Bone Load and Material Properties Summary

Type 1 collagen:
- Makes up 90% of the total collagen in bone.
- Also present in other connective tissues like tendons, ligaments, menisci, fibrocartilage, and joint capsules.
- Responsible for the tensile strength of the tissue.

Strength and Stiffness: These are important mechanical properties of bone.
Cortical vs. Cancellous Bone:
- Cortical bone is stiffer and can withstand greater stress but less strain compared to cancellous bone.
- Cortical bone fractures when strain exceeds 2% of its original length, while cancellous bone fractures at a greater length (75%).
Modulus of Elasticity:
- Cortical bone has a high modulus of elasticity (17-25 GPa), similar to wood. It exhibits high stiffness and low compliance.
- Trabecular bone has a low modulus of elasticity, low stiffness, and high compliance.
Bone is not as strong in tension as in compression, and its strength depends on density (mineralization) and the orientation of trabeculae relative to the load.
Bone is much stronger when the load is applied along its long axis.
Both cortical and trabecular bone can withstand greater stress and undergo less strain in compression than in tension.
The physiological response of trabecular bone to increased loading is hypertrophy. If loading is decreased or absent, the trabeculae become smaller and weaker.
The rate, frequency, duration, magnitude, and type of loading all affect bone.

Bone strength varies depending on the direction of loading (anisotropic).
When cortical bone is loaded in compression, longitudinal sections of the bone show the greatest strength.
Compressive strength of bone is about 250 MPa, which is greater than concrete (about 4 MPa) or wood (100 MPa) but less than steel (400 to 1500 MPa).

Varies with the speed of loading (strain-rate sensitivity).
Higher the speed of loading, the more energy the bone stores to failure.
Implications for fractures: Fast loading results in more energy stored, requiring more energy to be dissipated, leading to more damage.

Bone is laid down in areas of high stress and reabsorbed in areas of low stress.
Bone adapts to the stresses applied to it and the lack of stress applied to it.
Applies to all amounts of loading: too little, too much, or just right.

During development, bones can adapt and accommodate mechanical stresses and strains through force transmission.
Allows for eventual weight-bearing on legs and alternation in infant bony structure (e.g., hip anteversion).
Develops trabecular systems that correspond to the type of loading expected, usually compressive forces, bending, and gravity.
The neck of the femur undergoes bending stresses, with the superior aspect experiencing tensile stress and the inferior aspect experiencing compressive stress.

Microstructural deterioration of cortical and trabecular bone seen following 6 weeks of non-weight bearing (NWB) in otherwise healthy persons.
Decreased stiffness and decreased load-to-failure, especially in cortical bone.
Decreases persisted (improved but not to baseline) even after 13 weeks of full weight-bearing.

Produced by low repetition of high load or high repetition of relatively normal load (high impact aerobics).
Remodeling is outpaced by the fatigue process (not allowed to heal before the next failure or before adaptation to prevent failure can occur).
Repeated loadings, either high numbers of low loads (stress fracture) or one application of high load (fracture), can cause permanent strain and bone failure.
Bone loses stiffness and strength with repetitive loading as a result of creep strain. Creep strain occurs when a tissue is loaded again and again while the material is undergoing creep.

As babies become more upright, the loading on the bone changes to allow for standing, walking, running, etc.
Developmental issues or issues with the neuromuscular system can lead to changes in loading the bones and consequences of altered loading.

Heel spur: Relatively sustained tension through plantar fascia onto the attachment site on the calcaneus.
Fracture: Callus that forms increases the area around the fracture site, increasing the stiffness and strength of the bone.
Pins/screws: Create stress risers where stresses concentrate around the defect, decreasing stress distribution over the site. There is a cycle of decreased energy storage until bone remodels, increasing strength unless the pin/screw is removed, which creates another defect, decreasing strength/stiffness.
Bone graft: At donor site changes stresses.

Need to understand the relationship of the change to bone as a result of the change in articular cartilage.
Need to understand the concept and implications of focal loading.

Decreased mineral density leads to 'weaker' bones.
Need to know the implications for manual therapy with someone with osteoporosis.