Bone compressive strength: the influence of density and strain rate.

The compressive strength of bone is proportional to the square of the apparent density and to the strain rate raised to the 0.06 power. This relationship is applicable to trabecular and compact bone, and provides clinical guidelines for predicting bone strength on the basis of x-ray and densitometric examination.

Subperiosteal expansion and cortical remodeling of the human femur and tibia with aging.

Increases with aging in subperiosteal dimensions and second moments of area (measures of bending and torsional rigidity) in femoral and tibial cross sections are documented in an archeological sample from the American Southwest. Significant differences between cross-sectional sites and between sexes in the pattern of cortical remodeling with age are also present. These differences appear to be related to variations in the stress or strain levels in different regions of the femur and tibia which result from in vivo mechanical loadings of the lower limb.

Comparison of alendronate and sodium fluoride effects on cancellous and cortical bone in minipigs. A one-year study.

Fluoride stimulates trabecular bone formation, whereas bisphosphonates reduce bone resorption and turnover. Fracture prevention has not been convincingly demonstrated for either treatment so far. We compared the effects of 1-yr treatment of 9-mo-old minipigs with sodium fluoride (NaF, 2 mg/kg/d p.o.) or alendronate (ALN, 4 amino-1-hydroxybutylidene bisphosphonate monosodium, 1 mg/kg/d p.o.) on the biomechanical and histomorphometric properties of pig bones. As expected, NaF increased and ALN decreased bone turnover, but in these normal animals neither changed mean bone volume. NaF reduced the strength of cancellous bone from the L4 vertebra, relative to control animals, and the stiffness (resistance to deformation) of the femora, relative to the ALN group. In the ALN-treated animals, there was a strong positive correlation between bone strength and L5 cancellous bone volume, but no such correlation was observed in the NaF group. Furthermore, the modulus (resistance to deformation of the tissue) was inversely related to NaF content and there was a relative decrease in bone strength above 0.25 mg NaF/g bone. Moreover, within the range of changes measured in this study, there was an inverse correlation between bone turnover, estimated as the percentage of osteoid surface, and modulus. These findings have relevant implications regarding the use of these agents for osteoporosis therapy.

Biomechanical properties of the proximal femur determined in vitro by single-energy quantitative computed tomography.

To assess the use of quantitative computed tomography as an in vivo predictor of fracture in the osteoporotic hip, we examined the in vitro relationship between single-energy quantitative computed tomography data, calibrated for scanner drift, and the mechanical properties of trabecular bone from the proximal femur. For 49 samples, the apparent density and ultimate strength were measured and their functional relationship to the computed tomography data determined. Apparent density demonstrated a moderate linear correlation to the computed tomography numbers (R2 = 0.60), and the ultimate strength was related through a power law (R2 = 0.83). In addition, for 8 intact femora, average computed tomographic data from the sub-capital region were moderately correlated to the ultimate fracture load applied under controlled in vitro conditions (R2 = 0.64). The average fracture energy for these femora was 43 J, a value more than an order of magnitude less than the energy available in a fall from standing height, suggesting that fall mechanics are a more important determinant of fracture risk than has been previously thought. The relationship between the energy absorbed to failure and the computed tomography data was best described by a power law (R2 = 0.90). Based on these results, it appears that quantitative computed tomography provides a potentially useful approach for the direct estimate of that component of fracture risk that can be attributed to a reduction in bone strength.

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)

Maxillary molar extraction causes increased bone loss in the mandible of ovariectomized rats.

Although osteoporosis is a major public health concern, its effect on oral bone has not been determined. More important may be the effect of estrogen depletion on the response of oral bone to dental treatments such as tooth extraction or pathologic processes such as periodontal disease. Our objective was to determine if maxillary molar extraction increases mandibular bone loss in the ovariectomized compared with a sham-operated control. Fifty-three ovariectomized and 53 sham-operated 6-month-old Sprague-Dawley rats were randomly assigned to the following groups: (1) ovariectomized, adult; (2) sham-operated, adult; (3) ovariectomized, adult, extraction; (4) sham-operated, adult, extraction; (5) ovariectomized, old; (6) sham-operated, old; (7) ovariectomized, old, extraction; and (8) sham-operated, old, extraction. Fourteen days following ovariectomy, the extraction groups had their bilateral maxillary molars extracted. The adult and old rats were sacrificed 114 and 200 days postovariectomy, respectively. The right mandible was tested to failure in three point bending. The bone mineral density (BMD) of the left mandible was measured with high resolution dual energy X-ray absorptiometry. The area fraction and area moment of inertia of mandible sections were determined using image processing software. In the ovariectomized rats, maxillary molar extraction resulted in decreases (p < 0.05) in the failure load (21%), stiffness (39%), BMD (3%), and bone area fraction (8%) of the mandible. However, in the sham-operated rats, these decreases following maxillary molar extraction were less (p < 0.05) than those in the ovariectomized rats and only present in the mandibles of the aged rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of impact direction on the structural capacity of the proximal femur during falls.

As with any structure, the structural capacity of the proximal femur depends on the applied loads and these can vary as a function of impact direction during a fall. However, despite its potential importance in hip fracture risk assessment, the relative importance of impact direction is unknown. To investigate the role of impact direction in hip fracture, we developed a detailed finite element model of the proximal femur. We analyzed four loading configurations that represent a range of possible falls on the greater trochanter. Our results indicate that a change in the angle between the line of action of the applied force and the axis of the femoral neck from 0 degrees (representing a direct lateral impact) to 45 degrees (representing a posterolateral impact) reduced structural capacity by 26%. This weakening of the femur with changes in impact direction is comparable to the weakening associated with 2-3 decades of age-related bone loss. Our result elucidates the independent contribution of fall mechanics to hip fracture risk by identifying an aspect of the fall (the direction of impact) that is an important determinant of fall severity. The results can also be incorporated into a refined clinical method for assessment of hip fracture risk that accounts for the complex interactions between fall severity and bone fragility.

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)

Trochanteric bone mineral density is associated with type of hip fracture in the elderly.

Hip fractures can be separated into femoral neck (cervical or intracapsular) or trochanteric (extracapsular). Trochanteric fractures have been associated with up to twice the short-term mortality of cervical fractures in the elderly. Fracture type may be influenced by the fall direction and local differences in proximal femur strength properties. We previously demonstrated that fall characteristics and body habitus, in addition to femoral bone mineral density, play a dominant role in the prediction of hip fracture in elderly fallers. To examine the association of these determinants with hip fracture type, we assessed fall characteristics, body habitus, and site-specific bone mineral density measurements in 112 elderly hip fracture patients (85 women and 27 men, mean age 85 years) 1 week after an acute hip fracture. Trochanteric BMD was 13% lower in women and 11% lower in men for patients with trochanteric fracture than in those with femoral neck fracture (p < 0.01). A stepwise multiple logistic regression indicated that trochanteric BMD (decrease of 1.0 SD, adjusted OR 4.6, 95% Cl 2.0-9.5, p < 0.0001) and femoral neck BMD (increase of 1.0 SD, adjusted OR 3.0, 95% Cl 1.6-5.9, p = 0.0003) were independently associated with trochanteric fracture. Fall characteristics, body habitus, gender, and age were not associated with hip fracture type. We conclude that a relatively low trochanteric BMD or a high femoral neck BMD was associated with a trochanteric hip fracture and that site-specific trochanteric BMD determinations should be measured when assessing risk of trochanteric hip fractures in the elderly.

Effects of combined prostaglandin and alendronate treatment on the histomorphometry and biomechanical properties of bone in ovariectomized rats.

Prostaglandin E2 (PGE2) has been shown to stimulate both bone resorption and formation in experimental animals, leading to augmentation of trabecular and cortical bone. The amino bisphosphonate alendronate (ALN) is a potent inhibitor of bone resorption. The objectives of this study were to examine if PGE2 stimulation of bone formation was dependent on bone resorption and if the bone accrued as a result of PGE2 treatment contributed to bone strength. The 48 female Sprague-Dawley rats were assigned to six groups as follows: five groups (8/group) were ovariectomized at the age of 6 months. One group was sacrificed 2 months later to establish baseline conditions, and four groups were treated for 25 days with (1) vehicle, (2) PGE2 at 3 mg/kg/day, (3) ALN sc at 0.8 micrograms/kg/day, and (4) PGE2 + ALN at the respective doses. The sixth group served as nonovariectomized untreated controls. Histomorphometric analysis of 6-10 microns thick tibial sections after in vivo fluorochrome double labeling showed that treatment with PGE2 alone increased endocortical mineral apposition rate and bone formation rate, stimulated production of bone trabeculae in the marrow cavity, and increased cortical porosity. Combined ALN + PGE2 treatment prevented the resorption induced by PGE2 but not the stimulation of bone formation on endocortical and periosteal surfaces and resulted in a significant increase in cortical thickness. Consistent with these observations, the femoral midshaft tested to failure in three-point bending showed a significant increase in strength in the PGE2 + ALN group (181 +/- 15 N) compared to time 0 controls (145 +/- 23 N) or to the ovariectomized vehicle-treated group (141 +/- 28 N).(ABSTRACT TRUNCATED AT 250 WORDS)

Direct and computed tomography thickness measurements of the human, lumbar vertebral shell and endplate.

Although the trabecular bone of the human vertebral body has been well characterized, the thin "cortical" shell and endplate that surround the trabecular centrum have not. In addition, the accuracy of estimating the thickness of the shell and endplate using computed tomography (CT) has not been evaluated directly. To address these issues, we measured the thickness of the vertebral shell and endplate in the mid sagittal plane of 16 human L1 vertebral bodies using direct and CT based methods. Specimens were assigned to four equal sized groups based on age (middle-aged, mean age = 49 years; old, mean age = 84) and gender. We investigated the dependence of the shell and endplate thicknesses on age, gender, and anatomic region. Our findings indicate that the shell and endplate in vertebrae over age 45 are porous and often irregular, with an average thickness of approximately 0.35 mm. However, when measured from CT images, the vertebral shell and endplate appear significantly thicker, indicating that measurements based on clinical CT scans overestimate the thickness by a factor of at least two. In addition, our data indicated that, in the midsagittal plane, the anterior shell is thicker than the posterior shell or either endplate. Although these data indicated that thickness did not depend on age or gender, these particular findings are inconclusive given the small and heterogeneous sample we examined. We conclude that the so-called cortical shell and endplate of the vertebral body are thin (less than one-half of a millimeter) and porous, and perhaps are better thought of as thin membranes of fused trabeculae than as true cortices.

Etiology and prevention of age-related hip fractures.

Falls and fall-related injuries are among the most serious and common medical problems experienced by the elderly. Hip fracture, one of the most severe consequences of falling in the elderly, occurs in only about 1% of falls. Despite this, hip fracture accounts for a large share of the disability, death, and medical costs associated with falls. As measured by their frequency, influence on quality of life, and economic cost, hip fractures are a public health problem of crisis proportions. Without successful international initiatives aimed at reducing the incidence of falls and hip fractures, the implications for allocations of health resources in this and the next century are staggering. Identifying those at risk for harmful falls requires an understanding of what kinds of falls result in injury and fracture. In elderly persons who fall, in most of whom hip bone mineral density is already several standard deviations below peak values, fall severity (as reflected in falling to the side and impacting the hip) and body habitus are important risk factors for hip fracture and touch on a domain of risk entirely missed by knowledge of bone mineral density. These findings clearly suggest that factors related to both loading and bone fragility play important roles in the etiology of hip fracture. We provide a strategy, based on engineering approaches to fracture risk prediction, for determining the relative etiologic importance of loading and bone fragility and to summarize some of what is known about both sets of factors. We define a factor of risk, phi, as the ratio of the loads applied to the hip divided by the loads necessary to cause fracture and summarize available data on the numerator and the denominator of phi. We then provide an overview of the complex interplay between the risks associated with the initiation, descent, and impact phases of a fall, thereby suggesting an organized approach for evaluating intervention efforts being used to prevent hip fractures. The findings emphasize the continuing need for combined intervention strategies that focus on fall prevention, reductions in fall severity, and maintaining or increasing femoral bone mass and strength, either through targeted exercise programs, optimal nutrition (Ca, Vitamin D), and/or in the use of osteodynamic agents. By developing and refining the factor of risk, a property that captures both the contributions of bone density and the confounding influences of body habitus and fall severity, we believe these intervention strategies can be targeted more appropriately.

Fall direction, bone mineral density, and function: risk factors for hip fracture in frail nursing home elderly.

PURPOSE: To determine the importance of fall characteristics, body habitus, function, and hip bone mineral density as independent risk factors for hip fracture in frail nursing home residents. SUBJECTS AND METHODS: In this prospective, case-control study of a single, long-term care facility, we enrolled 132 ambulatory residents (95 women and 37 men) aged 65 and older, including 32 cases (fallers with hip fracture) and 100 controls (fallers with no hip fracture). Principal risk factors included fall characteristics, body habitus, measures of functional assessment, and hip bone mineral density by dual-energy X-ray absorptiometry. RESULTS: In multivariate analysis, including only those with knowledge of the fall direction (n=100), those who fell and suffered a hip fracture were more likely to have fallen sideways (odds ratio 5.7, 95% confidence interval [CI] 1.7 to 18, P= 0.004) and have a low hip bone mineral density (odds ratio 1.9, 95% CI 0.97 to 3.7, P=0.06) than those who fell and did not fracture. When all participants were included (n=132) and subjects who did not know fall direction were coded as not having fallen to the side, a fall to the side (odds ratio 3.9, 95% CI 1.3 to 11, P=0.01), low hip bone density (odds ratio 1.8, 95% CI 1.03 to 3, P=0.04), and impaired mobility (odds ratios 6.4, 95% CI 1.9 to 21, P=0.002) were independently associated with hip fracture. Sixty-seven percent of subjects (87% with and 62% without hip fracture) had a total hip bone mineral density greater than 2.5 SD below adult peak bone mass and were therefore classified as having osteoporosis using World Health Organization criteria. CONCLUSIONS: Among frail elderly nursing home fallers, the preponderance of whom are osteoporotic, a fall to the side, a low hip bone density, and impairment in mobility are all important and independent risk factors for hip fracture. These data suggest that, among the frailest elderly, measures to reduce the severity of a sideways fall and improve mobility touch on new domains of risk, independent of bone mineral density, that need to be targeted for hip fracture prevention in this high-risk group.

Local demineralization as a model for bone strength reductions in lytic transcortical metastatic lesions.

The structural consequences of bone density changes associated with lytic metastatic lesions were investigated using an experimental model of regular, lytic metastatic lesions in bone. Circular holes were drilled in the mid-diaphyseal cortex of paired adult canine femora. The region around the defect was demineralized in one bone of each pair with 0.8 N HCl. Specimens were tested to failure in four-point bending. Defect size was determined from conventional planar radiographs as the maximum apparent defect diameter divided by the periosteal diameter. Demineralization resulted in irregular defect geometries, which increased the maximum defect dimension 33% to 57% with respect to the original drill hole diameter. Demineralization resulted in additional strength reductions beyond those expected from the original drill hole alone. Despite the irregular demineralization patterns observed, strength reductions were in close agreement with those predicted from data for regular, nondemineralized holes (r2 = 0.93). The results demonstrate that irregular diaphyseal defect borders may not require more complex fracture risk predictors than can be determined from analytic and experimental studies of regular defect geometries. Our results also demonstrate that errors of over 100% can occur when measuring diaphyseal defect size from radiographs that are not optimally aligned with respect to the defect.

Effects of selected thermal variables on the mechanical properties of trabecular bone.

Osteoarticular allografts are commonly used in the treatment of segmental bone loss due to a wide resection of tumour. While the use of such grafts has met with considerable clinical success, fractures are a recognized complication of allograft use. Although trabecular bone can play an important structural role in the function of segmental allografts, few data exist on the effects of common storage and sterilization procedures on the mechanical properties of trabecular bone. To this end, we investigated with these experiments the effects of freezing at -20 degrees C, freezing at -70 degrees C, eight freeze-thaw cycles at -20 degrees C, freeze-drying, boiling and autoclaving on the compressive modulus and strength of bovine trabecular bone. Of these treatments, boiling and autoclaving were the only treatments to alter the properties of bovine trabecular bone, resulting in 26 and 58% reductions in strength, respectively. Autoclaving also significantly reduced the compressive modulus by 59%. From these data, freezing at temperatures between -20 and -70 degrees C does not appear to compromise the structural integrity of trabecular bone.

In vitro degradation of a poly(propylene fumarate)-based composite material.

We investigated the in vitro degradation of a novel degradable polymeric composite material being developed to function as a temporary replacement for trabecular bone. This material is based on a mixture of poly(propylene fumarate) cross-linked by N-vinyl-pyrrolidone and includes sodium chloride and beta-tricalcium phosphate. Using an in vitro test in simulated body fluids, the compressive strengths and compressive moduli of two composite materials increased with degradation time and remained above the minimum values acceptable for trabecular bone substitutes. A compressive strength of 21.3 (+/- 0.4) MPa and a compressive modulus of 696 (+/- 53) MPa were measured after twelve weeks for a composite material with initial strength of 18.0 (+/- 4.6) MPa and initial modulus of 113 (+/- 40) MPa. This unexpected phenomenon may prove to be useful for orthopaedic applications.

Evolution of bone transplantation: molecular, cellular and tissue strategies to engineer human bone.

Bone defects occur in a wide variety of clinical situations, and their reconstruction to provide mechanical integrity to the skeleton is a necessary step in the patient's rehabilitation. The current gold standard for bone reconstruction, the autogenous bone graft, works well in many circumstances. However, autograft reconstruction, along with the available alternatives of allogenous bone graft or poly(methylmethacrylate) bone cement, do not solve all instances of bone deficiency. Novel materials, cellular transplantation and bioactive molecule delivery are being explored alone and in various combinations to address the problem of bone deficiency. The goal of these strategies is to exploit the body's natural ability to repair injured bone with new bone tissue, and to then remodel that new bone in response to the local stresses it experiences. In general, the strategies discussed in this paper attempt to provide the reconstructed region with appropriate initial mechanical properties, encourage new bone to form in the region, and then gradually degrade to allow the new bone to remodel and assume the mechanical support function. Several of the concepts presented below are already finding clinical applications in early patient trials.

The ingrowth of new bone tissue and initial mechanical properties of a degrading polymeric composite scaffold.

Trabecular bone deficiency causes a dilemma at surgery in a variety of clinical situations, including trauma, tumor resection, and reconstruction. A synthetic material to replace trabecular bone would be biocompatible, provide temporary mechanical strength to the reconstructed region, and serve as a scaffold upon which new bone could grow (i.e., osteoconduction). In addition, it should serve as a carrier for osteoinductive biomolecules, degrade into nontoxic materials that the body can excrete via normal metabolic pathways, and allow the new bone to remodel along lines of local stress. A particulate filled composite based on an unsaturated linear polyester was designed as a candidate material for this application. The components are mixed with a monomer that cross links the double bonds of the unsaturated polyester. Degradation occurs via hydrolytic degradation of the backbone polymer's ester linkages. This strategy of prepolymer synthesis via condensation polymerization in the laboratory followed by cross linking the unsaturated prepolymer via radical polymerization at surgery offers design flexibility. The radical polymerization allows curing during surgery to facilitate reconstruction of various shaped defects. The laboratory synthesis of the prepolymer allows alterations of its composition and physical properties to effect desired properties in the resulting composite. This study investigates the effect of several composite material formulations on the in vitro mechanical properties and the associated in vivo histologic characteristics of the resulting material. The prepolymer molecular weight, presence of a leachable salt, and amount of cross linking monomer had strong effects on the resulting strength and modulus of the composite. These strengths were on the order of 5 MPa, a magnitude appropriate for consideration of the material as a temporary trabecular bone substitute. The in vivo studies in a rat proximal tibia model demonstrated progressive growth of new bone against the receding surface of the degrading material, and ingrowth of new bone trabeculae into the interior of the degrading specimen. The specimen was also well integrated with the surrounding bone, with no internal fibrosis. There was an absence of a foreign body inflammatory response to the presence of this material over a 5-week time span. This material may thus be an attractive candidate for temporary replacement of trabecular bone, facilitating both osteoconduction and osteoinduction.

Self-paced resistance training and walking exercise in community-dwelling older adults: effects on neuromotor performance.

BACKGROUND: Resistance-training intervention studies have demonstrated meaningful health benefits in older adults; however, most have used exercises performed at specific intensities on expensive equipment, which limit their widespread applicability. We tested whether two self-paced, less expensive exercise protocols could be effective and safe for modifying neuromotor performance and functional capacity in community-dwelling adults 65-95 years of age. METHODS: One hundred and thirty-one subjects were randomized to a novel resistance training, walking, or control group. Subjects determined their level of resistance or walking intensity (self-paced) on a session-by-session basis. Muscle strength, balance, reaction time, stair climbing speed, and a timed pen pickup task were measured before and after the intervention period. Exercisers met three times per week for 10 months. RESULTS: Significant improvements in tandem stance and single-legged stance with eyes open times and stair climbing speed were seen in both exercise groups. In addition, resistance trainers improved their muscle strength and ability to pick up an object from the floor and reduced the number of missteps taken during tandem walking, and walkers reduced tandem walking time. Controls showed no significant improvement in any variable. CONCLUSIONS: The two self-paced exercise protocols were effective at improving neuromotor performance and functional capacity in the study sample and show promise as a safe, effective, cost-efficient, acceptable exercise model for primary and secondary prevention in the general population of community-dwelling older adults.

Functional mobility discriminates nonfallers from one-time and frequent fallers.

BACKGROUND: Given that 90% of hip fractures result from a fall, individuals who fall frequently are more likely to be at greater risk for fracture than one-time fallers. Our aim was to determine whether performance variables associated with injurious falls could be used to distinguish frequent fallers from both one-time fallers and nonfallers. METHODS: A total of 157 men and women (77.4-5.4 years) were recruited and categorized into one of the following three groups based on falls status over the previous 12 months: nonfallers (n = 48), one-time fallers (n = 56), and frequent fallers (more than one fall) (n = 53). All subjects were evaluated on functional mobility and lower extremity strength and power. RESULTS: Using multivariate analysis of covariance with height as a covariate, nonfallers were significantly faster than both one-time and frequent fallers during the Get Up and Go (a test involving lower extremity strength and power, and mobility) and faster than one-time fallers on the Tandem Gait (p < .01). There were no significant differences between groups for other mobility variables or for laboratory measures of strength and power. Because one-time and frequent fallers were similar on all measures. they were grouped as "fallers" in discriminant analysis. The Get Up and Go discriminated between the fallers and nonfallers with a final Wilks's Lambda of .900 (p < .001) and correctly classified 72.4% of fallers and nonfallers before crossvalidation and 71.2% of the cases after validation. CONCLUSIONS: Given that the Get Up and Go discriminates between fallers and nonfallers and is associated with lower extremity strength and power, fall prevention strategies should focus on improving both functional mobility and lower extremity strength and power.