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.     

A biomechanical comparison of suture constructs used for coracoclavicular fixation

There is no consensus regarding surgical treatment for severely dislocated acromioclavicular joints. Although many treatments are suture-based, the suture materials and resulting suture-bone constructs have been subjected to limited systematic evaluation. This study identifies the strongest and least deforming suture construct among those commonly used for such repairs. Each suture-based repair was tested on a simulated clavicle and coracoid process with the skeletal components distracted until the suture failed to obtain tensile strength. Additional groups of sutures were subjected to cyclic loading to determine resistance to deformation. Panacryl braid had significantly greater tensile strength than all other constructs: Polydioxanone (PDS) braid, Mersilene tape, and Ethibond #5. Deformation after cyclic loading of Panacryl braid, PDS braid, and two strands of Mersilene tape was significantly less than that of the other constructs. A bioabsorbable suture loop, such as Panacryl, can act as a temporary internal splint, maintaining acromioclavicular joint reduction long enough for ligamentous healing during rehabilitation, and can avoid complications associated with permanent fixation materials. Panacryl braid deserves serious consideration for coracoclavicular fixation because of its strength, resistance to deformation, and bioabsorbable properties.     

寰椎侧块-枢椎椎板螺钉固定的有限元分析

目的:建立寰椎侧块-枢椎椎板螺钉固定的三维有限元模型并进行有限元分析,探讨其生物力学特性。方法:通过CT扫描获取1例健康成年男性寰枢椎图像信息,应用VTK软件及ABAQUS软件构建寰椎侧块-枢椎椎板螺钉固定的三维有限元模型,观察中立、前屈/后伸、侧弯、旋转、前后平移等载荷下固定系统的应力变化,分析寰枢侧块-枢椎椎板螺钉固定系统的生物力学特性。结果:所建立的有限元模型逼真地描绘了寰椎侧块-枢椎椎板螺钉固定系统的结构特点,共包含183363个节点(椎骨130982个,螺钉52381个),116082个单元(椎骨83776个,螺钉32306个)。在不同运动状态下,螺钉应力分布主要集中在螺钉置入骨质部分的根部周围和钉棒连接处。前屈载荷时,连接棒从头端至尾端的应力逐步减小,在寰椎侧块螺钉的钉棒连接处应力最大;其他载荷下连接棒应力分布从头端至尾端逐步增大,至枢椎椎板螺钉的钉棒连接处达到最大。后伸和旋转载荷下,螺钉存在明显的高应力区,各螺钉的应力最大值大于其他运动状态。结论:寰椎侧块-枢椎椎板螺钉固定系统固定时各螺钉在颈椎旋转及后伸时所受应力明显增加,术后应避免颈椎过度旋转及后伸,以减少螺钉松动和断裂的发生。     

Atlantoaxial fusion: a biomechanical analysis of two C1–C2 fusion techniques

BACKGROUND CONTEXT: Different atlantoaxial fusion techniques are used for instability. Transarticular screws are biomechanically superior to wiring techniques and equivalent to C1 lateral mass to C2 pedicle (C1LM-C2P) fixation. Recently, C1 lateral mass to C2 laminar (C1LM-C2L) fixation has been shown to have flexibility similar to C1LM-C2P fixation in flexion, extension, lateral bending, and axial rotation. PURPOSE: Compare the stiffness of C1LM-C2P with C1LM-C2L screw rod fixation. STUDY DESIGN: In vitro biomechanical study. OUTCOME MEASURES: Stiffness in flexion/extension, lateral bending, axial rotation, and anterior-posterior (AP) translation. METHODS: Eight fresh-frozen human cadaveric cervical spines (C1-C3) were tested intact and, after a type II odontoid fracture, were instrumented and tested with two fixation constructs: C1LM-C2P screws and C1LM-C2L screws. The testing involved flexion, extension, lateral bending, AP translation, and axial rotation. Stiffness was measured and compared with a repeated-measures analysis. RESULTS: C1LM-C2P was significantly stiffer than the intact in AP translation (p<.001), lateral bending (p=.001), and axial rotation (p=.002) and equivalent in flexion/extension (p=.09). C1LM-C2L was significantly stiffer than the intact in AP translation (p<.01) and axial rotation (p<.004) and equivalent in lateral bending (p<.71) and flexion/extension (p=.22). C1LM-C2P was stiffer than C1LM-C2L in right/left lateral bending (p<.001) and axial rotation (p=.009) and equivalent in AP translation (p=.06) and flexion/extension (p=.74). CONCLUSION: C1LM-C2P fixation is equivalent to C1LM-C2L fixation in flexion/extension and AP translation and superior in lateral bending and axial rotation.     

A biomechanical evaluation of press-fit stem constructs for tumor endoprosthetic reconstruction of the distal femur

This study was designed to assess the biomechanical parameters of the older Kotz Modular Femur Tibia Reconstruction (Stryker Inc, Mahwah, NJ) stem and the newer Restoration and the unfluted Global Modular Replacement System (Stryker Inc, Mahwah, NJ) uncemented stems for use with tumor endoprostheses as well as to assess the optimal reaming technique for insertion of these stems. Fresh-frozen adult femora or composite distal femora were implanted with the uncemented stems. Separate experiments were performed to compare reaming technique and bone resection level. All constructs were mechanically tested for axial compression, lateral bending, and torsional stiffness and torque to failure. Results showed that the biomechanical performance of all the stems were similar with respect to each parameter. Cylindrical reaming was associated with a significantly higher torque to failure than flexible reaming in the diaphysis (P = .006). Newer uncemented stems provide adequate initial biomechanical stability for implantation in the distal femur.     

A biomechanical analysis of static progressive elbow flexion splinting.

We all have many occasions to splint the stiff elbow, especially to overcome flexion contratures, but the functional requirement of 130+degrees of elbow flexion is also an important clinical reality requiring end range splinting in no small number of cases. The author presents an analysis of the theoretical forces acting upon the elbow in two types of splints at various angles of flexion.     

Differential biomechanical effects of injury and wiring at C1-C2.

STUDY DESIGN: An in vitro study compared the biomechanics of the upper cervical spine among three groups of cadaveric specimens, each with a different source of instability: transverse-alar-apical ligament disruptions, odontoid fractures, or odontoidectomies. The responses of the three groups were again compared after a uniform posterior cable and graft fixation was applied to the specimens. OBJECTIVES: To quantify and compare the effects of different injuries on atlantoaxial stability and to determine whether a single fixation technique effectively treats each injury. SUMMARY OF BACKGROUND DATA: Previous biomechanical studies of atlantoaxial instability have been focused on mechanisms of injury or on comparison among fixation types. METHODS: Cables and pulleys applied torques to human cadaveric C0-C6 specimens quasistatically while an optical system tracked three-dimensional angular and translational motion at C0-C1 and C1-C2. Specimens were tested immediately after injury, after posterior cable and graft fixation, and after 6000 cycles of fatigue. RESULTS: Odontoidectomies increased C1-C2 angular and translational range of motion significantly more than odontoid fractures or ligament disruptions, especially during flexion-extension. Odontoid fractures produced a slightly larger increase in C1-C2 angular range of motion than ligament disruptions but a smaller increase in C0-C1 range of motion. The different injuries affected the lax zone and the position of C1-C2 axis of rotation differently. Restabilization by posterior cable and graft reduced motion only moderately for each injury type. All three fixated injuries were susceptible to loosening from fatigue. CONCLUSION: The three different injuries produce different spinal biomechanical responses. To best promote fusion, posterior cable and graft fixation should be used with an adjunctive stabilizing technique to treat all three injuries.     

Image-guided anterior cervical corpectomy. A feasibility study

STUDY DESIGN: A feasibility study was performed to determine the efficacy of using image-guided frameless stereotaxy to perform anterior corpectomy of the cervical spine. OBJECTIVE: To assess the feasibility of using image-guided stereotaxy in performing anterior cervical corpectomy. SUMMARY OF BACKGROUND DATA: Anterior cervical decompression including discectomy and corpectomy is a commonly performed procedure. Particular concern about invasion of the vertebral artery arises while performing this procedure to gain maximal lateral decompression. At present, surgeons have only landmarks and experience to guide them in performance of this potentially dangerous procedure. METHODS: Four cadavers (average age, 40.3 years) were used. A lateral corpectomy trough was created in Group 1 by a standard technique using visual landmarks. In the second group of corpectomy troughs, an image-guided frameless stereotactic system was used. After completion of the experiment, each cadaver had a corpectomy trough at every level on one side performed in a standard manner and on the other with image guidance. Using the image guidance system, an independent observer measured the distance from the corpectomy trough (lateral border) to the medial border of the foramen transversarium. RESULTS: The average distance from the lateral border of the trough to the medial border of the foramen transversarium in the standard trough group was 5.10 mm (range, 1.72-7.71 mm), and the average distance from the medial border of the foramen transversarium to the image-guided trough was 4.34 mm (range, 3.34-5.48 mm). The trend of the comparison between the two troughs was toward significance at P = 0.08. CONCLUSIONS: Image-guidance provided improved accuracy when compared with that of a standard technique, implying clinical potential for image-guided corpectomy. Less variability is seen using an image-guided approach.     

Cervical stability after foraminotomy. A biomechanical in vitro analysis.

Laminectomy or facetectomy of the cervical spine, or both, may be needed for decompression of the spinal cord or of the nerve-roots. Acute stability of the cervical spine was tested after laminectomy and progressive staged foraminotomies in an in vitro model. Twelve cervical spines from human cadavera were used in the experiment. Biomechanical testing included the application of an axial load, the application of a flexion and extension moment, and the application of a torsional moment. Each specimen was tested intact, after laminectomy of the fifth cervical vertebra, and after progressive foraminotomy of the sixth cervical root. Foraminotomy was performed by resection of 25, 50, 75, and 100 per cent of the facet joint and capsule. Torsional stiffness decreased dramatically when more than 50 per cent of the facet had been resected. Statistically equivalent subsets were the intact specimen, laminectomy, 25 per cent facetectomy, and 50 per cent facetectomy in one subset, and 75 and 100 per cent facetectomy in the least-stiff subset. Flexion-moment testing showed that the posterior strain did not differ among three groups: the intact specimens, those that had been treated with laminectomy, and those that had been treated with a 25 per cent facetectomy. The 50 per cent facetectomy resulted in a 2.5 per cent increase in posterior strain, and the 75 or 100 per cent facetectomy, in a 25 per cent increase in posterior strain compared with the intact specimen. Segmental hypermobility of the cervical spine results if a foraminotomy involves resection of more than 50 per cent of the facet.(ABSTRACT TRUNCATED AT 250 WORDS)     

新型寰椎钩棒及椎板钩联合枢椎椎弓根螺钉内固定的生物力学

目的 评价自行研制的寰椎后弓环抱钩棒及椎板钩联合枢椎椎弓根螺钉内固定系统的生物力学稳定性,为临床应用提供实验依据. 方法 6具新鲜尸体枕颈部标本,采用脊柱三维运动试验机依次对其齿状突切断后(A组)、寰椎椎板钩联合枢椎椎弓根螺钉内固定(B组)、自制寰椎钩棒及椎板钩联合枢椎椎弓根螺钉内固定(C组)、寰椎椎弓根螺钉联合枢椎椎弓根螺钉内固定(D组)等4种状态进行三维运动范围测试;分析比较新型寰枢椎后路内固定系统的稳定性. 结果 B组后伸平均ROM明显大于C、D组,差异均有统计学意义(P＜0.05);C、D组后伸平均ROM差异无统计学意义(P＞0.05);而B、C、D组间在其他角度ROM(前屈、侧屈、旋转)差异均无统计学意义(P＞0.05).结论 采用自制寰椎后弓钩棒及椎板钩联合枢椎椎弓根螺钉进行内固定,可为失稳的寰枢关节提供良好的即刻和较长期稳定性.     

The mechanism of clavicular fracture. A clinical and biomechanical analysis

A consecutive series of 150 patients with clavicular fractures is presented. In 81% detailed information regarding the mechanism of the injury was available and, of these, 94% had fractured their clavicle from a direct blow on the shoulder; only 6% had fallen on the outstretched hand. This finding, at variance with commonly held views regarding the mechanism of this injury, was further investigated by biomechanical analysis of the forces involved in clavicular fractures. The biomechanical model supported the clinical findings.     

Junction kinematics between proximal mobile and distal fused lumbar segments: biomechanical analysis of pedicle and hook constructs

Background contextBiomechanical studies have demonstrated increased motion in motion segments adjacent to instrumentation or arthrodesis. The effects of different configurations of hook and pedicle screw instrumentation on the biomechanical behaviors of adjacent segments have not been well documented.PurposeTo compare the effect of three different fusion constructs on adjacent segment motion proximal to lumbar arthrodesis.MethodsSeven human cadaver lumbar spines were tested in the following conditions: 1) intact; 2) L4–L5-simulated circumferential fusion (CF); 3) L4–L5-simulated fusion extended to L3 with pedicle screws; and 4) L4–L5-simulated fusion extended to L3 with sublaminar hooks. Rotation data at L2–L3, L3–L4, and L4–L5 were analyzed using both load limit control (±7.5 N·m) and displacement limit control (truncated to the greatest common angular motion of the segments for each specimen).ResultsBoth the L3–L4 and L2–L3 motion segments above the L4–L5-simulated CF had significantly increased motion in all loading planes compared with the intact spine, but no significant differences were found between L3–L4 and L2–L3 motion. When the L3–L4 segment was stabilized with pedicle screws, its motion was significantly smaller in flexion, lateral bending, and axial rotation than when stabilized with sublaminar hooks. At the same time, L2–L3 motion was significantly larger in flexion, lateral bending, and axial rotation in the pedicle screw model compared with the sublaminar hook construct.ConclusionsThe use of sublaminar hooks to stabilize the motion segment above a circumferential lumbar fusion reduced motion at the next cephalad segment compared with a similar construct using pedicle screws. The semiconstrained hook enhancement may be considered if a patient is at a risk of adjacent segment disorders.     

Dislocation of rotating hinge total knee prostheses. A biomechanical analysis

BACKGROUND: Symptomatic instability and implant dislocation are occasionally encountered in patients with a rotating hinge total knee prosthesis. A biomechanical study of rotating hinge total knee implants was performed to determine the association between the design (length and taper) of the central rotational stem and the stability of the implant. METHODS: The stem lengths and tapers of knee implants made by seven manufacturers were measured. The tilting laxity of each design was tested by measuring the degree of tilting of the central rotational stem within the tibial housing that occurred with increasing amounts of distraction. The maximum amount of distraction that was possible before the stem dislocated was determined for each design. RESULTS: Implant designs with a short and/or markedly tapered central rotational stem had the greatest tilting, laxity, and instability of that stem. The Howmedica, Techmedica, Intermedics/Sulzer Medica, and Wright Medical Technology/ Dow Corning Wright designs required > or = 39 mm of distraction before they dislocated. The Biomet knee implant required 33 or 44 mm of distraction to dislocate, depending on the thickness of the polyethylene tray that was utilized. The S-ROM knee required only 26 mm of distraction before dislocation occurred. CONCLUSIONS: The measurements confirmed that the shorter the stem and the greater its taper, the greater the instability and laxity at any given amount of joint distraction.     

A biomechanical analysis of gait during pregnancy

BACKGROUND: There are many anatomical changes during pregnancy that could potentially lead to substantial alterations in gait. Gait deviations may contribute to a variety of musculoskeletal overuse conditions associated with pregnancy, such as low-back, hip, and calf pain. Because we are aware of little research on this topic, the purpose of this study was to objectively analyze gait during pregnancy. METHODS: Three-dimensional gait analysis was performed on fifteen women during the second half of the last trimester of pregnancy and again one year post partum. Selected kinematic and kinetic parameters for the pregnancy and one-year postpartum conditions were compared with use of paired t tests (95 percent significance level). RESULTS: Overall, gait kinematics were remarkably unchanged during pregnancy. No evidence of a so-called waddling gait during pregnancy was found. Maximum anterior pelvic tilt during gait increased a mean of 4 degrees during pregnancy, although individual subject-to-subject variation (range, an increase of 13 degrees to a decrease of 10 degrees) was observed. Significant increases in hip and ankle kinetic gait parameters, however, were observed during pregnancy (p < 0.05). CONCLUSIONS: Significant increases in kinetic gait parameters during pregnancy (p < 0.05) explain how gait motion remained relatively unchanged despite increases in body mass and width as well as changes in mass distribution about the trunk. This finding indicates that during pregnancy there may be an increased demand placed on hip abductor, hip extensor, and ankle plantar flexor muscles during walking.