Ultrasound
I-Chia Ko, MD
Medical Student
Eastern Virginia Medical School
Norfolk, Virginia, United States
John Norbury, MD
PM&R Physician
Virginia Commonwealth University
Williamsburg, Virginia, United States
Mary E. Caldwell, DO
Associate Program Director
Physicial Medicine & Rehabilitation Department, Virginia Commonweath University
Richmond, Virginia, United States
Carly Watson, MD
PM&R Resident physician
Virginia Commonwealth University
Richmond, Virginia, United States
I-Chia Ko, MD
EVMS
Norfolk, Virginia, United States
A 47-year-old male presented with 2 months of left posterior sharp leg pain, numbness, and tingling following total knee arthroplasty 1 year prior (2/27/2025). There was evidence of arthrofibrosis with flexion limited to only 45° post op, and improved to 95° after MUA (4/16/25). Patient reported lateral hamstring tenderness with a snapping sensation and worsening pain on flexion, with paresthesias to the thigh and buttock.
Initial orthopedic sent for EMG to rule out radiculopathy or mononeuropathy. Electrodiagnostic testing of the left leg showed no evidence of radiculopathy or neuropathy. Ultrasound revealed a hyperechoic area with posterior acoustic shadowing, and dynamic ultrasound demonstrated the structure compressing the myotendinous junction of the biceps femoris short head during knee flexion, reproducing the snapping and pain. At this point, the differential diagnosis included fabella versus loose body after the TKA. Comparison with presurgical radiographs confirmed a loose body mimicking fabella impinging on soft tissues.
Discussions: This case highlights the utility of ultrasound in diagnosing posterior knee pain pathology. Ultrasound provided real-time visualization of the osteophyte impinging on surrounding soft tissues, consistent with the patient's presented symptoms. We do note that this is only something to be considered in the differential exam for posterior knee pain.
Conclusions: Ultrasound should be considered to evaluate pathology, such as posterior knee impingement, as it can capture dynamic impingement not appreciated on EMG or nerve conduction study. Although rare, fabella syndrome may cause post-TKA posterior knee pain. Classically, the source of impingement is fabella, whereas in our case it was a loose body in the fabella region. Ultrasound offers valuable dynamic imaging to assess the soft tissues fabella or a loose body mimicking the fabella is interacting with. It provided essential diagnostic clarity in a multifactorial presentation when other diagnostic modalities failed to identify the structural etiology.
