Analysis of Otologic Drill Entanglement Using Slow Motion Videography

Objective Drill entanglement occurs when cotton or other materials become wrapped in the rotating shaft of a drill and can rapidly result in a “weed whacker” phenomenon, potentially causing devastating injury to surrounding structures. The situation can be particularly dangerous during retrosigmoid approaches for cerebellopontine angle tumors when cotton pledgets or other materials may be used to protect the cerebellum during drilling of the internal auditory canal. We aim to characterize the entanglement properties of several commonly used materials and demonstrate the traumatic consequences of this complication.

Setting Temporal bone laboratory.

Methods Four materials were used to simulate drill entanglement: cotton pledgets, TELFA (nonadherent dressing with cotton interior), BICOL (collagen sponge), and Gelfoam (gelatin sponge). These materials were trimmed to uniform size (0.25 in × 3 in), moistened with saline and placed onto a piece of ripe banana in a temporal bone holder to simulate the cerebellum. The shaft of an otologic drill operating at 60,000 RPM was passed over each material until entanglement occurred, and the drill was stopped once the spinning material reached a stable velocity. Trials were performed in duplicate and recorded using an Apple iPhone 6S at 240 frames-per-second, and further slowed up to 32× using commercially available software. Damage to the banana was assessed both by recording pre- and postentanglement weights and photo documentation.

Results Slow motion analysis revealed material-specific entanglement properties. Cotton and Gelfoam exhibited a slow spinning phase initially, followed by a rapid acceleration to maximum velocity. In contrast, TELFA and BICOL achieved maximum velocity almost immediately. Cotton and Gelfoam were relatively hard to entangle, requiring multiple passes of the shaft over the material, while TELFA and BICOL were easily caught in the drill shaft. Cotton, Gelfoam, and BICOL tended to remain intact while wrapping tightly around the drill shaft, while TELFA exploded in a shower of cotton debris. Cotton inflicted the most damage to the banana with a mean weight reduction of 1.45 g compared with <0.25 g for the other materials. The cotton pledget itself was at least four times the weight of the other materials (0.8 g vs. <0.2 g, respectively), which likely increased its capacity to induce trauma. Although similar weight reductions were not observed with TELFA, BICOL, or Gelfoam, video analysis demonstrated significant motion of the banana caused by contact with the spinning material.

Conclusion A variety of materials can be used to protect the cerebellum during retrosigmoid approaches to skull base lesions, all of which have distinct drill-entanglement properties. While some materials in this study were more difficult entangle, all materials tested have the potential to become caught in the drill and possibly cause significant trauma to the cerebellum, blood vessels, or internal auditory canal nerves. While the materials can be shielded by a retractor or suction, any exposed material poses a risk for entanglement. We therefore recommend that a high level of caution be employed when drilling in the presence of such materials.

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