Hydrostatic Excess Pore Pressure Research Articles

Simultaneous magnetic resonance imaging and consolidation measurement of articular cartilage.

Magnetic resonance imaging (MRI) offers the opportunity to study biological tissues and processes in a non-disruptive manner. The technique shows promise for the study of the load-bearing performance (consolidation) of articular cartilage and changes in articular cartilage accompanying osteoarthritis. Consolidation of articular cartilage involves the recording of two transient characteristics: the change over time of strain and the hydrostatic excess pore pressure (HEPP). MRI study of cartilage consolidation under mechanical load is limited by difficulties in measuring the HEPP in the presence of the strong magnetic fields associated with the MRI technique. Here we describe the use of MRI to image and characterize bovine articular cartilage deforming under load in an MRI compatible consolidometer while monitoring pressure with a Fabry-Perot interferometer-based fiber-optic pressure transducer.

Articular Cartilage Compression: How Microstructural Response Influences Pore Pressure in Relation to Matrix Health

Our research investigated the influence of degeneration on both the pore-pressure development and microstructural response of cartilage during indentation with a flat-porous-indenter. Experiments were conducted to link the mechanical and structural responses of normal and degenerate articular cartilage. We found that from the instant of loading the degenerate matrix generated a higher peak hydrostatic excess pore pressure in a shorter period of time than the normal matrix. Following the attainment of this peak value the pore pressure in both tissue groups then gradually decayed toward zero over time, thus demonstrating a classical consolidation response. The microstructural analysis provided a unique insight into the influence of degeneration on the mechanisms of internal stress-sharing within the loaded matrix. Both disruption of the articular surface and general matrix destructuring results in an altered deformation field in both the directly loaded and nondirectly loaded regions. It is argued that the higher levels of matrix shear combined with less of the applied load being redirected into the wider cartilage continuum accounts for the elevated levels of peak hydrostatic pore pressure generated in the degenerate matrix.

Consolidation responses of delipidized articular cartilage

Objective. To determine the role of articular cartilage lipids in its load-bearing function. Design. Normal and delipidised, bovine articular cartilage specimens were statically loaded and both the hydrostatic excess pore pressure and creep strain were measured. From this the compression stiffness of the skeletal structures of both types of matrices was determined. Background. It has been hypothesized that surfactant injection could relief osteoarthritis, but there is no study in the literature relating to the influence of lipids, the main ingredients of such products, on cartilage load-carriage. Methods. Articular cartilage specimens were obtained from the patellar grooves of 2–3 year old bovine animals. When required specimen delipidization was carried out using chloroform/methanol rinsing. Both normal and delipidised samples were loaded in the consolidometer and the hydrostatic excess pore pressure and strain were measured. Results. The transient patterns of the hydrostatic excess pore pressure were similar for both types of tissue, with a relatively insignificant increase of 2% in the maximum hydrostatic excess pore pressure of the delipidized samples relative to the normal intact specimens. The maximum creep strain of the delipidised specimens decreased by 10% on average relative to their normal intact counterparts, thereby indicating that delipidization causes stiffening of the cartilage matrix. Conclusion. The delipidized fluid-saturated articular cartilage is stiffer than its intact counterpart with consequence for cartilage compliance during function. Relevance Because osteoarthritis can be accompanied by lipid loss in cartilage, this study contributes to the further understanding of the disease with potential benefit for treatment.

Predictive rheological models for the consolidation behaviour of articular cartilage under static loading.

This paper evaluates an existing rheological model of articular cartilage and explores the representative capacity of other phenomenological models of the tissue's matrix within the framework of mechanical consolidation. A unique feature is the introduction of a swelling element in tandem with 'fluid-filled' hyperelastic rheological elements to predict the transient load-induced behaviour of the tissue and evaluate the role of swelling in determining its load-carriage mechanism. The rheological models proposed have been used to predict the dependence of the one-dimensional consolidation response of the articular cartilage matrix, and the results obtained have been compared with published experimental results. This comparison demonstrates that the hydrostatic excess pore pressure, especially in the initial stages of deformation cannot be predicted without an adequate representation of swelling and its non-linear interaction with mechanical governing parameters such as permeability and stiffness.

Experimental factors governing the internal stress state of the intervertebral disc

This paper presents an experimental study of the response of the intervertebral disc to short-term static-axial loading within the conceptual framework of mechanical consolidation as applied to swelling materials. An experimental technique developed previously for articular cartilage was used to load the disc in compression and measure simultaneously the matrix internal excess pore pressure under both radially constrained and radially unconstrained conditions. Specifically the short-term pattern of development of the hydrostatic excess pore pressure was investigated. The average ratio of the maximum excess pore pressure u i to the nominal applied stress for unconstrained loading was approximately 1.9 while for constrained loading it was about 1.4. This behaviour is interpreted in terms of the generation of a pore pressure in the nucleus which reflects a non-uniform partitioning of the applied load across the inhomogeneous structure of the disc. These results provide a quantitative insight into the role that physico-chemically generated swelling plays in determining the internal mechanical response of the disc to an externally applied load and suggest a quantitative means of investigating the important influence of degenerative changes on the internal stress state of the disc without needing to pierce the annular wall in order to insert a pressure sensing device.

Biomechanics of load-bearing of the intervertebral disc: an experimental and finite element model

This paper presents an experimental and finite element study of the biomechanical response of the intervertebral disc to static-axial loading in which classical consolidation theory was used to analyse its time-dependent response. A newly developed experimental technique was employed to load the disc in compression and measure simultaneously the matrix internal pressure and creep strain for the full consolidation process. It is shown that, upon loading, the disc develops a maximum hydrostatic excess pore pressure which gradually decays as water is exuded from the matrix. During this decay process, the applied load is progressively transferred to the solid components of the matrix until the load is borne in full by the solid at the end of consolidation. Material properties for the tissue were then obtained from the experimental stress-strain data and used in the finite element study in the development of a finite element solution based on Biot's theory of coupled solid-fluid interaction. An axisymmetric formulation was employed and the disc modelled as an anisotropic, non-linear poroelastic solid. A sensitivity analysis of the material properties for the structural components of the disc was carried out and the biomechanical response to compressive loading evaluated and compared to experimental data. The results show that the matrix permeability plays a significant role in determining the transient response of the tissue. Annualr disruptions of the disc were shown to result in an increase in the nuclear principal stresses suggesting that disrupted regions of the annulus fibrosus play a reduced role in load bearing.

Complex nature of stress inside loaded articular cartilage

We show that in the early stages of loading of the cartilage matrix extensive water exudation and related physicochemical and structural changes give rise to a distinctly consolidatable system. By enzymatically modifying the pre-existing osmotic condition of the normal matrix and measuring its hydrostatic excess pore pressure, we have studied the exact influence of physicochemistry on the consolidation of cartilage. We argue that the attainment of a certain minimum level of swelling stiffness of the solid skeleton, which is developed at the maximum hydrostatic excess pore pressure of the fluid, controls the effective consolidation of articular cartilage. Three related but distinct stresses are developed during cartilage deformation, namely (1) the swelling stress in the coupled proteoglycan/collagen skeleton in the early stages of deformation, (2) the hydrostatic excess pore pressure carried by the fluid component, and (3) the effective stress generated on top of the minimum value of the swelling stress in the consolidation stages following the attainment of the fluid's maximum pore pressure. The minimum value of the swelling pressure is in turn generated over and above the intrinsic osmotic pressure in the unloaded matrix. The response of the hyaluronidase-digested matrix relative to its intact state again highlights the important influence of the osmotic pressure and the coefficient of permeability, both of which are related to the volume fraction of proteoglycans on cartilage deformation, and therefore its ability to function as an effective stress-redistributing layer above the subchondral bone.

The Generalized Consolidation of Articular Cartilage: An Investigation of Its Near-Physiological Response to Static Load

This paper presents a study of the response of articular cartilage to compression whilst measuring simultaneously its strain and fluid excess pore pressure using a newly developed experimental apparatus for testing the tissue in its unconfined state. This has provided a comparison of the load-induced responses of the cartilage matrix under axial, radial and 3-D consolidation regimes. Our results demonstrate that the patterns of the hydrostatic excess pore pressure for axial and 3-D consolidation are similar, but differ significantly from that obtained under the more physiologically relevant condition in which the matrix exhibits radial consolidation when loaded either through a non-porous polished stainless steel indenter or an opposing cartilage disc. Based on the transient strain characteristics obtained under axial and unconfined compression we argue that consolidation is indeed the controlling mechanism of cartilage biomechanical function.

Is classical consolidation theory applicable to articular cartilage deformation?

In this paper, classical consolidation theory has been used to investigate the time-dependent response of articular cartilage to static loading. An experimental technique was developed to measure simultaneously the matrix internal pressure and creep strain under conditions of one-dimensional consolidation. This is the first measurement of the internal stress state of loaded cartilage. It is demonstrated that under static compression the applied load is shared by the components of the matrix (i.e. water, the proteoglycans, and the collagen fibrillar meshwork), during which time a maximum hydrostatic excess pore pressure is developed as initial water exudation occurs. This pressure decays as water is further exuded from the matrix and effective consolidation begins with a progressive transfer of the applied stress from water to the collagen fibrils and proteoglycan gel. Consolidation is completed when the hydrostatic excess pore pressure is reduced to zero and the solid components sustain in full the applied load.