Tissue-disruptive forces during median sternotomy

Background. Acute and chronic pain after median sternotomy is common and often underestimated. The mechanical retractors used for median sternotomy exert significant forces on the skeletal cage. We hypothesized that instrumented retractors could be developed to enable real-time monitoring and control of retraction forces, functions that may provide equivalent exposure with significantly reduced forces and tissue damage, and thus, less postoperative pain. Methods. We developed a novel instrumented retractor designed to enable real-time force monitoring during surgical retraction and then tested it by performing median sternotomies on 16 mature sheep. For 8 of these median sternotomies, retraction was performed to 7.5 cm at a standard "clinical pace" of 7.25 +/- 0.97 minutes without real-time monitoring of retraction forces. For the other 8 median sternotomies, we performed retraction to the same exposure using real-time visual force feedback and, consequently, a more deliberate pace of 12.05 +/- 1.73 minutes (P <.001). Retraction forces, blood pressure, and heart rate were monitored throughout the procedure. Results. Full retraction resulted in an average force of 102.99 +/- 40.68 N at the standard clinical pace, compared to 64.68 +/- 17.60 N with force feedback (a 37.2% reduction, P=.021). Standard retraction produced peak forces of 368.79 +/- 133.61 N, whereas force feedback yielded peak forces of 254.84 +/- 75.77 N (a 30.9% reduction, P=.1152). Heart rate was significantly higher during standard clinical retraction (P=.025). Conclusions. Use of the novel instrumented retractor resulted in lower average and peak retraction forces during median sternotomy. Moreover, these reduced retraction forces correlated to a reduction in animal stress, as documented by heart rate.