Seth W. Donahue

Strains in the metatarsals during the stance phase of gait: implications for stress fractures.

BACKGROUND Stress fractures of the metatarsals are common overuse injuries in athletes and military cadets, yet their etiology remains unclear. In vitro, high bone strains have been associated with the accumulation of microdamage and shortened fatigue life. It is therefore postulated that stress fractures in vivo are caused by elevated strains, which lead to the accumulation of excessive damage. We used a cadaver model to test the hypothesis that strains in the metatarsals increase with simulated muscle fatigue and plantar fasciotomy. METHODS A dynamic gait simulator was used to load fifteen cadaveric feet during the entire stance phase of gait under conditions simulating normal walking, walking with fatigue of the auxiliary plantar flexors, and walking after a plantar fasciotomy. Strains were measured, with use of axial strain-gauges, in the dorsal, medial, and lateral aspects of the diaphysis of the second and fifth metatarsals as well as in the proximal metaphysis of the fifth metatarsal. RESULTS When the feet were loaded under normal walking conditions, the mean peak strain in the dorsal aspect of the second metatarsal (-1897 microstrain) was more than twice that in the medial aspect of the fifth metatarsal (-908 microstrain). Simulated muscle fatigue significantly increased peak strain in the second metatarsal and decreased peak strain in the fifth metatarsal. Release of the plantar fascia caused significant alterations in strain in both metatarsal bones; these alterations were greater than those caused by muscle fatigue. After the plantar fasciotomy, the mean peak strain in the dorsal aspect of the second metatarsal (-3797 microstrain) was twice that under normal walking conditions. CONCLUSIONS The peak axial strain in the diaphysis of the second metatarsal is significantly (p < 0.0001) higher than that in the diaphysis of the fifth metatarsal during normal gait. The plantar fascia and the auxiliary plantar flexors are important for maintaining normal strains in the metatarsals during gait.

Osteoblastic cells have refractory periods for fluid-flow-induced intracellular calcium oscillations for short bouts of flow and display multiple low-magnitude oscillations during long-term flow.

Partitioning a daily mechanical stimulus into discrete loading bouts enhances bone formation in rat tibiae (J. Bone Mineral Res. 15(8) (2000) 1596). We hypothesized that a refractory period exists in primary rat osteoblastic cells, during which fluid-flow-induced [Ca(2+)](i) oscillations are insensitive to additional short bouts (2 min) of fluid flow. Because the frequency of [Ca(2+)](i) oscillations is believed to be important for regulating cellular activity and long-term fluid flow alters gene expression in bone cells, we also hypothesized that long-term (15 min) oscillating fluid flow produces multiple [Ca(2+)](i) oscillations in osteoblastic cells. Primary osteoblastic cells from rat long bones were exposed to 2 min of oscillating fluid flow that produced shear stresses of 2 Pa at 2 Hz. After a rest period of 5, 30, 60, 300, 600, 900, 1800, or 2700 s, the cells were exposed to a second 2-min bout of flow. A 600 s rest period was required to recover the percentage of cells responding to fluid flow and a 900 s rest period was required to recover the [Ca(2+)](i) oscillation magnitude. The magnitude and shape of the two [Ca(2+)](i) oscillations were strikingly similar for individual cells after a 900 s rest period. During 15 min of continuous oscillating flow, some individual cells displayed between 1 and 9 oscillations subsequent to the initial [Ca(2+)](i) oscillation. However, only 54% of the cells that responded initially displayed subsequent [Ca(2+)](i) oscillations during long-term flow and the magnitude of the subsequent oscillations was only 28% of the initial response.

A Comparative Study of Shear Stresses in Collagen-Glycosaminoglycan and Calcium Phosphate Scaffolds in Bone Tissue-Engineering Bioreactors

The increasing demand for bone grafts, combined with their limited availability and potential risks, has led to much new research in bone tissue engineering. Current strategies of bone tissue engineering commonly use cell-seeded scaffolds and flow perfusion bioreactors to stimulate the cells to produce bone tissue suitable for implantation into the patients body. The aim of this study was to quantify and compare the wall shear stresses in two bone tissue engineering scaffold types (collagen-glycosaminoglycan (CG) and calcium phosphate) exposed to fluid flow in a perfusion bioreactor. Based on micro-computed tomography images, three-dimensional numerical computational fluid dynamics (CFD) models of the two scaffold types were developed to calculate the wall shear stresses within the scaffolds. For a given flow rate (normalized according to the cross-sectional area of the scaffolds), shear stress was 2.8 times as high in the CG as in the calcium-phosphate scaffold. This is due to the differences in scaffold geometry, particularly the pore size (CG pore size approximately 96 microm, calcium phosphate pore size approximately 350 microm). The numerically obtained results were compared with those from an analytical method that researchers use widely experimentalists to determine perfusion flow rates in bioreactors. Our CFD simulations revealed that the cells in both scaffold types were exposed to a wide range of wall shear stresses throughout the scaffolds and that the analytical method predicted shear stresses 12% to 21% greater than those predicted using the CFD method. This study demonstrated that the wall shear stresses in calcium phosphate scaffolds (745.2 mPa) are approximately 40 times as high as in CG scaffolds (19.4 mPa) when flow rates are applied that have been experimentally used to stimulate the release of prostaglandin E(2). These findings indicate the importance of using accurate computational models to estimate shear stress and determine experimental conditions in perfusion bioreactors for tissue engineering.

Parathyroid hormone may maintain bone formation in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis.

SUMMARY Mechanical unloading of bone causes an imbalance in bone formation and resorption leading to bone loss and increased fracture risk. Black bears (Ursus americanus) are inactive for up to six months during hibernation, yet bone mineral content and strength do not decrease with disuse or aging. To test whether hibernating bears have biological mechanisms to prevent disuse osteoporosis, we measured the serum concentrations of hormones and growth factors involved in bone metabolism and correlated them with the serum concentration of a bone formation marker (osteocalcin). Serum was obtained from black bears over a 7-month duration that included periods of activity and inactivity. Both resorption and formation markers increased during hibernation, suggesting high bone turnover occurred during inactivity. However, bone formation appeared to be balanced with bone resorption. The serum concentration of parathyroid hormone (PTH) was higher in the hibernation (P=0.35) and post-hibernation (P=0.006) seasons relative to pre-hibernation levels. Serum leptin was lower (P<0.004) post-hibernation relative to pre-hibernation and hibernation periods. Insulin-like growth factor I (IGF-I) decreased (P<0.0001) during hibernation relative to pre-hibernation and reached its highest value during remobilization. There was no difference (P=0.64) in 25-OH vitamin D between the three seasons. Serum osteocalcin (bone formation marker) was significantly correlated with PTH, but not with leptin, IGF-I or 25-OH vitamin D. Osteocalcin and PTH were positively correlated when samples from all seasons were pooled and when only hibernation samples were considered, raising the possibility that the anabolic actions of PTH help maintain bone formation to prevent disuse osteoporosis. Prostaglandin E2 (PGE2) release from MC3T3 osteoblastic cells was significantly affected by treatment with bear serum from different seasons (i.e. hibernation versus active periods). The seasonal changes in PGE2 release showed trends similar to the seasonal changes in serum IGF-I. Since both PGE2 and IGF-I are associated with collagenous bone formation, it is possible that seasonal changes in a circulating factor influence IGF-I levels in vivo in bears and PGE2 release in osteoblastic cells in vitro. The significant decrease in serum leptin following arousal from hibernation may promote bone formation during remobilization, assuming there is a similar decrease in intracerebroventricular leptin. These findings support the idea that seasonal changes in the concentration of circulating molecules help regulate bone formation activity and may be important for preventing disuse osteoporosis in bears.

Bone strain and microcracks at stress fracture sites in human metatarsals

Microcracks in bone have been implicated in the development of stress fractures. The goal of this study was to evaluate bone strain and microcracks at locations where stress fractures are common (second metatarsal diaphysis) and rare (fifth metatarsal diaphysis) in an attempt to increase our understanding of the pathogenesis of stress fractures. A dynamic gait simulator was used to simulate normal walking with cadaver feet. The feet were loaded over the entire stance phase of gait and diaphyseal strains were recorded in second and fifth metatarsals. Microcrack density (Cr.Dn) and surface density (Cr.S.Dn) were determined in metatarsal cross sections from the contralateral feet. Bone strain was significantly higher in second metatarsals (-1897 +/- 613 microstrain) than in fifth metatarsals (-908 +/- 503 microstrain). However, second metatarsal Cr.Dn (0.23 +/- 0.15 #/mm(2)) was not significantly different from fifth metatarsal Cr.Dn (0.35 +/- 0.19 #/mm(2)). There was also no significant difference between Cr.S.Dn in second (17.64 +/- 10.99 microm/mm(2)) and fifth (26.70 +/- 15.53 microm/mm(2)) metatarsals. There were no significant relationships between the microcrack parameters and peak strain in either metatarsal. Cracks that occurred in trabecular struts (92 +/- 33 microm) were significantly longer than those found ending at cement lines (71 +/- 15 microm) and within osteons (57 +/- 16 microm). There were no significant relationships between the microcrack parameters and age in either metatarsal. Peak strain was more than twofold greater in second metatarsals than in fifth metatarsals for simulations of normal walking; however, microcrack parameters were unable to explain the greater incidence of second metatarsal stress fractures.

Biomechanical Consequences of Plantar Fascial Release or Rupture During Gait: Part I - Disruptions in Longitudinal Arch Conformation

To examine whether conformational changes induced by plantar fascial division may progress during gait, we loaded the feet of seven cadavers using an apparatus that simulates the actions of the extrinsic plantarflexors. We measured the effects of plantar fasciotomy at two instants in the terminal-stance phase of gait. Radiographic measurements of height of the arch, base length of the arch, and talo first-metatarsal angle were used to assess contributions to arch support made by the plantar fascia, tibialis posterior, peroneus longus and brevis, and digital flexor muscles. Complete fasciotomy caused significant collapse of the arch in the sagittal plane. Early in terminal stance, at the instant after heel-off, mean height of the arch decreased from 47 to 45 mm. Late in terminal stance, at the instant preceding contralateral heel strike, mean height of the arch decreased from 46 to 43. Effects of division of the central band, though significant, were mild. Medial base length of the arch increased from 163 to 167 mm in the absence of tibialis posterior contraction at late terminal stance. Arch-supporting abilities of the other extrinsic muscles were insignificant.

BIOMECHANICAL CONSEQUENCES OF PLANTAR FASCIAL RELEASE OR RUPTURE DURING GAIT. PART II : ALTERATIONS IN FOREFOOT LOADING

With a model using feet from cadavers, we tested the hypothesis that plantar fascial release or rupture alters the loading environment of the forefoot during the latter half of the stance phase of gait. The model simulated the position and loading environment of the foot at two instants: early in terminal stance immediately after heel-off and late in terminal stance just preceding contralateral heel strike. Eight feet were loaded at both positions by simulated plantar flexor contraction, and the distribution of plantar pressure was measured before and after progressive release of the plantar fascia. Strain in the diaphysis of the second metatarsal was also measured, from which the bending moments and axial force imposed on the metatarsal were calculated. Cutting the medial half of the central plantar fascial band significantly increased peak pressure under the metatarsal heads but had little effect on pressures in other regions of the forefoot or on second metatarsal strain and loading. Dividing the entire central band or completely releasing the plantar fascia from the calcaneus had a much greater effect and caused significant shifts in plantar pressure and force from the toes to beneath the metatarsal heads. These shifts were accompanied by significantly increased strain and bending in the second metatarsal. Complete fasciotomy increased the magnitude of strain in the dorsal aspect of the second metatarsal by more than 80%, suggesting that plantar fascial release or rupture accelerates the accumulation of fatigue damage in these bones. Altered forefoot loading may be a potential complication of plantar fasciotomy.

Grizzly bears (Ursus arctos horribilis) and black bears (Ursus americanus) prevent trabecular bone loss during disuse (hibernation)

Disuse typically causes an imbalance in bone formation and bone resorption, leading to losses of cortical and trabecular bone. In contrast, bears maintain balanced intracortical remodeling and prevent cortical bone loss during disuse (hibernation). Trabecular bone, however, is more detrimentally affected than cortical bone in other animal models of disuse. Here we investigated the effects of hibernation on bone remodeling, architectural properties, and mineral density of grizzly bear (Ursus arctos horribilis) and black bear (Ursus americanus) trabecular bone in several skeletal locations. There were no differences in bone volume fraction or tissue mineral density between hibernating and active bears or between pre- and post-hibernation bears in the ilium, distal femur, or calcaneus. Though indices of cellular activity level (mineral apposition rate, osteoid thickness) decreased, trabecular bone resorption and formation indices remained balanced in hibernating grizzly bears. These data suggest that bears prevent bone loss during disuse by maintaining a balance between bone formation and bone resorption, which consequently preserves bone structure and strength. Further investigation of bone metabolism in hibernating bears may lead to the translation of mechanisms preventing disuse-induced bone loss in bears into novel treatments for osteoporosis.

Contributions of active and passive toe flexion to forefoot loading.

Toe flexion during terminal stance has an active component contributed by the muscles that flex the toes and a passive component contributed by the plantar fascia. This study examined the relative importance of these two mechanisms in maintaining proper force sharing between the toes and forefoot. Thirteen nonpaired cadaver feet were tested in a dynamic gait stimulator, which reproduces the kinematics and kinetics of the foot, ankle, and tibia by applying physiologic muscle forces and proximal tibial kinematics. The distribution of plantar pressure beneath the foot was measured at the terminal stance phase of gait under normal extrinsic muscle activity with an intact plantar fascia, in the absence of extrinsic toe flexor activity (no flexor hallucis longus or flexor digitorum longus) with an intact plantar fascia, and after complete fasciotomy with normal extrinsic toe flexor activity. In the absence of the toe flexor muscles or after plantar fasciotomy the contact area decreased beneath the toes and contact force shifted from the toes to the metatarsal heads. In addition, pressure distribution beneath the metatarsal heads after fasciotomy shifted laterally and posteriorly, indicating that the plantar fascia enables more efficient force transmission through the high gear axis during locomotion. The plantar fascia enables the toes to provide plantar-directed force and bear high loads during push-off.

Six months of disuse during hibernation does not increase intracortical porosity or decrease cortical bone geometry, strength, or mineralization in black bear (Ursus americanus) femurs.

Disuse typically uncouples bone formation from resorption, leading to bone loss which compromises bone mechanical properties and increases the risk of bone fracture. Previous studies suggest that bears can prevent bone loss during long periods of disuse (hibernation), but small sample sizes have limited the conclusions that can be drawn regarding the effects of hibernation on bone structure and strength in bears. Here we quantified the effects of hibernation on structural, mineral, and mechanical properties of black bear (Ursus americanus) cortical bone by studying femurs from large groups of male and female bears (with wide age ranges) killed during pre-hibernation (fall) and post-hibernation (spring) periods. Bone properties that are affected by body mass (e.g. bone geometrical properties) tended to be larger in male compared to female bears. There were no differences (p>0.226) in bone structure, mineral content, or mechanical properties between fall and spring bears. Bone geometrical properties differed by less than 5% and bone mechanical properties differed by less than 10% between fall and spring bears. Porosity (fall: 5.5+/-2.2%; spring: 4.8+/-1.6%) and ash fraction (fall: 0.694+/-0.011; spring: 0.696+/-0.010) also showed no change (p>0.304) between seasons. Statistical power was high (>72%) for these analyses. Furthermore, bone geometrical properties and ash fraction (a measure of mineral content) increased with age and porosity decreased with age. These results support the idea that bears possess a biological mechanism to prevent disuse and age-related osteoporoses.