Sternal force distribution during median sternotomy retraction

Background: Median sternotomy is the access of choice in cardiac surgery. Sternal retractors exert significant forces on the thoracic cage and might cause considerable damage. The aim of this study was to determine the effects of retractor shape on local force distribution to obtain criteria for retractor design. Methods: Two types of sternal retractors (straight [SSR] and curved [CSR]) were equipped with force sensors. Force distribution, total force, and displacement were recorded to a spread width of 10 cm in 18 corpses (11 males and 7 females; age, 62 +/- 12 years). Both retractors were used in alternating sequence in 4 iterations in every corpse. Data were compared with respect to the different retractor blade shapes. Results: Maximum total forces for full retraction of both retractors resulted in 349.4 +/- 77.9 N. Force distribution during the first retraction for the cranial/median/caudal part of the sternum was 101.5 +/- 43.9/29.1 +/- 33.9/63.0 +/- 31.4 N for the SSR and 38.7 +/- 41.3/80.9 +/- 64.5/34.0 +/- 25.8 N for the CSR, respectively. During the 4 spreading cycles, the average force decreased from 224.6 +/- 61.3 N in the first to 110.8 +/- 39.8 N in the fourth iteration. The mean total force for the first retraction revealed 226.4 +/- 71.9 N for the CSR and 222.8 +/- 52.9 N for the SSR. Conclusions: The shape of sternal retractors considerably influences the force distribution on the sternal incision. In the SSR, forces on the cranial and caudal sternum are significantly higher than in the median section, whereas in the CSR, forces in the median section are highest.