Chapter 22 Chapter 22 Biomechanics of Hip and Vertebral Fractures Copyright © 2013 Elsevier Inc. All rights reserved.

FIGURE 22.1 Factors that affect the load-bearing capacity of bone: material properties (A); geometric characteristics, including size, shape, and material distribution (B); and the loading conditions, which includes the loading type and direction (e.g., compression or bending), loading location and loading rate (C). 2

Copyright © 2013 Elsevier Inc. All rights reserved. FIGURE 22.2 Example of a sideways fall elicited in a young subject by unexpected lateral movement of the sheet that the subject was standing on. Out of 42 subjects, 40 fell, 39 had impacts on the left hand and 38 had impacts on the left hip. Source: from Feldman and Robinovitch (2007), with permission [34]. 3

Copyright © 2013 Elsevier Inc. All rights reserved. FIGURE 22.3 Effect of trochanteric soft tissue thickness on (top panel) the force delivered to the femur and (bottom panel) the energy absorbed by soft tissue for a constant energy impact directed laterally on the hip. Source: from Robinovitch etal. (1995), with permission [47]. 4

Copyright © 2013 Elsevier Inc. All rights reserved. FIGURE 22.4 Loading conditions significantly affect failure loads in testing of cadaveric proximal femurs. In comparing a stance configuration and a sideways fall configuration in paired older adult femurs (left), the stance configuration is more than 300% stronger than the fall configuration. Source: data from Keyak et al. (1998) [40]. In comparing a slow (2 mm/s) and fast (100 mm/s) loading rate in paired older adult femurs in a sideways fall configuration (center), fast loading rate was about 20% stronger than slow loading rate. Younger adult femurs (right) were about 100% stronger than older adult femurs, but showed a similar increase with loading rate. Source: data from Courtney et al. (1995) [39] and (1994) [69]. 5

Copyright © 2013 Elsevier Inc. All rights reserved. FIGURE 22.5 A degenerated intervertebral disc increases load transfer through the neural arch during erect posture, leading to skeletal deterioration in the anterior third of the vertebral body and potentially increasing risk of anterior wedge vertebral fracture during forward bending. Source: adapted from Pollintine etal. (2004), with permission [116]. 6

Copyright © 2013 Elsevier Inc. All rights reserved. FIGURE 22.6 Pattern of compressive loading across the spine during relaxed standing; shows the loading effect of a mild or severe fracture simulated at T12. The loads were computed for a 65-year-old woman who weighs 73 kg and is 160 cm tall. 7

Copyright © 2013 Elsevier Inc. All rights reserved. FIGURE 22.7 Using a musculoskeletal model, we simulated an age-related increase in thoracic kyphosis for three different posture conditions: an uncompensated increase in kyphosis, an increase in kyphosis while making a postural adjustment that maintains the body’s center of mass in a stable location (in this case posterior tilting of the pelvis), and an increase in kyphosis while maintaining a congruent posture (where the lumbar and thoracic curves have a proportional amount of curvature and work to maintain a stable center of mass). A congruent posture almost completely mitigated kyphosis-related increases in loading. CA: Cobb angle; LL: lumbar lordosis; pT: pelvic tilt; TK: thoracic kyphosis. Source: adapted from Bruno etal. (2012), with permission [126]. 8

Copyright © 2013 Elsevier Inc. All rights reserved. FIGURE 22.8 A vertebra will fracture when the applied loads exceed strength. The schematic above shows some of the factors that affect vertebral loading and vertebral strength. 9