Biomechanical Properties of Bone

• Anisotropy results in different ultimate strength values under tensile, compressive, and shear loading, indicating bone's mechanical properties depend on the direction of loading.
• Stiffness or Young's modulus varies with the direction of loading, affecting a bone's ability to resist deformation.

Fracture and Healing

• Bone fractures occur from a single load exceeding the bone's ultimate strength or from repeated lower-magnitude loads, leading to fatigue.

Bone After Fracture: Callus

• Callus is a cuff of dense fibrous tissue or woven bone that forms around a fracture site during healing.
• Callus significantly increases the area and moment of inertia, enhancing strength and stiffness in bending and torsion during healing.

Biomechanical Properties of Bone: Viscoelasticity

• Viscoelasticity is critical in bone as it affects mechanical properties and clinical impact on fracture patterns and soft tissue damage.
• Increased strain rates make bone more brittle and less tolerant to strain.
• Bone is stronger and more resilient during brisk walking compared to slow walking.

Influence of Loading Rate

• Bone increases in strength, stiffness, and toughness with higher strain rates.
• Stiffness is thus more sensitive to loading rates than strength.
• The loading rate influences both the fracture pattern and the amount of soft tissue damage:
• Low loading rates dissipate energy through a single crack.
• High loading rates lead to bone comminution and extensive soft tissue damage.

Muscle Activity Influence

• High tensile stress is produced on the posterior aspect of the tibia during skiing, and reducing this stress protects the bone.
• Triceps Surae contraction reduces compressive stress on the posterior aspect, countering tensile stress and preventing tibia failure.
• Muscle contraction alters stress distribution in bone by reducing or eliminating tensile stress, producing high compressive stress.
• Muscle contraction in the hip joint produces effects similar to those in the tibia.

Hip Joint - Femoral Neck

• Bending moments are applied to the femoral neck, and tensile stress is produced on the superior cortex.
• Gluteus Medius contraction generates compressive stress that neutralizes tensile stress on the superior cortex, resulting in no net stress.
• Muscle contraction protects the femoral neck from bending-related failure.

Influence of Muscle Activity - Loads

• Daily activities induce varied loads.
• Fractures result from a combination of loads.
• Muscle contraction affects stress patterns:
• Produces compressive stress
• Neutralizes tensile stress
• Optimizes the bone's loading situation, enhancing strength.

Effects of Fatigue & Stress

• Fatigue can lead to bone fractures due to repeated load application over time.
• Loading prevents proper bone remodeling, weakening the bones involved.

Fatigue Fractures

• Bone fractures result from either a single high-magnitude load (immediate fracture) or repetitive lower-magnitude loading (fatigue fracture).
• Fatigue fractures occur in two types:
• Fatigue Type Stress Fracture: Occurs in normal bone due to excessive activity.
• Insufficiency Type Fatigue Fracture: Occurs in abnormal bone under normal activity.
• Fatigue fractures are classified as:
• Tensile Fatigue Fracture: Features transverse, fast-propagating cracks, caused by debonding of osteons.
• Compressive Fatigue Fracture: Features oblique, slow-propagating cracks.

Fatigue Fracture Case Studies

• Case Study 1 shows that a few repetitions of a high load exceeds the bone's capacity, leading to rapid failure before the bone can adapt or remodel.
• Case Study 2 shows that repeated loads during a short period surpass the time for bone remodeling, causing fatigue type stress fractures.

Concepts in Bone Fatigue

• Bone fatigue occurs faster, as applied load approaches yield strength, reducing necessary cycles to cause a fracture.
• Yield strength defines the number of repetitions needed to produce a fracture, which diminishes rapidly.
• Bone growth depends non-linearly on load intensity i.e. microstrain, with risks of fractures when strain goes above/below optimum levels.
• Anabolic effect occurs above the optimal load/strain ratio, increasing fracture risk.
• Bone resorption occurs below the optimal load/strain ratio.
• Bone resorption is the process by which bones are broken down and absorbed by the body, which leads to dissolution of bone mineral and degradation of the organic bone matrix.
• Bone fatigue is influenced by load magnitude, number of repetitions, and frequency of loading.
• Bone self-repairs, but fatigue fractures occur if the fatigue process outpaces remodeling.
• Too frequent loading prevents needed remodeling leading to failure.

Bone Adaptation

• Bone remodels in response to mechanical demands by altering its size, shape, and structure.
• It gains and loses cancellous or cortical bone based on the level of strain sustained.

Wolff's Law

• Bone adapts to loads by remodeling over time to become stronger in response to increased loading.
• Increased use/activity (stress) leads to bone formation.
• Disuse/inactivity lowers the bones resistance.
• The adaptation ensures bones develop strength where it's needed, reducing unnecessary mass in low-stress areas.

Geometry & Bone Behaviour

• Mechanical behavior changes based on : cross-sectional area, length, distribution of bone tissue
• Larger cross-sectional area/greater length/higher moment of inertia result in a stronger/stiffer bone
• In tension and compression: the load to failure/stiffness are proportional to cross-sectional area
• When bending: cross-sectional area/distribution of bone tissue around neutral axis affect mechanical behavior

Open Section Defects

• The larger the polar moment of inertia, the stronger and stiffer the bone in torsion.
• The factors that affect bone strength and stiffness in torsion are similar to those in bending.
• Distribution of bone tissue around a neutral axis.
• The larger the polar moment of inertia, the stronger and stiffer the bone in torsion.
• Stress raisers occur when the length of the defect is less than the bone diameter.
• Open section defects occur when the defect length exceeds the bone diameter, creating structural weaknesses.
• Magnitude of stresses produced at contact points in the structure is related to the length.
• Certain procedures/defects can produce defects that greatly weaken the bone, especially in torsion.

Disruptions to Bone

• A stress raiser is created when a small piece of bone is removed or a screw is inserted, reducing bone strength due to unevenly distributed stresses.
• Open section defects disrupt stress distribution within a bone, weakening its structural integrity.
• Surgical removal of bone pieces can weaken it, especially under torsion.

Implants

• Implants stabilize fractures but, after removal, reduce stimulus, leading to bone reabsorption/weakening.
• Stress shielding occurs when the implant remains attached, diminishing bone stimulus/strength, thus leading to bone weakening.
• Inadequate healing is the result

Bone Remodeling

• Hypertrophy occurs in normal adult bone, where increased mechanical stimulus make bone stronger & more dense, prevents fractures or structural failure

Aging impact

• Structural and mechanical changes over time, lead to bone become more fragile/susceptible to fractures.
• Changes include:
• Reduction in the cancellous bone, makes bones more fragile.
• Decrease in thickness of cortical bone.
• Diminution of bone toughness, increasing risk of breaks.
• Reduction in strength and stiffness.
• Bone loss particularly affects structure and bone mass decreases
• There can be lower longitudinal/transverse trabeculae
• Also bone becomes much less dense

Other Effects

• Estrogen is key for bone remodeling which drops after menopause
• Reduction in collagen cross-linking/bone density/strength/stiffness/toughness cause bone fragility with age.

Stress with Aging

• Loss due to aging can be based on estrogen and gender

• Stress is nearly the same. Stress & other aspects of Bone

• Influence of physical activity, inactivity & weight-bearing exercises can lead to bone loss

• Calcium affects density and deficiency of bone

Anisotropy in bones leads to varying ultimate strength values under different loading conditions. Bone fractures result from excessive single loads or repeated lower-magnitude loads. Callus formation enhances strength and stiffness during healing and Viscoelasticity affects mechanical properties.