Page 507 - First Aid for the USMLE Step 1 2020, Thirtieth edition [MedicalBooksVN.com]_Neat

Musculoskeletal, skin, and connective tissue ` pathology Musculoskeletal, skin, and connective tissue ` pathology section iii 463

Osteopetrosis Failure of normal bone resorption due to defective osteoclasts Ž thickened, dense bones that are
A prone to fracture. Mutations (eg, carbonic anhydrase II) impair ability of osteoclast to generate
acidic environment necessary for bone resorption. Overgrowth of cortical bone fills marrow space
Ž pancytopenia, extramedullary hematopoiesis. Can result in cranial nerve impingement and
palsies due to narrowed foramina.
X-rays show diffuse symmetric sclerosis (bone-in-bone, “stone bone” A ). Bone marrow transplant is
potentially curative as osteoclasts are derived from monocytes.

Osteomalacia/rickets Defective mineralization of osteoid B
(osteomalacia) or cartilaginous growth plates
A
(rickets, only in children). Most commonly due
to vitamin D deficiency.
X-rays show osteopenia and “Looser zones”
(pseudofractures) in osteomalacia, epiphyseal
widening and metaphyseal cupping/fraying in
rickets. Children with rickets have pathologic
bow legs (genu varum A ), bead-like
costochondral junctions (rachitic rosary B ),
craniotabes (soft skull).
 vitamin D Ž  serum Ca Ž  PTH secretion
2+
Ž  serum PO .
3−
4
Hyperactivity of osteoblasts Ž  ALP.

Osteitis deformans Also called Paget disease of bone. Common, Hat size can be increased due to skull
localized disorder of bone remodeling thickening A ; hearing loss is common due to
A
caused by  osteoclastic activity followed by auditory foramen narrowing.
 osteoblastic activity that forms poor-quality Stages of Paget disease:
bone. Serum Ca , phosphorus, and PTH ƒ Lytic—osteoclasts
2+
levels are normal.  ALP. Mosaic pattern of ƒ Mixed—osteoclasts + osteoblasts
woven and lamellar bone (osteocytes within ƒ Sclerotic—osteoblasts
lacunae in chaotic juxtapositions); long bone ƒ Quiescent—minimal osteoclast/osteoblast
chalk-stick fractures.  blood flow from  activity
arteriovenous shunts may cause high-output Treatment: bisphosphonates.
heart failure.  risk of osteosarcoma.

Avascular necrosis of Infarction of bone and marrow, usually very Branch of Watershed
bone painful. Most common site is femoral obturator artery zone (infarcted)
A head (watershed zone) A (due to insufficiency
of medial circumflex femoral artery). Causes
include Corticosteroids, Alcoholism, Sickle Medial femoral
circumflex
cell disease, Trauma, SLE, “the Bends” artery (posterior)
(caisson/decompression disease), LEgg-Calvé- Lateral femoral
Perthes disease (idiopathic), Gaucher disease, circumflex
Slipped capital femoral epiphysis—CASTS artery (anterior)
Bend LEGS.

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