In-vitro comparison of LC-DCP- and LCP-constructs in the femur of newborn calves - a pilot study.

BACKGROUND: To compare the biomechanical in-vitro characteristics of limited-contact dynamic compression plate (LC-DCP) and locking compression plate (LCP) constructs in an osteotomy gap model of femoral fracture in neonatal calves. Pairs of intact femurs from 10 calves that had died for reasons unrelated to the study were tested. A 7-hole LC-DCP with six 4.5 mm cortical screws was used in one femur and a 7-hole LCP with four 5.0 mm locking and two 4.5 mm cortical screws was used in the corresponding femur. The constructs were tested to failure by cyclic compression at a speed of 2 mm/s within six increasing force levels. RESULTS: The bone-thread interface was stripped in 21 of 80 cortical screws (26.3%) before a pre-set insertion torque of 3 Nm was achieved. Only 3 corresponding intact pairs of constructs could be statistically compared for relative structural stiffness, actuator excursion and width of the osteotomy gap. Relative structural stiffness was significantly greater, actuator excursion and width of the osteotomy gap were significantly smaller in the LCP constructs. While failure occurred by loosening of the screws in the LC-DCP constructs, locking constructs failed by cutting large holes in the soft distal metaphyseal bone. CONCLUSIONS: An insertion torque sufficient to provide adequate stability in femurs of newborn calves could not be achieved reliably with 4.5 mm cortical screws. Another limiting factor for both constructs was the weak cancellous bone of the distal fracture fragment. LCP constructs were significantly more resistant to compression than LC-DCP constructs.

The influence of different osteosynthesis configurations with locking compression plates (LCP) on stability and fracture healing after an oblique 45° angle osteotomy.

BACKGROUND: Locking compression plates are used in various configurations with lack of detailed information on consequent bone healing. STUDY DESIGN: In this in vivo study in sheep 5 different applications of locking compression plate (LCP) were tested using a 45° oblique osteotomy simulating simple fracture pattern. 60 Swiss Alpine sheep where assigned to 5 different groups with 12 sheep each (Group 1: interfragmentary lag screw and an LCP fixed with standard cortex screws as neutralisation plate; Group 2: interfragmentary lag screw and LCP with locking head screws; Group 3: compression plate technique (hybrid construct); Group 4: internal fixator without fracture gap; Group 5: internal fixator with 3mm gap at the osteotomy site). One half of each group (6 sheep) was monitored for 6 weeks, and the other half (6 sheep) where followed for 12 weeks. METHODS: X-rays at 3, 6, 9 and 12 weeks were performed to monitor the healing process. After sacrifice operated tibiae were tested biomechanically for nondestructive torsion and compared to the tibia of the healthy opposite side. After testing specimens were processed for microradiography, histology, histomorphometry and assessment of calcium deposition by fluorescence microscopy. RESULTS: In all groups bone healing occurred without complications. Stiffness in biomechanical testing showed a tendency for higher values in G2 but results were not statistically significant. Values for G5 were significantly lower after 6 weeks, but after 12 weeks values had improved to comparable results. For all groups, except G3, stiffness values improved between 6 and 12 weeks. Histomorphometrical data demonstrate endosteal callus to be more marked in G2 at 6 weeks. DISCUSSION AND CONCLUSION: All five configurations resulted in undisturbed bone healing and are considered safe for clinical application.

Mechanical anchorage and peri-implant bone formation of surface-modified zirconia in minipigs.

AIM: To test the hypothesis that peri-implant bone formation and mechanical stability of surface-modified zirconia and titanium implants are equivalent. MATERIALS AND METHODS: Twelve minipigs received three types of implants on either side of the mandible 8 weeks after removal of all pre-molar teeth: (i) a zirconia implant with a sandblasted surface; (ii) a zirconia implants with a sandblasted and etched surface; and (iii) a titanium implant with a sandblasted and acid-etched surface that served as a control. Removal torque and peri-implant bone regeneration were evaluated in six animals each after 4 and 13 weeks. RESULTS: The titanium surface was significantly rougher than both tested zirconia surfaces. Mean bone to implant contact (BIC) did not differ significantly between the three implant types after 4 weeks but was significantly higher for titanium compared with both zirconia implants after 13 weeks (p<0.05). Bone volume density (BVD) did not differ significantly at any interval. Removal torque was significantly higher for titanium compared with both zirconia surfaces after 4 and 13 weeks (p<0.001). The sandblasted and etched zirconia surface showed a significantly higher removal torque after 4 weeks compared with sandblasted zirconia (p<0.05); this difference levelled out after 13 weeks. CONCLUSIONS: It is concluded that all implants achieved osseointegration with similar degrees of BIC and BVD; however, titanium implants showed a higher resistance to removal torque, probably due to higher surface roughness.

Prediction of dental implant torque with a fast and automatic finite element analysis: a pilot study.

OBJECTIVES: Despite its importance, implant removal torque can be assessed at present only after implantation. This paper presents a new technique to help clinicians preoperatively evaluate implant stability. STUDY DESIGN: Planning software has been combined with an in-house finite element solver. Once the clinician has chosen the implant position on the planner, a finite element analysis automatically calculates the primary stability. The process was designed to be as simple and fast as possible for clinical use. This paper describes application of the method to the prediction of removal torque. A preliminary validation has been performed in both polyurethane foam and sheep bone. RESULTS: The predicted torque is quantitatively equivalent to experimental values with correlation coefficients of >0.7 in both materials. CONCLUSIONS: This preliminary study is a first step toward the introduction of finite element models in computer-assisted surgery. The fact that the process is fast and automatic makes it suitable for a clinical use.

Viscoelastic properties of the ovine posterior spinal ligaments are strain dependent.

BACKGROUND: The biomechanical role of the posterior spinal ligaments for spinal stability has been stated in previous studies. The investigation of the viscoelastic properties of human lumbar spinal ligaments is essential for the understanding of physiological differences between healthy and degenerated tissues. The stress-relaxation behavior of biological tissues is commonly described with the quasi-linear viscoelastic model of Fung, which assumes that the stress-relaxation response is independent of the applied strain. The goal of this study was to investigate the stress-relaxation response of ovine posterior spinal ligaments at different elongations to verify the above-mentioned hypothesis. METHODS: Twenty-four ovine lumbar spinal segments, consisting of only the supraspinous and interspinous ligaments and adjoining spinous processes, were elongated uniaxially to different strain levels within the physiological elastic region (5-20%). The experimental data were described with a non-linear viscoelastic model: the modified superposition method of Findley. FINDINGS: A linear dependency of the relaxation rate to the applied strains was observed on intact segments, when both ligaments were considered, as well as on each individual ligament. This result can be applied to the human spinal ligaments, due to similarities observed between the sheep and human spinal segment under physiological loading. INTERPRETATION: The non-linear viscoelastic modified superposition method of Findley is an appropriate model for describing the viscoelastic properties of lumbar spinal ligaments in vitro due to its ability to address variation in applied strain during the force relaxation measurements.

Biomechanical analysis of torsion and shear forces in lumbar and lumbosacral spine segments of nonchondrodystrophic dogs.

OBJECTIVE: To determine stiffness and load-displacement curves as a biomechanical response to applied torsion and shear forces in cadaveric canine lumbar and lumbosacral specimens. STUDY DESIGN: Biomechanical study. ANIMALS: Caudal lumbar and lumbosacral functional spine units (FSU) of nonchondrodystrophic large-breed dogs (n=31) with radiographically normal spines. METHODS: FSU from dogs without musculoskeletal disease were tested in torsion in a custom-built spine loading simulator with 6 degrees of freedom, which uses orthogonally mounted electric motors to apply pure axial rotation. For shear tests, specimens were mounted to a custom-made shear-testing device, driven by a servo hydraulic testing machine. Load-displacement curves were recorded for torsion and shear. RESULTS: Left and right torsion stiffness was not different within each FSU level; however, torsional stiffness of L7-S1 was significantly smaller compared with lumbar FSU (L4-5-L6-7). Ventral/dorsal stiffness was significantly different from lateral stiffness within an individual FSU level for L5-6, L6-7, and L7-S1 but not for L4-5. When the data from 4 tested shear directions from the same specimen were pooled, level L5-6 was significantly stiffer than L7-S1. CONCLUSIONS: Increased range of motion of the lumbosacral joint is reflected by an overall decreased shear and rotational stiffness at the lumbosacral FSU. CLINICAL RELEVANCE: Data from dogs with disc degeneration have to be collected, analyzed, and compared with results from our chondrodystrophic large-breed dogs with radiographically normal spines.

Comparative biomechanical investigation of a modular dynamic lumbar stabilization system and the Dynesys system.

The goal of non-fusion stabilization is to reduce the mobility of the spine segment to less than that of the intact spine specimen, while retaining some residual motion. Several in vitro studies have been conducted on a dynamic system currently available for clinical use (Dynesys). Under pure moment loading, a dependency of the biomechanical performance on spacer length has been demonstrated; this variability in implant properties is removed with a modular concept incorporating a discrete flexible element. An in vitro study was performed to compare the kinematic and stabilizing properties of a modular dynamic lumbar stabilization system with those of Dynesys, under the influence of an axial preload. Six human cadaver spine specimens (L1-S1) were tested in a spine loading apparatus. Flexibility measurements were performed by applying pure bending moments of 8 Nm, about each of the three principal anatomical axes, with a simultaneously applied axial preload of 400 N. Specimens were tested intact, and following creation of a defect at L3-L4, with the Dynesys implant, with the modular implant and, after removal of the hardware, the injury state. Segmental range of motion (ROM) was reduced for flexion-extension and lateral bending with both implants. Motion in flexion was reduced to less than 20% of the intact level, in extension to approximately 40% and in lateral bending a motion reduction to less than 40% was measured. In torsion, the total ROM was not significantly different from that of the intact level. The expectations for a flexible posterior stabilizing implant are not fulfilled. The assumption that a device which is particularly compliant in bending allows substantial intersegmental motion cannot be fully supported when one considers that such devices are placed at a location far removed from the natural rotation center of the intervertebral joint.

Biomechanical analysis of the three-dimensional motion pattern of the canine cervical spine segment C4-C5.

OBJECTIVE: To study the kinematics of cervical spine segment C(4)-C(5) and its association with disc dimensions and the coupled motion (CM) in relation to primary motion (PM). STUDY DESIGN: Cadaveric biomechanical study. ANIMALS: Cadavers of large breed dogs (>20 kg; n=11). METHODS: Spines were freed from muscles. Radiographs were taken orthogonal to the C(4)-C(5) disc space and disc thickness, endplate width, and height were measured. Spines were mounted on a simulator for 3-dimensional motion analysis. Data were recorded with an optoelectronic motion analysis system. Range of motion (ROM) and neutral zone (NZ) were determined in the direction of flexion/extension, left/right lateral bending, and left/right axial rotation, as well as the ROM of CM. RESULTS: ROM in flexion and extension was similar; there was no CM in flexion/extension. Left/right axial rotation and left/right lateral bending were coupled to the same side. CM was 1.72 and 3.56 times the ROM of the PM in lateral bending and axial rotation, respectively. Disc dimensions were positively correlated with body weight. Flexion/extension magnitude was significantly reduced for larger endplates, but axial rotation was not influenced. Lateral bending had no correlation with weight or disc dimensions. CONCLUSION: Left/right lateral bending and left/right axial rotation are coupled differently in the C(4)-C(5) segment in dogs compared with humans. CLINICAL RELEVANCE: The canine C(4)-C(5) spinal segment has unique motion coupling patterns that should be considered for dynamic implant designs.

Quantifying the influence of bone density and thickness on resonance frequency analysis: an in vitro study of biomechanical test materials.

PURPOSE: Resonance frequency analysis (RFA) offers the opportunity to monitor the osseointegration of an implant in a simple, noninvasive way. A better comprehension of the relationship between RFA and parameters related to bone quality would therefore help clinicians improve diagnoses. In this study, a bone analog made from polyurethane foam was used to isolate the influences of bone density and cortical thickness in RFA. MATERIALS AND METHODS: Straumann standard implants were inserted in polyurethane foam blocks, and primary implant stability was measured with RFA. The blocks were composed of two superimposed layers with different densities. The top layer was dense to mimic cortical bone, whereas the bottom layer had a lower density to represent trabecular bone. Different densities for both layers and different thicknesses for the simulated cortical layer were tested, resulting in eight different block combinations. RFA was compared with two other mechanical evaluations of primary stability: removal torque and axial loading response. RESULTS: The primary stability measured with RFA did not correlate with the two other methods, but there was a significant correlation between removal torque and the axial loading response (P < .005). Statistical analysis revealed that each method was sensitive to different aspects of bone quality. RFA was the only method able to detect changes in both bone density and cortical thickness. However, changes in trabecular bone density were easier to distinguish with removal torque and axial loading than with RFA. CONCLUSIONS: This study shows that RFA, removal torque, and axial loading are sensitive to different aspects of the bone-implant interface. This explains the absence of correlation among the methods and proves that no standard procedure exists for the evaluation of primary stability.

A robust and accurate two-stage approach for automatic recovery of distal locking holes in computer-assisted intramedullary nailing of femoral shaft fractures.

It has been recognized that one of the most difficult steps in intramedullary nailing of femoral shaft fractures is the distal locking - the insertion of distal transverse interlocking screws, for which it is necessary to know the positions and orientations of the distal locking holes (DLHs) of the intramedullary nail (IMN). This paper presents a robust and accurate approach for solving this problem based on two calibrated and registered fluoroscopic images. The problem is formulated as a two-stage model-based optimal fitting process. The first stage, nail detection, automatically estimates the axis of the distal part of the IMN (DP-IMN) by iteratively fitting a cylindrical model to the images. The second stage, pose recovery, resolves the translations and the rotations of the DLHs around the estimated axis by iteratively fitting the geometrical models of the DLHs to the images. An iterative best matched projection point (IBMPP) algorithm is combined with random sample strategies to effectively and robustly solve the fitting problems in both stages. We designed and conducted comprehensive experiments to validate the robustness and the accuracy of the present approach. Our in vitro experiments show on average less than 14 s execution time on a Linux machine, a mean angular error of 0.48 degrees (std = 0.21 degrees ), and a mean translational error of 0.09 mm (std = 0.041 mm). We conclude that the present approach is fast, robust, and accurate for distal locking applications.

Vertebral endplate trauma induces disc cell apoptosis and promotes organ degeneration in vitro.

There is a major controversy whether spinal trauma with vertebral endplate fractures can result in post-traumatic disc degeneration. Intervertebral discs, which are adjacent to burst endplates, are frequently removed and an intercorporal spondylodesis is performed. In any case, the biological effects within the discs following endplate fractures are poorly elucidated to date. The aim of our investigations was therefore to establish a novel disc/endplate trauma culture model to reproducibly induce endplate fractures and investigate concurrent disc changes in vitro. This model is based on a full-organ disc/endplate culture system, which has been validated by the authors before. Intervertebral disc/endplate specimens were isolated from Burgundy rabbits and cultured in standard media (DMEM/F12, 10%FCS). Burst endplate fractures were induced in half of the specimens with a custom-made fracture device and subsequently cultured for 9 days. The biological effects such as necrotic or apoptotic cell death and the expression of pro-apoptotic genes and other genes involved in organ degeneration, e.g. matrix metalloproteinases (MMPs) were analyzed. Cell damage was assessed by quantification of the lactate dehydrogenase (LDH) activity in the supernatant. The expression of genes involved in the cellular apoptotic pathway (caspase 3) and the pro-apoptotic proteins FasL and TNF-alpha were monitored. The results demonstrate that LDH levels increased significantly post trauma compared to the control and remained elevated for 3 days. Furthermore, a constant up-regulation of the caspase 3 gene in both disc compartments was present. The pro-apoptotic proteins FasL and TNF-alpha were up regulated predominantly in the nucleus whereas the MMP-1 and -13 transcripts (collagenases) were increased in both disc structures. From this study we can conclude that endplate burst fractures result in both necrotic and apoptotic cell death in nucleus and annulus tissue. Moreover, FasL and TNF-alpha expression by nucleus cells may lead to continued apoptosis induced by Fas- and TNF-alpha receptor bearing cells. In addition TNF-alpha over-expression has potentially deleterious effects on disc metabolism such as over-expression of matrix proteinases. Taken together, the short term biological response of the disc following endplate fracture exhibits characteristics, which may initiate the degeneration of the organ.

Accurate and reliable pose recovery of distal locking holes in computer-assisted intra-medullary nailing of femoral shaft fractures: a preliminary study.

OBJECTIVE: One of the difficult steps in intra-medullary nailing of femoral shaft fractures is distal locking - the insertion of distal interlocking screws. Conventionally, this is performed using repeated image acquisitions, which leads to considerable irradiation of the patient and surgical team. Virtual fluoroscopy has been used to reduce radiation exposure, but can only provide multi-planar two-dimensional projection views. In this study, two calibrated fluoroscopic images were used to automatically recover the positions and orientations of the distal locking holes (DLHs). The ultimate goal is to provide precise three-dimensional guidance during distal locking. METHODS: A model-based optimal fitting process was used to reconstruct the positions and orientations of the DLHs from two calibrated fluoroscopic images. No human intervention is required. A preliminary in vitro validation study was conducted to analyze the accuracy and reliability of the technique using images acquired from different viewpoints. The ground truths of the DLH were obtained by inserting a custom-made steel rod through the hole and then digitizing both the top and bottom centers of the rod using a sharp pointer. The recovery errors were computed by comparing the computed results to the ground truths. RESULTS: In all experiments, the poses of the DLHs could be recovered fully automatically. When the recovered positions and orientations of the DLHs were compared to their associated ground truths, a mean angular error of 0.5 degrees (STD = 0.2 degrees ), and a mean translational error of 0.1 mm (STD = 0.0 mm) were found. CONCLUSIONS: Accurate and reliable pose recovery of distal locking holes from two calibrated fluoroscopic images is achievable. Our preliminary in vitro experimental results demonstrate that the recovered poses of the distal locking holes are sufficiently accurate for intra-operative use.

Minimizing errors during in vitro testing of multisegmental spine specimens: considerations for component selection and kinematic measurement.

Apparatus-induced artifacts may invalidate standard spine testing protocols. Kinematic measurements may be compromised by the configuration of motion capture equipment. This study has determined: (1) the influence of machine design (component friction) on in vitro spinal kinetics; (2) the sensitivity of kinematic measurements to variations in the placement of motion capture markers. A spinal loading simulator has been developed to dynamically apply pure bending moments (three axes) with or without a simultaneous compressive preload. Two linear slider types with different friction coefficients, one with caged ball bearings and one with high-precision roller bearings on rails, were mounted and specimen response compared in sequential tests. Three different optoelectronic marker cluster configurations were mounted on the specimen and motion data was captured simultaneously from all clusters during testing. A polymer tube with a uniform bending stiffness approximately equivalent to a polysegmental lumbar spine specimen was selected to allow reproducible behavior over multiple tests. The selection of sliders for linear degrees of freedom had a marked influence on parasitic shear forces. Higher shear forces were recorded with the caged-bearing design than with the high-precision rollers and consequently a higher moment was required to achieve a given rotation. Kinematic accuracy varied with each marker configuration, but in general higher accuracy was achieved with larger marker spacings and situations where markers moved predominantly parallel to the camera's imaging plane. Relatively common alternatives in the mechanical components used in an apparatus for in vitro spine testing can have a significant influence on the measured kinematic and kinetics. Low-magnitude parasitic shear forces due to friction in sliders induces a linearly increasing moment along the length of the specimen, precluding the ideal of pure moment application. This effect is compounded in polysegmental specimens. Kinematic measurements are highly sensitive to marker design and placement, despite equivalent absolute precision of individual marker measurements, however marker configurations can be designed to minimize errors related to spatial distribution and system bias.