A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model

Digital smart internal fixation surgery for coronal process basal fracture with normal joint spaces or radius-shortening: Occult factor of radius-ulna load sharing

BACKGROUND

Reasonable postoperative humeroradial and humeroulnar joint spaces maybe an important indicator in biomechanical stability of smart internal fixation surgery for coronoid process basal fractures (CPBF). The aim of this study is to compare elbow articular stresses and elbow-forearm stability under smart internal fixations for the CPBF between normal elbow joint spaces and radius-shortening, and to determine the occult factor of radius-ulna load sharing.

METHODS

CT images of 70 volunteers with intact elbow joints were retrospectively collected for accurate three-dimensional reconstruction to measure the longitudinal and transverse joint spaces. Two groups of ten finite element (FE) models were established prospectively between normal joint space and radius-shortening with 2.0 mm, including intact elbow joint and forearm, elbow-forearm with CPBF trauma, anterior or posterior double screws-cancellous bone fixation, mini-plate-cancellous bone fixation. Three sets of physiological loads (compression, valgus, varus) were used for FE intelligent calculation, FE model verification, and biomechanical and motion analysis.

RESULTS

The stress distribution between coronoid process and radial head, compression displacements and valgus angles of elbow-forearm in the three smart fixation models of the normal joint spaces were close to those of corresponding intact elbow model, but were significantly different from those of preoperative CPBF models and fixed radius-shortening models. The maximum stresses of three smart fixation instrument models of normal joint spaces were significantly smaller than those of the corresponding fixed radius-shortening models.

CONCLUSIONS

On the basis of the existing trauma of the elbow-forearm system in clinical practice, which is a dominant factor affecting radius-ulna load sharing, the elbow joint longitudinal space has been found to be the occult factor affecting radius-ulna load sharing. The stability and load sharing of radius and ulna after three kinds of smart fixations of the CPBF is not only related to the anatomical and biomechanical stability principles of smart internal fixations, but also closely related to postoperative elbow joint longitudinal space.

Biomechanical comparison of two different compression screws for the treatment of odontoid fractures in human dens axis specimen

BACKGROUND

Lag screw osteosynthesis for odontoid fractures has a high rate of pseudoarthrosis, especially in elderly patients. Besides biomechanical properties of the different screw types, insufficient fragment compression or unnoticed screw stripping may be the main causing factors for this adverse event. The aim of the study was to compare two screws in clinical use with different design principles in terms of compression force and stability against screw stripping.

METHODS

Twelve human cadaveric C2 vertebral bodies were considered. Bone density was determined. The specimens were matched according to bone density and randomly assigned to two experimental groups. An odontoid fracture was induced, which were fixed either with a 3.5 mm standard compression screw or with a 5 mm sleeve nut screw. Both screws are certified for the treatment of odontoid fractures. The bone samples were fixed in a measuring device. The screwdriver was driven mechanically. The tests were analyzed for peak interfragmentary compression and screw-in torque with a frequency of 20 Hz.

FINDINGS

The maximum fragment compression was significantly higher with screw with sleeve nut at 346.13(SD ±72.35) N compared with classic compression screw at 162.68(SD ±114.13) N (p = 0.025). Screw stripping occurred significantly earlier in classic compression screw at 255.5(SD ±192.0)° rotation after reaching maximum compression than in screw with sleeve nut at 1005.2(SD ±341.1)° (p = 0.0039).

INTERPRETATION

Screw with sleeve nut achieves greater fragment compression and is more robust to screw stripping compared to classic compression screw. Whether the better biomechanical properties lead to a reduction of pseudoarthrosis has to be proven in clinical studies.

How to Optimize Pedicle Screw Parameters for the Thoracic Spine? A Biomechanical and Finite Element Method Study

STUDY DESIGN

Pedicle screw study.

OBJECTIVE

The selection of pedicle screw parameters usually involves the surgeon's analysis of preoperative CT imaging along with anatomical landmarks and tactile examination. However, there is minimal consensus on a standardized guideline for selection methods on pedicle screws. We aimed to determine the effects of thoracic screw diameter to pedicle width on pullout strength determined by cortical bone purchase.

METHODS

Biomechanical study performed with human cadaveric thoracic vertebrae and experimentally validated three-dimensional finite element model instrumented with pedicle screws of various diameters. We used a variable (SD/PW) ratio to express the screw selection. We hypothesized a positive correlation between the pullout load determined by the bone purchase and the SD/PW. This relationship was first investigated in a validated finite element model considering bone purchase related to the strength of an upper thoracic vertebra. Then, the correlation to the entire spine is evaluated.

RESULTS

The failure load ranged from 371.3 to 1601.0 N, respectively, for 3 and 6 mm screws. The determinant coefficient was increased to R2=.421 when a linear relationship between pullout load and the SD/PW ratio was used. The peak loads of 1216 and 1288N were found for an SD/PW ratio of .83.

CONCLUSION

We have found that the screw pullout load is more correlated to SD/PW than other pedicle measures for a maximized SD/PW ratio of .83. This particular value should be considered the upper limit of the indicated SD/PW ratio and a means to determine the optimal screw diameter to enhance pullout strength.

Biomechanical comparison of compact versus standard flute drill bits, and interlocking versus buttress thread self-tapping cortical bone screws in cadaveric equine third metacarpal condyle

OBJECTIVES

To compare (1) performance of compact versus standard flute drill bits, (2) screw insertion properties and (3) pullout variables between interlocking thread (ITS) and buttress thread (BTS) self-tapping screws in third metacarpi.

STUDY DESIGN

In vitro experimental study.

SAMPLE POPULATION

Paired third metacarpi from 11 Thoroughbreds aged 2-4 years.

METHODS

Screws were inserted into the lateral condylar fossae following bone preparation using the respective drill bit for each screw type. Screw pullout was achieved using a mechanical testing system. Density and porosity of bone surrounding screw holes was measured with microcomputed tomography following each pullout test. Drilling, screw insertion and pullout variables were compared between drill bit and screw types using repeated measures ANOVA. Linear regression analyses were used to characterize relationships between bone tissue properties and drill bit and screw outcomes.

RESULTS

Maximum torque power spectral density (PSD) was lower for compact flute drill bits. Insertion torque was 50% higher for ITS. BTS had 33% greater preyield stiffness and 7% greater mean yield force. Bone tissue properties affected measured variables similarly for both screw and drill bit types.

CONCLUSIONS

Lower torque PSD may increase durability of the compact flute drill bit. ITS had greater insertional torque, which may reflect greater bone engagement. BTS had greater resistance to axial pullout forces.

CLINICAL SIGNIFICANCE

Metacarpal bone provides a simple model for comparison of drill bit and screw designs. Use of ITS to repair equine fractures subject to predominantly tensile forces is not justified based on the results of this study.

Anatomical and Biomechanical Stability of Single/Double Screw-Cancellous Bone Fixations of Regan-Morry Type III Ulnar Coronoid Fractures in Adults: CT Measurement and Finite Element Analysis

OBJECTIVE

At present, it is still uncertain whether single screw has the same stability as double screws in the treatment of ulnar coronal process basal fracture (Regan-Morry type III). So, we aimed to compare the pull-out force and anti-rotation torque of anterior single/double screw-cancellous bone fixation (aSSBF, aDSBF) in this fracture, and further study the influencing factors on anatomical and biomechanical stability of smart screw internal fixations.

METHODS

A total of 63 adult volunteers with no history of elbow injury underwent elbow CT scanning with associated three-dimensional reconstruction that enabled the measurements of bone density and fixed length of the proximal ulna and coronoid. The models of coronal process basal fracture, aSSBF and aDSBF, were developed and validated. Using the finite element model test, the sensitivity analysis of pull-out force and rotational torque was carried out.

RESULTS

The pull-out force of aSSBF model was positively correlated with the density of the cancellous bone and linearly related to the fixed depth of the screw. The load pattern of pull-out force of aDSBF model was similar to that of aSSBF model. The ultimate torque of aDSBF model was higher than that of aSSBF model, but the load pattern of ultimate torque of both models was similar to each other when the fracture reset was satisfactory, and the screw nut attaches closely to coronoid process. Moreover, with enhancement of initial pre-tightening force, the increase of ultimate torque of both models was small.

CONCLUSIONS

In addition to three pull-out stability factors of smart screw fixations, fracture surface fitting degree and nut fitting degree are the other two important anatomical and biomechanical stability factors of smart screw fixations both for rotational stability. When all pull-out stability and rotational stability factors meet reasonable conditions simultaneously, single or double screw fixation methods are stable for the treatments of ulnar coronoid basal fractures.

THE FIXATION EFFECT OF DIFFERENT TYPES OF SCREWS IN THE WHOLE OSTEOPOROTIC LUMBAR VERTEBRAE: AN FEA STUDY

Purpose: The aim of this study is to explore how pedicle screws (PSs) and cortical bone trajectory (CBT) screws differ in fixation strength when implanted in L1–L5 with osteoporosis, providing support for choosing implants and trajectories in spine internal fixation surgeries. Methods: We filtered 30 lumbar segments out from CT images of eight osteoporotic participants and simulated PS or CBT screw implantation in each segment, generating 60 vertebra-screw assembly FE models. To evaluate the fixation effect, we performed a pull-out force test simulation in each model and analyzed the maximal pull-out force, pull-out stiffness, and equivalent stress of vertebrae and screws. Results: The maximal pull-out force of PS and CBT screws in L1–L5 was in the range of 905–1552 (N) and 587–1012 (N), while the pull-out stiffness was in the range of 1990–2617 (N/mm) and 1007–1681 (N/mm). The fixation strength of PS in L4 and L5 was higher ([Formula: see text]), while in L1–L3 PS and CBT screws are similar ([Formula: see text]). The maximal stress of vertebrae and screws when PS was pulled at 0.25[Formula: see text]mm was larger than that of CBT screws. Conclusions: For patients with moderate osteoporosis, it is recommended to insert PS into L4 and L5 to attain better fixation strength, but vertebrae are more prone to fracture. Consequently, under severe osteoporosis, the implantation of CBT screws should be considered first. Bone cement injection may be necessary to consolidate the screw-vertebrae interface with osteoporosis.

A Novel Screw Modeling Approach to Study the Effects of Screw Parameters on Pullout Strength

Screw loosening remains a prominent problem for osteoporotic patients undergoing pedicle screw fixation surgeries and is affected by screw parameters (e.g., diameter, pitch and thread angle). However, the individual and interactive effects of these parameters on screw fixation are not fully understood. Furthermore, current finite element modeling of an threaded screw is less computationally efficient. To address these issues, we (1) explored a novel "simulated threaded screw" approach (virtual threads assigned to the contact elements of a simplified screw) and compared its performance with threaded and simplified screws, and (2) examined with this approach the individual and interactive effects of altering screw diameter (5.5-6.5 mm), pitch (1-2 mm) and half-thread angle (20-30°) on pullout strength of normal vertebrae. Results demonstrated that the "simulated threaded screw" approach equivalently predicted pullout strength compared to the "threaded screw" approach (R2 = 0.99, slope = 1). We further found that the pullout strength was most sensitive to the change in screw diameter, followed by thread angle, pitch and interactions of diameter*pitch or diameter*angle. In conclusion, the "simulated threaded screw" approach can achieve the same predictive capability compared to threaded modeling of the screw. The current findings may serve as useful references for planning of screw parameters, so as to improve the complication of screw loosening.

Influence of thread design on anchorage of pedicle screws in cancellous bone: an experimental and analytical analysis

Threads of modern pedicle screws can vary greatly in design. It is difficult to assess which interplay of design features is particularly advantageous for screw anchorage. This study aims to increase the understanding of the anchorage behaviour between screw and cancellous bone. Pull-out tests of six pedicle screws in two sizes each were performed on three densities of biomechanical test material. More general screw characteristics were derived from the screw design and evaluated using the test data. Selected screws were tested on body donor material. Some screw characteristics, such as compacting, are well suited to compare the different thread designs of screws with tapered core. The combination of two characteristics, one representing bone compacting and one representing thread flank area, appears to be particularly advantageous for assessing anchorage behaviour. With an equation derived from these characteristics, the pull-out strength could be calculated very accurately (mean deviation 1%). Furthermore, findings are corroborated by tests on donor material. For screws with tapered core, the design demands for good anchorage against pull-out from cancellous bone change with material density. With sufficient bone quality, screws with a high compacting effect are advantageous, while with low bone density a high thread flank area also appears necessary for better screw anchorage.

3D pull-out finite element simulation of the pedicle screw-trabecular bone interface at strain rates

Biomedical experimental studies such as pull-out (PO), screw loosening experience variability mechanical properties of fresh bone, legal procedures of cadaver bone samples and time-consuming problems. Finite Element Method (FEM) could overcome experimental problems in biomechanics. However, material modelling of bone is quite difficult, which has viscoelastic and viscoplastic properties. The study presents a bone material model which is constructed at the strain rates with the Johnson-Cook (JC) material model, one of the robust constitutive material models. The JC material constants of trabecular bone are determined by the curve fitting method at strain rates for the 3D PO finite element simulation, which defines the screw-bone interface relationship. The PO simulation is performed using the Abaqus/CAE software program. Bone fracture mechanisms are simulated with dynamic/explicit solutions during the PO phenomenon. The paper exposes whether the strain rate has effects on the PO performance. Moreover, simulation reveals the relationship between pedicle screw diameter and PO performance. The results obtained that the maximum pull-out force (POF) improves as both the screw diameter and the strain rate increase. For 5.5 mm diameter pedicle screw POFs were 487, 517 and 1708 N at strain rate 0.00015, 0.015 and 0.015 s^-1, respectively. The FOFs obtained from the simulation of the other screw were 730, 802 and 2008 N at strain rates 0.00015, 0.0015 and 0.015, respectively. PO phenomenon was also simulated realistically in the finite element analysis (FEA).

Homogenized finite element models can accurately predict screw pull-out in continuum materials, but not in porous materials

BACKGROUND AND OBJECTIVE

Bone screw fixation can be estimated with several test methods such as insertion torque, pull-out, push-in and bending tests. A basic understanding of the relationship between screw fixation and bone microstructure is still lacking. Computational models can help clarify this relationship. The objective of the paper is to evaluate homogenized finite element (hFE) models of bone screw pull-out.

METHODS

Experimental pull-out tests were performed on three materials: two polyurethane (PU) foams having a porous microstructure, and a high density polyethylene (HDPE) which is a continuum material. Forty-five titanium pedicle screws were inserted to 10, 20, and 30 mm in equally sized blocks of all three materials (N = 5/group). Pull-out characteristics i.e. stiffness (S), yield force (F_y), peak pull-out force (F_ult) and displacement at F_ult (d_ult) were measured. hFE models were created replicating the experiments. The screw was modeled as a rigid body and 5 mm axial displacement was applied to the head of the screw. Simulations were performed evaluating two different conditions at the bone-screw interface; once in which the screw fitted the pilot hole exactly ("free-stressed") and once in which interface stresses resulting from the insertion process were taken into account ("pre-stressed").

RESULTS

The simulations representing the pre-stressed condition in HDPE matched the experimental data well; S, F_y, and F_ult differed less than 11%, 2% and 0.5% from the experimental data, respectively, whereas d_ult differed less than 16%. The free-stressed simulations were less accurate, especially stiffness (158% higher than the pre-stressed condition) and d_ult (30% lower than pre-stressed condition) were affected. The simulations representing PU did not match the experiments well. For the 20 mm insertion depth, S, F_y and F_ult differed by more than 104%, 89% and 66%, respectively from the experimental values. Agreement did not improve for 10 and 30 mm insertion depths.

CONCLUSIONS

We found that hFE models can accurately quantify screw pull-out in continuum materials such as HDPE, but not in materials with a porous structure, such as PU. Pre-stresses in the bone induced by the insertion process cannot be neglected and need to be included in the hFE simulations.

Biomechanical influence of the surgical approaches, implant length and density in stabilizing ankylosing spondylitis cervical spine fracture

Ankylosing spondylitis cervical spine fractures (ASCFs) are particularly unstable and need special consideration when selecting appropriate internal fixation technology. However, there is a lack of related biomechanical studies. This study aimed to investigate the biomechanical influence of the pattern, length, and density of instrumentation for the treatment of ASCF. Posterior, anterior, and various combined fixation approaches were constructed using the finite element model (FEM) to mimic the surgical treatment of ASCFs at C5/6. The rate of motion change (RMC) at the fractured level and the internal stress distribution (ISD) were observed. The results showed that longer segments of fixation and combined fixation approaches provided better stability and lowered the maximal stress. The RMC decreased more significantly when the length increased from 1 to 3 levels (302% decrease under flexion, 134% decrease under extension) than from 3 to 5 levels (22% decrease under flexion, 23% decrease under extension). Longer fixation seems to be more stable with the anterior/posterior approach alone, but 3-level posterior fixation may be the most cost-effective option. It is recommended to perform surgery with combined approaches, which provide the best stability. Long skipped-screwing posterior fixation is an alternative technique for use in ASCF patients.

Biomechanical comparison of three different compression screws for treatment of odontoid fractures evaluation of a new screw design

BACKGROUND

Lag screw osteosynthesis in odontoid fractures shows a high rate of pseudarthrosis. Biomechanical properties may play a role with insufficient fragment compression or unnoticed screw stripping. A biomechanical comparison of different constructed lag-screws was carried out and the biomechanical properties determined.

METHODS

Two identical compression screws with different pilot holes (1.25 and 2.5 mm), a double-threaded screw and one sleeve-nut-screw were tested on artificial bone (Sawbone, densities 10-30pcf). Fragment compression and torque were continuously measured using thin-film force sensors (Flexiforce A201, Tekscan) and torque sensors (PCE-TM 80, PCE GmbH).

FINDINGS

The lowest compression reached the double-threaded screw. Compression and sleeve-nut-screw achieved 214-298% and 325-546%, respectively, of the compression force of double-threaded-screw, depending on the test material. The pilot hole optimization led to a significant improvement in compression only in the densest test material. Screw stripping took place significantly later with increasing density of the test material on all screws. In compression screws this was done at a screw rotation of 180-270°, in sleeve nut screw at 270-720° and in double-threaded screws at 300-600° after reaching the maximum compression.

INTERPRETATION

Double-threaded screw is robust against screw stripping, but achieves only low fragment compression. The classic compression screws achieve better compression, but are sensitive to screw stripping. Sleeve-nut screw is superior in compression and as robust as double-threaded screw against screw stripping. Whether the better biomechanical properties lead to a reduction in pseudarthrosis must be proven in clinical trials.

Effect of the positional relationship between the interference screw and the tendon graft in the bone tunnel in ligament reconstruction

PURPOSE

To reveal the effects of the positional and length relationships between the interference screws (ISs) and the tendon graft in the bone tunnel on the fixation strength in ligament reconstruction.

METHODS

We compared three IS positions on the anterior (the Anterior group) or posterior (the Posterior group) or side (the Side group) of the tendon graft in relation to the pullout direction. The tendon graft was pulled at 0°, 30°, 60°, and 90° to the bone tunnel, and the maximum pullout load at each angle was compared among the groups. We also investigated the relationship between the length of the tendon graft and the length of the IS in the bone tunnel. The direction of the pullout force was the same as that of the Anterior group, and the maximum load was compared between groups in which the tendon graft was longer or shorter than the IS.

RESULTS

The maximum loads of the Anterior group were significantly greater than those of the Posterior and Side groups at the traction angles of 30° and 60°, respectively. An IS shorter than the tendon graft was found to provide significantly superior fixation strength compared to an IS longer than the tendon graft.

CONCLUSIONS

Better fixation strength was achieved when the IS was placed on the side of the anchorage tunnel on which the tendon graft was loaded and the IS was shorter than the tendon graft.

Individualized prediction of pedicle screw fixation strength with a finite element model

Pedicle screws are used for the treatment of a wide variety of spinal pathologies. A good screw holding power in bone is required for treatment success, but has so far not been predictable computationally. The goal of this study was to develop an automated tool able to predict patient-specific screw fixation strength through finite element simulation. We compared the simulation results with results from biomechanical pull-out tests performed on animal lumbar specimens. Experimental and simulation pull-out strengths were highly correlated [Formula: see text] and the mean error was 20.25%. The fixation strength was also associated to great extent with pull-out stiffness and strain energy, as well as the screw size and mean vertebral density.

A comparative study of tapped and untapped pilot holes for bicortical orthopedic screws – 3D finite element analysis with an experimental test

The aim of this study was to compare the screw-to-bone fixation strength of two insertion techniques: self-tapping screw (STS) and non-self-tapping screw (NSTS). Finite element analysis (FEA) was used for the comparison by featuring three tests (insertion, pull-out and shear) in a human tibia bone model. A non-linear material behavior with ductile damage properties was chosen for the modeling. To validate the numerical models, experimental insertion and pull-out tests were carried out using a synthetic bone. The experimental and numerical results of pull-out tests correlated well. Thread forming was successfully simulated during the insertion process of STS and NSTS. It is demonstrated that the STS generates higher insertion torque, induces a higher amount of stress after the insertion process and relatively more strength under the pull-out and shear tests than the NSTS. However, the NSTS induces more stiffness under the two tests (pull-out and shear) and less damage to the screw-bone interface compared to the STS. It is concluded that the use of STS ensures tighter bony contact and enables higher pull-out strength; however, the use of NSTS improves the stiffness of the fixation and induces less damage to the cortical bone-screw fixation and thus minimum risk is obtained in terms of bone necrosis.

Finite Element- and Design of Experiment-Derived Optimization of Screw Configurations and a Locking Plate for Internal Fixation System

Objectives

The optimization for the screw configurations and bone plate parameters was studied to improve the biomechanical performances such as reliable internal fixation and beneficial callus growth for the clinical treatment of femoral shaft fracture.

Methods

The finite element analysis (FEA) of internal fixation system under different screw configurations based on the orthogonal design was performed and so was for the different structural parameters of the locking plate based on the combination of uniform and orthogonal design. Moreover, orthogonal experiment weight matrixes for four evaluation indexes with FEA were analyzed.

Results

The analytical results showed the optimal scheme of screw configuration was that screws are omitted in the thread holes near the fracture site, and single cortical screws are used in the following holes to the distal end, while the double cortical screws are fixed in thread holes that are distal to the fracture; in the other words, the length of the screws showed an increasing trend from the fracture site to the distal end in the optimized configuration. The plate structure was optimized when thread holes gap reached 13 mm, with a width of 11 mm and 4.6 mm and 5 mm for thickness and diameter of the screw, respectively. The biomechanical performance of the internal fixation construct was further improved by about 10% based on the optimal strain range and lower stress in the internal fixation system.

Conclusions

The proposed orthogonal design and uniform design can be used in a more efficient way for the optimization of internal fixation system, which can reduce the simulation runs to about 10% compared with comprehensive test, and the methodology can be also used for other types of fractures to achieve better internal fixation stability and optimal healing efficiency, which may provide a method for an orthopedist in choosing the screw configurations and parameters for internal fixation system in a more efficient way.

Finite Element Analysis of Long Posterior Transpedicular Instrumentation for Cervicothoracic Fractures Related to Ankylosing Spondylitis

STUDY DESIGN

Biomechanical finite element model analysis.

OBJECTIVES

Spinal fractures related to ankylosing spondylitis (AS) are often treated by long posterior stabilization. The objective of this study is to develop a finite element model (FEM) for spinal fractures related to AS and to establish a biomechanical foundation for long posterior stabilization of cervicothoracic fractures related to AS.

METHODS

An existing FEM (consisting of 2 separately developed models) including the cervical and thoracic spine were adapted to the conditions of AS (all discs fused, C0-C1 and C1-C2 mobile). A fracture at the level C6-C7 was simulated. Besides a normal spine (no AS, no fracture) and the uninstrumented fractured spine 4 different posterior transpedicular instrumentations were tested. Three loads (1.5g, 3.0g, 4.5g) were applied according to a specific load curve.

RESULTS

All posterior stabilization methods could normalize the axial stability at the fracture site as measured with gap distance. The maximum stress at the cranial instrumentation end (C3-C4) was slightly greater if every level was instrumented, than in the skipped level model. The skipped level instrumentation achieved similar rotatory stability as the long multilevel instrumentation.

CONCLUSIONS

Skipping instrumentation levels without giving up instrumentation length reduced stresses in the ossified tissue within the range of the instrumentation and did not decrease the stability in a FEM of a cervicothoracic fracture related to AS. Considering the risks associated with every additional screw placed, the skipped level instrumentation has advantages regarding patient safety.

Effects of the Insertion Type and Depth on the Pedicle Screw Pullout Strength: A Finite Element Study

Purpose

The pedicle screw is a surgical device that has become widely used in spinal fixation and stabilization. Postsurgical complications such as screw loosening due to fatigue loading and screw breakage still need investigations. Clinical parameters such as the screw insertion type and depth, the bone density, and the patient degree of mobility greatly affect the mechanisms of the implant's failure/success.

Methods

The current finite element study focused on the prediction of the pedicle screw pullout strength under various conditions such as insertion type, insertion depth, bone quality, and loading mode.

Results

As depicted in this study, the preservation of the pedicle cortex as in the N1 insertion technique greatly enhances the pullout resistance. In addition, the higher the screw-anchoring depth, permitting to gear a maximum number of threads, the better the protection against premature breakouts of pedicle screws.

Conclusions

In agreement with experimental data, the type of insertion in which the first screw thread is placed immediately after the preserved pedicle cortex showed the best pullout resistance for both normal and osteoporotic bone.

Evaluating Pedicle-Screw Instrumentation Using Decision-Tree Analysis Based on Pullout Strength

Study Design

A biomechanical study of pedicle-screw pullout strength.

Purpose

To develop a decision tree based on pullout strength for evaluating pedicle-screw instrumentation.

Overview of Literature

Clinically, a surgeon’s understanding of the holding power of a pedicle screw is based on perioperative intuition (which is like insertion torque) while inserting the screw. This is a subjective feeling that depends on the skill and experience of the surgeon. With the advent of robotic surgery, there is an urgent need for the creation of a patient-specific surgical planning system. A learning-based predictive model is needed to understand the sensitivity of pedicle-screw holding power to various factors.

Methods

Pullout studies were carried out on rigid polyurethane foam, representing extremely osteoporotic to normal bone for different insertion depths and angles of a pedicle screw. The results of these experimental studies were used to build a pullout-strength predictor and a decision tree using a machine-learning approach.

Results

Based on analysis of variance, it was found that all the factors under study had a significant effect (p <0.05) on the holding power of a pedicle screw. Of the various machine-learning techniques, the random forest regression model performed well in predicting the pullout strength and in creating a decision tree. Performance was evaluated, and a correlation coefficient of 0.99 was obtained between the observed and predicted values. The mean and standard deviation of the normalized predicted pullout strength for the confirmation experiment using the current model was 1.01±0.04.

Conclusions

The random forest regression model was used to build a pullout-strength predictor and decision tree. The model was able to predict the holding power of a pedicle screw for any combination of density, insertion depth, and insertion angle for the chosen range. The decision-tree model can be applied in patient-specific surgical planning and a decision-support system for spine-fusion surgery.

Contribution to FE modeling for intraoperative pedicle screw strength prediction

Although the use of pedicle screws is considered safe, mechanical issues still often occur. Commonly reported issues are screw loosening, screw bending and screw fracture. The aim of this study was to develop a Finite Element (FE) model for the study of pedicle screw biomechanics and for the prediction of the intraoperative pullout strength. The model includes both a parameterized screw model and a patient-specific vertebra model. Pullout experiments were performed on 30 human cadaveric vertebrae from ten donors. The experimental force-displacement data served to evaluate the FE model performance. μCT images were taken before and after screw insertion, allowing the creation of an accurate 3D-model and a precise representation of the mechanical properties of the bone. The experimental results revealed a significant positive correlation between bone mineral density (BMD) and pullout strength (Spearman ρ = 0.59, p < 0.001) as well as between BMD and pullout stiffness (Spearman ρ = 0.59, p < 0.001). A high positive correlation was also found between the pullout strength and stiffness (Spearman ρ = 0.84, p < 0.0001). The FE model was able to reproduce the linear part of the experimental force-displacement curve. Moreover, a high positive correlation was found between numerical and experimental pullout stiffness (Pearson ρ = 0.96, p < 0.005) and strength (Pearson ρ = 0.90, p < 0.05). Once fully validated, this model opens the way for a detailed study of pedicle screw biomechanics and for future adjustments of the screw design.

Minimizing Pedicle Screw Pullout Risks: A Detailed Biomechanical Analysis of Screw Design and Placement

STUDY DESIGN

Detailed biomechanical analysis of the anchorage performance provided by different pedicle screw designs and placement strategies under pullout loading.

OBJECTIVE

To biomechanically characterize the specific effects of surgeon-specific pedicle screw design parameters on anchorage performance using a finite element model.

SUMMARY OF BACKGROUND DATA

Pedicle screw fixation is commonly used in the treatment of spinal pathologies. However, there is little consensus on the selection of an optimal screw type, size, and insertion trajectory depending on vertebra dimension and shape.

METHODS

Different screw diameters and lengths, threads, and insertion trajectories were computationally tested using a design of experiment approach. A detailed finite element model of an L3 vertebra was created including elastoplastic bone properties and contact interactions with the screws. Loads and boundary conditions were applied to the screws to simulate axial pullout tests. Force-displacement responses and internal stresses were analyzed to determine the specific effects of each parameter.

RESULTS

The design of experiment analysis revealed significant effects (P<0.01) for all tested principal parameters along with the interactions between diameter and trajectory. Screw diameter had the greatest impact on anchorage performance. The best insertion trajectory to resist pullout involved placing the screw threads closer to the pedicle walls using the straightforward insertion technique, which showed the importance of the cortical layer grip. The simulated cylindrical single-lead thread screws presented better biomechanical anchorage than the conical dual-lead thread screws in axial loading conditions.

CONCLUSIONS

The model made it possible to quantitatively measure the effects of both screw design characteristics and surgical choices, enabling to recommend strategies to improve single pedicle screw performance under axial loading.

Oblique Terminal Screw Placement: Does It Improve Fixation in Plate Osteosynthesis?

The technique of placing an oblique screw in the terminal hole of a plate to increase screw pullout strength is widely taught in the operating room. The origin of this technique is unclear; however, it may have been used simply as a means to identify radiographs and misinterpreted to have some biomechanical benefit. The objective of this study was to measure the structural effect of oblique terminal screw placement (OTSP) during plate osteosynthesis. Foam blocks and limited contact dynamic compression plates and screws were used along with a custom fixture device. The terminal screw was placed in either an oblique (30-degree angle outward) or perpendicular fashion. A load was applied perpendicular to the plate in cantilever bending until failure. The oblique screw construct was significantly weaker than the perpendicular screw construct (399N vs. 465N, P < 0.001), independent of the block of material used. Post hoc analysis showed that the screw angle (P < 0.001) was a significant determinant of the load required to cause screw pullout. OTSP led to a decrease in pullout strength compared with a perpendicular screw in a deformable foam medium similar in density to osteoporotic bone. In patients with poor bone quality, OTSP may create a suboptimal fracture fixation construct.

Stability and Spine Pedicle Screws Fixation Strength-A Comparative Study of Bone Density and Insertion Angle

STUDY DESIGN

Analysis of insertion angle and bone density on the pedicle screw fixation strength with a novel testing protocol that accounts for the articular processes.

OBJECTIVE

To analyze the relationship between pedicle screw fixation strength and bone mineral density for different transverse screw insertion angles.

SUMMARY OF BACKGROUND DATA

The stability of the screw can become compromised by demineralization of the vertebral bone due to diseases such as osteoporosis. A weakening of the bone-screw interface, and therefore, a decrease in the fixation strength of the screw, leads to an increased probability of instrument failure, most commonly by screw loosening or screw pullout.

METHODS

Using the ASTM F543 as reference, we performed pullout tests with an Instron mechanical testing machine of a posterior fixation construct mimicking two pedicle screws connected at a distance of 40 mm as suggested by the ASTM F1717 on four densities of polyurethane foam in accordance with the ASTM F1839-08 standard to simulate bone densities ranging from osteoporotic (5 pcf) to higher than normal (20 pcf) in four transverse insertion angles.

RESULTS

A linear regression with two independent variables was found to be Y = -354.8812 + 91.8102 × X₁ - 6.8747 × X₂ (X₁ = density [pcf], X₂ = angle [degrees]), with a correlation coefficient of 0.95 for all the experimental data.

CONCLUSIONS

Pedicle screw insertion angle and bone density are critical to pullout strength. However, in osteoporotic bone, the insertion angle has only a marginal influence on pullout strength.

LEVEL OF EVIDENCE

V.

A new alternative to expandable pedicle screws: Expandable poly-ether-ether-ketone shell

Screw pullout is a very common problem in the fixation of sacrum with pedicle screws. The principal cause of this problem is that the cyclic micro motions in the fixation of sacrum are higher than the other regions of the vertebrae that limit the osteo-integration between bone and screw. In addition to that, the bone quality is very poor at sacrum region. This study investigated a possible solution to the pullout problem without the expandable screws' handicaps. Newly designed poly-ether-ether-ketone expandable shell and classical pedicle screws were biomechanically compared. Torsion test, pullout tests, fatigue tests, flexion/extension moment test, axial gripping capacity tests and torsional gripping capacity tests were conducted in accordance with ASTM F543, F1798 and F1717. Standard polyurethane foam and calf vertebrae were used as embedding medium for pullout tests. Classical pedicle screw pullout load on polyurethane foam was 564.8 N compared to the failure load for calf vertebrae's 1264 N. Under the same test conditions, expandable poly-ether-ether-ketone shell system's pullout loads from polyurethane foam and calf vertebrae were 1196.3 and 1890 N, respectively. The pullout values for expandable poly-ether-ether-ketone shell were 33% and 53% higher than classical pedicle screw on polyurethane foam and calf vertebrae, respectively. The expandable poly-ether-ether-ketone shell exhibited endurance on its 90% of yield load. Contrary to poly-ether-ether-ketone shell, classical pedicle screw exhibited endurance on 70% of its yield load. Expandable poly-ether-ether-ketone shell exhibited much higher pullout performance than classical pedicle screw. Fatigue performance of expandable poly-ether-ether-ketone shell is also higher than classical pedicle screw due to damping the micro motion capacity of the poly-ether-ether-ketone. Expandable poly-ether-ether-ketone shell is a safe alternative to all other expandable pedicle screw systems on mechanical perspective.

Effect of bone material properties on effective region in screw-bone model: an experimental and finite element study

Background

There have been numerous studies conducted to investigate the pullout force of pedicle screws in bone with different material properties. However, fewer studies have investigated the region of effect (RoE), stress distribution and contour pattern of the cancellous bone surrounding the pedicle screw.

Methods

Screw pullout experiments were performed from two different foams and the corresponding reaction force was documented for the validation of a computational pedicle screw-foam model based on finite element (FE) methods. After validation, pullout simulations were performed on screw-bone models, with different bone material properties to model three different age groups (<50, 50–75 and >75 years old). At maximum pullout force, the stress distribution and average magnitude of Von Mises stress were documented in the cancellous bone along the distance beyond the outer perimeter pedicle screw. The radius and volume of the RoE were predicted based on the stress distribution.

Results

The screw pullout strengths and the load–displacement curves were comparable between the numerical simulation and experimental tests. The stress distribution of the simulated screw-bone vertebral unit showed that the radius and volume of the RoE varied with the bone material properties. The radii were 4.73 mm, 5.06 mm and 5.4 mm for bone properties of ages >75, 75 > ages >50 and ages <50 years old, respectively, and the corresponding volumes of the RoE were 6.67 mm³, 7.35 mm³ and 8.07 mm³, respectively.

Conclusions

This study demonstrated that there existed a circular effective region surrounding the pedicle screw for stabilization and that this region was sensitive to the bone material characteristics of cancellous bone. The proper amount of injection cement for augmentation could be estimated based on the RoE in the treatment of osteoporosis patients to avoid leakage in spine surgery.

Comparison study of the pullout strength of conventional spinal pedicle screws and a novel design in full and backed-out insertions using mechanical tests

Recently, new pedicle screw designs have been developed. However, these designs’ performances are still unclear, especially when backed out after insertion. The objective of this study was to investigate the performances of different screw designs when backed out from full insertion. Seven conventional designs of the pedicle screw and one novel design were inserted into polyurethane foam (0.32 g/cm3). All screws were first fully inserted (43 mm) and were backed out 360°. Axial pullout tests were performed and the reaction force was measured. The results showed that the conical screw of type 1 with a small inner diameter provided the highest pullout strength in both full insertion and backed-out insertion (2401.85 and 2169.82 N, respectively). However, this screw’s pullout strength significantly decreased (9.7%) when backed out from full insertion. There was no significant difference between the conical screw of type 1 with a small inner diameter and double duo core screw ( p > 0.01) in backed-out insertion. The cylindrical screw with a small diameter, dual inner core screw and double dual core screw also provided good results in both full insertion (2115.44, 2182.99 and 2226.93 N, respectively) and backed-out conditions (2065.80, 2014.28 and 1941.29 N, respectively). The increased pullout strength of the conical design could be due to the effect of bone compaction. However, the screw exhibited less consistent pullout strength when backed out when compared with the other designs. The conical screw should be inserted to the precise position without turning back, especially in osteoporosis patients. The dual inner core screw and double dual core screw could provide greater stability in both conditions. Care should be taken when using both the cylindrical screw with a small thread depth and the dual outer core screw.

Effect of pilot hole on biomechanical and in vivo pedicle screw-bone interface

PURPOSE

To experimentally study the influence of pilot hole diameter (smaller than or equal to the internal (core) diameter of the screw) on biomechanical (insertion torque and pullout strength) and histomorphometric parameters of screw-bone interface in the acute phase and 8 weeks after pedicle screw insertion.

METHODS

Fifteen sheep were operated upon and pedicle screws inserted in the L1-L3 pedicles bilaterally. The pilot hole was smaller (2.0 mm) than the internal diameter (core) of the screw on the left side pedicle and equal (2.8 mm) to the internal diameter (core) of the screw on the right side pedicle. Ten animals were sacrificed immediately (five animals were assigned to pullout strength tests and five animals were used for histomorphometric bone-screw interface evaluation). Five animals were sacrificed 8 weeks after pedicle screw insertion for histomorphometric bone-screw interface evaluation.

RESULTS

The insertion torque and pullout strength were significantly greater in pedicle screws inserted into pilot holes smaller than internal (core) diameter of the screw. Histomorphometric evaluation of bone-screw interface showed that the percentage of bone-implant contact, the area of bone inside the screw thread and the area of bone outside the screw thread were significantly higher for pilot holes smaller than the internal (core) diameter of the screw immediately after insertion and after 8 weeks.

CONCLUSION

A pilot diameter smaller than the internal (core) diameter of the screw improved the insertion torque and pullout strength immediately after screw insertion as well the pedicle screw-bone interface contact immediately and 8 weeks after screw placement in sheep with good bone mineral density.

Evaluation of a new approach for modelling the screw–bone interface in a locking plate fixation: A corroboration study

Computational modelling of the screw–bone interface in fracture fixation constructs is challenging. While incorporating screw threads would be a more realistic representation of the physics, this approach can be computationally expensive. Several studies have instead suppressed the threads and modelled the screw shaft with fixed conditions assumed at the screw–bone interface. This study assessed the sensitivity of the computational results to modelling approaches at the screw–bone interface. A new approach for modelling this interface was proposed, and it was tested on two locking screw designs in a diaphyseal bridge plating configuration. Computational models of locked plating and far cortical locking constructs were generated and compared to in vitro models described in prior literature to corroborate the outcomes. The new approach led to closer agreement between the computational and the experimental stiffness data, while the fixed approach led to overestimation of the stiffness predictions. Using the new approach, the pattern of load distribution and the magnitude of the axial forces, experienced by each screw, were compared between the locked plating and far cortical locking constructs. The computational models suggested that under more severe loading conditions, far cortical locking screws might be under higher risk of screw pull-out than the locking screws. The proposed approach for modelling the screw–bone interface can be applied to any fixation involved application of screws.

Time-lapsed imaging of implant fixation failure in human femoral heads

The failure mechanisms of bone-implant constructs are still incompletely understood, because the role of the peri-implant bone in implant stability is unclear. We hypothesized that implant fixation failure is preceded by substantial peri-implant bone failure. A new device was developed that combines mechanical testing of large bone-implant constructs with high-resolution peripheral quantitative computed tomography, following the principles of image-guided failure assessment (IGFA). In this study, we investigated the push-in failure behavior of dynamic hip screws (DHS) implanted in human cadaveric femoral heads. For the first time the fixation failure of a clinically used implant in human trabecular bone could be experimentally visualized at the microstructural level. The ultimate force was highly correlated with the peri-implant bone volume fraction (R(2)=0.85). We demonstrated that primary fixation failure of DHS implants was accompanied by trabecular bone failure in the immediate peri-implant bone region only. Such experimental data are crucial to enhance the understanding on the quality of the bone-implant interface and of the trabecular bone in the process of implant fixation failure. We believe that this newly developed device will be beneficial for the development of new implant designs, especially for use in osteoporotic bone.

Assessment of pedicle screw pullout strength based on various screw designs and bone densities-an ex vivo biomechanical study

BACKGROUND CONTEXT

The pedicle screw fixation system has been used for various spinal disorders. Many studies have been conducted on the fixation ability of the pedicle screw, but variable results have been reported based on bone qualities, pedicle screw properties, insertion techniques, and experimental methods.

STUDY DESIGN

An experimental and biomechanical study.

PURPOSE

To evaluate the geometric factors of screws affecting fixation ability after assessing pullout strength based on various pedicle screw designs and different bone densities.

METHODS

Nine types of pedicle screws were prepared according to the outer diameter shape (cylindrical or conical), the inner diameter shape (cylindrical or conical), and thread shape (V shape, buttress shape, and square shape). The pedicle screws were inserted into standardized polyurethane foams of Grades 5, 15, and 20. The pullout strength of each pedicle screw was determined using an MTS 858 machine (Material Testing System Corp., Minneapolis, MN, USA), and the results were analyzed statistically.

RESULTS

Pullout strength based on the shape of thread was the highest in the V shape and lowest in the square shape for all foam grades (p<.05). The outer cylindrical and inner conical shape of pedicle screw showed the highest pullout strength in Grades 5 and 15 foam (p<.05). An outer cylindrical and inner conical shape with a V-shaped thread showed the highest pullout strength in all foam grades (p<.05).

CONCLUSIONS

Pedicle screw with an outer cylindrical and inner conical configuration with a V-shaped thread may have maximum pullout strength, regardless of bone density.