Mechanical strength of a new plate compared to six previously tested opening wedge high tibial osteotomy implants

Background This study aimed to assess the mechanical static and fatigue strength provided by the FlexitSystem plate in medial opening wedge high tibial osteotomies (MOWHTO), and to compare it to six previously tested implants: the TomoFix small stature, the TomoFix standard, the ContourLock, the iBalance, the second generation PEEKPower and the size 2 Activmotion. Thus, this will provide surgeons with data that will help in the choice of the most appropriate implant for MOWHTO. Methods Six fourth-generation tibial bone composites underwent a MOWHTO and each was fixed using six FlexitSystem plates, according to standard techniques. The same testing procedure that has already been previously defined, used and published, was used to investigate the static and dynamic strength of the prepared bone-implant constructs. The test consisted of static loading and cyclical loading for fatigue testing. Results During static testing, the group constituted by the FlexitSystem showed a fracture load higher than the physiological loading of slow walking (3.7 kN > 2.4 kN). Although this fracture load was relatively small compared to the average values for the other Implants from our previous studies, except for the TomoFix small stature and the Contour Lock. During fatigue testing, FlexitSystem group showed the smallest stiffness and higher lifespan than the TomoFix and the PEEKPower groups. Conclusions The FlexitSystem plate showed sufficient strength for static loading, and average fatigue strength compared to the previously tested implants. Full body dynamic loading of the tibia after MOWHTO with the investigated implants should be avoided for at least three weeks. Implants with a wider T-shaped proximal end, positioned onto the anteromedial side of the tibia head, or inserted in the osteotomy opening in a closed-wedge construction, provided higher mechanical strength than implants with small a T-shaped proximal end, centred onto the medial side of the tibia head

The biomechanical effects of allograft wedges used for large corrections during medial opening wedge high tibial osteotomy

The inclusion of an allograft wedge during medial opening wedge high tibial osteotomy has been shown to lead to satisfactory time-to-union in larger corrections (>10°). Such large corrections are associated with greater incidences of intraoperative hinge fracture and reduced construct stability. The purpose of this study was to investigate the biomechanical stability that an allograft wedge brings to an osteotomy. Ten medium-size fourth generation artificial sawbone tibiae underwent 12 mm biplanar medial opening wedge high tibial osteotomy with a standard Tomofix plate. Five tibiae had an allograft wedge inserted into the osteotomy gap prior to plate fixation (allograft group). The gap in the remaining tibiae was left unfilled (control group). Each group underwent static compression testing and cyclical fatigue testing until failure of the osteotomy. Peak force, valgus malrotation, number of cycles, displacement and stiffness around the tibial head were analysed. Intraoperative hinge fractures occurred in all specimens. Under static compression, the allograft group withstood higher peak forces (6.01 kN) compared with the control group (5.12 kN). Valgus malrotation was lower, and stiffness was higher, in the allograft group. During cyclical fatigue testing, results within the allograft group were more consistent than within the control group. This may indicate more predictable results in large osteotomies with an allograft. Tibial osteotomies with allograft wedges appear beneficial for larger corrections, and in cases of intraoperative hinge fracture, due to the added construct stability they provide, and the consistency of results compared with tibial osteotomies without a graft

Etude biomécanique comparative de cinq différents systèmes de fixation utilisés dans les cas d'ostéotomies tibiales valgisantes: Essais expérimentaux et simulations numériques incluant les forces musculaires

This research project was carried out in partnership with the Orthopaedic and Traumatology service of the “clinique d’Eich” of the Centre Hospitalier de Luxembourg. The main objective consisted in a comparative biomechanical study of the stability of five different currently used implants for open-wedge high tibial osteotomy (HTO). The following implants were tested in the comparative biomechanical study: Contour Lock HTO, PEEKPower HTO, iBalance HTO, TomoFix standard and TomoFix small stature. The implants were chosen freely from the market and there has not been any funding or link to any of the manufacturers. The comparison was first made experimentally using static compression loading to failure and dynamic loading to failure tests, then computationally using simulations by mean of the finite element method. Muscle forces were predicted using musculoskeletal modeling and applied to the finite element models of the lower limb that simulated the stance phase of the gait cycle. The finite element models created included all the bones of the lower limb, except those of the foot, as well as the menisci, the articular cartilage layers of the knee and the patellar tendon which was modelled by springs. The comparative study using numerical simulations was done considering two separate loadings: (1) application of a compressive load on the tibial plateau and (2) consideration of muscle forces. The comparison of the two types of loading (1) and (2) showed that loading (1) used during the mechanical tests is compatible with a realistic loading of the tibia with the leg at 15 % of the gait cycle during slow walking. Observations from numerical simulations considering loading (2) emphasized the necessity to take into account the muscle forces in the testing protocols and implant design process. The results of the numerical simulations considering loading (1) were in line with the findings of the experimental study. All the implants tested showed sufficient stability during static loading. All the specimens failed due to fracture of the opposite cortical bone. In regards to the results of this study, the implant iBalance offered the best mechanical stability to the operated tibia, and the PEEKPower plate the worst. Simplifications were made to reduce the complexity of the different physical and numerical models; hence the transposition of the obtained results to clinical settings should be done with precaution. There is no conflict of interest in relation to this work.Ce projet de recherche s’est fait en partenariat avec le service Orthopédie et Traumatologie de la clinique d’Eich du Centre Hospitalier de Luxembourg. L’objectif majeur a consisté en une étude biomécanique comparative de la stabilité mécanique des cinq différents systèmes de fixation suivant, utilisés dans les cas d’ostéotomie tibiale de valgisation : Contour Lock HTO, PEEKPower HTO, iBalance HTO, TomoFix standard et TomoFix « small stature ». Les implants ont été librement choisis sans lien quelconque avec les fabricants et sans financement de la part de ceux-ci. La comparaison s’est faite de façon expérimentale à l’aide d’essais statiques et cycliques, puis numériquement en utilisant des simulations par la méthode des éléments finis. Les forces des muscles ont été déterminées à partir d’un modèle musculo-squelettique et appliquées aux modèles éléments finis du membre inférieur simulant la phase d’appui du cycle de la marche. Les modèles éléments finis réalisés comprennent les os du membre inférieur, hormis ceux du pied, ainsi que les ménisques, les couches de cartilage articulaire du genou et le tendon rotulien modélisé par des ressorts. L’étude comparative des implants par des simulations numériques s’est faite en considérant deux chargements distincts : (1) application d’une force compressive sur le plateau tibial et (2) considération des forces musculaires. La comparaison des deux types de chargement (1) et (2) a montré que le chargement (1) utilisé lors des essais en laboratoire est compatible avec un chargement réaliste du tibia, lors d’une marche lente, le membre inférieur se trouvant à 15 % du cycle de la marche. Les observations issues des simulations numériques considérant le chargement (2) ont montré la nécessité de tenir compte des forces musculaires dans les protocoles d’essais et des processus de conception des implants. Les résultats des simulations numériques considérant le chargement (1) ont été conformes aux résultats expérimentaux. Tous les implants testés ont été suffisamment résistants à l’endommagement lors du chargement statique. Les spécimens ont tous subi un endommagement dû à la fracture de l’os cortical opposé. Au regard des résultats de cette étude, l’implant iBalance a offert la meilleure stabilité mécanique au tibia opéré, et la plaque PEEKPower la moins bonne. Des simplifications pour réduire la complexité des modèles physiques et numériques ont été réalisées. Il faudrait donc procéder avec précaution lors du transfert des résultats obtenus dans des contextes cliniques. Il n’y a aucun conflit d’intérêt en relation à ce travail

Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy

Abstract Background This study aimed to investigate, by means of finite element analysis, the effect of a drill hole at the end of a horizontal osteotomy to reduce the risk of lateral cortex fracture while performing an opening wedge high tibial osteotomy (OWHTO). The question was whether drilling a hole relieves stress and increases the maximum correction angle without fracture of the lateral cortex depending on the ductility of the cortical bone. Methods Two different types of osteotomy cuts were considered; one with a drill hole (diameter 5 mm) and the other without the hole. The drill holes were located about 20 mm distally to the tibial plateau and 6 mm medially to the lateral cortex, such that the minimal thickness of the contralateral cortical bone was 5 mm. Based on finite element calculations, two approaches were used to compare the two types of osteotomy cuts considered: (1) Assessing the static strength using local stresses following the idea of the FKM-guideline, subsequently referred to as the “FKM approach” and (2) limiting the total strain during the opening of the osteotomy wedge, subsequently referred to as “strain approach”. A critical opening angle leading to crack initiation in the opposite lateral cortex was determined for each approach and was defined as comparative parameter. The relation to bone aging was investigated by considering the material parameters of cortical bones from young and old subjects. Results The maximum equivalent (von-Mises) stress was smaller for the cases with a drill hole at the end of the osteotomy cut. The critical angle was approximately 1.5 times higher for the specimens with a drill hole compared to those without. This corresponds to an average increase of 50%. The calculated critical angle for all approaches is below 5°. The critical angle depends on the used approach, on patient’s age and assumed ductility of the cortical bone. Conclusions Drilling a hole at the end of the osteotomy reduces the stresses in the lateral cortex and increases the critical opening angle prior to cracking of the opposite cortex in specimen with small correction angles. But the difference from having a drill hole or not is not so significant, especially for older patients. The ductility of the cortical bone is the decisive parameter for the critical opening angle

Static and dynamic differences in fixation stability between a spacer plate and a small stature plate fixator used for high tibial osteotomies – A biomechanical bone composite study

Background: The objective of the present study was to comparemechanical strength and stability of the newly designed spacer plate with the gold standard plate for the treatment of medial knee joint osteoarthritis. Materials and Methods: Ten fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomy (HTO) according to standard techniques, using five TomoFix plates and five Contour Lock plates. Static compression load to failure and load-controlled cyclical fatigue failure tests were performed. Forces and horizontal displacements were measured; plastic deformations and dynamic stiffness were determined. Results and Discussion: In all samples, rotation of the tibial head and fracture of the opposite cortex were observed. Behaviors of the specimens under static loading were comparable between groups. Cyclic testing revealed lateral significant higher stiffness untilfailure for the Contour Lock compared to the TomoFix plate. No visible implant failure was observed in any group. Conclusion: Considering the static analysis, both plates offered sufficient stability under physiologic loads of up to 3000N. The Contour Lock plate-fixated specimens showed a higher stability during the cyclic testing, supposedly due to the wider distance between the fixation screws

Biomechanics of new implants for HTO

Biomechanical characteristics of 5 tibial osteotomy plates for the treatment of medial knee joint osteoarthritis were examined. Fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomy, using TomoFix™ standard, PEEKPower®, ContourLock®, TomoFix™ small stature plates, and iBalance® implants. Static compression load to failure and load-controlled cyclic fatigue failure tests were performed. All plates had sufficient stability up to 2400 N in the static compression load to failure tests. Screw breakage in the iBalance® group and opposite cortex fractures in all constructs occurred at lower loading conditions. The highest fatigue strength in terms of maximal load and number of cycles performed prior to failure was observed for the ContourLock® group followed by the iBalance® implants, the TomoFix™ standard and small stature plates. PEEKPower® had the lowest fatigue strength. All plates showed sufficient stability under static loading. Compared to the TomoFix™ and the PEEKPower® plates, the ContourLock® plate and iBalance® implant showed a higher mechanical fatigue strength during cyclic fatigue testing, suggesting that both mechanical static and fatigue strength increase with a wider proximal T‑shaped plate design together with diverging proximal screws. Mechanical strength of the bone–implant constructs decreases with a narrow T‑shaped proximal end design and converging proximal screws (TomoFix™) or a short vertical plate design (PEEKPower®). Published results indicate high fusion rates and good clinical results with the TomoFix™ plate, which is contrary to our findings. A certain amount of interfragmentary motion rather than high mechanical strength and stiffness seem to be important for bone healing which is outside the scope of this paper

Static and fatigue strength of a novel anatomically contoured implant compared to five current open-wedge high tibial osteotomy plates

Abstract Background The purpose of the present study was to compare the mechanical static and fatigue strength of the size 2 osteotomy plate “Activmotion” with the following five other common implants for the treatment of medial knee joint osteoarthritis: the TomoFix small stature, the TomoFix standard, the Contour Lock, the iBalance and the second generation PEEKPower. Methods Six fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomy (HTO), according to standard techniques, using size 2 Activmotion osteotomy plates. All bone-implant constructs were subjected to static compression load to failure and load-controlled cyclic fatigue failure testing, according to a previously defined testing protocol. The mechanical stability was investigated by considering different criteria and parameters: maximum forces, the maximum number of loading cycles, stiffness, the permanent plastic deformation of the specimens during the cyclic fatigue tests, and the maximum displacement range in the hysteresis loops of the cyclic loading responses. Results In each test, all bone-implant constructs with the size 2 Activmotion plate failed with a fracture of the lateral cortex, like with the other five previously tested implants. For the static compression tests the failure occurred in each tested implant above the physiological loading of slow walking (> 2400 N). The load at failure for the Activmotion group was the highest (8200 N). In terms of maximum load and number of cycles performed prior to failure, the size 2 Activmotion plate showed higher results than all the other tested implants except the ContourLock plate. The iBalance implant offered the highest stiffness (3.1 kN/mm) for static loading on the lateral side, while the size 2 Activmotion showed the highest stiffness (4.8 kN/mm) in cyclic loading. Conclusions Overall, regarding all of the analysed strength parameters, the size 2 Activmotion plate provided equivalent or higher mechanical stability compared to the previously tested implant. Implants with a metaphyseal slope adapted to the tibia anatomy, and positioned more anteriorly on the proximal medial side of the tibia, should provide good mechanical stability

A finite element model of the lower limb during stance phase of gait cycle including the muscle forces

Abstract Background Results of finite element (FE) analyses can give insight into musculoskeletal diseases if physiological boundary conditions, which include the muscle forces during specific activities of daily life, are considered in the finite element modelling. So far, many simplifications of the boundary conditions are currently made. This study presents an approach for FE modelling of the lower limb for which muscle forces were included. Method The stance phase of normal gait was simulated. Muscle forces were calculated using a musculoskeletal rigid body (RB) model of the human body, and were subsequently applied to a FE model of the lower limb. It was shown that the inertial forces are negligible during the stance phase of normal gait. The contact surfaces between the parts within the knee were modelled as bonded. Weak springs were attached to the distal tibia for numerical reasons. Results Hip joint reaction forces from the RB model and those from the FE model were similar in magnitude with relative differences less than 16%. The forces of the weak spring were negligible compared to the applied muscle forces. The maximal strain was 0.23% in the proximal region of the femoral diaphysis and 1.7% in the contact zone between the tibia and the fibula. Conclusions The presented approach based on FE modelling by including muscle forces from inverse dynamic analysis of musculoskeletal RB model can be used to perform analyses of the lower limb with very realistic boundary conditions. In the present form, this model can be used to better understand the loading, stresses and strains of bones in the knee area and hence to analyse osteotomy fixation devices

Biomechanical properties of five different currently used implants for open-wedge high tibial osteotomy

Background: As several new tibial osteotomy plates recently appeared on the market, the aim of the present study was to compare mechanical static and fatigue strength of three newly designed plates with gold standard plates for the treatment of medial knee joint osteoarthritis. Methods: Sixteen fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomyn(HTO) according to standard techniques, using five TomoFix standard plates, five PEEKPower plates and six iBalance implants. Static compression load to failure and load-controlled cyclic fatigue failure tests were performed. Forces, horizontal and vertical displacements were measured; rotational permanent plastic deformations, maximal displacement ranges in the hysteresis loops of the cyclic loading responses and dynamic stiffness were determined. Results: Static compression load to failure tests revealed that all plates showed sufficient stability up to 2400 N without any signs of opposite cortex fracture, which occurred above this load in all constructs at different load levels. During the fatigue failure tests, screw breakage in the iBalance group and opposite cortex fractures in all constructs occurred only under physiological loading conditions (<2400 N). The highest fatigue strength in terms of maximal load and number of cycles performed prior to failure was observed for the ContourLock group followed by the iBalance implants, the TomoFix standard (std) and small stature (sm) plates. The PEEKPower group showed the lowest fatigue strength. Conclusions: All plates showed sufficient stability under static loading. Compared to the TomoFix and the PEEKPower plates, the ContourLock plate and iBalance implant showed a higher mechanical fatigue strength during cyclic fatigue testing. These data suggest that both mechanical static and fatigue strength increase with a wider proximal T-shaped plate design together with diverging proximal screws as used in the ContourLock plate or a closed-wedge construction as in the iBalance design. Mechanical strength of the bone-implant constructs decreases with a narrow T-shaped proximal end design and converging proximal screws (TomoFix) or a short vertical plate design (PEEKPower Plate). Whenever high mechanical strength is required, a ContourLock or iBalance plate should be selected