Effect of coronal plane acetabular correction on joint contact pressure in Periacetabular osteotomy: a finite-element analysis

Background The ideal acetabular position for optimizing hip joint biomechanics in periacetabular osteotomy (PAO) remains unclear. We aimed to determine the relationship between acetabular correction in the coronal plane and joint contact pressure (CP) and identify morphological factors associated with residual abnormal CP after correction. Methods Using CT images from 44 patients with hip dysplasia, we performed three patterns of virtual PAOs on patient-specific 3D hip models; the acetabulum was rotated laterally to the lateral center-edge angles (LCEA) of 30°, 35°, and 40°. Finite-element analysis was used to calculate the CP of the acetabular cartilage during a single-leg stance. Results Coronal correction to the LCEA of 30° decreased the median maximum CP 0.5-fold compared to preoperatively (p < 0.001). Additional correction to the LCEA of 40° further decreased CP in 15 hips (34%) but conversely increased CP in 29 hips (66%). The increase in CP was associated with greater preoperative extrusion index (p = 0.030) and roundness index (p = 0.038). Overall, virtual PAO failed to normalize CP in 11 hips (25%), and a small anterior wall index (p = 0.049) and a large roundness index (p = 0.003) were associated with residual abnormal CP. Conclusions The degree of acetabular correction in the coronal plane where CP is minimized varied among patients. Coronal plane correction alone failed to normalize CP in 25% of patients in this study. In patients with an anterior acetabular deficiency (anterior wall index < 0.21) and an aspherical femoral head (roundness index > 53.2%), coronal plane correction alone may not normalize CP. Further studies are needed to clarify the effectiveness of multiplanar correction, including in the sagittal and axial planes, in optimizing the hip joint’s contact mechanics.

Steam Turbine Casing Analyses to Determine Pressure and Temperature Limits

To ensure safe and reliable operation, steam turbine casings must have acceptable stresses and maintain sealing when subjected to internal pressures and temperatures. To show turbine casings acceptable, analysts conduct structural evaluations using finite element analysis (FEA) techniques. This paper outlines the analytical methods used to perform these types of analyses, provides analysis examples, and summarizes the process to create pressure and temperature limit maps. Finite element models of the main casing and steam chest are used to determine stresses and sealing of the casing horizontal split line and steam chest cover during normal operation. The sealing evaluations consider the sealing capabilities of the bolted joints when the casing is subjected to internal steam pressure and consider the effects of bolt stress relaxation at elevated temperatures, joint contact surface separation, and penetration of the internal pressure into the sealing surface. The acceptance criteria for the bolted joint sealing is based on the minimum width of the contacting surface and the minimum joint contact pressure. A series of analyses were conducted on the various models to create pressure and temperature limit maps, so that the design can be applied for the appropriate conditions. These maps plot maximum allowable working pressure (MAWP) versus maximum allowable working temperature (MAWT), and allow an application engineer to easily determine the acceptability of the casing for a particular application. An explanation of the process used to create the limit maps is presented.

Computational modelling of hip resurfacing arthroplasty investigating the effect of femoral version on hip biomechanics

Aim How reduced femoral neck anteversion alters the distribution of pressure and contact area in Hip Resurfacing Arthroplasty (HRA) remains unclear. The purpose of this study was to quantitatively describe the biomechanical implication of different femoral neck version angles on HRA using a finite element analysis. Materials and methods A total of sixty models were constructed to assess the effect of different femoral neck version angles on three different functional loads: 0°of hip flexion, 45°of hip flexion, and 90° of hip flexion. Femoral version was varied between 30° of anteversion to 30° of retroversion. All models were tested with the acetabular cup in four different positions: (1) 40°/15° (inclination/version), (2) 40°/25°, (3) 50°/15°, and (4) 50°/25°. Differences in range of motion due to presence of impingement, joint contact pressure, and joint contact area with different femoral versions and acetabular cup positions were calculated. Results Impingement was found to be most significant with the femur in 30° of retroversion, regardless of acetabular cup position. Anterior hip impingement occurred earlier during hip flexion as the femur was progressively retroverted. Impingement was reduced in all models by increasing acetabular cup inclination and anteversion, yet this consequentially led to higher contact pressures. At 90° of hip flexion, contact pressures and contact areas were inversely related and showed most notable change with 30° of femoral retroversion. In this model, the contact area migrated towards the anterior implant-bone interface along the femoral neck. Conclusion Femoral retroversion in HRA influences impingement and increases joint contact pressure most when the hip is loaded in flexion. Increasing acetabular inclination decreases the area of impingement but doing so causes a reciprocal increase in joint contact pressure. It may be advisable to study femoral neck version pre-operatively to better choose hip resurfacing arthroplasty candidates.

Effect of Vastus Medialis Loss on Rabbit Patellofemoral Joint Contact Pressure Distribution

Vastus medialis (VM) weakness is thought to alter patellar tracking, thereby changing the loading of the patellofemoral joint (PFJ), resulting in patellofemoral pain. However, it is challenging to measure VM force and weakness in human studies, nor is it possible to measure the associated mechanical changes in the PFJ. To obtain fundamental insight into VM weakness and its effects on PFJ mechanics, the authors determined PFJ loading in the presence of experimentally simulated VM weakness. Skeletally mature New Zealand White rabbits were used (n = 6), and the vastus lateralis, VM, and rectus femoris were stimulated individually through 3 custom-built nerve cuff electrodes. Muscle torque and PFJ pressure distribution were measured while activating all muscles simultaneously, or when the vastus lateralis and rectus femoris were activated, while VM was not, to simulate a quadriceps muscle strength imbalance. For a given muscular joint torque, peak pressures were greater and joint contact areas were smaller when simulating VM weakness compared with the condition where all muscles were activated simultaneously. The results in the rabbit model support that VM weakness results in altered PFJ loading, which may cause patellofemoral pain, often associated with a strength imbalance of the knee extensor muscle group.

Direct Validation of Human Knee-Joint Contact Mechanics Derived From Subject-Specific Finite-Element Models of the Tibiofemoral and Patellofemoral Joints

The primary aim of this study was to validate predictions of human knee-joint contact mechanics (specifically, contact pressure, contact area, and contact force) derived from finite-element models of the tibiofemoral and patellofemoral joints against corresponding measurements obtained in vitro during simulated weight-bearing activity. A secondary aim was to perform sensitivity analyses of the model calculations to identify those parameters that most significantly affect model predictions of joint contact pressure, area, and force. Joint pressures in the medial and lateral compartments of the tibiofemoral and patellofemoral joints were measured in vitro during two simulated weight-bearing activities: stair descent and squatting. Model-predicted joint contact pressure distribution maps were consistent with those obtained from experiment. Normalized root-mean-square errors between the measured and calculated contact variables were on the order of 15%. Pearson correlations between the time histories of model-predicted and measured contact variables were generally above 0.8. Mean errors in the calculated center-of-pressure locations were 3.1 mm for the tibiofemoral joint and 2.1 mm for the patellofemoral joint. Model predictions of joint contact mechanics were most sensitive to changes in the material properties and geometry of the meniscus and cartilage, particularly estimates of peak contact pressure. The validated finite element modeling framework offers a useful tool for noninvasive determination of knee-joint contact mechanics during dynamic activity under physiological loading conditions.