Screw Loads Research Articles

Low-speed instrumented drill press for bone screw insertion

Screw insertion torque is a widely used/effective method for quantifying fixation strength in orthopedic implant research for different screw geometries, implantation sites, and loads. This work reports the construction of an open-source instrumented benchtop screw insertion device for a total cost of $7545 ($492 + $7053 for equipped sensors), as well as validation of the device and an example use-application. The insertion device is capable of recording the axial load, rotational speed, and applied torque throughout the screw insertion process at 10 samples per second, as demonstrated in the validation test. For this combination of bone analog (20 PCF Sawbones©), screw, and loading, the resolution of the torque sensor was 25% of the maximum measured torque; a different model torque sensor would be required to meet ASTM F543-17, which specifies a resolution of 10% of the maximum torque. This system is optimized for fastener insertion at speeds of 120 rpm or less and axial loading up to 50 N.

Can semirigid fixation of the rostral instrumented segments prevent proximal junctional kyphosis in the case of long thoracolumbar fusions? A finite element study.

Proximal junctional kyphosis (PJK) is a relatively common complication following long instrumented posterior spinal fusion. Although several risk factors have been identified in the literature, previous biomechanical studies suggest that one of the leading causes is the sudden change in mobility between the instrumented and noninstrumented segments. The current study aims to assess the biomechanical effect of 1 rigid and 2 semirigid fixation techniques (SFTs) on developing PJK. Four T7-L5 finite element (FE) models were developed: 1) intact spine; 2) 5.5-mm titanium rod from T8 to L5 (titanium rod fixation [TRF]); 3) multiple rods from T8 to T9 connected with titanium rod from T9 to L5 (multiple-rod fixation [MRF]); and 4) polyetheretherketone rod from T8 to T9 connected with titanium rod from T9 to L5 (PEEK rod fixation [PRF]). A modified multidirectional hybrid test protocol was used. First, a pure bending moment of 5 Nm was applied to measure the intervertebral rotation angles. Second, the TRF technique's displacement from the first loading step was applied to the instrumented FE models to compare the pedicle screw stress values in the upper instrumented vertebra (UIV). In the load-controlled step, at the upper instrumented segment, the intervertebral rotation values relative to TRF increased by 46.8% and 99.2% for flexion, by 43.2% and 87.7% for extension, by 90.1% and 137% for lateral bending, and by 407.1% and 585.2% for axial rotation, in the case of MRF and PRF, respectively. In the displacement-controlled step, maximum pedicle screw stress values at the UIV level were highest in the case of TRF (37.26 MPa, 42.13 MPa, 44.4 MPa, and 44.59 MPa for flexion, extension, lateral bending, and axial rotation, respectively). Compared to TRF, in the case of MRF and PRF, the screw stress values were reduced by 17.3% and 27.7% for flexion, by 26.6% and 36.7% for extension, by 6.8% and 34.3% for lateral bending, and by 49.1% and 59.8% for axial rotation, respectively. FE analysis has shown that the SFTs increase the mobility at the upper instrumented segment and therefore provide a more gradual transition in motion between the instrumented and rostral noninstrumented segments of the spine. In addition, SFTs decrease the screw loads at the UIV level and hence could help reduce the risk for PJK. However, further investigations are recommended to evaluate the long-term clinical usefulness of these techniques.

Articular contact vs. embedding: Effect of simplified boundary conditions on the stress distribution in the distal radius and volar plate implant loading

Boundary conditions (BCs) are often simplified in experimental and numerical models simulating distal radius fractures and their treatments. The aim of this study was to investigate the effects of simplified BCs at the radiocarpal joint: (1) on the stress distribution in the intact distal radius, and (2) on the loading of a volar locking plate (VLP) used for distal radius fracture treatment. Finite element models of the distal radius with contact between carpals and cartilage were created as reference models for an intact bone and a fractured bone with VLP treatment. Four models with simplified BCs were compared to these reference models: One with embedding material instead of carpals, one with carpals tied to the radius; each loaded either uniaxially or with statically equivalent loading to the reference model. Differences in distal bone stress distributions and mechanical parameters of the VLP (fracture gap movement, plate peak stresses, distal screw loads) were generally largest for the uniaxially loaded, embedded model (up to 250 % in individual screw loads) and smallest for the model with tied carpals and statically equivalent loads (<25 % for all parameters). Differences were greatly reduced if statically equivalent loads were applied, but subchondral stress peaks were absent without carpals. In conclusion, implementing realistic resultant forces and moments is more important than the exact articular load distribution, but carpal bones should be included if subchondral bone stresses are analyzed. In this case, a tie constraint may replace articular contact modelling with acceptable accuracy if statically equivalent loading is applied.

Calculation and analysis on braided carbon/carbon composite pull-through fastener threads load distribution

Braided carbon/carbon composites are widely used in the aerospace field due to their superior performance. However, because braided carbon/carbon composite materials have anisotropy characteristics, the distribution of screw loads in the fastener connection structure will differ from that of metal. In view of the particularity of the braided carbon/carbon composite fasteners’ connection, this paper obtains the theoretical results of the load distribution of screw teeth through the homogenization of stiffness and establishes a finite element model for comparative verification. The trend of finite element results is the same as that of theoretical calculation results which can provide guidance for the engineering design of carbon/carbon composite fastener.

Optimal tether configurations and preload tensioning to prevent proximal junctional kyphosis: a finite element analysis.

Proximal junctional kyphosis (PJK) is, in part, due to altered segmental biomechanics at the junction of rigid instrumented spine and relatively hypermobile non-instrumented adjacent segments. Proper application of posteriorly anchored polyethylene tethers (i.e., optimal configuration and tension) may mitigate adjacent-segment stress and help prevent PJK. The purpose of this study was to investigate the impact of different tether configurations and tensioning (preloading) on junctional range-of-motion (ROM) and other biomechanical indices for PJK in long instrumented spine constructs. Using a validated finite element model of a T7-L5 spine segment, testing was performed on intact spine, a multilevel posterior screw-rod construct (PS construct; T11-L5) without tether, and 15 PS constructs with different tether configurations that varied according to 1) proximal tether fixation of upper instrumented vertebra +1 (UIV+1) and/or UIV+2; 2) distal tether fixation to UIV, to UIV-1, or to rods; and 3) use of a loop (single proximal fixation) or weave (UIV and/or UIV+1 fixation in addition to UIV+1 and/or UIV+2 proximal attachment) of the tether. Segmental ROM, intradiscal pressure (IDP), inter- and supraspinous ligament (ISL/SSL) forces, and screw loads were assessed under variable tether preload. PS construct junctional ROM increased abruptly from 10% (T11-12) to 99% (T10-11) of baseline. After tethers were grouped by most cranial proximal fixation (UIV+1 vs UIV+2) and use of loop versus weave, UIV+2 Loop and/or Weave most effectively dampened junctional ROM and adjacent-segment stress. Different distal fixation and use of loop versus weave had minimal effect. The mean segmental ROM at T11-12, T10-11, and T9-10, respectively, was 6%, 40%, and 99% for UIV+1 Loop; 6%, 44%, and 99% for UIV+1 Weave; 5%, 23%, and 26% for UIV+2 Loop; and 5%, 24%, and 31% for UIV+2 Weave.Tethers shared loads with posterior ligaments; consequently, increasing tether preload tension reduced ISL/SSL forces, but screw loads increased. Further attenuation of junctional ROM and IDP reversed above approximately 100 N tether preload, suggesting diminished benefit for biomechanical PJK prophylaxis at higher preload tensioning. In this study, finite element analysis demonstrated UIV+2 Loop and/or Weave tether configurations most effectively mitigated adjacent-segment stress in long instrumented spine constructs. Tether preload dampened ligament forces at the expense of screw loads, and an inflection point (approximately 100 N) was demonstrated above which junctional ROM and IDP worsened (i.e., avoid over-tightening tethers). Results suggest tether configuration and tension influence PJK biomechanics and further clinical research is warranted.

Limiting interpedicular screw displacement increases shear forces in screws: A finite element study

Background contextScrew loosening has been reported for non-fusion devices. Forces on pedicle screws could be related to kinematic parameters as the interpedicular displacement (ID), which consists of the displacement between superior and inferior screw heads from full extension to full flexion. PurposeTo investigate the relationship between ID and screw loosening for different designs of posterior implants using a finite element model. MethodsAn L3-sacrum previously validated spine FE model was used. Three-rod designs were considered in L4-L5 segment: a rigid screw-rod implant, a flexible one and a specific design with a sliding rod providing limited restrain in ID. In order to simulate intermediate configurations, the friction coefficient between the sliding rods and connectors were varied. The sacrum was rigidly fixed. Rotations (flexion-extension, lateral bending and axial rotation) were applied to L3, for each modeled configuration: intact, injured, injured with different implants. Model consistency was checked with existing experimental in vitro data on intact and instrumented segments. Screw loads were computed as well as ID. ResultsIn flexion-extension, the ID was less than 2mm for rigid (R) and flexible (F) constructs and 5.5mm for intact spine and the sliding implant (S3). Screw's shear forces were 272N, 153N, 43N respectively for R, F and S3 constructs. ConclusionsImplants that allow ID presented lower screws loads. A compromise between the ability of the implant to withstand compressive forces, which requires longitudinal stiffness, and its ability to allow ID could be important for future implant designs in order to prevent screw loosening.

The effect of posterior polyester tethers on the biomechanics of proximal junctional kyphosis: a finite element analysis.

OBJECTIVE Proximal junctional kyphosis (PJK) remains problematic following multilevel instrumented spine surgery. Previous biomechanical studies indicate that providing less rigid fixation at the cranial aspect of a long posterior instrumented construct, via transition rods or hooks at the upper instrumented vertebra (UIV), may provide a gradual transition to normal motion and prevent PJK. The purpose of this study was to evaluate the ability of posterior anchored polyethylene tethers to distribute proximal motion segment stiffness in long instrumented spine constructs. METHODS A finite element model of a T7-L5 spine segment was created to evaluate range of motion (ROM), intradiscal pressure, pedicle screw loads, and forces in the posterior ligament complex within and adjacent to the proximal terminus of an instrumented spine construct. Six models were tested: 1) intact spine; 2) bilateral, segmental pedicle screws (PS) at all levels from T-11 through L-5; 3) bilateral pedicle screws from T-12 to L-5 and transverse process hooks (TPH) at T-11 (the UIV); 4) pedicle screws from T-11 to L5 and 1-level tethers from T-10 to T-11 (TE-UIV+1); 5) pedicle screws from T-11 to L-5 and 2-level tethers from T-9 to T-11 (TE-UIV+2); and 6) pedicle screws and 3-level tethers from T-8 to T-11 (TE-UIV+3). RESULTS Proximal-segment range of motion (ROM) for the PS construct increased from 16% at UIV-1 to 91% at UIV. Proximal-segment ROM for the TPH construct increased from 27% at UIV-1 to 92% at UIV. Posterior tether constructs distributed ROM at the UIV and cranial adjacent segments most effectively; ROM for TE-UIV+1 was 14% of the intact model at UIV-1, 76% at UIV, and 98% at UIV+1. ROM for TE-UIV+2 was 10% at UIV-1, 51% at UIV, 69% at UIV+1, and 97% at UIV+2. ROM for TE-UIV+3 was 7% at UIV-1, 33% at UIV, 45% at UIV+1, and 64% at UIV+2. Proximal segment intradiscal pressures, pedicle screw loads, and ligament forces in the posterior ligament complex were progressively reduced with increasing number of posterior tethers used. CONCLUSIONS Finite element analysis of long instrumented spine constructs demonstrated that posterior tethers created a more gradual transition in ROM and adjacent-segment stress from the instrumented to the noninstrumented spine compared with all PS and TPH constructs. Posterior tethers may limit the biomechanical risk factor for PJK; however, further clinical research is needed to evaluate clinical efficacy.

The influence of bone density and anisotropy in finite element models of distal radius fracture osteosynthesis: Evaluations and comparison to experiments

Continuum-level finite element (FE) models can be used to analyze and improve osteosynthesis procedures for distal radius fractures (DRF) from a biomechanical point of view. However, previous models oversimplified the bone material and lacked thorough experimental validation. The goal of this study was to assess the influence of local bone density and anisotropy in FE models of DRF osteosynthesis for predictions of axial stiffness, implant plate stresses, and screw loads. Experiments and FE analysis were conducted in 25 fresh frozen cadaveric radii with DRFs treated by volar locking plate osteosynthesis. Specimen specific geometries were captured using clinical quantitative CT (QCT) scans of the prepared samples. Local bone material properties were computed based on high resolution CT (HR-pQCT) scans of the intact radii. The axial stiffness and individual screw loads were evaluated in FE models, with (1) orthotropic inhomogeneous (OrthoInhom), (2) isotropic inhomogeneous (IsoInhom), and (3) isotropic homogeneous (IsoHom) bone material and compared to the experimental axial stiffness and screw–plate interface failures. FE simulated and experimental axial stiffness correlated significantly (p<0.0001) for all three model types. The coefficient of determination was similar for OrthoInhom (R2=0.807) and IsoInhom (R2=0.816) models but considerably lower for IsoHom models (R2=0.500). The peak screw loads were in qualitative agreement with experimental screw–plate interface failure. Individual loads and implant plate stresses of IsoHom models differed significantly (p<0.05) from OrthoInhom and IsoInhom models. In conclusion, including local bone density in FE models of DRF osteosynthesis is essential whereas local bone anisotropy hardly effects the models׳ predictive abilities.

Biomechanical analysis of iliac screw fixation in spinal deformity instrumentation

BackgroundHigh rates of iliac screw fixation failures have been reported in spinopelvic instrumentations. The objective was to assess the iliac screw loads as functions of instrumentation variables. MethodsSpinopelvic instrumentations of six neuromuscular scoliosis were simulated using patient-specific modeling techniques to evaluate the intra- and postoperative iliac screw loads as functions of instrumentation variables: the combined use of sacral screws, the uses of lateral offset connectors and cross-rod connectors, and the iliac screw insertion point and trajectory. FindingsSacral screws, lateral connectors and the iliac screw insertion point had significant effects on iliac screw axial forces (69–297N) and toggle moments (0.8–2.9Nm) (p<0.05). The addition of sacral screws made the iliac screw forces lower for some functional loads but higher for other functional loads, and resulted in an increase of intraoperative screw forces when attaching the rods onto these additional screws. When lateral offset connectors were used, the toggle moments were 16% and 25% higher, respectively for the left and right sides. Inserting iliac through the sacrum resulted in 17% lower toggle moment compared to insertion through the iliac crest. Cross-rod connectors had no significant effect on the intraoperative iliac screw forces. Postoperative functional loading had an important effect (additional 34% screw axial force and 18% toggle moment). InterpretationIt is possible to reduce the iliac screw loads by adapting instrumentation variables and strategies. Reducing the loads could decrease the risk of failure associated with iliac screw fixations.

The Effects of Locked and Unlocked Neutralization Plates on Load Bearing of Fractures Fixed With a Lag Screw

Because locked plates as a neutralization device for lag screw fracture fixation have different biomechanics than that of unlocked plates, we investigated how this would affect lag screw load at the fracture site. The purpose of this study was to assess the load at a fracture site compressed with a lag screw when both locked and unlocked plates are used as neutralization devices. Nine cadaver femurs had a mid-shaft oblique fracture created and were fixed with a lag screw, incorporating load transducers at the fracture site and lag screw. Three neutralization plate constructs (a standard plate, a locked plate, and an offset locked plate) were sequentially applied and loaded. Loads at the fracture site and the lag screw were measured after sequential application of axial loads. Plate application to the lag screw fixations did not significantly change (P > 0.26) the load at the fracture site or on the lag screw that were approximately 200 N. The unlocked, locked, and offset locked plates behaved similarly. The addition of a load to the specimens did not change the lag screw loads but increased the average fracture loads by approximately 20% of the applied load. Unlocked and locked neutralization plates do not affect the initial compressive load across a fracture fixed by a lag screw, and both behave similarly in transferring the load when the fracture was loaded. For a well-fixed stable fracture fixed with a lag screw, there is no advantage in using a more expensive locked plate over a standard plate for neutralization purposes if adequate screw purchase can be achieved.

Biomechanical Evaluation of Parasagittal Occipital Plating

Biomechanical evaluation of occipitocervical instrumentation techniques. Compare methods of occipital instrumentation by quantifying load sharing of occipital screws and measuring motion across instrumented occipitocervical spines. Newer occipitocervical plate/screw systems that attach to longitudinal rods have been developed to improve fixation. These devices place screws in the center of occipital bone or off-midline. Midline plates offer screw purchase in thicker bone. Off-midline systems may increase the effective moment arm for torsional and lateral bending control. Measurement of screw loads within occipital plates is useful for determining optimal plate configuration. Ten cadaveric specimens (occiput-C4) were tested in flexion/extension (FE), lateral bending (LAT), and axial rotation (ROT) over +/-3 Nm pure moment. After intact testing, 4 occipitocervical fixation constructs were tested using washer load cells to assess loading across screws used to fix the plates to the occiput. Parasagittal occipital plates were positioned either convex or concave side facing medially. Each plate was first fixed using 3 screws (rostral, middle, caudal), then with the caudal screw eliminated (simulated failure). Range of motion (ROM) and peak screw loads are reported. ROM decreased from intact to any of the 4 fusion plate configurations in FE, LAT, and ROT (P << 0.05), but not between plate configurations. Screw load significantly decreased from medially convex to medially concave configurations in LAT, but no significant changes were observed in FE or ROT. With caudal screws removed, middle screws peak loads significantly increased in FE and LAT (P < 0.05), but not ROT. Occipital screw placement off-midline improves screw loads under lateral bending forces on occipitocervical constructs, though loads for FE and ROT are unchanged. As screws pullout, the loads may be redistributed, resulting in increased screw pullout forces above. Despite the improvement in screw loads for laterally based plates during lateral bending, overall ROM across the occipitocervical junction is unchanged.

The effect of pedicle morphometry on pedicle screw loading. A synthetic model.

Static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw bending load was studied as a function of pedicle morphometry. To determine how sagittal bending moment in pedicle screws is affected by changes in pedicle height, length, and width. An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for axially unstable fractures. The majority of screws fail in sagittal bending within the pedicle. To date, little is known of the exogenous factors that affect in situ loads incurred by pedicle screws. Synthetic vertebral analogues were fabricated, varying pedicle height, length, or width independently. Pedicle screws internally instrumented with strain gages were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture. Screw bending moments within the pedicle increased incrementally with increasing pedicle length, rising 30% as length increased from 8.0 mm to 12.0 mm. Screw moment increased 20% when pedicle height dropped below 15.0 mm, consistent with a threshold effect. Changes in pedicle width did not affect screw loads within the pedicle. In situ pedicle screw loads increased significantly as pedicle length increased and as pedicle height decreased. Pedicle screws instrumented internally with strain gages are an effective research instrument allowing measurement of in situ loading along the axis of the screw.

Improvements in Bio-Mechanical Adhesion of Screws Used in Medical Field: First Application in Spinal Surgery

Metallic screws are becoming more and more useful to join bones or for prosthesis support in orthopaedic and dental surgery. High biocompatible materials such as titanium alloys and hydroxyapatite ceramics are making possible the realization of stable fixation devices utilizable in load-bearing applications. The mechanical and biological anchorage of metallic screws to the bone depends on many factors: mechanical screws-bone thread matching, use of cements between bone and screw, chemical-physical treatments of screw surface, use of screw coverage films based on osteointegrating ceramics or active bioglasses, use of porous coverage films to induce bone ingrowth into the pore, and so on. The first step of the research, the aim of the present paper, is the comparison among screws of different shapes and geometric characteristics in order to find the best macromechanical system versus the different load conditions. Static and dynamic tests are applied to the screws mounted on segments of pig spine, in order to measure the mechanical characteristics of the system under tension (pull-out), bending on the principal planes along the axis of the screw and fatigue loads. Physical analysis, obtained using energetic charged particles, such as SEM, RBS, and AES, are used to characterize the screw surface compositions and morphology.

Sacral Screw Loads in Lumbosacral Fixation for Spinal Deformity

Fixation to the sacrum and pelvis is a problem in the operative treatment of spinal deformity. Previous testing of pedicle screws address axial pull-out strength, yet how screws are loaded in vivo remains unknown. The goals of this study were to determine the loads experienced by sacral screws when loaded as part of Cotrel-Dubousset (CD) sacral instrumentation and whether different anterior grafting methods would effect screw loads. Sacral screws were modified to become transducers capable of measuring axial and bending loads. The screw transducers were incorporated into the sacral fixation of CD instrumentation in seven calf spines. Specimens were loaded to simulate flexion. The sacral screws carried axial loads (1.1 N/[Nm of load]) and bending moments (1.1 Nm/[Nm of load]). The results suggest bending of the sacral screws may be important in their failure, and screw loading was not dependent on graft types used.

Determination of the power and load parameters of rubber extruders

The power input and screw loads are theoretically determined for the extrusion of materials with anomalous viscosity. The drive power is expended on the translational and circulatory flow of material in the helical channel. The axial load can be treated as the sum of the rubber pressure on the front of the screw and the axial component of the reaction. Circulatory flow results in an axial load component associated with the presence of a transverse pressure gradient.