OBJECTIVE: Topical negative pressure therapy has excellent healing effects in poststernotomy mediastinitis. Topical negative pressure therapy reduces bacterial counts, increases wound edge microvascular blood flow and granulation tissue formation, and facilitates healing. No study has yet been performed to examine the effect of topical negative pressure on the blood and fluid content in the sternal bone marrow, which is a crucial component in osteitis. METHODS: Eight pigs underwent median sternotomy, left internal thoracic artery harvesting, followed by topical negative pressure treatment. Magnetic resonance imaging was used to quantify both tissue fluid and/or blood content (T2-weighted short tau inversion recovery [T2-STIR]) and internal thoracic artery blood flow (flow quantification). RESULTS: Before application of topical negative pressure, the T2-STIR signal intensity ratio was lower for the left than for the right hemisternum (left, 1.3; right, 2.6), indicating lower levels of tissue fluid content on the left, devascularized side. On application of topical negative pressure, the T2-STIR signal intensity ratio increased immediately for both the sternal bone and the pectoral muscle (left hemisternum after 4 minutes of topical negative pressure: 2.3), leveled off after 4 minutes, and remained unchanged for the ensuing 40 minutes, suggesting movement of fluid and/or blood into the tissue of the wound edge. Topical negative pressure did not affect blood flow in the right internal thoracic artery. CONCLUSIONS: T2-STIR measurements show that topical negative pressure increases sternotomy wound edge tissue fluid and/or blood content. Topical negative pressure creates a pressure gradient that presumably draws fluid from the surrounding tissue to the sternal wound edge and into the vacuum source. This "endogenous drainage" may be one possible mechanism by which osteitis is resolved by topical negative pressure in poststernotomy mediastinitis.