Three-dimensional kinematic evaluation of Tightrope CCL in a canine in vitro cranial cruciate deficient stifle model.

The impact of surgical correction of cranial cruciate ligament-deficient stifles (CCDS) on the 3-dimensional (3D) kinematics of the canine stifle has been sparsely evaluated. Tightrope (TR) cranial cruciate ligament (CCL) has been proposed to restore baseline 3D kinematics in CCDS by using isometric points. We hypothesized that TR would restore baseline 3D kinematics of the stifle in our model. Ten pelvic limbs were used with a previously validated apparatus. Three experimental conditions were evaluated: i) intact stifle, ii) cranial cruciate ligament transection (CCLt), and iii) CCLt stabilized with TR; kinematic data was recorded. Tightrope CCL in CCDS did not limit sagittal flexion. Tightrope CCL neutralized internal rotation without restoring baseline curves, but it did not restore abduction, nor did it neutralize or restore cranial translation, but it did restore latero-medial and proximo-distal translations. In our model, TR without pre-conditioning of the FiberTape strands did not restore baseline stifle 3D kinematics and residual cranial translation could result in frequent meniscal tears.

L'impact de la correction chirurgicale d'une déficience du ligament croisé crânial du genou (CCDS) sur la cinématique du grasset canin a été peu étudié. La technique de restauration du ligament croisé crânial (CCL) appelée 'Tightrope' (TR) a été proposée pour restaurer la cinématique 3D lors de CCDS en utilisant des points isométriques. Nous avons émis l'hypothèse que la technique TR restaurerait la cinématique 3D d'origine du grasset dans notre modèle. Dix membres pelviens ont été utilisés avec un appareil préalablement validé. Trois conditions expérimentales furent évaluées : i) grasset intact, ii) transsection du ligament croisé crânial (CCLt), et iii) CCLt stabilisé par TR; et les données de cinématique furent enregistrées. La technique TR lors de CCL n'a pas limité la flexion sagittale. Cette technique neutralisait la rotation interne sans restaurer les courbes d'origine, mais elle ne restaurait pas l'abduction, ni ne neutralisait ou restaurait une translation crâniale, mais elle a restauré les translations latéro-médiale et proximo-distale. Dans notre modèle, la technique TR sans préconditionnement des bandes FiberTape n'a pas restauré la cinématique 3D d'origine et une translation crâniale résiduelle pourrait résulter en des déchirures fréquentes du ménisque.(Traduit par Docteur Serge Messier).

Biomechanical assessment of the stabilization capacity of monolithic spinal rods with different flexural stiffness and anchoring arrangement.

BACKGROUND: Spinal disorders can be treated by several means including fusion surgery. Rigid posterior instrumentations are used to obtain the stability needed for fusion. However, the abrupt stiffness variation between the stabilized and intact segments leads to proximal junctional kyphosis. The concept of spinal rods with variable flexural stiffness is proposed to create a more gradual transition at the end of the instrumentation. METHOD: Biomechanical tests were conducted on porcine spine segments (L1-L6) to assess the stabilization capacity of spinal rods with different flexural stiffness. Dual-rod fusion constructs containing three kinds of rods (Ti, Ti-Ni superelastic, and Ti-Ni half stiff-half superelastic) were implanted using two anchor arrangements: pedicle screws at all levels or pedicle screws at all levels except for upper instrumented vertebra in which case pedicle screws were replaced with transverse process hooks. Specimens were loaded in forward flexion, extension, and lateral bending before and after implantation of the fusion constructs. The effects of different rods on specimen stiffness, vertebra mobility, intradiscal pressures, and anchor forces were evaluated. FINDING: The differences in rod properties had a moderate impact on the biomechanics of the instrumented spine when only pedicle screws were used. However, this effect was amplified when transverse process hooks were used as proximal anchors. INTERPRETATION: Combining transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff-half superelastic) rods provided more motion at the upper instrumented level and applied less force on the anchors, potentially improving the load sharing capacity of the instrumentation.

Braided tubular superelastic cables provide improved spinal stability compared to multifilament sublaminar cables.

This study investigates the use of braided tubular superelastic cables, previously used for sternum closure following sternotomy, as sublaminar fixation method. It compares the biomechanical performance of spinal instrumentation fixation systems with regular sublaminar cables and proprietary superelastic cables. A hybrid experimental protocol was applied to six porcine L1-L4 spinal segments to compare multifilament sublaminar cables (Atlas, Medtronic Sofamor Danek, Memphis, TN) with proprietary superelastic cables. First, intact total range of motion was determined for all specimens using pure moment loading. Second, pure moments were imposed to the instrumented specimens until these intact total ranges of motion were reproduced. Compared to the intact specimens, the use of superelastic cables resulted in stiffer instrumented specimens than the use of multifilament cables for all the loading modes except axial torsion. Consequently, the superelastic cables limited the instrumented segments mobility more than the multifilament cables. Spinal instrumentation fixation systems using superelastic cables could be a good alternative to conventional sublaminar cables as it maintains a constant stabilization of the spine during loading.

Factors affecting intradiscal pressure measurement during in vitro biomechanical tests.

OBJECTIVES: To assess the reliability of intradiscal pressure measurement during in vitro biomechanical testing. In particular, the variability of measurements will be assessed for repeated measures by considering the effect of specimens and of freezing/thawing cycles. METHODS: Thirty-six functional units from 8 porcine spines (S1: T7-T8, S2: T9-T10, S3: T12-T11, S4: T14-T13, S5: L1-L2 and S6: L3-L4) have been used. The intervertebral discs were measured to obtain the frontal and sagittal dimensions. These measurements helped locate the center of the disc where a modified catheter was positioned. A fiber optic pressure sensor (measuring range: -0.1 to 17 bar) (360HP, SAMBA Sensors, Sweden) was then inserted into the catheter. The specimens were divided into 3 groups: 1) fresh (F), 2) after one freeze/thaw cycle (C1) and 3) after 2 freeze/thaw cycles (C2). These groups were divided in two, depending on whether specimens were subjected to 400 N axial loading or not. Ten measurements (insertion of the sensor for a period of one minute, then removal) were taken for each case. Statistical analyses evaluated the influence of porcine specimen and the vertebral level using a MANOVA. The effect of repeated measurements was evaluated with ANOVA. The difference between freeze/thaw cycles were analysed with U Mann-Whitney test (P≤0.05). RESULTS: Without axial loading, the F group showed 365 mbar intradiscal pressure, 473 mbar for the C1 group, and 391 mbar for the C2 group. With 400N axial load, the F group showed intradiscal pressure of 10610 mbar, the C1 group 10132 mbar, the C2 group 12074 mbar. The statistical analysis shows a significant influence of the porcine specimen (p<0.001), with or without axial loading and of the vertebral level with (p=0.048) and without load (p<0.001). The results were also significantly different between the freeze/thaw cycles, with (p<0.001) and without load (p=0.033). Repeated measurement (without load p = 0.82 and with p = 0.56) did not show significant influence. CONCLUSIONS: The results tend to support that freezing/thawing cycles can affect intradiscal pressure measurement with significant inter-specimen variability. The use of the same specimen as its own control during in vitro biomechanical testing could be recommended.

In-vitro assessment of the stabilization capacity of monolithic spinal rods with variable flexural stiffness: Methodology and examples.

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. To assess the stabilization capacity of such a spinal rod, in vitro experiments on porcine spine models are carried out. This paper describes the methodology followed to evaluate the effect of Ti-Ni rods compared to conventional titanium rods. Validation of the methodology and examples of results obtained are also presented.

Biomechanical analysis of traumatic mesenteric avulsion.

Mesenteric avulsion, corresponding to a tearing of intestine's root, generally results from high deceleration in road accidents. The biomechanical analysis of bowel and mesenteric injuries is a major challenge for injury prevention, particularly because seat belt restraint may paradoxically increase their risk of occurrence. The aim of this study was to identify the biomechanical behavior of mesentery and small bowel (MSB) tissue samples under dynamical loading conditions. A dedicated test bench was designed in order to perform tensile tests on fresh MSB porcine specimens, with quasi-static (1 mm/s) and dynamic (100 mm/s) loading conditions. The mechanical behavior of MSB specimens was investigated and compared to isolated mesenteric and isolated small bowel specimens. The results show a high sensitivity of MSB stiffness (1.0 ± 0.2 and 1.3 ± 0.3 N/mm at 1 and 100 mm/s, p = 0.001) and ultimate force (22 ± 5 and 35 ± 8 N at 1 and 100 mm/s, p = 0.001) to the loading rate but not for the displacement at failure. This leads to postulate on a failure criteria based on strain level regardless of the strain rate. These experimental results could be further used to develop refined finite element models and to further investigate on injury mechanisms associated to seat belt restraints, as well as to evaluate and improve protective devices.