What are the potential causes of hip pain in a patient post bipolar hemiarthroplasty (hip replacement surgery)?

Causes of Hip Pain Post Bipolar Hemiarthroplasty

The most common causes of pain after bipolar hemiarthroplasty are acetabular component loosening with migration (occurring in up to 21% of cases), polyethylene wear leading to osteolysis, periprosthetic infection, and soft tissue pathology including abductor tendon tears. 1, 2

Primary Mechanical Causes

Acetabular Component Loosening and Migration

• Acetabular-side failure is the predominant mechanical problem, accounting for 91% of mechanical failures in bipolar hemiarthroplasty 2
• Progressive migration of the bipolar shell into the acetabulum occurs in approximately 28-30% of cases at 10+ years, often presenting as groin pain 2, 3
• Acetabular reaming at the time of index surgery increases revision risk 6.4-fold compared to non-reamed acetabulum 2
• Radiographic signs include progressive lucencies >2mm at the bone-prosthesis interface and superior migration of the bipolar component 4

Polyethylene Wear and Osteolysis

• Eccentric wear of the polyethylene liner leads to osteolysis, which appears as expansile well-defined lucent lesions on imaging 4
• Catastrophic wear manifests as joint noise, increasing leg length discrepancy, and eventual femoral head subluxation within the cup 5
• Wear-induced synovitis causes joint distension and pain, particularly with weight-bearing 4
• CT with metal artifact reduction is 74.7% sensitive for detecting osteolysis, while MRI reaches 95.4% sensitivity 4

Femoral Component Issues

• Femoral loosening is less common (8% failure rate at 12 years) but presents as thigh pain with weight-bearing 2
• Periprosthetic femoral fractures can occur, particularly in osteoporotic bone or after low-energy trauma 6, 5

Infection

Clinical Presentation

• Chronic low-grade infection is a critical cause of acetabular protrusion and should be suspected in any protruded prosthesis following low-energy trauma 6
• Night pain or rest pain strongly suggests infection, while mechanical pain is predominantly weight-bearing related 1
• Warmth, erythema, fever, or systemic symptoms warrant immediate infection workup 1

Diagnostic Approach

• Image-guided hip aspiration for synovial fluid analysis (cell count, differential, culture) is the most useful test for confirming or excluding infection 1
• Combined WBC scan and sulfur colloid scan has 88-100% specificity for periprosthetic hip infection 4
• CRP >13.5 mg/L has 73-91% sensitivity for infection, though specificity is limited 1
• Joint distension on CT has 83% sensitivity and 96% specificity for infection 4

Soft Tissue Pathology

Abductor Mechanism Dysfunction

• Abductor tendon tears, tendonopathies, and trochanteric bursitis cause lateral hip pain 4
• MRI detects 21 soft tissue abnormalities including 12 tendonopathies and 2 tendon tears that SPECT/CT misses 4, 1
• Trochanteric surface irregularities >2mm on radiographs indicate abductor tendon abnormalities 7
• Greater trochanter avulsion can occur intraoperatively or postoperatively 3

Other Soft Tissue Causes

• Iliopsoas tendinopathy presents as anterior groin pain 4
• Heterotopic ossification can limit range of motion and cause impingement pain 1, 7
• Pseudotumor formation (particularly with metal-on-metal components) causes extracapsular soft tissue masses 8

Referred Pain

• Spine pathology is a common source of referred hip pain in this population 4, 1
• Image-guided anesthetic injection of the hip differentiates intraarticular hip pain from referred pain (especially lumbar spine) 4
• Bone SPECT/CT identified non-hip causes of pain (all in the spine) in 6 of 19 patients with painful prostheses 4, 1

Diagnostic Algorithm

Initial Workup

• Obtain AP pelvis and lateral hip radiographs comparing to prior postoperative films to assess for component migration, progressive lucencies, osteolysis, or periprosthetic fracture 1, 7
• Characterize pain pattern: night/rest pain suggests infection; weight-bearing pain suggests mechanical loosening; lateral pain suggests abductor pathology 1

When Infection is Suspected

• Proceed directly to image-guided hip aspiration for definitive diagnosis 1
• If aspiration is positive or highly suspicious, obtain combined WBC and sulfur colloid scan 1

When Infection is Excluded

• CT with metal artifact reduction is superior for quantifying osteolysis, assessing bone-implant interface, and detecting component loosening (sensitivity 84.85% vs 33.3-51.5% for radiographs) 4, 1
• MRI with metal artifact reduction sequences is superior for soft tissue evaluation including tendon tears, bursitis, and detecting osteolysis (95.4% sensitivity) 4, 1
• Avoid planar bone scans as they cannot differentiate aseptic loosening from other causes and provide insufficient diagnostic value 4, 1
• If advanced imaging is negative but pain persists, bone SPECT/CT identifies alternative pain sources including spine pathology in 68% of cases 4, 1

Critical Pitfalls

• Do not miss chronic infection in protruded prostheses—maintain high index of suspicion even years after surgery 6
• Groin pain after bipolar hemiarthroplasty may persist even after conversion to total hip arthroplasty in 17% of patients, so counsel patients appropriately 3
• Acetabular reaming during index bipolar hemiarthroplasty dramatically increases failure risk—this should influence surgical technique selection 2
• Younger patients (<40 years) and those with advanced disease (ARCO stage IV) have poor outcomes with bipolar hemiarthroplasty and should receive total hip arthroplasty instead 9