Experimental in vivo acute and chronic biomechanical and histomorphometrical comparison of self-drilling and self-tapping anterior cervical screws

Abstract

Purpose: Self-drilling screws (SDS) and self-tapping screws (STS) allow for quicker bone insertion and are associated with increased anchorage. This is an experimental in vivo comparison of anterior cervical SDS and STS in the post-insertion acute and chronic phases.

Methods: Thirty C2-C6 vertebrae from six Santa Inês hair sheep were used. Each screw design was randomly assigned to five of each spinal level. Insertion torque was measured using a torque device. Three animals were killed in each phase. Vertebrae were randomly assigned to pullout tests and histomorphometrical bone-screw interface evaluation (percent screw-bone contact and bone density inside and outside the threaded area). Statistical significance was set at P < 0.05.

Results: SDS insertion torque was greater than STS (P = 0.0001). SDS pullout strength was significantly greater than STS in the acute and chronic phases (P = 0.0001, 0.0003, respectively). SDS percent screw-bone contact and inside area bone density were significantly greater in both phases. No outside area bone density differences were observed in either phase.

Conclusions: SDS had higher insertion torque and better anchorage than STS in both phases. SDS percent bone-screw contact and inside area bone density were higher in both phases.

Fig. 1

Photograph of self-tapping (a) and self-drilling (b) anterior cervical screws

Fig. 2

Diagram illustrating the experimental set-up for pullout testing. Right details of the screw–pullout device connection

Fig. 3

Micrograph of the trabecular bone–implant interface illustrating the histomorphometrical measurement. The dotted line indicates the bone–implant contact. The smaller rectangle delimits the area selected for the measurement of bone percentage inside the screw thread. The larger rectangle delimits the cancellous bone area outside the screw thread. Alizarin red and Stevenel’s blue staining. ×25 magnification

Fig. 4

Graph showing the insertion torque (mean and standard deviation) of self-tapping screws (STS) and self-drilling screws (SDS) in the acute phase. The asterisk (*) indicates a statistically significant difference (P < 0.0001)

Fig. 5

Graph showing the pullout strength (mean and standard deviation) of self-tapping (STS) and self-drilling (SDS) screws after insertion (acute phase) and 8 weeks later. The asterisks indicate a statistically significant difference (*P < 0.0001 and ** P < 0.0003)

Fig. 6

Graph showing the percentage of bone inside the screw thread area (mean and standard deviation) of self-tapping screws (STS) and self-drilling screws (SDS) after insertion (acute phase) and 8 weeks later. The asterisks indicate a statistically significant difference (*P < 0.0005 and **P < 0.013)

Fig. 7

Graph showing the percentage of bone–implant contact (mean and standard deviation) of self-tapping screws (STS) and self-drilling screws (SDS) after insertion (acute phase) and 8 weeks later. The asterisks indicate a statistically significant difference (*P < 0.004 and **P < 0.04)

Fig. 8

Graph showing the percentage of bone outside the screw thread area (mean and standard deviation) of self-tapping screws (STS) and self-drilling screws (SDS) after insertion (acute phase) and 8 weeks later. No statistical difference was found

Fig. 9

Micrographs of the trabecular bone–implant interface illustrating the histomorphometric results in the acute and chronic phases. a Self-tapping screw (STS) in the acute phase. b STS in the chronic phase (8 weeks). c Self-drilling screw (SDS) in the acute phase. d SDS in the chronic phase (8 weeks). Alizarin red and Stevenel’s blue staining. ×25 magnification

Fig. 10

Micrographs of the trabecular bone–implant interface illustrating the histomorphometrical results in the acute and chronic phases. a Self-tapping screws (STS) in the acute phase. b STS in the chronic phase (8 weeks). c Self-drilling screws (SDS) in the acute phase. d SDS in the chronic phase (8 weeks). Alizarin red and Stevenel’s blue staining. ×25 magnification