In an older adult two weeks after total hip arthroplasty who now has sudden 10/10 medial and lateral right knee pain with posterior knee‑to‑calf bruising, what is the most likely diagnosis and how should it be urgently evaluated and managed?

Urgent Evaluation for Periprosthetic Femur Fracture

This patient most likely has a periprosthetic femur fracture, and immediate plain radiographs (AP and lateral views of the entire femur including hip and knee) must be obtained urgently to confirm the diagnosis and guide surgical management.

Most Likely Diagnosis

The clinical presentation is highly suspicious for a periprosthetic femur fracture occurring during sleep, which explains:

• Sudden onset of severe 10/10 pain in the right knee and upper leg after an initial period of excellent recovery 1
• Posterior knee-to-calf bruising indicating significant hemorrhage tracking distally from a proximal fracture site 1
• Pain in both medial and lateral aspects of the knee, which represents referred pain from the femoral shaft fracture 2, 3

The timing (2 weeks post-operatively) and mechanism (occurring during sleep without trauma) are classic for a stress-related periprosthetic fracture, particularly in the context of recent hip arthroplasty 1.

Immediate Diagnostic Evaluation

Obtain stat plain radiographs of the entire right femur including:

• AP and lateral views of the hip (to assess the prosthesis) 2, 3
• AP and lateral views of the femur (full length) 1
• AP and lateral views of the knee 2, 3

Critical imaging principle: The entire bone length must be visualized before decision-making, as failure to diagnose the full extent of femoral pathology increases risks of adverse outcomes and need for additional surgery 1.

If plain radiographs are negative but clinical suspicion remains high (which is unlikely given this dramatic presentation), MRI of the femur should be obtained within 48-72 hours to detect occult fractures 3.

Urgent Management Algorithm

If Periprosthetic Fracture is Confirmed:

Immediate stabilization measures:

• Non-weight bearing status immediately 1
• Multimodal analgesia including peripheral nerve block (strong recommendation) 1
• Supplemental oxygen administration 1
• Assess hemodynamic stability and obtain hemoglobin level 1

Surgical planning considerations:

• The fracture location relative to the prosthesis determines fixation strategy 1
• Unstable fractures require cephalomedullary nail fixation (strong recommendation) 1
• If fracture extends to involve both the hip prosthesis and distal femur, combined approaches may be necessary (revision arthroplasty with spanning plate fixation) 1

Anticoagulation Management:

Critical consideration: If the patient was on enoxaparin for VTE prophylaxis (as recommended post-operatively) 1, this must be addressed before surgery:

• Hold anticoagulation immediately 1
• If urgent surgery is required and the patient is on therapeutic anticoagulation, reversal may be necessary 4
• The bruising pattern suggests significant hemorrhage, which may indicate ongoing bleeding risk 1

Alternative Diagnoses to Exclude

While periprosthetic fracture is most likely, the differential includes:

Deep venous thrombosis (DVT) with hemorrhagic complications:

• However, DVT typically does not cause 10/10 pain or this degree of bruising 5, 6, 7
• Asymptomatic DVT occurs in 34-37.6% of hip arthroplasty patients but is usually detected on screening, not with acute dramatic presentation 6
• The posterior bruising pattern is more consistent with fracture-related hemorrhage than DVT 6

Hematoma without fracture:

• Less likely given the severity of pain and bilateral knee involvement 1
• Would still require imaging to exclude fracture 2, 3

Critical Pitfalls to Avoid

Do not assume this is simply postoperative pain or DVT without obtaining radiographs first, as delayed diagnosis of periprosthetic fracture significantly worsens outcomes and may necessitate more extensive revision surgery 1.

Do not obtain only hip radiographs – the entire femur must be visualized as the fracture may be in the diaphysis or distal femur, remote from the prosthesis 1.

Do not delay surgical consultation – periprosthetic fractures in the early postoperative period require urgent orthopedic evaluation for operative planning 1.

Assess for anemia and transfusion needs – the visible bruising suggests significant blood loss, and hemoglobin should be checked immediately with transfusion if Hb <8 g/dL with symptoms 1.