Skeletal+System

Locomotion and rigidity of the body is controlled by 208 skeletal bones and 501 separate muscles. The act of picking up an object is balanced by power exerted to muscles, the weight of the object supported by the bone and the neurological signals that update the sensory system by contracting and relaxing muscle cells and fibrils.

==

Type of Bones==


 * ~ Bone Types ||~ Features ||
 * Long Bones || Have a thin cortical outer layer to compensate for torsional stress. Supported by trabaculae which grow along the direction of stress. ||
 * Short Bones ||  ||
 * Flat Bones ||  ||

Composition
Bone consists of three general components:
 * 1) living cells (osteocytes, osteoblasts, osteoclasts)
 * 2) non-living organic matter (collagen)
 * 3) non-living inorganic matter (HA crystals)

Living Cells
Osteoblasts - bone forming cells Osteoclasts -bone resorbing cells Osteocytes - ossefied bone cells

Non-living Organic Matter
Collagen is polymeric, tough protein that is a large composition of bone, There are 13 types of collagen divided among 3 classes.
 * Class 1 - 300 nm triple helix
 * Class 2 - basement membranes
 * Class 3 - short chain

Type I collagen is most important to bone as it can be materialized. Bone minerialization is the process of HA crystal formation between collagen fibrils resulting in higher stiffness (1-2GPa) and tensile strengths (50-100 MPa).

Non-Living Inorganic Matter
A series of hydroxyapatite (HA) crystals nucleate within the pores of collagen fibers. The HA composition is Ca5(PO4)3OH and aligned along the longitudinal fibral axis. The matrix is reinforced which provides a tougher, stronger composite an the nanometric scale. A hierarchy of structures are made from these fundamental building blocks.

Hierarchical Structure
A series of morphological structures are formed based on the arrangement and compositions of cells, collagen fibers and the state of mineralization. Different mechanical properties for each are observed.

__Mineralized__ Structural units called osteons are formed from collagen fibrils. The osteons form parallel to the length of the bone, which lends to the anisotropic mechanical properties.
 * ~ Types ||~ Strength ||~ Structure ||~ Properties ||
 * Woven bone || Weak ||  ||   ||
 * Cancellous (aka traebecular or spongy) || Intermediate || Porous || Low elastic modulus ||
 * Cortical (aka compact) || Strong || Dense || High elastic modulus ||

__Unmineralized__
 * ~ Types ||~ Features ||
 * Cartilage ||  ||
 * Tendons ||  ||
 * Ligaments ||  ||

Bone Mechanics
Bone has a higher strain to failure in compression than in tension. It is debated whether bone growth induction is greater in compression or tension. The quality of bone depends on age, gender and disease state. The density of cortical lining is proportional to general mechanics. It is weakest for elderly, osteoporotic female partients.

Fracture
There are nice classes of bone fracture summarized below.



Tendons and Ligaments
Tendons and ligaments are composites of collagen fibrils embedded in a proteogylcan/elastin extracellular matrix. Fibroblasts are the dominant cell-type and are parallel to Type II collagen fibrils.

Tendons - connects bone to soft tissue (muscle, eye). Consists of 86% Type II collagen. Ligament - connects two bones. Consists of 70% Type II collagen.

Fasicles are bundles of collagen fibrils in tendons bound by glycoproteins and water. There is limited blood supply to tendons and ligaments, but enough to feed fibroblasts. Common sport injuries involve damage to the microvascular system.



There is a creep behavior in tendons and ligaments where the elastic limit can be shifted 2-3x through approximately 10 load cycles.

Cartilage
Articular cartilege ismost important with chondrocytes encased in matrices or collagen fibers, proteoglycans and interstitial water. The stresses exerted on the articular surfaces are absorbed by the cartilage by displacing the water away from the area of stress.
 * ~ Type ||~ Examples ||~ Structure ||~ Features ||
 * Elastic cartilage || ear, nose ||  ||   ||
 * fribrocartilage || intervertebral space ||  ||   ||
 * articular cartilage || articulating ends || Lamellar chondrcytes || Protects joints from abrasion ||

BMPs
Bone morphogenic proteins are members of the TGF-beta superfamily. BMPs have mitigenic stimulating properties for new bone formation and mutagenic properties, which enable the transformation of connective tissue into new bone. As much as 3-3.5 mg of BMP is sufficient to accelerate bone growth and subsequent bone healing of non-union fractures. BMPs sometimes require porous carriers to focus the local effect at sufficient concentrations. There is merit for hesitation in using certain polymeric carriers such as collagen, which when under pressure can release BMP into the epidural cavity, increasing risk of bone formation in undesired areas of the body.