The geometry of the knee in the sagittal plane

A geometric model of the tibio-femoral joint in the sagittal plane has been developed which demonstrates the relationship between the geometry of the cruciate ligaments and the geometry of the articular surfaces. The cruciate ligaments are represented as two inextensible fibres which, with the femur and the tibia, are analysed as a crossed four-bar linkage. The directions of the ligaments at each position of flexion are calculated. The instant centre, where the flexion axis crosses the parasagittal plane through the joint, lies at the intersection of the cruciates. It moves relative to each of the bones during flexion and extension. The successive positions of the flexion axis relative to a fixed femur and to a fixed tibia are deduced. The shapes of articular surfaces which would allow the bones to flex and extend while maintaining the ligaments each at constant length are calculated and are found to agree closely with the shapes of the natural articular surfaces. The calculated movements of the contact point between the femur and the tibia during flexion also agree well with measurements made on cadaver specimens. The outcome is a geometric simulation of the tibio-femoral joint in the sagittal plane which illustrates the central role played by the cruciate ligaments in the kinematics of the knee and which can be used for the analysis of ligament and contact forces.     

Single flexion-axis selection influences femoral component alignment and kinematics during knee simulation

The objective of this paper is to investigate the effect of the implant geometry and alignment on its surface displacement, and to show that the proposed method of alignment can be used to improve the consistency of total knee replacement alignment for knee simulation. Poor femoral flexion-axis selection in the alignment process can possibly alter the intended design's functionality and introduce significant anterior/posterior (A/P), and proximal/distal (P/D) displacements. In the study, four multi-axis femoral components from each of two different manufacturers, NKII and 3DKnee, were used. A custom-built femoral alignment and surface measurement instrument was used to adjust and locate the single femoral axis position for each implant, which would optimally minimize their P/D displacements and A/P translations. The aligned NKII implants yielded a mean implant maximum P/D and A/P contact point shifts of 0.577 +/- 0.078 mm (+/- std. dev.) and of 2.325 +/- 0.243 mm between 0 and 60 degrees of flexion, which was significantly different from the aligned 3DKnee, 0.415 +/- 0.157 mm and 0.800 +/- 0.1512mm (p<0.0001, p<0.0001). Future work is needed to quantify the effect of femoral flexion axis selection on resulting long-term wear, damage areas, and soft tissue loading during simulation.     

A dynamic model of simulating stress distribution in the distal femur after total knee replacement

The aim of this study has been to develop a dynamic model of the knee joint after total knee replacement (TKR) to analyse the stress distribution in the distal femur during daily activities. Using MSC/ADAMS and MSC/MARC software, a dynamic model of an implanted knee joint has been developed. This model consists of the components of the knee prosthesis as well as the bones and ligaments of the knee. The femur, tibia, fibula, and patella have been modelled as mixed cortico-cancellous bone. The distal part of femur has been modelled as a flexible body with springs used to simulate the ligaments positioned at their anatomical insertion points. With this dynamic model a gait cycle was simulated. Stress shielding was identified in the distal femur after TKR, which is consistent with other investigators' results. Interestingly, higher stresses were found in the bone adjacent to the femoral component peg. This dynamic model can now be used to analyse the stress distribution in the distal femur with different load conditions. This will help to improve implant designs and will allow comparison of prostheses from different manufacturers.     

The biomechanics of a validated finite element model of stress shielding in a novel hybrid total knee replacement

This study proposes a novel hybrid total knee replacement (TKR) design to improve stress transfer to bone in the distal femur and, thereby, reduce stress shielding and consequent bone loss. Three-dimensional finite element (FE) models were developed for a standard and a hybrid TKR and validated experimentally. The Duracon knee system (Stryker Canada) was the standard TKR used for the FE models and for the experimental tests. The FE hybrid device was identical to the standard TKR, except that it had an interposing layer of carbon fibre-reinforced polyamide 12 lining the back of the metallic femoral component. A series of experimental surface strain measurements were then taken to validate the FE model of the standard TKR at 3000 N of axial compression and at 0 degreeof knee flexion. Comparison of surface strain values from FE analysis with experiments demonstrated good agreement, yielding a high Pearson correlation coefficient of R(2)= 0.94. Under a 3000N axial load and knee flexion angles simulating full stance (0O degree, heel strike (200 degrees, and toe off (600 degrees during normal walking gait, the FE model showed considerable changes in maximum Von Mises stress in the region most susceptible to stress shielding (i.e. the anterior region, just behind the flange of the femoral implant). Specifically, going from a standard to a hybrid TKR caused an increase in maximum stress of 87.4 per cent (O0 degree from 0.15 to 0.28 MPa), 68.3 per cent (200 degrees from 1.02 to 1.71 MPa), and 12.6 per cent (600 degrees from 2.96 to 3.33 MPa). This can potentially decrease stress shielding and subsequent bone loss and knee implant loosening. This is the first report to propose and biomechanically to assess a novel hybrid TKR design that uses a layer of carbon fibrereinforced polyamide 12 to reduce stress shielding.     

Size and shape of the resection surface geometry of the osteoarthritic knee in relation to total knee replacement design

This study examines the resection surface geometry of the femur, tibia, and patella in relation to the design of total knee implants. Using a technique known as principal component analysis (PCA), the variation in the resection geometry of the knee was summarized. Of the total variation of the knee, 58 per cent was due to variation in size and 14 per cent was due to varying femoral intercondylar notch width. A PCA was performed on each bone separately and it was found that 60 per cent, 76 per cent and 71 per cent of variation was due to size for the femur, tibia, and patella respectively. Femoral and tibial size were highly correlated (r = 0.95) while patellar size had poorer correlation with both femoral and tibial size (r < 0.7). Simple linear dimensions (femoral epicondylar width or tibial mediolateral width) were reliable indicators of knee size. The effect of shape variation, which is generally not accounted for in implant design, was measured. The resected surfaces of each subject were compared with a model of the resection surfaces of the knee which varied in size but not shape. The maximum overhang and underhang of the model on the resection surfaces were measured. There was average maximum model overhang of 3.6 mm and underhang of 3.9 mm in the femur, 2.3 mm overhang and 1.9 mm underhang in the tibia, and 2.6 mm overhang and 2.5 mm underhang in the patella. The maximum coverage that an implant can be expected to provide for a population is quantified. Implant designs which include some shape as well as size variation improve on the implant fit.     

In-vitro measurement of the human knee joint motion during quadriceps leg raising

This paper represents a three-dimensional motion analysis of the human knee joint under given conditions of loading and constraint. As the knee is extended by a known force applied to the quadriceps tendon, relative displacements of the femur, tibia, and patella are measured using a video motion analysis system. The most prominent motion of the tibia is external rotation and anterior displacement relative to the femur during knee extension. The patellar flexion angle decreases from 70° to 0°. The moment arm of the knee extensor mechanism exhibits a characteristic bell shape which peaks somewhere in the 40°–60° region of flexion. In general, the quadriceps force results primarily from an increase in the torque exerted by the weight of the lower leg. In the range of 20°–60°, the quardricep force needed to extend the leg remains relatively constant. As the knee approaches full extension, the moment arm decreases due to the fact that the posterior capsule and the ACL begin to tighten in this region. Consequently, the quadriceps force increases rapidly.     

A quantitative method of effective soft tissue management for varus knees in total knee replacement surgery using navigational techniques

Total knee replacement (TKR) has become the standard procedure in management of degenerative joint disease with its success depending mainly on two factors: three-dimensional alignment and soft-tissue balancing. The aim of this work was to develop and validate an algorithm to indicate appropriate medial soft tissue release during TKR for varus knees using initial kinematics quantified via navigation techniques. Kinematic data were collected intraoperatively for 46 patients with primary end-stage osteoarthritis undergoing TKR surgery using a computer-tomography-free navigation system. All patients had preoperative varus knees and medial release was made using the surgeon's experience. Based on these data an algorithm was developed. This algorithm was validated on a further set of 35 patients where it was used to define the medial release based on the kinematic data. The post-operative valgus stress angles for the two groups were compared. These results showed that the algorithm was a suitable tool to indicate the type of medial release required in varus knees based on intra-operatively measured pre-implant valgus stress and extension deficit angles. It reduced the percentage of releases made and the results were more appropriate than the decisions made by an experienced surgeon.     

A model of human knee ligaments in the sagittal plane. Part 1: Response to passive flexion.

The development of a mathematical model of the knee ligaments in the sagittal plane is presented. Essential features of the model are (a) the representation of selected cruciate ligament fibres as isometric links in a kinematic mechanism that controls passive knee flexion and (b) the mapping of all other ligament fibres between attachments on the tibia and femur. Fibres slacken and tighten as the ligament attachment areas on the bones move relative to each other. The model is used to study the shape and fibre length changes of the cruciate and collateral ligaments in response to passive flexion/extension of the knee. The model ligament shape and fibre length changes compare well qualitatively with experimental results reported in the literature. The results suggest that when designing and implanting a ligament replacement with the aim of reproducing the natural fibre strain patterns, the surgeon must not only implant through the natural attachment areas but must also maintain the natural fibre mapping and render all fibres just tight at the appropriate flexion angle.     

Muscle-ligament interactions at the knee during walking.

A two-dimensional mathematical model of the knee is used with gait analysis to calculate muscle, cruciate ligament and tibio-femoral contact forces developed at the knee during normal level walking. Ten normal adult subjects--four females and six males--participated. The knee model is based upon a four-bar linkage comprising the femur, tibia and two cruciate ligaments. It takes account of the rolling and sliding of the femur on the tibia during flexion/extension and the changes in direction of the ligaments and muscle tendons. We considered forces transmitted by six elements: quadriceps, hamstrings, gastrocnemius, anterior and posterior cruciate ligaments, and tibio-femoral contact. The equations of mechanics can be used to determine the absolute values of only three of the knee forces simultaneously, so that twenty limiting solutions of three of the six forces were considered. A limiting solution was rejected if any of the three forces were negative, corresponding to compressive muscle or ligament forces, or tensile contact forces. These constraints always reduced and at times removed the redundancy of the knee structures. The high incidence of predicted single muscle activity, supported by electromyography, suggested that the ligaments play a significant role in load transmission during gait. The temporal patterns of muscle and ligament activity and ligament force magnitudes were sensitive to the choice of model parameters. The analysis showed that each of four possible minimum principles of muscle selection--minimal muscle force, muscle stress, ligament force and contact force--was unlikely to be valid throughout the walking cycle.     

计算机辅助全膝置换手术系统的创新设计

传统的全膝置换手术存在着夹具种类繁多、操作繁琐、股骨力学难以确定等问题。本文设计了一套新型的适用于全膝置换手术的装夹具，包括可调式的支撑臂及伺服电机驱动机械臂，并在实验室中搭建了计算机辅助全膝置换手术系统。实验证明该结构不仅彻底地解决了困扰传统机器人辅助手术中患者腿部的固定装夹问题，而且整个系统结构紧凑，大大地方便了医生的手术干预.     

A laboratory-level surgical robot system for minimal invasive surgery (MIS) total knee arthroplasty

Recently, commercially available computer-aided surgical robot systems have been introduced to enable surgeons to improve the accuracy of cutting and alignment in total knee arthroplasty (TKA). The minimal invasive surgery (MIS) in the TKA has been increased since MIS can improve the surgical outcomes such as the recovery time and hospital stay by reducing the incision in surgery. However, current surgical robot systems do not provide the MIS TKA capability. In this study, we developed a laboratory-level surgical robot system to cut the bone from the lateral direction of the knee joint to provide MIS TKA. TKA experiments with saw bones of femur were compared between the conventional approach and the MIS approach. The incision length for the working space of the cutting tool and bone cutting time were decreased in lateral direction bone cutting. In addition, the cutting surface was smoother and the ranges of motion of all six joints were decreased in lateral direction bone cutting. Moreover, the cutting accuracy in terms of the lengths and angles of five cutting planes in femur were smaller in lateral approach than those in conventional approach. Therefore, consideration of bone cutting from the lateral direction in robotic TKA surgery should be necessary to improve the surgical outcomes in knee arthroplasty. The developed surgical robot system could be a platform of various orthopedic robotic surgeries.     

A sequentially-defined stiffness model of the knee

A new procedure is proposed to obtain a stiffness model of the knee, i.e. a model of the joint when static external loads are applied to the tibia and femur. A sequential approach is used to generalise a kinematic model of the passive motion of the articulation previously presented by the authors. The procedure is devised in such a way that the restraining function of the articular components which guide the passive motion of the joint is preserved after generalisation. As a result, the new stiffness model can replicate the passive motion with the same accuracy as the previous kinematic model, and it can also replicate the relative motion of the tibia and femur when the knee is loaded by static external loads. The proposed procedure is applied to a specimen and the relevant stiffness model is devised. The motion of the model under several loading conditions is then compared with the original motion of the specimen and with data obtained from the literature, in order to show the accuracy of the model and the potential of the proposed procedure.     

In vitro characterization of the relationship between the Q-angle and the lateral component of the quadriceps force

Although the Q-angle is routinely measured, the relationship between the Q-angle and the lateral component of the quadriceps force acting on the patella is unknown. Five cadaver knees were flexed on a knee simulator with a normal Q-angle, and flexed after increasing and decreasing the Q-angle by shifting the quadriceps origin laterally and medially, respectively. The motion of the femur, tibia and patella was tracked from 20 to 90 degrees of flexion using electromagnetic sensors. The motion of landmarks used to quantify the Q-angle was tracked to determine the 'dynamic Q-angle' during flexion. The lateral component of the force applied by the actuator secured to the quadriceps tendon was also quantified throughout flexion. Increasing the initial Q-angle significantly (p < 0.05) increased the dynamic Q-angle and the lateral force exerted through the quadriceps tendon throughout flexion. Decreasing the initial Q-angle significantly decreased the dynamic Q-angle at 90 degrees of flexion and significantly decreased the lateral force exerted through the quadriceps tendon from 20 to 40 degrees of flexion. Even though the dynamic Q-angle changes during flexion, an abnormally large initial Q-angle can be an indicator of an abnormally large lateral force acting on the patella during flexion.     

Objective measurement of knee laxity and stiffness with reference to knee injury diagnosis. Part 1: Design considerations and apparatus

Apparatus capable of objectively evaluating the laxity of the knee in vivo has been developed. The equipment consisted of a microcomputer-controlled machine, into which the leg was firmly clamped. The mechanical properties of the knee were measured by slowly applying a load to the tibia, while the femur was held stationary, and monitoring the resulting displacement of the tibia. Three separate tests could be performed: anterior-posterior drawing, varus-valgus rotation and tibial rotation. The tests were carried out on both legs of each subject, making six tests in all. The forces versus displacement (or torque versus rotation) took the form of a hysteresis loop. From these a total of 24 variables describing the stiffness, laxity and visco-elastic properties of the knee were calculated.     

Long-term implant-bone fixation of the femoral component in total knee replacement

Success of total knee replacement (TKR) depends on the prosthetic design. Aseptic loosening of the femoral component is a significant failure mode that has received little attention. Despite the clinical relevance of failures, no protocol is available to test long-term implant-bone fixation of TKR in vitro. The scope of this work was to develop and validate a protocol to assess pre-clinically the fixation of TKR femoral components. An in vitro protocol was designed to apply a simplified but relevant loading profile using a 6-degrees-of-freedom knee simulator for 1,000000 cycles. Implant-bone inducible micromotions and permanent migrations were measured at three locations throughout the test. After test completion, fatigue damage in the cement was quantified. The developed protocol was successfully applied to a commercial TKR. Additional tests were performed to exclude artefacts due to swelling or creep of the composite femur models. The components migrated distally; they tilted towards valgus in the frontal plane and in extension in the sagittal plane. The migration patterns were consistent with clinical roentgen-stereophotogrammetric recordings with TKR. Additional indicators were proposed that could quantify the tendency to loosen/stabilize. The type and amount of damage found in the cement, as well as the migration patterns, were consistent with clinical experience with the specific TKR investigated. The proposed pre-clinical test yielded repeatable results, which were consistent with the clinical literature. Therefore, its relevance and reliability was proved.     

Morphological analysis of the human knee joint for creating custom-made implant models

Restoration of normal knee joint function is the goal of total knee replacement (TKR) surgery. However, due to the various size and shapes of human knee joint of every individual, a ready-made commercial implant may not conform to a patient anatomy for meeting specific patient needs. Since mismatched implants often cause a severe balancing problem and a short-term durability, customizing an implant is a unique solution for patients with a deformity or an abnormal anatomy. This paper presents a methodology that creates a customized knee implant model by extracting typical 3-dimensional (3D) geometrical parameters of the knee. All of the derived parameters are directly used to define the geometry of implant. Software system was developed to extract knee parameters and determine implants for individual. Surgeons are also able to simulate the whole process of surgery with the system, so that they know what the custom-made implant should be for the patient prior to surgery. The feasibility and verification of a custom-made knee implant is described with case study. It indicates that the proposed system is applicable in the early stage of implant design process.     

膝关节三维模型的构建

通过对逆向工程技术的研究,提出了一种构建膝关节三维模型的新方法。首先,分别对已有膝关节假体原型和志愿者的股骨远端及胫骨近段进行数据的采集;其次,利用逆向工程软件Geomagic和Mimics分别对膝关节假体原型的点云数据和股骨远端和胫骨近端的CT图像进行处理,然后利用三维软件PRO/E进行再设计;最后按顺序在PRO/E软件中装配成膝关节三维模型。模型的建立为进一步进行膝关节生物力学分析提供了方法与平台,同时也为人工膝关节模型数据库的建立提供了方法。     

Design of a stability-detecting device for dental implants

Resonance frequency (RF) analysis technology was used to design a dental implant stability detector. The device uses a miniature-sized electromagnetic triggering rod to elicit vibration in a dental implant. Vibrational signals were recorded via an acoustic receiver. To assess the in vivo performance of the test apparatus, animal models were used. Implants were placed in the left tibia of 12 rabbits using a conventional surgical procedure. Standard 3.2 mm x 8 mm implants were placed in each test tibia with pre-tapping cavities of 3.2 mm and 3.7 mm diameters to simulate either a 'well-fitting' or a 'loosely fitting' situation. The RF values of the test implants were detected by the newly developed device which was directly mounted on the healing abutments of the implants. The results showed that the RF values of the implants under well-fitting conditions significantly increased (p < 0.01) 3 weeks after surgery and reached a plateau at around 6-7 weeks. Meanwhile implants with higher initial RF values had shorter healing times and higher final RF values at the plateau. Based on these findings, it was concluded that the idea of using the current designed device for detecting the degree of bone healing during the osseointegration process seems feasible.     

Three-dimensional dynamic model of the knee

A three-dimensional dynamic model of the knee was developed to study the interactions between the articulating surfaces of the bones and the geometrical and mechanical properties of the ligaments. The contact-surface geometry of the distal femur, proximal tibia, and patella was modeled by fitting polynomials to each of the eight articular surfaces. Twelve elastic elements were used to describe the function of the ligamentous and capsular structures of the knee. The origin and insertion sites of each model ligament were obtained from cadaveric data reported for an average-size knee. The response of the model to both anterior-posterior drawer and axial rotation suggests that the geometrical and mechanical properties of the model ligaments approximate the behavior of real ligaments in the intact knee. Comparison of the model’s response with experimental data obtained from cadaveric knee extension indicate further that the three-dimensional model reproduces the response of the real knee during movement.     

Muscle-ligament interactions at the human knee (I)

A three-dimensional dynamic model of the knee was developed to study the interactions between the articulating surfaces of the bones and the geometrical and mechanical properties of the ligaments. The contact-surface geometry of the distal femur, proximal tibia, and patella was modeled by fitting polynomials to each of the eight articular surfaces. Twelve elastic elements were used to describe the function of the ligamentous and capsular structures of the knee. The origin and insertion sites of each model ligament were obtained from cadaveric data reported for an average-size knee. The response of the model to both anterior-posterior drawer and axial rotation suggests that the geometrical and mechanical properties of the model ligaments approximate the behavior of real ligaments in the intact knee. Comparison of the model’s response with experimental data obtained from cadaveric knee extension indicate further that the three-dimensional model reproduces the response of the real knee during movement.