Residency · Residency · Orthopedic Surgery
Instability After Total Hip Arthroplasty
Introduction
Instability and dislocation remain among the most common complications following total hip arthroplasty (THA), accounting for a significant proportion of revision procedures. The reported incidence ranges from 1 to 5% after primary THA and up to 10 to 25% after revision THA. Understanding the risk factors, mechanisms, and management strategies is critical for preventing and treating this challenging problem.
Risk Factors
Patient-Related Factors
Patient-related factors include advanced age (over 75 years) with associated deconditioning and poor muscle tone, female sex (wider pelvis and increased range of hip motion), neuromuscular disorders (Parkinson disease, cerebrovascular disease, dementia, cognitive impairment), prior hip surgery (disrupted soft tissue envelope and scar formation), substance abuse (alcohol and narcotics impair compliance with precautions), and abductor deficiency (prior trochanteric nonunion, muscle atrophy, superior gluteal nerve injury).
Surgical Factors
The most common modifiable surgical factor is component malposition. The posterior approach has historically been associated with higher dislocation rates, though modern capsular repair mitigates this. Inadequate soft tissue tension from failure to restore offset and leg length, impingement (component-to-component, bone-to-component, or bone-to-bone), and trochanteric nonunion or failure of abductor repair all contribute.
Component Position
The acetabular component targets 40 plus or minus 10 degrees of abduction and 15 to 25 degrees of anteversion (the Lewinnek safe zone). Combined anteversion (cup plus stem) should aim for 25 to 50 degrees total. Excessive anteversion predisposes to posterior dislocation, while excessive retroversion predisposes to anterior dislocation. Recent evidence suggests the "safe zone" is not universally protective, as functional pelvic tilt and spinopelvic mobility affect cup orientation dynamically.
Spinopelvic Relationship
Pelvic tilt changes from standing to sitting significantly alter functional cup orientation. Patients with spinal fusion or sagittal imbalance have limited ability to adjust pelvic tilt, placing them at higher dislocation risk. A fused or stiff lumbar spine results in a relatively fixed pelvis, causing the cup to become more anteverted when sitting (posterior dislocation risk) and more retroverted when standing (anterior dislocation risk). Preoperative assessment of spinopelvic mobility with standing and sitting lateral radiographs is increasingly recommended for at-risk patients.
Evaluation of the Dislocating THA
History
The evaluation includes timing of the first dislocation relative to surgery, position and activity at the time of dislocation (flexion/adduction/internal rotation suggests posterior dislocation; extension/external rotation suggests anterior), number of recurrences and interval between events, and compliance with hip precautions.
Imaging
An AP pelvis radiograph assesses cup abduction, anteversion, and femoral stem alignment. A cross-table lateral radiograph evaluates cup anteversion. CT is the gold standard for measuring component version using metal artifact reduction protocols. Standing and sitting lateral radiographs assess spinopelvic mobility.
Assessment for Infection
PJI must always be excluded with ESR, CRP, and joint aspiration before planning revision surgery.
Management
Closed Reduction
Closed reduction is the first-line treatment for acute dislocation, performed under sedation or general anesthesia with fluoroscopic guidance. Stability is assessed through a range of motion after reduction. A hip abduction brace is applied for 6 to 12 weeks. Approximately 65 to 70% of patients who dislocate once will not dislocate again after closed reduction and bracing.
Nonoperative Management
Nonoperative management includes hip precautions and activity modification, abduction bracing for 6 to 12 weeks, and physical therapy focusing on abductor strengthening. It is appropriate for first-time dislocators with adequately positioned components.
| Strategy | Indication | Mechanism |
|---|---|---|
| Component repositioning | Malpositioned cup or stem | Corrects combined anteversion |
| Head/liner exchange | Adequate component position; small head | Increases head-neck ratio and jump distance |
| Dual-mobility bearing | Recurrent instability; high-risk patient | Inner + outer articulation; maximizes ROM before impingement |
| Constrained liner | Severe soft tissue/abductor deficiency | Locking ring captures femoral head |
| Trochanteric advancement | Abductor insufficiency | Increases abductor tension and moment arm |
Revision Surgery
Revision is indicated for recurrent dislocation (two or more episodes) or when a clear modifiable surgical factor is identified. Component repositioning corrects a malpositioned cup or stem to improve combined anteversion, with navigation or robotic assistance used for precision. Head and liner exchange increases head size (from 28 to 32 or 36 mm) to improve the head-neck ratio and jump distance. A constrained liner may be used if soft tissue deficiency prevents adequate tension, or conversion to a dual-mobility construct may be performed.
Dual-Mobility Bearings
Dual-mobility bearings have an inner articulation between a small head and a polyethylene liner and an outer articulation between the liner and the metal shell. They dramatically reduce dislocation rates to less than 1 to 2% in revision for instability and are increasingly used in primary THA for high-risk patients. Intraprosthetic dislocation (IPD) is rare with modern designs.
Constrained Liners
A locking ring captures the femoral head within the polyethylene. This option is reserved for severe soft tissue deficiency or abductor dysfunction when no other option is viable. Higher rates of mechanical failure, liner dissociation, and acetabular loosening result from increased constraint at the bone-implant interface.
Trochanteric Advancement
Trochanteric advancement is indicated when abductor insufficiency is the primary cause, advancing the greater trochanter distally and laterally to increase abductor tension. It is technically demanding with variable union rates.
Prevention Strategies
Prevention includes meticulous component positioning using intraoperative guides, navigation, or robotics; posterior capsule and short external rotator repair when using the posterior approach; larger femoral head sizes (32 or 36 mm) to maximize jump distance; dual-mobility bearings in high-risk patients (elderly, neuromuscular disease, revision, prior spine fusion); preoperative spinopelvic assessment in patients with prior lumbar fusion or sagittal imbalance; and comprehensive patient education regarding hip precautions and safe movement patterns.
Clinical Pearls
Component malposition is the most common correctable cause of THA instability, and CT measurement of component version is essential in the workup of recurrent dislocation. The spinopelvic relationship is increasingly recognized as a major contributor to instability, and patients with lumbar fusion or flatback deformity require special attention to cup positioning. Dual-mobility constructs have become the preferred solution for revision of recurrent instability and for primary THA in high-risk patients. Periprosthetic joint infection must always be ruled out before proceeding with revision surgery for instability.
References
- Lewinnek GE, Lewis JL, Tarr R, et al. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am. 1978;60(2):217-220.
- Abdel MP, von Roth P, Jennings MT, et al. What safe zone? The vast majority of dislocated THAs are within the Lewinnek safe zone for acetabular component position. Clin Orthop Relat Res. 2016;474(2):386-391.
- Heckmann N, McKnight B, Stefl M, et al. Late dislocation following total hip arthroplasty: spinopelvic imbalance as a causative factor. J Bone Joint Surg Am. 2018;100(21):1849-1858.
- Grazioli A, Ek ET, Rudiger HA. Biomechanical concept and clinical outcome of dual mobility cups. Int Orthop. 2012;36(12):2411-2418.