Lateral Stress Radiographs of Lateral Compression Type 1 Pelvic Injuries, With the Injured Side Down or Up, Reliably Measure Occult Instability.

OBJECTIVES

To evaluate the interobserver reliability of measured displacement and occult instability of minimally displaced lateral compression type 1 (LC1) fractures on lateral stress radiographs (LSRs) and to compare differences in displacement between LSR with the injured side down (ID) and up (IU).

DESIGN

Retrospective review.

SETTING

Urban Level 1 trauma center.

PATIENTS/

PARTICIPANTS

Twenty-three adult patients with minimally displaced (<1 cm) LC1 injuries.

INTERVENTION: None.

MAIN OUTCOME MEASUREMENTS: Three orthopaedic surgeons measured the distance between the radiographic teardrops on LSR and supine anteroposterior pelvic radiographs to calculate dynamic fracture displacement. The interobserver reliability of the measured displacement, a continuous variable, was assessed by calculating the intraclass correlation coefficient. The interobserver reliability of occult instability (≥10 mm of displacement on LSR), a categorical variable, was assessed by calculating the kappa value. Matched-pairs analysis was performed to calculate the mean difference of measurements between observers and between ID and IU LSR.

RESULTS

The interobserver reliability of the measured displacement was excellent (intraclass correlation coefficient 0.93). The mean difference in measurements between observers ranged from -1.8 to 0.96 mm. The mean difference in the measured displacement between ID and IU LSRs for each observer ranged from -0.6 to 0.3 mm. There was 83% (19/23 cases) agreement on the presence of occult instability (≥10 mm of displacement on LSR) on both ID and IU LSRs. The interobserver reliability of occult instability was moderate (kappa 0.76).

CONCLUSIONS

Measured fracture displacement and occult instability of minimally displaced LC1 injuries were reliably measured and identified on LSR, regardless of the laterality.