Eliminating projection error of measuring Pauwels' angle in the femur neck fractures by CT plane manipulation.

Objectives: The aim for the present study was to determine whether projection error of measuring Pauwels' angle in young femur neck fracture could be eliminated by CT plane manipulation.

Methods: Clinical data of displaced femur neck fractures in young adults aged 20 to 64years old (13 females and 17 males) were retrospectively analyzed. Their average age was 47.9years (range: 22-64years; SD: 11.3). Using modified measurement method for Pauwels' angle using central line of the shaft as a guideline, the angle of a conventional coronal CT image was measured. CT images were imported into Mimics® software. The scanning plane was then reformatted parallel to the neck axis to eliminate projection error of injured limb. Measured angles were classified into three types (I<30°; II, 30-50°; and III>50°) and differences were analyzed.

Results: Average Pauwels' angle was 52.9° (range: 28.6-68.3°; SD: 9.9; type II, 17 cases; type III, 13 cases) for conventional CT images and 68.7° (range: 29.8-91.2°; SD: 13.4; type II, 1 cases; type III, 29 cases) for reformatted CT images. Difference between these two measurements on average was 15.7° (range: 1.2-34.9°; SD: 7.3).

Conclusions: Reformatting CT scanning plane by manipulating the proximal fragment to be parallel with the neck axis of the distal neck-shaft fragment is a convenient and reliable technique for eliminating the projection error of measuring Pauwels' angle in the femur neck fractures.

Level Of Evidence: IV, cohort study.