gms | German Medical Science

Objectives: Pedicle screws have earned high acceptance in a broad variety of spinal surgeries. Nevertheless, placement in the thoracic spine remains a challenge due to the difficult anatomical conditions, particularly if deformities are present. Several freehand insertion techniques have been described to properly identify the pedicle entry point and to minimize the risk of screw misplacement. However, some techniques might compromise pedicle screw anchorage, while allowing safe screw placement.

This study investigated pedicle screw anchorage after screw placement using the new “Modified Slide” technique (MS) and compared it to the original Slidetechnique by Vialle et al., the Funneltechnique as proposed by Gaines et al. as well as the conventional technique.

Methods: Forty-five fresh frozen human thoracic vertebrae (Th4-Th11; mean age: 69.4 ± 14.0 years; mean BMD: 99.97 ± 22.96 mg/ccm) were used in this study. For pairwise comparisons, one randomly selected pedicle of each vertebra was instrumented using the new MS technique. The contralateral side was instrumented using either the Funneltechnique, the original Slidetechnique or the conventional technique resulting in three groups of 15 vertebrae, respectively. Each screw was tested using cyclic cranio-caudal loading with an initial load ranging from -50 N to +50 N with stepwise increasing compressive load (5 N every 100 cycles) until screw loosening was detected. A Wilcoxon signed-rank test was then performed to compare anchorage.

Results and conclusion: Screws implanted using the MS technique sustained significantly higher loads than those using the original Slide technique (460 N ± 160.4 vs. 253 N ± 95.2, p < 0.001) and the Funnel technique (440 N ± 157.58 vs. 261 N ± 105.16, p < 0.001) but lower loads than conventionally implanted screws (442 N ± 94 vs. 546 N ± 177.4, p = 0.008) (Figure 1 [Fig. 1]).

In terms of screw anchoring, the MS technique is superior to the Funnel technique and the original Slide technique. Even though screws implanted using the new „Modified Slide“ technique sustained significantly lower loads than conventional implanted screws, they can sustain higher loads than those measured in daily activities while still allowing safe screw implantation. Therefore the „Modified Slide“ technique could be a helpful assistance technique for safe pedicle screw placement.