Basic Science


Basic Science

Raymond Hsu, Matthew E. Deren, and Richard M. Terek

I. Bone and Joint Physiology

1. Cell types (Table 1.1, Fig. 1.1)

• Osteoblasts

a. Originate from mesenchymal lineage; produce type I collagen; produce alkaline phosphatase

b. Wnt/β-catenin pathway: Wnt protein binds and activates lipoprotein receptor-related protein (LRP) 5/6 at the cell surface and activates an intracellular cascade involving translocation of β-catenin into the nucleus to activate transcription of genes that control osteoblast differentiation.

c. Runx2 (Cbfa1) and Osx: transcription factors required for differentiation of mesenchymal stem cells into osteoblasts

d. Secrete receptor activator of nuclear factor kappa B ligand (RANKL) and macrophage colony-stimulating factor (MCSF) to activate osteoclasts.

e. Stimulated by estrogens and 1,25-(OH)2-vitamin D; produce osteocalcin

f. Downregulated by glucocorticoids, prostaglandins, leptin, parathyroid hormone (PTH)

• Osteoclasts

a. Originate from monocyte/macrophage lineage

b. Use lysosomal enzymes including cathepsin K, matrix metalloproteinase, and carbonic anhydrase to resorb bone

Carbonic anhydrase produces hydrogen ions that are pumped into the ruffled border.

c. Directly inhibited by calcitonin

d. Responsible for pathological absorption of bone in multiple myeloma and metastatic disease

e. Stimulated by interleukin-1 (IL-1) and RANKL

• Osteocytes

a. Originate as osteoblasts that become trapped in matrix

b. 90% of cells in mature skeleton

c. Canaliculi have gap junctions for communication between osteocytes.

d. Stimulated by calcitonin, inhibited by PTH

2. Bone matrix

• 60–70% inorganic: compressive strength; 25–30% organic: tensile strength (90% collagen); 5–8% water

• Osteocalcin: expressed by mature osteoblasts, specific marker of osteoblast phenotype and differentiation, involved in calcium homeostasis. Levels increase with increasing bone mineral density when treating osteoporosis.


• Osteonectin: calcium-binding glycoprotein secreted by platelets and osteoblasts

• Mineralization occurs as crystals form a lattice in the hole zones between collagen fibrils. The formation of the critical nucleus requires the most energy in this process.

3. Types of bone (Fig. 1.2)

• Lamellar: normal, mature, cortical, or cancellous

a. Cortical

Majority of skeleton

Higher Young’s modulus

Connection of haversian canals

Cement lines = outer border of osteon

♦ Interstitial lamellae connect osteons

b. Cancellous

Higher turnover

Less dense

Modeled along stress lines

Wolff’s law: form follows function, bone responds to stress by increasing formation

• Woven: random; pathological or immature

• Biopsy of fracture callus can be confused with osteosarcoma because they both have woven bone.

4. Blood supply of bone (two supplies)

• Nutrient artery: traverse cortices through nutrient foramen, arise from major arteries, supply inner two thirds of cortex; high-pressure system

• Periosteum: capillaries supplying outer one third of cortex; low-pressure system

• Metaphyseal-epiphyseal blood supply

a. Growth plate supplied by perichondral artery (nutrients) and epiphyseal artery (supplying proliferative zone of physis)

5. Bone formation (three types)

• Endochondral: cartilage model replaced by bone

a. Occurs in nonrigid fracture callus, physis, and formation of long bones

Associated with type X collagen

Sox-9: key gene for regulation of chondrogenesis, expressed earliest in endochondral ossification

• Intramembranous: mesenchymal stem cells differentiate into osteo-blasts to form bone

a. Occurs in formation of flat bones (skull, clavicle), healing of fractures repaired with semirigid stability (plating), and distraction osteogenesis

• Appositional: Laying new bone on preexisting bone by osteoblasts occurs in bone remodeling and periosteal enlargement of bone (creating increased thickness).

6. Physis (Fig. 1.3)

• Zones of physis:

a. Reserve/resting: coordinates organization and development of chondrocytes, marked by abundant matrix and cell inactivity

b. Proliferative: longitudinal growth of cells, high oxygen tension and ionized calcium, significant endoplasmic reticulum from aerobic metabolism

c. Hypertrophic: enlarged cells, calcification of the matrix, alkaline phosphatase, and type X collagen abundant

d. Maturation: removal of mineralized cartilage and formation of primary spongiosa

• Groove/zone of Ranvier: periphery of physis responsible for appositional bone growth of the physis and provides structural stability in first 2 years of life

• Perichondral ring of La Croix: thick fibrous band surrounding and providing stability to physis

• Achondroplasia: affects the proliferative zone

• Gigantism: affects the proliferative zone by action of growth hormone

• Physeal fractures occur through the zone of provisional calcification in the hypertrophic zone.

7. Fracture repair

• Types of healing (dependent on rigidity/strain)

a. Primary (“cutting cones,” haversian remodeling)

Requires contact and absolute stability (compression plates)

b. Intramembranous healing (direct bone formation, no intermediate)

Semirigid fixation (locked plating, intramedullary nails)

c. Endochondral healing (cartilage intermediate and then bone formation)

Nonrigid fixation (casts, external fixation)

Intramedullary nails: combination of endochondral and intramembranous based on stability and bony contact

d. In general, less rigid fixation is associated with more callous (endochondral).

• Stages of fracture healing

a. Reactive/inflammation (24–72 hours)

Hematoma provides source of growth factors and fibroblasts and mesenchymal precursors of osteoblasts.

Inhibition of cyclooxygenase-2 (COX-2) in mice and rabbits increases time to healing.

b. Repair (2 weeks)

Callous formation, type and amount depend on extent of immobilization

Nonrigid fixation: initial soft callous formation from fibroblasts followed by chondroblasts (type II collagen and then type I collagen). Type X collagen expressed by hypertrophic chondrocytes as matrix undergoes endochondral calcification.

Rigid fixation: minimal callous, primarily haversian remodeling

Protein deprivation in rats limits callous formation.

c. Remodeling (7 years)

Wolff’s law: remodeling in response to mechanical stress

8. Biological fracture treatments

• Bone morphogenic proteins (BMPs)

a. Extracellular proteins that belong to the transforming growth factor-β (TGF-β) family act by binding serine-threonine kinase surface receptors that activate intracellular signaling molecules called SMADs.

BMP-2: used to treat acute open tibial fractures

BMP-7: used to treat tibial nonunions

• Smoking/nicotine decreases blood flow and callous strength while increasing time to union and risk of nonunion.

• Low-intensity pulsed ultrasound stimulation (LIPUS) produces nanomotion to stimulate bone formation.

a. 30 mW/cm2 pulsed-wave at 1.0 kHz

b. Increases intracellular calcium, increasing proteoglycan synthesis

c. Decreases time to union in nonoperatively treated radial shaft, distal radius, scaphoid, and tibial fractures

d. No demonstrated benefit for intramedullary fixed tibia shafts

• Capacitative coupling (CC) stimulation uses electrodes with an alternating current to create an electric field to stimulate bone formation.

a. Stimulates transmembrane calcium translocation through voltage-gated calcium channels

b. Calcium activates calmodulin and upregulates cytokines for bone formation.

• Direct current stimulation reduces local oxygen concentration and increases local tissue pH, decreasing osteoclast and increasing osteoblast activity.

• Nonunion treatment

a. Hypertrophic: adequate biology, inadequate immobilization

Treatment: increase mechanical stability (e.g., compression plating of previously nailed fracture)

b. Atrophic: inadequate biology

Treatment: take down of nonunion, restabilize, and bone graft/BMP

9. Bone grafts

• Properties:

a. Osteoconductive: provides structural framework for bone growth

b. Osteoinductive: contains growth factors that stimulate bone growth

c. Osteogenic: contains cells that produce bone (osteoblasts or mesenchymal stem cells)

• Autograft: osteoconductive, osteoinductive, and osteogenic

a. Gold standard

b. Cancellous: less structural integrity, more osteoconductive, rapid incorporation by creeping substitution

c. Cortical: more structural support, incorporates slowly by remodeling of haversian system (cutting cones)

Includes vascularized bone grafts

d. Iliac crest bone graft: anterior harvest has higher complication rate than posterior harvest (can be cancellous or cortical).

e. Femoral intramedullary reaming contents equivalent to iliac crest cancellous autograft

f. Bone marrow aspirate: only other osteogenic source

• Allografts: osteoconductive (osteoinductive depends on processing)

a. Antigenicity dependent on cell-surface glycoproteins and matrix macromolecules.

b. Fresh used for osteochondral defects

c. Frozen bulk allograft used in tumor reconstruction and revision arthroplasty.

Some osteoinduction

Higher immunogenicity and risk of disease transmission

d. Freeze dried much more common

Osteoconductive, no osteoinduction but decreased immunogenicity/disease

Fewer impactions to maximal stiffness compared with fresh-frozen, therefore possibly more mechanically efficient

e. Routinely screened for HIV, hepatitis B, hepatitis C, and syphilis

f. Risk of transmission (estimated from blood transmission)

HIV: 1:1,000,000–1,500,000

Hepatitis C: 1:100,000

Hepatitis B: 1:50,000–60,000

g. Massive cortical structural allograft

Only the ends are incorporated by creeping substitution (“cutting cones”)

May eventually be encapsulated by callous but the bulk of the graft remains avascular

Stress fractures occur in ~ 25% (no remodeling)

h. Cartilage allograft

Cartilage architecture maintained for first 2–3 years

Cartilage graft remains completely acellular

Pannus of host fibrocartilage can form over the graft

i. Demineralized bone matrix: produced by acid extraction

Removal of inorganic component exposes more osteoinductive proteins, but efficacy of proteins is also partially lost in processing.

• Synthetics: osteoconductive only

a. Calcium phosphate and calcium sulfate

High compressive strength

Low tensile/sheer/torsional properties

Ca phosphate: very slow resorption (1 year), cement form available, popular for subchondral support of fractures

Ca sulfate: fast resorption (4–12 weeks), essentially plaster of Paris, can cause increased serous drainage from surgical incisions

b. Subsets of calcium phosphate

Tricalcium phosphate

♦ Resorbs more rapidly, less compressive strength, and weaker than hydroxyapatite

♦ Partially converted to hydroxyapatite

Hydroxyapatite: Ca10(PO4)6(OH)2

♦ Ceramic preparation very resistant to resorption

♦ Can be converted from calcium carbonate (marine coral)

♦ Porous preparation enables neovascularization and appositional new bone growth.

♦ Resorbed by foreign body giant cell

10. Bone metabolism

• Calcium homeostasis (Fig. 1.4)

a. Requirements for Ca intake (Table 1.2)

b. Calcitonin: directly inhibits osteoclasts, decreases serum calcium

• Hormone effects and interactions

a. Estrogen

Most important hormone for peak bone mass in females

Inhibits bone absorption and increases bone formation

Associated decreased risk of heart disease and increased risk of endometrial and breast cancer

b. Corticosteroids

Decreases gut absorption of calcium, increases bone loss (decreases bone formation from inhibition of osteoblast collagen synthesis)

c. Thyroid hormones

Thyroxine: high doses can result in osteoporosis

Affect physeal growth by increasing chondrocyte growth, collagen X synthesis, and alkaline phosphatase; increases both proliferation and hypertrophy of the growth plate

d. Growth hormone

Insulin-like growth factor-I (IGF-I) induces linear growth by engendering proliferation in physis.

• Growth factor signaling types

a. Autocrine: affects the same cell that secreted the growth factor

b. Paracrine: affects adjacent cells

c. Endocrine: affects cells at distant site

11. Metabolic bone disease (Table 1.3)

• Malignancy/metastasis

Tumor cells secrete parathyroid hormone–related protein (PTHrP), interleukins, macrophage inflammatory protein (MIP), tumor necrosis factor-α (TNF-α), prostaglandin E2 (PGE2) to activate osteoblastic production of RANKL, or directly secrete RANKL. Increased ratio of RANKL/osteoprotegerin (OPG) activates osteoclasts.

Bone resorption by osteoclasts releases TGF-β from bone matrix, feeding back to tumor cells to release more PTHrP (leading to increased lysis).

• Hyperthyroidism

a. Causes hypercalcemia by increased production of calcitonin from thyroid parafollicular clear cells that binds to osteoclasts to decrease activity and number, resulting in increased serum calcium

• Vitamin D toxicity

a. Excessive vitamin D intake leads to increased 25(OH)-vitamin D, with subsequent increased intestinal absorption of calcium and resultant hypercalcemia.

b. Treatment: correct intake of vitamin D

• Hypoparathyroidism

a. Decreased PTH production by parathyroid chief cells leads to decreased serum calcium, increased phosphate, decreased 1,25-(OH)2-vitamin D levels


b. Treatment: calcium and vitamin D supplementation

• Pseudohypoparathyroidism

a. Genetic disorder of ineffective PTH receptor causing normal to elevated PTH levels

• Albright hereditary osteodystrophy

a. A form of pseudohypoparathyroidism due to a defective maternal GNAS1 gene with resultant exostoses, short fourth and fifth metacarpals and metatarsals, brachydactyly, obesity, low intelligence, short stature

• Vitamin D deficiency rickets/osteomalacia

a. Rickets: children with open growth plates

Widened growth plates from accumulation of nonmineralized osteoid and cartilage

Causes widening of the anterior ribs (rachitic rosary)

b. Osteomalacia: adults with closed growth plates

c. Mechanism: dietary deficiency of vitamin D leads to decreased intestinal calcium absorption.

d. Results in increased PTH, increased bone resorption (alkaline phosphatase)

e. Low to normal serum calcium, low phosphate, low Vitamin D

f. Treatment: 5,000 IU vitamin D daily

• Primary hyperparathyroidism

a. Primary pathology is increased PTH, such as from a parathyroid adenoma

b. Accumulation of fibrous tissue in metaphysis can mimic widened growth plates of rickets.

c. Erosions around growth plate

d. Brown tumor of hyperparathyroidism

• Secondary hyperparathyroidism (renal osteodystrophy)

a. Primary pathology is renal failure

Inability to convert vitamin D3 to active calcitriol leads to hypocalcemia and osteomalacia

Failure to adequately excrete phosphate leading to uremia-related phosphate retention

b. Insoluble calcium phosphate forms, removing calcium from the circulation.

c. Low serum calcium/high serum phosphate causes secondary hyperparathyroidism.

d. Two types:

High bone turnover: increased PTH, parathyroid hyperplasia leading to osteitis cystica; continues after correction of kidney disease

Low bone turnover: common in dialysis, low PTH, and low bone formation (frontal bossing, genu varum, metaphyseal enlargement)

e. “Rugger jersey” spine: sclerosis of end plates of the vertebrae seen on X-ray

• Osteogenesis imperfecta

a. Mutation in collagen type 1 (COL1A1 or COL1A2 genes)

b. Hearing defects, blue sclera, fractures, scoliosis, poor dentition

c. Olecranon apophysis fractures relatively common

d. Bisphosphonates reduce bone pain and fracture incidence, increase bone density and overall function

• Hereditary vitamin D–dependent rickets

a. Genetics: autosomal recessive

b. Type I: loss of function mutation in 25-hydroxyvitamin D hydroxylase gene [decreased levels of 1,25-(OH)2-vitamin D]

c. Type II: defective intracellular receptor for 1,25-(OH)2-vitamin D3 [increased levels of 1,25-(OH)2-vitamin D3]

d. Decreased serum calcium and phosphorus, increased PTH, increased alkaline phosphatase

• X-linked hypophosphatemic (vitamin D–resistant) rickets

a. Genetics: X-linked dominant

b. Mechanism: mutated PHEX gene (X-chromosome) causing inability of renal proximal tubules to reabsorb phosphate (phosphate diabetes)

c. Low serum phosphorus, elevated alkaline phosphatase, normal PTH levels, low or normal calcium levels

d. Treatment: High-dose vitamin D3

• Oncogenic osteomalacia

a. Mesenchymal tumors secrete fibroblast growth factor-23 (FGF-23) or phosphatonin, which inhibits phosphate reabsorption and increases excretion at the proximal renal tubules.

• Hypophosphatasia

a. Genetics: autosomal recessive

b. Mechanism: Defect in tissue-nonspecific isoenzyme of alkaline phosphatase leading to decreased levels of alkaline phosphatase and hypomineralization

c. Diagnosis by elevated urinary phosphoethanolamine

• Osteoporosis

a. Chronic progressive disease associated with low bone mass and decreased bone strength

b. Genetics: multiple associated polymorphisms in genes, including calcitonin receptor, estrogen receptor-1, vitamin D receptor, type 1 collagen α-chain, IL-1, IL-10, IGF-II, TGF-β, TNF-α, TNF receptor 2

c. Bone structure with age:

Loss of density in both cortical and cancellous bone but more in cancellous (thinned trabeculae, decreased interconnections)

Decreasing cortical thickness and enlarging medullary canal diameter in long bones

Estrogen most important hormone for peak bone mass, which usually occurs at between 16 and 25 years of age

d. Bone mineral density (BMD):

Dual-energy X-ray absorptiometry (DEXA) testing: determines bone density (defined as standard deviations) in hip and lumbar spine

Recommended for all women of ages 65 and older and all men of ages 70 and older

T-score is in comparison to a healthy 25-year-old of same sex and ethnicity (peak bone age)

♦ Osteopenia: T-score between –1 and –2.5

♦ Osteoporosis: T-score ≤ –2.5

Z-score is in comparison to same age, sex, and ethnicity.

♦ For diagnosis of metabolic bone diseases

Osteoarthritis can falsely elevate spinal BMD values.

e. Workup after a fragility fracture:

DEXA, 25-OH vitamin D levels, calcium levels

Metabolic workup and arrange follow-up with osteoporosis clinic

f. History of any fragility fracture (spine, hip, or wrist)

Most predictive of future fractures (more so than vitamin D level, T-score, family history, or other risk factors)

Vertebral body fractures

♦ Most predictive of future vertebral body fractures (as compared with hip and wrist fractures)

♦ Higher overall mortality than previously recognized

♦ Overall mortality twice that of controls

♦ Greater increase in mortality risk in men than women and with younger age

g. FRAX (fracture risk assessment tool) score

Developed by World Health Organization (WHO)

Calculates clinical risk of fracture using BMD at femoral neck, body mass index (BMI), current smoking activity, history of parental hip fracture, and prior personal history of fracture before age 50

Does not use BMD of spine

h. Medications that increase risk of osteoporosis:

Oral corticosteroids

Androgen-deprivation therapy, aromatase inhibitors

Protease inhibitors

Selective serotonin reuptake inhibitors, prolactin-raising antiepileptics (carbamazepine, phenytoin, valproic acid)

i. Dietary treatments:

Daily calcium intake for osteoporosis treatment/prevention: 1,000–1,500 mg (starting from age 9) (only lactating women require more: 2,000 mg/day)

Daily vitamin D intake for adults > 50 years: 1,000 units

♦ With age, decreased dietary intake, decreased conversion via the skin, and decreased conversion in the kidney

Protein-enriched diet

j. Pharmacological treatments

Bisphosphonates (see dedicated section below)

Teriparatide (Forteo) (recombinant 1–34 amino acid sequence at the N-terminus of parathyroid hormone)

♦ Activates osteoblasts, which release RANKL and IL-6 to activate osteoclasts

♦ Intermittent dosing: increased coupling of osteoblast activity to osteoclast resorption, net bone formation (maximum treatment 2 years)

♦ Continuous dosing: net bone resorption

Calcitonin: directly inhibits osteoclasts

Denosumab: anti-RANKL monoclonal antibody

• Lead toxicity

a. Stored in bone and released slowly over decades

b. Inhibits parathyroid hormone–related peptide (PTHrP) causing decreased bone mineral density

• Osteopetrosis: abnormal osteoclast number and function

a. decreased bone turnover and remodeling (fractures, Erlenmeyer flask deformity)

b. CLCN7 and TC1RG1 genes

• Scurvy: deficiency of vitamin C, which is required for cross-linking during collagen synthesis

a. Bleeding from fragile capillaries

b. Growth plate affected primarily at primary spongiosa

c. Radiographs show dense band at metaphyseal/growth plate junction: white line of Frankel.

• Fibrodysplasia ossificans progressiva (FOP): characterized by massive spontaneous heterotopic bone formation

a. Altered BMP-4 signal transduction

b. Diagnosis is clinical; biopsy worsens process

12. Heterotopic ossification

• Bone formation in extraskeletal tissue

• Risk factors: prolonged ventilator time, brain injury, spinal cord injury, neurologic compromise, burns, blast injury, and amputation through zone of injury

• Prophylaxis: irradiation with 700 cG or indomethacin 25 mg oral t.i.d. for 6 weeks

13. Bisphosphonates

• Pyrophosphate analogues that inhibit osteoclast resorption of bone

• Accumulate in high concentrations in bone due to affinity for hydroxyapatite crystals, then taken up by osteoclasts

• Nitrogen-containing

a. Alendronate/Fosamax, pamidronate/Aredia, risedronate/Actonel, zoledronate/Zometa

b. Inhibit farnesyl diphosphate synthase (FPPS), preventing protein prenylation of small guanosine triphosphatases (GTPases) in the cholesterol synthetic pathway

Also inhibits geranylgeranyl diphosphate synthase (GGPPS) and undecaprenol diphosphate synthase (UPPS)

• Non–nitrogen containing

a. Etidronate/Didronel, clodronate, tiludronate

b. Metabolized form replaces terminal pyrophosphate of adenosine triphosphate (ATP), which forms an analogue that competes with ATP and causes osteoclast apoptosis

• Indications: osteoporosis, history of fragility fractures, osteogenesis imperfecta, Paget’s disease, metastatic bone disease, idiopathic hyperphosphatasia, avascular necrosis

• Results for osteoporosis:

a. Vertebral fractures: 65% reduction after 1 year and 40% reduction after 3 years

b. Nonvertebral fractures: 40% reduction after 3 years

• Metabolism:

a. Minimal gastrointestinal (GI) absorption (recommendation: take 1 hour prior to meal)

b. Excreted by kidneys

• Complications:

a. Subtrochanteric stress reaction and fractures:

Symptoms: lateral thigh pain

Imaging: lateral cortical thickening, beaking, “dreaded black line” (stress fracture)

Treatment: discontinuation of bisphosphonate, contralateral imaging, consider prophylactic intramedullary fixation for impending fracture

b. Osteonecrosis of the jaw

c. Reduced fusion rates in spine fusion surgery

d. Osteopetrosis-like bone when used in children

14. The joint

• Articular cartilage (Fig. 1.5)

a. Deep zone: highest concentration of proteoglycans and lowest concentration of water

b. Overall concentrations: water > collagen > proteoglycan > noncollagenous proteins > chondrocytes.

c. Collagen oriented parallel in superficial zone and perpendicular in the calcified zone

d. 65–80% water

Water effectively stress shields matrix from compression.

e. Osteoarthritis changes in cartilage versus changes in aging (Table 1.4)

f. Proteoglycans: 10–15% of wet weight, viscoelastic with molecular twotiered brush-like structure (Fig. 1.6)

Hyaluronate: a complex sugar, composes the core

Aggrecan: major proteoglycan in cartilage, aggregates onto hyaluronic acid with link proteins

Glycosaminoglycan chains: attached to the core aggrecan

♦ Chondroitin sulfate and keratin sulfate

♦ Glucosamine serves as substrate for formation of chondroitin sulfate.

♦ Increase in the knee with moderate exercise

• Effect of aging:

a. Decreased proteoglycan synthesis and water

b. Decreased chondrocyte number

c. Keratin sulfate increases until age 30, then levels off.

d. Chondroitin sulfate decreases.

• Collagen types throughout the body:

a. Type I: major form in tendon, bone, and meniscus

b. Type II: major collagen of articular cartilage

Very stable, half-life > 25 years

Adults have only a 5% rate of synthesis in articular cartilage as compared with teenagers.

Jun 28, 2018 | Posted by in ORTHOPEDIC | Comments Off on Basic Science

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