A kinematic model for the design of a bicondylar mechanical knee

In this paper we present a design methodology for a bicondylar joint that mimics many of the physical mechanisms in the human knee. We replicate the elastic ligaments and sliding and rolling joint surfaces. As a result the centre of rotation and moment arm from the quadriceps changes as a function of flexion angle in a similar way to the human knee. This leads to a larger moment arm in the centre of motion, where it is most needed for high load tasks, and a smaller moment arm at the extremes, reducing the required actuator displacement. This is anticipated to improve performance:weight ratio in legged devices for tasks such as stair accent and sit-to-stand. In the design process ligament attachment positions, femur profile and ligament lengths were taken from cadaver studies. This information was then used as inputs to a simplified kinematic computer model in order to design a valid profile for a tibial condyle. A physical model was then tested on a custom built squatting robot. It was found that although ligament lengths deviated from the designed values the robot moment arm still matched the model to within 6.1% on average. This shows that the simplified model is an effective design tool for this type of joint. It is anticipated that this design, when employed in walking robots, prostheses or exoskeletons, will improve the high load task capability of these devices. In this paper we have outlined and validated a design method to begin to achieve this goal

The effect of coronal splits on the structural stability of bi-condylar tibial plateau fractures: a biomechanical investigation

Introduction!#!Surgical treatment of bi-condylar tibial plateau fractures is still challenging due to the complexity of the fracture and the difficult surgical approach. Coronal fracture lines are associated with a high risk of fixation failure. However, previous biomechanical studies and fracture classifications have disregarded coronal fracture lines.!##!Materials and methods!#!This study aimed to develop a clinically relevant fracture model (Fracture C) and compare its mechanical behavior with the traditional Horwitz model (Fracture H). Twelve samples of fourth-generation tibia Sawbones were utilized to realize two fracture models with (Fracture C) or without (Fracture H) a coronal fracture line and both fixed with lateral locking plates. Loading of the tibial plateau was introduced through artificial femur condyles to cyclically load the fracture constructs until failure. Stiffness, fracture gap movements, failure loads as well as relative displacements and rotations of fracture fragments were measured.!##!Results!#!The presence of a coronal fracture line reduced fracture construct stiffness by 43% (p = 0.013) and decreased the failure load by 38% from 593 ± 159 to 368 ± 63 N (p = 0.016). Largest displacements were observed at the medial aspect between the tibial plateau and the tibial shaft in the longitudinal direction. Again, the presence of the coronal fracture line reduced the stability of the fragments and created increased joint incongruities.!##!Conclusions!#!Coronal articular fracture lines substantially affect the mechanical response of tibia implant structures specifically on the medial side. With this in mind, utilizing a clinically relevant fracture model for biomechanical evaluations regarding bi-condylar tibial plateau fractures is strongly recommended

Fracture fixation of complex tibial plateau fractures

Bi-condylar tibial plateau fractures with the highest frequency in 40-to-60-year-old patients accounts for 35% of all tibial plateau fractures. Surgical treatment of this fracture remains challenging due to the multi-planar articular comminution and the subsequent severe soft tissue injuries. Preoperative planning is meaningfully affected by recognition of the exact location of the fracture fragments, in particular the posteromedial fragment created by the coronal split. Despite the 50% incidence rate of this crucial fracture line, it has been disregarded in the established fracture classifications like the AO or Schatzker systems as well as in the previous biomechanical studies. For this reason, there is still a controversy regarding an ideal fixation strategy for the complex tibial plateau fractures. Therefore, through both experimental and the numerical investigations, this study aimed to develop a coronal fracture model based on the clinical data to ultimately address this concern. The experiments focused on comparison of the structural stability between the coronal fracture model and the traditional Horwitz model. The numerical simulations, which were established based on the validation of these fracture models, evaluated the effects of the fracture morphology on the stress distributions within the implant. Both study parts revealed that the coronal split remarkably reduced the global axial stiffness and displacement of the bone-implant structure, significantly destabilized the medial side of the tibia, as well as noticeably changed the stress distributions within the locking plates and screws. Furthermore, the lateral locking plate cannot adequately stabilize this fracture, and a double plating method including a supplemental medial plate is required. Consequently, it is highly recommended to apply the coronal fracture model of bi-condylar tibial plateau fractures for biomechanical tests, which aimed to compare different fixation methods, as well as for numerical studies, which focused on finding the optimum plate position, screw direction or plate design.Bicondyläre Tibiaplateaufrakturen machen 35% aller Tibiaplateaufrakturen aus, mit der höchsten Häufigkeit im Alter von 40-60 Jahren. Die chirurgische Behandlung dieser Fraktur bleibt aufgrund der multiplanaren Gelenkzertrümmerung sowie der nachfolgenden schweren Weichteilverletzungen eine Herausforderung. Das Erkennen der genauen Lage der Frakturfragmente wird bei der präoperative Planung, insbesondere durch die koronale Spaltung des entstandenen posteromedialen Fragments, erheblich beeinträchtigt. Trotz einer Inzidenzrate von 50% für diese kritische Frakturlinie wurde sie in den etablierten Frakturklassifikationen wie dem AO- oder Schatzker-System sowie in früheren biomechanischen Studien nicht berücksichtigt. Aus diesem Grund gibt es nach wie vor eine Kontroverse über eine ideale Fixationsstrategie für komplexe Tibiaplateaufrakturen. Daher zielte diese Studie, die aus experimentellen und numerischen Teilen besteht, darauf ab, auf der Grundlage der klinischen Daten ein koronales Frakturmodell zu entwickeln, um diesem Anliegen letztlich Rechnung zu tragen. Der experimentelle Teil konzentrierte sich auf den Vergleich der strukturellen Stabilität zwischen dem koronalen Frakturmodell und dem traditionellen Horwitz-Modell. Im numerischen Teil, der auf der Grundlage der Validierung dieser Bruchmodelle erstellt wurde, werden die Auswirkungen der Bruchmorphologie auf die Spannungsverteilungen innerhalb der Implantatkomponenten bewertet. Beide Teile dieser Studie zeigten, dass die koronale Bruchlinie die globale axiale Steifigkeit und Verschiebung der Knochen-Implantat-Struktur bemerkenswert reduziert, die mediale Seite der Tibia signifikant destabilisiert und die Spannungsverteilung innerhalb der Verriegelungsplatten und -schrauben merklich verändert hat. Außerdem kann die laterale Verriegelungsplatte diese Fraktur nicht ausreichend stabilisieren, so dass eine doppelte Verplattung einschließlich einer zusätzlichen medialen Platte erforderlich ist. Daher wird dringend empfohlen, das koronale Frakturmodell für bikondyläre Frakturen des Tibiaplateaus anzuwenden, sowohl bei biomechanischen Tests, bei denen verschiedene Fixationsmethoden verglichen werden, als auch für numerische Studien, bei denen es darum geht, die optimale Plattenposition, Schraubenrichtung oder das optimale Plattendesign zu finden

A sliding stem in revision total knee arthroplasty provides stability and reduces stress shielding: An RSA study using impaction bone grafting in synthetic femora

Contains fulltext : 87345.pdf (publisher's version ) (Open Access)BACKGROUND AND PURPOSE: In the reconstruction of unicondylar femoral bone defects with morselized bone grafts in revision total knee arthroplasty, a stem extension appears to be critical to obtain adequate mechanical stability. Whether stability is still assured by this reconstruction technique in bicondylar defects has not been assessed. The disadvantage of relatively stiff stem extensions is that bone resorption is promoted due to stress shielding. We therefore designed a stem that would permit axial sliding movements of the articulating part relative to the intramedullary stem. METHODS: This stem was used in the reconstruction with impaction bone grafting (IBG) of 5 synthetic distal femora with a bicondylar defect. A cyclically axial load was applied to the prosthetic condyles to assess the stability of the reconstruction. Radiostereometry was used to determine the migrations of the femoral component with a rigidly connected stem, a sliding stem, and no stem extension. RESULTS: We found a stable reconstruction of the bicondylar femoral defects with IBG in the case of a rigidly connected stem. After disconnecting the stem, the femoral component showed substantially more migrations. With a sliding stem, rotational migrations were similar to those of a rigidly connected stem. However, the sliding stem allowed proximal migration of the condylar component, thereby compressing the IBG. INTERPRETATION: The presence of a functional stem extension is important for the stability of a bicondylar reconstruction. A sliding stem provides adequate stability, while stress shielding is reduced because compressive contact forces are still transmitted to the distal femoral bone.1 juni 201

Early experience with the NexGen® CR-Flex Mobile knee arthroplasty system: results of 2-year follow-up

We evaluated our initial results in 57 patients who received the NexGen® CR-Flex Mobile knee system using the standard anterior approach in a prospective study. The bicondylar surface implant was cemented in position (Palacos®) without posterior patellar resurfacing. The clinical outcome and perioperative and post-operative complications were documented over 24 months of its use. Overall, after two years, good results were obtained for the categories of pain and ROM (range of motion), and for the HSS (knee society score) (pre-operative: 42/57; post-operative: 87/80). No pathological radiological findings were made during this period. Two patients, however, felt that the primary operation had not been successful because of lateral patellar tilt. This was corrected with revision surgery. It was remarkable that our patients achieved greater than 100° flexion within the first 14 days of the immediate post-operative period. Evaluation and comparison of the scores with those of conventional bicondylar surface replacement systems showed no relevant differences

Musculoskeletal modeling and finite element analysis of the proximal juvenile femur

The influence of mechanical loading on bone modelling and remodelling has been, and still is the subject of many studies. It is widely accepted that the internal structure of long bones is orientated to the strains experienced throughout activities, and the morphometry of the bones are as a result of the loading. Although other influences play a role in bone development including, hormonal, nutritional and genetic. The internal structure is orientated in such a way that it transfers the loads experienced without being excessive in weight, providing an efficient weight bearing structure. Many researchers have analysed the adult femur but little work has been undertaken to understand femoral development in juveniles. Therefore the aim this work was to develop an understanding of the mechanical stresses and strains that the femur experiences during growth.The juvenile femur changes dramatically throughout growth. These changes occur from prenatal through to full maturity. The most notable include the ossification from a highly cartilaginous structure in the early years of development, to bone at ~18 years old, an increase in the length and angle of the neck, a change in the shaft torsion and a change in the bicondylar angle. Similarly, the development of movement patterns and locomotion in humans changes significantly throughout growth. Movement is restricted in utero, in neonates the movement begins to engage muscular activity, at 6 months a baby is usually able to sit upright; 9 months crawling begins; by 1 year old there is the ability to walk without support and at 4 years old an adult like gait pattern has developed. Full adult gait pattern has been documented to be achieved between 8-11 years old.In this work through gait analysis and musculoskeletal modelling the loads which the femur experiences at specific stages/ages of bipedal locomotion are analysed. Finite element analyses were then performed to develop an understanding of the stresses and strains of the proximal juvenile femur in relation to the attainment and development of bipedal gait. This was achieved by evaluating changes in these mechanical stresses and strains throughout different ages, relating them to the variations discovered in the gait patterns.Digitisation of the femora was performed on four specimens; prenatal, 3 years old, 7 years old and an adult. Following the scanning of the specimens in a micro CT scanner, some restoration to the damaged samples was required. Furthermore the dry samples were incomplete, and the models were needed to be modelled to accurately resemble fully intact femurs. The CT scans contained the full shaft however were missing the fully articulated proximal femur, due to the dry nature of the specimens the cartilages were absent. MRI scans which contained the femoral head data but were missing the full shaft were merged with the CT data to create a fully articulated femur for use in subsequent modelling.Gait analysis was performed on five children aged from 3-7 years old, with an average of five adults gait data used for comparison. The analysis showed that kinematic data was similar between all ages, however kinetic results revealed some differences. Ground reaction force in the 3 year old showed a higher heel strike compared to a higher toe off observed in adult during the gait cycle, indicating a lack of control in the 3 year old. Furthermore the 3 year old, compared to the other ages, had different values in joint moments. These joint moment results in particular played a role in the muscle forces produced from the musculoskeletal modelling.To obtain the muscle force data required for the FEA, musculoskeletal models were built. Testing the reliability of the musculoskeletal model was performed comparing the kinematic and kinetic data from the musculoskeletal modelling against the data obtained from the motion capture system. A good agreement was found between these data sets with the kinematics having the largest difference in the ankle plantar flexion of 8.6°. The kinetic results revealed almost exact matches. Further testing was attempted between the muscle force data and collected EMG. The collected EMG matched reported EMG in the literature and the onset and offset times of muscle activity corresponded well to muscle force peaks produced in the musculoskeletal model. Comparisons between the EMG and force through calculating the EMG as a force were inconclusive, although a degree of accuracy was shown but a more comprehensive method is required. It was concluded that with the accuracy of the kinematic and kinetic results the musculoskeletal modelling was accurate enough to give a true representation of physiological muscle forces to be modelled during FEA.Analysis of the musculoskeletal modelling results in the children revealed that the 3 year old had the highest significance between all the age groups. With the greatest significance in the hip flexors and abductors throughout the gait cycle. Joint reaction forces as a percentage of bodyweight were found to be much higher in the juvenile models. The adult model had a value of 265% bodyweight whereas the 3 year old showed a reaction force of 537% bodyweight. These differences observed in the musculoskeletal modelling had a direct effect on the FEA because the loads calculated here were applied to the finite element models to evaluate the effects that these would have on the stresses and strains during growth and development of the femur.FE models were built to represent a 3 year old, 7 year old and adult femur. Age specific loads calculated over 100% of a gait cycle, were applied to the models. The stress/strain analysis revealed some differences between the models but in general the areas exposed to high and low strain levels were similar. The similarities could suggest that each model was structurally adapted to the loads the femur regularly experiences. The thesis was successful in evaluating the stress and strain distribution apparent in the developing femur. However the work would be advanced by evaluating models from age ranges with a much more varied movement pattern i.e. crawling. This would increase an understanding of the structural optimisation of the femur

The Roles of Morphology and Posture on Gait Mechanics

Humans walk with an upright posture, extended limbs during stance, and a double-peaked vertical ground reaction force. Our closest living relatives, chimpanzees, sometimes walk bipedally but do so with a flexed, abducted hind limb. Previous researchers have studied humans walking with a crouched, chimpanzee-like gait pattern to try to infer how extinct human ancestors walked. However, it is not clear if the way humans perform this crouched posture gait would be similar to the way a species that is adapted to walk with a crouched posture would walk. The purpose of this dissertation was to investigate the impact of morphology and posture on gait mechanics in humans and chimpanzees. We investigated how human subjects perform different types of crouched posture walking and the degree to which human crouched posture walking converges to that of bipedal chimpanzee gait. The results from the first study indicate that crouched posture human gait does become more similar to chimpanzee gait, with more chimpanzee-like hip and knee flexion, and hip abduction patterns. However, differences between species persisted as the humans walking with a crouched posture did not have a single-peaked ground reaction force or as much pelvis transverse plane rotation. In the second study, we investigated how the major muscle groups in the lower limbs induce center of mass accelerations across different human postures and between humans and chimpanzees. Our results showed that when humans walk with a crouched posture, they rely on their gluteus maximus and vastus group to a greater extent to produce vertical accelerations than when humans walk with a normal posture. When comparing between species, we found that the chimpanzees rely less on their vastus muscle group and more on their gluteus maximus to induce vertical accelerations than humans walking with a crouched posture. The differences between humans and chimpanzees that persist when humans walk with a crouched posture in gait kinematics, ground reaction forces, and muscle function suggest that human crouched posture walking does not approximate a gait pattern of a chimpanzee and therefore should be used with caution when trying to understand the evolution of human bipedalism

Mismatch between trochlear coronal alignment of arthritic knees and currently available prosthesis: a morphological analysis of 4116 knees and 45 implant designs

Purpose: In up to a fifth of total knee replacements (TKR), surgeons are not capable of achieving good clinical and functional results. Despite comprehensive diagnostic workup, an underlying cause is not always identified in these patients. The purpose of this study is to compare native and prosthetic trochlear anatomies, to evaluate a potential source of morphologic mismatch and theoretically, of poor clinical outcomes. Methods: Native trochlear angles of 4116 knee CTs from 360 Knee Systems database of arthritic pre-operative TKR patients were evaluated. A semi-automated tridimensional analysis was performed to define the native trochlear angle in the coronal plane (NTA) among other 142 parameters. An active search was conducted to identify currently available TKR models; prosthetic trochlear orientation in the coronal plane (PTA) was extracted from the technical data provided by manufacturers. Results: The mean native trochlear angle (NTA) was 1.6° ± 6.6° (valgus) with a range from − 23.8° (varus) to 30.3°(valgus). A valgus NTA was present in 60.6% of the knees and 39.4% of them had a varus NTA. 89 TKR models were identified; trochlear details were available for 45 of them, of which 93% were designed with a valgus orientation of the prosthetic trochlear angle (PTA) and 6.9% showed a neutral (0°) PTA. Varus alignment of PTA was not present in any system. Angular numeric values for PTA were available for 34 models; these ranged from 0° to 15° of valgus, with a median value of 6.18° (SD ± 2.88°). Conclusion: This study shows a significant mismatch between native and prosthetic trochlear angles. A relevant proportion of the studied knees (41.45%) fall out of the trochlear angle range of currently available implants; representing a potential source for biomechanical imbalance. While further research is warranted to fully understand the clinical implications of the present study, manufacturers may need to take these findings into account for future implant designs. Level of evidence: Level III, retrospective cohort study