20-Bio-A4 Anatomy and Physiology, December 2013, Q2

Question 2 (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) & b) Longitudinal (L) vs. transverse (T) stress-strain curves. Cortical bone is strongly anisotropic: its osteons and the collagen/mineral lamellae within them run predominantly along the bone's long axis, so loading along L engages the full fibre-reinforced stiffness and strength of the tissue, while loading along T is resisted mainly by the weaker cement lines between osteons.

Figure (sketch) — cortical-bone stress-strain response along the longitudinal axis (L, solid) vs. the transverse axis (T, dashed). L shows a steeper elastic slope (higher stiffness), a higher yield stress, a visible plastic region, and a higher ultimate strength; T is markedly less stiff, yields and fails at much lower stress, and shows almost no plastic region before an abrupt (brittle) fracture.

In both curves the initial straight-line portion from the origin is the elastic region (fully recoverable deformation, slope = Young's modulus); it ends at the yield point, where the curve visibly bends over as permanent (plastic) deformation begins. The curved segment beyond yield is the plastic region, and its peak stress is the ultimate strength. For L, the plastic region and the gap between yield and ultimate strength are both clearly visible (bone is markedly stronger and moderately more ductile along its long axis); for T, yield and ultimate strength sit almost on top of each other and the specimen fractures with very little plastic deformation — a much more brittle response, consistent with loading across (rather than along) the osteons.

c) Gait abnormality (Figure 2). The photograph shows the subject in single-leg stance on the left leg (right leg swinging), with the pelvis dropped on the right (swing) side and the trunk leaning laterally over the stance (left) hip, using the wall/door frame for support. This is the classic (compensated) Trendelenburg gait — a positive Trendelenburg sign on the left stance hip.

d) Muscles to strengthen. The Trendelenburg sign indicates the stance-side (left) hip abductors are too weak to hold the pelvis level against body weight during single-leg support. Strengthening should target the left hip abductors: primarily gluteus medius and gluteus minimus, with tensor fasciae latae as a synergist.

Topic: Cortical-bone anisotropy and the Trendelenburg sign.

Key relations: stress-strain response = f(loading direction relative to osteon orientation); single-leg-stance pelvic equilibrium requires the stance-hip abductor moment to balance the body-weight moment about the stance hip (Trendelenburg test logic).

Why this works: osteons (and their internal lamellae) are aligned along the bone's long axis, so tensile/compressive load along L is carried directly by this fibre-reinforced structure, while transverse load must cross the much weaker interstitial cement lines between osteons — hence lower stiffness, strength, and ductility in T. In gait, when the stance-side abductors cannot generate enough moment to hold the pelvis level, the pelvis drops contralaterally unless the trunk shifts over the stance hip to reduce the required abductor moment (compensated Trendelenburg lurch).

Common pitfall: drawing the T curve merely as a shorter version of the L curve without also flattening its initial (elastic) slope, and — for the gait case — strengthening the swing-side hip rather than the true deficit on the stance side.

Source: Nordin & Frankel, Basic Biomechanics of the Musculoskeletal System, ch. 2 (bone mechanical anisotropy) & ch. on gait analysis (Trendelenburg gait).

Question 2 of 4

Question 2 (15 marks)