Computed Tomography Scan Detection of Intra-Articular Air in the Ankle Joint: A Cadaveric Study

The diagnosis of traumatic arthrotomy is often accomplished using the saline load test. The introduction of large volumes of saline into the joint is uncomfortable for the patient and has been reported to have low sensitivity and specificity. Computed tomography (CT) scan has been previously validated as a fast and reliable method of detecting free air in the knee joint. The primary objective of this study was to evaluate the use of CT scan for the detection of intra-articular free air in the ankle joint.

Eighteen fresh-frozen cadaver distal lower extremities were thawed, and then CT scans were obtained at their baseline harvested state. The tibiotalar joint was injected with 0.05cc free air, and then repeat CT scans of each specimen were obtained. Specimens with instrumentation, intra-articular air, or ambiguous free air prior to intervention were excluded from the study. Scans were performed from the mid leg through the entire foot at institution standard radiation dose (120 kV, 170 mA) and a slice thickness of 0.6 mm. Images were anonymized, randomized, and sent to 4 reviewers (2 orthopedic surgeons, 1 musculoskeletal radiologist, and 1 radiology resident) for evaluation of free air. Sensitivity and specificity of CT to detect free air were calculated, and Fleiss's Kappa coefficient was used to determine interobserver reliability.

The sensitivity and specificity of CT to detect free air in cadaver ankles was 100%. Twenty-two CT scans (11 natural state and 11 with simulated air arthrotomies) were correctly identified for the presence or absence of intra-articular free air by all reviewers with kappa coefficient 1.0 (complete agreement).

Computed tomography is a fast, reproducible method for detecting small volumes of free air in the ankle and may offer clinical benefit in evaluation of traumatic arthrotomy of the ankle.