the cause.

• Early IPD (under 3 months) - usually technical error: incomplete liner seating during surgery, neck-on-liner impingement levering the liner off the head, or component malposition
• Late IPD (over 3 months) - polyethylene liner wear, liner deformation over time, and older-generation liner retention designs

Presentation. The patient feels the hip "go out", but it is a different sensation from a typical dislocation, and there may be pain, clicking, mechanical symptoms or shortening, often after a preceding dislocation and its reduction. The hip may appear reduced on the X-ray, because the liner is still in the shell and the head has moved medially.

Radiographs.

• AP pelvis: the femoral head appears medialised or eccentric in the cup, having lost its concentric relationship
• Lateral: the head is not concentrically positioned in the liner
• "Double-density" sign: overlapping shadows of the head and the liner rim
• The liner's radio-opaque marker wire displaced away from the head, or the liner absent from the shell altogether
• CT confirms the liner position when the radiograph is equivocal, and finds a migrated liner

Management is operative in every presentation. The rim cannot re-capture the head by closed means, so the treatment is open reduction with liner exchange, urgently rather than electively. Inspect the retentive rim, exchange the mobile component, assess for neck-on-liner impingement, and revise the shell if it is malpositioned or has been scored by metal-on-metal contact. Recurrent IPD is treated the same way, with the search for impingement or malposition; late IPD with wear needs liner exchange, with shell revision considered if the shell is worn or malpositioned.

Prevention. Meticulous technique to ensure complete liner seating, component positions that avoid impingement, and modern liner designs with enhanced retention mechanisms.

What it is. In a true (extra-articular) dislocation the head and liner leave the shell together, giving the classic shortened, rotated limb. It can usually be reduced closed and is managed in principle as a standard THA dislocation, but the reduction carries a hazard specific to this implant: a forceful closed reduction can lever the liner's retentive rim off the femoral head, leaving the liner in the shell and the head bare, and turn a reducible dislocation into an iatrogenic IPD, an implant failure.

The reported pattern. Reported cases share a sequence: bedside reduction, no anaesthesia, no fluoroscopy, and a post-reduction film read as satisfactory.

Recurrent true dislocation. Assess component position, offset, impingement and abductor function, and revise accordingly.

The differential diagnosis of the painful or "unstable-feeling" dual mobility hip, with the features that distinguish each entry and the next step:

Hip instability is the leading early cause of revision THA worldwide, and dual mobility has become a globally accepted strategy to address it. The picture below is evidence-led rather than tied to any single health system.

Global epidemiology and burden of instability

• Dislocation is among the commonest indications for early revision THA; pooled comparative data show dual-mobility cups cut dislocation roughly six-fold versus fixed-bearing cups (dislocation risk ratio 0.16) (Romagnoli et al. Int Orthop 2018; DOI).
• Dual mobility was conceived by Gilles Bousquet in Saint-Etienne, France (early 1970s); long-term modern-generation cementless cups now show no intraprosthetic dislocation or osteolysis at 10-16 years (Gaillard/Batailler et al. J Arthroplasty 2019; DOI).
• Use is highest in France and continental Europe, rising in Australasia and North America (FDA clearance for modular dual mobility, 2009).

Major guidance, side by side

Registry evidence (instability and survivorship)

Global practice variation

• High-resource settings: dual mobility increasingly routine for revision instability and elderly femoral neck fracture; debate persists about routine use in young, high-demand primary THA owing to polyethylene wear concerns.
• Limited-resource settings: higher implant cost and supply constraints limit uptake; standard fixed-bearing THA with optimised component position and head size remains the mainstay, with dual mobility reserved for highest-risk hips.
• Bearing trends: modern HXLPE liners and improved retention mechanisms have largely resolved the first-generation IPD/wear problem; ceramic-head dual mobility is under evaluation but carries limited long-term data.