Dislocation & Instability

Hip Dislocations and Instability after Arthroplasty

 Incidence and Timing

• Reported incidence ranges from 0.2% to 10% globally, with large modern registries showing ~2.3% dislocation rate within 2 years after primary THA.

• Half of dislocations (≈52%) occur in the first 3 months, and >80% within 2 years postoperatively.

• Recurrent instability is common — 57% of patients with an initial dislocation experience recurrence, and 11% have ≥5 events, often necessitating revision surgery.

Etiopathogenesis and Risk Factors

 Patient Factors

• Age <65 years and female sex are independent risk factors.

• Obesity (BMI >30) and high comorbidity burden (Elixhauser index ≥3) correlate with higher dislocation rates.

• Neuromuscular disorders, cognitive decline, and inflammatory arthropathy increase postoperative instability risk.

 Surgical Factors

• Posterior approach historically carried higher risk, yet modern evidence shows no difference in dislocation rates between posterior, lateral, or anterior approaches when soft tissue repair is adequate.

• Component positioning remains crucial — excessive cup anteversion, inclination >45°, or combined offset malalignment significantly increase instability.

• Femoral head size: larger diameters (≥36 mm) reduce dislocation risk by improving impingement-free motion arcs.

Implant-Related Factors

• Cemented fixation and metal-on-poly or metal-on-metal bearings are associated with higher instability compared to ceramic-on-poly.

• Dual-mobility cups have emerged as effective solutions for high-risk patients.

Hip Precautions and Rehabilitation

Recent systematic reviews found no statistical difference between restricted vs unrestricted postoperative protocols (2.2% vs 2.0% dislocation rates).
➡️ Early mobilization and functional recovery improve patient satisfaction without increasing risk.Traditional precautions — avoiding >90° flexion, adduction, and internal rotation — have not been shown to reduce dislocation risk following posterior-approach THA.

 Mechanisms of Instability

1. Soft-tissue insufficiency (capsular laxity, abductor deficiency).

2. Component malalignment (excessive anteversion/retroversion).

3. Impingement (bony or prosthetic).

4. Head–neck ratio mismatch or short offset.

5. Neurologic or proprioceptive deficits.

Management Algorithm

Initial episode:

• Closed reduction under sedation → radiographic assessment for component positioning and fracture.

• Activity modification + physiotherapy.

Recurrent dislocation:

• CT-based evaluation of implant orientation.

• Consider dual-mobility constructs, constrained liners, or component revision when malposition or soft-tissue insufficiency confirmed.

Chronic instability:

• Multidisciplinary approach — surgical correction of malalignment, soft-tissue reconstruction, or revision arthroplasty.

Prevention Principles

1. Accurate component positioning is the strongest modifiable factor.

2. Repair posterior capsule and short external rotators when using posterior approach.

3. Assess combined anteversion intraoperatively.

4. Use larger heads (≥36 mm) to increase jump distance.

5. Consider dual mobility or constrained liners in high-risk or revision cases.

💡 Soft-tissue balance and version alignment matter more than approach choice.

Postoperative Protocols

Traditional restrictions (avoiding >90° flexion, adduction, or internal rotation) do not significantly reduce dislocation rates.
Modern rehabilitation emphasizes:

• Early mobilization

• Functional independence

• Education on safe movement patterns

Diagnosis and Evaluation

• Radiographs: confirm reduction, component orientation, or periprosthetic fracture.

• CT scan: assess anteversion, inclination, and bone coverage.

• MRI (metal-artifact reduction): evaluate soft-tissue or abductor insufficiency.

Clinical Pearls

• Most dislocations occur early — meticulous soft-tissue repair and orientation are more impactful than postoperative restrictions.

• Dual mobility or constrained liners should be considered for revision cases or high-risk primary THAs.

• Dynamic stability testing intraoperatively (flexion, rotation, extension) predicts postoperative behavior better than static visual assessment.

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References

1. Gillinov SM et al. Incidence, Timing, and Predictors of Hip Dislocation After Primary THA for OA. J Am Acad Orthop Surg. 2022;30:1047–1053.

2. Crompton J, Osagie-Clouard L, Patel A. Do Hip Precautions After Posterior-Approach THA Affect Dislocation Rates? Acta Orthop. 2020;91:687–692.

3. Dargel J et al. Surgical approach and risk of dislocation. Clin Orthop Relat Res. 2014.

4. Peters RM et al. Effect of reduced hip precautions on dislocation and function after THA. Bone Joint J. 2019.