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Objective:
Ipsilateral femoral neck fractures are known to occur in 1 - 9% of high energy femoral shaft fractures with up to a third of these femoral neck fractures being missed on initial presentation due to the fracture being nondisplaced. Delays in diagnosis can result in eventual displacement of the fracture, increasing risk for fracture nonunion and femoral head avascular necrosis. A recent study using MRI to screen for femoral neck fractures in patients with high energy shaft fractures found radiographically occult femoral neck fractures in 8% of patients with high energy shaft fractures. The purpose of this study is to describe the MRI characteristics of these radiographically occult ipsilateral femoral neck fractures.

Materials and Methods:
Single reviewer retrospective review of all screening pelvic MRIs was performed in patients with high energy femoral shaft fractures and no femoral neck fracture identified on CT presenting to our level 1 trauma center from September 2018 through December 2021. Screening pelvis MRI was comprised of coronal STIR and T1 sequences with slice thickness of 3.0 mm and spacing of 3.3 mm. All patients also underwent a thin slice (2 mm) pelvic CT as part of trauma evaluation. Location, orientation, and STIR/T1 imaging characteristics of femoral neck fractures were recorded along with location and STIR/T1 characteristics of bone marrow edema/contusion of the femoral head, neck, and trochanters.

Results:
Screening MRI was performed in 427 patients with 446 high energy femoral shaft fractures. Bone marrow contusions were present in the femoral head/neck in 43% (193/446) of cases with 84% (n = 162) of these patients having contusion of the greater trochanter. Seven percent (31/446) of cases had an ipsilateral femoral neck fracture: 15 subcapital, 2 transcervical, 10 basicervical, 4 intertrochanteric. All fractures were incomplete with 68% (n = 21) of fractures extending across more than 50% of the femoral neck. Ninety-four percent (16/17) of subcapital/transcervical femoral neck fractures had a vertical orientation while 86% (12/14) basicervical/intertrochanteric fractures had a transverse orientation. 61% (n=19) of fractures had no surrounding marrow edema, 29% (n = 9) had surrounding edema visible only on STIR, and 10% (n = 3) had edema visible on STIR and T1. A fracture line was visible only on the STIR sequence in 10% (n = 3) of cases.

Conclusion:
This study supports the use of MRI to screen for occult femoral neck fractures in patients with high energy femoral shaft fractures and is the first study to describe the MRI characteristics of these fracture. A large majority of these fractures occur in two patterns, either vertically oriented fractures through the subcapital/transcervical region or transverse fractures through the basicervical/intertrochanteric region. The fractures typically occur without associated marrow edema, are incomplete, and may only be visible on the STIR sequence, therefore, careful attention must be paid to the femoral neck on screening MRI to avoid missing this injury.