What is the most appropriate initial imaging study for a patient with a swollen thigh and buttock pain 10 days after hip replacement surgery?

Initial Imaging for Swollen Thigh and Buttock Pain 10 Days Post-Hip Replacement

Start with plain radiographs (X-ray) of the hip and pelvis as the initial imaging study, followed immediately by ultrasound of the thigh to evaluate for deep vein thrombosis (DVT), which is the most critical time-sensitive diagnosis to exclude in this clinical scenario. 1

Primary Diagnostic Approach

Initial Radiographic Assessment

• Plain radiographs of the hip are the universally recommended first-line imaging for any symptomatic hip prosthesis, regardless of the specific complaint 1
• Radiographs can identify periprosthetic fractures, component loosening, dislocation, heterotopic ossification, and other mechanical complications that may present with thigh swelling and buttock pain 1
• However, radiographs have significant limitations: they are neither sensitive nor specific for infection (normal in 50% of infected prostheses), and they cannot reliably detect soft tissue pathology or DVT 1

Critical Time-Sensitive Consideration: DVT Evaluation

• The 10-day postoperative timeframe places this patient at peak risk for deep vein thrombosis, which is the most dangerous diagnosis to miss 2, 3
• DVT occurs in approximately 58% of patients after total hip replacement, with 43% of cases detected by postoperative day 2 and continuing risk through day 10-14 2, 3
• Ultrasound of the thigh is the appropriate next imaging study to evaluate for DVT, as thigh vein thrombosis is significantly more common on the operated side and produces symptoms in 23% of cases 2
• Swollen thigh is a classic presentation of DVT, and buttock pain can occur with proximal extension 2, 3

Algorithmic Decision-Making After Initial Radiographs

If Radiographs Show Mechanical Complications

• CT without IV contrast is the next appropriate study if radiographs suggest but do not definitively show periprosthetic fracture, component malposition, or intra-articular fragments 1, 4
• CT provides superior assessment of joint congruence, component positioning, and acetabular fractures 4

If Radiographs Are Normal or Non-Diagnostic

• Proceed with ultrasound of the thigh first to exclude DVT, given the clinical presentation and timing 2, 3
• If ultrasound is negative for DVT and infection is suspected (given the 10-day timeframe when surgical site infections commonly present), consider image-guided aspiration or MRI without contrast as complementary procedures 1
• MRI without contrast is appropriate for evaluating soft tissue complications including abductor tendon injury, trochanteric bursitis, hematoma, or pseudotumor (though less likely with non-metal-on-metal prostheses) 1

Critical Pitfalls to Avoid

Do Not Delay DVT Evaluation

• The combination of swollen thigh and the 10-14 day postoperative window is classic for DVT, which carries significant morbidity and mortality risk if missed 2, 3
• Thigh vein thrombosis (as opposed to isolated calf DVT) is more likely to be symptomatic and requires immediate anticoagulation 2

Do Not Rely on Radiographs Alone

• Normal radiographs do not exclude infection, DVT, soft tissue hematoma, or early component loosening 1
• Radiographs should be viewed as a screening tool that guides subsequent imaging, not as a definitive study 1

Avoid Inappropriate Advanced Imaging as Initial Study

• CT and MRI are not appropriate initial imaging studies for symptomatic hip prostheses—radiographs must come first 1
• Bone scans, PET/CT, and image-guided aspiration have no role as initial imaging procedures 1

Summary of Recommended Sequence

1. X-ray of hip and pelvis (initial screening for mechanical complications) 1
2. Ultrasound of thigh (to exclude DVT, given clinical presentation and timing) 2, 3
3. CT without contrast or MRI without contrast (if radiographs and ultrasound are non-diagnostic, choice depends on whether mechanical vs. soft tissue pathology is suspected) 1, 4