Non-Neoplastic Bone

Question 1

Bone Physiology

Answer 1

• Bone tissue is formed by intramembranous ossification or by endochondral ossification
• The original or model tissue is gradually destroyed and replaced with bone tissue
• Woven bone is primarily formed and later converted to lamellar bone by subsequent remodeling

Remodelling:
* Osteoclasts
* RANKL binds to surface receptor RANK
* Macrophage colony-stimulating factor works with RANK –> activates functional osteoclasts
* Osteoprotegerin (OPG) –> decoy receptor for RANKL –. prevents bone reabsorption

Question 2

Achondroplasia

Answer 2

• Most common skeletal dysplasia
• Major cause of dwarfism
• Gain-of-function –> FGFR3 gene –> normally inhibits endochondral growth
• Shortened proximal extremities
• Enlarged head, bulging forehead
• Depression at the root of the nose
• Relatively normal trunk length

Question 3

Thanatophoric dysplasia

Answer 3

• Most common lethal from of dwarfism
• Gain-of-function –> FGFR3 gene
• Different to achondroplasia –> greater increases in FGFR3 signalling
• Micromellic limbs
• Frontal bossing, relative macrocephaly
• Small chest cavity
• Bell shaped abdomen
• Underdeveloped thoracic cavity –> respiratory insufficiency
• Frequently die at birth or soon after
• Reduced chondrocyte proliferation and disorganisation in the zone of proliferation

Question 4

Osteogenesis imperfecta

Answer 4

• Also called brittle bone disease
• Autosomal dominant
• One of the most common congenital connective tissue matrix diseases
• Disease of type I collagen
• Mutations in genes coding for alpha 1 and 2 collagen chains
• Marked cortical thinning and attenuation of trabeculae,
• Skeletal abnormalities may be mild (reduced amounts of normal collagen) or severe/lethal (abnormal polypeptide chains cannot form collagen triple helix); associated with short stature and increased fractures (hundreds of minor / major fractures during childhood, usually in lower limb, often involving growth plate fragmentation around knees)
• Blue sclera: due to translucent sclera and visualization of choroid
• Hearing loss: sensorineural defect and impeded conduction due to abnormalities of middle ear bones
• Dental imperfections: small, misshapen, blue - yellow teeth, due to dentin deficiency

Types
Type I:
* usually acquired mutation
* decreased synthesis of pro-alpha I chain
* abnormal pro-alpha I or pro-alpha 2 chain
* autosomal dominant
* normal lifespan with increased fractures during childhood but decreasing after puberty

Type II:
* usually autosomal recessive
* unstable triple helix
* abnormally short pro-alpha I chain
* abnormal or insufficient pro-alpha 2 chain
* uniformly fatal due to extraordinary bone fragility with multiple intrauterine fractures

Type III:
* autosomal dominant or recessive
* altered structure of pro-peptides of pro-alpha 2 chain
* impaired formation of triple helix
* growth retardation, but otherwise like type I

Type IV:
* autosomal dominant
* abnormally short pro-alpha 2 chain
* unstable triple helix
* short stature, but otherwise like type I

Types I and IV have A and B variants
B variants are associated with dental deformaties

Question 5

Osteopetrosis

Answer 5

• Also called marble bone disease
• Rare, hereditary, diffuse and symmetric skeletal sclerosis (increased bone density) caused by osteoclast dysfunction
• Bones have “stone-like” quality but are abnormality brittle and fracture like chalk
• One cause is deficiency of carbonic anhydrase II –> required by osteoclasts and renal tubular cells to excrete hydrogen ion –> deficiency causes failure to solubilize and resorb matrix and failure to acidify urine
• Associated with anemia and hepatosplenomegaly since reduced bone marrow
• Erienmeyer flask deformity –> lack a medullary canal, ends of long bones are bulbous
• Treatment is bone marrow transplant

Types:

“malignant”
* autosomal recessive
* detected in utero due to fractures, anemia, hydrocephaly, cranial nerve problems, infections, hepatosplenomegaly

“benign”
* autosomal dominant
* repeated fractures, mild cranial nerve deficits, anemia

Molecular:
* Albers-Schonberg disease –> mild autosomal dominant, mutation of CLCN7. Chloride channel.
* CAII –> Resessive. Carbonic acid. <5%. Severe.
* TCIRG1 –> Resessive. Proton pump. 60%. severe
* CLCN7 –> Resessive version. chloride channel. 15%. Severe
* gl/gl –> Resessive. <5%. Severe

Question 6

Osteopenia and Osteoporosis

Answer 6

1. Osteopenia
Definition: Osteopenia is a condition where bone mineral density (BMD) is lower than normal but not low enough to be classified as osteoporosis.

Bone Density: It is considered a “precursor” to osteoporosis. The bone density is between 1 and 2.5 standard deviations below the average for a healthy young adult.

Risk: People with osteopenia are at an increased risk of developing osteoporosis, but they are not as likely to experience fractures as those with osteoporosis.

Diagnosis: Diagnosed with a bone mineral density test (DEXA scan), where a T-score between -1.0 and -2.5 indicates osteopenia.

2. Osteoporosis
Definition: Osteoporosis is a more severe condition characterized by weakened bones that are more prone to fractures, even from minor falls or injuries.

Bone Density: In osteoporosis, bone density is 2.5 standard deviations or more below the average for a healthy young adult.

Risk: Osteoporosis significantly increases the risk of bone fractures, particularly in areas like the hips, spine, and wrists. Fractures can occur even with very little trauma.

Diagnosis: Diagnosed with a bone mineral density test (DEXA scan), where a T-score of -2.5 or lower indicates osteoporosis.

Causes and Risk Factors of Osteoporosis:
1. Age
Age-related Bone Loss: As we age, bone resorption (breakdown of bone tissue) outpaces bone formation, leading to a gradual decrease in bone density. This process accelerates after menopause in women and as men age beyond 70.
Menopause: For women, the drop in estrogen levels after menopause accelerates bone loss, contributing to osteoporosis –> worse in bones with increased surface area –> Vertebra

2. Gender
   Women are at Higher Risk: Women are more likely to develop osteoporosis due to hormonal changes after menopause, when estrogen levels decrease. Estrogen plays a critical role in maintaining bone density.
   Men: Though osteoporosis is less common in men, they are also at risk, especially as they age and their testosterone levels decline.
3. Family History and Genetics
   Genetic Factors: A family history of osteoporosis or fractures increases your risk. Specific genetic variations can affect how well your body builds bone and maintains bone density.
   Ethnicity: People of European and Asian descent are at higher risk for developing osteoporosis compared to those of African descent.
4. Hormonal Imbalances
   * Estrogen Deficiency (in Women): Estrogen is a key hormone for maintaining bone density. A decrease in estrogen levels, such as during menopause, significantly increases the risk of osteoporosis.
   * Testosterone Deficiency (in Men): Low testosterone levels in men also contribute to bone loss.
   * Thyroid Hormone Imbalances: Overproduction of thyroid hormones (hyperthyroidism) can lead to increased bone loss. Inappropriate thyroid hormone treatment (e.g., for hypothyroidism) can also have this effect.
   * Parathyroid Hormone (PTH) Overproduction: Hyperparathyroidism (overactive parathyroid glands) leads to increased bone resorption, resulting in decreased bone density.
5. Medical Conditions
   Certain chronic conditions can contribute to the development of osteoporosis:
   * Rheumatoid Arthritis:
   * Celiac Disease:
   * Chronic Kidney Disease:
   * Diabetes:
   * Multiple Myeloma:
   * Anorexia or Bulimia:
6. Medications
   Certain medications can increase the risk of osteoporosis by affecting bone metabolism:
   * Corticosteroids:
   * Anticonvulsants:
   * Proton Pump Inhibitors (PPIs):
   * Aromatase Inhibitors:
   * Certain Chemotherapy Drugs:
7. Nutritional Deficiencies
   * Calcium Deficiency:
   * Vitamin D Deficiency:
   * Malnutrition:
8. 8. Sedentary Lifestyle
      Physical Inactivity:
      Bed Rest:
9. Smoking
10. Alcohol Consumption
11. Low Body Weight

Question 7

Osteomalacia and Rickets

Answer 7

• Osteomalacia refers to the softening of the bones due to a deficiency of vitamin D, calcium, or phosphate.
• This leads to improper mineralization of bone tissue, causing the bones to become weak and prone to pain and fractures.
• Primarily caused by vitamin D deficiency, which impairs calcium absorption from the intestines and leads to abnormal bone mineralization.
• It can also be caused by calcium or phosphate deficiencies, or by other conditions affecting bone metabolism (e.g., chronic kidney disease).
• Rickets is the equivalent condition in children and is also caused by vitamin D deficiency.

Question 8

Brown tumor of bone

Answer 8

• Metabolic bone disease
• Manifestation of hyperparathyroidism
• Caused by increased osteoclastic activity and fibroblastic proliferation
• The term brown tumor derives from the color, which is caused by the vascularity, hemorrhage and deposits of hemosiderin
• Elevated blood calcium or parathyroid hormone levels
• Low serum phosphorus
• Radiologically osteolytic lesion
• Histologically giant cell rich lesion

Micro:
* Lobular pattern composed of groups and clusters of osteoclast-like multinucleated giant cells
* Vascular fibroblastic stroma
* Hemorrhage and hemosiderin deposits

Question 9

Paget disease of Bone

Answer 9

• Chronic disease of bones
• Episodes of increased bone resorption (osteoclast activity, osteolytic phase)
• Followed by excessive mixed osteoblast and osteoclast activity (mixed phase) leading to disordered, poorly formed bone with increased density (osteosclerotic phase) and increased likelihood of fractures
• Essentially, accelerated bone turnover making disordered bone
• Most are over age 55; rare before age 40
• Patients can present as polyostotic (multiple bones) or monostotic (one bone)
• Most common sites:
  Spine and pelvis (30 - 75%)
  Sacrum (30 - 60%)
  Skull (25 - 65%)
  Femur (25 - 25%)
• Multiple phases are present
• poorly formed bone
• increased osteoclast activity (bites in the bone)
• osteoblastic rimming

Diagnosis:
* Diagnosis is usually made radiographically
* Serum alkaline phosphatase (ALP) can be helpful if positive and used to monitor disease progression
* Increased urinary hydroxyproline

Radiology:
Early:
* radiolucency
* Well defined lytic lesion is called osteoporosis circumscripta and is fairly specific for Paget disease when occurring in the frontal bone

Late:
* increased bone density, cortical thickening with marked tunneling
* increased prominence of trabeculae, increased microfractures, loss of distinction between cortex and medulla
* may have sharp demarcation between normal and affected bone; may extend into soft tissue if florid disease
* Long bone can become bowed

Pathophysiology:
* Disease of osteoclasts
* Receptor activator of nuclear factor kappa Β ligand (RANKL), osteoclast differentiation
* Macrophage colony stimulating factor (MCSF), macrophage differentiation
* Nuclear factor kappa B (NFκB), apoptosis regulation
Osteoprotegrin, bone remodeling

Molecular:
* SQSTM1 mutations are the most common in familial Paget disease of bone
* TNFRSF11A gene encoding RANKL leads to increased osteoclast activity

Micro:
* Major findings are haphazard layers of lamellar bone with woven bone
* irregular cement lines between layers of lamellar bone
* thickened trabeculae with osteoblastic rimming and areas of bone resorption (bite marks) with abnormal osteoclasts
* peritrabecular fibrosis and increased vascularity of marrow

Early:
* primarily woven bone
* Focal mosaic pattern of lamellar bone, resembles jigsaw puzzle with prominent irregular cement lines
* Osteoclasts present at surface of bone with bite marks into the bone
* Osteoclasts are increased and abnormal with up to 100 nuclei
* peritrabecular fibrosis, increased vascularity

Middle phase:
* enlarged trabeculae with very irregular shapes and abnormal cement lines
* prominent osteoblast activity

Late:
* significantly decreased osteoblast and osteoclast activity
* thick trabeculae and thicker bones with mosaic pattern of cement lines
* fine fibrosis of marrow

All phases may be identified within a single specimen

Treatment:
* Bisphosphonates
* Calcitonin

Question 10

Fracture

Answer 10

• Fracture is a break in bone
• Open (or compound) fracture occurs when the bone breaks through the skin
• Closed (or simple) fracture occurs when the bone is broken but the skin remains intact

Other descriptions or types of fracture include:
* Displaced (ends of bone are not aligned)
* Greenstick (incomplete fracture where a portion of bone is broken, causing the other side to bend)
* Transverse (break in a straight line across the bone), commonly associated with Paget disease and osteopetrosis
* Spiral (break spirals around bone, common in twisting injury)
* Oblique (break is diagonal across bone)
* Compression (bone is crushed)
* Comminuted (bone broken in 3 or more places with many fragments present)
* Segmental (bone broken in 2 places with floating piece of bone)
* Avulsion (fragment of bone breaks where ligament or tendon inserts)

Healing:
* Fracture leads to disruption of blood vessels which causes hemorrhage
* Hematoma forms between 2 ends of bone, creating a fibrin mesh that seals the site
* Portions of bone and soft tissue with disrupted blood supply undergo necrosis

Stage 1: inflammation
* Tissue damage activates wound healing pathways
* Growth factors and cytokines (e.g., TGFβ, PDGF, FGF2, VEGF, M-CSF, IL1, IL6, BMPs, TNFα) are released
* Clot / thrombus formation, inflammatory cell recruitment, mesenchymal stem cell migration, granulation tissue formation and debris removal
* Dead bone resorption begins at 3 days

Stage 2: soft callus formation
* Periosteal pluripotent cells differentiate into chondroblasts
* Chondroblasts and fibroblasts create a semirigid soft callus, offering mechanical support and serving as a template
* Growth factors (e.g., TFG2, TFGβ3, PDGF, FGF1, IGF, BMP2, BMP4, BMP5, BMP6 and VEGF) play a role
* Early callus is reminiscent of disorganized fetal cartilage
* Periosteal inner layer promotes intramembranous bone growth

Stage 3: hard callus formation
* Elevated osteoblastic activity leads to new vessel formation and woven bone formation in the peripheral soft callus
* New bone replaces the soft callus and forms along lines of stress

Stage 4: remodeling
* Involves orderly bone resorption by osteoclasts and lamellar bone formation by osteoblasts
* RANK ligand signaling and cytokines play an important role

Question 11

Avascular necrosis

Answer 11

• Avascular necrosis (AVN) is a disease that results from temporary or permanent loss of blood supply to bone at watershed zones where collateral circulation is lacking
• Pathologically, AVN is characterized by death of bone cells and marrow elements and manifests as a localized area of bone infarction, usually at subchondral locations
• Slowly progressive groin pain, which may radiate to the buttock or knee and a limited range of motion
• Preiser disease: osteonecrosis of the scaphoid
• Keinbock disease: osteonecrosis of the lunate
• Legg-Calvé-Perthes disease: idiopathic osteonecrosis of femoral epiphysis in children
• Dysbaric osteonecrosis / caisson disease: osteonecrosis in undersea divers

Radiology:
* Plain Xray (insensitive in early stages)
* Patchy areas of lucency (bone resorption) and sclerosis (partly due to saponification of fatty acids released from necrotic adipocytes in the marrow)
* Sclerosis at the rim (new bone formation)
* Curvilinear subchondral lucency (crescent sign) indicative of subchondral fracture and impending articular collapse
* Double line sign on MRI corresponds to the inner granulation tissue zone and outer rim of reactive bone

Micro:
* Earliest histological sign: diffuse coagulative necrosis of marrow elements including hematopoietic cells, adipocytes and endothelial cells
* Later, purplish aggregates of amorphous material appear in the marrow representing fat saponification (due to binding of released fatty acids with calcium)
* Osteocyte necrosis in cancellous bone trabeculae
* Manifests initially as pyknotic nuclei and later as uniformly empty osteocyte lacunae
* Loss of osteocytes may not be complete until 2 - 4 weeks after the onset of ischemia
* Osteoblastic activity also evident with appositional bone formation seen over necrotic nonresorbed trabeculae (creeping substitution)
* Marrow has fat necrosis and calcium depositis –> more sensitive indicator of necrosis than bone.

Question 12

Ankylosing Spondylitis

Answer 12

• chronic inflammatory arthritis primarily affecting the spine
• form of spondyloarthritis (a group of inflammatory diseases that affect the spine and pelvis)
• Over time, this inflammation can cause the affected bones to fuse together, leading to stiffness and loss of mobility.

Causes and Risk Factors:
Genetics:
* The most significant genetic risk factor for AS is the presence of the HLA-B27 gene.
* However, not everyone with this gene will develop AS, and many people with AS do not have this gene.

Age:
* AS typically begins in young adulthood, often between the ages of 15 and 30.
* It is rare in older adults.

Gender:
* Men are more likely to develop AS than women, and they often experience more severe symptoms.

Family History:
* A family history of ankylosing spondylitis or other related conditions (e.g., psoriatic arthritis, reactive arthritis) increases the risk of developing AS.

Question 13

Osteomyelitis

Answer 13

• Classified based on:
• Mechanism of infection (Hematogenous vs non-hematogenous)
• Duration of illness (acute, subacute, chronic)

Infectious agents:
* Staph aureus
* Coagulase gram negative bacilli
* Group A streptococci
* Strep Pneumoniae
* E Coli
* Pseudomonas
* Klebsiella
* Salmonella –> in sickle cell

Clinic:
* Gradual onset of symptoms over several days
* Dull pain at the involved site
* Tenderness, warmth, erythema and swelling
* Fever and rigors may also be present
* Leukocytosis on complete blood count
* Elevated inflammatory markers: erythrocyte sedimentation rate (ESR) and C reactive protein (CRP)
* Blood cultures are positive in 50 - 60% of cases
* Bone aspirate cultures may be positive when blood cultures are negative

Sequestrum
* dead piece of bone
* gradually separated from living bone by granulation tissue

Involucrum:
* Sleeve of living tissue created by periosteum
* deposited around sequestrum

Micro:
Patterns of acute osteomyelitis

Osseous changes:
* Osteonecrosis: bone trabeculae with visually empty osteocyte cavities are detectable as a criterion for necrotic bone tissue especially with EDTA decalcification
* The bone trabeculae have irregular contours and are fragmented
* They may be fractured and completely necrotic (so called bone sequester)
* There are intramedullary granulocyte infiltrates and fibrin exudates
In bone tissue with a haemopoietic function (e.g. axial skeleton) there is a reduced or complete lack of haemopoiesis

Soft tissue changes:
* Soft tissue necrosis: criteria for soft tissue necrosis are apoptoses, a tissue eosinophilia, fibrin exudations and a confining texture of the tissue

Inflammatory infiltrate pattern:
* Neutrophilic granulocyte infiltrate: diffuse and grouped deposits (so called microabscesses, ≥ 5 granulocytes) of segmented neutrophilic granulocytes in the usually highly oedematous medullary spaces
* The neutrophilic granulocytes are PAS cytoplasmic
* CD15 positivity

Patterns of chronic osteomyelitis

Osseous changes:
* Bone neogenesis:
* Medullary space fibrosis with ectatic sinus
* The medullary space tissue shows fibrosing with granulation tissue formation

Soft tissue changes:
* there is fibrosing with granulation tissue formation

Inflammatory infiltrate pattern:
* Lymphocyte / macrophage / plasma cell infiltrate