Bone-screw Interface Research Articles

Overpowering the Previously Posterior Instrumented Cervical Spine With Cage-Assisted Anterior Cervical Discectomy and Fusion: A Cadaveric Study

PurposeCervical spines previously posteriorly instrumented and fused with a kyphotic deformity represent a surgical challenge. Current treatment strategies include C7 pedicle subtraction osteotomy or a posterior-anterior-posterior approach, which carry the risk of significant complications. The objective of this study was to attempt to achieve lordosis with multiple anterior cervical discectomy and fusion (ACDF) cages to overpower the posterior instrumentation. MethodsFour adult cadaveric specimens were selected and underwent C3–C7 posterior laminectomy with posterior instrumentation in a kyphotic alignment using a 3.5-mm titanium screw-rod system. Next, ACDF from C3 to C7 was performed with 15° lordotic cages to restore cervical lordosis. Posterior instrumentation was then inspected for failure. Fluoroscopic images were obtained to calculate total construct lordosis and change in segmental lordosis. CT scans were obtained after ACDF to assess for loosening, instrumentation failure, endplate damage, or impaction. Bone mineral density was calculated on CT scans. ResultsAge ranged from 59 to 82, and all specimens were male. No gross instrumentation failure was observed. Mean pre-ACDF lordosis between C3 and C7 was 0° (–5° to 5°). Post-ACDF lordosis increased to 37° (35°–38°). Mean segmental lordosis achieved with no endplate destruction was 13.1° (8°–17°). T scores for the cadavers were –0.5, –0.5, –3.2, and –5.1. Two levels of impaction were observed (12.5%). Failure of bone screw interface occurred in the cadaver, with a T score of –5.1 in the middle of the construct. ConclusionOur study demonstrates the validity of overpowering posterior instrumentation through multiple level ACDF with lordotic cages. This may obviate the need to perform posterior-anterior-posterior procedures. Level of EvidenceLevel III

Thursday, September 27, 2018 1:05 PM–2:05 PM Lumbar Spine Surgery: What You Need to Know: 111. Fibular allograft as a salvage option in revision pedicle screw fixation

BACKGROUND CONTEXT In the setting of revision spine surgery for pseudoarthrosis, a variable amount of pedicle bone loss may be present. This is a result of screw loosening and subsequent screw track widening. Often the bone quality may be poor secondary to aging, malnutrition, previous infection and smoking. In these difficult cases, multiple techniques have been utilized to enhance the construct strength. Current options for revision pedicle screw fixation include cement augmented screw fixation and custom-made large diameter screws. PURPOSE The purpose of this study was to present the authors’ experience using fibular allograft in the setting of bone loss with loose pedicle screw revision. STUDY DESIGN/SETTING Retrospective. PATIENT SAMPLE A total of 10 patients. OUTCOME MEASURES CT scan lumbar spine ODI. METHODS In patients with extensive screw track dilation, fibular allograft was obtained and cut into small, longitudinal strips—approximately 3 mm in thickness. The length of each fibular allograft strut was trimmed to coincide with the length of the former screw, allowing for overhang. Allograft strut was inserted into the screw track along the wall. The screw was then advanced between the allograft, and after full insertion of the screw, the fibular allografts were then trimmed. RESULTS This technique was successfully used in 10 consecutive patients during revision lumbar spine surgeries for pseudoarthrosis. All patients had loose pedicle screws with significant bone loss around the screw tracks. Excellent purchase was achieved in all patients. CT scan at the last follow up showed incorporation of the fibular allograft and sound fusion in all patients except one. CONCLUSIONS Fibular allograft is a viable option in the setting of revision pedicle screw fixation and has several advantages when compared to currently available methods. It avoids the complications associated with cement augmented pedicle screws. In addition, it can be used impromptu, when other options are not immediately available. This technique can be employed without specialized instrumentation at significant cost savings when compared to custom made screws. Finally, this technique allows for biologic fixation at the screw-bone interface. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Optimization of Monoenergetic Extrapolations in Dual-Energy CT for Metal Artifact Reduction in Different Body Regions and Orthopedic Implants

To define an optimal monoenergetic extrapolation (ME) in dual-energy computed tomography (DECT) for metal artifact reduction (MAR) including different body regions and orthopedic implants. DECT scans were acquired with dual-source CT (SOMATOM Force, Siemens, Germany) at tube voltage A 80-100 kV/B Sn150 kV from 39 patients (mean 54.1 ± 20.7 years, 23 male vs. 16 female) with orthopedic implants ranging from wires to joint implants. Scans were assembled in four groups based on scan regions and volume. Single- and weighted-energy images at a ratio of 0.3 and MEs at 100, 130, 160, and 190 keV were produced using vendor-specific postprocessing software (Syngo.Via, Siemens, Germany). Artifact degree was assessed quantitatively by metal-induced Hounsfield unit changes in relation to reference tissues. Visibility of screw-bone interface, hardware integrity, adjacent bone, and soft tissues were visually rated on a four-point Likert scale (0, none; 3, strong artifacts with nondiagnostic quality). Optimal energy was visually determined by side-by-side comparisons. Artifact degree was statistically compared between regions and energies. Metal-induced attenuation changes were most severe in large scan volume groups for all energies. Reference tissue attenuation outside metal artifacts was not affected by ME (p = 0.57). Independent of region, ME at 130-190 keV quantitatively performed significantly better for MAR than the remainder. ME 130 keV showed the highest frequency (54%) in optimal energy ratings based on qualitative image criteria. DECT significantly reduces image artifacts in patients with orthopedic hardware and prospective choice of ME at 130 keV may suit best for optimal MAR, independent of region or implant.

How a pilot hole size affects osteosynthesis at the screw–bone interface under immediate loading

An inappropriate pilot hole size (PHS) is one of several factors that affects the stiffness of the screw–bone fixation. The present study uses finite element models to investigate the effect of varying the PHS on the biomechanical environment of the screw–bone interface of the fractured bone, after the screw insertion and under the immediate body weight pressure (BWP). Four PHS from 71% up to 85% of the screw external diameter (SED) were considered for analysis. A non linear material behaviour of the bone with ductile damage properties was used in the study. To validate the numerical models, an experimental pull-out test was carried out using a synthetic bone. The results of the insertion process demonstrated that the relatively smaller holes (71% and 75.5% of SED) increased the insertion torque value within the recommended level, caused more bone radial extension deformation and maximized the contact area between the bone threads and the screw, in comparison to the PHS higher than 80% of SED. Under the immediate BWP after osteosynthesis, the stress level exceeds the elastic limit and becomes high enough to initiate the ductile damage of the bone. Also, enlarging PHS from 71% to 75.5% of SED increased the bone microdisplacement at the screw–bone interface from 75 up to 100 μm, and that reduced the stiffness of the fixation.

Implant Density at the Apex Is More Important Than Overall Implant Density for 3D Correction in Thoracic Adolescent Idiopathic Scoliosis Using Rod Derotation and En Bloc Vertebral Derotation Technique.

Biomechanical analysis of 3D correction and bone-screw forces through numerical simulations of scoliosis instrumentation with different pedicle screw patterns. To analyze the effect of different screw densities and distributions on 3D correction and bone-screw forces in adolescent idiopathic scoliosis (AIS) instrumentation. Instrumentation constructs with various numbers of pedicle screws and patterns have been proposed for thoracic AIS instrumentation. However, systematic biomechanical studies have not yet been completed on the appropriate screw patterns for optimal 3D correction. Patient-specific biomechanical models of the spine were created for 10 AIS cases (Lenke 1). For each case, surgical instrumentation patterns were computationally simulated using respectively a reference screw pattern (two screws per level fused) and six alternative screw patterns with fewer screws. Simulated surgical maneuvers and model definition were unchanged between simulations except the number and distribution of screws. 3D correction and bone-screw forces were compared. A total of 140 posterior instrumentations were computationally simulated. Mean corrections in the coronal and sagittal planes with alternative screw patterns were within 4° to the reference pattern. Increasing screw density in the apical region from one to two screws per level improved percent apical vertebral rotation (AVR) correction (r = 0.887, P < 0.05). Average bone-screw force associated with the reference screw pattern was 243N ± 54N and those with the alternative screw patterns were 11% to 48% lower. Compared with the reference maximal screw density pattern, alternative screw patterns allowed similar corrections in the coronal and sagittal planes. AVR correction was strongly correlated with screw density in the apical region; AVR correction varied significantly with screw patterns of the same overall screw density when an en bloc vertebral derotation technique was simulated. High screw density tended to overconstrain the instrumented spine and resulted in higher forces at the bone-screw interface. N/A.

Biomechanical analysis of the durability of a modified S1 vertebrae transpedicular screws insertion technique

BackgroundOne of the most important elements of the transpedicular screw implantation technique, which enables a strong screw-bone interface, is the precise choice of the site of screw insertion and the screw's trajectory. Due to the complex biomechanics of the lumbosacral interface and different shape of the sacrum, fixation of this segment remains a challenge for surgeons. Because of this, Kubaszewski et al. proposed a modified technique in which the entry point for screw insertion in the S1 vertebra is changed. MethodsSix human cadaver specimens of the S1 vertebrae were examined. Two transpedicular screws were inserted into the body of each examined vertebra using two implantation methods with different screw entry points and trajectories. The screws were subjected to cyclic preloading, followed by the pull-out test. The ultimate pull-out force, displacement, stiffness, and failure energy were measured. FindingsThe average pull-out force obtained for the standard method of implantation was 498 N (SD 201), whereas for the modified technique, it was 1308 N (SD 581). Displacement of the inserted screws in the new method was 36% higher than in the case of the standard method. This method is also characterized by the greater stiffness of the obtained interface and greater failure energy than the normally used technique. InterpretationThe obtained results demonstrate that the use of the new technique of implantation significantly increases the strength of the obtained screw-bone interface. It should also increase the success rate of the performed fixations and increase the safety of such fixations in clinical practice.

Local bone quality measurements correlates with maximum screw torque at the femoral diaphysis

BackgroundSuccessful fracture fixation depends critically on the stability of the screw-bone interface. Maximum achievable screw torque reflects the competence of this interface, but it cannot be quantified prior to screw stripping. Typically, the surgeon relies on the patients' bone mineral density and radiographs, along with experience and tactile feedback to assess whether sufficient compression can be generated by the screw and bone. However, the local bone quality would also critically influence the strength of the bone-screw interface. We investigated whether Reference Point Indentation can provide quantitative local bone quality measures that can inform subsequent screw-bone competence. MethodsWe examined the associations between the maximum screw torque that can be achieved using 3.5 mm, 4.5 mm, and 6.5 mm diameter stainless steel screws at the distal femoral metaphysis and mid-diaphysis from 20 cadavers, with the femoral neck bone mineral density and the local measures of bone quality using Reference Point Indentation. FindingsIndentation Distance Increase, a measure of bone's resistance to microfracture, correlated with the maximum screw stripping torque for the 3.5 mm (p < 0.01; R = 0.56) and 4.5 mm diameter stainless steel screws (p < 0.01; R = 0.57) at the femoral diaphysis. At the femoral metaphysis, femoral neck bone mineral density significantly correlated with the maximum screw stripping torque achieved by the 3.5 mm (p < 0.01; R = 0.61), 4.5 mm (p < 0.01; R = 0.51), and 6.5 mm diameter stainless steel screws (p < 0.01; R = 0.56). InterpretationReference Point Indentation can provide localized measurements of bone quality that may better inform surgeons of the competence of the bone-implant interface and improve effectiveness of fixation strategies particularly in patients with compromised bone quality.

Experimental and numerical investigation into the influence of loading conditions in biomechanical testing of locking plate fracture fixation devices.

ObjectivesSecondary fracture healing is strongly influenced by the stiffness of the bone-fixator system. Biomechanical tests are extensively used to investigate stiffness and strength of fixation devices. The stiffness values reported in the literature for locked plating, however, vary by three orders of magnitude. The aim of this study was to examine the influence that the method of restraint and load application has on the stiffness produced, the strain distribution within the bone, and the stresses in the implant for locking plate constructs.MethodsSynthetic composite bones were used to evaluate experimentally the influence of four different methods of loading and restraining specimens, all used in recent previous studies. Two plate types and three screw arrangements were also evaluated for each loading scenario. Computational models were also developed and validated using the experimental tests.ResultsThe method of loading was found to affect the gap stiffness strongly (by up to six times) but also the magnitude of the plate stress and the location and magnitude of strains at the bone-screw interface.ConclusionsThis study demonstrates that the method of loading is responsible for much of the difference in reported stiffness values in the literature. It also shows that previous contradictory findings, such as the influence of working length and very large differences in failure loads, can be readily explained by the choice of loading condition.Cite this article: A. MacLeod, A. H. R. W. Simpson, P. Pankaj. Experimental and numerical investigation into the influence of loading conditions in biomechanical testing of locking plate fracture fixation devices. Bone Joint Res 2018;7:111–120. DOI: 10.1302/2046-3758.71.BJR-2017-0074.R2.

Effect of Pedicle Fill on Axial Pullout Strength in Spinal Fixation After Rod Reduction.

Rod reduction to pedicle screws is used for a variety of spinal fixation procedures; however, it can alter the integrity of the screw-bone interface. The authors investigated the effect of pedicle fill (ratio of pedicle screw diameter to pedicle diameter) on the strength of the screw-bone interface after simulated rod reduction on 17 vertebrae (3 thoracolumbar spine specimens). Pedicle diameter was measured with standard clinical computed tomography scan protocols. The authors determined the minimum pedicle diameter for each level. Polyaxial pedicle screws were surgically placed bilaterally with a freehand technique with standard clinical anatomic landmarks. The pedicle pairs were instrumented with pedicle screws of predetermined diameter, 1 with greater than 80% fill and 1 with less than 80% fill. A simulated reduction maneuver was performed with a 5-mm gap followed by an axial pullout test to assess screw interface strength. Comparison of insertion torque between less than 80% fill and greater than 80% fill did not show significant increases. A significant difference in pullout load (P=.043) occurred with greater than 80% fill (791±637 N) compared with less than 80% fill (636±492 N). No significant difference in stiffness was noted (P=.154) with pedicle fill of greater than 80% (427±134 N/mm) compared with less than 80% (376±178 N/mm). The current findings support the use of greater than 80% pedicle fill for optimal screw anchoring in pedicle screw-based constructs involving rod reduction. Surgeons should consider placing screws that can safely fill vertebral pedicles, especially at the apex of the curve and the proximal and distal levels of constructs, where excessive forces are imparted to the screws. [Orthopedics. 2017; 40(6):e990-e995.].

Biomechanical Study of Fused Lumbar Spine Considering Bone Degeneracy Using FEA

Pedicle screw fixation is a universally accepted technique in the surgical treatment of spinal disorder. The existence of high mechanical stresses on the vertebral body due to loading/bone degeneracy on patients having vertebral implants may result in malfunctioning of the pedicle screws. The present work aims to predict the effect of various pedicle screws used in vertebral implants considering bone degeneracy and to select the screw for improved performance. A three-dimensional (3D) intact and fused model of L2–L3–L4 is generated for biomechanical and FE study. To consider degeneracy, bones were categorized into four groups, i.e., weak bone, normal bone, strong bone and very strong bone. The developed FE model was subjected to various axial compressive loads (270, 330, 450 and 510 N), and pure moment of 10 Nm is applied at the top surface of the L2 vertebra, while the lower surface of the L4 vertebra remains fixed. Three different diameters of the pedicle screw (5.5, 6.0 and 6.5) were considered to identify localized stress generation at bone–screw interface. Results prevail that pedicle screw of 6.0 mm diameter, 45 mm length enables minimum stress distribution at the bone–screw interface considering flexion, extension, bending and twist. The observations and data shown in the results may support researchers and clinical practitioners to identify accurate pedicle screw for vertebral implants so as to minimize screw failure/loosening. The work may help to design new vertebral implants and to identify the after-effects of implantation with varying bone densities.

Biomechanics of anterior plating failure in treating distractive flexion injury in the caudal subaxial cervical spine

BackgroundOperative level is a potential biomechanical risk factor for construct failure during anterior fixation for distractive flexion injury. No biomechanical study of this concept has been reported, although it is important in clinical management. MethodsTo explore the mechanism of this concept, a previously validated three-dimensional C2-T1 finite element model was modified to simulate surgical procedure via the anterior approach for treating single-level distractive flexion injury, from C2-C3 to C7-T1. Four loading conditions were used including no-compression, follower load, axial load, and combined load. Construct stability at the operative level was assessed. FindingsUnder these loading conditions with the head's weight simulated, segmental stability decreases when the operative level shifts cephalocaudally, especially at C6-C7 and C7-T1, the stress of screw-bone interface increases cephalocaudally, and in the same operative level, the caudal screws always carries more load than the cephalad ones. All these predicted results are consistent with failure patterns observed in clinical reports. In the contrast, under other loading conditions without the weight of head, no obvious segmental divergence was predicted. InterpretationThis study supports that the biomechanical mechanism of this phenomenon includes eccentric load from head weight during sagittal movements and difference of moment arms. Our study suggests that anterior fixation is not recommended for treating distractive flexion injury at the caudal segments of the subaxial cervical spine, especially at C6-C7 and C7-T1, because of the intrinsic instability in these segments. Combined posterior rigid fixation with anterior fixation should be considered for these segments.

250 - Analyses of a Novel Screw Retention Technology: A Biomechanical and Histological Assessment Utilizing an Ovine Posterior Lumber Fusion Model

BACKGROUND CONTEXT: Screws are the most commonly used implant in orthopedic surgery. Unfortunately, screws do not always operate effectively and “loosen” which causes a loss of fixation that can significantly disrupt the healing process. Screw failure can occur in any type of procedure and is especially apparent in procedures that treat patients with poor bone quality (eg, osteoporotic bone). There is up to a 15% revision rate in spine fusion procedures related to screw loosening. This study aimed to address the underlying problem with screw failure: the bone-screw interface. Utilizing a unique bio-textile, a braided sleeve was placed around screws to increase the surface area of contact between the screw and the bone while allowing bone growth through the sleeve which we theorized would enhance the engagement of the screw in simulated poor purchase scenarios, thus prevent loosening.

Impact of bone quality on the performance of integrated fixation cage screws

Background ContextCommercially available lumbar integrated fixation cages (IFCs) have variable designs. For example, screw-based designs have up to four screws inserted at different locations across the vertebral end plate as well as at different angles in the sagittal and transverse planes. This is important as end plate and trabecular bone quality may vary across the vertebra and may affect the screw's fixation ability, particularly if bone purchase at the bone-screw interface is poor. PurposeThis study aimed to evaluate whether variations in local bone quality surrounding IFC screws inserted at different locations in the vertebrae would affect their mechanical performance. Study DesignThis study is an in vitro human cadaveric biomechanical analysis. Materials and MethodsFourteen lumbar (L3 and L4) vertebrae from 10 cadavers (age: 76±10 years, bone mineral density: 0.89±0.17 g/cm2) were used for this study. Pilot holes (3.5-mm diameter×15-mm length) representing three different IFC screw orientations (lateral to medial [LM], midsagittal [MS], and medial to lateral [ML]) were created in vertebrae using an IFC guide and bone awl. The screw locations and trajectories chosen are representative of commercially available IFC designs. These pilot holes were then imaged with high-resolution microcomputed tomography to obtain a three-dimensional structure of the bone surrounding the pilot hole. Local bone morphology was then quantified by evaluating a 3-mm-thick circumferential volume surrounding the pilot hole. Integrated fixation screws were implanted into pilot holes while recording maximum screw insertional torques. Screws were toggled in the cranial direction from 10 to 50 N for first 10,000 cycles, and the maximum load was increased by 25 N for every 5,000 cycles for a total of 25,000 cycles. ResultsTotal bone volume (BV) and trabecular bone volume fraction surrounding ML screws were significantly greater (p<.03) compared with those around MS screws and LM screws. The maximum insertional torque for ML screws were greater (p=.06) than LM and significantly greater (p<.02) than MS screws. The number of cycles to failure for the ML screw was significantly greater (p<.04) than that for the LM and the MS screws. Total BV (R2≤46.2%, p<.03) and the maximum insertional torque (R2≤59.6%, p<.03) provided better correlations to screw loosening compared with all the other bone quality parameters. ConclusionsOur findings indicate that bone quality in the vertebral body varied spatially depending on the orientation and the insertion location of the IFC screw. These alterations in local bone quality significantly affected the screw's ability to fixate to bone. These variations in bone quality may be assessed intraoperatively using screw insertional torque measurements. By understanding available bone purchase at the bone-implant interface, the appropriate implant design can be selected to maximize the fixation strength.

Biomechanical tensile strength analysis for medial patellofemoral ligament reconstruction

BackgroundMedial patellofemoral ligament (MPFL) reconstruction is a surgery for acute and chronic dislocating patella. Several surgical techniques have been described. No biomechanical study has compared suture anchors, interference screws, and suspensory cortical fixation for MPFL reconstruction using human gracilis allograft. MethodsTwelve human cadaver knees were used for the analysis of five MPFL reconstruction techniques on the femur (F) and patella (P): suspensory cortical (SC), interference screw (IS) and suture anchor (SA) fixation (SC-F/SC-P, SC-F/IS-P, SC-F/SA-P, IS-F/SC-P, IS-F/IS-P). Each method was examined six times, each using a new human gracilis allograft. The force necessary for 50% patellar displacement and 100% patellar displacement were recorded for each method. Additionally, we examined the peak force to fixation failure for all methods. Patella dislocation or loss of fixation was considered failure. ResultsSC-F/SC-P, IS-F/SC-P, and SC-F/IS-P required force to failure greater than that of the native MPFL. The SC-F/IS-P required the largest force to failure. The SC-F/SA-P fixation technique required significantly less force to failure (P<0.05) than the native MPFL and significantly less force to failure (P<0.05) than all four other fixation techniques. All methods of fixation employing an interference screw failed secondary to graft pullout at the interference screw-bone interface. Methods employing suture anchors and two suspensory cortical fixations failed at the graft-suture anastomosis. ConclusionSC-F/SC-P, IS-F/SC-P, and SC-F/IS-P fixations were found to be stronger than the native MPFL, with the strongest being SC-F/IS-P.

Influence of Hydroxyapatite Stick on Pedicle Screw Fixation in Degenerative Lumbar Spine: Biomechanical and Radiologic Study.

A prospective, within-patient, left-right comparative study. To evaluate the efficacy of hydroxyapatite (HA) stick augmentation method by comparing the insertional torque of the pedicle screw in osteoporotic and nonosteoporotic patients. Unsatisfactory clinical outcomes after spine surgery in osteoporotic patients are related to pedicle screw loosening or pull-outs. HA, as a bone graft extender, has a possibility to enhance the fixation strength at the bone-screw interface. From November 2009 to December 2010, among patients who required bilateral pedicle screw fixation for lumbar spine surgery, 22 patients were enrolled, who recieved unilateral HA stick augmentation and completed intraoperative insertional torque measurement of each pedicle screws. On the basis of preoperative evaluation of bone mineral density, patients with osteoporosis had 2 HA sticks inserted unilaterally, and 1 stick for patients without osteoporosis. Pedicle screw loosening and pull-outs were assessed using 12-month postoperative CT scans and follow-up radiographs. Clinical evaluation was done preoperatively and at 1 year postoperatively, based on Visual Analog Scale score, Oswestry Disability Index, and Short Form-36 Health Survey. Regardless of bone mineral density, the average torque value of all pedicle screws with HA stick insertion (HA stick inserted group) was significantly higher than that of all pedicle screws without HA insertion (control group) (P<0.0001). Same results were seen in the HA stick inserted subgroups and the control subgroups within both of the osteoporosis group (P=0.009) and the nonosteoporosis group (P=0.0004). There was no statistically significant difference of the rate of pedicle screw loosening in between the HA stick inserted group and the control group. Clinical evaluation also showed no statistically significant difference in between patients with loosening and those without. The enhancement of initial pedicle screw fixation strength in osteoporotic patients can be achieved by HA stick augmentation.

Résistance à l’arrachement d’une fixation hybride (Tape Locking Screw™) dans la reconstruction du ligament croisé antérieur. Une étude biomécanique sur os humain et porcin

IntroductionA novel hybrid anterior cruciate ligament (ACL) reconstruction technique known as Tape Locking Screw™ (TLS) is gaining popularity. Utilizing a suspension-type construct in conjunction with an interference screw, this technique has demonstrated successful initial clinical results with the use of quadruple hamstring graft. However, there is currently limited data available on the biomechanical strength of this fixation. This study investigates the pullout strength of the constructing human distal femora as well as in a porcine model. The construct is tested in isolation, without the use of any graft. We hypothesized that the pullout strength of this construct would be similar to or better than current fixation systems available. Materials and methodsThe Tape Locking Screw hybrid fixation system was implanted into 22 fresh frozen human distal femora (50–89 years old) randomized to 10×20mm titanium or polyether ether ketone (PEEK) screws by a single sports fellowship trained orthopedic surgeon. Given that the graft is secured to polyethylene terephthalate tape within the construct, the construct was implanted without any graft in order to isolate the device for biomechanical testing. After implantation, a tensile force was applied directly to the loop of tape at a loading rate of 5mm/min using an electromechanical testing system. The failure load was calculated from the resultant load-displacement curve. Specimens were then visually examined for mode of failure. Similar biomechanical tests were performed on 16 porcine femora. ResultsIn the human model, the mean pullout strength was 523±269N with the PEEK screw and 578±245N with the titanium screw. In the porcine femur model, mean strength was 616±177 N with PEEK, 584±245N with titanium. There was no statistically significant difference in failure loads between these four groups. Tape slippage at the screw bone interface was the primary mode of failure in all the groups tested. DiscussionOur results demonstrate that the hybrid technique provides excellent pullout strength in comparison to other soft-tissue ACL fixation methods, with tape slippage being the mode of failure in all specimens tested. This data, in addition to the advantages of the TLS system, support its consideration in the armamentarium of constructs available for soft-tissue ACL reconstruction. Type of studyLaboratory controlled study level 2.

Resolution of screw radiolucency following instrumentation with a hybrid rigid to dynamic stabilization system

Abstract Introduction Screw loosening, a common complication of patients undergoing a semi-rigid fixation, is associated with the radiological finding of lucency within bone surrounding the implanted screw. Case report A clinical and radiologic review is presented of a patient who underwent rigid fixation with L4-5 interbody fusion and L3-4 posterolateral dynamic stabilization with Zimmer Dynamic-to-Optima (DTO) hybrid fusion to non-fusion system (Zimmer Spine, Minneapolis, MN) with hydroxyapatite coated pedicle screws that exhibited lucency on radiographic studies which subsequently disappeared. An L4-5 interbody fusion and L3-4 posterolateral dynamic stabilization was performed. The patient did well clinically postoperatively. At 4 months postoperatively, X-rays of the lumbar spine demonstrated the appearance of lucency around the left L3 pedicle screw, which began to resolve at 10 months, and continued to resolve on subsequent CT scan 1 year postoperatively. The patient's clinical status continued to improve, with no complications. Conclusions The development of radiolucent lines around pedicle screws several months postoperatively may be due to loosening of the screws or due to a stress response at the bone-screw interface. The subsequent disappearance of this finding would suggest the ongrowth of bone at the osteoconductive hydroxyapatite coating or the protection of the bone-screw interface afforded by dynamic stabilization.

Introducing the \u201cBone-Screw-Fastener\u201d for improved screw fixation in orthopedic surgery: a revolutionary paradigm shift?

BackgroundConventional screws used for fracture fixation in orthopedic surgery continue to rely on the historic buttress thread design. While buttress screws generally provide solid resistance against unidirectional axial loading forces, their design suffers from several limitations, as the buttress thread does not adequately resist multiaxial forces. Furthermore, the buttress screw is prone to stripping at the bone-screw interface and can cause microfracturing of the surrounding bone due to its thread design. Standard buttress screws are therefore at risk of adverse postoperative outcomes secondary to failure of bone fixation. A new patented Bone-Screw-Fastener was recently designed that is based on an interlocking thread technology. This new fastener provides distributive forces from the threads onto the bone and therefore resists loads in multiple directions. The underlying concept is represented by a “female thread” bone cutting technology designed to maximize bone volume, preserve bone architecture, and create a circumferential interlocking interface between the implant and bone that protects the thread from stripping and from failing to multiaxial forces.Presentation of the hypothesisWe hypothesize that the new Bone-Screw-Fastener overcomes the classic shortcomings of conventional orthopedic screws with buttress threads by ease of insertion, improved bone preservation, increased resistance to off-axis multidirectional loading forces and to stripping of the threads. These advanced biomechanical and biological properties can potentially mitigate the classic limitations of conventional buttress screws by providing better resistance to implant failure under physiological loads, preserving bone biology, and thus potentially improving patient outcomes in the future.Testing the hypothesisThe presumed superiority of the new fastener will require testing and validation in well-designed prospective multicenter randomized controlled trials (RCTs), using the conventional buttress screw as control.Implications of the hypothesisOnce validated in multicenter RCTs, the new Bone-Screw-Fastener may drive a change in paradigm with regard to its innovative biomechanical principles and biologic bone preservation for surgical applications requiring screw fixation.

Pullout strength of a novel hybrid fixation technique (Tape Locking Screw™) in soft-tissue ACL reconstruction: A biomechanical study in human and porcine bone

IntroductionA novel hybrid anterior cruciate ligament (ACL) reconstruction technique known as Tape Locking Screw™ (TLS) is gaining popularity. Utilizing a suspension-type construct in conjunction with an interference screw, this technique has demonstrated successful initial clinical results with the use of quadruple hamstring graft. However, there is currently limited data available on the biomechanical strength of this fixation. This study investigates the pullout strength of the construct in human distal femora as well as in a porcine model. The construct is tested in isolation, without the use of any graft. We hypothesized that the pullout strength of this construct would be similar to or better than current fixation systems available. Materials and methodsThe Tape Locking Screw hybrid fixation system was implanted into twenty-two fresh frozen human distal femora (50–89 years old) randomized to 10×20mm titanium or polyether ether ketone (PEEK) screws by a single sports fellowship trained orthopedic surgeon. Given that the graft is secured to polyethylene terephthalate tape within the construct, the construct was implanted without any graft in order to isolate the device for biomechanical testing. After implantation, a tensile force was applied directly to the loop of tape at a loading rate of 5mm/min using an electromechanical testing system. The failure load was calculated from the resultant load–displacement curve. Specimens were then visually examined for mode of failure. Similar biomechanical tests were performed on sixteen porcine femora. ResultsIn the human model, the mean pullout strength was 523±269N with the PEEK screw and 578±245N with the titanium screw. In the porcine femur model, mean strength was 616±177N with PEEK, 584±245N with titanium. There was no statistically significant difference in failure loads between these four groups. Tape slippage at the screw bone interface was the primary mode of failure in all the groups tested. DiscussionOur results demonstrate that the hybrid technique provides excellent pullout strength in comparison to other soft-tissue ACL fixation methods, with tape slippage being the mode of failure in all specimens tested. This data, in addition to the advantages of the TLS system, support its consideration in the armamentarium of constructs available for soft-tissue ACL reconstruction. Level of evidenceLaboratory controlled study level 2.

A personalized 3D-printed prosthetic joint replacement for the human temporomandibular joint: From implant design to implantation

Personalized prosthetic joint replacements have important applications in cases of complex bone and joint conditions where the shape and size of off-the-shelf components may not be adequate. The objective of this study was to design, test and fabricate a personalized 3D-printed prosthesis for a patient requiring total joint replacement surgery of the temporomandibular joint (TMJ). The new ‘Melbourne’ prosthetic TMJ design featured a condylar component sized specifically to the patient and fixation screw positions that avoid potential intra-operative damage to the mandibular nerve. The Melbourne prosthetic TMJ was developed for a 58-year-old female recipient with end-stage osteoarthritis of the TMJ. The load response of the prosthesis during chewing and a maximum-force bite was quantified using a personalized musculoskeletal model of the patient's masticatory system developed using medical images. The simulations were then repeated after implantation of the Biomet Microfixation prosthetic TMJ, an established stock device. The maximum condylar stresses, screw stress and mandibular stress at the screw-bone interface were lower in the Melbourne prosthetic TMJ (259.6MPa, 312.9MPa and 198.4MPa, respectively) than those in the Biomet Microfixation device (284.0MPa, 416.0MPa and 262.2MPa, respectively) during the maximum-force bite, with similar trends also observed during the chewing bite. After trialing surgical placement and evaluating prosthetic TMJ stability using cadaveric specimens, the prosthesis was fabricated using 3D printing, sterilized, and implanted into the female recipient. Six months post-operatively, the prosthesis recipient had a normal jaw opening distance (40.0 mm), with no complications identified. The new design features and immediate load response of the Melbourne prosthetic TMJ suggests that it may provide improved clinical and biomechanical joint function compared to a commonly used stock device, and reduce risk of intra-operative nerve damage during placement. The framework presented may be useful for designing and testing customized devices for the treatment of debilitating bone and joint conditions.