Page 850 - Textbook of Pathology, 6th Edition

834 MORPHOLOGIC FEATURES. There are pathological Osteogenesis Imperfecta
fractures of the involved bone due to infarcts. Most Osteogenesis imperfecta is an autosomal dominant or reces-
common sites are the ones where the disruption in blood sive disorder of synthesis of type I collagen that constitutes
supply is at end-arterial circulation. The infarcts mainly 90-95% of bone matrix. The disorder, thus, involves not only
involve the medulla of the long bone in the diaphysis. This the skeleton but other extra-skeletal tissues as well
is because the nutrient arteries supply blood to sinusoids containing type I collagen such as sclera, eyes, joints,
of the medulla and the inner cortex after penetrating the ligaments, teeth and skin. The skeletal manifestations of
cortex, while the cortex is relatively unaffected due to osteogenesis imperfecta are due to defective osteoblasts
collateral circulation. which normally synthesise type I collagen. This results in
Grossly, the lesional area shows a wedge-shaped area of thin or non-existent cortices and irregular trabeculae (too
infarction in the subchondral bone under the convex little bone) so that the bones are very fragile and liable to
surface of the joint. multiple fractures. The growth plate cartilage is, however,
Microscopically, the infracted medulla shows saponified normal. The condition may be evident at birth (osteogenesis
marrow fat. The overlying cartilage and the cortex of the imperfecta congenita) when it is more severe, or may appear
long bones are relatively unaffected. during adolescence (osteogenesis imperfecta tarda) which
Longterm sequelae of osteonecrosis include occurrence is a less incapacitating form. Extraskeletal lesions of osteo-
of malignant tumours in this location such as osteosarcoma, genesis imperfecta include blue and translucent sclerae,
malignant fibrous histiocytoma and fibrosarcoma etc. hearing loss due to bony abnormalities of the middle and
inner ear, and imperfect teeth.
FRACTURE HEALING
Osteopetrosis
Fracture of the bone initiates a series of tissue changes which
eventually lead to restoration of normal structure and Osteopetrosis, also called marble bone disease, is an autosomal
function of the affected bone. Fracture of a bone is commonly dominant or recessive disorder of increased skeletal mass or
associated with injury to the soft tissues. The various types osteosclerosis caused by a hereditary defect in osteoclast
of fractures and their mechanism of healing are discussed function. The condition may appear in 2 forms: autosomal
along with healing of specialised tissues in Chapter 6 recessive (malignant infantile form) and autosomal dominant
(page 171). (benign adult form). Failure of normal osteoclast function of
bone resorption coupled with continued bone formation and
SECTION III
DISORDERS OF BONE GROWTH AND DEVELOPMENT endochondral ossification results in net overgrowth of
(SKELETAL DYSPLASIAS) calcified dense bone (too much bone) which occupies most of
the available marrow space. Despite increased density of the
A number of abnormalities of the skeleton are due to dis- bone, there is poor structural support so that the skeleton is
ordered bone growth and development and are collectively susceptible to fractures. Besides the skeletal abnormalities,
termed skeletal dysplasias. These include both local and the infantile malignant form is characterised by effects of
systemic disorders. marrow obliteration such as anaemia, neutropenia, thrombo-
Local defects involve a single bone or a group of bones cytopenia, hepatosplenomegaly with extramedullary
such as: absence or presence in diminished form, fused with haematopoiesis, hydrocephalus and neurologic involvement
neighbouring bones (e.g. syndactyly), and formation of extra with consequent deafness, optic atrophy and blindness.
bones (e.g. supernumerary ribs). Metabolically, hypocalcaemia occurs due to defective
Systemic Pathology
However, more importantly, skeletal dysplasias include osteoclast function.
systemic disorders involving particular epiphyseal growth Histologically, the number of osteoclasts is increased
plate. These include: achondroplasia (disorder of which have dysplastic, bizarre and irregular nuclei and
chondroblasts), osteogenesis imperfecta (disorder of are dysfunctional.
osteoblasts), osteopetrosis (disorder of osteoclasts) and foetal
rickets (disorder of mineralisation). Though multiple METABOLIC AND ENDOCRINE BONE DISEASES
exostoses (osteochondromas) is a hereditary lesion, it is
described later along with solitary sporadic exostosis on page A large number of metabolic and endocrine disorders
843. produce generalised skeletal disorders. These include the
following:
Achondroplasia 1. Osteoporosis—Resulting from quantitative reduction in
otherwise normal bone.
Achondroplasia is an autosomal dominant genetic abnor-
mality. There is selective interference with normal endo- 2. Osteomalacia and rickets—Characterised by qualitative
chondral ossification at the level of epiphyseal cartilaginous abnormality in the form of impaired bone mineralisation due
growth plates of long bones. Thus, the long bones are to deficiency of vitamin D in adults and children respectively
abnormally short but the skull grows normally leading to (page 249).
relatively large skull. Achondroplasia is the commonest 3. Scurvy—Caused by deficiency of vitamin C resulting in
cause of dwarfism. subperiosteal haemorrhages (page 251).