In Vitro and In Vivo Evaluation of Ultrasound-Triggered Release From Novel Spinal Device

Objectives-Bacterial infection following spinal fusion is a major clinical concern with up to 20% incidence. An ultrasound-triggered bulk-release system to combat postsurgical bacterial survival was designed and evaluated. Methods-Polylactic acid (PLA) clips were loaded with vancomycin (VAN) and microbubbles (Sonazoid, GE HealthCare) in vitro. Stability was determined over 14 days. VAN-loaded clips were submerged in water and insonated using a Logiq E10 scanner (GE HealthCare) with a curvilinear C6 probe. Doppler-induced VAN release was quantified using spectrophotometry. For in vivo testing, clips were loaded with methylene blue (MeB) solution and Sonazoid. These clips were implanted into a rabbit along the spine at L2 and L5, as well as a pig at L1 and L3, then insonated in Doppler mode using the C6 probe. Results-Sonazoid microbubbles were better preserved when incubated in VAN compared with distilled water at 4 degrees C, 25 degrees C, and 37 degrees C incubation temperatures (P = .0131). Contrast enhancement was observed from both solutions when incubated at 4 degrees C storage conditions. Insonated clips achieved average cumulative VAN release of 101.8 +/- 2.8% (81.4 +/- 2.8 mg) after 72 hours. Uninsonated clips had only 0.3 +/- 0.1% (0.3 +/- 0.1 mg) average cumulative VAN release (P < .0001). Clips retrieved from the rabbit did not rupture with insonation nor produce MeB staining of surrounding tissues. In the pig, the PLA film was visibly ruptured and MeB tissue was observed following insonation, whereas the uni-nsonated clip was intact. Conclusion-These results demonstrate ultrasound-triggered release of an encapsulated prophylactic solution and provide an important proof-of-concept for continuing large animal evaluations for translational merit.