Effect of Insertional Direction of Pedicle Screw on Screw Loosening: A Biomechanical Study on Synthetic Bone Vertebra under a Physiology-like Load.

OBJECTIVES: It is now understood that pedicle screw loosening at the implant-bone interface can lead to poor screw-bone interface purchase and decreased fixation stability. Previous biomechanical tests used cadaveric vertebrae and pull-out or torque loads to assess the effect of the insertional direction of pedicle screws on screw loosening. However, these tests faced challenges in matching biomechanical differences among specimens and simulating in vivo loads applied on pedicle screws. This study aimed to evaluate the effect of the insertional direction of pedicle screws on screw loosening using tension-compression-bending loads and synthetic bone vertebrae. METHODS: Polyaxial pedicle screws were inserted into nine synthetic bone vertebrae in three directions (three samples per group): cranial, parallel, and caudad (-10°, 0°, +10° of the pedicle screw rod to the upper plane of the vertebra, respectively). Pedicle screws in the vertebrae were loaded using a polyethylene block connected to a material testing machine. Tension-compression-bending loads (100N-250N) with 30,000 cycles were applied to the pedicle screws, and displacements were recorded and then cycle-displacement curve was drawn based on cycle number. Micro-CT scans were performed on the vertebrae after removing the pedicle screws to obtain images of the screw hole, and the screw hole volume was measured using imaging analysis software. Direct comparison of displacements was conducted via cycle-displacement curve. Screw hole volume was analyzed using analysis of variance. The correlation between the displacement, screw hole volume and the direction of pedicle screw was assessed by Spearman correlation analysis. RESULTS: The smallest displacements were observed in the caudad group, followed by the parallel and cranial groups. The caudad group had the smallest screw hole volume (p < 0.001 and p = 0.009 compared to the cranial and parallel groups, respectively), while the volume in the parallel group was greater than that in the cranial group (p = 0.003). Correlation analysis revealed that the insertional direction of the pedicle screw was associated with the displacement (p = -0.949, p < 0.001) and screw hole volume (p = -0.944, p < 0.001). CONCLUSION: Strong correlations were found between the insertional direction of the pedicle screw and relevant parameters, including displacement and screw hole volume. Pedicle screw insertion in the caudad direction resulted in the least pedicle screw loosening.

Correlation between direction of pedicle screw and restoration of lumbar degenerative scoliosis in degenerative lumbar spondylolisthesis: a retrospective study.

Background: Inferior clinical outcomes have been reported in patients with degenerative lumbar spondylolisthesis (DLS) accompanied by lumbar degenerative scoliosis, but little attention has been paid to its radiologic assessment or preoperative planning. The aim of this study was to analyze the effect of transforaminal lumbar interbody fusion on patients with DLS and lumbar degenerative scoliosis and explore the surgical aspects benefiting the restoration of lumbar degenerative scoliosis. Methods: All patients with DLS and lumbar degenerative scoliosis undergoing single-level unilateral transforaminal lumbar interbody fusion surgery between July 1, 2015, and April 30, 2021, were screened in this retrospective cohort study. Clinical outcomes including visual analog scale (VAS), Oswestry disability index (ODI), and radiographic parameters of sagittal and coronal alignment, cage spatial locations, and angle of pedicle screw (parallel, cranial, and caudad angle) were assessed. Coronal asymmetry was demonstrated by the intervertebral height difference between the medial and lateral margins of indexed intersegmental space. The correlations between Δintervertebral height difference (postoperative intervertebral height difference-preoperative intervertebral height difference) and radiographic parameters and clinical outcomes were analyzed by univariable, multivariable, mediation, and correlation analyses. Significance was set at a bilateral P<0.05. Results: A total of 57 included patients were followed up for a minimum of 1 year. Reduction of VAS, ODI, and improvement of radiographic parameters were found after surgery. The cranial angle of the lower pedicle screw positively correlated with Δintervertebral height difference restoration (b=0.54; standard error=0.11; P<0.001). Conclusions: Transforaminal lumbar interbody fusion surgery appears to be an effective approach to improving the radiographic and clinical outcomes of patients with DLS and lumbar degenerative scoliosis. The cranial direction of the lower pedicle screws in single-level unilateral transforaminal lumbar interbody fusion surgery may be associated with a better postoperative restoration of lumbar degenerative scoliosis.

Lumbar segment-dependent soft tissue artifacts of skin markers during in vivo weight-bearing forward-Backward bending.

Traditional optical motion capture (OMC) with retroreflective markers is commonly used to measure joint kinematics but was also reported with unavoidable soft tissue artifacts (STAs) when quantifying the motion of the spine. Additionally, the patterns of the STA on the lumbar spine remain unclear. This study aimed to 1) quantify the in vivo STAs of the human lower back in three-dimensional directions during weight-bearing forward-backward bending and 2) determine the effects of the STAs on the calculated flexion angles between the upper and lower lumbar spines and adjacent vertebrae by comparing the skin marker (SM)- and virtual bone marker (VM)-based measurements. Six healthy volunteers were imaged using a biplanar radiographic system, and thirteen skin markers were mounted on every volunteer's lower back while performing weight-bearing forward-backward bending. The STAs in the anterior/posterior (AP), medial/lateral (ML), and proximal/distal (PD) directions were investigated. The flexion angles between the upper and lower lumbar segments and adjacent intervertebral segments (L2-L5) throughout the cycle were calculated. For all the participants, STAs continuously increased in the AP direction and exhibited a reciprocal trend in the PD direction. During flexion, the STA at the lower lumbar region (L4-L5: 13.5 ± 6.5 mm) was significantly higher than that at the upper lumbar (L1-L3: 4.0 ± 1.5 mm) in the PD direction (p < 0.01). During extension, the lower lumbar (L4-L5: 2.7 ± 0.7 mm) exhibited significantly less STAs than that exhibited by the upper lumbar region (L1-L3: 6.1 ± 3.3 mm) (p < 0.05). The STA at the spinous process was significantly lower than that on both sides in the AP direction (p < 0.05). The present results on STAs, based on dual fluoroscopic measurements in healthy adult subjects, presented an anatomical direction, marker location, and anatomic segment dependency, which might help describe and quantify STAs for the lumbar spine kinematics and thus help develop location- and direction-specific weighting factors for use in global optimization algorithms aimed at minimizing the effects of STAs on the calculation of lumbar joint kinematics in the future.

Is reduction or arthrodesis in situ the optimal choice for adolescent spondylolisthesis?-a systematic review and meta-analysis.

BACKGROUND: Controversy remains about the choice of reduction or arthrodesis in situ for surgical management of adolescent spondylolisthesis, while no systematic review and meta-analysis were performed to determine which one is the optimal surgical choice. The study aims to compare outcomes of the two surgical strategies for adolescent spondylolisthesis. METHODS: A comprehensive search was performed through PubMed, Web of Science, Cochrane Library, Embase, OVID/MEDLINE, CBM, CNKI, and Wanfang with a cutoff date of May 21st, 2021. Search terms included "spondylolisthesis", "in situ" and "reduction". Included studies had following characteristics: (I) participants: adolescents with spondylolisthesis. (II) Intervention: reduction following arthrodesis. (III) Control: arthrodesis in situ. (IV) Outcomes: postoperative clinical and/or radiographic results. (V) Study design: randomized controlled trial (RCT), cohort or case-control study. Data were analyzed with Review Manager 5.4, and risk of bias assessment of studies was assessed via Newcastle-Ottawa quality assessment scale (NOS). RESULTS: Six cohort studies were included, with NOS scores of all ≥6. There were no significant differences regarding operative time [mean difference (MD) =152.62; 95% [confidence interval (CI)]: -54.02 to 359.26; I2=96%; P=0.15], blood loss (MD =786.61; 95% CI: -646.82 to 2,220.04; I2=90%; P=0.28), patient satisfaction (MD =1.98; 95% CI: 0.72 to 5.43; I2=0%; P=0.18), neurological complications (MD =1.02; 95% CI: 0.25 to 4.18; I2=0%; P=0.98), or total complications (MD =0.59; 95% CI: 0.29 to 1.19; I2=0%; P=0.14). However, patients undergoing reduction achieved better radiographic results: fusion rate (MD =3.09; 95% CI: 1.22 to 7.84; I2=40%; P=0.02), postoperative pseudarthrosis (MD =0.35; 95% CI: 0.15 to 0.79; I2=24%; P=0.01), percentage of slippage (MD =-20.58; 95% CI: -26.32 to -14.84; I2=0%; P<0.00001), and slipping angle (MD =-10.05; 95% CI: -14.55 to -5.54; I2=0%; P<0.0001). And no overt publication bias was found in the studies. DISCUSSION: Both reduction and arthrodesis in situ in adolescent spondylolisthesis are safe and demonstrate good clinical outcomes. However, reduction showed better radiographic results and was associated with less pseudarthrosis, better relief of disability, and improvements in self-image. In conclusion, reduction may be the optimal choice compared with arthrodesis in situ, but further verification of these findings is recommended using RCTs.