Hip Replacement Dislocation and Instability: Revision Surgery in Birmingham
Dislocation is one of the most common early complications of hip replacement, occurring when the artificial femoral ball separates from the acetabular socket. While a single dislocation can often be treated by manipulation under anaesthetic, recurrent dislocation indicates significant joint instability requiring specialist revision surgery. Mr Shakir Hussain, Consultant Orthopaedic Surgeon at the Royal Orthopaedic Hospital Birmingham, manages hip replacement instability with dual mobility implants and complex component revision.
What is hip replacement instability?
Hip replacement instability is a condition in which the artificial femoral head separates from the acetabular cup, either partially (subluxation) or completely (dislocation). Unlike knee instability, which typically involves ligamentous laxity and a feeling of giving way, hip dislocation is a dramatic, visible event: the ball comes entirely out of the socket, producing immediate severe pain and inability to weight bear.
Dislocation occurs in approximately 1 to 3 per cent of primary hip replacements, making it one of the most common early failure modes. The risk is highest in the first few months after surgery, before scar tissue has formed around the joint to supplement soft tissue stability. However, instability can present at any time after surgery, including many years later, if components migrate or soft tissues attenuate over time.
There are two main patterns of instability, determined largely by the surgical approach used and the direction of component malposition:
- Posterior instability is the most common pattern, associated with the posterior surgical approach. The hip dislocates when placed in a combination of flexion, adduction, and internal rotation (for example, bending forward while the knee points inward). The femoral head levers out of the back of the cup.
- Anterior instability is less common and associated with the anterior or direct lateral approach. The hip dislocates in extension and external rotation (for example, stepping backwards or lying with the leg externally rotated in bed).
A third pattern, late instability, can develop years after a successful primary replacement when soft tissues gradually stretch and thin, component positions shift subtly due to wear or settling, or effective femoral head size decreases as polyethylene liner wear reduces the jump distance. Identifying which pattern is present guides surgical planning for revision.
What are the signs of hip replacement instability?
Acute dislocation causes sudden severe pain in the hip and groin with immediate inability to walk. Before frank dislocation, patients may describe a feeling of the hip slipping or apprehension with certain movements. Recurrent episodes requiring emergency department visits are the clearest sign that revision surgery is needed.
Sudden severe hip pain (acute dislocation)
Immediate severe pain in the hip, groin, or buttock with sudden inability to walk or bear weight. This is a medical emergency requiring urgent hospital assessment and manipulation under sedation or anaesthetic.
Visible limb deformity
During an acute dislocation, the leg often adopts a characteristic position: shortened, internally rotated, and adducted (for posterior dislocation). Clothing may visibly show one leg lying differently from the other.
Feeling of hip giving way
Before frank dislocation, many patients describe a sensation of the hip slipping or giving way with certain movements, particularly bending forward past 90 degrees or crossing the legs while seated.
Apprehension with deep hip flexion
Patients develop fear and guarding with movements that approach the limits of joint stability, including putting on shoes and socks, rising from low chairs, and sitting in car seats.
Recurrent dislocations requiring hospital visits
Multiple presentations to an emergency department for hip manipulation under sedation are the clearest indication that the underlying instability has not been corrected and that revision surgery is required.
Leg length discrepancy during episodes
The dislocated hip appears shorter than the normal side during an acute event due to the femoral head riding superiorly. After successful reduction, leg lengths return to their baseline.
What causes hip replacement instability?
Recurrent instability nearly always reflects an underlying mechanical problem: component malposition (most commonly insufficient cup anteversion), use of a small femoral head offering limited jump distance, or soft tissue insufficiency from abductor weakness or posterior capsule damage. Patient factors including cognitive impairment and neuromuscular conditions also contribute.
Understanding the cause of instability is essential because the surgical solution must address the specific mechanical failure. The main causes include:
- Component malposition. The acetabular cup must be implanted within a safe zone of inclination (typically 40 to 45 degrees) and anteversion (typically 15 to 20 degrees). Cups that are retroverted (pointing too far backward) predispose to posterior dislocation. Cups implanted in excessive anteversion predispose to anterior dislocation. Femoral stem malrotation compounds the problem by reducing combined anteversion below the optimal range.
- Inadequate femoral head size. Jump distance, the margin of movement required before the ball can lever out of the cup, is proportional to femoral head diameter relative to cup opening size. Smaller femoral heads (28 mm or less in older implant systems) carry significantly higher dislocation rates than modern 32 to 36 mm heads.
- Posterior capsule and short external rotator damage. The posterior surgical approach requires division of the posterior capsule and short external rotators to access the hip. If these structures are not repaired at closure, posterior stability depends entirely on component position and head size.
- Abductor muscle weakness. The gluteus medius and minimus provide dynamic stability to the hip. Abductor weakness from nerve injury, prior surgery, or muscle atrophy reduces the compressive force across the joint and increases dislocation risk.
- Patient factors. Cognitive impairment (inability to follow movement restrictions), neuromuscular conditions affecting muscle tone, and a history of recurrent falls all substantially increase dislocation risk and influence whether revision surgery is appropriate.
- Liner wear reducing effective head size. In late instability, polyethylene wear reduces the effective femoral head diameter and thus the jump distance, destabilising a joint that was previously stable. This is an indication for liner exchange with conversion to a larger head or dual mobility design.
How is hip replacement instability investigated?
Standing AP pelvis and lateral hip X-rays assess cup orientation (inclination and anteversion angles) and femoral stem positioning. CT with 3D reconstruction precisely measures combined anteversion. Gait and balance assessment identifies muscle weakness. Fluoroscopic stress testing is used in selected complex cases.
Accurate diagnosis of the underlying cause of instability requires systematic investigation. Mr Hussain will typically request the following:
- AP pelvis X-ray. Assesses cup inclination angle, overall limb length, and offset. The Lewinnek safe zone for cup inclination (30 to 50 degrees) can be measured on this view.
- Lateral hip X-ray. Required to assess cup anteversion on plain radiographs, though the cross-table lateral view has significant measurement variability.
- CT with 3D reconstruction. The gold standard for measuring true cup anteversion, femoral stem anteversion, and combined anteversion. This investigation is essential in planning revision surgery, as it identifies the exact angular correction required.
- Gait and balance assessment. Functional assessment of hip abductor strength, limb length, and gait pattern. Significant abductor weakness may need to be addressed at revision surgery via trochanteric advancement.
- Fluoroscopic stress testing. In selected cases, dynamic fluoroscopy under sedation can confirm the pattern of instability (anterior versus posterior) and assess whether the hip can be stabilised without surgery if component orientation is acceptable.
How is hip replacement instability treated?
A first dislocation is treated with closed reduction under anaesthetic. Revision surgery is indicated for recurrent dislocation or clear component malposition. The choice between isolated cup revision, head size exchange, and dual mobility cup insertion depends on the underlying cause identified on CT and fluoroscopic assessment.
The treatment pathway for hip replacement instability is determined by whether this is a first or recurrent event, whether component malposition has been identified, and the patient's overall fitness for further surgery.
Component Revision and Cup Reorientation
Where CT identifies clear component malposition, surgical correction of cup orientation is the primary strategy. The cup is revised to place it within the optimal safe zone, and the femoral head is exchanged to the maximum size compatible with the cup to maximise jump distance. The posterior capsule and short external rotators are reconstructed where anatomically possible to restore passive posterior stability.
- Cup revised to correct inclination and anteversion
- Femoral head exchanged to maximum compatible size
- Posterior soft tissue reconstruction where possible
- Stem retained if well-fixed and correctly positioned
- Combined anteversion restored to the optimal 25 to 45 degree range
Dual Mobility Cup Conversion
A dual mobility cup provides a second articulating surface: the femoral head moves within a polyethylene insert, and that insert can also rotate within the metal shell. This creates an effective femoral head size substantially larger than the actual head, dramatically increasing the jump distance required for dislocation. Dual mobility cups have reduced dislocation rates to under 1 to 2 per cent even in high-risk revision cases and are now the preferred solution for recurrent instability at specialist centres.
- Two articulating surfaces for intrinsic anti-dislocation design
- Effective head size dramatically increased without oversizing the implant
- Suitable even when component malposition cannot be fully corrected
- Excellent outcomes data across multiple long-term studies
- Constrained liner reserved for the most severe or neuromuscular cases
For elderly or medically frail patients who are not fit for further major surgery, conservative measures including hip bracing, abductor strengthening physiotherapy, and careful movement education can reduce dislocation frequency. However, these do not address the underlying mechanical cause and are not a long-term solution in patients who have repeated dislocations.
Mr Hussain performs complex revision hip replacement for the full spectrum of instability patterns at the Royal Orthopaedic Hospital Birmingham. See the revision surgery page for full details of the operative approach, implant options, and recovery.
What are the outcomes of revision surgery for hip instability?
Dual mobility cups reduce dislocation rates to under 1 to 2 per cent at specialist centres, compared with 3 to 7 per cent with conventional revision implants. Patient satisfaction after dual mobility revision for instability exceeds 90 per cent in published series. Outcomes are best when the underlying cause has been accurately identified and addressed at a specialist revision centre.
The results of revision surgery for hip replacement instability depend critically on correctly identifying and addressing the underlying cause. Dual mobility conversion has the strongest evidence base for recurrent instability and is now the most commonly used solution in specialist revision practice. Key published outcomes include:
Revision surgery outcomes are best when patients are managed at a specialist centre with access to the full range of revision implant systems, including dual mobility cups, constrained liners, and proximal femoral replacements. The Royal Orthopaedic Hospital Birmingham carries all these systems and has dedicated revision arthroplasty pathways that support complex case planning and post-operative rehabilitation.
Specialist expertise in hip replacement instability in Birmingham
Consultant at the Royal Orthopaedic Hospital
Mr Hussain practises at the Royal Orthopaedic Hospital Birmingham, one of Europe's largest specialist orthopaedic centres, with the full range of revision implants including dual mobility systems, constrained liners, and proximal femoral replacements.
3,000+ arthroplasty cases from 5,000+ procedures
A high operative volume including complex primary and revision cases requiring dual mobility implants. Read more about Mr Hussain's training and background.
British Hip Society Travelling Fellowship at ENDO-Klinik Hamburg
Specific training in complex revision hip arthroplasty including instability management under Professor Thorsten Gehrke and Professor Mustafa Citak, two of the world's leading revision hip surgeons.
Complex revision hip surgery expertise
Mr Hussain performs revision hip replacement for the full spectrum of failure modes including instability, loosening, and infection. See the revision surgery page for full details.
Doctify Outstanding Patient Experience 2024, 2025, and 2026
Awarded in three consecutive years, recognising consistently high patient-reported outcomes and communication throughout the surgical journey.
4.98 out of 5 from verified reviews on Doctify. Outstanding Patient Experience Award 2024, 2025, and 2026.
Frequently asked questions about hip replacement dislocation
For more questions about surgery, recovery, fees, and what to expect, see the full frequently asked questions page or read recent patient testimonials.