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Session: Poster session I
Session starts: Thursday 24 January, 15:00

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Gert Kraaij (Biomechanics and Imaging Group, Dept of Orthopaedics, Leiden University Medical Center, Netherlands & Medical Instruments, Dept of Biomechanical Engineering, Delft University of Technology, Netherlands)
Steven den Dunnen (Medical Instruments, Dept of Biomechanical Engineering, Delft University of Technology, Netherlands)
Jenny Dankelman (Medical Instruments, Dept of Biomechanical Engineering, Delft University of Technology, Netherlands)
Rob Nelissen (Biomechanics and Imaging Group, Dept of Orthopaedics, Leiden University Medical Center, Netherlands)
Edward Valstar (Biomechanics and Imaging Group, Dept of Orthopaedics, Leiden University Medical Center, Netherlands & Medical Instruments, Dept of Biomechanical Engineering, Delft University of Technology, Netherlands)

Abstract:
Introduction: Worldwide about 1.5 million hip prostheses are implanted annually and this number is growing due to the longer life expectancy of people [1]. Within the first ten years approximately 10% of the hip prostheses fail because of aseptic loosening [2]. An alternative for the current invasive revision surgery, which requires complete removal of the loosened prosthesis and insertion of a new prosthesis, is a new Minimally Invasive Hip Refixation (MIHR) procedure. An important step in this procedure is the removal of the soft peri-prosthetic interface tissue. In a previous study laser and coblation were evaluated [3], however heat generation was an issue. This can be solved by using a coolant. But why not using the coolant itself, for water jet cutting, to cut the tissue? In medical field, the current applications of water jet are limited, but it is already used for liver resections. In experimental examinations Rau et al [4] found a pressure of 3-4 MPa (30-40 bar) and a nozzle diameter of 0.1 mm to be very effective to dissect normal liver tissue. The effect of a water jet on a material is influenced by the pressure and nozzle diameter combination. The aim of this pilot study is to evaluate the effect of the nozzle diameter on the required minimum water jet pressure to perforate soft elastic materials with different thicknesses. Materials and Methods: Water jets with nozzle diameters of 0.1, 0.2 and 0.6 mm were applied to specimens with thicknesses of 3, 6 and 9 mm and consisting of interface tissue substitutes: ballistic gel and silicone (Dragon Skin Pro). A specimen was placed under the water jet cutting head and by trial and error the minimum pressure was found to perforate the specimen. A low starting pressure was chosen and the water jet was activated for 2 seconds. At a new spot the water jet was activated again while the pressure was increased with 0.5 MPa. This procedure was repeated until the water jet pressure was high enough to perforate the specimen. To be sure the right minimum pressure was found, we performed three additional trials respectively with the minimum pressure and minimum pressure minus 0.5 MPa bar. Results: In Figure 1 the resulting minimum pressures for ballistic gel and silicone are shown. Minimum pressures for silicone were higher compared to those for ballistic gel. Pressure was (linear) increasing with increasing specimen thickness. A larger nozzle diameter resulted in a lower minimum water jet pressure. The largest drop in minimum pressure was found between a 0.1 and 0.2 mm nozzle. Discussion/Conclusion: This pilot study shows that using larger nozzles results in reduced minimum water jet pressures to perforate soft elastic material. However, research with human interface tissue is necessary to find the optimal combination of nozzle diameter and associated minimum water jet pressure for interface dissection. Currently we are collecting human interface tissue to test water jet dissection. References: [1] R.J. Looney et al., Arthritis Research and Therapy, 4:59–63, 2002. [2] H. Malchau et al., The wellcemented total hip arthroplasty: Theory and practice, chapter 11, pages 291–299. Springer Berlin Heidelberg, 2005. [3] Kraaij et al., Medical Engineering & Physics, 34:370-377, 2012 [4] Rau et al., HPB , 10: 275-280, 2008