Purpose Sternal instability, a complication arising for some patients after sternotomy for cardiac surgery affects their later quality of life and cost of care. However, there are currently few guidelines for its diagnosis, quantification, and monitoring. Ultrasound equipment with associated software for calculating selected video-monitor distances provides one way of quantifying the extent of sternal separation. Description This study evaluated the validity and reliability of an ultrasound measurement made by attaching the head of the unit to an extensible stand. First the procedure was tested with bony sterna, and second in the examination of the chests of 8 patients with sternal instability. Evaluation Reliability estimation of the ultrasound measure on bony sterna gave ICC (2, 1) values >0.99, and reliability estimates for the sternal separation measure in the patient group were ICC (2, 1) values between 0.90 and 0.93. Conclusions Therefore gap measurements taken by ultrasound can objectively reflect the extent of bony separation occurring in a group of cardiac surgery patients experiencing sternal instability.