Comparison of three different screw trajectories in osteoporotic vertebrae: a biomechanical investigation

Percutaneous pedicle screw placement with a mini-open decompression versus open surgery in the treatment of lumbar spondylolisthesis: one-year results of a randomised controlled trial

PURPOSE

Symptomatic lumbar spondylolisthesis is usually treated with fusion surgery when conservative methods fail. However, traditional open decompression and fusion involves a large skin incision and muscle detachment. Therefore, minimally invasive techniques have been developed to reduce tissue damage, potentially leading to less postoperative pain and earlier resumption of activities. The purpose of this study was to compare percutaneous versus open pedicle screw placement in patients receiving lumbar midline decompression due to symptomatic lumbar spondylolisthesis focusing on short-term low back pain.

METHODS

A randomised controlled trial was conducted in 2 Dutch hospital from 2015 to 2020. Participants with spondylolytic or degenerative lumbar spondylolisthesis were randomised into percutaneous pedicle screw placement with a mini-open decompression (mini-open), or conventional open surgery with instrumented fusion (open). The primary outcome measure was short-term low back pain after 2 weeks, measured by a visual analogue scale. Leg pain, disability and quality of life were recorded at 2 and 6 weeks, 3 and 6 months and 1 year. Surgical variables, including complications, were recorded. Analyses were performed in the intention-to-treat population.

RESULTS

In total, 169 participants were included and randomised to mini-open (n = 81) or standard open surgery (n = 88). No statistically or clinically significant differences were found between groups in terms of primary or secondary outcomes. Surgery duration, blood loss, hospital stay, and complications were also similar between groups.

CONCLUSIONS

This study detected no difference in outcome between mini-open compared to open surgery in patients with spondylolisthesis. The hypothetical advantage of reduced short-term low back pain, less blood loss and better clinical outcome could not be confirmed.

Biomechanical impact of cortical bone vs. traditional pedicle screw trajectories: a finite element study on lumbar spinal instrumentation

Background

Pedicle screw fixation using the cortical bone trajectory (CBT) enhances stability by engaging cortical bone, offering a valuable alternative to the traditional pedicle screw trajectory (TT). This study used finite element analysis to compare L4-5 instrumentation with CBT and TT screws, investigating whether the increased cortical bone engagement in CBT improves stability but makes it more susceptible to fatigue failure.

Methods

A L3-sacrum model was generated using anonymized CT patient data, validated against existing studies, showing consistent ROM (range of motion) values. A mono-segmental L4-5 instrumentation with an interbody fusion cage was configured with both TT and CBT models, differentiated for healthy and osteoporotic bone (reduced Young's modulus). Both models were exposed to simulated biomechanical loading conditions (compression, flexion, extension, lateral bending, and rotation) to calculate screw loosening and breakage risk. Screw loosening was assessed by measuring micro-movements within the screw hole, while screw breakage was evaluated based on maximum stress values and their frequency at the same locations.

Results

In both healthy and osteoporotic bone, the CBT model exhibited smaller micro-movements compared to the TT model across all motions. For maximum stress in healthy bone, CBT showed lower stress during right rotation but higher stress in the other six motions. In osteoporotic bone, CBT stress exceeded TT stress in all conditions. The TT model in healthy bone showed stress concentrations at three locations, while CBT distributed stress across five sites. In osteoporotic bone, CBT showed stress at three locations, while TT distributed stress at four. Notably, in the TT model, maximum stress occurred at the screw head in six of seven movements, whereas in the CBT model, three movements showed maximum stress at the screw head and three at the screw tail.

Conclusion

CBT screws, by traversing three cortical layers, achieve greater integration with the vertebral bone compared to TT screws, thus reducing the risk of screw loosening. Although this increases the maximum stress on the screws, the stress is more evenly distributed, with the screw tail helping to reduce the risk of breakage.

Development and Validation of Deep Learning Preoperative Planning Software for Automatic Lumbosacral Screw Selection Using Computed Tomography

Achieving precise pedicle screw placement in posterior lumbar interbody fusion (PLIF) is essential but difficult due to the intricacies of manual preoperative planning with CT scans. We analyzed CT data from 316 PLIF patients, using Mimics software for manual planning by two surgeons. A deep learning model was trained on 228 patients and validated on 88 patients, assessing planning efficiency and accuracy. Automatic planning successfully segmented and placed screws in all 316 cases, significantly outperforming manual planning in speed. The Dice coefficient for segmentation accuracy was 0.95. The difference in mean pedicle transverse angle (PTA) and pedicle sagittal angle (PSA) for automatic planning screws compared to manual planning screws was 1.63 ± 0.83° and 1.39 ± 1.03°, respectively, and these differences were either statistically comparable or not significantly different compared to the variability of manual planning screws. The average Dice coefficient of implanted screws was 0.63 ± 0.08, and the consistency between automatic screws and manual reference screws was higher than that of internal screws (Dice 0.62 ± 0.09). Compared with manual screws, automatic screws were shorter (46.58 ± 3.09 mm) and thinner (6.24 ± 0.35 mm), and the difference was statistically significant. In qualitative validation, 97.7% of the automatic planning screws were rated Gertzbein-Robbins (GR) Class A and 97.3% of the automatic planning screws were rated Badu Class 0. Deep learning software automates lumbosacral pedicle screw planning, enhancing surgical efficiency and accuracy.

Side specific differences of Hounsfield-Units in the osteoporotic lumbar spine

Background

Gold standard for determining bone density as a surrogate parameter of bone quality is measurement of bone mineral density (BMD) by dual energy X-ray absorptiometry (DXA), most commonly performed on the lumbar spine (L1-L4). Computed tomography (CT) data are often available for surgical planning prior to spine procedures, but currently this information is not standardized for bone quality assessment. Besides, measuring the Hounsfield-Units (HU) is also of great importance in the context of biomechanical studies. This in vitro study aims in comparing BMD from DXA and HU based on diagnostic CT scans. In addition, methods are presented to quantify local density variations within bones.

Methods

One hundred and seventy-six vertebrae (L1-L4) from 44 body donors (age 84.0±8.7 years) were studied. DXA measurements were obtained on the complete vertebrae to determine BMD, as well as axial CT scans with a slice thickness of 1 mm. Using Mimics Innovation Suite image processing software (Materialise NV, Leuven, Belgium), two volumes (whole vertebra vs. spongious bone) were formed for each vertebra, which in turn were divided in their left and right sides. From these total of six volumes, the respective mean HU was determined. HU of the whole vertebra and just spongious HU were compared with the BMD of the corresponding vertebrae. Side specific differences were calculated as relative values.

Results

Whole bone and spongious HU correlated significantly (P>0.001; α=0.01) with BMD. A positive linear correlation was found, which was more pronounced for whole bone HU (R=0.72) than for spongious HU (R=0.62). When comparing the left and right sides within each vertebra, the HU was found to be 10% larger on average on one side compared to the opposite side. In some cases, the difference of left and right spongious bone can be up to 170%. There is a tendency for the side comparison to be larger for the spongious HU than for the whole vertebra.

Conclusions

Determination of HU from clinical CT scans is an important tool for assessing bone quality, primarily by including the cortical portion in the calculation of HU. Unlike BMD, HU can be used to distinguish precisely between individual regions. Some of the very large side-specific gradients of the HU indicate an enormous application potential for preoperative patient-specific planning.

Biportal endoscopic-assisted cortical bone trajectory screw placement and lumbar interbody fusion

BACKGROUND

Endoscopically assisted screw fixation with lumbar interbody fusion is rarely performed. We succeeded in implanting the cortical bone trajectory (CBT) screws under the guidance of unilateral biportal endoscopy (UBE).

METHOD

We attempted endoscopically assisted screw fixation in a patient with degenerative spondylolisthesis. Through a third portal, ipsilateral CBT screws were implanted without complications.

CONCLUSIONS

We successfully performed unilateral biportal endoscopic lumbar interbody fusion (ULIF) with CBT and reversed CBT screws. Compared with percutaneous pedicle screw (PPS) placement, this procedure is a minimally invasive, endoscopic alternative that allows precise screw placement.

Bone density optimized pedicle screw insertion

Background: Spinal fusion is the most common surgical treatment for the management of degenerative spinal disease. However, complications such as screw loosening lead to painful pseudoarthrosis, and are a common reason for revision. Optimization of screw trajectories to increase implant resistance to mechanical loading is essential. A recent optimization method has shown potential for determining optimal screw position and size based on areas of high bone elastic modulus (E-modulus). Aim: The aim of this biomechanical study was to verify the optimization algorithm for pedicle screw placement in a cadaveric study and to quantify the effect of optimization. The pull-out strength of pedicle screws with an optimized trajectory was compared to that of a traditional trajectory. Methods: Twenty-five lumbar vertebrae were instrumented with pedicle screws (on one side, the pedicle screws were inserted in the traditional way, on the other side, the screws were inserted using an optimized trajectory). Results: An improvement in pull-out strength and pull-out strain energy of the optimized screw trajectory compared to the traditional screw trajectory was only observed for E-modulus values greater than 3500 MPa cm3. For values of 3500 MPa cm3 or less, optimization showed no clear benefit. The median screw length of the optimized pedicle screws was significantly smaller than the median screw length of the traditionally inserted pedicle screws, p < 0.001. Discussion: Optimization of the pedicle screw trajectory is feasible, but seems to apply only to vertebrae with very high E-modulus values. This is likely because screw trajectory optimization resulted in a reduction in screw length and therefore a reduction in the implant-bone interface. Future efforts to predict the optimal pedicle screw trajectory should include screw length as a critical component of potential stability.

[Osteoporotic vertebral body fractures]

The number of osteoporotic fractures of the spine is increasing. These fractures are associated with elevated morbidity and mortality. This article provides an overview of the special features of these fractures, the diagnostic procedure, their classification, and the conservative and surgical treatment options. For the mostly elderly patients, it is important to treat the underlying disease and to address associated problems such as frailty and sarcopenia. To meet this growing medical and socio-economic challenge, a holistic interdisciplinary and interprofessional treatment approach is required.

Supraspinous ligament arc tangent guided freehand thoracic pedicle screw insertion technique: high parallelism between screws and upper endplate

Research objective

To propose a technique for placing pedicle screws in the thoracic spine using the Supraspinous ligament Arc Tangent (SLAT) as a guide to increase the safety and stability of screw placement.

Content and methods

A retrospective analysis of postoperative anteroposterior and lateral x-ray images was performed for 118 patients with thoracic spine diseases who received conventional freehand technique from January 2016 to May 2020 and SLAT-guided technique since June 2020 to present. The diagnoses included thoracic spinal stenosis, deformity, fractures, infections, and tumors. The angle between the screw and the upper endplate was categorized as grade 1 (0°-5°), grade 2 (5°-10°), and grade 3 (>10°). Three surgeons with more than 10 years of experience in spinal surgery measured the angle between the screw and the upper endplate in the lateral view. Chi-square test was used for statistical analysis, and p < 0.05 was considered statistically significant.

Results

A total of 1315 pedicle screws were placed from T1 to T12 in all patients. In the conventional freehand technique group, 549 screws were grade 1, 35 screws were grade 2, and 23 screws were grade 3. In the SLAT-guided freehand technique group, 685 screws were grade 1, 15 screws were grade 2, and 8 screws were grade 3. The data of each group was p < 0.05 by Chi-squared test, which was statistically significant, indicating that the SLAT-guided freehand technique resulted in a higher rate of parallelism between the screws and the upper endplate. All patients underwent intraoperative neurophysiological monitoring, immediate postoperative neurological examination, postoperative x-ray examination, and assess the eventual recovery. The screws were safe and stable, and no complications related to pedicle screw placement were found.

Conclusion

The SLAT-guided freehand technique for placing pedicle screws in the thoracic spine can achieve a higher rate of screw-upper endplate parallelism, making screw placement safer and more accurate. Our method provides a convenient and reliable technique for most spinal surgeons, allowing for increased accuracy and safety with less fluoroscopic guidance.

Biomechanical comparison of pedicle screw fixation strength among three different screw trajectories using single vertebrae and one-level functional spinal unit

Three key factors are responsible for the biomechanical performance of pedicle screw fixation: screw mechanical characteristics, bone quality and insertion techniques. To the best of the authors' knowledge, no study has directly compared the biomechanical performance among three trajectories, i.e., the traditional trajectory (TT), modified trajectory (MT) and cortical bone trajectory (CBT), in a porcine model. This study compared the pullout strength and insertion torque of three trajectory methods in single vertebrae, the pullout strength and fixation stiffness including flexion, extension, and lateral bending in a one-level instrumented functional spinal unit (FSU) that mimics the in vivo configuration were clarified. A total of 18 single vertebrae and 18 FSUs were randomly assigned into three screw insertion methods (n = 6 in each trajectory group). In the TT group, the screw converged from its entry point, passed completely inside the pedicle, was parallel to the superior endplate, was located in the superior third of the vertebral body and reached to at least the anterior third of the vertebral body. In the MT group, the convergent angle was similar to that of the TT method but directed caudally to the anterior inferior margin of the vertebral body. The results of insertion torque and pullout strength in single vertebrae were analyzed; in addition, the stiffness and pullout strength in the one-level FSU were also investigated. This study demonstrated that, in single vertebrae, the insertion torque was significantly higher in CBT groups than in TT and MT groups (p < 0.05). The maximal pullout strength was significantly higher in MT groups than in TT and CBT groups (p < 0.05). There was no significant difference in stiffness in the three motions among all groups. The maximal pullout strength in FSUs of MT and CBT groups were significantly higher than the TT groups (p < 0.05). We concluded that either MT or CBT provides better biomechanical performance than TT in single vertebrae or FSUs. The lack of significance of stiffness in FSUs among three methods suggested that MT or CBT could be a reasonable alternative to TT if the traditional trajectory was not feasible.

Development and validation of an automated planning tool for navigated lumbosacral pedicle screws using a convolutional neural network

BACKGROUND CONTEXT

Navigation and robotic systems have been increasingly applied to spinal instrumentation but dedicated screw planning is a time-consuming prerequisite to tap the full potential of these techniques.

PURPOSE

To develop and validate an automated planning tool for lumbosacral pedicle screw placement using a convolutional neural network (CNN) to facilitate the planning process.

STUDY DESIGN/SETTING

Retrospective analysis and processing of CT and screw planning data randomly selected from a consecutive registry of CT-navigated instrumentations from a single academic institution.

PATIENT SAMPLE

Data from 179 cases was processed for CNN training and validation (155 for training, 24 for validation) leveraging a total of 1182 screws (1052 for training, 130 for validation).

OUTCOME MEASURES

Quantitative and qualitative (Gertzbein-Robbins classification [GR]) validation via comparison of automatically and manually planned reference screws, inter-rater and intra-rater variability.

METHODS

Annotated data from CT-navigated instrumentation was used to train a CNN operating in a vertebra instance-based approach employing a state-of-the-art U-Net framework. Internal five-fold cross-validation and external validation on an independent cohort not previously involved in training was performed. Quantitative validation of automatically planned screws was performed in comparison to corresponding manually planned screws by calculating the minimal absolute difference (MAD) of screw head and tip points, length and diameter, screw direction and Dice coefficient. Results were evaluated in relation to inter-rater and intra-rater variability of manual screw planning.

RESULTS

Automated screw planning was successful in all targeted 130 screws. Compared with manually planned screws as a reference, mean MAD of automatically planned screws was 4.61±2.27 mm for screw head, 3.96±2.19 mm for tip points and 5.51±3.64° for screw direction. These differences were either statistically comparable or significantly smaller when compared with interrater variability of manual screw planning (p>.99 for head point and direction, p=.004 for tip point, respectively). Mean Dice coefficient of 0.61±0.16 indicated significantly greater agreement of automatic screws with the manual reference compared with interrater agreement (Dice 0.56±0.18, p<.001). Automatically planned screws were marginally shorter (MAD 3.4±3.2 mm) and thinner (MAD mean 0.3±0.6 mm) compared with the manual reference, but with statistical significance (p<.0001, respectively). Automatically planned screws were GR grade A in 96.2% in qualitative validation. Planning time was significantly shorter with the automatic approach (0:41 min vs. 6:41 min, p<.0001).

CONCLUSIONS

We derived and validated a fully automated planning tool for lumbosacral pedicle screws using a CNN. Our validation showed noninferiority to manual screw planning and provided sufficient accuracy to facilitate and expedite the screw planning process. These results offer a high potential to improve workflows in spine surgery when integrated into navigation or robotic assistance systems.

CT-Navigated Spinal Instrumentations-Three-Dimensional Evaluation of Screw Placement Accuracy in Relation to a Screw Trajectory Plan

Background and Objectives: In the literature, spinal navigation and robot-assisted surgery improved screw placement accuracy, but the majority of studies only qualitatively report on screw positioning within the vertebra. We sought to evaluate screw placement accuracy in relation to a preoperative trajectory plan by three-dimensional quantification to elucidate technical benefits of navigation for lumbar pedicle screws. Materials and Methods: In 27 CT-navigated instrumentations for degenerative disease, a dedicated intraoperative 3D-trajectory plan was created for all screws. Final screw positions were defined on postoperative CT. Trajectory plans and final screw positions were co-registered and quantitatively compared computing minimal absolute differences (MAD) of screw head and tip points (mm) and screw axis (degree) in 3D-space, respectively. Differences were evaluated with consideration of the navigation target registration error. Clinical acceptability of screws was evaluated using the Gertzbein−Robbins (GR) classification. Results: Data included 140 screws covering levels L1-S1. While screw placement was clinically acceptable in all cases (GR grade A and B in 112 (80%) and 28 (20%) cases, respectively), implanted screws showed considerable deviation compared to the trajectory plan: Mean axis deviation was 6.3° ± 3.6°, screw head and tip points showed mean MAD of 5.2 ± 2.4 mm and 5.5 ± 2.7 mm, respectively. Deviations significantly exceeded the mean navigation registration error of 0.87 ± 0.22 mm (p < 0.001). Conclusions: Screw placement was clinically acceptable in all screws after navigated placement but nevertheless, considerable deviation in implanted screws was noted compared to the initial trajectory plan. Our data provides a 3D-quantitative benchmark for screw accuracy achievable by CT-navigation in routine spine surgery and suggests a framework for objective comparison of screw outcome after navigated or robot-assisted procedures. Factors contributing to screw deviations should be considered to assure optimal surgical results when applying navigation for spinal instrumentation.

Finite element study on whether posterior upper wall fracture is a risk factor for the failure of short-segment pedicle screw fixation in the treatment of L1 burst fracture

PURPOSE

To establish the finite element model of T12 and L2 (T12-L2) pedicle screw fixation for severe L1 burst fracture, and quantitatively simulate and analyze the screw stress and vertebral displacement in different degrees of L1 posterior upper wall fracture (PUWF), and evaluate whether PUWF degree is a risk factor for fixation failure.

METHODS

The data of 6 healthy volunteers were used to establish a finite element model of T12-L2 pedicle screw fixation for type A severe burst fractures. The stress and displacement of the conventional and Schanz pedicle screws for the different degrees of PUWF (including the anterior upper wall of the vertebral canal and the bipedicle) were evaluated.

RESULTS

The maximum stress and L1 displacement of conventional and Schanz pedicle screws were positively correlated with the severity of the PUWF (P<0.05). During anterior flexion, the maximum stress of conventional pedicle screws for 70% type I were 538.3±59.75MPa and the maximum stress of Schanz pedicle screws for 90% type Ⅱ, 90% type Ⅲ and 70% type IV fractures were close to the fatigue threshold. The maximum stress during anterior flexion were significantly higher than those during posterior extension, bending and rotation (P<0.05).

CONCLUSION

The posterior upper wall fracture of vertebral body (VB) of type A burst fracture is not an independent risk factor for the failure of short-segment pedicle screw fixation (SSPSF). Anterior flexion of type A fractures combined with severe PUWF of VB was a risk factor for the failure of SSPSF.