A device for measuring sternal bone connectivity using vibration analysis techniques
Objectives: Stability of bone splitting sternotomy is essential for normal healing after open cardiac surgery. Mechanical vibration transmittance may offer a means for early detection of separation of bone (diastasis) in the sternotomy and prevent further complications. This paper describes the technical implementation and validation of vibration analysis-based prototype device built for measuring sternal bone connectivity after sternotomy. Methods: An in-house built measurement system, Sternal Vibration Device, consisting of actuator, sensor, and main controller and signal acquisition unit was designed and manufactured. The system was validated, and three different test settings were studied in mockups (polylactide rods in ballistic gel) and in two human sternums: intact, stable wire fixation, and unstable wire fixation with a gap mimicking bone diastasis. The transmittance of vibration stimulus across the median sternotomy was measured. Results: The validation showed that the force produced by the actuator was stable and the sensor could be calibrated to precisely measure the acceleration values. The vibration transmittance response to material cut and sternotomy was evident and detectable in the 20Hz - 2kHz band. The transmittance decreased when the connectivity between the sternal halves became unstable. The trend was visible in all the settings. Conclusion: Technical solutions and description of validation process were given. The device was calibrated and the vibration transmittance analysis differentiated intact and cut polylactide rod. In the sternum, intact bone, wire fixation with exact apposition, and with a gap were identified separately. Although further studies are needed to assess the accuracy of the method to detect different levels of diastases, the method appears to be feasible.
Steps to reproduce
Three mockups were manufactured to simulate intact, split and steel wire fixed, and split sterni. A 3D printed 20 mm x 5 mm x 200 mm PLA rod (PLA silver gray 2.85 mm filament by Ultimaker B.V., Geldermalsen, The Netherlands) modeled the rib-sternum-rib combination. The rods were embedded into 150 mm x 60 mm x 210 mm ballistic gel blocks (10 % ballistic gelatin by Clear Ballistics, Fort Smith, AR, USA) to a depth of 5 mm (Figure 3). The actuator to sensor distance was 6 cm. Five repeated measurements were made on the mockups. For each measurement the power in the 20 Hz - 2 kHz band was integrated. Two adult human cadavers were used to test the device in a realistic setting. The study consisted of three states of in situ sterni and modeled them before and after surgery: baseline (intact sternum), stable tight wire fixation closure (sternum bound tightly with surgical steel wires) and unstable wire fixation with a gap (wires slightly loose and sternal halves moving; mimicking a complication inducing sternal instability). The skin was covered with an adhesive plastic film. After each manipulation of the cadaver the soft tissue was closed by suturing. The sternal midline and costal cartilage level 3 were marked on the skin. The actuator and sensor were placed on the costal cartilage bilaterally 3 cm from the sternal midline and perpendicular to the skin. Five repeated measurements were recorded, and the measurement power calculated as above.