Bone-screw Interface Research Articles

Finite element analysis of the pelvis after modular hemipelvic endoprosthesis reconstruction

Dear Editor We read the article by Zhou et al. with great interest [1]. The authors conducted a study to investigate the biomechanics of the pelvis reconstructed with a modular hemipelvic prosthesis using finite element (FE) analysis. At the end of the study, they found that stress distribution of the postoperative pelvis was similar to that in the normal pelvis at three different static positions and that reconstruction with a modular hemipelvic prosthesis yielded good biomechanical characteristics. We appreciate their study, but we believe that the study has some limitations that would essentially effect the conclusion. The main criticism of this article is that the authors neglected to present data that establishes model validity. The second criticism is the lack of details concerning the quality of the FE analysis. Without these details, the results cannot be judged as being or not being valid. We believe that the study should include details on the FE mesh: element type, element edge length relative to smallest geometric features and mesh sensitivity analyses [2, 3]. The paper should also include a figure of the mesh showing the element distribution for a qualitative evaluation of mesh quality. Third, there is no detail on how the acetabular cup, CS fixator, pubic plate and screw–bone interface was modeled. Fourth, it is not clear how the authors modeled the contact surfaces between the femoral head and the acetabulum. Fifth, we could not determine whether or not articular cartilage, trabecular and subchondral bone were included in this model. Sixth, we believe that it would be more appropriate to clarify the boundary conditions precisely for the three different loadings that were modeled. Also, lack of experimental validation, justification of simplifying assumptions of mechanical properties and interface conditions are other drawbacks of the study. It is not clear why the authors chose that particular patient to model for their study. If the others were chosen, would they find similar results? Though the authors mentioned as one of the limitations that the strength of the periacetabular muscle was not considered because it was greatly affected by the widespread resection and could not be calculated accurately, we did not understand whether all 21 muscles [2] inserting onto the pelvic bone or only some that were resected were or were not incorporated in the model. Physiological loads occur under anatomical circumstances; thus, simulations of them should also contain the appropriate model of anatomic structures. Because the muscle forces have a stabilizing effect on pelvic-load transfer, analysis without muscle forces could result in higher stresses at certain locations than in the situation where muscle forces are included [2]. Muscle forces have considerable influence on stress patterns in the pelvic bone also. Analysis without muscle forces show that load transfer is entirely directed along the axis from the sacroiliac joint to the pubic symphysis. The ischial bone and the superior part of the iliac bone remain virtually unloaded, whereas the pubic bone is more highly stressed than in if muscle forces were included. The values and directions of the hip-joint reaction force and muscle forces in the three positions considered—sitting, standing on two feet and standing on the foot of the affected side—would also be expected to be different [2]. We believe that lack of all these details we mention limits the validity of the study and that it would be appropriate to clarify the issues pointed out herein in order to make more clinically relevant conclusions.

Effects of misalignment on static torsional strength of anterior cervical plate systems

Background contextThere is little understanding of cervical plate misalignment as a risk factor for plate failure at the plate-screw-bone interface. PurposeTo assess the torsional strength and mode of failure of cervical plates misaligned relative to the midsagittal vertical axis. Study designPlastic and foam model spine segments were tested using static compression and torsion to assess effects of misaligned and various lengths anterior cervical plate (ACPs). MethodsDifferent length ACPs and cancellous fixed angle screws underwent axial torsional testing on a servo-hydraulic test frame at a rate of 0.5°/s. A construct consisted of one ACP, four screws, one ultrahigh–molecular weight polyethylene inferior block, and one polyurethane foam superior block. Group 1 had ACPs aligned in the midsagittal vertical axis, group 2 plates were positioned 20° offset from the midline, and group 3 had the ACP shifted 5 mm away and 20° offset from midline. Torques versus angle data were recorded. The failure criterion was the first sign of pullout determined visually and graphically. ResultsGroup 1 had a more direct screw pullout during failure. For the misaligned plates, failure was a combination of the screws elongating the holes and shear forces acting between the plate and block. The misaligned plates needed more torque to failure. The failure torque was 50% reduced for the longer versus the shorter plates in the neutral position. Graphically shown initial screw slippage inside the block preceded visual identification of slippage in some cases. ConclusionsWe observed different failure mechanisms for neutral versus misaligned plates. Clinically, misalignment may have the benefit of needing more torque to fail. Misalignment was a risk factor for failure of the screw-bone interface, especially in longer plate constructs. These comparisons of angulations may be a solid platform for expansion toward a more applicable in vivo model.

In Vivo Study of Pedicle Screw Augmentation Using Bioactive Glass in Osteoporosis Sheep

Augmentation of pedicle screws with bioactive glass (BG) was performed in osteoporotic ovine spine in vivo. Biomechanical tests, micro-computed tomography (CT) analysis, and histologic observation were performed. To investigate the biomechanical stability of pedicle screws augmented by BG in osteoporotic sheep and observe the bone-screw interface histologically. There is little information on the long-term biomechanical performance and screw-bone interfacial bonding of pedicle screws augmented with BG in osteoporotic spine in vivo. Twelve months after ovariectomy combined with methylprednisolone injection, 8 adult female sheep were randomly divided into 2 groups (3- and 6-mo time point groups). In each time point group, pedicles were randomly selected from the lumbar spine (L1-L6) and implanted with (1) pedicle screw alone; (2) pedicle screw augmented by polymethylmethacrylate; or (3) pedicle screw augmented by BG. Three and 6 months after implantation, animals were labeled with tetracycline and calcein before being killed. Then vertebrae with pedicle screws were obtained, and a micro-CT scan, histologic analysis, and biomechanical tests were performed. Three months after implantation, micro-CT reconstruction showed that microstructural parameters of the BG group were significantly better compared with those in the other 2 groups (P<0.05). Histologic observation revealed that bone trabeculae around the screws in the BG group were more in number and denser than those in the control group. The average mineral apposition rate of the bone in the BG group was also higher than that in the other 2 groups (P<0.05). The mechanical properties in the BG group were also significantly higher than that in the control group. Six months after implantation, similar results except mineral apposition rate can be obtained among different groups. BG can significantly improve bone microstructure of the interface in osteoporosis condition and increase the hold strength of the pedicle screw.

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

To experimentally study the influence of pilot hole diameter (smaller than or equal to the internal (core) diameter of the screw) on biomechanical (insertion torque and pullout strength) and histomorphometric parameters of screw-bone interface in the acute phase and 8 weeks after pedicle screw insertion. Fifteen sheep were operated upon and pedicle screws inserted in the L1-L3 pedicles bilaterally. The pilot hole was smaller (2.0 mm) than the internal diameter (core) of the screw on the left side pedicle and equal (2.8 mm) to the internal diameter (core) of the screw on the right side pedicle. Ten animals were sacrificed immediately (five animals were assigned to pullout strength tests and five animals were used for histomorphometric bone-screw interface evaluation). Five animals were sacrificed 8 weeks after pedicle screw insertion for histomorphometric bone-screw interface evaluation. The insertion torque and pullout strength were significantly greater in pedicle screws inserted into pilot holes smaller than internal (core) diameter of the screw. Histomorphometric evaluation of bone-screw interface showed that the percentage of bone-implant contact, the area of bone inside the screw thread and the area of bone outside the screw thread were significantly higher for pilot holes smaller than the internal (core) diameter of the screw immediately after insertion and after 8 weeks. A pilot diameter smaller than the internal (core) diameter of the screw improved the insertion torque and pullout strength immediately after screw insertion as well the pedicle screw-bone interface contact immediately and 8 weeks after screw placement in sheep with good bone mineral density.

A finite element analysis of bone plates available for prophylactic internal fixation of the radial osteocutaneous donor site using the sheep tibia model

IntroductionThe strengthening effect of prophylactic internal fixation (PIF) with a bone plate at the radial osteocutaneous flap donor site has previously been demonstrated using the sheep tibia model of the human radius. This study investigated whether a finite element (FE) model could accurately represent this biomechanical model and whether stress or strain based failure criteria are most appropriate. MethodsAn FE model of an osteotomised sheep tibia bone was strengthened using 4 types of plates with unilocking or bicortical screw fixation. Torsion and 4-point bending simulations were performed. The maximum von Mises stresses and strain failure criteria were studied. ResultsThe strengthening effects when applying stress failure criteria [factor 1.76–4.57 bending and 1.33–1.80 torsion] were comparable to the sheep biomechanical model [factor 1.73–2.43 bending and 1.54–2.63 torsion]. The strongest construct was the straight 3.5mm stainless steel unilocking plate. Applying strain criteria the strongest construct was the straight 3.5mm stainless DCP plate with bicortical screw fixation. ConclusionsThe FE model was validated by comparison with the sheep tibia model. The complex biomechanics at the bone-screw interface require further investigation. This FE modelling technique may be applied to a model of the human radius and other sites.

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

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

The biomechanical effect of artificial and human bone density on stopping and stripping torque during screw insertion

Orthopedic surgeons apply torque to metal screws manually by “subjective feel” to obtain adequate fracture fixation, i.e. stopping torque, and attempt to avoid accidental over-tightening that leads to screw–bone interface failure, i.e. stripping torque. Few studies have quantified stripping torque in human bone, and only one older study from 1980 reported stopping/ stripping torque ratio. The present aim was to measure stopping and stripping torque of cortical and cancellous screws in artificial and human bone over a wide range of densities. Sawbone blocks were obtained having densities from 0.08 to 0.80g/cm3. Sixteen fresh-frozen human femurs of known standardized bone mineral density (sBMD) were also used. Using a torque screwdriver, 3.5-mm diameter cortical screws and 6.5-mm diameter cancellous screws were inserted for adequate tightening as determined subjectively by an orthopedic surgeon, i.e. stopping torque, and then further tightened until failure of the screw–bone interface, i.e. stripping torque. There were weak (R=0.25) to strong (R=0.99) linear correlations of absolute and normalized torque vs. density or sBMD. Maximum stopping torques normalized by screw thread area engaged by the host material were 15.2N/mm (cortical screws) and 13.4N/mm (cancellous screws) in sawbone blocks and 20.9N/mm (cortical screws) and 6.1N/mm (cancellous screws) in human femurs. Maximum stripping torques normalized by screw thread area engaged by the host material were 23.4N/mm (cortical screws) and 16.8N/mm (cancellous screws) in sawbone blocks and 29.3N/mm (cortical screws) and 8.3N/mm (cancellous screws) in human femurs. Combined average stopping/ stripping torque ratios were 80.8% (cortical screws) and 76.8% (cancellous screws) in sawbone blocks, as well as 66.6% (cortical screws) and 84.5% (cancellous screws) in human femurs. Surgeons should be aware of stripping torque limits for human femurs and monitor stopping torque during surgery. This is the first study of the effect of sawbone density or human bone sBMD on stopping and stripping torque.

Locking plate fixation for calcaneal fractures of Sanders type IV: a finite element analysis

Objective To evaluate the biomechanical performance of a calcaneal locking plate in fixation of calcaneal fractures of Sanders type IV through finite element（FE） analysis. Methods A healthy male volunteer was enrolled for this study who were 26 years old and 70 kg in weight. A 3D FE model of calcaneal fracture of Sanders type IV fixated with a ealcaneal locking plate （Synthes） was reconstructed using software Mimics 13.0 and Geomagic 10. 0 based on the 3D CT images of the volunteer＇s both feet. An axial load of 700 N was applied through the talus. The stress distribution and peak stress of fracture fragments and internal fixator, and the maximum stress point and maximum displacement area of the calcaneus and internal fixator as well, were analyzed. Results The lateral stresses on the calcaneus were concentrated on the 4 areas： the medial side of the subtalar joint （15. 224 MPa）, the bone-screw interface below the subtalar joint （13. 083 MPa）, the bone-screw interface beside the heel （9. 786 MPa） and the bone-screw interface beside the calcaneocuboid joint （7. 632 MPa）. The peek stress on the plate was about 260. 1 MPa, distributed on the fracture line between the posterior fragment and the median-anterior fragment. The maximal displacements occurred on the sustentaculum tali （0. 403 mm）, the interface between the posterior fragment and median fragment （0. 320 mm） and the interface between the anterior fragment and median fragment （0. 319 ram）. The maximal displacement of the plate-screw occurred on the distal side of the proximal screws, especially the one used to fixate the sustentaculum tali, about 0. 390 mm. Conclusions Since fixation with a locking plate for calcaneal fractures of Sanders type IV may lead to a high risk of traumatic arthritis, it is suggested that primary external fixation followed by subtalar fusion be a better choice or appropriate modifications be made of the current locking plate to achieve better mechanical performance. Key words: Calcaneal fracture; Finite element analysis; Computer simulation; Fracturefixation, internal

The impact of a distal expansion mechanism added to a standard pedicle screw on pullout resistance. A biomechanical study

Background contextSpinal deformity surgery in elderly patients is associated with an increased risk of implant loosening due to failure at the screw-bone interface. Several techniques can be used to increase the screw anchorage characteristics. Cement-augmented screw fixation was shown to be the most efficient method; however, this technique is associated with a risk of complications related to vertebral cement deposition and leakage. Hence, there is a need to further elaborate the alternative screw augmenting techniques to reduce the indications for bone cement. PurposeTo analyze surgical alternatives to cement augmentation, the present study sought to quantify the impact of a distal expansion mechanism added to a standard pedicle screw on an axial pullout resistance. Study designA biomechanical laboratory study on the uniaxial pullout resistance of a standard pedicle screw versus a customized pedicle screw with a distal expansion mechanism. MethodsA total of 40 vertebrae from seven fresh-frozen human specimens were harvested and subjected to a computed tomography scanning and an analysis of the bone mineral density (BMD). The vertebrae were instrumented with a standard 6.0-mm pedicle screw and a modified 6.0-mm pedicle screw with a distal expansion mechanism added. The actual working length of both screws inside the vertebrae was identical. The distal expansion mechanism made up one-fifth of the shaft length. The accuracy of the screw insertion was assessed using biplanar radiographs and by inspection. Analysis of resistance to pullout was performed by a coaxial alignment of the pedicle screws and attachment to an electromechanical testing machine. The pullout rate was 5 mm/min, and the load-displacement curve was recorded until the force of the pullout resistance peaked. The peak load-to-failure was measured in Newtons and reported as the ultimate failure load. With each test, the mode of failure was noted and analyzed descriptively. ResultsA total of 17 vertebrae with matched pairs of standard and expansion pedicle screws were eligible for the final statistical analysis. The BMD of the vertebrae tested was 0.67±0.19 g/cm³. The screw length was 50 mm, and the actual working length of both screws was 40.3±4.2 mm. The ultimate failure load of the standard screw was 773.8±529.4 N and that of the expansion screw was 910.3±488.3 N. Statistical analysis revealed a strong trend toward an increased failure load with the expansion screw (p=.06). The mean increase of the ultimate failure load was 136.5±350.4 N. Abrupt vertebral fracture at the vertebral body-pedicle junction and the pedicle occurred seven times with the expansion screw and only five times with the standard screw (p=.16). ConclusionsOur study indicates that adding a distal expansion mechanism to a standard pedicle screw increases the failure load by one-fifth. Modern expansion screws might offer an intermediate solution for the augmentation of screw-rod constructs in osteoporotic bone while reducing the need for cement-augmented screws and avoiding the related risks.

Biomechanical Analysis of Corrective Forces in Spinal Instrumentation for Scoliosis Treatment

Computer modeling and simulations to analyze correction forces at the bone-screw interface in scoliosis instrumentation. To derive the minimum corrective forces applied on vertebrae through pedicle screws to achieve desired scoliosis corrections and evaluate the actual bone-screw forces associated with 3 types of pedicle screws (monoaxial, polyaxial, and dorsoaxial). The optimum screw pattern has not been established in the literature. The final bone-screw forces in scoliosis instrumentation consist of "true corrective forces" (i.e., the minimum forces required to achieve the desired corrections without considering adequate rod seating at all pedicle screws) and "extra forces" (EF) (i.e., supplementary forces applied to ensure proper rod seating when the attachment of some screws is not in compliance with the attachment of their neighboring screws; they have no benefit to overall corrections). METHODS.: Using patient-specific computer models, true corrective forces were estimated for 10 spinal instrumentation cases. EF were computed by simulating the instrumentations of the 10 cases using respectively monoaxial, polyaxial, and dorsoaxial screws. The average true corrective forces were 50 ± 30 N. The average bone-screw forces were 229 ± 140 N, 141 ± 99 N, and 103 ± 42 N, respectively, for monoaxial, polyaxial, and dorsoaxial screws; the averages of the EF magnitudes were 205 ± 136 N, 125 ± 93 N, and 65 ± 39 N, respectively. Bone-screw forces to achieve desired corrections can be minimized. However, EF are inevitable to secure the locking of all screws. Higher EF were associated with pedicle screws, with less degrees of freedom for connecting screw body to rod, that is, monoaxial followed by polyaxial and then by dorsoaxial screws.

Biomechanical Comparison of Endplate Forces Generated by Uniaxial Screws and Monoaxial Pedicle Screws

Current surgical treatment of idiopathic scoliosis involves the use of various segmental instrumentation. Various pedicle screws have allowed for improved correction. Although monoaxial screws have improved rotational control compared with polyaxial screws, their use may increase screw-bone interface or vertebral endplate forces if not inserted in an exactly straight trajectory. Uniaxial screws potentially decrease these forces while retaining the advantages of monoaxial screws with respect to better rotational control. The purpose of this study was to compare the vertebral endplate forces with monoaxial or uniaxial screws when being reduced to a rod. Thirty-two plastic, surrogate T11 vertebrae were prepared with monoaxial (n=16) or uniaxial (n=16) screws. Screw angles relative to inferior vertebral endplates were assessed with lateral radiographs. The vertebrae were fixed to a load cell, and loads were measured as the screw was reduced to a rod. Monoaxial screws demonstrated a linear progression of endplate force with increasing screw angle. Uniaxial screws demonstrated minimal endplate force until approximately 20°, coinciding with screwhead excursion angle. As this maximum excursion angle was passed, uniaxial screws demonstrated a force slope similar to the monoaxial screws.The measured endplate forces should be equivalent to forces at the screw-bone interface. The reduced force with uniaxial screws is expected to have less cranial-caudal plow potential as the screw is coupled to a rod for deformity correction. This could have potential implications for screw failure, especially in less dense bone.

Catastrophic Failure After Open Reduction Internal Fixation of Femoral Neck Fractures With a Novel Locking Plate Implant

To determine if the use of a novel proximal femoral locking plate could reduce the incidence of femoral neck shortening and improve clinical outcomes after open reduction internal fixation (ORIF) for femoral neck fractures as compared with historical controls. Single surgeon, retrospective case-control study. Academic level I trauma center. Twenty-one femoral neck fractures treated with the posterolateral femoral locking plate (Synthes, Inc, Paoli, PA) were eligible for inclusion. Eighteen met inclusion/exclusion criteria with a mean follow-up of 16 months. ORIF of femoral neck fracture with the posterolateral femoral locking plate. This consists of a side plate with multiple locking screws directed into the femoral head at converging/diverging angles and a single shaft screw. Intraoperative compression was achieved with partially threaded screws before locking screw insertion. Maintenance of reduction was assessed by comparing immediate postoperative and final follow-up radiographs. Clinical outcome was assessed with Harris Hip Scores after 1 year. Complications and secondary operations were noted. Seven (36.8%) of 18 patients experienced catastrophic failure. Five of these patients required total hip replacement, whereas the remaining 2 died before further treatment. The remaining 11 patients (61.1%) achieved bony union; the average displacement of the center of the head did not differ when compared with historical controls (0.78 mm inferiorly, 1.62 mm medially, and 2.4 degrees of increased varus vs. 0.86 mm, 1.23 mm, and 0.6 degree). Complications in this group include 1 instance of screw fracture, 2 total hip replacements, and a peri-implant subtrochanteric femur fracture. The average patient age and proportion of displaced fractures did not differ between the historical control and experimental groups. Fracture displacement was strongly associated with catastrophic failure in the experimental group only. Average Harris Hip Scores was significantly worse compared with that of historical controls (67.9 vs. 84.7, P = 0.05). ORIF of femoral neck fractures using a locking plate construct yielded unacceptably poor outcomes in this patient population. We hypothesize that the stiffness of this construct prevents any fracture site micromotion, placing the mechanical burden on the implant, which can result in failure at the bone-screw interface or fatigue failure of the implant itself.

Design and biomechanical study of bone cement injectable canulated pedicle screw

Objective To investigate the design and mechanical properties of bone cement injectable canulated pedicle screw (CICPS) so as to provide a safe and effective internal fixation for osteoporotic spinal disorder. Methods ( 1 ) Bone cement injection test was performed in vitro,and within osteoporotic cancellous bone models and osteoporotic vertebrae respectively.The distribution of bone cement and screw-bone interface were observed by X-ray films and CT.(2) Ten CICPSs already injected with bone cement and ten conventional pedicle screws were respectively examined by shear strength test.(3) CICPS in the cancellous bone models was augmented with 2-3 ml of bone cement.Then,the maximum axial pull-out strength of the CICPS was measured and were compared with that of conventional screws. Results Bone cement overflowed from each side hole of the CICPS and distributed only around the front of screws in an even and extensive way.Also,no bone cement leakage was founded.The mean shear load of CICPS and conventional screws was ( 10 600.8 ± 360.1 ) N and ( 15 458.1 ± 31 1.4) N respectively ( P ＜0.05 ).The mean maximum axial force at pull-out was ( 209.3 ± 13.3 ) N for the CICPS and ( 27.0 ± 5.0) N for the conventional screws ( P ＜ 0.05 ). Conclusions By dispersing bone cement uniformly,CICPS reduces the risk of cement leakage and significantly improves the pullout strength of screws in osteoporotic vertebrae.CICPS with good efficacy and safety provides theoretic basis for its clinical application. Key words: Osteoporosis; Biomechanics; Poymethyl mathacrylate; Pedicle screw

Biomechanical measurements of cortical screw stripping torque in human versus artificial femurs

Femur fracture plates are applied using cortical bone screws. Surgeons do this manually by subjective 'feel' without monitoring torque. Few studies have quantified stripping torque in human bone. No studies have measured stripping torque in the artificial bones from Sawbones (Vashon, WA, USA) that are frequently used in biomechanical studies. The present aim was to measure stripping torque of cortical screws in human versus artificial femurs. Sixteen fresh-frozen human femurs and eight artificial femurs were used. Using a digital torque screwdriver, each femur had a 3.5-mm diameter uni-cortical screw manually inserted into the anterior midshaft until failure of the screw-bone interface. Results were normalized by cortical thickness and the screw-bone interfacial area. There were no statistical differences in human versus artificial data, respectively, for stripping torque (1741 +/- 442 N.mm, 2012 +/- 176 N.mm, p = 0.11), stripping torque/thickness (313 +/- 59 N, 305 +/- 30 N, p = 0.74), and stripping torque area (28.5 +/- 5.3 N/mm, 27.8 +/- 2.8 N/mm, p = 0.74). Artificial unicortical thickness (6.6 + 0.3 mm) was greater than human thickness (5.6 +/- 1.1 mm) (p = 0.02). For human specimens, there was a moderate linear correlation of absolute and normalized stripping torque versus standardized bone mineral density (R > or = 0.32) and clinical T-score (R = 0.29), but not with age (R < or = 0.29). Surgeons should be aware of the stripping torque limits for human femurs and potentially take steps to monitor these values during surgery. The artificial femurs being increasingly used in research accurately replicate human cortical properties during screw insertion. To date, this is the first series of human femurs evaluated for cortical screw stripping.

Oblique Lumbar Interbody Fixation

In vitro spine biomechanics. To determine the biomechanical properties of oblique lumbar interbody fixation (OLIF) in human cadaveric spines. OLIF has been used for stabilization of degenerative spondylolisthesis at the lumbosacral junction. Biomechanical properties and mode of failure of OLIF as a standalone device for motion segments without sagittal deformity has not yet been investigated. We hypothesize that the biomechanical properties of OLIF will be comparable with the contemporary standard of pedicle screw (PS) fixation. Randomly matched motion segments from L1 to L5 were allocated into 2 groups: (A) OLIF (group 1, n=5) or (B) PS (group 2, n=5). The intact and instrumented motion segments with and without anterior interbody graft were first tested under a combination of 200N axial compression and 5 Nm bending moments in flexion-extension and in lateral bending. Range of motion (ROM) and neutral zone were determined and compared between intact, OLIF and PS. A final load to failure test was carried out for each motion segment in either flexion or extension. OLIF resulted in reduction of flexion-extension ROM to 36%±14% of intact whereas PS resulted in reduction to 27%±22% of intact. The reduction of lateral bending ROM were 32%±13% and 32%±24% of intact with OLIF and PS. There were no significant difference in ROM between OLIF and PS (P=0.39). The mean failure loads with OLIF and PS in flexion were 1284 and 1158N, and in extension were 1879 and 1934N, respectively. Failure occurred at the ventral screw bone interface without pedicle fracture. These results indicate that stiffness and load to failure of the OLIF is comparable with PS fixation. OLIF failure occurred ventrally through the anterior cortical rim without concomitant pedicle fracture.

Late infections after dynamic stabilization of the lumbar spine with Dynesys

Dynamic stabilization of the spine was developed as an alternative to rigid fusion in chronic back pain to reduce the risk of adjacent segment degeneration. Dynamic neutralization system (Dynesys, Zimmer CH) is one of the most popular systems available, but some midterm studies show revision rates as high as 30 %. Some late infectious complications in our patients prompted us to review them systematically. Propionibacterium recently has been shown to cause subtle infections of prosthetic material. Here, we report on a consecutive series of 50 Dynesys implants. In a median follow-up of 51 months (range 0-91), we identified 12 infectious and 11 non-infectious complications necessitating reoperation or removal of the implant in 17 patients. Material infections occurred after a median of 52 months (2-77) and were due to Propionibacterium alone (n = 4) or in combination (n = 3) in seven out of 11 patients. Clinical presentation combines new or increasing pain associated with signs of screw loosening on conventional X-rays; however, as many as 73.5 % of patients present some degree of screw loosening without being at all symptomatic of infection. The high rate of late infections with low-grade germs and the frequency of screw loosening signs made us suspect a lack of integration at the bone-screw interface. Surgeons should be suspicious if the patient presents a combination of new or increasing pain and signs of screw loosening, and aggressive revision is recommended in these cases.

Does screw–bone interface modelling matter in finite element analyses?

The effect of screw–bone interface modelling strategies was evaluated in the setting of a tibial mid-shaft fracture stabilised using locking plates. Three interface models were examined: fully bonded interface; screw with sliding contact with bone; and screw with sliding contact with bone in an undersized pilot hole. For the simulation of the last interface condition we used a novel thermal expansion approach to generate the pre-stress that the bone would be exposed to during screw insertion. The study finds that the global load-deformation response is not influenced by the interface modelling approach employed; the deformation varied by less than 1% between different interaction models. However, interface modelling is found to have a considerable impact on the local stress–strain environment within the bone in the vicinity of the screws. Frictional and tied representations did not have significantly different peak strain values (<5% difference); the frictional interface had higher peak compressive strains while the tied interface had higher tensile strains. The undersized pilot hole simulation produced the largest strains. The peak minimum principal strains for the frictional interface were 26% of those for the undersized pilot hole simulation at a load of 770 N. It is concluded that the commonly used tie constraint can be used effectively when the only interest is the global load-deformation behaviour. Different contact interface models, however, alter the mechanical response around screw holes leading to different predictions for screw loosening, bone damage and stress shielding.

A Study of Low Elastic Modulus Expandable Pedicle Screws in Osteoporotic Sheep

Low elastic modulus expandable pedicle screw (L-EPS) was inserted into osteoporotic sheep. Biomechanical tests, micro-CT analysis and histologic observation were performed. To find out whether the L-EPS can further improve fixation strength compared to the expandable pedicle screws (EPS) in osteoporotic sheep spine. The screw-bone interface is also detected by micro-computed tomography (CT) and histologic techniques. There are some studies on EPS in vivo; however, no earlier study has focused on the elastic modulus of EPS, especially in osteoporosis. Twelve months after ovariectomy, bilateral pedicles of lumbar vertebrae (L1 to L5) of 4 female sheep were fixed with EPSs. The L-EPS and EPS were randomly placed into each pedicle and then were expanded. Six months later, the sheep were sacrificed and biomechanical tests, micro-CT analysis, and histologic observation were conducted on the isolated specimen vertebrae. Twelve months after ovariectomy, animal model of osteoporosis was established successfully. The axial pull-out strength in L-EPS group was significantly enhanced compared with that in the EPS group (P < 0.05). Micro-CT reconstruction and analysis showed that there was more bone around the L-EPS group compared with those in EPS group (P < 0.05), meanwhile the more homogeneous bone formation distribution around the screws was found in the L-EPS group. Histologic observation showed that newly formed bone extended along the expandable fissures and grew into the center of EPS; meanwhile, the more direct contact and the less fibrous tissue on the screw-bone interface were observed in the L-EPS group. The L-EPS can further improve the biomechanical fixation strength of EPS in the osteoporotic sheep. The screw elastic matching with surrounding bone is helpful to distribute stress uniformly, relieve the stress shielding effect, and strengthen the screw-bone interface. Although the experience with the L-EPS is very limited and preliminary, results to date indicate that it is of value in treating patients with osteoporosis and warrants further study.

Posterior lumbar fusion by peek rods in degenerative spine: preliminary report on 30 cases

Pre-curved peek rods to support posterior lumbar fusion have been available in the market since 4 years. Potential advantages using this new technology are increased load sharing on the anterior column promoting interbody fusion, reduced stress on bone-screw interface decreasing the rate of screw mobilization and, in the long term, reduced incidence of adjacent level disc degeneration. The authors retrospectively reviewed 30 cases in which posterior fusion was supported by peek rods, analyzing early complications, rate of fusion and clinical outcome. At an average follow-up of 18 months, both clinical and radiographic results were satisfactory with only one case requiring surgical revision for a mechanical complication. The semi-rigid systems can now be considered a viable option in the lumbar degenerative disease, although clinical evaluations are necessary in the longer term.

Experimental in vivo acute and chronic biomechanical and histomorphometrical comparison of self-drilling and self-tapping anterior cervical screws

Self-drilling screws (SDS) and self-tapping screws (STS) allow for quicker bone insertion and are associated with increased anchorage. This is an experimental in vivo comparison of anterior cervical SDS and STS in the post-insertion acute and chronic phases. Thirty C2-C6 vertebrae from six Santa Inês hair sheep were used. Each screw design was randomly assigned to five of each spinal level. Insertion torque was measured using a torque device. Three animals were killed in each phase. Vertebrae were randomly assigned to pullout tests and histomorphometrical bone-screw interface evaluation (percent screw-bone contact and bone density inside and outside the threaded area). Statistical significance was set at P < 0.05. SDS insertion torque was greater than STS (P = 0.0001). SDS pullout strength was significantly greater than STS in the acute and chronic phases (P = 0.0001, 0.0003, respectively). SDS percent screw-bone contact and inside area bone density were significantly greater in both phases. No outside area bone density differences were observed in either phase. SDS had higher insertion torque and better anchorage than STS in both phases. SDS percent bone-screw contact and inside area bone density were higher in both phases.