Incomplete insertion of pedicle screws triggers a higher biomechanical risk of screw loosening: mechanical tests and corresponding numerical simulations.

Screw loosening is a widely reported issue after spinal screw fixation and triggers several complications. Biomechanical deterioration initially causes screw loosening. Studies have shown that incomplete insertion of pedicle screws increases the risk of screw breakage by deteriorating the local mechanical environment. However, whether this change has a biomechanical effect on the risk of screw loosening has not been determined. This study conducted comprehensive biomechanical research using polyurethane foam mechanical tests and corresponding numerical simulations to verify this topic. Pedicle screw-fixed polyurethane foam models with screws with four different insertion depths were constructed, and the screw anchoring ability of different models was verified by toggle tests with alternating and constant loads. Moreover, the stress distribution of screw and bone-screw interfaces in different models was computed in corresponding numerical mechanical models. Mechanical tests presented better screw anchoring ability with deeper screw insertion, but parameters presented no significant difference between groups with complete thread insertion. Correspondingly, higher stress values can be recorded in the model without complete thread insertion; the difference in stress values between models with complete thread insertion was relatively slight. Therefore, incomplete thread insertion triggers local stress concentration and the corresponding risk of screw loosening; completely inserting threads could effectively alleviate local stress concentration and result in the prevention of screw loosening.

Poor bone mineral density aggravates adjacent segment's motility compensation in patients with oblique lumbar interbody fusion with and without pedicle screw fixation: An in silico study.

Objective: Motility compensation increases the risk of adjacent segment diseases (ASDs). Previous studies have demonstrated that patients with ASD have a poor bone mineral density (BMD), and changes in BMD affect the biomechanical environment of bones and tissues, possibly leading to an increase in ASD incidence. However, whether poor BMD increases the risk of ASD by aggravating the motility compensation of the adjacent segment remains unclear. The present study aimed to clarify this relationship in oblique lumbar interbody fusion (OLIF) models with different BMDs and additional fixation methods. Methods: Stand-alone (S-A) OLIF and OLIF fixed with bilateral pedicle screws (BPS) were simulated in the L4-L5 segment of our well-validated lumbosacral model. Range of motions (ROMs) and stiffness in the surgical segment and at the cranial and caudal sides' adjacent segments were computed under flexion, extension, and unilateral bending and axial rotation loading conditions. Results: Under most loading conditions, the motility compensation of both cranial and caudal segments adjacent to the OLIF segment steeply aggravated with BMD reduction in S-A and BPS OLIF models. More severe motility compensation of the adjacent segment was observed in BPS models than in S-A models. Correspondingly, the surgical segment's stiffness of S-A models was apparently lower than that of BPS models (S-A models showed higher ROMs and lower stiffness in the surgical segment). Conclusion: Poor BMD aggravates the motility compensation of adjacent segments after both S-A OLIF and OLIF with BPS fixation. This variation may cause a higher risk of ASD in OLIF patients with poor BMD. S-A OLIF cannot provide instant postoperative stability; therefore, the daily motions of patients with S-A OLIF should be restricted before ideal interbody fusion to avoid surgical segment complications.

[Biomechanical affect of percutaneous transforaminal endoscopic discectomy on adjacent segments with different degrees of degeneration:a finite element analysis].

OBJECTIVE: To investigate the biomechanical affect of percutaneous transforaminal endoscopic discectomy(PTED) on adjacent segments with different degrees of degeneration and related risk of adjacent segment diseases (ASD) caused by this operation. METHODS: A healthy male adult volunteer was selected, and the lumbosacral vertebra image data was obtained by CT scan, and the external contour of the bone structure was reconstructed. On this basis, the external contour of the bone structure was fitted by using the smooth curve in 3D-CAD software, and the complete three-dimensional finite element modelof the non degenerate L3-S1 segment and the degenerative models of the L3-L4 and L5-S1 segment were drawn forward. In L4, L5 segment simulating PTED surgery through the removal of right part of articular process and nucleus pulposus and anulus fibrosus. After PTED was simulated in the L4-L5 segment and the risk of ASD has been evaluated by six changes of biomechanical indicators in flexion, extension, left and right lateral bending, left and right axial rotation conditions. RESULTS: In the finite element model without adjacent segmental disc degeneration, the annulus fibrosus von Mises stress and intradiscal pressure of the PTED model showed only a slight increase under most stress conditions, and a slight decrease in a few conditions, and there was no significant change trend before and after surgery. In the original degenerated adjacent segment disc model, the biomechanical indicators related to disc degeneration in the pre- and post-PTED model showed significant deterioration, leading to an increased risk of potential adjacent spondylopathy. CONCLUSION: PTED surgery will not lead to the significant deterioration of postoperative biomechanical environment of non-degeneration adjacent intervertebral discs, and the original degeneration of adjacent intervertebral discs is a important risk factor for ASD.