Biomedical subjects

Ian A Stokes

Publications and source records attributed to Ian A Stokes.

7 recordsLinked to original sources

PubMed2005

Modulation of vertebral and tibial growth by compression loading: diurnal versus full-time loading.

PURPOSE: This study was designed to determine whether the amount of endochondral growth response to mechanical compression and the underlying growth mechanism differed with night-time or day-time loading, relative to full-time loading. METHODS: Mechanical compression (nominally 0.1 MPa stress) was applied across tibial and tail vertebral growth plates of growing Sprague-Dawley rats. Four groups of animals (five per group) were used: 24/24 h (full-time loading); 12/24 h (day-loading); 12/24 h (night-loading); and 0/24 h (sham instrumented). Contralateral tibiae and adjacent vertebrae served as within-animal controls. The animals were euthanized after eight days. Growth plates were processed for quantitative histology to measure 24-h growth, total and BrdU-positive proliferative zone chondrocyte counts, and hypertrophic chondrocytic enlargement in the growth direction. RESULTS: Growth as a percentage of within-animal control averaged 82% (full-time); 93% (day-loading); 90% (night-loading); 100% (sham) for vertebrae. For proximal tibiae it averaged 70% (full-time); 84% (day-loading); 86% (night-loading); 89% (sham). Reduced amount of hypertrophic chondrocytic enlargement explained about half of this effect in full-time loaded growth plates, but was not significantly altered in half-time loaded growth plates. The remaining variation in growth was apparently explained by reduced total numbers of proliferative zone chondrocytes. These findings indicate that sustained compression loading suppressed growth more than intermittent loading at both anatomical locations.

Animals↗

PubMed2003

Physiological axial compressive preloads increase motion segment stiffness, linearity and hysteresis in all six degrees of freedom for small displacements about the neutral posture.

The stiffness of motion segments, together with muscle actions, stabilizes the spinal column. The objective of this study was to compare the experimentally measured load-displacement behavior of porcine lumbar motion segments in vitro with physiological axial compressive preloads of 0, 200 and 400 N equilibrated in a physiological fluid environment, for small displacements about the neutral posture. These preloads are hypothesized to increase stiffness, hysteresis and linearity of the load-displacement behavior. At each preload, displacements in each of six degrees of freedom (+/-0.3 mm AP and lateral translations, +/-0.2 mm axial translation, +/-1 degrees lateral bending and +/-0.8 degrees flexion/extension and torsional rotations) were imposed. The resulting forces and moments were recorded. Tests were repeated after removal of posterior elements. Using least squares, the forces at the vertebral body center were related to the displacements by a symmetric 6 x 6 stiffness matrix. Six diagonal and two off-diagonal load-displacement relationships were examined for differences in stiffness, linearity and hysteresis in each testing condition. Mean values of the diagonal terms of the stiffness matrix for intact porcine motion segments increased significantly by an average factor of 2.2 and 2.9 with 200 and 400 N axial compression respectively (p<0.001). Increases for isolated disc specimens averaged 4.6 and 6.9 times with 200 and 400 N preload (p<0.001). Changes in hysteresis correlated with the changes in stiffness. The load-displacement relationships were progressively more linear with increasing preload (R(2)=0.82, 0.97 and 0.98 at 0, 200 and 400 N axial compression respectively). Motion segment and disc load-displacement behaviors were stiffer, more linear and had greater hysteresis with axial compressive preloads.

Animals↗

PubMed2002

Measurement of a spinal motion segment stiffness matrix.

The six-degrees-of-freedom elastic behavior of spinal motion segments can be approximated by a stiffness matrix. A method is described to measure this stiffness matrix directly with the motion segment held under physiological conditions of axial preload and in an isotonic fluid bath by measuring the forces and moments associated with each of the six orthogonal translations and rotations. The stiffness matrix was obtained from the load-displacement measurements by linear least squares assuming a symmetric matrix. Results from a pig lumbar spinal motion segment in an isotonic bath, with and without a 500 N axial preload, showed a large stiffening effect with axial preload.

Animals↗

PubMed2002

Enlargement of growth plate chondrocytes modulated by sustained mechanical loading.

BACKGROUND: Mechanical compression and distraction forces are known to modulate growth in vertebral growth plates, and they have been implicated in the progression of scoliosis. This study was performed to test the hypothesis that growth differences produced by sustained compression or distraction loading of vertebrae are associated with alterations in the amount of increase in the height of growth plate chondrocytes in the growth direction. METHODS: Compression or distraction force of nominally 60% of body weight was maintained for four weeks on a caudad vertebra of growing rats by an external apparatus attached, by means of transcutaneous pins, to the two vertebrae cephalad and caudad to it. Growth of the loaded and control vertebrae was measured radiographically. After four weeks, the animals were killed and histological sections of the loaded and control vertebrae were prepared to measure the height of the hypertrophic zone (average separation between zonal boundaries), the mean height of hypertrophic chondrocytes, and the amount of increase in cell height in the growth direction. RESULTS: Over the four weeks of the experiment, the growth rates of the compressed and distracted vertebrae averaged 52% and 113% of the control rates, respectively. The reduction in the growth rate of the compressed vertebrae was significant (p = 0.002). In the compressed vertebrae, the height of the hypertrophic zone, the mean chondrocyte height, and the amount of increase in cell height averaged 87%, 85%, and 78% of the control values, respectively, and all were significantly less than the corresponding control values. In the distracted vertebrae, these measurements did not differ significantly from the control values. The height of the hypertrophic zone and the mean chondrocyte height correlated with the growth rate (r (2) = 0.29 [p = 0.03] and r (2) = 0.23 [p = 0.06], respectively), when each variable was expressed as a proportion of the control value. The percentage changes in the measurements of the chondrocytic dimensions relative to the control values were smaller than the percentage changes in the growth rates, a finding that suggested that the rate of chondrocytic proliferation was also modulated by the mechanical loading. CONCLUSIONS: Mechanical loading of tail vertebrae in rats modulated their growth rate, which correlated with changes in the height of hypertrophic chondrocytes. The effects of compression were greater than those of distraction. CLINICAL RELEVANCE: Information about the growth rate and chondrocytic response to mechanical loads in rat vertebrae undergoing mechanically modulated growth will be helpful in determining how human vertebral growth might respond to altered loading states during progression or treatment of scoliosis and other growth-related angular skeletal deformities.

Animals↗

PubMed2002

Growth plate chondrocyte enlargement modulated by mechanical loading.

Mechanical compression and distraction forces are known to modulate growth in vertebral growth plates, and are implicated in the progression of scoliosis. The amount of cellular enlargement is thought to be a key variable correlating with differing growth at different anatomical sites. This work tested the hypothesis that growth differences produced by mechanical loading are associated with alterations in the amount of cell height increase in the growth direction. Compression force (3 animals) or distraction force (4 animals) of 60% bodyweight was maintained on a caudal vertebra of growing rats with an external apparatus attached to vertebrae adjacent to the experimental level via transfixing pins. Growth of loaded and control vertebrae was measured from weekly radiographs. After four weeks, animals were euthanized and the loaded vertebrae and adjacent control vertebrae were excised for histological sections from which the following were measured: height of the hypertrophic zone (the average separation between boundaries of the zone); Mean Hypertrophic chondrocytic height; slope of the linear regression of cell height vs. position in the zone, (the amount of cell enlargement). Distracted and compressed vertebra growth averaged 110% and 53% of control levels. The percentage changes in the measures of chondrocyte dimensions, relative to control values, were smaller than the percentage changes in growth velocity. Also, the effects were of smaller magnitude for distracted vertebrae than for compressed vertebrae. Growth plate zonal height, slope and mean cell height all correlated significantly (p<0.05) with growth rate (expressed as a percentage of control) (Pearson r = 0.69, 0.37 and 0.41 respectively. Thus mechanical loading of tail vertebrae modulated their growth rate, that in turn correlated with changes in the amount of hypertrophic chondrocyte height increase. The effects for compression were greater than for distraction.

Animals↗

PubMed2002

Mechanical modulation of vertebral and tibial growth: diurnal versus full-time loading.

UNLABELLED: The aim of this study was to determine whether the amount of growth response to mechanical compression and the underlying mechanism differed with night-time or day-time loading, relative to full time loading. Mechanical compression (nominally 0.1 MPa stress) was applied across tibial and tail vertebral growth plates of growing Sprague-Dawley rats. Four groups of animals were tested: 24/24 hour (full-time loading); 12/24 hour (day-loading); 12/24 hour (night-loading); and 0/24 hour (sham instrumented), 4 or 5 animals per group. After 8 days animals were euthanized and the growth plates were processed for quantitative histology of loaded and within-animal control growth plates to measure 24-hour growth, total and BrdU-positive proliferative zone chondrocyte counts, and hypertrophic chondrocyte enlargement in the growth direction. RESULTS: Growth as a percentage of within-animal control averaged 82% (full-time); 93% (day-loading); 90% (night-loading); 100% (sham) for vertebrae. For proximal tibiae it averaged 70% (full-time); 84% (day-loading); 86% (night-loading); 89% (sham). Reduced amount of hypertrophic chondrocytic enlargement explained about half of this effect in full-time compressed growth plates, but was not significantly altered in half-time loaded growth plates. The remaining variation in growth was apparently explained by reduced total numbers of proliferative zone chondrocytes. The BrdU labeling index demonstrated an opposite trend, which was not statistically significant. In half-time loaded growth plates the proliferative zone cell count change predominated.

Age Factors↗

PubMed2002

Motion segment stiffness measured without physiological levels of axial compressive preload underestimates the in vivo values in all six degrees of freedom.

UNLABELLED: Axial preload is known to alter the mechanical properties of spinal motion segments. The objective of this study was to compare the experimentally measured load-displacement behavior (stiffness, hysteresis and linearity) of porcine lumbar motion segments in vitro with physiological axial compressive preloads of 0, 200 and 400 N equilibrated in a physiological fluid environment. At each preload, displacements in each of six degrees of freedom (+/- 0.3 mm AP and lateral translations, +/- 0.2 mm axial translation, +/- 1 degrees lateral bending and +/- 0.8 degrees flexion/extension and torsional rotations) were imposed. The resulting forces and moments were recorded. Tests were repeated after removal of posterior elements. Using least squares, the forces at the vertebral body center were related to the displacements by a symmetric 6x6 stiffness matrix. The stiffness, hysteresis area and linearity of six diagonal and two off-diagonal load-displacement relationships were examined for differences between preload conditions. RESULTS: Mean values of the diagonal terms of the stiffness matrix for intact porcine motion segments increased significantly by an average factor of 2.2 and 2.9 with 200 and 400 N axial compression respectively (p<0.001). Increases for isolated disc specimens averaged 4.6 and 6.9 times with 200 and 400 N preload (p<0.001). Changes in hysteresis correlated with the changes in stiffness. The load-displacement relationships were progressively more linear with increasing preload (R2 = 0.82, 0.97 and 0.98 at 0, 200 and 400 N axial compression respectively). Motion segment and disc load-displacement behaviors were stiffer, more linear and had greater hysteresis with axial compressive preloads.

Animals↗