A New Technique for Placing Pedicle Screws Into Bone Cement-Strengthened Vertebrae Involving Drilling and Melting

Vertebroplasty with non-degradable polymethyl methacrylate bone cement is a common procedure in spine surgery. However, this bone cement reinforcement of the vertebral body could affect subsequent spinal surgeries, especially for pedicle screw insertion. This study proposes a novel method of inserting pedicle screws into bone cement-reinforced vertebral bodies through thermal softening via Kirschner wires (K-wires) drilling, and conducts preliminary in vitro experiments to assess its feasibility and safety.

This study includes bone cement block experiments and in vitro goat bone experiments. The bone cement block experiment utilized a CNC machine to drill into bone cement blocks with varying K-wire diameters, rotational speeds, and feed rates, followed by the insertion of pedicle screws. The highest temperature during the procedure and the pull-out strength of the pedicle screws were recorded for different groups. A two-way ANOVA was used for comparative analysis. The goat bone experiment consisted of an experimental group and a control group. The control group had screws inserted along the pedicle after drilling. In the experimental group, screws were inserted after softening the old bone cement within the vertebral body using a 3.5 mm K-wire for drilling. The highest temperature during the procedure was recorded for the experimental group, and pull-out tests were conducted on the screws of both groups after the procedure. The pull-out results from the goat bone experiment were analyzed using Student's t-test.

Bone cement block experiment: Drilling with K-wires caused a significant temperature increase in the bone cement blocks, with temperature rises at 4 mm ranging from 28.1°C to 75.9°C. The maximum pull-out loads across all groups ranged from 2455.053 to 15201.94 N. In the goat bone experiment, the experimental group showed temperature increases of 8.38°C ± 3.07°C beneath the pedicle, 11.18°C ± 1.42°C in the spinal canal, and 8.26°C ± 3.46°C anterior to the vertebral body during drilling. The average maximum loads for the experimental and control groups were 910.5504 ± 221.6544 N and 294.229 ± 40.3475 N, respectively, indicating a statistically significant difference between the two groups (p = 0.0001).

The experimental results demonstrate that screws can be inserted into bone cement after thermal softening via K-wire drilling, achieving good pull-out resistance. In the goat bone experiment, the average temperature increase around the vertebral body was measured to be below 10°C, indicating a low risk of thermal damage to the surrounding tissues.