Mechano-Hypoxia Conditioning of Engineered Human Meniscus.

Meniscus fibrochondrocytes (MFCs) experience simultaneous hypoxia and mechanical loading in the knee joint. Experimental conditions based on these aspects of the native MFC environment may have promising applications in human meniscus tissue engineering. We hypothesized that in vitro "mechano-hypoxia conditioning" with mechanical loading such as dynamic compression (DC) and cyclic hydrostatic pressure (CHP) would enhance development of human meniscus fibrocartilage extracellular matrix in vitro. MFCs from inner human meniscus surgical discards were pre-cultured on porous type I collagen scaffolds with TGF-ß3 supplementation to form baseline tissues with newly formed matrix that were used in a series of experiments. First, baseline tissues were treated with DC or CHP under hypoxia (HYP, 3% O2) for 5 days. DC was the more effective load regime in inducing gene expression changes, and combined HYP/DC enhanced gene expression of fibrocartilage precursors. The individual treatments of DC and HYP regulated thousands of genes, such as chondrogenic markers SOX5/6, in an overwhelmingly additive rather than synergistic manner. Similar baseline tissues were then treated with a short course of DC (5 vs 60 min, 10-20% vs 30-40% strain) with different pre-culture duration (3 vs 6 weeks). The longer course of loading (60 min) had diminishing returns in regulating mechano-sensitive and inflammatory genes such as c-FOS and PTGS2, suggesting that as few as 5 min of DC was adequate. There was a dose-effect in gene regulation by higher DC strains, whereas outcomes were inconsistent for different MFC donors in pre-culture durations. A final set of baseline tissues was then cultured for 3 weeks with mechano-hypoxia conditioning to assess mechanical and protein-level outcomes. There were 1.8-5.1-fold gains in the dynamic modulus relative to baseline in HYP/DC, but matrix outcomes were equal or inferior to static controls. Long-term mechano-hypoxia conditioning was effective in suppressing hypertrophic markers (e.g., COL10A1 10-fold suppression vs static/normoxia). Taken together, these results indicate that appropriately applied mechano-hypoxia conditioning can support meniscus fibrocartilage development in vitro and may be useful as a strategy for developing non-hypertrophic articular cartilage using mesenchymal stem cells.

A geometric approach to study the contact mechanisms in the patellofemoral joint of normal versus patellofemoral pain syndrome subjects.

The biomechanics of the patellofemoral (PF) joint is complex in nature, and the aetiology of such manifestations of PF instability as patellofemoral pain syndrome (PFPS) is still unclear. At this point, the particular factors affecting PFPS have not yet been determined. This study has two objectives: (1) The first is to develop an alternative geometric method using a three-dimensional (3D) registration technique and linear mapping to investigate the PF joint contact stress using an indirect measure: the depth of virtual penetration (PD) of the patellar cartilage surface into the femoral cartilage surface. (2) The second is to develop 3D PF joint models using the finite element analysis (FEA) to quantify in vivo cartilage contact stress and to compare the peak contact stress location obtained from the FE models with the location of the maximum PD. Magnetic resonance images of healthy and PFPS subjects at knee flexion angles of 15°, 30° and 45° during isometric loading have been used to develop the geometric models. The results obtained from both approaches demonstrated that the subjects with PFPS show higher PD and contact stresses than the normal subjects. Maximum stress and PD increase with flexion angle, and occur on the lateral side in healthy and on the medial side in PFPS subjects. It has been concluded that the alternative geometric method is reliable in addition to being computationally efficient compared with FEA, and has the potential to assess the mechanics of PFPS with an accuracy similar to the FEA.

Compositional and material properties of rat bone after bisphosphonate and/or Strontium ranelate drug treatment.

PURPOSE: We investigated elemental strontium and/or bisphosphonate drug incorporation upon the compositional and biomechanical properties of vertebral bone, in a rat model of Osteoporosis secondary to ovariectomy. METHODS: Six month old female rats were ovariectomized (OVX) and divided into untreated OVX-Vehicle, OVX-RIS (Risedronate bisphosphonate [BP] treated), OVX-SrR (Strontium Ranelate [Protos®] treated), combination OVX-RIS+SrR, and sham-operated controls. After 16 weeks of treatment, rats were euthanized and lumbar vertebra were dissected. Micro-Computed Tomography (micro-CT), Electron Probe Micro-Analysis (EPMA), mechanical testing in compression and nano-indentation testing were then undertaken to evaluate bone morphometry, elemental composition, material properties and strength. RESULTS: Bone Volume was significantly reduced in the OVX-Vehicle (133±10 mm(3)) compared with OVX-RIS (169±22 mm(3)), OVX-SrR (145±2 mm(3)), and OVX-RIS+SrR (172±8 mm(3)). EPMA mapped elemental Sr deposition to the periosteal surface of cortical bone (50-100 µm thick), endosteal trabecular surfaces (20 µm thick), as well as to both vertebral growth plates. The atomic ratios of (Ca+Sr)/P were significantly reduced with SrR treatment (2.4%-6.6%), indicating Sr incorporation into bone mineral. No significant differences were measured in vertebral bone reduced modulus by nano-indentation. Conversely, all BP-dosed groups had significantly increased structural bone strength. CONCLUSIONS: Thus, we conclude that BP drugs dominate the conservation of trabecular geometry and structural strength in OP rats, whereas Sr drugs likely influence bone volume and material composition locally.

Congruency effects on load bearing in diarthrodial joints.

Modelling load bearing in diarthrodial joints is challenging, due to the complexity of the materials, the boundary and interface conditions and the geometry. The articulating surfaces are covered with cartilage layers that are filled with a fluid that plays a major role in load bearing [Mow, V.C., Holmes, M.H., Lai, W.M. (1984) "Survey article: fluid transport and mechanical properties of articular cartilage: a review", Journal of Biomechanics 17(5), 377-394]. Researchers have tended to approximate joint geometry using axisymmetry [Donzelli, P.S., Spilker, R.L., Ateshian, G.A., Mow, V.C. (1999) "Contact analysis of biphasic transversely isotropic cartilage layers and correlations with tissue failure", Journal of Biomechanics 32, 1037-1047], often with a rounded upper articulating surface, creating a form of Hertz problem [Donzelli, P.S., Spilker, R.L., Ateshian, G.A., Mow, V.C. (1999) "Contact analysis of biphasic transversely isotropic cartilage layers and correlations with tissue failure", Journal of Biomechanics 32, 1037-1047]. However, diarthrodial joints (shoulder, hip and knee) are equipped with peripheral structures (glenoid labrum, acetabular labrum and meniscus, respectively) that tend to deepen the joint contact and thus cause initial contact to be established at the periphery of the joint rather than "centrally". The surface geometries are purposefully incongruent, and the incongruency has a significant effect on the stresses, pressures and pressure gradients inside the tissue. The models show the importance of the peripheral structures and the incongruency from a load-bearing perspective. Joint shapes must provide a compromise between demands for load-bearing, lubrication and the supply of nutrients to the chondrocytes of the cartilage and cells of the peripheral structures. Retention and repair of the functionality of these peripheral structures should be a prime consideration in any surgical treatment of an injured joint.

A novel application of the principles of linear elastic fracture mechanics (LEFM) to the fatigue behavior of tendon tissue.

BACKGROUND: Experiments on the fatigue of tendons have shown that cyclic loading induces failure at stresses lower than the ultimate tensile strength (UTS) of the tendons. The number of cycles to failure (Nf) has been shown to be dependent upon the magnitude of the applied cyclic stress. METHOD OF APPROACH: Utilizing data collected by Schechtman (1995), we demonstrate that the principles of Linear Elastic Fracture Mechanics (LEFM) can be used to predict the fatigue behavior of tendons under cyclic loading for maximum stress levels that are higher than 10% of the ultimate tensile strength (UTS) of the tendon (the experimental results at 10% UTS did not fit with our equations). CONCLUSIONS: LEFM and other FM approaches may prove to be very valuable in advancing our understanding of damage accumulation in soft connective tissues.