An experimental device for catheter insertion and fixation to the skull was designed, constructed, and tested under both ex-vivo and in-vivo conditions. The aim was to evaluate its capability for accurate cannulation of the ventricle and to assess the strength of its fixation to the skull.

The fixation device is comprised of a 3-legged base with each leg containing a single 1.6 mm self-tapping titanium screw that provides even distribution of force on the skull (Fig. 1). The small diameter titanium screws are inserted directly through the skin into the outer table of the cranium and do not require additional incisions.

Fixation device main parts assembly

To provide 2 degrees of freedom (DOF), the fixation base was designed as a socket that houses a trajectory control ball which allows full control of the planned insertion after cranial fixation is accomplished (Fig. 2A). Apart from the titanium screws, all components were 3D printed using Stereolithography (SLA) printer (Formlabs, Somerville, MA, USA) with designated biocompatible material (Formlabs Biomed-Amber and Biomed Durable resins).

Fixation device mechanism schematics. A: Trajectory ball 2 angular degrees of freedom (DOF). B: Ball mechanism at unlocked state. C: Ball mechanism at locked state

After the trajectory is determined, it is then locked by using the locking handle (Fig. 2B-C). Once the catheter is inserted, the catheter guide, ball, and cap mechanism (Fig. 3A-D) are used to secure the catheter at its target site without obstructing the flow of CSF and without requiring the use of sutures. The additional design of the catheter guide ears and locking pin within the locking socket further assist in the prevention of accidental removal when the cap is on (Fig. 3D).

Catheter Guide (green), trajectory ball (blue), catheter (orange) and cap (red) assembly. A: Trajectory Ball, Catheter Guide and Cap in exploded view. B: Catheter guide is inserted to the trajectory ball pressing the guide ears while inserting. C: The catheter is inserted through the catheter guide to the desired depth. D: Cap is placed over the catheter and locks it in place. The cap also prevents the catheter guide from being removed by mechanically blocking the guide ears

The placement of the fixation device is comprised of multiple steps as shown in Fig. 4A-E. In our experiments, the EVD guide is initially positioned over Kocher’s point equivalent on the lamb and fixed to the skull using three 1.6 mm diameter titanium screws. The screws are inserted into the skull to the desired depth while keeping the fixation base away from the scalp to prevent pressure and potential necrosis and tissue damage when EVD drainage is required for extended periods of time. After the EVD guide is fixed to the skull, the trajectory ball is used to select the desired insertion trajectory for the catheter. The trajectory is locked in position using the locking handle (Fig. 4A-B). The drill guide is used to create a burr hole craniostomy that will be aiming directly at the desired target. The diameter of the drill guide cylinder enables drilling of a craniostomy with the exact diameter necessary for placement of the desired EVD type which enables the appropriate fit for the catheter. Once the craniostomy is complete, the drill guide is exchanged for the catheter guide which is then inserted and locked into the trajectory ball (Fig. 4B-C). The EVD is inserted in standard fashion to the desired measured depth as determined by preoperative imaging (Fig. 4D). The cap is locked to the catheter guide to prevent unintentional disconnection and inadvertent pull-back from the ventricular space while allowing free flow of CSF through its inner diameter (Fig. 4E). A catheter loop can additionally be used to further ensure catheter coupling to the guide base.

Fixation Procedure Steps. A: the fixation device is positioned, and the insertion trajectory is determined. B: the trajectory ball is locked in place, the drill guide inserted, and a drill is used in the desired trajectory. C: the catheter guide is inserted. D-E: the catheter is inserted to the desired depth and the cap is locked to the catheter guide

The bench-side ex-vivo experiments were designed to evaluate the ability of the fixation device to facilitate the insertion and secure fixation of ventricular catheters.

These experiments included both basic functionality testing and pull-out strength tests that were conducted at the Bio-Inspired and Medical Robotics Lab at the department of Mechanical Engineering at Ben Gurion University of the Negev (BGU) in Be’er-Sheva, Israel, during 2023. The ex-vivo fixation functionality experiments were completed using 3D printed simulation skull models created from Computed Tomography (CT) scans of the human head printed in ABS material. These experiments did not provide any measurable data but played a crucial role in ensuring that troubleshooting and identifying design improvements were tested ex-vivo. During these trials, the senior author was able to successfully insert the EVD in a short time frame (3–5 min) from initial fixation of the guide to the simulated skull to securing the inserted catheter to the guide.

Two sets of ex-vivo experiments were designed to test the ball pullout and the catheter pullout strength using an Enpaix EFG500 digital force gauge, with an accuracy of ± 0.1 kgf. In the first set of experiments the fixation device was attached to the fixed base using the 3 titanium screws. The force gauge was connected to the trajectory ball using a designated fitting and pulled until the ball exited the base. The maximal force in each pull was recorded. The ball pullout strength was tested 30 times and statistical significance was evaluated using Student’s t-test with a confidence interval (CI) of 95%. In the second set of experiments, the catheter pullout strength was tested in 2 different methods. These included the direct catheter pull and the safety loop method, as shown in Fig. 5. The direct pull was measured 30 times at three different angles (10 measurements per angle). Statistical significance was evaluated in the same manner as the ball pull test.

Fixation device used without (A) and with (B) a safety loop. Free catheter pull angles are marked in A

The in-vivo experiments were conducted to evaluate the performance of the fixation device in a live setting. These experiments tested the device’s ability to enable accurate ventricular cannulation, measured the pullout force of the fixation ball for comparison with ex-vivo results, and assessed the skull fixation strength.

All experiments conducted in this work followed the guidelines for animal research and were approved by the Institutional Animal Care and Use Committee (IACUC) through CCHMC (2021-0008) and JUMISC (ES100370001499).

The fixation device underwent preliminary in-vivo testing in February 2023 on a single day of life (DOL) 1 control male lamb at Cincinnati Children’s Hospital Medical Center (CCHMC) in Cincinnati, Ohio, USA. During this test, a single fixation device was used, and a ventricular catheter was inserted into the ventricular system. An endoscope (CLARUS NeuroPen, Minneapolis, MN, USA) was subsequently inserted to provide visual confirmation of entry into the ventricle. The subject then was imaged with T1 and T2-weighted magnetic resonance imaging (MRI) sequences to further validate the catheter placement before the postmortem analysis.

Another set of in-vivo experiments were conducted in November 2023 at Jesús Usón Minimally Invasive Surgery Centre (JUMISC) in Caceres, Spain. The experiments were conducted on three DOL 0 infant hydrocephalic lambs (1 male 2 female) at delivery on placental support using ex-utero intrapartum treatment (EXIT) procedure. Hydrocephalus was created by intracisternal injection of a BioGlue in the fetal stages which induced obstructive hydrocephalus at birth by blockage of the CSF pathways resulting in ventriculomegaly without causing an neuroinflammatory response as described previously by Soner et al. [20, 21].

Hydrocephalus was evaluated by fetal MRI obtained prior to each EXIT procedure. Six bilaterally placed EVD guides and catheters were tested. For each subject two separate 2.8 mm catheters (Integra, Princeton, NJ, USA) were inserted using two fixation devices (one left sided and one right sided). Two subjects underwent post procedure MRIs to validate correct placement of the catheter. Furthermore, all subjects had post-mortem examinations including evaluation of the fixation device in terms of screw placement and strength of screw fixation. Finally, we performed a gross examination of the skull, cortical mantle, and trajectory into the ventricles to evaluate for potential complications such as intraparenchymal injury or hemorrhage.

The pullout strength experiments were completed with the same Enpaix EFG500 digital force gauge. Testing of each subject occurred postmortem. As we applied a force to the fixation device’s ball using the designated fitting, the ball exited from the base prior to the screws being detached from the skull. This experiment allowed us to measure the maximum pulling force that the ball can resist. In addition, after each ball pull, the remaining base was pulled from a single screw to test individual screw pullout strength from the skull. Of note, the screw pullout test was performed after the trajectory ball pullout test. As a result, the maximal pullout force of each individual screw was reduced. Therefore, we would expect the actual single screw pullout force to be at minimum equal to the measured value we report. The pullout strength was tested 5 times for the fixation device base and 5 times for a single screw.