Preliminary tribological evaluation of nanostructured diamond coatings against ultra-high molecular weight polyethylene.

BACKGROUND: Some loss of joint prostheses has been attributed to osteolytic loosening associated with debris from wear of polyethylene articulating against metal alloys. Reduced polyethylene wear has been reported with ceramics serving as an alternative counterface. METHODS: Nanostructured Diamond (NSD) coatings were deposited onto Ti6Al4V by microwave plasma-assisted chemical vapor deposition, with both hydrogen-rich (H-NSD) and helium-rich (He-NSD) feedgas mixtures. Pin-on-disk wear tests of polyethylene against NSD and CoCr were performed in serum lubrication at body temperature. Scanning electron microscopy was used to examine surface morphology, and nanoindentation was used to determine hardness and modulus of the polyethylene wear surfaces. Raman spectroscopy, surface roughness, and wettability analyses of the NSD coatings were performed. RESULTS: Raman spectroscopy confirmed sp(2) and sp(3) bonded carbon in the NSD coatings. No significant differences in wear factors were found between polyethylene on H-NSD, He-NSD, and CoCr, despite higher roughness and friction coefficients for the He-NSD and H-NSD coatings, compared with CoCr. Contact angles for the diamond coatings were reduced following the wear tests, indicating that these surfaces became more hydrophilic. Numerous small protuberances were observed on pins articulated against CoCr, and a single, large protuberance was observed in polyethylene-on-NSD. These features were conjectured to be reconsolidated polyethylene particles. Nanoindentation modulus and hardness of the worn polyethylene surfaces were lower for polyethylene-on-diamond than for polyethylene-on-CoCr. CONCLUSIONS: As a counterface to polyethylene, NSD-coated Ti6Al4V produced wear factors comparable to CoCr in the present pin-on-disk tests, a promising step towards its use in joint replacement bearing applications.

A biomechanical study of periacetabular defects and cement filling.

Periacetabular bone metastases cause severe pain and functional disability in cancer patients. Percutaneous acetabuloplasty (PCA) is a minimally invasive, image-guided procedure whereby cement is injected into lesion sites. Pain relief and functional restoration have been observed clinically; however, neither the biomechanical consequences of the lesions nor the effectiveness of the PCA technique are well understood. The objective of this study was to investigate how periacetabular lesion size, cortex involvement, and cement modulus affect pelvic bone stresses and strains under single-legged stance loading. Experiments were performed on a male cadaver pelvis under conditions of intact, periacetabular defect, and cement-filling with surface strains recorded at three strain gage locations. The experimental data were then employed to validate three-dimensional finite element models of the same pelvis, developed using computed tomography data. The models demonstrated that increases in cortical stresses were highest along the posterior column of the acetabulum, adjacent to the defect. Cortical stresses were more profoundly affected in the presence of transcortical defects, as compared to those involving only trabecular bone. Cement filling with a modulus of 2.2 GPa was shown to restore cortical stresses to near intact values, while a decrease in cement modulus due to inclusion of BaSO(4) reduced the restorative effect. Peak acetabular contact pressures increased less than 15% for all simulated defect conditions; however, the contact stresses were reduced to levels below intact in the presence of either cement filling. These results suggest that periacetabular defects may increase the vulnerability of the pelvis to fracture depending on size and cortical involvement and that PCA filling may lower the risk of periacetabular fractures.

Biomechanical response of the pubic symphysis in lateral pelvic impacts: a finite element study.

Automotive side impacts are a leading cause of injuries to the pubic symphysis, yet the mechanisms of those injuries have not been clearly established. Previous mechanical testing of isolated symphyses revealed increased joint laxity following drop tower lateral impacts to isolated pelvic bone structures, which suggested that the joints were damaged by excessive stresses and/or deformations during the impact tests. In the present study, a finite element (FE) model of a female pelvis including a previously validated symphysis sub-model was developed from computed tomography data. The full pelvis model was validated against measured force-time impact responses from drop tower experiments and then used to study the biomechanical response of the symphysis during the experimental impacts. The FE models predicted that the joint underwent a combination of lateral compression, posterior bending, anterior/posterior and superior/inferior shear that exceeded normal physiological levels prior to the onset of bony fractures. Large strains occurred concurrently within the pubic ligaments. Removal of the contralateral constraints to better approximate the boundary conditions of a seated motor vehicle occupant reduced cortical stresses and deformations of the pubic symphysis; however, ligament strains, compressive and shear stresses in the interpubic disc, as well as posterior bending of the joint structure remained as potential sources of joint damage during automotive side impacts.

Three-dimensional finite element models of the human pubic symphysis with viscohyperelastic soft tissues.

Three-dimensional finite element (FE) models of human pubic symphyses were constructed from computed tomography image data of one male and one female cadaver pelvis. The pubic bones, interpubic fibrocartilaginous disc and four pubic ligaments were segmented semi-automatically and meshed with hexahedral elements using automatic mesh generation schemes. A two-term viscoelastic Prony series, determined by curve fitting results of compressive creep experiments, was used to model the rate-dependent effects of the interpubic disc and the pubic ligaments. Three-parameter Mooney-Rivlin material coefficients were calculated for the discs using a heuristic FE approach based on average experimental joint compression data. Similarly, a transversely isotropic hyperelastic material model was applied to the ligaments to capture average tensile responses. Linear elastic isotropic properties were assigned to bone. The applicability of the resulting models was tested in bending simulations in four directions and in tensile tests of varying load rates. The model-predicted results correlated reasonably with the joint bending stiffnesses and rate-dependent tensile responses measured in experiments, supporting the validity of the estimated material coefficients and overall modeling approach. This study represents an important and necessary step in the eventual development of biofidelic pelvis models to investigate symphysis response under high-energy impact conditions, such as motor vehicle collisions.

Contact pressures in the flexed hip joint during lateral trochanteric loading.

Acetabular fractures are an especially problematic outcome of motor vehicle side impacts. While fracture type has been correlated with impact direction and femoral orientation, actual contact pressures in the hip joint have not been quantified for lateral loading conditions. In the present study, we used pressure sensitive film to measure contact areas and pressures in seven hip joints from four cadavers under quasi-static lateral loading through the greater trochanter. The aim was to quantify the interactions of the femoral head with the acetabulum associated with variations in femoral orientation. Three angles of hip flexion (80 degrees , 90 degrees , 100 degrees ) and hip abduction (-10 degrees , 0 degrees , 10 degrees ) were tested, producing nine test orientations for each joint. We observed that contact areas, pressures, and forces varied significantly with femoral orientation for the adducted hip. The principal locations of load transmission were in the anterior and posterior regions of the acetabulum. For the abducted femur, contact pressures were concentrated anteriorly, and with increased adduction, anterior contact pressures diminished while posterior and superior pressures increased. The movement of pressure sites was consistent with mechanisms of acetabular fractures described by Letournel and Judet and provides new data for validation of finite element models of the pelvis in side impact.

Effects of trochanteric soft tissues and bone density on fracture of the female pelvis in experimental side impacts.

Pelvic fractures continue to be a source of morbidity and mortality for occupants in motor vehicle side impacts, especially among women. Previous studies have produced fracture tolerances for the female pelvis, yet the roles of soft tissues and bone quality remain unclear. Presently, we studied the influence of trochanteric soft tissue thickness (T) and total hip bone mineral density (BMD) on pelvic fracture of 10 female human pelves subject to lateral impact centered over the greater trochanter. Multiple impacts of increasing severity were performed and impact force, energy, impulse, compression, and viscous criteria were quantified. BMD and T were found to be additive predictors of the fracture force. For a given BMD, the force to fracture was significantly higher than that found previously using isolated pelvic bones. Impulse was found to positively correlate with T; however, maximum compression, viscous criterion, and energy to fracture were independent of BMD and T. The force tolerance at 25% probability of fracture found presently (3.16 kN) is substantially below previously reported values. The results suggest that the trochanteric soft tissue thickness and total hip BMD have a significant bearing on fracture outcome for the female pelvis in automotive side impact.