Standard Screw Research Articles

Biomechanics of posterior instrumentation in L1–L3 lateral interbody fusion: Pedicle screw rod construct vs. transfacet pedicle screws

BackgroundThe use of pedicle screws is the gold standard for supplemental posterior fixation in lateral interbody fusion. Information about the performance of transfacet pedicle screws compared to standard pedicle screws and rods in the upper lumbar spine with or without a lateral interbody fusion device in place is limited. MethodsFifteen fresh frozen human cadaveric lumbar spine segments (T12-L4) were studied using standard pure moment flexibility tests. Specimens were divided into two groups to receive either bilateral transfacet pedicle screws (n=8) or bilateral pedicle screws (n=14). Stability of each motion segment (L1–L2 and L2–L3) was evaluated intact, with posterior instrumentation with an intact disc, with posterior instrumentation and a lateral interbody fusion device in place, and following cyclic loading with the interbody device and posterior instrumentation still in place. Both raw values of motion (range of motion, lax zone and stiff zone) and normalized mobility (ratios to intact) were analyzed for each case. FindingsIn terms of immediate stability, transfacet pedicle screws performed equivalent to similarly sized pedicle screws, both with intact disc and with lateral interbody fusion device in all directions of loading. Stability following cyclic loading decreased significantly during lateral bending and axial rotation. InterpretationPosterior fixation with transfacet pedicle screws provides equivalent immediate stability to similarly sized pedicle screws. However, in the presence of a lateral interbody fusion device, pedicle screws seem to resist loosening more and may be a better option for fusion in the upper lumbar spine.

Is There an Optimal Proximal Locking Screw Length in Retrograde Intramedullary Femoral Nailing? Can We Stop Measuring for These Screws?

Insertion of locking screws through the proximal thigh while locking retrograde femoral nails is arguably more difficult and traumatic to local tissues than locking at other intramedullary nail sites. The purpose of this study was to evaluate whether a "standard" screw length for proximal interlocking of retrograde nails is possible, therefore assessing whether the act of measuring for these screws can be omitted. This article retrospective evaluates screw position and estimated proximal locking screw length in patients undergoing retrograde nailing using a large radiographically measured computed tomography cohort, with validation through a smaller clinical cohort. According to these data, it seems reasonable to skip depth gauge measurement during anteroposterior interlocking of retrograde femoral nails and insert a standard length screw based on location relative to the lesser trochanter. This should decrease the amount of local trauma to the patient at the locking screw site while increasing operating room efficiency by avoiding what can often become a difficult step during the procedure.

PMMA-augmented SI screw: a biomechanical analysis of stiffness and pull-out force in a matched paired human cadaveric model

IntroductionCurrent literature data and clinical experience show that the number of pelvic fractures continuously rises due to the increasing elderly population. In the elderly with suspected osteoporosis additional implant augmentation with bone cement seems to be an option to avoid secondary displacement. There are no reported biomechanical data in the literature comparing the fixation strength (and anchorage) of standard and augmented SI screws so far.The purpose of this study was to assess the biomechanical performance of cement-augmented versus non-augmented SI screws in a human cadaveric pelvis model. Material and MethodsSix human cadaveric pelvises preserved with the method of Thiel were used in this study. Each pelvis was split to a pair of 2 hemi-pelvises, assigned to 2 different groups for instrumentation with one non-augmented or one contralateral cement-augmented SI screw, placed in the body of S1 in a randomized fashion. The osteosynthesis followed a standard procedure with 3D controlled percutaneous iliosacral screw positioning. A biomechanical setup for a quasistatic pullout test of each SI screw was used. Construct stiffness and maximum pullout force were calculated from the load-displacement curve of the machine data. Statistical evaluation was performed at a level of significance p = .05 for all statistical tests. ResultsStiffness and pullout force in the augmented group (501.6 N/mm ± 123.7, 1336.8 N ± 221.1) were significantly higher than in the non-augmented one (289.7 N/mm ± 97.1, 597.7 N ± 115.5), p = .04 and p = .014, respectively. BMD influenced significantly the pullout force in all study groups. ConclusionCement augmentation significantly increased the fixation strength in iliosacral screw osteosynthesis of the sacrum in a biomechanical human cadaveric model.

PROSPECTIVE COMPARISON OF FOUR NEW DF-4 DEFIBRILLATOR LEADS

BackgroundOver the past few years, new DF-4 connectors have been introduced to facilitate lead connection to the device header and to reduce the size of the device. So far, there is limited publication regarding their performance, and no direct comparison exists between the different DF-4 leads.MethodThis is a prospective non-randomized study of all consecutive patients with implantation of a new DF-4 lead system at a single experimented tertiary care university centre between October 2010 and February 2015. The following single-coil DF-4 leads were evaluated: St. Jude Medical Durata 7122Q, Medtronic Sprint Quattro Secure 6935M, Boston Scientific Endotak Reliance 4-Site 0293, and Boston Scientific Reliance 4-Front 0693. This study evaluated the acute and first follow-up DF-4 lead performance as well as early peri-procedural adverse events.ResultsA total of 808 patients (age 63±12 years, 80% male, LVEF 31±12%, 58% ischemic heart disease, 35% biventricular devices) underwent implantation of a new DF-4 lead successfully. The acute and first follow-up R-wave detection and threshold were excellent for all leads in the 792 patients with complete available data (Table). A higher number of early lead revisions were required with the Boston Scientific leads compared to other leads (p=0.023). These were mostly related to lead dislodgement. The last 71 Boston Scientific 0693 leads were implanted using a standard screw activator instead of the new Boston device. There were 2 lead revisions with this new strategy (p=0.129). Rates of pneumothorax and infection requiring extraction were 0.4% and 0.8% respectively.Conclusion BackgroundOver the past few years, new DF-4 connectors have been introduced to facilitate lead connection to the device header and to reduce the size of the device. So far, there is limited publication regarding their performance, and no direct comparison exists between the different DF-4 leads. Over the past few years, new DF-4 connectors have been introduced to facilitate lead connection to the device header and to reduce the size of the device. So far, there is limited publication regarding their performance, and no direct comparison exists between the different DF-4 leads. MethodThis is a prospective non-randomized study of all consecutive patients with implantation of a new DF-4 lead system at a single experimented tertiary care university centre between October 2010 and February 2015. The following single-coil DF-4 leads were evaluated: St. Jude Medical Durata 7122Q, Medtronic Sprint Quattro Secure 6935M, Boston Scientific Endotak Reliance 4-Site 0293, and Boston Scientific Reliance 4-Front 0693. This study evaluated the acute and first follow-up DF-4 lead performance as well as early peri-procedural adverse events. This is a prospective non-randomized study of all consecutive patients with implantation of a new DF-4 lead system at a single experimented tertiary care university centre between October 2010 and February 2015. The following single-coil DF-4 leads were evaluated: St. Jude Medical Durata 7122Q, Medtronic Sprint Quattro Secure 6935M, Boston Scientific Endotak Reliance 4-Site 0293, and Boston Scientific Reliance 4-Front 0693. This study evaluated the acute and first follow-up DF-4 lead performance as well as early peri-procedural adverse events. ResultsA total of 808 patients (age 63±12 years, 80% male, LVEF 31±12%, 58% ischemic heart disease, 35% biventricular devices) underwent implantation of a new DF-4 lead successfully. The acute and first follow-up R-wave detection and threshold were excellent for all leads in the 792 patients with complete available data (Table). A higher number of early lead revisions were required with the Boston Scientific leads compared to other leads (p=0.023). These were mostly related to lead dislodgement. The last 71 Boston Scientific 0693 leads were implanted using a standard screw activator instead of the new Boston device. There were 2 lead revisions with this new strategy (p=0.129). Rates of pneumothorax and infection requiring extraction were 0.4% and 0.8% respectively. A total of 808 patients (age 63±12 years, 80% male, LVEF 31±12%, 58% ischemic heart disease, 35% biventricular devices) underwent implantation of a new DF-4 lead successfully. The acute and first follow-up R-wave detection and threshold were excellent for all leads in the 792 patients with complete available data (Table). A higher number of early lead revisions were required with the Boston Scientific leads compared to other leads (p=0.023). These were mostly related to lead dislodgement. The last 71 Boston Scientific 0693 leads were implanted using a standard screw activator instead of the new Boston device. There were 2 lead revisions with this new strategy (p=0.129). Rates of pneumothorax and infection requiring extraction were 0.4% and 0.8% respectively. Conclusion

Design analysis of a standard injection screw for plasticising polycarbonate resins

Abstract The industrial use of plastic injection moulding machines is widespread. However, few studies have examined the injection screw, which is one of the key components of moulding machines. Studies have demonstrated that a properly designed injection screw improves both the moulding quality and the production rate. Factors that affect the plasticisation properties of conventional standard reciprocating screws include the screw geometry, the screw operation settings, and the processed resins. An ideal standard reciprocating screw exhibits a high plasticising capacity and excellent melt temperature homogeneity; however, these properties typically conflict. Through simulation analysis, this study investigated the optimal design of a standard reciprocating injection screw used for plasticising polycarbonate resins. First, the Taguchi method was integrated with a commercial simulation programme to identify the key control factors affecting the plasticising rate of a screw and the temperature uniformity of the melt. Simulation results revealed that the screw diameter, rotation speed, metering channel depth, ratio of the screw length to the screw diameter, and compression ratio substantially influence performance. Consequently, grey relational analysis was adopted to optimise the design of an injection screw that ensures sufficient quality according to the plasticisation rate and the homogeneity of molten plastic.

What would be the annual cost savings if fewer screws were used in adolescent idiopathic scoliosis treatment in the US?

There is substantial heterogeneity in the number of screws used per level fused in adolescent idiopathic scoliosis (AIS) surgery. Assuming equivalent clinical outcomes, the potential cost savings of using fewer pedicle screws were estimated using a medical decision model with sensitivity analysis. Descriptive analyses explored the annual costs for 5710 AIS inpatient stays using discharge data from the 2009 Kids' Inpatient Database (Healthcare Cost and Utilization Project, Agency for Healthcare Research and Quality), which is a national all-payer inpatient database. Patients between 10 and 17 years of age were identified using the ICD-9-CM code for idiopathic scoliosis (737.30). All inpatient stays were assumed to represent 10-level fusions with pedicle screws for AIS. High screw density was defined at 1.8 screws per level fused, and the standard screw density was defined as 1.48 screws per level fused. The surgical return for screw malposition was set at $23,762. A sensitivity analysis was performed by varying the cost per screw ($600-$1000) and the rate of surgical revisions for screw malposition (0.117%-0.483% of screws; 0.8%-4.3% of patients). The reported outcomes include estimated prevented malpositioned screws (set at 5.1%), averted revision surgeries, and annual cost savings in 2009 US dollars, assuming similar clinical outcomes (rates of complications, revision) using a standard- versus high-density pattern. The total annual costs for 5710 AIS hospital stays was $278 million ($48,900 per patient). Substituting a high for a standard screw density yields 3.2 fewer screws implanted per patient, with 932 malpositioned screws prevented and 21 to 88 revision surgeries for implant malposition averted, and a potential annual cost savings of $11 million to $20 million (4%-7% reduction in the total cost of AIS hospitalizations). Reducing the number of screws used in scoliosis surgery could potentially decrease national AIS hospitalization costs by up to 7%, which may improve the safety and efficiency of care. However, such a screw construct must first be proven safe and effective.

Biomechanical Analysis of the Proximal Adjacent Segment after Multilevel Instrumentation of the Thoracic Spine: Do Hooks Ease the Transition?

Study Design Biomechanical cadaveric study. Objective Clinical studies indicate that using less-rigid fixation techniques in place of the standard all-pedicle screw construct when correcting for scoliosis may reduce the incidence of proximal junctional kyphosis and improve patient outcomes. The purpose of this study is to investigate whether there is a biomechanical advantage to using supralaminar hooks in place of pedicle screws at the upper-instrumented vertebrae in a multilevel thoracic construct. Methods T7–T12 spines were biomechanically tested: (1) intact; (2) following a two-level pedicles screw fusion from T9 to T11; and after proximal extension of the fusion to T8–T9 with (3) bilateral supra-laminar hooks, (4) a unilateral hook + unilateral screw hybrid, or (5) bilateral pedicle screws. Specimens were nondestructively loaded while three-dimensional kinematics and intradiscal pressure at the supra-adjacent level were recorded. Results Supra-adjacent hypermobility was reduced when bilateral hooks were used in place of pedicle screws at the upper-instrumented level, with statistically significant differences in lateral bending and torsion (p < 0.05 and p < 0.001, respectively). Disk pressures in the supra-adjacent segment were not statistically different among top-off techniques. Conclusions The use of supralaminar hooks at the top of a multilevel posterior fusion construct reduces the stress at the proximal uninstrumented motion segment. Although further data is needed to provide a definitive link to the clinical occurrence of PJK, this in vitro study demonstrates the potential benefit of “easing” the transition between the stiff instrumented spine and the flexible native spine and is the first to demonstrate these results with laminar hooks.

Fenestrated Screws Augmented With PMMA Increase the Pullout Strength of Sacral Pedicle Screws.

Biomechanics. The objective of this study was to compare the fixation in osteoporotic specimens of a bicortical sacral pedicle screw to the following constructs: a unicortical solid screw augmented with polymethylmethacrylate (PMMA) (TRACT), a unicortical fenestrated screw augmented with PMMA (FEN), and a bicortical screw/sacral alar screw combination (PED/ALA). Rigid posterior sacral fixation of osteoporotic spines is problematic due to poor bone quality. Options to improve fixation include bicortical or tricortical screw trajectory, addition of S2 alar screws, or PMMA augmentation. Eleven osteoporotic cadaveric sacra were potted in 2-part filler compound. As a baseline fixation method, a 6.5-mm bicortical pedicle screw was placed into 1 pedicle of each sacra. The contralateral side was randomly assigned to receive a unicortical fenestrated 6.5-mm pedicle screw augmented with PMMA, a unicortical solid 6.5-mm pedicle screw augmented with PMMA, or a bicortical pedicle screw/alar screw construct. Pedicle screws were failed in cantilever pullout using a materials testing machine. Data were normalized into a test index by taking the ratio of the test screw pullout strength to the intraspecimen bicortical control screw pullout strength. The fixation strengths as expressed by the test index of the TRACT, FEN, and PED/ALA groups were 101%, 157%, and 167%, respectively. Although there was no statistical difference detected between the FEN and PED/ALA groups, the TRACT group had a trend toward lower pullout strength than the FEN (P=0.06) or PED/ALA (0.06) groups. Although underpowered, this study did not detect biomechanical inferiority for any of the 3 test configurations when compared with standard bicortical S1 screws. The data indicate that the fixation strength of a fenestrated, PMMA-augmented construct or a combined S1 pedicle/S2 alar screw construct may be better than either standard bicortical or unicortical PMMA screw-tract augmented screws.

A Technique of Superficial Medial Collateral Ligament Reconstruction Using an Adjustable-Loop Suspensory Fixation Device

This report describes superficial medial collateral ligament reconstruction of the knee using a novel method of graft fixation with the ACL Tightrope RT (Arthrex, Naples, FL). After tibial fixation with either a standard interference screw or staple, femoral fixation of the semitendinosus tendon is performed with the adjustable-loop suspensory fixation device, which allows for both initial graft tensioning and re-tensioning after cyclical knee range of motion. This provides the ability for the graft to accommodate for resultant soft-tissue creep and stress relaxation, thereby allowing for optimal soft-tissue tension and reduction in laxity at the end of the procedure.

Biomechanical evaluation of a second generation headless compression screw for ankle arthrodesis in a cadaver model.

Many types of screws, plates, and strut grafts have been utilized for ankle arthrodesis. Biomechanical testing has shown that these constructs can have variable stiffness. More recently, headless compression screws have emerged as an evolving method of achieving compression in various applications but there is limited literature regarding ankle arthrodesis. The aim of this study was to determine the biomechanical stability provided by a second generation fully threaded headless compression screw compared to a standard headed, partially threaded cancellous screw in a cadaveric ankle arthrodesis model. Twenty fresh frozen human cadaver specimens were subjected to simulated ankle arthrodesis with either three standard cancellous-bone screws (InFix 7.3mm) or with three headless compression screws (Acumed Acutrak 2 7.5mm). The specimens were subjected to cyclic loading and unloading at a rate of 1Hz, compression of 525 Newtons (N) and distraction of 20N for a total of 500 cycles using an electromechanical load frame (Instron). The amount of maximum distraction was recorded as well as the amount of motion that occurred through 1, 10, 50, 100, and 500 cycles. No significant difference (p=0.412) was seen in the amount of distraction that occurred across the fusion site for either screw. The average maximum distraction after 500 cycles was 201.9μm for the Acutrak 2 screw and 235.4μm for the InFix screw. No difference was seen throughout each cycle over time for the Acutrak 2 screw (p-value=0.988) or the InFix screw (p-value=0.991). Both the traditional InFix type screw and the second generation Acumed Acutrak headless compression screws provide adequate fixation during ankle arthrodesis under submaximal loads. There is no demonstrable difference between traditional cannulated partially threaded screws and headless compression screws studied in this model.

Allograft Reconstruction of the Lisfranc Ligament.

For Lisfranc injuries, screw fixation of the medial and middle columns is currently the standard of treatment. The purpose of this study was to biomechanically evaluate the use of allograft for a severed Lisfranc ligament compared to standard screw fixation and the intact condition. Six pairs of fresh-frozen cadaveric lower extremities were prepared with reflective marker arrays and cyclically loaded to simulate partial weight bearing under 4 sequential testing conditions: (1) intact ligament, (2) disrupted ligament, (3) tendon allograft reconstructed ligament, and (4) rigid screw fixation. The relative displacement between the medial cuneiform and the second metatarsal was assessed via motion analysis. A mixed model analysis of variance was used to determine the significance (α = .05) of displacement differences. Mean displacements and 95% confidence intervals for each condition were as follows: (1) intact 9.1 (7.1-11.2) mm, (2) cut ligament 9.4 (7.4-11.5) mm, (3) allograft fixation 8.8 (6.8-10.9) mm, and (4) screw fixation 8.2 (6.2-10.3) mm. There were no significant differences among the specimens according to condition. Allograft fixation provided adequate strength and stability and did not differ significantly compared to intact or screw fixation. Level V: Bench testing.

Axial pullout strength comparison of different screw designs: fenestrated screw, dual outer diameter screw and standard pedicle screw.

BackgroundThe pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA. In the present study, the pullout strength of an innovative fenestrated screw augmented with PMMA was investigated and was compared to unaugmented fenestrated, standard and dual outer diameter screw.MethodsTwenty four thoracolumbar vertebrae (T10-L5, age 60 to 70 years) from three cadavers were implanted with the four different pedicle screws. Twelve screws of each type were instrumented into either left or right pedicle with standard screw paired with unaugmented and dual outer diameter screw paired with augmented fenestrated screw in any given vertebra. Axial pullout testing was conducted at a rate of 5 mm/min. Force to failure (Newtons) for each pedicle screw was recorded.ResultsThe augmented fenestrated screws had the highest pullout strength, which represented an average increase of 149%, 141%, and 78% in comparison to unaugmented, standard, and dual outer diameter screws, respectively. Pullout strength of unaugmented screws was comparable to that of standard screws, however it was significantly lower than dual outer diameter screws.ConclusionsFenestrated screws augmented with PMMA improve the fixation strength and result in significantly higher pullout strength compared to dual outer diameter, standard and unaugmented fenestrated screws. Screws with dual outer diameter provided enhanced bone-screw purchase and may be considered as an alternative technique to increase the bone-screw interface in cases where augmentation using bone cement is not feasible. Unaugmented screws can be left in the pedicle even without cement and provide similar pullout strength to standard screws.

Biomechanical Analysis of a Novel Pedicle Screw Anchor Designed for the Osteoporotic Population

The biomechanical study was performed to investigate the effect of a novel pedicle screw anchor in increasing the pullout strength of pedicle screws. Ten lumbar vertebral bodies with a weighted average T-score of -2.13 were used. Pedicle screws of 4.5 mm diameter and 25 mm length were inserted in to one pedicle randomly and matched with an anchor in the corresponding pedicle. Fatigue testing was performed by applying an axial load of ±200N to the screw tulip, along the axis of the rod, at a rate of 0.5 Hz for 1,000 cycles. After fatigue loading was completed, all screws underwent axial pullout testing at a rate of 0.1 mm/sec until failure. A paired two sample for means t-test was performed to determine a significant difference between the two groups (p ≤ 0.05). Following fatigue testing, the axial displacement at the 1,000 cycle point for the anchor and non-anchor group was 1.4 ± 0.7mm and 2.9 ± 1.2mm, respectively. The anchor group had significantly lower axial displacement compared to the non-anchor group (p ≤ 0.01). The group with the anchor reached an average maximum load of 702 ± 373N. The average yield load for the non-anchor group was 421 ± 293N. The anchor group yield load was significantly greater than the non-anchor group (p ≤ 0.01). A novel anchor for standard pedicle screws resulted in significantly less axial movement during fatigue and a greater failure force compared a screw with no anchor. The anchor may provide a stronger bone-to-screw interface, than a non-anchor screw, without the complications of cement augmentation.

Biomechanical Comparison of 2 Different Pedicle Screw Systems During the Surgical Correction of Adult Spinal Deformities

Study DesignA biomechanical spine model was used to evaluate the impact of screw design on screw–vertebra interface loading during simulated surgical corrections of adult scoliosis. ObjectivesTo evaluate differences in screw–vertebra interface forces during adult scoliosis correction between favored angle (FA) screws with extension tabs and standard polyaxial screws while varying deformity severity and curve rigidity. Summary of Background DataPedicle screws enable surgeons to safely and effectively realign spinal deformities. The risk of perioperative screw pullout increases when presented with adult deformities that have less flexible spines and lower bone mineral density. An FA screw with reduction tabs is believed to enable surgical techniques permitting load distribution on multiple screws, thereby reducing screw pullout potential. MethodsThe researchers constructed 3 finite element spine models from adult scoliosis patients. Mechanical properties of intervertebral discs were modeled to reflect less flexible adult spines and their stiffness was varied to evaluate impact on screw–vertebra forces. Models simulated scoliosis surgery according to clinical data using FA or polyaxial screws. Forces measured at the screw–vertebra interface were monitored and compared for each patient with FA and then polyaxial screws. ResultsSimulations using FA screws reduced screw–vertebra interface forces significantly compared with polyaxial screws. Favored angle screws caused 18%, 14%, and 16% reductions in peak forces and 29%, 35%, and 22% reductions in average forces compared with polyaxial screws for patients 1, 2, and 3, respectively. Favored angle screws also provided consistent relative reduction in average forces by 28% when varying properties of intervertebral discs among 8, 10, and 12 MPa. ConclusionsUsing a virtual finite element platform, FA screws reduced screw–vertebra interface forces encountered during simulated correction of less flexible adult scoliosis compared with standard polyaxial screws. These results show a potential benefit of using this modified screw design to reduce screw–vertebra forces and potential intraoperative pullout failures.

Prevention of proximal junctional kyphosis after posterior surgery of Scheuermann kyphosis: an operative technique.

A prospective randomized study. To introduce an operative technique that prevents proximal junctional kyphosis (PJK) in Scheuermann disease after a segmental posterior spinal instrumentation and fusion. PJK is the progression of kyphotic deformity at the proximal end of a construct >10 degrees, and it can be seen up to 30% after posterior Scheuermann kyphosis surgery. After posterior fusion the biomechanics of the spine changes and the loss of motion at the fused levels is compensated by increased motion at other unfused segments. As a result significant amount of additional force is placed on the proximal junction. With our operative technique, we aimed to have a smooth passage from rigid to mobile segments and to decrease the stress on proximal junction during cantilever reduction to prevent PJK. A total of 60 consecutive patients (mean age: 18.27±3.19, male/female: 28/32) who were surgically treated for Scheuermann kyphosis in our institution were recruited into this study and were prospectively evaluated. Patients were divided into 2 groups according to upper-most screw fixation technique. In group 1, a standard screw insertion technique was used (ST group). The technique was modified in group 2 (MT group), leaving 2 threads out of the posterior cortex. There were 29 patients in group 1 (ST) and 31 patients in group 2 (MT). Patients had an average follow-up time of 24.2 months (range, 19-48 mo). Evaluated radiographic parameters were preoperative and postoperative kyphosis angle, and proximal junctional angle (PJA) at last visit. PJA was defined as the angle between the caudal endplate of the upper instrumented vertebra and the cephalad endplate of 2 suprajacent vertebrae above the upper instrumented vertebra. PJA exceeding 10 degrees was accepted as PJK. Quality of life measurement was assessed preoperatively and postoperatively with SF-36 questionnaire. Correction amounts in ST group and MT group were 46.8% and 43.7%, respectively, which was statistically insignificant. The mean PJA was 8.08±2.96 degrees and 4.44±1.55 degrees in ST and MT groups, respectively, which demonstrated a statistically significant difference (P=0.001). Five patients in ST group had a PJA exceeding 10 degrees (PJK), whereas PJK was not seen in MT group (P=0.022). The improvement in physical component summary of SF-36 was significantly better in MT group; however, mental component summary was similar in both groups. This study introduces a new technique that may have an effect in preventing PJK. Our results seem to be satisfactory, but additional studies with more patients and longer follow-up times are needed to further delineate the feasibility of this technique.

Dynamization at the near cortex in locking plate osteosynthesis by means of dynamic locking screws: an experimental study of transverse tibial osteotomies in sheep.

Locking plates are widely used in fracture fixation, mainly for meta-diaphyseal fractures, comminuted fractures, fractures with a critical-size bone defect, periprosthetic fractures, osteotomies, and fractures in osteoporotic bone. The aim of this animal study was to evaluate the effect on bone-healing of dynamization of locking plate constructs by means of new 5.0-mm dynamic locking screws (in the DLS group), which allow near-cortex micromotion, compared with a more rigid construct utilizing standard bicortical locking-head screws (in the LS group). Use of dynamic locking screws allows modulation of the stiffness of existing locking compression plate systems via parallel interfragmentary micromotion. A standardized diaphyseal tibial osteotomy (90°, 3-mm fracture gap) was performed and stabilized with a six-hole large-fragment locking compression plate in twelve female sheep (six in each group). Radiographs were made postoperatively and then weekly from week three until sacrifice at nine weeks. Macroscopic, biomechanical, histologic, and radiographic assessments and microcomputed tomography were performed. The callus in the tested specimens in the DLS group had better biomechanical stability, with a significantly greater maximum failure moment (mean and standard deviation [SD] as a percentage of intact, 55.15 ± 20.65 compared with 26.80 ± 14.96 in the LS group; p = 0.021). The DLS group also had greater periosteal callus volume at the near cortex (mean volume and SD as a percentage of the tibial shaft volume, 36.21% ± 10.08% compared with 18.98% ± 8.61% in the LS group; p = 0.026) and in the intercortical region (mean volume and SD as a percentage of the bone volume of the tibial shaft, 3.56% ± 0.52% compared with 2.64% ± 0.98% in the LS group; p = 0.045), as shown by microcomputed tomography. The DLS group also had significantly greater torsional stiffness (mean and SD as a percentage of intact, 84.88 ± 13.51 compared with 58.89 ± 20.61 in the LS group; p = 0.027). Controlled micromotion and nearly homogeneous interfragmentary strain at the fracture site, together with the stable bicortical fixation achieved by the new dynamic locking screw, led to more uniform callus formation, significantly more callus formation at the near cortex, and biomechanically more competent bone-healing compared with use of rigid locking plate constructs with locking-head screws.

Cortical and Standard Trajectory Pedicle Screw Fixation Techniques in Stabilizing Multisegment Lumbar Spine with Low Grade Spondylolisthesis.

Cortical screw (CS) fixation has been recently proposed as an alternative to the standard pedicle screw (PS) fixation technique. Biomechanical studies involving individual screw pullout and single level motion segment stabilization showed comparable performance of both techniques. However, whether this new fixation technique can be applied to the stabilization of multilevel lumbar segments with significant destabilization has been unclear. To compare stability of CS fixation to the traditional PS fixation in an unstable 3 level spondylolisthesis model. This is a biomechanical study comparing cortical trajectory pedicle screw fixation to traditional trajectory pedicle screw fixation in an unstable cadaveric model using nondestructive flexibility test. Eight fresh frozen cadaveric lumbar spines (T12- S1) were obtained. After intact baseline testing, a 3-level lowgrade spondylolisthesis was simulated at the L1-4. Each specimen was instrumented with the PS and CS fixation systems. Standard nondestructive flexibility test was performed. Range of motion at each level was compared between the constructs during flexion-extension, lateral bending, and axial rotation. The destabilization model significantly increased the ROM in all planes (P<0.05). Both fixation techniques provided significant reduction in the ROM (P<0.05). There was no significant difference in ROM between the PS and CS groups in any of planes (P>0.05). Cortical trajectory pedicle screw fixation provided stabilization to multilevel lumbar segment with low-grade spondylolisthesis comparable to the standard trajectory pedicle screw construct.

Balancing Rigidity and Safety of Pedicle Screw Fixation via a Novel Expansion Mechanism in a Severely Osteoporotic Model

Many successful attempts to increase pullout strength of pedicle screws in osteoporotic bone have been accompanied with an increased risk of catastrophic damage to the patient. To avoid this, a single-armed expansive pedicle screw was designed to increase fixation strength while controlling postfailure damage away from the nerves surrounding the pedicle. The screw was then subsequently tested in two severely osteoporotic models: one representing trabecular bone (with and without the presence of polymethylmethacrylate) and the other representing a combination of trabecular and cortical bone. Maximum pullout strength, stiffness, energy to failure, energy to removal, and size of the resulting block damage were statistically compared among conditions. While expandable pedicle screws produced maximum pullout forces less than or comparable to standard screws, they required a higher amount of energy to be fully removed from both models. Furthermore, damage to the cortical layer in the composite test blocks was smaller in all measured directions for tests involving expandable pedicle screws than those involving standard pedicle screws. This indicates that while initial fixation may not differ in the presence of cortical bone, the expandable pedicle screw offers an increased level of postfailure stability and safety to patients awaiting revision surgery.

Facet Wedge Fixation in Lumbar Spine Degenerative Pathology: A New Option for Minimally-Invasive Posterior Approach?

In recent years spinal surgery has undergone a drastic change of course, mainly due to the increasing use of minimally invasive and percutaneous techniques for the treatment of lumbar spine degenerative pathology. This has led to the development of techniques more effective (for the pathology) and less invasive (for the patient). Recently the technological progress focused on a very specific vertebral fusion technique: the facet joint fixation. This technique has already been described in past as available with the assistance of autologous bone chips into the facet joint; nowadays it has been brought up to date by the development and use of an implantable device that promotes the bone fusion of the joint complex: the Facet-Wedge®. The employment of this device seems to offer a minimally invasive choice to the standard posterior screws fixation techniques for the treatment of selected cases of lumbar degenerative pathologies. Obviously their application has to be evaluated on the basis of a careful preoperative radiological planning. Up to date there aren’t in literature clinical trials about the effective of this device and this technique. The biomechanical meaning of this device, the possibility of implanting in percutaneous technique and in combined instrumentation with anterior cages or different posterior techniques, could reveal that this technique might be an effective and feasible alternative, which needs to be addressed in further studies.

A Survey on Cortical Bone Trajectory for Spinal Fusions

There have been a number of developments in screw design and implantation techniques over recent years, including proposal of an alternative trajectory entitled as cortical bone trajectory (CBT). Cortical bone trajectory has been investigated in recent medical treatments as an alternative for screw fixation aimed at increasing purchase of pedicle screws in higher density bone. CBT screw insertion follows a lateral path in the transverse plane and caudocephalad path in the sagittal plane. This technique has been advocated because it is reportedly less invasive, improves screw−bone purchase and reduces neurovascular injury. Furthermore CBT pedicle screw fixation provides stabilization to multilevel lumbar segment with low-grade spondylolisthesis comparable to the standard trajectory pedicle screw construct. However, these claims have not been supported by robust clinical evidence. Recent investigations focus on evaluations of CBT as a pioneer method.