Objective measurement of knee laxity and stiffness with reference to knee injury diagnosis. Part 2: Results

Equipment capable of objective knee analysis has been used to obtain data from 85 'normal' healthy knees, 47 patients suffering with knee disorders, and three cadaveric knee joints. Among the 'normals' it was found that there was a correlation between body weight and stiffness and laxity. A lower stiffness and higher laxity was recorded at 20 degrees of knee flexion than at 90 degrees. Using relative-paired difference analysis the variables affected by different injuries in patients were identified and are presented. In a separate analysis a multi-variate technique is used to interpret the data. The technique could be used to predict or diagnose knee injury, and as such may be highly useful to clinicians.     

Short-term repeatability of joint space width measurements using a magnetic resonance imaging compatible knee positioning device

The purpose of this study was to evaluate a magnetic resonance imaging (MRI) compatible knee positioning device to aid in minimizing intratechnologist and intertechnologist differences of minimum joint space width (JSW) measurements. Five subjects were scanned by two separate technologists, with and without an MRI-compatible positioning device. A semi-automated program calculated the minimum JSW of the tibiofemoral and patellofemoral joints. The scan-to-scan repeatability was evaluated from measurements between serial scans without subject repositioning, and the intratechnologist and intertechnologist repeatabilities were evaluated when the subject was removed from the magnet and repositioned by an individual technologist. The root mean square (RMS) error of the JSW measurements was also calculated. All measures of scan-to-scan repeatability and intratechnologist repeatability were unchanged with the MRI-compatible positioning device. The intertechnologist repeatability decreased from 0.70 to 0.42 mm, and the RMS error was significantly reduced (P = 0.0006) from 0.26 to 0.15 mm for the tibiofemoral joint. The variability of patellofemoral JSW measurements increased when using the positioning device; however, the increases were not statistically significant. The intertechnologist repeatability increased from 1.55 to 1.79 mm, and the RMS error increased from 0.58 to 0.73 mm. The MRI-compatible positioning device was successful at reducing JSW measurement variability at the tibiofemoral joint. The increase in measurement variability at the patellofemoral joint may be due to local incongruities of the articular surfaces. An MRI-compatible positioning device may be beneficial for quantitative longitudinal studies evaluating knee joint health.     

An optimal control model for determining articular contact forces at the human knee during rising from a static squat position

A two-dimensional dynamic model of the knee joint was incorporated into a four-segment, eight-muscle model of the human body to determine the muscle, ligament, and articular contact forces transmitted at the knee as humans stand up from a static squatting position. Our optimal control model predicted peak tibiofemoral contact forces 8 times as high as body weight. Furthermore, ligament forces, especially those in the anterior-cruciate, were nearly body weight as knee flexion approached 90 degrees. Ligament and tibiofemoral contact loads were dominated by the forces exerted by muscles during the movement.     

Validation of the soft tissue restraints in a force-controlled knee simulator

In vitro testing of total knee replacements (TKRs) is important both at the design stage and after the production of the final components. It can predict long-term in vivo wear of TKRs. The two philosophies for knee testing are to drive the motion by displacement or to drive the motion by force. Both methods have advantages and disadvantages. For force control an accurate simulation of soft tissue restraints is required. This study was devised to assess the accuracy of the soft tissue restraints of the force-controlled Stanmore knee simulator in simulating the restraining forces of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). In order to do this, human cadaver knee joints were subjected to the ISO Standard Walking Cycle. The resulting kinematics were monitored when the soft tissue structures were intact, when the ACL and PCL were resected, and when they were simulated by springs positioned anteriorly and posteriorly. The stiffness of the springs was determined from the literature. Two different stiffnesses of springs were used which were 7.24 N/mm (designated as soft springs) and 33.8 N/mm (designated as hard springs). All the intact knees showed displacements that were within the range of the machine. Cutting the ACL and PCL resulted in anterior and posterior motion and internal external rotation that were significantly greater than the intact knee. Results showed that when the ACL and PCL were cut hard springs positioned anterior and posterior to the knee returned the knee to near normal anterior-posterior (AP) motion. Using hard springs in the posterior position in any condition reduced rotational displacements. Therefore using springs in a force-controlled simulator is a compromise. More accuracy may be obtained using springs that are of intermediate stiffness.     

KIN-Nav navigation system for kinematic assessment in anterior cruciate ligament reconstruction: features, use, and perspectives

In this paper a new navigation system, KIN-Nav, developed for research and used during 80 anterior cruciate ligament (ACL) reconstructions is described. KIN-Nav is a user-friendly navigation system for flexible intraoperative acquisitions of anatomical and kinematic data, suitable for validation of biomechanical hypotheses. It performs real-time quantitative evaluation of antero-posterior, internal-external, and varus-valgus knee laxity at any degree of flexion and provides a new interface for this task, suitable also for comparison of pre-operative and post-operative knee laxity and surgical documentation. In this paper the concept and features of KIN-Nav, which represents a new approach to navigation and allows the investigation of new quantitative measurements in ACL reconstruction, are described. Two clinical studies are reported, as examples of clinical potentiality and correct use of this methodology. In this paper a preliminary analysis of KIN-Nav's reliability and clinical efficacy, performed during blinded repeated measures by three independent examiners, is also given. This analysis is the first assessment of the potential of navigation systems for evaluating knee kinematics.     

Effect of ultra-high molecular weight polyethylene thickness on contact mechanics in total knee replacement

One of the important design parameters in current knee joint replacements is the thickness of the ultra-high molecular weight polyethylene (UHMWPE) tibial insert, yet there is no clear definition of the upper limit of the 'thick' polyethylene insert. Using one design knee implant and subjecting it to the physiological loads encountered throughout the gait cycle, measurements of the lengths of contact imprints generated were compared with the corresponding theoretical predictions for different insert thicknesses under the same applied load. Multiple regression analysis was applied to test whether the dimensions of contact imprints are influenced by UHMWPE thickness. Good agreement was obtained between the theoretical predictions and the experimental measurements of the dimensions of contact imprints when the knee was at 60 degrees flexion. Therefore, it was possible to estimate the contact pressure at the articulating surface using the theoretical model. Contact imprint dimensions increased with increasing applied load. Statistical analysis of the experimental data revealed that, at 0 degree flexion, the overall imprint dimensions increased as the UHMWPE thickness increased from 8 to 20 mm. However, the increment was not significant when the thickness subinterval 10-15 mm was considered. Furthermore, at 60 degrees flexion, thickness was not a significant factor for the overall imprint dimensions. No evidence was found from the data to suggest that an increment in polyethylene thickness over 10 mm would significantly reduce the contact imprint dimensions. These findings suggest that thicker inserts can be avoided, as they require unnecessary bone resection.     

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.     

Mechanics of the passive knee joint. Part 2: interaction between the ligaments and the articular surfaces in guiding the joint motion

The aim of this study was to examine how the interaction between ligament tensions and contact forces guides the knee joint through its specific pattern of passive motion. A computer model was built based on cadaver data. The passive motion and the ligament lengthening and force patterns predicted by the model were verified with data from the literature. The contribution of each ligament and contact force was measured in terms of the rotational moment that it produced about the tibial medial plateau and the anterior-posterior (AP) force that it exerted on the tibia. The high tension of the anterior cruciate ligament (ACL) and the geometric constraints of the anterior horns of the menisci were found to be key features that stabilized the knee at full extension. The mutual effect of the cruciates was found as the reason for the screw-home mechanism at early flexion. Past 300, the AP component of contact force on the convex geometry of the lateral tibial plateau and tension of the lateral collateral ligament (LCL) were identified as elements that control the joint motion. From 60 degrees to 90 degrees, reduction in the tension of the ACL was determined as a reason for continuation of the tibial anterior translation. From 90 degrees to 120 degrees, increase in the tension of the posterior cruciate ligament and the AP component of the contact force on the convex geometry of the lateral tibial plateau pushed the tibia more anteriorly. This anterior translation was limited by the constraining effects of the ACL tension and the AP component of the contact force on the medial meniscus. The important guiding role observed for the LCL suggests that it should not be overlooked in knee models.     

Biological activity of polyethylene wear debris produced in the patellofemoral joint

Polyethylene wear is considered a threat to the long-term survival of total knee replacements. The aim of this study was to investigate the contribution that resurfacing the patella makes to wear debris-induced osteolysis following total knee replacement. Ultra-high molecular-weight polyethylene wear particles were isolated from simulator lubricant. Particle shape, size, and volume distributions were recorded allowing the osteolytic potential of the wear debris produced in the patellofemoral joint to be estimated using the concept of specific biological activity and functional biological activity. Values were compared with those reported for the tibiofemoral joint. Specific biological activity for the patellofemoral joint was not significantly different from the values for the tibiofemoral joint of total knee replacement devices, and therefore, has a similar potential to stimulate osteolytic cytokine release from macrophages. Functional biological activity was significantly lower for the patellofemoral joint compared with the tibiofemoral joint. Functional biological activity was significantly lower for the patellofemoral joint compared with the fixed bearing and rotating platform total knee replacement devices. However, as patellar resurfacing is commonly fitted as part of a total knee replacement system, this results in a 20% increase in overall functional biological activity for the system. Therefore, implanting a patellar resurfacing will increase the potential for osteolysis in the knee.     

Development of a model-based Roentgen stereophotogrammetric analysis system to measure polyethylene wear in unicompartmental arthroplasty

One of the most important causes of failure in unicompartmental knee replacement (UKR) is polyethylene wear. The aim of this study was to develop and assess a novel Roentgen stereophotogrammetric analysis (RSA)-based method for the measurement of linear wear suitable for UKR. Model-based RSA was used to estimate the linear wear of polyethylene bearings in UKR. A phantom was used to validate the method using in vitro measured bearing thicknesses and the linear wear of ten control bearings was estimated in vivo. Computer aided design (CAD) models for the UKRs were used in the model-based RSA system. There was no statistically significant difference between the estimated and measured bearing thicknesses using the CAD models (p = 0.386). The precision of the linear wear measurement, expressed as the standard deviation of the difference between the estimated and measured bearing thickness was 0.163 mm. The bias (mean difference) was 0.030 mm. The use of RSA to measure in vivo wear in a UKR has been shown to be accurate in a phantom, and has been verified with in vivo measured controls. The technique does not require surgical implantation of marker balls and can be used retrospectively.     

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.     

A computerized bioskills system for surgical skills training in total knee replacement

Although all agree that the results of total knee replacement (TKR) are primarily determined by surgical skill, there are few satisfactory alternatives to the 'apprenticeship' model of surgical training. A system capable of evaluating errors of instrument alignment in TKR has been developed and demonstrated. This system also makes it possible quantitatively to assess the source of errors in final component position and limb alignment. This study demonstrates the use of a computer-based system to analyse the surgical skills in TKR through detailed quantitative analysis of the technical accuracy of each step of the procedure. Twelve surgeons implanted a posterior-stabilized TKR in 12 fresh cadavers using the same set of surgical instruments. During each procedure, the position and orientation of the femur, tibia, each surgical instrument, and the trial components were measured with an infrared coordinate measurement system. Through analysis of these data, the sources and relative magnitudes of errors in position and alignment of each instrument were determined, as well as its contribution to the final limb alignment, component positioning and ligament balance. Perfect balancing of the flexion and extension gaps was uncommon (0/15). Under standardized loading, the opening of the joint laterally exceeded the opening medially by an average of approximately 4 mm in both extension (4.1 +/- 2.1 mm) and flexion (3.8 +/- 3.4 mm). In addition, the overall separation of the femur and the tibia was greater in flexion than extension by an average of 4.6 mm. The most significant errors occurred in locating the anterior/posterior position of the entry point in the distal femur (SD = 8.4 mm) and the correct rotational alignment of the tibial tray (SD = 13.2 degrees). On a case-by-case basis, the relative contributions of errors in individual instrument alignments to the final limb alignment and soft tissue balancing were identified. The results indicate that discrete steps in the surgical procedure make the largest contributions to the ultimate alignment and laxity of the prosthetic knee. Utilization of this method of analysis and feedback in orthopaedic training is expected rapidly to enhance surgical skills without the risks of patient exposure.     

Mechanical characterization of fourth generation composite humerus

Mechanical data on upper extremity surrogate bones, supporting use as biomechanical tools, is limited. The objective of this study was to characterize the structural behaviour of the fourth-generation composite humerus under simulated physiologic bending, specifically, stiffness, rigidity, and mid-diaphysial surface strains. Three humeri were tested in four-point bending, in anatomically defined anteroposterior (AP) and mediolateral (ML) planes. Stiffness and rigidity were derived using load-displacement data. Principal strains were determined at the anterior, posterior, medial, and lateral surfaces in the humeral mid-diaphysial transverse plane of one specimen using stacked rosettes. Linear structural behaviour was observed within the test range. Average stiffness and rigidity were greater in the ML (918 +/- 18 N/mm; 98.4 +/- 1.9 Nm2) than the AP plane (833 +/- 16 N/mm; 89.3 +/- 1.6 Nm2), with little inter-specimen variability. The ML/AP rigidity ratio was 1.1. Surface principal strains were similar at the anterior (5.41 micro epsilon/N) and posterior (5.43 micro epsilon/N) gauges for AP bending, and comparatively less for ML bending, i.e. 5.1 and 4.5 micro epsilon/N, at the medial and lateral gauges, respectively. This study provides novel strain and stiffness data for the fourth-generation composite humerus and also adds to published construct rigidity data. The presented results support the use of this composite bone as a tool for modelling and experimentation.     

The stability of the femoral component of a minimal invasive total hip replacement system

In this study, the initial stability of the femoral component of a minimal invasive total hip replacement was biomechanically evaluated during simulated normal walking and chair rising. A 20 mm diameter canal was created in the femoral necks of five fresh frozen human cadaver bones and the femoral heads were resected at the smallest cross-sectional area of the neck. The relatively short, polished, taper-shaped prostheses were cemented centrally in this canal according to a standardized procedure. A servohydraulic testing machine was used to apply dynamic loads to the prosthetic head. Radiostereophotogrammetric analysis was used to measure rotations and translations between the prosthesis and bone. In addition, the reconstructions were loaded until failure in a static, displacement-controlled test. During the dynamic experiments, the femoral necks did not fail and no macroscopical damage was detected. Maximal values were found for normal walking with a mean rotation of about 0.2 degrees and a mean translation of about 120 microm. These motions stabilized during testing. The mean static failure load was 4714 N. The results obtained in this study are promising and warrant further development of this type of minimal invasive hip prosthesis.     

Knee and hip joint forces--sensitivity to the degrees of freedom classification at the knee

Previous research has demonstrated that the number of degrees of freedom (DOF) modelled at a given joint affects the antagonistic muscle activity predicted by inverse dynamics optimization techniques. This higher level of muscle activity in turn results in greater joint contact forces. For instance, modelling the knee as a 3 DOF joint has been shown to result in higher hip and knee joint forces commensurate with a higher level of muscular activity than when the knee is modelled with 1 DOF. In this study, a previously described musculoskeletal model of the lower limb was used to evaluate the sensitivity of the knee and hip joint contact forces to the DOF at the knee during vertical jumping in both a 1 and a 3 DOF knee model. The 3 DOF knee was found to predict higher tibiofemoral and hip joint contact forces and lower patellofemoral joint contact forces. The magnitude of the difference in hip contact force was at least as significant as that found in previous research exploring the effect of subject-specific hip geometry on hip contact force. This study therefore demonstrates a key sensitivity of knee and hip joint contact force calculations to the DOF at the knee. Finally, it is argued that the results of this study highlight an important physiological question with practical implications for the loading of the structures of the knee; that is, the relative interaction of muscular, ligamentous, and articular structures in creating moment equilibrium at the knee.     

Investigation of wear of knee prostheses in a new displacement/force-controlled simulator

The performance of two knee simulators designed by ProSim (Manchester, UK) was evaluated by comparison of the wear seen in the press-fit condylar (PFC) Sigma (DePuy) knee prosthesis. Twelve specimens of the same design and manufacturing specification, were subjected to a wear test of 2 x 10(6) cycles duration using bovine serum as a lubricant. The anterior/posterior displacement and internal/external rotation inputs were based on the kinematics of the natural knee. International Standards Organization (ISO) standards were used for the flexion and axial load. The wear rates and wear scar areas were compared across all stations. The mean wear rates found were 17.6+/-5 mm3/10(6) cycles for stations 1 to 6 and 19.6+/-4 mm3/10(6) cycles for stations 7 to 12, resulting in an overall mean wear rate of 18.1+/-3 mm3/10(6) cycles. The differences between the two simulators were not significant. The average wear scar area seen on inserts from stations I to 6 was calculated at 32.4+/-1 per cent of the intended articulating surface. Similarly on stations 7 to 12 the average wear scar area was 30.7+/-3 per cent. The wear scars seen were a good physiological representation of those found from clinical explant data. This study has shown good repeatability from the simulator, both within and between the simulators.     

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.     

Tibiofemoral cartilage thickness distribution and its correlation with anthropometric variables

The objective of this study was to determine tibiofemoral cartilage thickness distribution, and to investigate the relationship between cartilage geometry and anthropometric variables. In this study, 20 magnetic resonance examinations of the knee from normal individuals were reconstructed to provide three-dimensional models of the knee joint, including bony and cartilage surfaces. Three regions were defined on the articular surface, and the cartilage thickness distribution along each of these was determined. Statistically significant differences between femoral and tibial regions were examined using the paired Student t test in Microsoft Excel. Correlations were investigated using the correlation tool in Microsoft Excel. The average tibial cartilage thickness was found to be 2.76 mm and the average femoral cartilage thickness was 2.75 mm. Significant correlations exist between the tibia cartilage thickness and body height (R = 0.60; P < 0.05) and weight (R = 0.64; P < 0.05). Significant correlations exist between the femoral cartilage volume and the body height (R = 0.736; P < 0.01) and weight (R = 0.855; P < 0.01). It is suggested that the distribution and correlations of cartilage distribution indicate adaptation in response to mechanical loading. Information regarding cartilage thickness and volume distribution as found in this study may be useful in diagnosing and monitoring cartilage loss in patients with degenerative joint disease.     

Influence of the varus-valgus instability on the contact of the femoro-tibial joint

The varus-valgus instability of the knee joint is mainly due to ruptured or lax collateral ligaments. The purpose of this investigation was to study the influence of the varus-valgus instability on the contact pressures of the femoro-tibial joint. Six fresh knee specimens of human cadavers were tested to measure the contact pressure on the tibia plateau of the knee joint at varus or valgus alignment under various loads and at full extension. Pressure transducers and Bourdon tube pressure gauges were used simultaneously for recording pressure. At neutral alignment of the knee with the menisci intact, the peak pressure increased linearly with forces up to 4 MPa. With increasing varus alignment, the peak contact pressure on the medial plateau not covered by the menisci increased up to a maximum of 7.3 MPa at 5 degrees varus, and at 5 degrees valgus, the peak pressure on the lateral plateau was 7.8 MPa. After total meniscectomy, the contact pressure increased up to a maximum of 7.4 MPa at a force of 2700 N. With increasing varus alignment, the contact pressure on the medial plateau increased to 8.1 MPa at 5 degrees varus and on the lateral plateau 9.2 MPa at 5 degrees valgus.     

Modelling femoral curvature in the sagittal plane: a cadaveric study

This study examined the possibility of representing the mid third of the human femur with two straight sections. This portion of the femur visually has a distinct curvature, which can potentially present problems when considering implant stem designs to be introduced in this region. Sixteen femora were sectioned at 10 mm intervals along the femoral shaft in the mid third region (35-65 per cent of femoral length). Photographic records were obtained of each section against a consistent axis system to which all coordinates were referenced. The position of the centre of the medullary canal cross-sectional area along the femur, in relation to fixed orthogonal planes, has been analysed; the outer anterior cortex was also modelled. The results showed that the medullary centre of area plots and the anterior cortex coordinates are suitably modelled as two straight lines. For each bone it was possible to define the intersection point between the two straight sections (point of angulation), and the subtended angle between these sections (angle of incidence). The average point of angulation for the medullary plots occurred at 57 per cent along the femur, while the mean angle of incidence was 6.5 degrees. The anterior surface had an average point of angulation at 58 per cent along the femur with the mean angle of incidence being 22.2 degrees. The centre-line of the medulla was also found to be almost parallel to the outer anterior surface for sections distal to the point of angulation. It is proposed therefore, that this difference in angulation is the result of medullary expansion/cortical thinning towards the proximal extremity of the femur, causing the straight-line model of the medulla to angulate less than the outer anterior cortex.