James J. Mason

Patellofemoral joint forces

In this review of patellofemoral joint forces as they might apply to implant design, methodologies for estimating forces on the patella and estimates of the forces, as reported in the literature, are summarized. Two methodologies exist for studying joint loads; one that measures kinematics in-vivo and uses analysis to estimate the joint loads and another that measures ground reaction forces and uses analysis to estimate the joint loads. In both these analyses many assumptions are required with varying degrees of uncertainty; here, those assumptions are examined with data from the published literature. The topics covered include: relationships between quadriceps forces and patellofemoral forces or patella ligament forces, relationships between knee joint moments and quadriceps forces, knee joint moments in various gaits, relationships between patellofemoral forces and lateral subluxation forces, and relationships between patella forces and inferior-superior forces. In many cases, there is little data on patella forces during normal activities, in other cases, there are some discrepancies in reported patella forces, i.e. during squat.

Compressive properties of trabecular bone in the distal femur.

Early loosening and implant migration are two problems that lead to failures in cementless (press-fit) femoral knee components of total knee replacements. To begin to address these early failures, this study determined the anterior-posterior mechanical properties from four locations in the human distal femur. Thirty-three cylindrical specimens were removed perpendicular to the press-fit surface after the surgical cuts on 10 human cadaveric femurs (age 71.5+/-14.2 years) had been made. Compression testing was performed that utilized methods to reduce the effects of end-artifacts. The bone mineral apparent density (BMAD), apparent modulus of elasticity, yield and ultimate stress, and yield and ultimate strain were measured for 28 cylindrical specimens. The apparent modulus, yield and ultimate stress, and yield and ultimate strain each significantly differed (p<0.05) in the superior and inferior locations. Linear and power law relationships between superior and inferior mechanical properties and BMAD were determined. The inferior apparent modulus and stresses were higher than those in the superior locations. These results show that the press-fit fixation characteristics of the femoral knee component differ on the anterior shield and posterior condyles. This information will be useful in the assignment of mechanical properties in finite element models for further investigations of femoral knee components. The property-density relations also have applications for implant design and preoperative assessment of bone strength using clinically available tools.

Hydrophilic–hydrophobic hydrogels for cartilage replacement

A new class of hydrogels combining both hydrophilic and hydrophobic structures are presented. Hydrogels have been investigated as cartilage replacement materials due to the high water content potentially leading to low friction, low wear, and rubbery or pliable nature similar to native cartilage. Unfortunately, many of these hydrogels lack the required shear, tear, and creep strength necessary to be used as cartilage replacement materials. The new hydrogels presented here utilize hydrophobic domains to reinforce the structure and provide higher tear, shear, and creep strengths versus traditional hydrogels without sacrificing water content, low friction and pliability. The mechanical properties of some of these gels are reported, and it is shown that the strength and creep resistance are greatly improved through the addition of hydrophobic segments without negatively impacting the friction. As such, these new gels may be a candidate for use to repair or replace articular cartilage.

Improved fatigue life of acrylic bone cements reinforced with zirconia fibers.

Poly(methyl methacrylate) (PMMA) bone cements have a long and successful history of use for implant fixation, but suffer from a relatively low fracture and fatigue resistance which can result in failure of the cement and the implant. Fiber or particulate reinforcement has been used to improve mechanical properties, but typically at the expense of the pre-cured cement viscosity, which is critical for successful integration with peri-implant bone tissue. Therefore, the objective of this study was to investigate the effects of zirconia fiber reinforcement on the fatigue life of acrylic bone cements while maintaining a relatively low pre-cured cement viscosity. Sintered straight or variable diameter fibers (VDFs) were added to a PMMA cement and tested in fully reversed uniaxial fatigue until failure. The mean fatigue life of cements reinforced with 15 and 20 vol% straight zirconia fibers was significantly increased by approximately 40-fold, on average, compared to a commercial benchmark (Osteobond) and cements reinforced with 0-10 vol% straight zirconia fibers. The mean fatigue life of a cement reinforced with 10 vol% VDFs was an order of magnitude greater than the same cement reinforced with 10 vol% straight fibers. The time-dependent viscosity of cements reinforced with 10 and 15 vol% straight fibers was comparable to the commercial benchmark during curing. Therefore, the addition of relatively small amounts of straight and variable diameter zirconia fibers was able to substantially improve the fatigue resistance of acrylic bone cement while exhibiting similar handling characteristics compared to current commercial products.

Initial fixation of a femoral knee component: an in vitro and finite element study

Loosening is the primary cause of total knee arthroplasty implant failure; therefore, to investigate this failure mode, femoral knee components were implanted in vitro on three cadaveric femurs. Bone-implant finite element (FE) models were created to predict the initial fixation of the interface of each femur. Initial fixation of the femoral knee component was successfully measured with the strain-gauged implants. Specimen-specific FE models were calibrated using the in vitro strain measurements and used to assess initial fixation. Initial fixation was shown to increase with bone density. The geometry of the implant causes the distal femur to deform plastically. It also causes higher stresses in the lateral side and higher pressures on the lateral surfaces. The implementation of plasticity in the bone material model in the FE model decreased these strains and pressures considerably from a purely elastic model, which demonstrated the importance of including plasticity.

High Strain Rate Testing of Bovine Trabecular Bone

In spinal vertebral burst fractures, the dynamic properties of the trabecular centrum, which is the central region of porous bone inside the vertebra, can play an important role in determining the failure mode. If the failure occurs in the posterior portion of the vertebral body, spinal canal occlusion can occur and ejected trabecular bone can impact the spinal cord resulting in serious injury. About 15% of all spinal cord injuries are caused by such burst fractures. Unfortunately, due to the uniqueness of burst fracture injuries, postinjury investigation cannot always accurately assess the degree of damage caused by these fractures. This research makes an effort to begin understanding the governing effects in this important bone fracture event. Measurements of the dynamic deformation response of bovine trabecular bone with the marrow intact and marrow removed using a modified split-Hopkinson pressure bar apparatus are reported and compared with quasistatic deformation response results. Because trabecular bone is more compliant and lower in strength than cortical bone, typical Hopkinson pressure bar experimental techniques used for high strain rate testing of harder materials cannot be applied. Instead, a quartz-crystal-embedded, split-Hopkinson pressure bar developed for testing compliant, low strength materials is used. Care is taken into account for the orthotropic properties in the bone by testing only along the principle material axes, determined through microcomputed tomography. In addition, shaping of the stress wave pulse is used to ensure a constant strain rate and homogeneous specimen deformation. Results indicate that the strength of trabecular bone increases by a factor of approximately 2-3 when the strain rate increases from 10(-3) s(-1) to 500 s(-1) and that the bone fractures beyond a critical strain.

Patellofemoral interactions in walking, stair ascent, and stair descent using a virtual patella model

Restoration of normal patella kinematics is an important clinical outcome of total knee arthroplasty. Failure of the patella within total knee systems has been documented and, upon occurrence, often necessitates revision surgery. It is thus important to understand patella mechanics following implantation, subject to load states that are typically realized during walking and other gaits. Here, a computational model of the patella is developed and used to examine the effects of walking, stair ascent, and stair descent on the development of stress and contact pressure in the patella throughout the gait cycle. Motion of the patella was governed by a combination of kinematic and force control, based on knee flexion and patellofemoral joint reaction force data from the literature. Unlike most previous analyses of full gait, quasi-static equilibrium was enforced throughout the cycle. Results indicate that, though peak forces vary greatly between the three gaits, maximum contact pressure and von Mises stress are roughly equivalent. However, contact area is larger in stair ascent and descent than walking, as patellofemoral loading, implant geometry, and polyethylene yield increase conformity between the femoral component and patella. Additionally, maximum contact pressure does not coincide with maximum load except for the case of walking. Though specific to the implant design considered here, this result has important ramifications for patella testing and emphasizes the need to characterize patella mechanics throughout gait.

Mice with a heterozygous Lrp6 deletion have impaired fracture healing

Bone fracture non-unions, the failure of a fracture to heal, occur in 10%–20% of fractures and are a costly and debilitating clinical problem. The Wnt/β-catenin pathway is critical in bone development and fracture healing. Polymorphisms of linking low-density lipoprotein receptor-related protein 6 (LRP6), a Wnt-binding receptor, have been associated with decreased bone mineral density and fragility fractures, although this remains controversial. Mice with a homozygous deletion of Lrp6 have severe skeletal abnormalities and are not viable, whereas mice with a heterozygous deletion have a combinatory effect with Lrp5 to decrease bone mineral density. As fracture healing closely models embryonic skeletal development, we investigated the process of fracture healing in mice heterozygous for Lrp6 (Lrp6 +/−) and hypothesized that the heterozygous deletion of Lrp6 would impair fracture healing. Mid-diaphyseal femur fractures were induced in Lrp6 +/− mice and wild-type controls (Lrp6 +/+). Fractures were analyzed using micro-computed tomography (μCT) scans, biomechanical testing, and histological analysis. Lrp6 +/− mice had significantly decreased stiffness and strength at 28 days post fracture (PF) and significantly decreased BV/TV, total density, immature bone density, and mature area within the callus on day-14 and -21 PF; they had significantly increased empty callus area at days 14 and 21 PF. Our results demonstrate that the heterozygous deletion of Lrp6 impairs fracture healing, which suggests that Lrp6 has a role in fracture healing.

The Effect of Entrapped Bone Particles on the Surface Morphology and Wear of Polyethylene

Clinically retrieved highly cross-linked ultrahigh molecular weight polyethylene (HXPE) acetabular liners have demonstrated scratching, whereas conventional ultrahigh-molecular-weight polyethylene (UHMWPE) implants show a smoother surface early after implantation. In the present study, the potential of bone particles and soft tissues, rather than cement, to scratch the articular surface of HXPE and UHMWPE (gamma radiated) acetabular components was evaluated; multiple bone particles located at the articular surface for 3600 simulated walking cycles replicated the scratches observed on retrieved implants. By remelting, these scratches were confirmed to be due to plastic deformation of the polyethylene, not wear. Furthermore, it was shown using wear testing that these scratches did not affect the subsequent wear rate of HXPE or conventional UHMWPE. Wear rates of scratched conventional and cross-linked polyethylene were not significantly different from unscratched conventional and cross-linked polyethylene, respectively.

Modeling Machining at High Speeds as a Fluid Mechanics Problem

Even though many models for machining exist, most of them are for low-speed machining, where momentum is negligible and material behavior is well approximated by quasi-static plastic constitutive laws. In machining at high speeds, momentum can be important and the strain rate can be exceedingly high. For these reasons, a fluid mechanics approach to understanding high-speed, very high-speed, and ultra-high-speed machining is attempted here. Namely, a potential flow solution is used to model the behavior of the material around a sharp tool tip during machining at high speeds. It is carefully argued that the potential flow solution is relevant and can be used as a first approximation to model the behavior of a metal during high-speed, very high-speed, or ultra-high-speed machining events; and at a minimum, the potential flow solution is qualitatively useful in understanding mechanics of machining at high speeds and above. Interestingly, the flow solution predicts that there is a stagnation point on the rake face, not at the tool tip as is usually assumed. Because the stagnation point is not at the tool tip, the flow solution predicts a significant amount of deformation in the workpiece resulting in large residual strains that may lead to a temperature rise on the finished surface.Copyright