Biomedical subjects

Theo H Smit

Publications and source records attributed to Theo H Smit.

27 records · Page 2Linked to original sources

PubMed2003

Strain-derived canalicular fluid flow regulates osteoclast activity in a remodelling osteon--a proposal.

The concept of bone remodelling by basic multicellular units is well established, but how the resorbing osteoclasts find their way through the pre-existing bone matrix remains unexplained. The alignment of secondary osteons along the dominant loading direction suggests that remodelling is guided by mechanical strain. This means that adaptation (Wolff's Law) takes place throughout life at each remodelling cycle. We propose that alignment during remodelling occurs as a result of different canalicular flow patterns around cutting cone and reversal zone during loading. Low canalicular flow around the tip of the cutting cone is proposed to reduce NO production by local osteocytes thereby causing their apoptosis. In turn, osteocyte apoptosis could be the mechanism that attracts osteoclasts, leading to further excavation of bone in the direction of loading. At the transition between cutting cone and reversal zone, however, enhanced canalicular flow will stimulate osteocytes to increase NO production, which induces osteoclast retraction and detachment from the bone surface. Together, this leads to a treadmill of attaching and detaching osteoclasts in the tip and the periphery of the cutting cone, respectively, and the digging of a tunnel in the direction of loading.

Biomechanical Phenomena↗

PubMed2003

Inertial shear forces and the use of centrifuges in gravity research. What is the proper control?

Centrifuges are used for 1 x g controls in space flight microgravity experiments and in ground based research. Using centrifugation as a tool to generate an Earth like acceleration introduces unwanted inertial shear forces to the sample. Depending on the centrifuge and the geometry of the experiment hardware used these shear forces contribute significantly to the total force acting on the cells or tissues. The inertial shear force artifact should be dealt with for future experiment hardware development for Shuttle and the International Space Station (ISS) as well as for the interpretation of previous space-flight and on-ground research data.

Acceleration↗

PubMed2002

Bioabsorbable poly-L-lactic acid cages for lumbar interbody fusion: three-year follow-up radiographic, histologic, and histomorphometric analysis in goats.

STUDY DESIGN: Long-term evaluation was performed for bioabsorbable poly-L-lactic acid cages in a goat interbody fusion model. OBJECTIVE: To assess the radiographic, histologic, and histomorphometric characteristics of poly-L-lactic acid cages during 3 years of follow-up evaluation. SUMMARY OF BACKGROUND DATA: Failed cage fusions may be related to cage design and material in addition to the surgical technique used. To overcome material-related complications and to explore the potential benefits of bioabsorbable cages, poly-L-lactic acid cages have been designed. METHODS: For this study, 36 Dutch milk goats underwent a lumbar interbody fusion procedure (L3-L4). Two types of custom-made cage devices were impacted with bone graft and implanted: poly-L-lactic acid cages (n = 30) and titanium cages (n = 6). Sequential harvesting of surgically managed motion segments (intervals: 3, 6, 12, 24, and 36 months) was performed for analysis. RESULTS: In poly-L-lactic acid specimens, permanent interbody fusion could be achieved within 6 months after surgery with maintenance of cage height. Titanium specimens showed no interbody fusion within this period. Radiographic follow-up evaluation (6-36 months) showed interbody fusion in 86% (19/22) of poly-L-lactic acid specimens, as compared with 33% (2/6) of titanium specimens. After 36 months of implantation, in one half of the specimens, poly-L-lactic acid cages were completely absorbed. Bone histomorphometry showed complete bone remodeling after 2 years of follow-up evaluation. During the study period, no local or distant adverse histologic effects were observed. CONCLUSIONS: The current study showed that poly-L-lactic acid cage devices are feasible for lumbar interbody fusion. New poly-L-lactic acid cages designed for clinical practice might be a viable alternative to current nonabsorbable cage devices.

Absorbable Implants↗

PubMed2002

The use of poly-L-lactic acid in lumbar interbody cages: design and biomechanical evaluation in vitro.

Cage design and cage material may play a crucial role in the incidence of postoperative complications reported with current non-absorbable interbody cage devices. Bioabsorbable poly-L-lactic acid cage devices may have potential benefits. The purpose of this study was to determine the required strength of poly-L-lactic acid cages for use in experimental goat studies and to evaluate the mechanical properties of different cage designs in situ. The yield and ultimate strength of native goat motion segments (L1-L6) were determined; the yield strength was used as a design parameter for the cages. The mechanical behaviour of two types of poly-L-lactic acid cages, the influence of endplate perforation, differences between toothed and smooth cages, and the influence of cage filling were biomechanically tested and compared to native motion segments. Only axial compression until failure of the motion segments was performed. Dual energy X-ray absorptiometry was used to determine bone mineral content. The yield and ultimate strength of the native motion segments were 3.5 and 7.0 kN, respectively. Based on these data, flexible and stiff poly-L-lactic acid cages were designed with strengths of 3.5 and 7 kN, respectively. Poly-L-lactic acid cages, whether with or without bone graft and perforating the endplates, did not reduce the compressive strength of motion segments as compared to native segments. However, toothed titanium cages, with the same geometry, negatively influenced the segments' compressive strength, which effect was reduced using smooth titanium cages.

Absorbable Implants↗

PubMed2002

The effect of cage stiffness on the rate of lumbar interbody fusion: an in vivo model using poly(l-lactic Acid) and titanium cages.

STUDY DESIGN: A goat interbody fusion model using poly-(L-lactic acid) and titanium cages was designed to evaluate the effect of cage stiffness on lumbar interbody fusion. OBJECTIVE: To investigate the effect of cage stiffness on the rate of interbody fusion. SUMMARY OF BACKGROUND DATA: Various types of cages considerably exceed the stiffness of vertebral bone, which ultimately may lead to postoperative complications. To avoid these complications, poly-(L-lactic acid) cages with limited stiffness have been designed. The mechanical integrity of the cages remains intact for at least 6 months. METHODS: Interbody fusions were performed at L3-L4 of 15 Dutch milk goats, and one of three cages was randomly implanted: 1) a titanium cage (n = 3), 2) a stiff poly-(L-lactic acid) cage (n = 6), or 3) a flexible poly-(L-lactic acid) cage (n = 6). Interbody fusion was assessed radiographically by three independent observers 3 and 6 months after surgery. RESULTS: At 3 months, all the poly-(L-lactic acid) specimens showed ingrowth of new bone, but with radiolucency in the fusion mass. At 6 months, solid arthrodesis was observed in four of six poly-(L-lactic acid) specimens, advanced ingrowth in one specimen, and infection in one specimen. Titanium cages showed ingrowth of bone, but with radiolucency in the fusion mass. Interbody fusion using poly-(L-lactic acid) cages showed a significantly higher rate statistically (P = 0.016) and more complete fusion than titanium cages of the same design. CONCLUSIONS: The reduced stiffness of poly-(L-lactic acid) cages showed enhanced interbody fusion, as compared with titanium cages after 6 months. Bioabsorbable poly-(L-lactic acid) cages thus may be a viable alternative to current interbody cage devices, thereby avoiding the concomitant problems related to their excessive stiffness. However, the bioabsorbability of the poly-(L-lactic acid) cages awaits investigation in a long-term study currently underway.

Absorbable Implants↗

PubMed2002

The use of a quadruped as an in vivo model for the study of the spine - biomechanical considerations.

Animal models in spine research are often criticized for being irrelevant to the human situation due to the horizontal position of the spine. Whether this is justified from a biomechanical point of view can be questioned. The purpose of the study reported here was to provide arguments that a quadruped can be a valuable in vivo model for the study of the spine in spite of its horizontal position. Relevant literature is reviewed, and biomechanical analyses were made of the standing and walking quadruped. Further, the vertebral trabecular bone architecture was quantitatively analysed by computer and interpreted in the light of Wolff's law. Due to the fact that spinal segments cannot withstand substantial bending moments, additional tensile forces from muscles and ligaments are necessary to control the posture of a quadruped spine. As a consequence, the spine is mainly loaded by axial compression. The trabeculae in a goat's vertebral body were found to course horizontally between its anterior and posterior endplates, implying that the main load within the vertebral body was indeed an axial compression force. The density of the vertebrae of quadrupeds is higher than that of human vertebrae, suggesting that the quadruped has to sustain higher axial compression stresses. The quadruped spine is mainly loaded along its long axis, just like the human spine. The quadruped can thus be a valuable animal model for spine research. An important point of difference is the higher axial compression stress in quadrupeds, which leads to higher bone densities in the vertebrae. This puts some limitations on the transferability of the results of animal experiments to the human situation.

Animals↗

PubMed2002

Estimation of the poroelastic parameters of cortical bone.

Cortical bone has two systems of interconnected channels. The largest of these is the vascular porosity consisting of Haversian and Volkmann's canals, with a diameter of about 50 microm, which contains a.o. blood vessels and nerves. The smaller is the system consisting of the canaliculi and lacunae: the canaliculi are at the submicron level and house the protrusions of the osteocytes. When bone is differentially loaded, fluids within the solid matrix sustain a pressure gradient that drives a flow. It is generally assumed that the flow of extracellular fluid around osteocytes plays an important role not only in the nutrition of these cells, but also in the bone's mechanosensory system. The interaction between the deformation of the bone matrix and the flow of fluid can be modelled using Biot's theory of poroelasticity. However, due to the inhomogeneity of the bone matrix and the scale of the porosities, it is not possible to experimentally determine all the parameters that are needed for numerical implementation. The purpose of this paper is to derive these parameters using composite modelling and experimental data from literature. A full set of constants is estimated for a linear isotropic description of cortical bone as a two-level porous medium. Bone, however, has a wide variety of mechanical and structural properties; with the theoretical relationships described in this note, poroelastic parameters can be derived for other bone types using their specific experimental data sets.

Bone Matrix↗

PubMed2002

A case for strain-induced fluid flow as a regulator of BMU-coupling and osteonal alignment.

Throughout life, human bone is renewed continuously in a tightly controlled sequence of resorption and formation. This process of bone remodeling is remarkable because it involves cells from different lineages, collaborating in so-called basic multicellular units (BMUs) within small spatial and temporal boundaries. Moreover, the newly formed (secondary) osteons are aligned to the dominant load direction and have a density related to its magnitude, thus creating a globally optimized mechanical structure. Although the existence of BMUs is amply described, the cellular mechanisms driving bone remodeling-particularly the alignment process-are poorly understood. In this study we present a theory that explains bone remodelling as a self-organizing process of mechanical adaptation. Osteocytes thereby act as sensors of strain-induced fluid flow. Physiological loading produces stasis of extracellular fluid in front of the cutting cone of a tunneling osteon, which will lead to osteocytic disuse and (continued) attraction of osteoclasts. However, around the resting zone and the closing cone, enhanced extracellular fluid flow occurs, which will activate osteocytes to recruit osteoblasts. Thus, cellular activity at a bone remodeling site is well related to local fluid flow patterns, which may explain the coordinated progression of a BMU.

Bone Remodeling↗

PubMed2002

Inertial shear force and the impact on facilities for the International Space Station.

Inertial shear force is a surface force that is generated in centrifuges especially with attached samples on flat surfaces and plays a significant role in gravitational and space research. The magnitude of this force is proportional to the radius of the centrifuge and surface area of the sample compartment. In gravitational research we want to study the impact of weight onto a system. However, the force of inertial shear is perpendicular to the gravity vector, hence, results may be obscured or even misinterpreted by this artifact.

Journal Article↗