Minimally invasive cochlear implant surgeries require a very high accuracy of at least 0.5 mm when drilling a canal through the most critical spot, the facial recess [1]. Important anatomical structures in the vicinity of the minimal invasive access path, including the facial nerve and the chorda tympani must be preserved. We conducted a quantitative verification study on five temporal bone specimens using the patient specific miniature stereotactical frame called „Microtable“. The Microtable is developed at Vanderbilt University (Nashville, TN) and evaluated in a multi center study together with the Medical School Hannover (MHH). After classical mastoidectomy and opening of the facial recess we measured the target error at the facial recess in an extra „post-OP“ CBCT (Cone Beam Computed Tomography) scan with a sham drill bit inserted.
First, a patient specific trajectory is planned based on the segmented anatomical structures and the position of the spherical markers on top of the bone anchors (screws). Second, the holes are milled (on a 4-DOF CNC milling machine) at the needed locations and to a depth that defines the orientation of the platform as previously planned. Third, the ”legs“ of the table are inserted, fixed, and the table is mounted onto the spherical anchors [3].
In this verification study, we produced individual Microtables for five human temporal bone specimens, performed classical mastoidectomy, opened the facial recess (was done by an experienced ENT surgeon), and measured the target error between the planned and the actual trajectory at the facial recess. To quantify the accuracy, we registered the CBCT scan from the planning phase with a second CBCT ”post-op“ scan and measured the distance between a sham drill bit seen in the CBCT image and the planned drilling (Figure 2, right). This study captures the accumulated errors: (1.) The detection of the positions of the spherical markers, (2.) the firmness of the bone anchors, (3.) the screw connections from bone anchor to spherical marker, (4.) the accuracy of gripper attachment to the spheres, (5.) the clearance of the driller (or sham drill bit), and (6.) the tightening of the legs onto the platform. 3
Endoscopic view (Figure 1, middle-right) confirmed that the sham drill bit successfully reached a clinically adequate target location and critical anatomical structures remained undamaged for all five evaluated temporal bone specimens. In the registered CBCT scans (Figure 2, right), the mean distance between planned and actual trajectory at the facial recess was 0.46 mm ± 0.18 mm (SD). Registration and resolution of the CBCT scans yields an estimated measurement precision of 0.1 to 0.2 mm. The individual results are listed in Table 1.
For the specimen named TB03b, we identified the source of the unsatisfying large deviation of 0.75 mm to be a result of a worn-out milling head which has been used to create at least four other Microtables before TB03b. One consequence of the abrasion of the milling head is that the insertion of the grippers (the legs) into the Microtable platform requires large forces. With the new milling head we then produced M57, M58 and M59 where the insertion of the grippers was unproblematic. # Specimen name Target error at facial recess in mm 1 TB02 0.40 mm 2 TB03b 0.75 mm 3 M57 0.43 mm 4 M58 0.50 mm 5 M59 0.21 mm mean 0.46 mm
SD 0.18 mm Table 1: The target error at the facial recess (which is the maximal distance of the planned drilling path to the actual drilling path)
The Microtable has already proven its accuracy earlier in controlled lab environments. It is one of the most promising approaches to enable minimal invasive access to the cochlea in the future. Therefore, the main goal of this study was not to push the theoretical possible accuracy further but instead to quantify and assess the precision under realistic conditions at our lab – which is not the lab the Microtable has been developed at. Clearly, we have to improve on the overall accuracy and identify at which point of the mechanical chain (see section 2 „Material and Methods“) the largest error was introduced and how the construction process can perhaps be optimized.
While we can report to have reached the necessary accuracy (=0.5 mm) for 80 percent of our specimens, we clearly have to improve and investigate what has been the main cause of the larger deviations from the planned trajectory. Additionally to the worn out end-mill for TB03b, we found that the sham drill bit had a small clearance, which could amount for 0.2 to 0.3 mm error alone. We hope to decrease this clearance by a proper drill guide. Another 0.1 mm error may stem from the registration of the two CBCT scans that we used to measure the error.
The in vitro study done by the skilled team at Nashville (TN) [2] with another type of milling machine reports a target error of 0.31 ± 0.10 mm.
I would like to thank our friends at Vanderbilt University for the cooperation and support (the custom made milling machine, material supply and teaching), especially Ramya Balachandran for her latest visit. Further, thanks to Marcel Kluge for his support with the milling machine. 4 5 6