Biomedical subjects

W C Hayes

Publications and source records attributed to W C Hayes.

At least 91 records · Page 5Linked to original sources

PubMed1993

Geometric variables from DXA of the radius predict forearm fracture load in vitro.

The purpose of this investigation was to determine the cross-sectional geometry of the radius in female and male cadaveric specimens using dual-energy X-ray absorptiometry (DXA), to measure the accuracy of this technique compared with a digitizing procedure, and to measure the correlation between these DXA-based geometric variables and the load required to produce a forearm fracture. Paired intact forearms were scanned at a distal site and at a site approximately 30% of the forearm length from the distal end. The cross-sectional area and the moments of inertia of two sections at 10 and 30% of the forearm length were computed from the X-ray attenuation data. One member of each pair was then sectioned at the 30% location, which is mostly cortical bone, and the section was traced on a digitizing pad. The other forearm was loaded to failure in a servohydraulic materials test system. The DXA-based area and moment of inertia at 30% correlated significantly with the digitized results (r2 = 0.93 for area; r2 = 0.95 for moment; P < 0.001). The conventional bone mineral density from DXA did not associate significantly with failure load, but the minimum moment of inertia and the cross-sectional area at 10% correlated in a strong and significant manner with the forearm fracture force (r2 = 0.67 for area; r2 = 0.66 for moment; P < 0.001). The determination of radial bone cross-sectional geometry, therefore, should have better discriminatory capabilities than bone mineral density in studies of bone fragility and fracture risk.

Absorptiometry, Photon↗

PubMed1993

Read my hips: measuring trochanteric soft tissue thickness.

There is considerable uncertainty about which factors best predict hip fractures in the elderly. We have previously shown that a global measure of body habitus, the body mass index (BMI), is an important predictor of hip fracture in this age group. BMI may serve as a surrogate measure of trochanteric soft tissue thickness. To determine a more local assessment of trochanteric soft tissue, we compared trochanteric soft tissue thickness by ultrasound to other local measures of trochanteric soft tissue thickness (by dual-energy X-ray absorptiometry [DXA] and hip circumference) and global measures of body habitus [body mass index (BMI), bioelectrical impedance analysis (BIA), hip/waist circumference] in 50 healthy, community-dwelling women [72 +/- 4 years old (mean +/- SD)]. Ultrasound was highly correlated with DXA trochanteric soft tissue (r2 = 0.815, P < 0.0001), hip circumference (r2 = 0.810, P < 0.0001), BMI (r2 = 0.721, P < 0.0001), and BIA (r2 = 0.743, P < 0.0001). Trochanteric soft tissue thickness measured by DXA was also significantly (P < 0.0001) correlated with BMI and BIA. We conclude that local assessments of trochanteric soft tissue thickness, including DXA and hip circumference, provide an easy and noninvasive technique to assess trochanteric soft tissue thickness.

Absorptiometry, Photon↗

PubMed1993

Continuous alendronate treatment throughout growth, maturation, and aging in the rat results in increases in bone mass and mechanical properties.

Alendronate (4-amino-1-hydroxybutylidene bisphosphonate) is a novel amino bisphosphonate that is being developed for the treatment of osteolytic bone disorders such as osteoporosis. As part of a 2-year carcinogenicity study, we investigated the morphologic and biomechanical effects of long-term alendronate (ALN) therapy, given throughout skeletal growth, maturation, and aging, on rat vertebrae and femora. Three treatment groups, receiving either deionized water, low- (1.00 mg/kg), or high-dose (3.75 mg/kg) ALN, were given daily oral treatment for 105 weeks. Results from mechanical tests indicate that ALN therapy (in males) increased the vertebral ultimate compressive load by 96% in the high- and 51% in the low-dose groups when compared with controls. ALN similarly increased the male ultimate femoral bending load by 59% in the high- and 31% in the low-dose groups. Vertebrae and femora from female rats treated with both high- and low-dose ALN also failed at significantly higher loads than controls, but no differences were seen between low- and high-dose groups. Morphologic analysis of both male and female vertebrae revealed a dose-dependent increase in area fraction of bone. Rats receiving high-dose ALN had a greater area fraction of bone than those receiving low doses. Both groups were greater than controls. Thus, the administration of ALN resulted in increased femoral cortical bending load when compared with control animals, as well as increased vertebral ultimate compressive load commensurate with a dose-related preservation of vertebral bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗

PubMed1993

Role of trabecular morphology in the etiology of age-related vertebral fractures.

Osteoporotic compression fractures of the spine differ from most other age-related fractures in that they usually are associated with minimal trauma and with loads no greater than those encountered during normal activities of daily living. With aging and osteoporosis, there is progressive resorption of bone, resulting in reductions in bone density, thinning of trabeculae, and loss of trabecular contiguity. These changes in trabecular bone structure are associated with losses in bone strength which are disproportionate to the reductions in bone mass alone. To explain this disproportionate loss of bone strength, the prevailing opinion is that density reductions in the vertebral centrum are accompanied by a reduction in the number of trabeculae, by preferential resorption of horizontal trabeculae, and by hypertrophy of the remaining vertical trabeculae. To evaluate this view of vertebral morphology, we performed three-dimensional stereological analysis of trabecular bone extracted from midsagittal sections of first lumbar vertebral bodies from 12 donors spanning an age of 27-81 years. We found that both the number (R2 = 0.63, P < 0.01) and thickness (R2 = 0.91, P < 0.01) of trabeculae decreased linearly with density (as expressed by bone volume fraction) whereas the spacing between the trabeculae (R2 = 0.61, P < 0.01) increased reciprocally. There were more vertical trabeculae with transverse trabeculae at all densities, and the number of vertical trabeculae changed with density at twice the rate of the number of transverse trabeculae (P < 0.001). These data do not support the prevailing view that there is preferential resorption of horizontal trabeculae or hypertrophy of the remaining vertical trabeculae.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Trabecular bone modulus and strength can depend on specimen geometry.

We performed a series of uniaxial compression tests on wet bovine trabecular bone to compare both modulus and strength when measured using 2:1 aspect ratio (10 mm long, 5 mm diameter) cylinders (n = 30) and 5 mm cubes (n = 29). We also compared the correlation coefficients in the resulting modulus-density and strength-density regressions and the standard errors of the estimate. When comparing the mean values of modulus and strength for each group, the confounding variations in apparent density were accounted for with an analysis of covariance. The Fisher's Z transformation was used to compare the correlation coefficients statistically. Results from the analysis of covariance indicated that the modulus and strength of the cubes were higher by 36% (p < 0.01) and 18% (p < 0.05), respectively, with respect to the 2:1 cylinder values. The correlation coefficients in the modulus-density and strength-density regressions were not sensitive to the regression model (linear versus power law). However, correlation coefficients for both modulus-density and strength-density regressions were higher (p < 0.05) for the 2:1 cylinders (r = 0.90, modulus; r = 0.94, strength) than for the cubes (r = 0.57, modulus; r = 0.82, strength). In addition, the standard errors of the estimate in both modulus and strength were substantially lower for the 2:1 cylinders. These data indicate that both modulus and strength can depend on the specimen geometry when using conventional compression testing techniques. We conclude, therefore, that inter-study comparisons of modulus and strength may be invalid if these confounding effects of different specimen geometries are not addressed. Our data also indicate that density can better explain the observed variance in modulus and strength when 2:1 cylinders are used as opposed to cubes. Using this phenomenon as a rationale for choosing a standard specimen gometry, we recommend that the 2:1 cylinder be used as a standard specimen in studies designed to determine the effects of various treatments on the uniaxial compressive modulus and strength of trabecular bone.

Animals↗

PubMed1993

Evaluation of finite element analysis for prediction of the strength reduction due to metastatic lesions in the femoral neck.

Between 30 and 70% of almost one million new cancer patients diagnosed each year will develop osseous metastases. Clinicians are faced with the difficult task of determining which patients require prophylactic stabilization to prevent pathologic fracture. The objective of this study was to test the ability of macroscopic finite element models to predict the fracture strength of the proximal femur with a lesion in the femoral neck. Drill hole defects in human cadaver femora were used to simulate lesions that penetrate one cortex of the femoral neck. Based on the first of two series of in vitro experiments, the fracture strength of a femur with a lesion that penetrates either the inferior-medial or superior-lateral cortex of the neck is approximately 45% less than the fracture strength of the paired intact femur; based on the second series, the fracture strength with the inferior-medial lesion is approximately 20% less than the fracture strength with the superior-lateral lesion. A series of three-dimensional finite element models were used to predict the fracture strength for anterior and posterior lesions, as well as the inferior-medial and posterior-lateral lesions tested in vitro. Based on a direct comparison of the strengths predicted by the finite element models to the measured in vitro fracture strengths, the finite element models performed poorly. In particular, the application of an anisotropic strength criterion to the stresses predicted by the models resulted in a considerable underestimation of the percentage reduction in the in vitro fracture strength. This may reflect a fundamental inability of a linear, macroscopic continuum-based analysis to predict accurately the fracture strength of a bone structure as complex as the proximal femur. However, despite this lack of agreement in absolute fracture strength, the general trends for gait and stair ascent loading for the inferior-medial and superior-lateral lesions were consistent with the in vitro data. The greatest reduction in strength was predicted for the inferior-medial lesion, followed by the anterior lesion and then the superior-lateral lesion, and the least reduction in strength was predicted for the posterior lesion. Most importantly, the predicted strength ratio varied considerably as a function of the applied loads. Any metastatic lesions of the femoral neck may be especially sensitive to some particular activity, making it difficult to determine precisely the risk of fracture.

Aged↗

PubMed1993

Biomechanics of pin and screw fixation of femoral neck fractures.

Using an in vitro cadaveric femoral neck osteotomy model and a paired experimental design, we compared fixation stiffness and yield load for three- and four-pin configurations for both Knowles pins and Asnis screws and for three Asnis screws versus an AO-ASIF sliding hip screw. Repeat measure analyses of variance indicated a significant increase in stiffness (p < 0.05) with the use of three versus four pins; the yield load did not depend significantly on the number of pins. Fixation was significantly stiffer (p < 0.01) and stronger (p = 0.02) for Knowles pins if the osteotomy was oblique rather than transverse to the femoral neck; the opposite was found for Asnis screws. Femoral fixed with hip screws were significantly stiffer (p < 0.01) than the contralateral side fixed with three Asnis screws. No significant difference in yield load was observed, however. Clinically, it appears from these findings that the use of three Asnis screws or three Knowles pins provides modest increases (averaging 4%) in fixation stiffness with no degradation in strength when compared with four screws or pins. Parallel placement of Asnis screws also provides advantages over nonparallel Knowles pins by allowing controlled impaction of the femoral head. While three Asnis screws provide fixation stiffness approximately 40% less than that found with a sliding hip screw, there is no compromise in fixation strength.

Aged↗

PubMed1993

A 20-year perspective on the mechanical properties of trabecular bone.

We have reviewed highlights of the research in trabecular bone biomechanics performed over the past 20 years. Results from numerous studies have shown that trabecular bone is an extremely heterogeneous material--modulus can vary 100-fold even within the same metaphysis--with varying degrees of anisotropy. Strictly speaking, descriptions of the mechanical properties of trabecular bone should therefore be accompanied by specification of factors such as anatomic site, loading direction, and age. Research efforts have also been focused on the measurement of mechanical properties for individual trabeculae, improvement of methods for mechanical testing at the continuum level, quantification of the three-dimensional architecture of trabecular bone, and formulation of equations to relate the microstructural and continuum-level mechanical properties. As analysis techniques become more sophisticated, there is now evidence that factors such as anisotropy and heterogeneity of individual trabeculae might also have a significant effect on the continuum-level properties, suggesting new directions for future research. Other areas requiring further research are the time-dependent and multiaxial failure properties at the continuum level, and the stiffness and failure properties at the lamellar level. Continued research in these areas should enhance our understanding of issues such as age-related bone fracture, prosthesis loosening, and bone remodeling.

Aged↗

PubMed1993

Effect of screw diameter, insertion technique, and bone cement augmentation of pedicular screw fixation strength.

This study investigated (1) the effect of screw diameter and insertion technique in lumbar vertebrae, and insertion site in the sacrum, on the axial pullout force and transverse bending stiffness of pedicle screws, and (2) the effect of bone cement augmentation using polymethylmethacrylate (PMMA) and the biodegradable composite, poly(propylene glycol-fumarate) on axial pullout force and transverse bending stiffness of pedicle screws inserted into lumbar vertebrae. The axial pullout force and transverse bending stiffness of a 6.25-mm Steffee screw and a 6-mm Kluger screw did not differ significantly in vertebral bodies of similar equivalent bone mineral density. The axial pullout force of Schanz screws was significantly increased with a 1-mm increase in screw diameter. However, there was no significant increase in transverse bending stiffness. In the sacrum, an approach through the S1 facet produced significantly higher axial pullout forces and transverse bending stiffness than the approach described by Harrington and Dickson. PMMA and a biodegradable composite bone cement poly(propylene glycol-fumarate) both increased the axial pullout force. PMMA also increased the transverse bending stiffness.

Biomechanical Phenomena↗

PubMed1993

Strength reductions from trabecular destruction within thoracic vertebrae.

An in vitro model of metastatic lesions in thoracic vertebrae was used to determine if the reduction in vertebral cross-sectional area could be used to predict the associated strength reduction. Defects, entirely within the trabecular bone, were created in alternating vertebrae of unembalmed human thoracic spines. The adjacent vertebrae were tested intact and served as controls. Defect size was determined as the cross-sectional area of the defect divided by the nominal cross-sectional area of the vertebral body midplane. Vertebrae were tested to failure using combined axial-flexion loads. For each spine, a linear regression was determined between the cross sectional area of the superior endplate and the load at failure for the intact vertebrae. The intact strength of bodies with defects was estimated from this regression. The normalized strength of thoracic vertebrae with trabecular defects was linearly related to the reduction in cross-sectional area (normalized failure load = 1.0-Ad/Ai, r2 = 0.51; Ad = cross-sectional area of defect; and Ai = intact cross-sectional area at midplane). The data suggest that the strength reduction due to lytic defects within the centrum of thoracic vertebrae is proportional to the cross-sectional area of bone resorbed.

Aged↗

PubMed1993

Cervical injuries under flexion and compression loading.

Cervical spine segmental tests were performed to determine the specific patterns of initial cervical injury in response to loading just beyond the point of structural failure. Well-defined combinations of flexion rotation and compression translation were applied to segments with varying degrees of disc and facet degeneration. Twelve cervical spine specimens (from human cadavers ages 52-91 years), each consisting of three vertebrae (two motion segments) from the middle (C2-C5) or lower (C5-T1) regions, were subjected to pure flexion rotation (seven specimens) or to combined flexion rotation and axial compression translation (five specimens). Specimens were sectioned and dissected to determine the patterns of structural failure. Pure flexion, and combined flexion and compression produced similar patterns of injuries. The disc was the most commonly injured structure, with annular injuries noted in 8 of the 12 specimens, and with anterior herniation of the nucleus occurring in two specimens. Wedge fractures and posterior ligament injuries were noted in both specimen groups and with both modes of loading. We conclude that similar patterns of initial anterior bony compressive failure and posterior ligamentous failure can occur with both flexion and with combined flexion-compression, without axial or lateral rotation, at low rates of loading. Anterior cervical disc herniations were produced in both middle and lower cervical segments.

Aged↗

PubMed1992

Mechanical properties of trabecular bone within and adjacent to osseous metastases.

Despite radiographic and histologic evidence of trabecular bone density changes within and adjacent to osseous metastases, there currently exist no data to demonstrate whether these changes are important in predicting the risk of fracture. To determine if these density changes result in significant reductions in mechanical properties, trabecular bone specimens were prepared from lower thoracic and lumbar vertebrae from two cadavers with radiographic, gross, and histologic evidence of lytic and/or blastic osseous metastases. Each specimen was classified as normal, lytic, or blastic based on appearance in fine-grain radiographs of 8-9 mm thick coronal plane sections. Specimens were tested to failure in uniaxial compression, and tissue and apparent densities were measured. Mean tissue densities were within normal ranges. The mean apparent density for all specimens combined was within the normal range for human vertebrae, and the mean apparent density for radiographically normal (0.131 g/ml) and lytic (0.111 g/ml) specimens was less than the mean apparent density of blastic (0.182 g/ml) specimens (p < 0.02). The moduli of lytic and blastic specimens were less than for normal specimens (p < 0.025). The strength of lytic specimens was less than normal (p = 0.057), but the strength of blastic specimens was not (p > 0.1). Apparent density explained significant fractions of the variations in both modulus (p < 0.001) and strength (p < 0.001). The data suggest that blastic changes associated with osseous metastases to trabecular bone disrupt the normal dependence of trabecular mechanical properties on apparent density, but lytic changes do not.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗

PubMed1992

Effects of 4-amino-1-hydroxybutylidene bisphosphonate on bone biomechanics in rats.

Bisphosphonates inhibit osteoclast-mediated bone resorption, but their effects on the mechanical behavior of bone remain uncertain. This study investigated the effects of 4-amino-1-hydroxybutylidene bisphosphonate (AHBuBP) on the biomechanical and morphologic properties of bone in ovariectomized rats. Sprague-Dawley rats (four groups, n = 6) were ovariectomized at 3 months of age. From 7 to 13 months, the groups received vehicle or 0.28, 2.8, or 28 micrograms/kg of AHBuBP twice weekly through subcutaneous injection. An additional group of control animals (n = 6) received neither surgery nor drug. We determined the stiffness, yield, and ultimate loads of the femoral midshaft, the sixth lumbar (L6) vertebra, and the femoral neck. Geometric properties of the cortical bone were measured from digitized images of the tibial diaphysis at the level of the synostosis. The area fraction of trabecular bone was determined through the midsagittal plane of the fifth lumbar (L5) vertebra. There were no significant differences in the structural properties of the femoral neck and midshaft, with the exception that the medium-dose group had a greater ultimate load than the vehicle group for the femoral midshaft in bending. Cross-sectional analysis of the tibia did not show significant differences in the inertial properties or area. Ovariectomy caused a significant reduction in the stiffness and ultimate load of L6 and in the area fraction of trabecular bone of L5.(ABSTRACT TRUNCATED AT 250 WORDS)

Alendronate↗

PubMed1992

Role of loads and prosthesis material properties on the mechanics of the proximal femur after total hip arthroplasty.

The objective of this study was to establish the role of loads and prosthesis material properties on the mechanics of the proximal femur after total hip arthroplasty. We developed a three-dimensional finite element model of an intact human femur and the same femur with a conventional collared straight-stem femoral component. Using published data, we defined two sets of loading conditions: one that represented three phases of gait, and one that represented four different extreme loads. The four extreme loads were based on the peak joint contact forces that occur during stair ascent and isometric contraction of various muscle groups. The model was analyzed with three different material properties for the prosthesis, including cobalt-chromium alloy, titanium alloy, and a carbon fiber-reinforced polymer (CFRP) laminate. We assumed that the implant was stable, with rigid bonding, collar contact, and no cement. To address femoral component loosening, we examined the shear stresses at the implant-bone interface; to address adaptive bone remodeling, we examined the principal stresses in the supporting cortical bone relative to those in the intact femur. Our analyses of the various loading conditions demonstrated large out-of-plane bending movements and torsional moments, especially for the load representing stair ascent. Based on stepwise multiple regressions, the maximum shear stresses at the implant-bone interface in the distal region were dependent on the total applied axial force and torsion; the maximum shear stresses in the proximal region were dependent on the axial component of the joint contact force alone. Reduction in the prosthesis stiffness, by substitution of the CFRP material properties, resulted in lower interface shear stresses at the distal end of the stem and higher interface shear stresses at the more proximal sections, consistent with the findings of others. We fit equations, based on composite beam theory, to the maximum implant-bone interface shear stresses and the cortical bone principal stresses as a function of the axial modulus of the prosthesis. These equations can be used to estimate the maximum stresses at the interface and in the cortical bone that would be predicted by similar models, for the same prosthesis constructed of alternative materials, relative to the stresses in the intact femur. The nonlinear nature of these relationships was such that the cortical bone stresses changed more rapidly, as a function of the prosthesis modulus, for lower values of elastic modulus, especially in the more proximal sections.

Biomechanical Phenomena↗

PubMed1992

In vitro hyperextension injuries in the human cadaveric cervical spine.

To investigate the relationship between the type of hyperextension injuries and the conditions producing them, nine cervical specimens (occiput to T1) were loaded to failure in tension at a fixed extension angle of 30 degrees. Under these loading conditions, specimens failed at average tensile loads and extension moments of 499 +/- 148 (SD) N and 4.0 +/- 3.1 Nm, respectively. Failure occurred at an average tensile displacement of 18.8 +/- 7.7 mm. The anterior longitudinal ligament ruptured and the intervertebral disc failed in at least one level in all specimens. In four specimens, the disc failed at an additional level, leaving the anterior longitudinal ligament intact at that site. With one exception, all injuries occurred in the lower cervical spine (C5-C6 and C6-C7), the region most often injured in vivo. The location of the injuries was associated with the degree of degeneration of the facet joints and the discs. The discs of the lower cervical spine were significantly more degenerated than those at the C2-C3 level. In addition, the degree of disc degeneration in the noninjured discs was significantly less than in the injured discs. These data help quantify the threshold of injury and the patterns of tissue damage resulting from hypertension loading of the cervical spine.

Aged↗

PubMed1992

Finite element stress analysis of simulated metastatic lesions in the lumbar vertebral body.

A three-dimensional finite element model of a lumbar vertebral body was developed to study the effects of geometry, material properties and loading conditions on stresses in the presence of metastatic lesions. Parameters studied included location and size of the lesion, modulus of the cortical and trabecular bone within and near the lesion, generalized osteoporosis and load distribution. The results, expressed as ratios of peak values of displacement and stress, relative to a normal baseline case, indicated that the location of a defect which did not penetrate the cortex had a minor influence on the peak displacement and stresses, as did the presence of lesions occupying less than 40% of the volume of the vertebral centrum. A lesion occupying 40% of the centrum volume increased the endplate displacement by 2.9 times, the peak tensile stress in the cortical shell by 2.2 times, and the peak von Mises stress in the endplate by 2.8 times. When this lesion penetrated the cortex, these values increased to 3.8, 3.3 and 4.4 times, respectively. The most severe case involved a defect penetrating the anterior cortex, osteoporotic bone properties and anteriorly eccentric loading. In this case, the peak values increased to 8.4, 3.4 and 5.9 times their baseline values, respectively. The results are consistent with a model of the vertebral body as a stiff frame of cortical bone surrounding a relatively compliant core of trabecular bone. Only variations in geometry and properties large enough to lessen significantly the structural stiffness affect the peak stresses and displacements.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗

PubMed1992

A biomechanical study of the fatigue characteristics of thoracolumbar fixation implants in a calf spine model.

Clinical failures of internal fixation implants for the treatment of the thoracolumbar spine are generally attributed to fatigue. Few studies, however, have characterized changes in fixation rigidity with time or subjected spine-implant fixation constructs to fatigue loading until failure. Fatigue characteristics of five dorsally applied spinal fixation implants were determined using lumbosacral calf spines, with an L3 vertebrectomy, loaded cyclically in combined compression (maximum 605 N) and flexion (maximum 16 Nm) for up to 100,000 cycles. Displacement transducers monitored motion at the site of instability and at the segment above the implants. Flexibility and strain at these segments were then calculated. A one-way analysis of variance showed that there were no significant differences in flexibility of the five fixation constructs (P greater than .05). A multiple Bonferroni test revealed that the AO and Kluger fixateur interne and Steffee plates, with fixation at L2 and L4, allowed significantly more strain (P less than .01) across the site of instability than did Harrington rods and Luque plates with fixation at L1, L2, L4, and L5. There were no significant differences between fixation constructs in initial strain above the implants. After 10,000 cycles, however, there were significant increases in strain across the segment above the Luque and Harrington implants (P less than .05). Failure of the AO Schanz screw occurred in three of six constructs at a mean of 73,300 cycles. The Steffee screws failed in four of five constructs at a mean of 20,800 cycles. The rods of the Kluger fixateur interne broke in four of five constructs at a mean of 47,800 cycles, and one screw slipped at 11,000 cycles. There were no metal failures in the Harrington or Luque implants.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Holding power and reinforcement of cancellous screws in human bone.

The authors report an in vitro biomechanical evaluation of a biodegradable material that might be used for the reinforcement of surgical screws in fractures involving severely osteoporotic bone. The material is a particulate composite with a matrix phase consisting of a hydrolyzable prepolymer, polypropylene fumarate (PPF), crosslinked with methacrylate monomer, and a particulate phase consisting of tricalcium phosphate and calcium carbonate. Pullout force and stripping load of cancellous screws were determined along with screw pullout force before and after reinforcement with either polymethylmethacrylate (PMMA) or PPF composite. Pullout force was moderately correlated (R2 = 0.59) with apparent density by a power law relationship of the form 0.065p1.37-1.77. Stripping load was strongly correlated (R2 = 0.91) with apparent density by a power law of the form 0.13p1.35-93.8. Mean pullout force before and after reinforcement with PMMA was 382 +/- 100 N (mean +/- standard deviation) and 879 +/- 315 N, respectively. Mean pullout force before and after reinforcement with PPF composite was 571 +/- 294 N and 829 +/- 354 N, respectively. Although the increase in pullout force with cement reinforcement was highly significant in both cases, the magnitude of the increase did not depend on the type of cement. Thus PPF seems to provide reinforcement that is equivalent to that provided by PMMA.

Aged↗