Heidi Ploeg

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.

Mice lacking pten in osteoblasts have improved intramembranous and late endochondral fracture healing.

The failure of an osseous fracture to heal (development of a non-union) is a common and debilitating clinical problem. Mice lacking the tumor suppressor Pten in osteoblasts have dramatic and progressive increases in bone volume and density throughout life. Since fracture healing is a recapitulation of bone development, we investigated the process of fracture healing in mice lacking Pten in osteoblasts (Ocn-cretg/+;Ptenflox/flox). Mid-diaphyseal femoral fractures induced in wild-type and Ocn-cretg/+;Ptenflox/flox mice were studied via micro-computed tomography (µCT) scans, biomechanical testing, histological and histomorphometric analysis, and protein expression analysis. Ocn-cretg/+;Ptenflox/flox mice had significantly stiffer and stronger intact bones relative to controls in all cohorts. They also had significantly stiffer healing bones at day 28 post-fracture (PF) and significantly stronger healing bones at days 14, 21, and 28 PF. At day 7 PF, the proximal and distal ends of the Pten mutant calluses were more ossified. By day 28 PF, Pten mutants had larger and more mineralized calluses. Pten mutants had improved intramembranous bone formation during healing originating from the periosteum. They also had improved endochondral bone formation later in the healing process, after mature osteoblasts are present in the callus. Our results indicate that the inhibition of Pten can improve fracture healing and that the local or short-term use of commercially available Pten-inhibiting agents may have clinical application for enhancing fracture healing.

Apparent elastic modulus of ex vivo trabecular bovine bone increases with dynamic loading

Although it is widely known that bone tissue responds to mechanical stimuli, the underlying biological control is still not completely understood. The purpose of this study was to validate required methods necessary to maintain active osteocytes and minimize bone tissue injury in an ex vivo three-dimensional model that could mimic in vivo cellular function. The response of 22 bovine trabecular bone cores to uniaxial compressive load was investigated by using the ZETOS bone loading and bioreactor system while perfused with culture medium for 21 days. Two groups were formed, the “treatment” group (n = 12) was stimulated with a physiological compressive strain (4000 µε) in the form of a “jump” wave, while the “control” group (n = 10) was loaded only during three measurements for apparent elastic modulus on days 3, 10, and 21. At the end of the experiment, apoptosis and active osteocytes were quantified with histological analysis, and bone formation was identified by means of the calcium-binding dye, calcein. It was demonstrated that the treatment group increased the elastic modulus by 61%, whereas the control group increased by 28% (p<0.05). Of the total osteocytes observed at the end of 21 days, 1.7% (±0.3%) stained positive for apoptosis in the loaded group, whereas 2.7% (±0.4%) stained positive in the control group. Apoptosis in the center of the bone cores of both groups at the end of 21 days was similar to that observed in vivo. Therefore, the three-dimensional model used in this research permitted the investigation of physiological responses to mechanical loads on morphology adaptation of trabecular bone in a controlled defined load and chemical environment.

3D Elastomeric Scaffolds Fabricated by Casting in Micro End Milled Moulds

It is known that conventional scaffold manufacturing techniques have low reproducibility and control of the micro-architecture features. Although there have been advances in bone tissue engineering fabrication, there is no consensus on the optimized parameter designs or clear understanding of the microfluidic interactions required for tissue regeneration. In this work, we introduce a new inexpensive fabrication method of producing pore designs of 3D-elastomeric structures with high controlled geometry of orthogonal arrays. The present fabrication method utilizes a permanent and reusable micro-machined mould along with a micro-casted process to efficiently fabricate diverse 3D feature directly. This fabrication method, without multiple process steps, would be suitable to support experiments of controlled environment for flow effects in 3D bone scaffolds.

Nano-Mechanical Properties of Bioceramic Bone Scaffolds Fabricated at Three Sintering Temperatures

Bone grafting is an exceptionally common procedure used to repair bone defects within orthopaedics, craniofacial surgery and dentistry. It is estimated that 2.2 million grafting procedures are performed annually worldwide [1] and maintain a market share of

FACTORIAL ANALYSIS PREDICTS PARAMETER INTERACTIONS IN FE MODEL WITH REVISION IMPLANT

7 billion in the United States alone [2]. There has been a considerable rise in the interest of using bioactive ceramic materials, such as hydroxyapatite and tricalcium phosphate (TCP), to serve as synthetic replacements for autogenous bone grafts, which suffer from donor site morbidity and limited supply [3]. These ceramic materials (which can be formed into three-dimensional scaffolds) are advantageous due to their inherent biocompatibility, osteoconductivy, osteogenecity and osteointegrity [2].Copyright

Ligament property optimization within a virtual biomechanical knee

Long stemmed implants are frequently utilized during revision total knee arthroplasties (TKA) to reduce load at the proximal tibial bone implant interface. Yet, the surgeon performing the revision TKA is faced with many decisions including the mode of fixation and stem configurations. There is a growing interest among surgeons to better quantify the effects of stem parameters. Past finite element [Au, 2005], in-vitro [Stern, 1997], and clinical [Fehring, 2003] studies have quantified individual parameters, but no consensus has been reached. These past studies quantified their results based on one parameter at a time analysis. Yet factorial analysis allows us to not only investigate the effect of individual parameters, but also how these parameters interact with each other. Therefore the goal of this study was to perform a three factor factorial analysis to investigate which parameters have the largest effect on maximum micromotion, and if there are any interactions among these parameters.

Evaluation of the Creep Properties of Acrylic Bone Cement at the Macro and Nanoscale

Specimen-specific modeling of the knee can be an effective tool for understanding knee mechanics [1–2]. It can also serve as a design tool for orthopaedic implant design through enhancing understanding of mechanics in the reconstructed knee [3], particularly when used in conjunction with instrumented components that record in vivo joint forces [4]. Techniques for developing specimen-specific computational geometric models of hard tissue and soft tissue are fairly commonplace, using imaging tools such as computed tomography (CT) and magnetic resonance (MR) in conjunction with software tools for image processing. Determination of specimen-specific material properties relies on measuring kinematics of the tissue associated with a defined load, either in vivo or in vitro, selecting an appropriate material model, and estimating values of the parameters of the model that closely match the experimental data. The goal of this work was to utilize inverse finite element (FE) analysis to determine material parameters of ligaments in a specimen-specific model of the knee, using both local and global optimization algorithms.Copyright