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1 Basic Science ofBone andCartilage Metabolism
ab
19
Fig. 1.28 Radiographs demonstrating the deformed
femur in a patient with osteogenesis imperfecta. Note the
typical slender bones excessively thin cortices (a, b).
Additionally, note the postoperative images after con-
material then “piles up” in the skeleton, making it
appear very dense radiographically (Fig.1.29).
Despite the fact that the bones look extremely
dense and, indeed, lack a medullary canal, they
are biomechanically very weak. This results in
frequent pathologic fractures. An additional complication is the displacement of marrow elements
from the long bones. This results in a myelophthisic anemia (pancytopenia), and in turn generates extramedullary hematopoiesis. Thus,
hepatosplenomegaly as well as a prominent forehead are usually seen in these patients.
Paget’s Disease
Sir James Paget described a syndrome of
unknown etiology that bears his name. The initial
description referred to the condition as “osteitis
deformans,” with increased osteoclastic bone
resorption contributing to abnormal bone remodeling (Fig.1.30). The syndrome is most common
trolled osteotomies to realign the bones, (c, d) stabilized
with telescoping intramedullary rods that allow for continued bone growth. (From Lovell and Winter’s Pediatric
Orthopaedics, Chapter 6, Figure9)
in individuals of European descent and in patients
typically over the age of 55.
There is strong evidence pointing to a slow
virus (paramyxovirus or respiratory syncytial
virus) as the cause of Paget’s disease. In the setting of increased osteoclastic resorption, bone
formation and bone resorption dramatically
increase. The two processes occur alternately
rather than simultaneously in any given bone,
with the net effect being bones of increased
density with marked trabecular thickening
(Fig.1.31).
The skull, pelvis, spine, tibia, and femur are
the favorite targets of this process. Sadly, and not
unlike osteopetrosis, the pagetic bones are
mechanically weak, making pathologic fracture a
frequent complication. Despite the presence of
abundant quantities of bone, it is poorly formed
and the mineral and matrix are poorly integrated.
Bone pain, spinal stenosis, and hearing decit
(resulting from impingement of disorganized,
overgrown bone in the skull on the eighth cranial
20
M. J. Kelly and J. N. Delahay
Fig. 1.29 Osteopetrosis. As
can be imagined, when the same
pathologic changes of
hyperdense bone occur in the
long bones (a), there is an
almost complete absence of
marrow cavity (b),
unsurprisingly contributing to
the nding of pancytopenia.
(From Lovell and Winter’s
Pediatric Orthopaedics, Chapter
6, Figure21A and B)
a b
Fig. 1.30 Paget’s disease of bone. Microscopic view
showing disorganized bony trabeculae with variable
thickness and brotic bone marrow. (From Practical
Orthopedic Pathology: A Diagnostic Approach, 2015
Elsevier, Deyrup and Siegal, Figure16-8)
nerve) are frequent problems in these patients.
Additionally, it is now well described that these
patients are at increased risk for secondary
Pagetoid osteosarcomas, and while only occurring in less than 1% of Pagetoid patients, the
prognosis associated with this condition is
dismal.
Several different therapeutic approaches have
been attempted. Currently, bisphosphonates are
the frequently employed therapeutic agent. Much
like in osteoporosis, they are aimed at osteoclast
inhibition, used in an attempt to inhibit bone
resorption and also to a lesser degree to block
bone mineralization. The rationale again is to
“freeze the skeleton,” and thereby decrease bone
turnover. Cyclic treatment regimens allow new
bone to become mineralized while decreasing the
osteoclastic activity. The serum alkaline phosphatase level provides a reliable way of monitoring the response to treatment, since it is elevated
in the presence of active bone turnover. After
bisphosphonates, calcitonin is another treatment
alternative, working through its direct effect on
osteoclast cells. Of note, teriparatide, a PTH analogue, is actually contraindicated in Paget’s disease, due to an associated increased risk of
Pagetoid osteosarcoma mentioned above noted in
animal studies.
1 Basic Science ofBone andCartilage Metabolism
21
Fig. 1.31 Paget’s disease of bone. X-ray showing dense
sclerotic changes with areas of osteolysis. Clearly, through
this disease bone, the patient has fractured their hip.
(From Practical Orthopedic Pathology: A Diagnostic
Approach, 2015 Elsevier, Deyrup and Siegal, Figure16-7)
Arthritis
It is important to recall that a diarthrodial joint
includes three tissues: bone, cartilage, and
synovium. Each of the arthritic diseases tends to
impact one of these tissues, with changes in the
other two resulting as secondary phenomena. The
radiographic and microscopic changes encountered represent a composite of the result of the
initial injury and the organism’s attempt at repair
of that injury.
Noninammatory Arthritis:
Osteoarthritis
The most common overall, osteoarthritis can be
primary or secondary, if one considers the degenerative joint disease that can follow trauma or
Fig. 1.32 Osteoarthritis of the knee with radiographic
evidence of three of the four cardinal ndings seen most
evidently in the medial femorotibial compartment—narrowing of the joint space, osteophyte formation, and subchondral sclerosis. The fourth ndings, cysts, are not
clearly seen. (From Orthopedic Surgery: Principles of
Diagnosis and Treatment, Figure18.7)
other primary events. The process itself targets
the articular cartilage. Whether the initial event is
mechanical or biochemical remains controversial, however, recent data suggests an increase in
proteolytic enzymes and inammatory cytokines
as the primary driving force resulting is progressive damage to the articular surface. The secondary bone changes that occur are reparative in
nature. Joint space narrowing, subchondral
sclerosis, osteophytes, and subchondral cysts,
therefore, are the four classic radiographic
changes (Fig.1.32).
Since this is most typically a disease of
weight-bearing joints, the hip and knee are the
joints that usually require orthopedic care. That
said, with aging of the population, and newer
technological advancements, osteoarthritis of the
shoulder, elbow, ankle and even wrist have gained
more attention. Total joint arthroplasty has
22
Fig. 1.33 Rheumatoid arthritis in the knee. Note the
symmetric joint space narrowing, generalized osteopenia
on both sides of the joint, and an absence of osteophytes.
(From Orthopedic Surgery: Principles of Diagnosis and
Treatment, Figure18-10)
become the mainstay of surgical management in
these patients, producing reliable long-term
results.
Inammatory Arthritis: Rheumatoid
Arthritis
Rheumatoid arthritis targets the synovial membrane as the site for the immunologic process that
is the root mechanism of this disease. Driven by a
cell-mediated immune response, the synovium
sees microvascular proliferation, and becomes
hyperplastic and hypertrophic. The thickened
synovium (now referred to as a pannus) rst
destroys the articular cartilage by enzymatic degradation, and follows with destruction of the
underlying bone by pressure necrosis and erosion. Unlike osteoarthritis, repair changes are, for
M. J. Kelly and J. N. Delahay
Fig. 1.34 Radiograph of both hands of a patient with
long-standing Rheumatoid arthritis. Osteoporosis in all
bones is marked. The wrist joints show advanced destruction. There is dislocation of the metacarpophalangeal
joints of all ngers. (From Bogumill GP. Orthopaedic
Pathology: A Synopsis with Clinical Radiographic
Correlation. Philadelphia, PA: Saunders; 1984. Reprinted
with permission)
the most part, abortive. The radiograph reects
this overall atrophic process. Soft tissue swelling,
osteopenia on both sides of the joint (periarticular osteopenia), and bone erosions are the standard ndings (Fig.1.33).
Joint destruction is generally symmetric and
much more global than with osteoarthritis, with
severe joint space narrowing, erosive changes,
and deformity, classically in the hands (Fig.1.34).
While in the recent past extensive alterations in
normal anatomy usually necessitated multiple
joint arthroplasties over the patient’s lifetime,
incredible advances in the pharmacologic treatment of rheumatoid arthritis with the disease
modifying anti-rheumatic drugs has greatly
decreased the need for surgical intervention in
this patient population.
Metabolic Arthritides: Crystalline Arthropathy
The common denominator of the metabolic
arthritides is the deposition of crystals or metabolic byproducts in or around joints. Destructive
changes in these joints necessitate rheumatologic
and frequently orthopedic care.
ab
1 Basic Science ofBone andCartilage Metabolism
23
Gout
In gout, monosodium urate crystals are deposited
in and around the joints. Finding these crystals in
joint uid is the diagnostic sine qua non of this
metabolic imbalance. An intense chemical synovitis and bony erosions can occur. Typically, the
rst metatarsophalangeal joint is the classic site,
but certainly the process can present in any joint,
including the spine. The rapid onset and signs of
acute inammation should suggest the diagnosis,
which is best conrmed by arthrocentesis. The
nding of needle-like, negatively birefringent
crystals under polarized light conrms the diagnosis. The treatment is usually medical, typically
with anti-inammatory drugs in the acute setting,
or uric acid-reducing medications (allopurinol,
colchicine) on a chronic basis.
Pseudogout
Pseudogout is one of the many causes of chondrocalcinosis (simply, calcication of cartilage) and
should not be considered synonymous with it. The
presence of weakly positively birefringent crystals,
rhomboid in shape, attests to the diagnosis. These
calcium pyrophosphate dihydrate crystals are radiopaque and, as such, can be viewed on standard
radiographs as calcication of brocartilage,
including the menisci of the knee or the triangular
brocartilage complex of the wrist (Fig.1.35).
Similar to gout, treatment frequently revolves
around anti-inammatory drugs, colchicine, or
intra-articular steroid injections.
Ochronosis
Ochronosis is a metabolic arthropathy resulting
from a rare inborn error of metabolism. The specic metabolic error is an absence of homogentisic acid oxidase, which subsequently results in
an accumulation of homogentisic acid intraarticularly. This pathological by-product targets
the articular cartilage for its deposition, which
gets stiffened and loses its resiliency in its presence. The net result is ssuring and brillation of
the articular surface, changes that radiographically and pathologically mimic osteoarthritis.
The unique feature of this condition is the pigment associated with the by-product that stains
the cartilage black, thereby accounting for the
blackish tinge of the earlobes and the tips of the
nose seen in these patients (Fig.1.36).
At this time, no primary treatment options for
this condition exist, and these patients are treated
symptomatically.
Fig. 1.35 (a, b) X-ray
of the knee in a patient
with pseudogout crystals
on aspiration. Note the
chondrocalcinosis due to
deposition of calcium in
the menisci, made of
brocartilage. (From
Orthopedic Imaging: A
Practical Approach. 7E.
2021. Chapter 15,
Figure15.44)
24
Fig. 1.36 Gross spinal pathology specimen in a patient
with ochronosis, showing black pigmentation within the
vertebral disc spaces, which are narrowed. (From
Orthopedic Imaging: A Practical Approach. 7E. 2021.
Chapter 15, Figure15.60)
M. J. Kelly and J. N. Delahay
Vascular Disease
This diagnostic category is a somewhat diverse
grouping of clinical entities that are best considered under this heading lest they be overlooked.
Circulatory Disease: Avascular Necrosis
Afictions of the vascular tree, especially the arterial side, tend to produce similar lesions in bone,
despite the etiology. Bone deprived of a portion of
its blood supply becomes necrotic, like all other tissues (Fig.1.37). This disease process is referred to
as osteonecrosis, or avascular necrosis (AVN).
Depending on the extent of the vascular involvement, the infarcts can range from small areas of
bony necrosis in the metaphysis, which are clinically inconsequential, to extensive involvement at
the ends of the long bones, precipitating signicant
degenerative joint disease (Figs.1.38 and 1.39).
The radiographic appearance of dead bone is
essentially that of sclerosis. In truth, the dead
tissue is incapable of changing its density since no
viable cells exist. Rather, the viable bone adjacent
to the necrotic segment develops a reactive hyper-
ab c
Fig. 1.37 Avascular necrosis of the femoral head, a gross
specimen. Note the well demarcated necrotic wedge adjacent to the articular surface. (a) The articular cartilage is
intact, and remains convex with normal shape. In (b), the
bony architecture remains intact. The pathology specimen
(c) shows infarcted, necrotic bone without osteoblastic or
osteoclastic activity. (From Orthopedic Imaging: A
Practical Approach. 7E. 2021. Chapter 4, Figure4.86)
1 Basic Science ofBone andCartilage Metabolism
25
Fig. 1.40 Avascular necrosis of the femoral head in a
45-year-old woman who had sustained a traumatic hip
dislocation a few weeks prior. Note the clear crescent sign
(black arrow) which depicts a cleft beneath the articular
cartilage resulting from compression fractures of dead trabeculae. (From Orthopedic Imaging: A Practical
Approach. 7E. 2021. Chapter 4, Figure4.90)
Fig. 1.38 Radiograph of the proximal humerus in a
patient with a history of deep sea diving. The sclerotic
area represents infarction of the marrow cavity with the
formation of calcium soaps and new bone from the reparative margins. Although apparent radiographically, these
lesions were clinically inconsequential. (From Bogumill
GP. Orthopaedic Pathology: A Synopsis with Clinical
Radiographic Correlation. Philadelphia, PA: Saunders;
1984. Reprinted with permission)
Fig. 1.39 Advanced, severe degenerative joint disease in
bilateral hips secondary to avascular necrosis of the femoral heads in a young adult patient with a history of previous bilateral traumatic hip dislocations. (From Orthopedic
Imaging: A Practical Approach. 7E. 2021. Chapter 4,
Figure4.93)
emia and resorbs. The area of necrosis then
appears to be more dense on the radiograph—socalled relative radiodensity. There is also some
compaction of dead trabeculae, as well as marrow
necrosis with subsequent saponication and calcication of the dead fat, which additionally
explains the sclerotic changes seen on radiographs.
A number of vaso-occlusive phenomena can
cause AVN.Although AVN can involve any number of different sites, the femoral head is by far
the most typical (Fig.1.40).
Etiologies of AVN can be grouped by
causation:
1. Trauma: damage to vessels supplying the seg-
ment of bone in question (i.e., fractures of the
femoral neck and scaphoid).
2. Occlusive phenomena:
(a) Emboli: fat in alcoholism and pancreatitis
or nitrogen bubbles in Caisson’s disease
26
M. J. Kelly and J. N. Delahay
(b) Stasis: coagulopathies and hemoglobin-
opathies
(c) External constriction: vasculitis (i.e., sys-
temic lupus erythematosus), inammatory bowel disease
(d) External compression: lysosomal storage
diseases (i.e., Gaucher’s and Fabry’s),
where stored material compresses intraosseous arterioles
3. Medications: antiretroviral agents for treatment of HIV (i.e., protease inhibitors)
4. Idiopathic (causative factor is unknown):
steroid- induced AVN and Chandler’s disease
Hematologic Syndromes
The genetic hemoglobinopathies, although not
truly circulatory diseases, are best remembered in
this group. Sickle cell disease and to a lesser
degree thalassemia produce skeletal changes primarily through two mechanisms: myeloid hyperplasia and vaso-occlusive phenomena. Because
of the anemia these patients suffer, there is a drive
to increase medullary hematopoiesis, and this
results in the dilation of bony contours to accommodate a marrow driven to produce more blood.
Widening of the diploe of the skull, dilation of
the small bones of the hands and feet, and
increased trabecular markings are all radiographic hallmarks of this process. The vaso-
occlusive effect of these distorted red cells causes
bone infarcts similar to those previously discussed. However, in a select group of patients,
the infarcts are frequently painful and a component of the “painful crisis.” The stasis, sludging,
and necrotic bone creates a comfortable environment for bacterial invasion, accounting for the
increased incidence of osteomyelitis in these
patients.
Hemophilia is a congenital bleeding disorder
due to a deciency in a necessary clotting factor.
Hemophilia A is due to a deciency in antihemophilic factor VII, while hemophilia B is due to a
deciency in plasma thromboplastin, or factor
IX.These patients present with excessive bleeding into the joints, most commonly the knee and
elbow. These recurrent hemarthroses eventually
lead to hemophilic arthropathy, which tends to
mimic rheumatoid arthropathy with diffuse, erosive changes (Fig.1.41).
Separate from the arthritic changes incurred,
this disorder carries with it a high risk of developing muscle hematomas, which can precipitate
compartment syndrome when occurring in
enclosed spaces. When this hemorrhage occurs
into the iliopsoas muscle, which resides alongside the femoral nerve in the stout iliopsoas
sheath, hip pain with an associated femoral
nerve palsy is the result. Administration of the
decient clotting factor is the mainstay of
treatment.
1 Basic Science ofBone andCartilage Metabolism
27
Fig. 1.41 Hemophilic
arthropathy of the knee
and elbow, resulting
from recurrent
intra-articular bleeding
episodes in one’s
lifetime. (From
Orthopedic Imaging: A
Practical Approach. 7E.
2021. Chapter 15,
Figure15.72)
a
b
cd
Neurodevelopmental Disorders
The nal diagnostic category discussed in this
chapter may be the most heterogeneous of all.
However, there exists a common theme that ties
this eclectic mix of clinical states together—the
end result of their pathologies precipitates muscle imbalance and resultant musculoskeletal
deformity. An attempt is made to describe them
generically and use examples from each category
to underscore their impact on the skeleton.
Neurologic Diseases
The decit produced by neurologic diseases can be
either sensory, motor, or central in origin. The level
of involvement will determine the skeletal changes.
Central nervous system decits are typied by
cerebral palsy. Most commonly caused by prematurity or prenatal anoxia, resulting damage to
the cerebral cortex of the newborn leads to damage to neural tissue that normally inhibits or
damps muscular tone. Without normal inhibitory
inuences, these muscles become spastic. While
the associated encephalopathy remains static in
nature, muscle spasticity (which on the other
hand, is quite dynamic) existing over a protracted
period results in muscle imbalance around joints.
Ultimately, contractures and chronic joint deformities, such as subluxations and dislocations,
will follow. The hip, for example, is of particular
concern in the spastic child (Fig.1.42).
Poliomyelitis is an example of a motor deficit disease. Viral damage to the anterior horn
cells of the spinal cord and brainstem results
28
Fig. 1.42 Pelvis X-ray in a child with spastic cerebral
palsy. Clearly, the right proximal femur is subluxated
from the hip joint, and the femoral head is poorly covered
by the malformed acetabulum. (From Orthopedic Surgery:
Principles of Diagnosis and Treatment, Figure11.17)
in focal motor weakness in various muscle
groups in the extremities. Sensation, however,
is maintained. Unfortunately, bone deprived of
normal muscle loading tends to become
osteopenic. In addition, the variable nature of
the involvement causes muscle imbalance
around joints, with resultant deformities of
bone and joint.
Sensory decits may result in neuropathic
arthritis. Joints deprived of proprioception (a
sense of awareness of the position of the body in
space) are rapidly destroyed (Fig.1.43).
The aggressive sequence of microtrauma,
recurrent effusions, ligamentous incompetence, articular damage, and severe degenerative joint disease is the fate of patients with
tertiary syphilis, diabetes, pernicious anemia,
leprosy, and heavy metal intoxications. When
occurring in the shoulder, specically, one
must also consider cervical syringomyelia as
the primary cause. Although proprioception is
the initial sensory component lost, pain ber
decit usually follows, resulting in destroyed,
but painless joints.
Spina bida, or myelodysplasia, results from
the failure of closure of the fetal spinal cord, and
results in mixed decits. This congenital defect
combines motor and sensory decits to produce
skeletal changes that parallel both. Osteopenia
M. J. Kelly and J. N. Delahay
Fig. 1.43 X-ray of the left shoulder in a patient with a
cervical syrinx, with destruction, dislocation, periarticular
ossication, and partial resorption of the humeral head
and glenoid fossa. This is a painless, destroyed joint.
(From Neuropathic arthropathy caused by syringomyelia,
a Journal of Neurosurgery Clinical Article)
and joint deformity culminate in the orthopedic
manifestations not limited to long bone fractures, spinal deformity, dysplastic hip joints,
knee and foot deformities, and advanced joint
destruction. The joints, as expected, are insensate, a fact that only compounds the clinical
problems.
Developmental/Congenital Defects
It is important to remember that congenital
defects (present at birth) need not be genetic and
vice versa. However, any process that impacts on
the growing skeleton, whether it be congenital or
developmental, can be expected to produce
changes. These changes can generally be
expected to be alterations in the conguration of
the bone itself. Shortening, bowing, or angular
deformities may be seen. Changes in bone density may or may not be seen.