What is the strongest predictor of hip instability after Total Hip Arthroplasty (THA)?

Strongest Predictors of Hip Instability After Total Hip Arthroplasty

The strongest predictor of hip instability after total hip arthroplasty (THA) is postoperative abductor weakness, which increases the risk of recurrent dislocation by over 7 times. 1

Key Risk Factors for Hip Instability

Patient-Related Factors

• Abductor Muscle Status: Postoperative abductor weakness is the most significant risk factor (HR 7.48) for recurrent dislocation after revision THA 1
• Lumbopelvic Stiffness: Patients with lumbopelvic stiffness have a significantly higher risk (HR 6.03) of hip instability 1
• BMI: Being overweight (BMI between 25-30 kg/m²) increases dislocation risk by over 4 times 1
• Medical Comorbidities: Higher American Society of Anesthesiologists (ASA) grade is associated with increased instability risk (HR 2.72) 1
• Age: Advanced age is associated with higher dislocation rates 2

Surgical and Technical Factors

• Component Size and Position: Increasing the acetabular component size by >1mm significantly decreases dislocation risk 1
• Specialized Liners: Paradoxically, the use of specialized liners has been associated with increased risk of recurrent dislocation (HR 5.39 to 10.55) in revision cases 1
• Trochanteric Status: Trochanteric nonunion is a significant risk factor for subsequent dislocation 3
• Femoral Head Size: Smaller head diameters (22mm) are associated with higher instability rates compared to larger heads (28mm and 32mm) 3

Imaging Evaluation of Hip Instability

Initial Imaging

• Radiographs: Should be the first imaging modality for assessment of a symptomatic hip prosthesis 4
• Component Position Assessment: Radiographs help evaluate component positioning, which is critical in assessing instability risk 4
• Comparison Studies: Serial radiographs are useful but may be affected by differences in hip flexion or rotation 4

Advanced Imaging

• CT with Metal Artifact Reduction (MAR): Provides detailed assessment of component position and potential complications 5
• MRI with Metal Artifact Reduction Sequences (MARS): Enables evaluation of soft tissues around the prosthetic hip, particularly useful for assessing the pseudocapsule and tendons 4
• Ultrasound: Valuable for assessing soft tissues adjacent to hip arthroplasties without being affected by prosthetic artifacts 4

Trends in Hip Instability

• Instability rates following primary THA have declined from approximately 4% in 2000 to 1.6% in 2018 6
• Instability is no longer the leading cause of THA failure, now ranking fourth after infection, periprosthetic fracture, and mechanical loosening 6
• The one-year cumulative incidence of recurrent dislocation after revision for instability is 8.7%, increasing to 18.8% at five years and 31.9% at ten years 1

Clinical Pearls and Pitfalls

• Soft Tissue Tension: The extent of soft-tissue dissection is likely the most important variable in revision instability, as head size and trochanteric nonunion are related to "soft-tissue tension" 3
• Elevated Rim Liners: Use of elevated rim liners is associated with significant decreases in dislocation following revision of femoral and acetabular components 3
• Misdiagnosis Risk: In patients with hip prostheses, extracapsular disease associated with adverse reactions to metal debris (ARMD) could be misinterpreted as trochanteric bursitis or other conditions 7
• Treatment Success: Surgical intervention for dislocation has variable success rates, with many patients experiencing recurrent instability even after surgical treatment 1

By understanding and addressing these risk factors, particularly focusing on abductor muscle function and appropriate component positioning, surgeons can work to minimize the risk of instability following total hip arthroplasty.