Bizarre Parosteal Osteochondromatous Proliferation (Nora’s Lesion) of the Forefoot: Case Report and Review of the Literature

Abstract

Bizarre parosteal osteochondromatous proliferation also known as Nora’s lesion is a benign, surface growing tumor of unknown etio-pathogenesis most commonly involving small tubular bones of the hand and feet. It is a rare entity with less than 200 cases being reported so far in international literature. Clinically as well as radiologically, it closely mimics other surface growing tumors of the bone. This resemblance to other lesions and its rare occurrence often leads to its misdiagnosis. Recurrence rate is high after excision and varies from 20% to 55% in different series, so accurate diagnosis and proper preoperative planning to remove the lesion en-block is the key. Histopathology is the key to the diagnosis. Presently, research is going on about chromosomal aberrations that have been found to be associated with this entity. We here present a twenty year old male with Nora’s lesion of proximal phalanx of the fourth toe that was preoperatively mistaken for an osteochondroma or surface chondroma.

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KUWAIT MEDICAL JOURNAL 49
March 2017
Kuwait Medical Journal 2017; 49 (1): 49 - 54
Bizarre Parosteal Osteochondromatous Proliferation
(Nora’s Lesion) of the Forefoot: Case Report
and Review of the Literature
ABSTRACT
Abedullah Bhat1,2, Nadeem Ali1, Sonali Sharma1
1Department of Orthopaedics, SHKM Government Medical College Mewat, Haryana, India
2Department of Radiodiagnosis, ASCOMS, Jammu, J & K, India
Bizarre parosteal osteochondromatous proliferation,
also known as Nora’s lesion, is a benign, surface growing
tumor of unknown etio-pathogenesis, most commonly
involving small tubular bones of the hand and feet. It is a
rare entity with less than 200 cases being reported so far in
international literature. Clinically as well as radiologically,
it closely mimics other surface growing tumors of the bone.
This resemblance to other lesions and its rare occurrence
often leads to its misdiagnosis. Recurrence rate is high after
excision and varies from 20% to 55% in dierent series, so
accurate diagnosis and proper preoperative planning to
remove the lesion en-block is the key. Histopathology is
the key to the diagnosis. Presently, research is going on
about chromosomal aberrations that have been found to be
associated with this entity.
We here present a twenty year old male with Nora’s lesion of
proximal phalanx of the fourth toe that was preoperatively
mistaken for an osteochondroma or surface chondroma.
KEY WORDS: blue bone, BPOP, chondroma, nora, osteochondroma, parosteal
Address correspondence to:
Dr Nadeem Ali, Mughal Mohalla, Lalbazar, Srinagar, Jammu and Kashmir, India – 190023. Phone: +911942423211, +919812962979, Email: drnadeeem@
gmail.com
INTRODUCTION
Bizarre parosteal osteochondromatous proliferation
(BPOP) or Nora’s lesion was rst described in literature
by Nora et al in 1983 as a very rare benign, parosteal,
exophytic neoplasm of unknown etio-pathogenesis
usually involving short tubular bones of extremities
and less commonly, long bones and skull-facial bones
[1-4]. So far less than 200 cases of BPOP have been
reported in international literature[2,5,6]. Clinically
and radiologically, Nora’s lesion mimics numerous
benign and malignant lesions. Because of its rare
occurrence and resemblance with other lesions, the
diagnosis of this entity is dicult and often missed[7,8].
Currently, marginal excision is the treatment of choice
but recurrence rate is high[8,9]. Histopathology of the
specimen is the key to conrm the diagnosis[10,11]. Here
we present a case of BPOP of forefoot in a 20 year old
male that was misdiagnosed as an osteochondroma or a
periosteal chondroma preoperatively. Histopathology
of the excised specimen however, had features of
BPOP.
CASE REPORT
Twenty year old male patient presented to us with
chief complaint of progressively increasing swelling of
left foot and problems in shoe wear for the last three
years. There was no history of trauma in the past. There
was no history of similar swelling in any other part of
the body. None of the family members had history
of such swellings. On clinical examination, there was
a lobulated swelling 4 cm × 2.5 cm at the base of the
second toe and rst web space of the left foot (Fig 1).
The overlying skin was normal. On palpation, swelling
had bone like consistency and was immobile. There
was no neurovascular decit of the toes. Movements
of toes were within normal range.
Case Report

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50
Routine baseline investigations and inammatory
markers were within normal range. Chest radiograph
was unremarkable. Radiograph of the left forefoot had
opacity with ne trabeculae suggestive of a bone like
mass arising from proximal phalanx of second toe
with scalloping of the outer surface of the cortex of
the phalanx (Fig 2). There was no cortical destruction,
erosions or periosteal reaction of the phalanx. From this
clinico-radiological picture, a dierential diagnosis
of an osteochondroma and periosteal chondroma
was made. Magnetic resonance imaging (MRI) of left
foot was advised but the patient refused because of
nancial constraints. Marginal excision of the tumor
was planned.
The tumor was approached dorsally. The surface
of the mass was smooth, lobulated and had greyish
tinge (Fig 3). The mass was aached to the shaft of
proximal phalanx of second toe, from which it was
excised (Fig 3). The surrounding periosteum was
excised and the supercial dorsal cortex of phalanx
was shaved o by a chisel. Excessive skin was excised
and the wound was closed (Fig 4). The excised mass
was sent for histopathologic examination (Fig 4). The
histopathology showed a disorganised mixture of
cartilage, bone and brous tissue. Cartilage component
Fig 1: Clinical photographs of the forefoot swelling
Fig 2: Antero-posterior and lateral view radiographs of foot showing
a calcied mass arising from proximal phalanx of the second toe.
There is scalloping of the cortex of the phalanx (boom row)
was composed of large chondrocytes which were
occasionally binucleate. There was immature bone
trabeculae interspersed in brous tissue. Underlying
the cartilage was a blue staining zone with islands
of chondrocytes. Haematopoietic tissue was absent
in between trabeculae; instead there was connective
tissue element consisting of elongated spindle cells.
However, nuclear atypia and areas of necrosis were
absent. This microscopic picture led to nal diagnosis
of Nora’s lesion. The patient was followed up for one
year and there were no features of recurrence, after
which follow up was lost.
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KUWAIT MEDICAL JOURNAL 51
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DISCUSSION
Bizarre parosteal osteochondromatous proliferation
(BPOP) is a rare tumor which was rst described in
1983 by Nora FE et al and hence the eponym Nora’s
lesion[1]. Series of 35 cases by Nora FE et al in 1983, 65
cases by Meneses et al in 1993, 24 cases by Dhondt et
al in 2006, 22 cases by Berber O in 2011, 13 cases by
Joseph J et al in 2011and 12 cases by Abramovici L in
2002 are the only large series of BPOP in international
Fig 3: Tumor exposed from dorsal surface of the foot (upper row). Cortical surface after excision (boom row)
literature and the rest of the literature is limited to
isolated case reports and small case series[1, 3, 4, 9, 11,12].
BPOP is a benign lesion composed of cartilage,
bone and brous tissue that presents as an exophytic
growth from the cortical surface of small tubular
bones of hand and feet[2,13]. Hands are four times more
commonly involved than feet. Proximal phalanges,
middle phalanges, metacarpals, and metatarsals are
the sites of predilection. Cases with involvement of
long bones, skull bones, clavicle, maxilla, mandible
and sesamoid have been reported in literature[3,4, 13-23].
It can occur at any age, with patients between 4 and 78
years being reported, with highest incidence between
20 to 35 years[8, 24-26]. Frequency is equal in both sexes[25,
27]. Presentation is that of a swelling that grows over
months to years with or without pain[4, 11, 28].
The exact aetio-pathogenesis of the lesion is not
known[8,29]. Many believe trauma as a trigger, and
consider, it represents a reactive lesion in response
to trauma like orid reactive periostitis and Turrets
exostosis[30,31]. However, history of trauma is an
inconsistent feature. In the series by Nora et al, no case
had history of trauma, while as in that of Meneses et
al and Joseph J et al, 30% and around 20% had history
of antecedent trauma respectively[1,4, 9]. Moreover, the
discovery of cytogenetic aberrations in these lesions
favours a neoplastic aetiology rather than a reactive
one. Dierent chromosomal aberrations have been
reported, but t (1:17) (q32; q31) constitutes specic
translocation for BPOP[27,32-35].
Radiology demonstrates a well demarcated
mineralized mass arising from the periosteal
cortical surface with maintenance of integrity of the
underlying cortex. Cortico-medullary continuity,
a feature of osteochondroma is characteristically
absent[8,11,18,25]. However, cases of histologically
proven BPOP with cortico-medullary continuity have
been reported in literature[8,36]. Cortical scalloping,
a feature of periosteal chondroma, cortical aring
and periosteal reaction seen in osteosarcoma are
absent. In our case, presence of ne trabeculae in
Fig 4: Final appearance after closure of the wound (upper row).
Excised mass specimen (boom row)

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52
the mass on radiography favoured the diagnosis
of osteochondroma. However, cortical scalloping
and absence of cortico-medullary continuity on the
other hand suggested periosteal chondroma. BPOP
with scalloping of the outer surface of cortex have
been reported in literature.[9] BPOP with atypical
radiological ndings have been reported in the
past[5,8,37,38]. Unfortunately a CT and or MRI could not
be obtained in our case for conrming the ndings.
Nora’s lesion can mimic a number of reactive,
benign and malignant lesions of the bone. The reactive
lesions which resemble BPOP include orid reactive
periostitis, subungal exostosis, Turret exostosis
and myositis ossicans. Benign tumors of bone
showing resemblance to BPOP are osteochondroma
and surface chondroma. Surface chondrosarcoma,
periosteal and parosteal osteosarcoma are the
malignant bone tumors that can be mistaken for a
BPOP[7,39,40,41]. Dierentiation from malignant lesions
is important for treatment purpose in order to avoid
unwanted destructive surgery and from benign ones
to prevent recurrence[42].
Histopathology is conrmatory and dierentiates
BPOP from other lesions[10,11,43]. It consists of a
disorganised proliferation of cartilage, bone and
brous tissue[4,40]. The lesion typically consists of four
layers, namely surface layer of thick bro connective
tissue with scaered scanty chondroblasts, underlying
irregular cartilaginous cap with atypical, bizarre and
occasionally bi-nucleate chondrocytes, zone of ‘blue
bone’ considered hallmark of BPOP having islands
of atypical chondrocytes still present but decrease in
number until they disappear in the deepest layer and
lastly the zone of mature pink bone which contain
osteoblast rimmed trabeculae[2-4,41,44]. Both blue and
pink bone encloses a myxoid connective tissue stroma
in the intratrabecular spaces in place of hematopoietic
tissue, which is present in osteochondroma and hence
a dierentiating feature[44]. Atypical mitosis or cellular
atypia is characteristically absent[6, 11].
Surgery is the mainstay of treatment, even in
asymptomatic cases[45]. There is controversy regarding
treatment with advocates of both marginal as well as
radical excision[11]. Local recurrence rate after surgery
is high, ranging from 22 to 55% in dierent series and
hence some surgeons favour the radical approach
of treatment[7,11]. However, marginal excision
still remains the mainstay of treatment[9]. Radical
surgical approach is recommended for tumors with
aggressive behaviour and intramedullary extension[8].
Recurrence is seen within months to two years after
primary excision[6]. Local excision is advocated for
recurrence rather than an aggressive approach[4,26].
Complete removal of pseudo-capsule, resection of
periosteum underneath the tumor and decortications
of underlying bone is the key to prevent recurrence[5,8].
Nora’s lesion is a benign lesion and metastasis is not a
feature. It is not pre-malignant[22]. However, one case
of malignant transformation to brosarcoma has been
reported in the literature[46].
CONCLUSION
Nora’s lesion of the bone is a rare lesion that
can easily be misdiagnosed as osteochondroma or a
surface chondroma. Presence of atypical features like
scalloping of cortex, cortico-medullary continuity,
aring of the cortex with the tumor can further
make radiological diagnosis dicult. Nora’s lesion
should always be kept as a dierential in case of
surface growing tumors of short tubular bones of
the extremities. Histopathology of the excised lesion
is conrmatory. Presently, detection of cytogenetic
aberrations in the lesion is under research and may
be of great benet in the near future.
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