We scrutinised the computed tomography (CT) capsular sign, which refers to the anterior capsular distension of the hip, to determine whether we can use it as an additional clue for detecting occult ipsilateral femoral neck fracture (IFNF) before operation.

We retrospectively examined 79 patients who suffered high-energy femoral shaft fracture and were evaluated with preoperative CT for occult IFNF. The occult IFNF was defined as a fracture that was not diagnosed confirmatively during preoperative evaluation. Thirteen patients were included in the occult IFNF group and 66 were included in the femoral neck intact group. A side-to-side difference of more than 1 mm of capsular distension was considered a positive CT capsular sign.

Among 13 patients in the occult IFNF group, 6 had a hairline fracture (bony window images) and 12 had a positive CT capsular sign (soft tissue window images) on preoperative CT scans. The presence of IFNF was more significantly associated with the CT capsular sign than with the hairline fracture (p = 0.031). Among the 66 patients in the femoral neck intact group, the CT capsular sign was false positive in 4.

The CT capsular sign can be used to detect the presence of occult IFNF in high-energy trauma patients with femoral shaft fractures. During the preoperative evaluation of IFNF, surgeons must pay extra attention to the presence of occult IFNF when the CT capsular sign is positive.