Temperature Prediction Model for Bone Drilling Based on Density Distribution and In Vivo Experiments for Minimally Invasive Robotic Cochlear Implantation

Abstract

Surgical robots have been proposed ex vivo to drill precise holes in the temporal bone for minimally invasive cochlear implantation. The main risk of the procedure is damage of the facial nerve due to mechanical interaction or due to temperature elevation during the drilling process.To evaluate the thermal risk of the drilling process, a simplified model is proposed which aims to enable an assessment of risk posed to the facial nerve for a given set of constant process parameters for different mastoid bone densities. The model uses the bone density distribution along the drilling trajectory in the mastoid bone to calculate a time dependent heat production function at the tip of the drill bit. Using a time dependent moving point source Green's function, the heat equation can be solved at a certain point in space so that the resulting temperatures can be calculated over time. The model was calibrated and initially verified with in vivo temperature data. The data was collected in minimally invasive robotic drilling of 12 holes in four different sheep. The sheep were anesthetized and the temperature elevations were measured with a thermocouple which was inserted in a previously drilled hole next to the planned drilling trajectory. Bone density distributions were extracted from pre-operative CT data by averaging Hounsfield values over the drill bit diameter. Post-operative µCT data was used to verify the drilling accuracy of the trajectories. The comparison of measured and calculated temperatures shows a very good match for both heating and cooling phases. The average prediction error of the maximum temperature was less than 0.7°C and the average root mean square error was approximately 0.5°C. To analyze potential thermal damage, the model was used to calculate temperature profiles and cumulative equivalent minutes at 43°C at a minimal distance to the facial nerve. For the selected drilling parameters, temperature elevation profiles and cumulative equivalent minutes suggest that thermal elevation of this minimally invasive cochlear implantation surgery may pose a risk to the facial nerve, especially in sclerotic or high density mastoid bones. Optimized drilling parameters need to be evaluated and the model could be used for future risk evaluation.

Making robots mill bone more like human surgeons: Using bone density and anatomic information to mill safely and efficiently

“…Second, lower cutting forces are expected to reduce the rate of thermal energy transferred to the surrounding bone, which could lead to heat-related trauma to the underlying vital structures. This latter implication is especially important in light of recent work that suggests high temperatures induced by bone drilling may cause thermal injury to the facial nerve [4], [21]. …”

Pre-operative Screening and Manual Drilling Strategies to Reduce the Risk of Thermal Injury During Minimally Invasive Cochlear Implantation Surgery

“…linear velocity, spindle speed, etc.) as well as the bone density at that point 14 , which can be approximated by the CT scan intensity in Hounsfield Units (HU) (see Figure 3). It is important to note that intensity can vary between scanners and thus yield inconsistent results; the scanners can be calibrated by using test scans with a phantom or by comparing scans of the same patient/specimen.…”

“…These recent results from Labadie et al 15 and Feldmann et al 14 indicate that controlling the heat generated during drilling in minimally invasive CI surgery is critical for the approach to be a viable alternative to the current clinical standard. The differences between the approaches for drilling the minimally invasive tunnel discussed above (manual vs. automated drill advancement) are particularly relevant for the reduction of heat.…”

“…The automated drilling approach provides better control over the drilling parameters and supports the integration of additional sensors, which enables optimization of the drilling process and redundant safety monitoring. 12-14 …”

“…Feldman et al tested a previously developed minimally invasive CI robotic system 7,8 on live sheep and measured the temperature several millimeters from the drilling trajectory using thermocouples inserted into the bone. 14 They then used the acquired temperature data, planned trajectory information, and image data to fit a patient-specific CT image-based thermal model and predicted the temperatures at other locations, including the facial nerve. Their results indicated that temperature elevation is strongly dependent on bone density and dangerously high temperatures are likely to occur for patients with high bone density in the nerve region if safety measures (e.g.…”