Ex-vivo biomechanical analysis of an original repair of canine calcaneal tendon rupture using a synthetic implant as mechanical support fixed by sutures in the proximal tendinous part and by an interference screw in the bone distal part.

Background: Rupture of the common calcaneal tendon is the second most frequent tendon rupture in dogs and may lead to severe lameness and pain. Surgical repair consists of re-apposition of the damaged tendon ends using sutures, but this type of repair is not always possible especially if the tendon has retracted. Tendon augmentation with an ultra-high molecular weight polyethylene (UHMWPE) implant is a recent solution to support the sutures and allow the repair of the canine calcaneal tendon. However, its biomechanical fixation strength remains untested for this pathology. Aim: To evaluate the biomechanical fixation strength of a UHMWPE implant for the repair of the canine calcaneal tendon. Methods: Ex-vivo biomechanical study was carried out on eight cadaveric hindlimbs from four adult dogs. Hindlimbs were tested under two independent modalities: proximal tendinous fixation (PTF) and distal calcaneus fixation (DCF), using a testing machine. PTF was achieved by eight simple interrupted polypropylene sutures performed through the UHMWPE implant. The latter was sandwiched inside the gastrocnemius tendon, which had previously been incised over about 5 cm longitudinally, and through the tendon of the superficial digital flexor. DCF was performed using an interference screw, which locked the UHMWPE implant into a calcaneus tunnel drilled perpendicularly. Results: Yield, failure load, and linear stiffness (mean ± SD) for the DCF modality were 920 ± 139 N, 1,007 ± 146 N, and 92 ± 15.21, respectively, which were greater than for the PTF modality (663 ± 92 N, 685 ± 84 N and 25.71 ± 5.74, respectively, p < 0.05). Failure modes were different between fixation modalities: for PTF it was suture breakage (n = 7/8), while for DCF it was implant damage and slippage (n = 8/8). Conclusion: The biomechanical fixation strength of the UHMWPE implant was greater for DCF than that of PTF, and should be suitable for calcaneal tendon repair in dogs. The clinical prediction of rupture of this calcaneal tendon repair will occur at the level of the PTF.

Implications of range of motion requirements for the laxity of ligaments in a lumbar finite element model.

Finite element models of the lumbar spine often adopt ligament properties from tensile tests without accounting for possible differences between testing and in situ initial ligament length. Such differences could result in laxities or preloads at the beginning of a simulation that would affect the ligament forces, tangent stiffness, and the posture at which they fail. In vivo and in vitro human experimental data reported laxities or preloads. However, laxities or preloads, which could also result from postural differences, are often neglected in simulation studies. This study proposes a numerical methodology to identify ranges of ligament laxities or preloads compatible with the selected tensile ligament properties, the model, and the range of motion (RoM) the model aims to simulate. The approach assumes that ligaments should remain in a safe elongation range for the complete RoM, and that each ligament should play a significant mechanical role in at least one load case. The methodology was applied to the functional spinal unit (FSU) models using the RoM from healthy subjects and ligament properties from the literature. Without laxity, some ligaments reached their elongation at failure within the RoM. Laxity ranges varied considerably (from -9.2 mm preload to 10.7 mm laxity) and flexion was the most critical load case to determine them. Their effect on the mobility response was also assessed. The effect on the mobility of a FSU was also assessed. While the proposed method cannot determine an exact laxity value, it is simple and it can be applied to any model to identify a plausible range of ligament initial length.

Biomechanical cyclic loading test of a synthetic ligament fixation system used for intra-articular stabilization of deficient canine stifles.

Background: Cranial cruciate ligament rupture (CCLr) is the most common cause of hind limb lameness in dogs. Currently, surgical management of CCLr is mostly performed using tibial osteotomy techniques to modify the biomechanical conformation of the affected stifle. These surgical techniques have a significant complication rate, associated with persistent instability of the stifle which may lead to chronic postoperative pain. Over the last decade, studies have been published on various techniques of anatomical caudal cruciate ligament reconstruction in veterinary practice, using physiological autografts or woven synthetic implants. Aim: The aim of this ex vivo biomechanical study is to investigate the ex vivo dynamic biomechanical behavior of a synthetic implant [ultrahigh molecular weight polyethylene (UHMWPE) implant] fixed with interference screws for the treatment of CCLr in dogs, according to a fatigue protocol (48 hours per test). Methods: Seven stifles from four skeletally mature canine cadavers were implanted with the synthetic implant. It was fixed with four interference screws inserted in transversal and oblique tunnels in both the distal femur and the proximal tibia. For each case, 100,000 cycles were performed at 0.58 Hz, with traction loads ranging from 100 to 210 N. Results: Neither screw-bone assembly rupture nor a pull-out issue was observed during the dynamic tests. Linear stiffness of the implants associated with a fixation system with four interference screws increased over time. The final displacement did not exceed 3 mm for five of the seven specimens. Five of the seven synthetic implants yielded to a lengthening in functional range (0-3 mm). Linear stiffness was homogeneous among samples, showing a strong dynamic strength of the interference screw-based fixations of the UHMWPE implant in the femoral and tibial bones. Conclusion: This study completes the existing literature on the biomechanical evaluation of passive stifle stabilization techniques with a testing protocol focused on cyclic loading at a given force level instead of driven by displacement. These biomechanical results should revive interest in intra-articular reconstruction after rupture of the CCLr in dogs.

Dynamic estimation of soft tissue stiffness for use in modeling socket, orthosis or exoskeleton interfaces with lower limb segments.

Modeling the interface between the lower limb segments and a socket, orthosis or exoskeleton is crucial to the design, control, and assessment of such devices. The present study aimed to estimate translational and rotational soft tissue stiffness at the thigh and shank during daily living activities performed by six subjects. Smooth orthogonal decomposition (SOD) was used on skin marker trajectories and fluoroscopy-based knee joint kinematics to compute stiffness coefficients during squatting, sitting and rising from a chair, level walking, and stair descending. On average, for all subjects and for all activities, in the anatomical directions observed, the translational and rotational stiffness coefficients for the shank were, respectively, 1.4 ± 1.99kN/m (median and interquartile range) and 41.5 ± 34.3Nm/deg. The results for the thigh segment were 1.79 ± 2.73kN/m and 30.5 ± 50.4Nm/deg. As previously reported in the literature dealing with the soft tissue artifact - considered as soft tissue deformation in this study - the computed stiffness coefficients were dependent on tasks, subjects, segments, and anatomical directions. The main advantage of SOD over previous methods lies in enabling estimation of a task-dependent 6 × 6 stiffness matrix of the interface between segments and external devices, useful in their modeling and assessment.

Ligaments Laxity and Elongation at Injuryin Flexed knees during Lateral Impact Conditions.

The knee is one of the regions of interest for pedestrian safety assessment. Past testing to study knee ligament injuries for pedestrian impact only included knees in full extension and mostly focused on global responses. As the knee flexion angle and the initial ligament laxity may affect the elongation at which ligaments fail, the objectives of this study were (1) to design an experimental protocol to assess the laxity of knee ligaments before measuring their elongation at failure, (2) to apply it in paired knee tests at two flexion angles (10 and 45 degrees). The laxity tests combined strain gauges to measure bone strains near insertions that would result from ligament forces and a custom machine to exercise the knee in all directions. Failure was assessed using a four-point bending setup with additional degrees of freedom on the axial rotation and displacement of the femur. A template was designed to ensure that the two setups used the exact same starting position. The protocol was applied to six pairs of knees which were tested until the failure of all ligaments. In the laxity tests, a higher compliance of the knee was observed at 45 degrees compared to 10 degrees. Minimum lengths associated with the beginning of bone loading were also successfully identified for the collateral ligaments, but the process was less successful for the cruciate ligaments. The failure tests suggested increased elongation and length at failure for the ligaments and their bundles at 45°. This could be consistent with the higher compliance in static test, but the minimum lengths identified on the collaterals did not explain this difference during failure. The results highlight the possible relationship between position, laxity and elongation at failure in a lateral loading and provide a dataset including 3D coordinates of insertions to continue the investigation using a modelling approach. Perspectives are also outlined to improve upon the laxity determination protocol.

A comparative biomechanical study of the Distal Tibia Nail against compression plating for the osteosynthesis of supramalleolar corrective osteotomies.

The Distal Tibia Nail (DTN; Mizuho, Japan) has demonstrated higher biomechanical stiffness to locking plates in previous research for A3 distal tibia fractures. It is here investigated as a fixation option for supramalleolar corrective osteotomies (SMOT). Sixteen Sawbones tibiae were implanted with either a DTN (n = 8) or Medial Distal Tibia Plate (MDTP; n = 8) and a SMOT simulated. Two surgical outcome scenarios were envisaged: "best-case" representing an intact lateral cortex, and "worst-case" representing a fractured lateral cortex. All samples were subjected to compressive (350 N, 700 N) and torsional (± 4 Nm, ± 8 Nm) testing. Samples were evaluated using calculated construct stiffness from force-displacement data, interfragmentary movement and Von Mises' strain distribution. The DTN demonstrated a greater compressive stiffness for the best-case surgical scenario, whereas the MDTP showed higher stiffness (p < 0.05) for the worst-case surgical scenario. In torsional testing, the DTN proved more resistant to torsion in the worst-case surgical setup (p < 0.05) for both ± 4 Nm and ± 8 Nm. The equivalent stiffness of the DTN against the MDTP supports the use of this implant for SMOT fixation and should be considered as a treatment option particularly in patients presenting vascularisation problems where the MDTP is an inappropriate choice.

A Method to Use Kriging With Large Sets of Control Points to Morph Finite Element Models of the Human Body.

As developing finite element (FE) human body models for automotive impact is a time-consuming process, morphing using interpolation methods such as kriging has often been used to rapidly generate models of different shapes and sizes. Kriging can be computationally expensive when many control points (CPs) are used, i.e., for very detailed target geometry (e.g., shape of bones and skin). It can also lead to element quality issues (up to inverted elements) preventing the use of the morphed models for finite element simulation. This paper presents a workflow combining iterative subsampling and spatial subdivision methodology that effectively reduces the computational costs and allows for the generation of usable models through kriging with hundreds of thousands of control points. As subdivision introduces discontinuities in the interpolation function that can cause distortion of elements on the boundaries of individual subdivision areas, algorithms for smoothing the interpolation over those boundaries are proposed and compared. Those techniques and their combinations were tested and evaluated in a scenario of mass change on the detailed 50th percentile male model of the global human body models consortium (GHBMC): the model, which has body mass index (BMI) 25.34, was morphed toward a statistical surface model of a person with body mass index 20, 22.7 and 35. 234 777 control points were used to successfully morph the model in less than 15 min on an office PC. Open source implementation is provided.

Geometrical and Mechanical Characterization of the Abdominal Fold of Obese Post Mortem Human Subjects for Use in Human Body Modelling.

Obese vehicle occupants sustain specific injury patterns in case of accidents in which the interaction between the seat belt and the abdomen may play a role. This study aimed to collect geometrical characteristics and to investigate the mechanical responses of the abdomen of obese subjects. Four Post Mortem Human Subjects (PMHS) with BMI ranging from 31 to 46 kg/m2 were collected. CT-scans performed in the seated position revealed that the antero-posterior depth of the abdominal fold (from the inguinal region to the most anterior point of the abdominal surface) was much greater (170 mm max., 127 mm average) than the thickness of subcutaneous adipose tissues (85 max., 38 mm in average). Each PMHS was subjected to three infra-injurious antero-posterior belt pulls in a seated posture with a lap belt positioned (C1) superior to the umbilicus, (C2) inferior to the umbilicus, (C3) inside the abdominal fold between the abdomen and the thigh. During the C1 and C2 tests, the belt moved cranially, and the abdominal fold opened widely especially in C2. Forces remained below 1800 N, for maximum applied displacements ranging from 89 to 151 mm for C1 and C2, and 37 to 66 mm for C3. Finally, sled tests were conducted on two PMHS seated on a semi-rigid seat and restrained by a three-point belt equipped with pretensioners and a 3.5 kN force limitation at the shoulder. The first PMHS (BMI 39 kg/m2) was tested at 49 km/h (39 g peak) and sustained severe injuries (AIS 4 pelvis dislocation, AIS 3 bilateral femur fractures) attributed to the combined loading of the seat and lap belt force (about 11 kN and 7 kN, respectively). The second PMHS (BMI 46 kg/m2) was subjected to a 29 km/h test (8 g plateau) and sustained no injury. The lap belt slid inside the abdominal fold in the first case and deformed the lower abdomen in the second, providing limited restraint forces during that interaction and leading to a large body excursion for the first test. The results highlight the possible relevance of the abdominal fold at the abdomen thigh junction to model and study the restraint conditions of obese occupants using Human Body Models (HBM).

Kinematics of the Normal Knee during Dynamic Activities: A Synthesis of Data from Intracortical Pins and Biplane Imaging.

Few studies have provided in vivo tibiofemoral kinematics of the normal knee during dynamic weight-bearing activities. Indeed, gold standard measurement methods (i.e., intracortical pins and biplane imaging) raise ethical and experimental issues. Moreover, the conventions used for the processing of the kinematics show large inconsistencies. This study aims at synthesising the tibiofemoral kinematics measured with gold standard measurement methods. Published kinematic data were transformed in the standard recommended by the International Society of Biomechanics (ISB), and a clustering method was applied to investigate whether the couplings between the degrees of freedom (DoFs) are consistent among the different activities and measurement methods. The synthesised couplings between the DoFs during knee flexion (from 4° of extension to -61° of flexion) included abduction (up to -10°); internal rotation (up to 15°); and medial (up to 10 mm), anterior (up to 25 mm), and proximal (up to 28 mm) displacements. These synthesised couplings appeared mainly partitioned into two clusters that featured all the dynamic weight-bearing activities and all the measurement methods. Thus, the effect of the dynamic activities on the couplings between the tibiofemoral DoFs appeared to be limited. The synthesised data might be used as a reference of normal in vivo knee kinematics for prosthetic and orthotic design and for knee biomechanical model development and validation.

Comparison of Kriging and Moving Least Square Methods to Change the Geometry of Human Body Models.

Finite Element Human Body Models (HBM) have become powerful tools to study the response to impact. However, they are typically only developed for a limited number of sizes and ages. Various approaches driven by control points have been reported in the literature for the non-linear scaling of these HBM into models with different geometrical characteristics. The purpose of this study is to compare the performances of commonly used control points based interpolation methods in different usage scenarios. Performance metrics include the respect of target, the mesh quality and the runability. For this study, the Kriging and Moving Least square interpolation approaches were compared in three test cases. The first two cases correspond to changes of anthropometric dimensions of (1) a child model (from 6 to 1.5 years old) and (2) the GHBMC M50 model (Global Human Body Models Consortium, from 50th to 5th percentile female). For the third case, the GHBMC M50 ribcage was scaled to match the rib cage geometry derived from a CT-scan. In the first two test cases, all tested methods provided similar shapes with acceptable results in terms of time needed for the deformation (a few minutes at most), overall respect of the targets, element quality distribution and time step for explicit simulation. The personalization of rib cage proved to be much more challenging. None of the methods tested provided fully satisfactory results at the level of the rib trajectory and section. There were corrugated local deformations unless using a smooth regression through relaxation. Overall, the results highlight the importance of the target definition over the interpolation method.

Multi-objective optimisation for musculoskeletal modelling: application to a planar elbow model.

One of the open issues in musculoskeletal modelling remains the choice of the objective function that is used to solve the muscular redundancy problem. Some authors have recently proposed to introduce joint reaction forces in the objective function, and the question of the weights associated with musculo-tendon forces and joint reaction forces arose. This question typically deals with a multi-objective optimisation problem. The aim of this study is to illustrate, on a planar elbow model, the ensemble of optimal solutions (i.e. Pareto front) and the solution of a global objective method that represent different compromises between musculo-tendon forces, joint compression force, and joint shear force. The solutions of the global objective method, based either on the minimisation of the sum of the squared musculo-tendon forces alone or on the minimisation of the squared joint compression force and shear force together, are in the same range. Minimising either the squared joint compression force or shear force alone leads to extreme force values. The exploration of the compromises between these forces illustrates the existence of major interactions between the muscular and joint structures. Indeed, the joint reaction forces relate to the projection of the sum of the musculo-tendon forces. An illustration of these interactions, due to the projection relation, is that the Pareto front is not a large surface, like in a typical three-objective optimisation, but almost a curve. These interactions, and the possibility to take them into account by a multi-objective optimisation, seem essential for the application of musculoskeletal modelling to joint pathologies.

Intraoperative three dimensional correction during in situ contouring surgery by using a numerical model.

STUDY DESIGN: A numerical study was conducted by simulating in situ contouring (ISC) surgery. OBJECTIVE: To quantify intraoperative correction during ISC surgery. SUMMARY OF BACKGROUND DATA: Surgical techniques correcting scoliosis, like the ISC one, lead to a complex 3-dimensional correction of the spine. Using motion analysis devices to analyze the effect of intraoperative surgical maneuvers was tedious and limited the study to the kinematics of exposed vertebrae. An alternative method consisted in simulating the surgical gestures. However, proposed models were based on rigid instrumentations, and focused attention on specific gestures of the rod-rotation and the distraction techniques through operator-dependent simulations. METHODS: This study included 10 patients with severe idiopathic scoliosis treated by ISC surgery. From a patient-specific finite-element model (T1-L5 and pelvis), all main steps of the ISC surgery were automatically simulated. A specific algorithm was developed to determine the sequences of bending maneuvers according to the rod shapes chosen by the surgeon. The accuracy of the automated surgery simulation was assessed regarding the virtual postoperative spinal configuration and postoperative clinical data. For each maneuver, vertebral kinematics was computed as well as the evolution of various clinical parameters. RESULTS: The bending maneuvers of both the first and the second rods provided complementary effects inside, but also outside the fused spinal area. These main maneuvers combined the intraoperative spinal corrections induced by maneuvers specific to the rod-rotation surgery. CONCLUSION: The automated patient-specific simulation of ISC surgery may improve the understanding of the main mechanisms involved in the scoliosis surgical correction.

In vivo distribution of spinal intervertebral stiffness based on clinical flexibility tests.

STUDY DESIGN: A numerical study was conducted to identify the intervertebral stiffness of scoliotic spines from spinal flexibility tests. OBJECTIVE: To study the intervertebral 3-dimensional (3D) stiffness distribution along scoliotic spine. SUMMARY OF BACKGROUND DATA: Few methods have been reported in literature to quantify the in vivo 3D intervertebral stiffness of the scoliotic spine. Based on the simulation of flexibility tests, these methods were operator-dependent and could yield to clinically irrelevant stiffnesses. METHODS: This study included 30 patients surgically treated for severe idiopathic scoliosis. A previously validated trunk model, with patient-specific geometry, was used to simulate bending tests according to the in vivo displacements of T1 and L5 measured from bending test radiographs. Differences between in vivo and virtual spinal behaviors during bending tests (left and right) were computed in terms of vertebral rotations and translation. An automated method, driven by a priori knowledge, identified intervertebral stiffnesses in order to reproduce the in vivo spinal behavior. RESULTS: Because of the identification of intervertebral stiffnesses, differences between in vivo and virtual spinal displacements were drastically reduced (95% of the differences less than +/-3 mm for vertebral translation). Intervertebral stiffness distribution after identification was analyzed. On convex side test, the intervertebral stiffness of the compensatory curves increased in most cases, whereas the major curve became more flexible. Stiffness singularities were found in junctional zones: these specific levels were predominantly flexible, both in torsion and in lateral bending. CONCLUSION: The identification of in vivo intervertebral stiffness may improve our understanding of scoliotic spine and the relevance of patient-specific methods for surgical planning.

Intraoperative three-dimensional correction during rod rotation technique.

STUDY DESIGN: A numerical study was conducted by simulating the Cotrel-Dubousset (CD) surgery. OBJECTIVE: To quantify intraoperative correction during CD surgery. SUMMARY OF BACKGROUND DATA: Very few methods have been reported in literature to analyze the effect of intraoperative surgical gestures, and none considers the three-dimensional correction of the entire spine during the main surgical gestures. Intraoperative frontal radiographs limit analysis to two-dimensional correction, and movement tracking devices focus the kinematics study of specific vertebrae in the instrumented area only. METHODS: This study included 20 patients, mean age 15 years, with severe idiopathic scoliosis treated by CD surgery. A patient-specific finite-element model (T1-L5 and pelvis), based on preoperative stereo-radiography and flexibility test radiographs, was constructed for each patient. An automated algorithm simulated all the main steps of the CD surgery. For each step, vertebral kinematics was exported to compute the evolution of various clinical parameters. Coherence of the simulations was evaluated by comparing the virtual postoperative spinal configuration with postoperative in vivo data. RESULTS: The CD surgery affected the vertebral levels inside but also outside the fused spinal area, in a three-dimensional complex kinematics. Every intraoperative maneuver contributes to scoliosis correction. The second rod insertion, focused on the apical vertebra, leading to a global modification of all the curves. CONCLUSIONS: The automated patient-specific simulation of CD surgery may improve our understanding of surgical biomechanics. Therefore, it could increase the relevance of preoperative surgery planning.

The effects of posture and subject-to-subject variations on the position, shape and volume of abdominal and thoracic organs.

In this study, the thorax and the abdomen of nine subjects were imaged in four postures using a positional MRI scanner. The four postures were seated, standing, forward-flexed and supine. They were selected to represent car occupants, pedestrians, cyclists and a typical position for medical imaging, respectively. Geometrical models of key anatomical structures were registered from the imaging dataset using a custom registration toolbox. The analysis of the images and models allowed the quantification of the respective effects of posture and subject-to-subject variation on the position, shape and volume of the abdominal organs, skeletal components and thoracic cavity. In summary, except for the supine posture, the organ volumes and their positions in the spinal frame were mostly unaffected by the posture. The supine posture was associated with a motion of all solid organs of up to 39 mm (interpostural maximum for the liver, n=9), and a reduction of the thoracic cavity volume of up to 1300 cm3. Subject-to-subject variations were especially large for the volume of the spleen (variations between 120 and 400 cm3) and the position of the kidneys. As a result, subject-to-subject variations were larger than most postural effects. Other results include values of parameters that can help positioning human models such as positions, volumes and inertial properties of organs as well as skeletal parameters. Overall, this study suggests that subject-to-subject variations and the use of supine geometrical data can be problematic for finite element modeling of the abdomen for injury prediction.