Colliding jets provide depth control for water jetting in bone tissue.

In orthopaedic surgery, water jet drilling provides several advantages over classic drilling with rigid drilling bits, such as the always sharp cut, absence of thermal damage and increased manoeuvrability. Previous research showed that the heterogeneity of bone tissue can cause variation in drilling depth whilst water jet drilling. To improve control over the drilling depth, a new method is tested consisting of two water jets that collide directly below the bone surface. The expected working principle is that after collision the jets will disintegrate, with the result of eliminating the destructive power of the coherent jets and leaving the bone tissue underneath the focal point intact. To assess the working principle of colliding water jets (CWJ), the influence of inhomogeneity of the bone tissue on the variation of the drilling depth and the impact of jet time (t) on the drilling depth were compared to a single water jet (SWJ) with a similar power. 98 holes were drilled in 14 submerged porcine tali with two conditions CWJ (impact angle of 30° and 90°) and SWJ. The water pressure was 70MPa for all conditions. The water jet diameter was 0.3 mm for CWJ and 0.4 mm for SWJ. t was set at 1, 3, 5 and 8s. Drilling depth and hole diameter were measured using microCT scans. A non-parametric Levene's test was performed to assess a significant difference in variance between conditions SWJ and CWJ. A regression analysis was used to determine differences in influence of t on the drilling depth. Hole diameter differences were assessed using a one way Anova. A significance level of p<0.05 was set. Condition CWJ significantly decreases the drilling depth variance caused by the heterogeneity of the bone when compared to SWJ. The mean depth for CWJ was 0.9 mm (SD 0.3 mm) versus 4.8 mm (SD 2.0) for SWJ. t affects the drilling depth less for condition CWJ (p<0.01, R=0.30) than for SWJ (p<0.01, R=0.46). The impact angle (30° or 90°) of the CWJ does not influence the drilling depth nor the variation in depth. The diameters of the resulting holes in the direction of the jets is significantly larger for CWJ at 90° than for 30° or a single jet. This study shows that CWJ provides accurate depth control when water jet drilling in an inhomogeneous material such as bone. The maximum variance measured by using the 95% confidence interval is 0.6 mm opposed to 5.4 mm for SWJ. This variance is smaller than the accuracy required for bone debridement treatments (2-4 mm deep) or drilling pilot holes. This confirms that the use of CWJ is an inherently safe method that can be used to accurately drill in bones.