Structure and function of bone
3 types of long bone
-*diaphysis*/Shaft -*metaphysis*/flaring of the bone -*epiphysis*/ends of the bone
Osteoclast
-*responsible for bone breakdown* -extremely large and are derived from hematopoietic precursors -similar to monocytes and macrophages -*produce tartrate resistant acid phosphatase* -live in regions of bone resorption pits called * howship's lacunae* -have a ruffled border, which forms when they bind the surfaces of bone
metaphyseal complex
-Arise from periarticular plexuses that penetrate directly into the metaphyseal region
Clinical implications of the vascular supply to bone
-Bone needs healthy blood supply to heal fractures and revitalized damaged areas -*injury to the blood supply* either from fracture or iatrogenic* can cause delays in healing* -bones with more robust blood supply = femur and humerus = quick healing and more predictable healing
Blood supply to the bone
-Critical for maintenance, healing, and function
Osteoblasts
Bone forming cells -drive signaling for bone breakdown -make collagen type I -produce osteocalcin and various other extracellular matrix proteins - produce alkaline phosphatase -derived from marrow stromal cell or mesenchymal stem cells
Mechanical properties of cortical bone
More stiff than cancellous bone -has maximum strength and stiffness for* axial loads*
Mechanical properties of trabecular bone
Much* lower stiffness and strength* -able to absorb a large amount of energy and allows larger deformations before failure -*more compliant *than cortical bone -*important for function around joints*
3 major bone cell types
Osteoblast, osteoclast, osteocyte
Responsible for basic coupling or formation degradation signaling
Osteoblasts
osteocytes
Osteoblasts that become surrounded by bone matrix -derived from marrow stromal cell or mesenchymal stem cells/pluripotent marrow cells -most numerous of bone cells -characterized by extensive canaliculi that establish communication between adjacent osteocytes and the canals the a gap junctions
Nutrient arteries
Pass directly into the diaphysis through nutrient foramen in the diaphyseal cortex entering the intramedullary canal -*supply the inner two thirds of the diaphyseal cortex*
Bisphosphonates
Prescribed for osteoporosis and are incorporated into bone by osteoblasts -synthetic analogs of inorganic phosphate -once placed in bone they disrupt bone degradation by disrupting the ruffled border of the osteoclasts or by causing apoptosis of the osteoclasts
3 main sources of blood supply to long bones
-Nutrient arteries -arteries in the periosteal membrane -metaphyseal complex
osteocyte function
-Participate in signaling in communicating the various loads and strains in the bone to guide formation and degradation
osteon
-also called Haversian system -cylindrical structures 3-6 mm long and 150-200 µm in diameter -*center contains blood vessels and poorly myelinated nerve fibers* -surrounding lamellae contain osteocytes
Percent of body sodium and magnesium stored by bone
65%
Percent of body's phosphorus stored by bone
85%
Percent of body's calcium stored by bone
98-99%
makes RANK-ligand and OPG
????
Butterfly
Area in which pressure was applied
Quick healing properties of bone such as the femur and humerus
Attributed to larger soft tissue envelope of highly vascular muscle tissue surrounding the bone
axiallyLoading bone
Sheer type fracture
Continuous exposure to PTH
Signals bone degradation by working to the osteoblast
act at the osteoblast level
Vitamin D parathyroid hormone interleukin 1 prostaglandins
porosity of trabecular bone
between 30-90%
osteoblast
cell that makes bone -master regulators of cell turnover, bone making and breaking -arise from MSC cells
Osteoprotegerin (OPG)
competitive inhibitor of RANK ligand
nerves in long bone
enter from the periosteum alongside the arteries and are found along the blood vessels and bone in the bony canal system
osteocytes
make up 90% of bone cells -quiescent -function is unclear; calcium homeostasis? -MSC origin
Prostate cancer
rankl <<< OPG
osteoclast
remove bone ruffled border come from osteopoeitic stem cells
osteon/haversian system
single blood vessel unmyelinated bone
Simple fractures
spiral oblique Simple
Bending bone
tension type fracture
cancellous
trabecular
oblique fracture
uneven bending
Bones with much *longer healing times* that are more predisposed to *non-unions*
*Tibia* → largely subcutaneous * talus* → largely covered in a vascular cartilage
Point at which force is applied to butterfly fractures
-Force/bending occurs adjacent to the butterfly -transverse fracture occurs on the tension side
trabecular bone
-Found primarily in the metaphyseal region of long bones -described as a collection of plates and or rods -structures have a maximum thickness of approximately 200 µm -space between the struts of the trabeculi make the bone porous and allow it to be filled with marrow -architecture designed to *maximize strength and minimize weight*
3 types of bone in the human body
-Long bones → femur, tibia, humorous -small or short bones → vertebra, sternum, carpal bones and tarsi -flat bones → skull, scapula, pelvis, and mandible
cortical bone
-compact bone -extremely dense with very low porosity -thickwalled cylinder surrounded by intramedullary canals -*acts as a loadbearing segment* -arranged into *osteons*
Intra-membranous ossification
-flat bones -form through loose condensation's of mesenchymal tissue, which give rise to bone directly
bone composition
-inorganic matrix → 60-70% -collagen → about 90% of the organic matrix -non-collagenous protein → 8% -water → 5-8%
Endochondral ossification
-long and short bones -formation starts with a collagenous model which is then replaced by bone
bone loaded for failure
-rarely fails in compression -more likely to fail in tension or shear
periosteal membrane arteries
And to the bone at various points and supply the outer 1/3rd of the cortex of the diaphysis
Comminuted fractures
Can be caused by - high-speed torsion -multiple points of bending - crush
Intermittent exposure to PTH
Causes bone formation -principal mechanism used in the drug Forteo prescribed for osteoporosis
3 forces acting on bone
Compression, tension, and shear bone is strongest against compression >tension >sheer
Transverse fracture
Pure bending
Turns on osteoclast activity when signaled by osteoblasts
RANK ligand
normal stromal cells
RANKL = OPG
Giant cell tumors
RANKL > opg
Breast cancer
RANKL >> opg
Multiple myeloma
RANKL >> opg
spiral fracture
Torsion
2 major tissue organizations of mature bone
Trabecular/cancellous bone cortical bone
Butterfly fractures
Usually result from bending and torsion or bending and compression
Osteocalcin
Very specific marker for osteoblast activity
cortical/compact
stiffer, stronger less forgiving