Thirty-six children were managed with cyclical intravenous pamidronate for
the treatment of low bone mineral density between 1999 and 2005 at the Women's
and Children's Hospital in North Adelaide, Australia. The primary diagnoses
included osteogenesis imperfecta (twenty patients), corticosteroid-dependent
asthma (three patients), fibrous dysplasia (two patients), cerebral palsy (two
patients), Legg-Calvé-Perthes disease (two patients), and Marfan
syndrome, mitochondrial disease, idiopathic osteopenia, general myopathy,
Crohn disease, liver disease, and homocystinuria (one patient each).
The dosing protocol varied for different conditions. Children with
osteogenesis imperfecta received six doses at one-month intervals, four doses
at two-month intervals, two doses at three-month intervals, one dose at a
four-month interval, and two doses at six-month intervals, for a total of
three years of planned treatment.
The effects of pamidronate therapy were monitored with use of a protocol in
which spinal radiographs were made at six-month intervals, dual-energy x-ray
absorptiometry scans were made at six-month intervals, and renal ultrasound
scans were made at one-year intervals. Limb radiographs were requested as
clinically indicated, with the main indications being suspected fractures or
bone pain.
Approval for the study was obtained from the ethics committee at Women's
and Children's Hospital. A retrospective review of case notes and radiographs
was carried out. We collected demographic, clinical, and radiographic data,
including gender, age, the bone that was studied with radiographs, the
diagnosis and indication, the number of cycles of intravenous pamidronate
treatment, the number of bands, and the band intervals.
We attempted to correlate the number of cycles of intravenous pamidronate
administration and the dosing intervals with the number of bands seen on
radiographs and their spacing. The rate of physeal growth in the present
series of children was estimated from the radiographs by correlating the band
intervals with the dosing regimen. The radiographic method that we used had
been assessed as showing 15% magnification on conventional radiographs, and
the measurements were corrected
accordingly20. The
distance between the first zebra line and the physis was measured for the
distal part of the femur and the proximal part of the tibia, and the time
interval was determined on the basis of the dates of the first dose of
pamidronate and the radiographic examination. Patients with a time interval of
less than four months between pamidronate treatment and radiographic imaging
were excluded as it was not possible to determine the growth rate accurately
from measurements on the radiograph of =2 mm. The longitudinal growth rates
of the proximal tibial physis and the distal femoral physis were plotted
against the average ages of the children during the treatment period.
Of the thirty-six children who were managed with cyclical intravenous
pamidronate infusions, nine patients with closed physes had no bands on
radiographs. One child with cerebral palsy had no radiographs and was
excluded. Parallel lines or bands of increased bone mineral density were
observed on the radiographs of the remaining twenty-six children managed with
pamidronate (see Appendix). The lines were perpendicular to the axis of growth
and spanned the width of the long bones studied, arising from and matching the
physeal contours. We propose calling these lines zebra lines in view
of the characteristic banding pattern observed.
Zebra lines were observed on all radiographs of all growing patients who
underwent cyclical therapy with intravenous pamidronate. Zebra lines did not
develop in skeletally mature patients.
Zebra lines appeared wherever substantial bone growth was occurring
(Fig. 1). We observed multiple
bands in the epiphyseal, metaphyseal, and apophyseal regions of bones (Figs.
1,
2,
3,
4). The most prominent bands
were observed in the metaphyseal region of rapidly growing bones, such as the
distal part of the femur and the proximal part of the tibia. Finer and more
densely spaced lines were observed in slowly growing bones, such as the pelvis
and the calcaneus (Fig. 2).
Epiphyseal and apophyseal bands were aligned circumferentially with the
surface of the epiphysis or apophysis, representing epiphyseal or apophyseal
growth from the secondary ossification centers (e.g., the distal femoral
epiphysis and the iliac crest apophysis)
(Fig. 3).
The banding pattern observed on radiographs depended on various factors,
including the number of doses of intravenous pamidronate, the frequency of
administration, the rate of growth of the child, and the site or bone under
observation.
Number of Doses of Intravenous Pamidronate. The number of bands
observed on radiographs correlated with the number of treatment cycles of
intravenous pamidronate. Minor discrepancies were attributed to difficulty in
discerning separate lines on radiographs in some patients with frequent dosing
regimens.
Frequency of Administration. The more frequent the doses of
pamidronate, the closer the lines (Figs.
1,
2,
3,
4).
Rate of Growth of the Child. Children nearing the end of the
pubertal growth spurt had lines that merged into one another with a decreasing
band interval. This finding was suggestive of growth deceleration.
Site or Bone Under Observation. Bone growth as a calculation of
band interval with time was highest in the distal part of the femur and the
proximal part of the tibia.
Zebra lines appeared radiographically similar to Harris growth arrest
lines. Harris described transverse striations at the ends of the diaphyses due
to illness, which remain steadfastly parallel and equidistant from the physis
until growth and remodeling cause them to
disappear21-25.
Zebra lines were noted to become less distinct with time, eventually
disappearing as the metaphysis remodeled into the diaphysis over three to four
years. In three patients, preexisting Harris growth arrest lines in the distal
part of the femur and proximal part of the tibia outlasted subsequent zebra
lines following cyclical pamidronate treatment
(Fig. 4), which may suggest
that Harris growth arrest lines persist for a longer duration than zebra lines
do. Additional studies are warranted to confirm these observations.
The distribution of zebra lines was noted to be more widespread in the
epiphyseal, apophyseal, and metaphyseal regions of all growing bones, whereas
Harris growth arrest lines are seen in the metaphyses of rapidly growing
bones, such as the distal part of the femur, proximal part of the tibia,
distal part of the tibia, and proximal part of the humerus. In addition,
Harris growth arrest lines may be localized to an individual bone, as
following a major
fracture24. Ten
children in the present study sustained a femoral or tibial shaft fracture
after the start of pamidronate infusions. No Harris growth arrest lines were
observed between the regular bands secondary to the pamidronate infusions.
This observation suggests that children receiving pamidronate may not have
formation of Harris growth arrest lines following a major fracture.
The physeal growth rates that were measured were within two standard
deviations of the growth rates in normal
children26,27.
Thus, there was no evidence to suggest a deceleration of bone growth in
children receiving pamidronate (Figs.
5-A and
5-B).
Zebra lines also were observed as distinct bands on axial, coronal, and
sagittal computed tomographic and magnetic resonance imaging scans, with the
same orientation, parallel to the physis, representing a plate of sclerotic
bone rather than pericortical thickening (Figs.
6-A and
6-B).
Bisphosphonate therapy is the current treatment of choice for conditions
associated with low bone mineral density, such as osteogenesis imperfecta,
cerebral palsy, prolonged corticosteroid therapy, and idiopathic
osteopenia1-16.
Treatment with bisphosphonates has resulted in clinical improvement for
children and adolescents with moderate to severe forms of osteogenesis
imperfecta, with a reduced frequency of fractures and consequent deformity.
Furthermore, pain levels are reduced, with an improvement in mobility and
quality of
life1,2,6-8,10-14.
Bisphosphonates are synthetic pyrophosphate analogs in which the oxygen of
the phosphate-oxygen-phosphate (P-O-P) skeleton has been replaced with a
carbon (P-C-P). This allows the addition of two side chains to the carbon
molecule. The biological action is in the two carbon side chains, referred to
as
R1
and
R2.
The affinity for the hydroxyapatite bone mineral is the property of the
R1 side chain, and
the biological activity on osteoclasts is dependent on the
R2 side
chain19. The newer,
more potent bisphosphonates, so-called third-generation bisphosphonates (e.g.,
alendronate, risedronate, zoledronate, ibandronate), have a nitrogen group as
one of the side
chains19.
Bisphosphonates irreversibly bind to, inactivate, and induce apoptosis of
osteoclasts. In addition, they decrease osteoclast recruitment,
differentiation, and action. Bisphosphonates work by inhibiting the mevalonate
pathway, which is responsible for cholesterol
synthesis19. The
inactivated osteoclast is then incorporated into the bone matrix. The
half-life of bisphosphonates in adults has been estimated to be greater than
ten years; to our knowledge, there have been no studies of bisphosphonates in
children that have spanned ten
years19.
The deposition of new bone, indicative of linear growth, is evident on the
post-treatment radiographs of long bones and the axial skeleton in prepubertal
children. Transverse opacities delineate treatment cycles, with the distance
between these transverse lines representing new bone
growth9,13,17-19.
Harris growth arrest lines are seen radiographically in the metaphysis and
proximal part of the diaphysis of long bones in children following systemic
disorders such as severe infection, cyclical chemotherapy and malnutrition, or
localized insults such as fractures of long bones. These lines correspond with
transverse trabecular plates of increased radiodensity, which parallel the
contours of the contiguous physis. Histologically, there is a thickened
transversely interconnected trabecular network, with more typical
longitudinally oriented trabecular bone on either side. Following a temporary
slowdown or cessation of rapid longitudinal bone formation in the primary
spongiosa, the trabeculae thicken and fuse with each other transversely. When
normal growth is resumed, longitudinally oriented trabeculae with interspersed
marrow elements are formed at the physis and are progressively displaced from
the physis as the bone lengthens. Narrower hypertrophic and columnar zones
manifest the overall slowdown of endochondral ossification. A primary slowdown
of cartilage growth and maturation is reflected in the pattern of trabecular
bone transformation. A dense layer of bone is formed, creating a distinct
plate of densely calcified bone that is apparent on radiographs following
resumption of normal physeal growth—hence the term growth recovery
lines24.
Zebra lines appear in the metaphyses of long bones following the cyclic
administration of bisphosphonates in children. Bisphosphonates are deposited
in the inorganic matrix and bind to exposed collagen in the bone matrix in the
primary spongiosa. Inhibition of osteoclastic activity by the bisphosphonates
results in uncoupling of bone remodeling so that a narrow zone of primary
spongiosa persists. Further growth of the physis results in the appearance of
normal bone—hence the banded appearance with each dose of
bisphosphonates. In the study by Rauch et al., histological analysis of
sclerotic lines in an iliac bone biopsy specimen from a child with
osteogenesis imperfecta who was receiving cyclical pamidronate showed a
decreasing proportion of calcified cartilage with increasing distance from the
growth plate28.
This finding suggests that the sclerotic lines seen on radiographs do not
represent "frozen" bars of growth plate cartilage but rather
horizontal trabeculae undergoing turnover. New mineralized bone is gradually
substituted for the old bone containing calcified cartilage, but the size and
shape of these trabeculae are not changed.
Zebra lines are similar in appearance to Harris growth arrest
lines29. However,
they represent failure of remodeling of the primary spongiosa to secondary
spongiosa at the physis rather than a growth arrest
phenomenon19.
Different mechanisms may be involved in the formation of zebra lines and
Harris growth arrest lines, which may explain why the latter appear to outlast
zebra lines when both are present in the same bone. In addition, children
receiving daily doses of bisphosphonates show continuous zones of metaphyseal
sclerosis corresponding with the duration of treatment. This suggests that
physeal growth is still occurring, with a relative increase in bone formation
as can occur with uncoupling of osteoblastic and osteoclastic activity. Subtle
metaphyseal undertubulation is observed in some patients and may result from
reduced metaphyseal remodeling secondary to decreased osteoclastic
activity19.
Both Harris growth arrest lines and zebra lines progressively move away
from the physis, and, as such, both can act as indicators of a growth
disturbance in the
physis24.
In addition, both tend to disappear as they move into the diaphysis
secondary to bone remodeling. However, in our study, zebra lines were noted to
disappear earlier than Harris growth arrest lines did.
In 1963, Anderson et al. measured the femoral and tibial lengths on serial
radiographs of growing children and determined that the growth rates of the
distal femoral and proximal tibial physes were 71% and 57%, respectively,
relative to the entire length of the
bones26. The
proportion of growth that accrued from either end of the bone was determined
with use of Harris growth arrest
lines26. The length
measurements included the epiphyses at both ends of the
bone26. We measured
the distance between the zebra lines and the physis and obtained physeal
growth rates for the distal part of the femur and proximal part of the tibia.
These measurements compare well with those obtained by Anderson et
al.26,27
and suggest that there is no substantial deceleration in growth in children
receiving cyclical pamidronate therapy. However, this method has inherent
limitations, as we did not take separate epiphyseal growth into account. In
addition, we assumed that that the distance between zebra lines correlates
with the rate of bone growth. Determination of the actual rate of bone growth
would require the measurement of the change in bone length over time on serial
radiographs26,27.
The effects of the underlying disease, concurrent fractures, and other
treatments on growth in the children in our study have to be considered when
their growth rates are compared with growth rates in normal children.
Currently, there are no longitudinal radiographic data in the literature to
allow one to calculate growth rates in these conditions.
Transverse bands in the metaphysis can also be found in children with a
number of other conditions such as heavy metal intoxication, treated leukemia,
healing rickets, and chronic anemia. Lead lines have been described in
children with lead poisoning. These usually disappear spontaneously within
four years and may provide a marker for the subsequent rate of bone
growth30.
In conclusion, cyclical pamidronate therapy results in distinctive
radiographic findings in the growing skeleton. We propose that the pattern of
metaphyseal banding be called zebra lines. These lines can also be
used as an indicator of bone growth.
A table showing clinical details on all of the patients in the study is
available with the electronic versions of this article, on our web site at
(go to
the article citation and click on "Supplementary Material") and on
our quarterly CD-ROM (call our subscription department, at 781-449-9780, to
order the CD-ROM). ?