Abstract This study investigates the biomechanics of the spine after insertion of vertebral body tethering (VBT) with different cord pretensions. For that purpose, a Finite Element model of the average thoracolumbar spine was stepwise calibrated and validated. The VBT instrumentation was inserted in the left side of the L1-L2 segment with different cord pretensions. As a second test, the L1-L2 segment was submitted to an external pure moment of 6 Nm in left and right lateral bending. The range of motion (ROM) for the spine with VBT was determined with respect to its initial post VBT position. Pretension forces of 100 N and 300 N resulted in a change of scoliotic angle of 2.7° and 5.3° to the left side of the spine, respectively. The ROM of the native spine was 4.5° in right lateral bending and reduced to 1.8° and 1.4° for the cases of the spine with a cord pretension of 100 N and 300 N, respectively. In left lateral bending, the absolute ROM of the native spine was 4.6°. For the cases of a cord pretension of 100 N and 300 N, the spine bent 1.9° and 0.8° to the left side from its initial post VBT position, respectively.

Abstract Aiming at multiscale numerical investigations of fibrous connective tissues, the present work proposes a finite strain viscoelastic model suitable to represent the local mechanical response of living cells in conjunction with finite element simulations of nanoindentation tests. The material model is formulated in a thermodynamically consistent framework based on a variational constitutive approach. As a case of study, a numerical investigation of the local compressible response of fibroblast cells is addressed. Moreover, a set of cyclic, stress relaxation and creep experiments are simulated in order to investigate the capability of the model to predict rate dependence of cells, showing sound agreement with experimental data. The proposed model may be used as a suitable tool for a better understanding of stiffness and energetic dissipation of cells.

Abstract Introduction The method of graft fixation is critical in anterior cruciate ligament (ACL) reconstruction surgery. Success of surgery is totally dependent on the ability of the implant to secure the graft inside the bone tunnel until complete graft integration. The principle of EndoButton is based on the cortical suspension of the graft. The Cross-Pin is based on graft expansion. The aim of this study was to evaluate the biomechanical performance of EndoButton and Bio Cross-Pin to fix the hamstring graft at femoral side of porcine knee joints and evaluate whether they are able to support of loading applied on graft during immediate post-operative tasks. Methods Fourteen ACL reconstructions were carried out in porcine femurs fixing superficial flexor tendons with Titanium EndoButton (n = 7) and with 6 × 50 mm HA/PLLA Bio Cross-Pin (n = 7). A cyclic loading test was applied with 50-250 N of tensile force at 1 Hz for 1000 cycles. The displacement was measured at 20, 100, 500 and 1000 load cycles to quantify the slippage of the graft during the test. Single-cycle load-to-failure test was performed at 50 N/mm to measure fixation strength. Results The laxity during cyclic loading and the displacement to failure during single-cycle test were lower for the Bio Cross-Pin fixation (8.21 ± 1.72 mm) than the EndoButton (11.20 ± 2.00 mm). The Bio Cross-Pin (112.22 ± 21.20 N.mm–1) was significantly stiffer than the EndoButton fixation (60.50 ±10.38 N.mm–1). There was no significant difference between Bio Cross-Pin (failure loading: 758.29 ± 188.05 N; yield loading: 713.67 ± 192.56 N) and EndoButton strength (failure loading: 672.52 ± 66.56 N; yield loading: 599.91 ± 59.64 N). Both are able to support the immediate post-operative loading applied (445 N). Conclusion The results obtained in this experiment indicate that the Bio Cross-Pin technique promote stiffer fixation during cyclic loading as compared with EndoButton. Both techniques are able to support the immediate post-operative loading applied.

La ruptura del ligamento cruzado anterior (LCA) es la lesión más común de la rodilla humana. Cuando se requiere cirugía, es de mucha ayuda para los cirujanos definir científicamente el mejor punto de inserción del injerto, para que pueda tener una funcionalidad similar a la de un LCA intacto. Para esto, es crucial la estimación de la fuerza que actúa en el ligamento (o injerto) en respuesta a una carga externa aplicada sobre la rodilla. Esta fuerza es llamada fuerza in-situ. El objetivo de esta investigación es evidenciar la relevancia del comportamiento hiperelástico de los ligamentos cruzados en el modelaje bidimensional de la rodilla. Para ello, se propone una metodología secuencial de modelaje basándose en teoría de mecanismos y el método de Davies. En una primera aproximación, los ligamentos cruzados son considerados como cuerpos rígidos; en una segunda aproximación, como cuerpos con comportamiento hiperelástico. Esas dos aproximaciones son comparadas. El modelo proporciona informaciones que permiten asistir el planeamiento pre-operatorio, mediante la simulación de las posiciones y la fuerzas in-situ del LCA. La metodología propuesta consiste en cuatro pasos y considera un procedimiento experimental realizado mediante un manipulador robótico que obtiene las fuerzas in-situ. Las fuerzas in-situ experimentales son usadas para validar el modelo propuesto. Además de apoyar al planeamiento pre-operatorio, el modelo permite verificar dos hipótesis biomecánicas relevantes: 1. Para la simulación de la fuerza in-situ del LCA, el modelaje de los ligamentos cruzados como barras rígidas, presenta resultados semejantes a los del modelaje que considera el comportamiento hiperelástico (más elaborado). 2. Las fuerzas in-situ del LCA pueden ser aproximadas satisfactoriamente, cuando la rodilla es modelada como un mecanismo bidimensional de 4-barras. Con base en los resultados puede concluirse que las fuerzas obtenidas por simulaciones que consideran el comportamiento hiperelástico de los ligamentos cruzados, son muy próximas a aquellas fuerzas obtenidas en simulaciones que consideran los ligamentos cruzados como cuerpos rígidos. También se puede observar que los resultados simulados son bastante similares a los resultados experimentales, lo que es relevante considerando que el modelaje propuesto es simplificado.

The rupture of the anterior cruciate ligament (ACL) is the most common injury of the human knee. [A1] When surgery is required, it is helpful for orthopedic surgeons to scientifically define the best position for the graft, which approximates the functionality of an intact ACL. To accomplish that, it is crucial to estimate the force acting on the ligament (or graft) in response to an external load applied to the knee. This force is called the in-situ force. The objective of this research is to evidence the relevance of the hyperelastic behavior of cruciate ligaments in the two-dimensional modeling of the knee. To achieve this, a sequential method of modeling is proposed based on the theory of mechanisms and Davies' method. In a first approach the cruciate ligaments are treated as rigid bodies, and in a second approach, as hyperelastic bodies. These two approaches are then compared. The model provides information to assist the preoperative planning, by simulation of the ACL positions and in-situ forces. The proposed methodology consists of four steps and an experimental procedure performed by a robotic manipulator to obtain the in-situ forces. Experimental in-situ forces are used to validate the proposed model. Besides helping the preoperative planning, the model allows verifying two relevant biomechanical hypotheses: 1. During the simulation of the ACL in-situ force, the modeling of the cruciate ligaments as rigid links shows similar results to the modeling, which considers the hyperelastic behavior (more complex). 2. The ACL in-situ force can be well approximated when the knee is modeled as a two-dimensional four bar mechanism. Based on the results it can be concluded that the forces obtained by simulations that consider the hyperelastic behavior of the cruciate ligaments are close to the forces obtained by simulations that consider the cruciate ligaments as rigid bodies. It can also be noted that the simulated results are quite similar to the experimental results, which is important considering that the proposed model is simplified.

Introduction The rupture of the anterior cruciate ligament (ACL) is the most common type of knee injury. Reconstructive surgery is the ‘gold standard’ treatment. During the immediate post-operative period, the fixation of the graft is entirely dependent on the ability of the grafted implant to be secured inside the bone tunnel under the cyclical loads associated with daily tasks. Poor fixation can lead to graft slippage, thus impairing the healing and integration of the graft. The aim of this study was to evaluate the biomechanical performance of tendon graft fixation devices with metallic and bioabsorbable interference screws. Methods Twenty ACL reconstructions were carried out in porcine tibias using deep flexor tendons to fix 9 × 20 mm metallic (n=10) and PLLA 70/30 bioabsorbable screws (n=10). To verify the ability of a construct to resist immediate postoperative (PO) rehabilitation protocols for immediate load bearing, a cyclic loading test was applied with 50-250 N of tensile force at 1 Hz for 1000 cycles, and the displacement was measured at 10, 50, 100, 500 and 1000 load cycles to quantify the slippage of the graft during the test. After the cyclic loading test, a single-cycle load-to-failure test was applied. Results The slippage of the graft using metallic screws did not differ (P = 0.616) from that observed when using bioabsorbable screws. Conclusion The results obtained in this experiment indicate that metallic screws may promote a similar amount of graft slippage during low cyclic loading as bioabsorbable screws. Additionally, there was no difference in the biomechanical performance of these two types of screws during high failure loads.

OBJETIVO: Por ser a articulação mecanicamente mais solicitada de nossa estrutura e pelo grande número de lesões associadas, motivaram a construção de um modelo tridimensional da articulação do joelho humano para simular a cinemática da articulação e obter as solicitações mecânicas nos principais ligamentos durante o movimento de flexão do joelho. Essas informações podem futuramente ser empregada como ferramenta de apoio à decisão médica em ortopedia, fornecendo subsídios na escolha do procedimento cirúrgico. MÉTODOS: Método dos Elementos Finitos foi utilizado para construir um modelo biomecânico, tridimensional, da articulação do joelho. Nesse modelo com seis graus de liberdade é aplicado movimento de flexão/extensão sendo os demais cinco graus de liberdade governados pelas interações entre os componentes da articulares. RESULTADOS: Foram obtidas informações dos movimentos, das rotações interna/externa e adução/abdução, das translações anterior/posterior, lateral/medial e superior/inferior e dos esforços nos quatro principais ligamentos articulares, no decorrer de um amplo movimento de flexão/extensão. Estes valores foram comparados, de forma qualitativa, com valores equivalentes obtidos na literatura. CONCLUSÃO: A análise de resultados permitiu observar que vários aspectos cinemáticos são satisfatoriamente reproduzidos. A pré-carga inicial dos ligamentos e o posicionamento das inserções ligamentares no modelo mostraram-se variáveis relevantes nos resultados.

OBJECTIVE: The knee joint is the part of our structure upon which most mechanical demands are placed and a large number of lesions are associated to it. These factors motivated the construction of a three-dimensional model of the human knee joint in order to simulate joint kinematics and obtain the mechanical demands on the main ligaments during knee flexion movements. METHODS: The finite elements method was used to build a three-dimensional, biomechanical model of the knee joint. In this model with six degrees of freedom, the flexion/extension movement is applied, while the other five degrees of freedom are governed by the interactions between joint components. RESULTS: Data was collected on the movements, on the internal/external and adduction/ abduction rotations, on the anterior/posterior, lateral/medial and upper/lower translations, and on the forces acting upon the four main joint ligaments, during a wide flexion/extension movement. These values were qualitatively compared with comparable values available in the literature. CONCLUSIONS: It was observed through an analysis of the results that several kinematic aspects are satisfactorily reproduced. The initial pre-load of the ligaments and the positioning of the ligament insertions in the model were shown to be relevant variables in the results.