1116
U N I T 1 2
Musculoskeletal Function
fractures in the weakened bones, especially those in
the distal radius and the proximal femur. In contrast
to osteoporosis, it is not a significant cause of hip frac-
tures. There may be delayed healing and poor retention
of internal fixation devices. Osteomalacia usually is
accompanied by a compensatory or secondary hyper-
parathyroidism stimulated by low serum calcium lev-
els. Parathyroid hormone reduces renal absorption of
phosphate and removes calcium from the bone. Serum
calcium levels are only slightly reduced in osteomalacia.
Diagnostic measures are directed toward identify-
ing osteomalacia and establishing its cause. Diagnostic
methods include x-ray studies, laboratory tests, bone
scan, and bone biopsy. X-ray findings typical of osteo-
malacia are the development of transverse lines or pseu-
dofractures called
Looser zones.
These apparently are
caused by stress fractures that are inadequately healed
or by the mechanical inadequacy of penetrating nutri-
ent vessels.
4
A bone biopsy may be done to confirm the
diagnosis of osteomalacia in a person with nonspecific
osteopenia who shows no improvement after treatment
with exercise, vitamin D, and calcium.
The treatment of osteomalacia is directed at the
underlying cause. If the problem is nutritional, restor-
ing adequate amounts of calcium and vitamin D to the
diet may be sufficient. The elderly and persons with an
inadequate dietary intake or sunlight exposure may
require supplemental vitamin D. Supplemental vitamin
D is specific for vitamin D–resistant rickets, but large
doses usually are needed to overcome the resistance to
calcium absorption action and to prevent renal loss of
phosphate. The biologically active forms of vitamin D,
25-OH vitamin D (calciferol) or 1,25-(OH)
2
vitamin
D (calcitriol), are available for use in the treatment of
osteomalacia resistant to vitamin D (i.e., osteomalacia
resulting from chronic liver disease and kidney failure). If
osteomalacia is caused by malabsorption, the treatment
is directed toward correcting the primary disease. For
example, adequate replacement of pancreatic enzymes is
of paramount importance in pancreatic insufficiency. In
renal tubular disorders, the treatment is directed at the
altered renal physiology.
Rickets
Rickets is a metabolic bone disorder characterized by
a failure or delay in calcification of the cartilaginous
growth plate in children whose epiphyses have not yet
fused.
28–30
It is also manifested by widening and defor-
mation of the metaphyseal regions of long bones and a
delay in the mineralization of trabecular, endosteal, and
periosteal bone surfaces. Rickets is not a reportable dis-
ease; however, case reports and hospital records suggest
that, in the United States, its incidence has been rising
during the past three decades.
31
There are several forms of rickets, including nutri-
tional rickets, vitamin D–dependent rickets, and vitamin
D–resistant rickets. As with osteomalacia in the adult,
rickets can result from kidney failure; malabsorptive
syndromes such as celiac disease and cystic fibrosis; and
medications such as anticonvulsants, which impair the
activation of vitamin D in the liver; and aluminum-con-
taining antacids, which bind phosphorus and prevent its
absorption.
Etiology and Pathogenesis.
Nutritional rickets results
from inadequate sunlight exposure or inadequate intake
of vitamin D, calcium, or phosphate. Although uncom-
mon in developed countries of the world, rickets still
can occur, particularly when an infant is solely breast
fed, is dark skinned, or has limited exposure to sunlight.
Modern conditions of dress, lifestyle, and recommenda-
tions regarding sun avoidance to reduce skin cancer risk
may prevent a large proportion of the population from
producing adequate amounts of vitamin D. In 2003,
the American Academy of Pediatrics recommended a
vitamin D supplement for breast-fed infants who do
not consume at least 500 mL/day of vitamin D–fortified
formula/beverages and non–breast-fed infants who do
not consume greater than 500 mL of vitamin D–forti-
fied beverages.
31
The supplementation should start dur-
ing the first 2 months of life and continue throughout
childhood and adolescence.
Vitamin D–dependent rickets can result from abnor-
malities in the gene coding for the enzyme that con-
verts inactive vitamin D to the active form of vitamin
D, or from an autosomal disorder caused by mutations
in the vitamin D receptor.
29
Vitamin D–resistant rick-
ets involves hypophosphatemia or a decrease in serum
phosphate levels, the most common form being caused
by mutations of the phosphate-regulating gene on the
X chromosome.
29
The gene mutation causes renal wast-
ing of phosphate at the proximal tubular level of the
kidney.
Clinical Features.
Rickets is characterized by changes
in the growing bones of children with overgrowth of the
epiphyseal cartilage due to inadequate provisional calci-
fication and failure of the cartilage cells to disintegrate.
Bones become deformed; ossification at the epiphyseal
plates is delayed and disordered, resulting in widening of
the epiphyseal cartilage plate. Any new bone that does
grow is unmineralized. The conformation of the gross
skeletal changes depends on the severity and duration
of the rachitic process, and in particular on the stresses
to which the individual bones are subjected. During the
nonmobile stage of infancy, the head and chest undergo
the greatest stresses. The skull is enlarged and soft, clo-
sure of the fontanels is delayed, and the teeth are slow
to develop. When an ambulating child develops rickets,
deformities are likely to affect the spine, pelvis, and long
bones (i.e., tibia), causing, most notably, lumbar lordo-
sis and bowing of the legs (Fig. 44-3). The ends of long
bones and ribs are enlarged. The thorax may be abnor-
mally shaped, with prominent rib cartilage (i.e., rachitic
rosary). The child usually has stunted growth, with a
height sometimes far below the normal range. Weight
often is not affected so that the children, many of whom
present with a protruding abdomen (i.e., rachitic pot-
belly), have been described as presenting a Buddha-like
appearance when sitting.
