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Computed Tomography Scan to Detect Intra-articular Air in the Knee Joint: A Cadaver Study to Define a Low Radiation Dose Imaging Protocol

Objectives: To determine the lowest estimated radiation dose necessary to reproducibly detect intra-articular air in the knee joint of a cadaver model. Methods: Ten adult fresh-frozen cadaver knees with intact joint capsules were thawed and scanned at 5 decreasing radiation doses (decreasing by approximately half from 8.42 to 0.74 mGy) after introducing increasing volumes (0, 0.1, 0.3, 0.5, 0.7, and 0.9 cc) of intra-articular air. Scans were performed using 2.0-mm slice thickness from the distal one-third of the femur to the proximal one-third of the tibia. Sagittal and coronal reconstructions of each scan using 1.0-mm slice thickness were rendered. All scans were reviewed by (1) a single attending radiologist, (2) a single attending orthopedic surgeon, and (3) a single chief resident, for the presence of intra-articular air. Results: The sensitivity and specificity of computed tomography scan to detect intra-articular air at each volume of intra-articular air (0.1, 0.3, 0.5, 0.7, and 0.9 cc) was 100% at 0.74 mGy—the radiation threshold dose (scan parameters: voltage 80 kV, current: 33 mA, and scan time: 12.17 seconds). The effective radiation dose at 0.74 mGy for a CT scan of the knee is approximately 0.10 mSV. Conclusions: Computed tomography scan to detect traumatic knee arthrotomies can be successfully accomplished at a threshold radiation dose of 0.74 mGy and for an intra-articular volume of 0.1 cc of air. This low radiation dose protocol and volume of intra-articular air should be taken into consideration with future studies evaluating the use of CT scan to detect traumatic arthrotomies.