Factor analysis of the effectiveness of transfixation and rod characteristics on the TSRH screw-rod instrumentation

The effects of Texas Scottish Rite Hospital (TSRH) hardware parameters (rod length and diameter and cross-link) and their interaction on the stiffness of the TSRH pedicle screw-rod construct were evaluated. Four TSRH screws were assembled in pairs to two polymethyl-methacrylate blocks to resemble a one-level or more corpectomy model and the construct underwent nondestructive torsional, extension, and flexion loading. In every loading test, each construct was tested using TSRH rods of different lengths (10, 15, and 20 cm) and diameters (4.9 and 6.5 mm) and different cross-links (TSRH and two new types made for this experiment). We compared the stiffness of the construct without cross-linking with that with single or double TSRH cross-linking, or either the closed new-type cross-link (closed NTC) or the open new-type cross-link (open NTC) using factor analysis. There was no axial slipping of one rod versus the other up to a force of 100 kg. The stiffness of the construct in all three loading modes increased as the rod length decreased, the rod diameter increased, and the construct was augmented with a cross-link. The closed NTC provided the greatest stiffness and the single TSRH provided the least stiffness. Unaugmented 10-cm-long rods showed two or three times more torsional stiffness than did that of the longer unaugmented rods independent of rod diameter. In addition, the closed NTC offered the maximal increase in flexion stiffness of the construct with thick rods and 10-, 15-, and 20-cm-long rods at a maximum of 40%, 27%, and 30%, respectively. This rigid closed NTC increased the extension stiffness of the same construct with 10- and 15-cm-long rods at 40% and 6%, respectively, whereas it had no influence on the extension stiffness of 20-cm-long rods.     

Segmental pedicle screw fixation or cross-links in multilevel lumbar constructs. a biomechanical analysis.

BACKGROUND CONTEXT: The placement of segmental pedicle screws and cross-links in short segment posterior pedicle screw constructs has been shown to increase the construct stiffness in some planes. To date, no studies have looked at the contributions of segmental pedicle screw and cross-link placement in longer constructs. PURPOSE: To evaluate the influence of segmental pedicle screw and/or cross-link placement on flexion/extension, lateral bending and axial torsion stiffness in two- and three-level posterior pedicle screw fixation constructs. STUDY DESIGN/SETTING: An in vitro biomechanical analysis of two- and three-level posterior pedicle screw constructs with and without segmental fixation and/or cross-links was performed using calf lumbar spines. Stiffness of the constructs was compared. METHODS: Six calf lumbar specimens were used to test stiffness in one-, two- and three-level posterior pedicle screw fixation constructs in 12 configurations. A custom-made, four-axis spine simulator applied pure cyclical (+/-5 Nm) flexion/extension, lateral bending and axial torsion moments at 0.1 Hz under a constant 50-N axial compressive load. The stiffness of each construct was calculated about each axis of rotation. Data were analyzed using nonparametric techniques with statistical significance determined at alpha less than .05. RESULTS: The stiffness of the instrumented spines were significantly greater than the noninstrumented intact spines in all loading conditions for one-, two- and three-level constructs. There were no significant changes in flexion/extension stiffness with the addition of either the cross-links or the segmental pedicle screws. In lateral bending, the addition of segmental pedicle screws significantly increased the stiffness in the two- and three-level constructs. The addition of two cross-links increased lateral bending stiffness in the longer three-level constructs, with little change in the two-level constructs. In axial torsion, the progressive addition of cross-links showed a tendency toward increased stiffness in both the two- and three-level constructs. Segmental pedicle screws further increased torsional stiffness of the longer, three-level constructs. CONCLUSIONS: As the use of segmental spinal instrumentation progresses from one to two and three levels, the contribution of cross-links and segmental pedicle screws to the overall construct stiffness increases.     

Torsional stability of cross-link configurations: a biomechanical analysis.

BACKGROUND CONTEXT: Cross-link systems have been used to augment segmental spinal instrumentation since the earliest introduction of these fixation systems. Although transverse cross-links have little impact on sagittal motion of spinal constructs, cross-linkage does affect torsional rigidity. Despite the wide variety of cross-link designs, almost all have been configured as transverse devices. The relative mechanical benefit of different cross-link configurations is not known. PURPOSE: The purpose of this study was to compare the torsional stability of three different cross-link configurations. STUDY DESIGN: Biomechanical analysis of segmental instrumentation constructs using porcine spines. METHODS: Thoracic porcine spines (T4 to T10) were instrumented with 6.5-mm conical pedicle screws and 7.0-mm connecting rods from T5 to T9. Terminal vertebrae were embedded in polymethylmethacrylate (PMMA) after a T7 corpectomy. Four cross-link configurations were tested in a randomized manner: Un-cross-linked Control (CONT); Transverse Rod-Rod (RR); Transverse Screw-Screw (SS); and Diagonal Screw-Screw (DX) Cross-links. The specimens were rotated to 3 Nm at a rate of 0.2 degrees/s and cycled six times with data acquisition over the final two cycles. Stiffness, rotation, and energy data were normalized to each control. A Newman-Keuls repeated measures analysis of variance was used to infer significant differences. RESULTS: Diagonal cross-link configurations provided the most rigid construct. Transverse cross-links did not significantly change torsional behavior compared with the unlinked control. Rotation and energy expended were not significantly greater torsional stiffness compared with other constructs tested (p<.01). CONCLUSIONS: The diagonal cross-link configuration provided increased torsional stiffness as compared with unlinked or transverse configurations. This observation should be considered in future cross-link designs.     

Torsional rigidity of scoliosis constructs

STUDY DESIGN: A biomechanical study of the rigidity of various scoliosis constructs instrumented with and without caudal pedicle screw anchors and with none, one, or two cross-link devices. OBJECTIVES: To determine whether the increased torsional rigidity provided by distal pedicle screw fixation might make cross-linking unnecessary. SUMMARY OF BACKGROUND DATA: Pedicle screws and cross-linking devices have been shown to increase the structural rigidity of spinal constructs. Their relative contributions to scoliosis construct rigidity has not been determined. METHODS: "Short" (T2-T11) and "long" (T2-L3) scoliosis constructs were mounted on an industrially fabricated spine model and tested in a hydraulic testing machine. Four different short and four different long constructs were tested: hooks only, hooks with concave side thoracic sublaminar wires, hooks with distal pedicle screw anchors, and hooks, distal pedicle screw anchors, and concave thoracic sublaminar wires. There were four iterations for each construct tested: no cross-links, one superior cross-link at T4-T5, one inferior cross-link at T9-T10, and two cross-links. Torsional rigidity was tested by applying a rotational torque at T2. Vertebral body motion was recorded with a three-dimensional video analysis system. RESULTS: Constructs with distal pedicle screws were statistically more rigid in torsion than those with hooks as distal anchors. The additional torsional rigidity from one or more cross-links was negligible compared with that provided by pedicle screws. CONCLUSIONS: With pedicle screws as distal anchors in scoliosis constructs, cross-linking with one or two devices adds very little additional rotational stiffness and may be unnecessary in many cases.     

Biomechanical evaluation of diagonal fixation in pedicle screw instrumentation.

STUDY DESIGN: Flexibility tests and finite element analyses were performed for the biomechanical evaluation of diagonal transfixation in pedicle screw instrumentation. OBJECTIVE: To assess the biomechanical advantages of diagonal transfixation compared with conventional horizontal transfixation. SUMMARY AND BACKGROUND DATA: A few pedicle screw instrumentation systems allow the use of cross-links in the diagonal direction. Such a diagonal transfixation is anticipated to improve the surgical construct stability, but its biomechanical qualities have not been completely evaluated. METHODS: Flexibility tests were performed on 10 calf lumbar spines (L2-L5). Specimens were subjected to pure moments up to 8.2 Nm in flexion, extension, lateral bending, and extension while the resulting movements of L3 and L4 were measured by a three-dimensional motion analysis system. The tested cases included (1) intact, (2) pedicle screw fixation without transfixation after total removal of the L3-L4 disc, (3) pedicle screw fixation with diagonal transfixation, and (4) pedicle screw fixation with horizontal transfixation. Three-dimensional finite element models of the tested surgical constructs were also developed by use of three-dimensional beam elements to investigate the effect of diagonal transfixation and horizontal transfixation on the construct stability and the corresponding stress changes in the screws. RESULTS: When compared with no transfixation, horizontal transfixation significantly improved the lateral bending and axial rotation stability by 15.7% and 13.9%, respectively, but there was no improvement of stability in flexion and extension. By contrast, diagonal transfixation significantly improved the flexion and extension stability by 12% and 10.7%, respectively, but not the lateral bending and axial rotation stability in comparison with no transfixation. Comparison between horizontal transfixation and diagonal transfixation showed that the stabilizing effect of diagonal transfixation was greater in flexion and extension (13% and 11%, P < 0.01) than that of horizontal transfixation but smaller in lateral bending (11%, P < 0.05) and axial rotation (6.6%, P > 0.1). Finite element model predictions of the motion changes were similar to the changes observed in flexibility tests. In horizontal transfixation, the load changes, compared with no transfixion, were a 0.02% increase in flexion-extension, a 27.5% increase in lateral bending, and a 58% decrease in axial rotation, and the magnitudes of the moments applied on both the right and left pedicle screws were identical. However, when diagonal transfixation was achieved by connecting the left superior screw and the right inferior screw, the loads in the left screw were increased by 11.5% in flexion-extension, 43.6% in lateral bending, and 7.9% in axial rotation, whereas the loads in the right screw were decreased by 10.9% in flexion-extension, increased by 0.06% in lateral bending, and decreased by 18.1% in axial rotation. CONCLUSIONS: The results of this study showed that diagonal transfixation provides more rigid fixation in flexion and extension but less in lateral bending and axial rotation in comparison with horizontal transfixation. Furthermore, greater stresses in the pedicle screws were predicted in the diagonal transfixation model. These limitations of diagonal transfixation should be considered carefully for clinical application.     

Comparison of lumbosacral fixation devices

Fusion of L4 and L5 to the sacrum has a high incidence of success. Using conventional methods, nonunion is common when long scoliosis fusions are extended to the sacrum. Three methods of instrumentation for fusing the lumbar spine to the sacrum were compared on a spine simulator test stand. Harrington distraction rods from the sacral ala to L1, Luque rods from L1 to the sacrum, and Harrington compression rods from L1 to the sacrum were tested. The use of a spine instrumentation test stand discounted biologic variation in spinal structure. Sequential loading of each test stand-instrumentation construct in torsion, flexion, extension, and lateral bending gave stiffness constants (Ks) for each test mode. Test values had reproducibility of greater than 94%. Ks illustrates the inability of Harrington distraction rods to the sacrum to resist flexion and torsion, but the ability to resist lateral bend and extension. Harrington compression rod and Luque rod constructs have equivalent stiffness in flexion and torsion. Harrington compression rods efficiently resist extension, and Luque rods resist lateral bending. Harrington distraction rods have limited use in lumbosacral junction fixation other than to correct and resist lateral bending.     

Is galvanic corrosion between titanium alloy and stainless steel spinal implants a clinical concern?

BACKGROUND CONTEXT: Surgeons are hesitant to mix components made of differing metal classes for fear of galvanic corrosion complications. However, in vitro studies have failed to show a significant potential for galvanic corrosion between titanium and stainless steel, the two primary metallic alloys used for spinal implants. Galvanic corrosion resulting from metal mixing has not been described in the literature for spinal implant systems. PURPOSE: To determine whether galvanic potential significantly affects in vitro corrosion of titanium and stainless steel spinal implant components during cyclical compression bending. STUDY DESIGN/SETTING: Bilateral spinal implant constructs consisting of pedicle screws, slotted connectors, 6.35-mm diameter rods and a transverse rod connector assembled in polyethylene test blocks were tested in vitro. Two constructs had stainless steel rods with mixed stainless steel (SS-SS) and titanium (SS-Ti) components, and two constructs had titanium rods with mixed stainless steel (Ti-SS) and titanium (Ti-Ti) components. METHODS: Each construct was immersed in phosphate-buffered saline (pH 7.4) at 37 C and tested in cyclic compression bending using a sinusoidal load-controlling function with a peak load of 300 N and a frequency of 5 Hz until a level of 5 million cycles was reached. The samples were then removed and analyzed visually for evidence of corrosion. In addition, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to evaluate the extent of corrosion at the interconnections. RESULTS: None of the constructs failed during testing. Gross observation of the implant components after disassembly revealed that no corrosion had occurred on the surface of the implants that had not been in contact with another component. The Ti-Ti interfaces showed some minor signs of corrosion only detectable using SEM and EDS. The greatest amount of corrosion occurred at the SS-SS interfaces and was qualitatively less at the SS-Ti and Ti-SS interfaces. CONCLUSIONS: The results from this study indicate that when loaded dynamically in saline, stainless steel implant components have a greater susceptibility to corrosion than titanium. Furthermore, the galvanic potential between the dissimilar metals does not cause a discernible effect on the corrosion of either. Although the mixture of titanium alloy with stainless steel is not advocated, the results of this study suggest that galvanic corrosion is less pronounced in SS-Ti mixed interfaces than in all stainless steel constructs.     

A comparative biomechanical study of spinal fixation using Cotrel-Dubousset instrumentation

A biomechanical study was performed comparing the stiffness and stability of Cotrel-Dubousset (CD) spinal instrumentation with that of segmentally wired Harrington distraction rods and segmentally wired Luque rods under conditions of single-level instability. The axial and torsional stiffness coefficients of each system were determined on a customized geometric spine simulator fashioned from stainless steel. The relative stability of each instrumentation system was then compared by mounting the fixation systems on bovine thoracic spines from 12-week-old calves, destabilized by anterior vertebrectomy to create simulated two column instability. Thirteen spines were tested. Each specimen was tested under axial and torsional loading conditions while monitoring with a personal computer-based data acquisition system was performed. The stability of first- and second-level CD instrumentation was tested on the bovine specimens. First-level CD instrumentation involved double-hook fixation one level above and below the level of instability. Second-level CD instrumentation involved fixation two levels above and below the level of instability without fixation at the intermediate level. In axial loading, double-level wired Harrington distraction rods, double-level wired Luque rods, and first-level CD rods were 26.5%, 18.4%, and 21.5%, respectively, as stable as second-level CD instrumentation. In torsion, double-level Harrington, double-level Luque, and second-level CD rods were 13%, 64%, and 34%, respectively, as stable as first level CD instrumentation. Locking hooks, double-hook configurations, and stabilizing transverse traction devices of the CD contributed to its greater stability.(ABSTRACT TRUNCATED AT 250 WORDS)     

Which posterior instrumentation is better for two-level anterior lumbar interbody fusion: translaminar facet screw or pedicle screw?

Purpose

To determine whether translaminar facet screws can provide stability equivalent to pedicle screws and whether the two posterior instrumentations have the same influence on the adjacent segments in two-level anterior lumbar interbody fusion.

Methods

In a biomechanical study conducted, we used 12 fresh human lumbar spines and tested an intact spine with a stand-alone two-level anterior lumbar interbody fusion and anterior fusion augmented with pedicle screws or translaminar facet screws, under 400 N compressive preloads and 7.5 N m moments in flexion, extension, axial rotation and lateral bending, and measured the stiffness of the operated level, range of motion and intradiscal pressure at the adjacent levels.

Results

We found a significant increase in the stiffness of the segments operated, range of motion and intradiscal pressure at the adjacent superior segment in the stand-alone two-level anterior lumbar interbody fusion during flexion, axial rotation and lateral bending, but a decrease in extension, when compared with the intact spine. The stiffness of operated segments, range of motion and intradiscal pressure in the adjacent segment are significantly higher in the two-level anterior lumbar interbody fusion augmented with posterior instrumentation than in the stand-alone two-level anterior lumbar interbody fusion. There was no significant difference between the two augmented constructs except that, at the adjacent superior segment, the intradiscal pressure was more in the construction augmented with a pedicle screw than with a translaminar facet screw in flexion.

Conclusions

Translaminar facet screws can provide stability equivalent to pedicle screws, but their influence on the adjacent segments is relatively lower; therefore, we suggest that translaminar facet screws be the choice in the optimal posterior instrumentation in a two-level anterior lumbar interbody fusion.     

Biomechanical study of lumbar pedicle screws in a corpectomy model assessing significance of screw height

BACKGROUND: We tested the hypothesis that a pedicle screw construct's height is an important factor in strengthening a screw-rod system. METHODS: Six corpectomy constructs were made, each using two ultra-high-molecular-weight polyethylene blocks, 6.5-mm pedicle screws, and two 6.35-mm rods. Pedicle screws were placed at +10-, +5-, 0-, and -5-mm depths in relation to the dorsal surface of the corpectomy model. Nondestructive testing was performed in flexion/extension and in torsion. RESULTS: For all modes tested, the screw-rod constructs continued to increase in stiffness as the height of the construct was lowered, and this was statistically significant at all heights tested (P < 0.001). The stiffness increased 232% when comparing flexion at +10 and -5 mm and increased 231% in extension from +10 to -5 mm. The torsional stiffness increased 171% when comparing +10 and -5 mm. CONCLUSIONS: Thus, lower-profile instrumentation systems should be used to take advantage of this by decreasing the size and bulkiness of the implants while increasing the strength of the construct.     

A comparison of magnetic and radiographic imaging artifact after using three types of metal rods: stainless steel,titanium, and vitallium

Background contextAfter spinal fusion surgery, postoperative management often includes imaging with either computed tomography (CT) or magnetic resonance imaging (MRI) to assess the spinal canal and nerve roots. The metallic implants used in the fusion can cause artifact that interferes with this imaging, reducing their diagnostic value. Stainless steel is known to produce large amounts of artifact, whereas titanium is known to produce significantly less. Other alloys such as vitallium are now being used in spinal implants, but their comparison to titanium and stainless steel has not been well documented in the orthopedic literature. Titanium is a desirable metal because of its light weight and lower production of artifact on imaging, although it is not as stiff as stainless steel. Vitallium is proposed as a replacement for titanium because it has stiffness similar to stainless steel, while still being as light as titanium.PurposeThe purpose of this study was to compare the amount of artifact produced on MRI and CT by three types of spinal implants: stainless steel, titanium, and vitallium.Study designA prospective experimental design was used to compare three types of spinal implants used in posterior spinal fusion surgery.Outcome measuresThe resulting images were evaluated by a radiologist to measure the amount of artifact (in millimeters) and by an orthopedic surgeon to assess the diagnostic quality (on a Likert scale).MethodsA porcine torso was used for repeated MRI and CT scans before and after implantation with pedicle screws and rods made of the three metals being studied.ResultsImages produced after the insertion of vitallium rods and titanium screws as well as those with titanium rods and screws were found to have less artifact and a better overall diagnostic quality than those produced with stainless steel implants. Overall, there was not a difference between the amount of artifact in the spinal images with vitallium and titanium rods, with the exception of a few trials that showed small but statistically significant differences between the two metals, where titanium had slightly better images.ConclusionsIf vitallium rods are used in posterior spinal surgery in place of implants made of titanium or stainless steel, any postoperative imaging of the spine using MRI or CT should have amounts of artifact that are similar to titanium and better than stainless steel.     

双节段后路腰椎椎体间融合术单侧椎弓根钉固定的生物力学稳定性

目的 分析对模拟双节段腰椎后路椎体间融合术(PLIF)采用单侧椎弓根钉固定(单侧固定)的生物力学稳定性.方法 将6具新鲜成人尸体腰椎标本(L2～S2)分别制备成L4～S1的PLIF模型,应用MTS 858实验机模拟产生屈伸、侧弯、轴向旋转,并按初始状态、单侧不稳、单侧不稳-单侧固定、双侧不稳-单侧固定、双侧不稳-双侧固定、双侧不稳的顺序进行测试,动态摄取记录各个节段角位移运动范围(ROM)与中性区值(NZ).结果 单侧不稳-单侧固定屈伸、侧弯、轴向旋转方向ROM值依次为2.53±1.12、4.03±2.19、2.78±1.00,NZ值依次为1.14±0.70、1.96±1.13、1.28±0.71,均显著小于初始状态(P<0.05),相比双侧不稳-双侧固定,各方向ROM与NZ值分别增加60.13%与17.52%、315.46%与243.86%、8.17%与6.20%,但差异无统计学意义(P>0.05).双侧不稳-单侧固定侧弯与旋转状态ROM与NZ值较双侧不稳-双侧固定显著增加(P<0.05).结论 单侧固定对人腰椎标本模拟双节段单侧PLIF可提供与双侧固定相似的生物力学稳定性,而对于模拟双节段双侧PLIF则单侧固定在大多数三维运动方向上不能提供足够的力学稳定性.

Abstract：

Objective To analyze the biomechanical efficacy of unilateral pedicle screw fixation on human cadaveric lumbar spine model simulated by two-level posterior lumbar interbody fusion (PLIF). Methods Six fresh-frozen adult human cadaveric lumbar spine motion segments (L2-S2) were simulated to unilateral/bilateral L4-S1 PLIF constructs augmented by unilateral/bilateral pedicle screw fixation sequentially and respectively. All configurations were tested by MTS 858 in the following sequential construct order: the intact, UI (unilateral instability), UIUF1C (unilateral instability via unilateral pedicle screw fixation plus one cage) , BIUF1C (bilateral instability via unilateral pedicle screw fixation plus one cage) , BIBF1C (bilateral instability via bilateral pedicle screw fixation plus one cage) and BI (bilateral instability without pedicle screw and cage). Each specimen was nondestructively tested in flexion/extension, lateral performed between different simulated constructs with One Way of ANOVA and Post hoc LSD tests. Results BIBF1C had the lowest ROM and NZ of L4-S1 fusion segments in all loading models, which were significantly lower than those of any uninstmmented construct (the intact, UI and BI) (P < 0. 05). In flexion/extension, lateral bending, and axial rotation, the ROM of UIUF1C was respectively 2.53 ± 1. 12, 4.03 ± 2. 19, 2. 78 ±1.00 and the NZ of UIUF1C was respectively 1.14 ±0.70, 1.96 ±1. 13, 1.28 ±0.71, which were significantly lower than those of the intact (P <0. 05). Compared to BIBF1C, the ROM and NZ were respectively increased 60.13% and 17.52% in flexion/extension, 315.46% and 243.86% in lateral bending, 8. 17% and 6. 20% in axial rotation, however, there were no significant differences between these two constructs (P > 0. 05). In lateral-bending and axial rotation, the ROM and NZ of BIUF1C were significantly higher than those of BIBF1C (P < 0. 05). In flexion/extension, the ROM and NZ of BIUF1C were higher than those of BIBF1C but there were no significant differences (P >0. 05). Compared to the intact, BIUF1C had lower ROM and NZ except for higher NZ in axial rotation, and there were significant differences only in flexion/extension (P < 0. 05). Conclusions All tested two-level unilateral fixation on simulated human cadaveric model with unilateral PLIF can achieve similar initial biomechanical stability in comparison with two-level bilateral pedicle screw fixation. However in most test modes, two-level unilateral pedicle screw fixation on simulated human cadaveric model with bilateral PLIF can not achieve enough biomechanical efficacy in comparison with two-level bilateral pedicle screw fixation.     

Texas Scottish Rite Hospital rod instrumentation for thoracic and lumbar spine trauma

The authors present their experience with 28 patients who had incurred unstable thoracic or lumbar spine fractures and who were intraoperatively stabilized with the Texas Scottish Rite Hospital (TSRH) universal instrumentation system. These patients were treated over a 1-year period and reflect an evolving insight into the treatment of thoracic and lumbar spine trauma with universal instrumentation. The TSRH instrumentation system appears equivalent to the more established Cotrel-Dubousset system in most respects. The construct design of the TSRH system facilitates the safe application of a rigid spinal implant. No cases of instability or pseudoarthrosis were observed during an average follow-up period of 9 months, (minimum 3 months). As the surgical treatment plan evolved, shorter and more compact constructs were increasingly utilized. There were no cases of instrumentation failure, regardless of the number of spinal levels fused or the number of levels instrumented. The value of using short rods when possible is emphasized: they may decrease the incidence of delayed instability and discomfort related to loosening at the hook/bone interface compared to that observed when long-rod systems are used in association with short spine fusions causing a fusion/instrumentation mismatch.     

Mechanical consequences of rod contouring and residual scoliosis in sublaminar segmental instrumentation

The mechanical performance of contoured Luque rods in a neuromuscular model of spine deformity was examined to define an upper limit of deformity above which rod stresses would exceed the endurance limit for 316L stainless steel and therefore predict fatigue failure. Bovine constructs varying from 0-120 degrees scoliosis were loaded axially, with strain recordings obtained at the apex of the curve. Relatively low loads produced enough tensile stress to contemplate implant fatigue in all except the nondeformed (0 degrees) construct. Construct stiffness was found to decrease rapidly in spines with greater than 38 degrees deformity. In addition, data on patients who had suffered rod fracture from four different centers were found to compare favorably with experimental observations. We conclude that the vulnerability of Luque rod constructs to implant failure, from a mechanical standpoint, is greater than is generally assumed. Cross-linking of rods was found to increase stiffness. Methods to decrease tensile stresses in the implants and increase stiffness include external immobilization, larger diameter rods, and procedures to enhance correction.     

Biomechanical comparison of flexible stainless steel and titanium nails with external fixation using a femur fracture model

There are several options available for surgical stabilization of pediatric femoral shaft fractures. The purpose of this study was to compare the stability afforded by Ender stainless steel nails, titanium elastic nails, and one-plane unilateral external fixators for the fixation using a synthetic adolescent midshaft femur fracture model. The anterior-posterior (sagittal plane) bending, lateral (coronal plane) bending, torsional, and axial stiffness values were calculated using 6 different fixation configurations. These included pairs of 3.5-mm-diameter Ender nails with and without distal locking, 3.5- and 4.0-mm-diameter titanium elastic nails as well as single- and double-stacked monolateral external fixators. Eight synthetic femur models, 4 each with simulated transverse and comminuted fracture patterns, were sequentially tested for stability afforded by the various fracture fixation configurations. External fixation exhibited significantly greater control of anterior-posterior angulation compared with all flexible-nailing systems. Although Ender nails were slightly superior to titanium nails in control of sagittal plane angulation, this was not statistically significant. Compared with the external fixation constructs, all 4 flexible nail constructs demonstrated higher torsional stability. For prevention of axial shortening, all fixation methods were similar for the transverse fracture pattern, whereas external fixation was superior to flexible nails in the comminuted fracture model. No significant benefit was demonstrated with double stacking of external fixators. These findings may help guide clinicians choose the optimal fixation method for treatment of pediatric femoral shaft fractures.     

Knee mechanics after repair of the anterior cruciate ligament A cadaver study of ligament augmentation

An experimental knee-testing system was used to investigate the immediate postoperative mechanical state in knees with nonaugmented and augmented repairs of the anterior cruciate ligament. Ligament, repair tissue, and augmentation forces were measured using buckle transducers, and joint motion was measured using an instrumented spatial linkage during the application of 90 N anteriorly-directed tibial loads to seven fresh knee specimens at 0-90 degrees of flexion. Force and motion data were collected from each knee with an intact and excised anterior cruciate ligament, and after performing (1) a nonaugmented repair and an augmented repair using the Ligament Augmentation Device (3M Company) placed either (2) anatomically through the lateral femoral condyle or (3) in the over-the-top position.

The forces in the nonaugmented repair and the repair with the augmentation in the two positions were greater than the forces in the intact anterior cruciate ligament with the knee under the same anterior loads; this difference from normal was not significant with the over-the-top augmentation. With the augmentation anatomically placed, the load sharing did not reduce the force in the repair tissue as compared with the nonaugmented case. The over-the-top augmentation, on the other hand, lowered the repair tissue forces at extension while avoiding high repair tissue forces in flexion. The tibia was consistently in an externally rotated configuration compared with normal in both the unloaded and anterior load states with all three repair procedures.     

Biomechanical comparison of stainless steel and titanium nails for fixation of simulated femoral fractures

Flexible intramedullary nails are commonly used to treat femoral fractures in children. This study evaluated the biomechanical differences between stainless steel and titanium nails when securing transverse and comminuted fractures in a synthetic femur model. Retrograde flexible stainless steel and titanium nails placed in a divergent "C" pattern were mechanically tested, and axial rotation and compression stiffness were analyzed with a two-way ANOVA. Rotational stability was significantly greater for titanium nails than stainless steel nails for both fracture patterns. Axial compression stiffness was significantly greater for titanium nails than stainless steel nails for both fracture patterns. There was no statistical difference between materials for axial "failure" load that produced 5 mm of shortening. Titanium intramedullary nails were more stable than stainless steel nails in torsion and axial compression. Both materials stabilized simulated fractures at levels beyond physiologic non-weight-bearing loads without permanent deformation.     

Knee mechanics after repair of the anterior cruciate ligament. A cadaver study of ligament augmentation

An experimental knee-testing system was used to investigate the immediate postoperative mechanical state in knees with nonaugmented and augmented repairs of the anterior cruciate ligament. Ligament, repair tissue, and augmentation forces were measured using buckle transducers, and joint motion was measured using an instrumented spatial linkage during the application of 90 N anteriorly-directed tibial loads to seven fresh knee specimens at 0-90 degrees of flexion. Force and motion data were collected from each knee with an intact and excised anterior cruciate ligament, and after performing (1) a nonaugmented repair and an augmented repair using the Ligament Augmentation Device (3M Company) placed either (2) anatomically through the lateral femoral condyle or (3) in the over-the-top position. The forces in the nonaugmented repair and the repair with the augmentation in the two positions were greater than the forces in the intact anterior cruciate ligament with the knee under the same anterior loads; this difference from normal was not significant with the over-the-top augmentation. With the augmentation anatomically placed, the load sharing did not reduce the force in the repair tissue as compared with the nonaugmented case. The over-the-top augmentation, on the other hand, lowered the repair tissue forces at extension while avoiding high repair tissue forces in flexion. The tibia was consistently in an externally rotated configuration compared with normal in both the unloaded and anterior load states with all three repair procedures.     

A biomechanical study of 3 different types of sublaminar wire used for constructs in the thoracic spine

A biomechanical study was carried out on 3 different types of sublaminar wire used in constructs to secure the thoracic spine: stainless steel monofilament wire (steel wire), titanium cable (cable), and ultra-high molecular weight polyethylene tape (tape). Two experiments were carried out. Experiment 1: Thirty-one fresh human thoracic vertebrae classified as osteoporotic (bone mineral density of <0.8 g/cm2) were used. The steel wire, cable, or tape was placed sublaminarly and a tensile force was applied until the steel wire, cable, or tape cut 5 mm through the lamina, and the force at this point was noted. Experiment 2: Seven fresh human thoracic spines (T7-T10) were biomechanically tested as follows: axial compression (250 N), flexion (7.5 Nm), extension (7.5 Nm), left lateral bending (7.5 Nm), right lateral bending (7.5 Nm), left axial torsion (10 Nm), and right axial torsion (10 Nm). This sequence was applied to the intact spine. The spine was then de-stabilized and then restabilized using one or other of the 3 different types of sublaminar wires. The biomechanical testing was then repeated on the restabilized spine and stiffness curves were generated. In the laminar cut-through test, the cut-through force for tape was higher than that for either steel wire or cable. In the biomechanical stiffness testing, there was no significant difference between the 3 different sublaminar wiring constructs in any of the loading modes tested. The results of both experiments suggest that tape is as good, if not better, than steel wire or cable as a sublaminar wiring construct material.     

Effect of spinal construct stiffness on early fusion mass incorporation. Experimental study

The relationship between initial spinal construct stiffness and the stiffness of the resulting fusion mass was studied by performing standardized 10-segment posterior spinal fusions in goats. Animals were divided into 5 groups based on type of spinal construct, using rods of different diameters (3.2 mm, 4.8 mm, 6.4 mm) with or without rigid crosslinking to produce constructs of different stiffnesses. Stiffness data on 28 animals were obtained by removing the spines en bloc, at 6 or 12 weeks postoperatively, and performing load-deformation testing in axial and torsional loading to determine the stiffness of the fusion masses (rods removed). The initial construct stiffnesses were also compared by ex vivo testing on spine specimens to correlate initial construct stiffness with eventual fusion mass stiffness. In axial testing, results showed stiffer fusion masses from larger diameter rod constructs compared with smaller rod constructs. This was similar to results of control testing on spine specimens ex vivo. Rigid crosslinking did not produce stiffer fusions in axial testing, due to a technical limitation of the button-wire implants used to segmentally fix the rods at each vertebra. In torsional testing, stiffer fusion masses resulted from using larger rods, and rigid Crosslinking also produced the stiffest fusion masses, which was consistent with ex vivo testing. In general, larger diameter (stiffer) rods produced stiffer fusion masses, and no evidence of stress shielding was found.