Australasian Radiology

(2003)

47

, 181–183

Case Report

Occult epiphyseal bone abscess: Lessons for the unwary

Michael Solomon,

1

Michael Stening,

1

Samuel Macdessi,

1

Christine Shearman,

2

John Pereira,

2

Chee Chung Hiew

2

and Hans Van der Wall

3

1

Department of Pediatric Orthopedics, Sydney Children’s Hospital,

2

Department of Radiology, Prince of Wales Hospital, and

3

Department of Nuclear Medicine, Concord Hospital, Sydney, Australia

SUMMARY

Two male children presented with increasing pain in the right knee and constitutional symptoms. Biochemical markersof inflammation were elevated. Plain radiography was reported as normal and bone scintigraphy was consistent withsynovitis of the right knee in the first case. The second child underwent aspiration of the knee with drainage of turbidfluid 1 week after antibiotics. Slow response to therapy led to MRI and CT scanning in the second child, revealing anepiphyseal abscess. Review of the scintigraphic studies in the first child raised the possibility of osteomyelitis of thedistal right femur. Further imaging was undertaken with MRI and CT scanning confirming an epiphyseal bone abscess.Failure of diagnosis of an epiphyseal bone abscess by combined plain radiography and scintigraphy has not previouslybeen reported and provides a number of valuable lessons.

Key words:

bone abscess; computed tomography scan; epiphyseal osteomyelitis; magnetic resonance imaging;

scintigraphy.

INTRODUCTION

Haematogenous osteomyelitis is primarily a disease of the

metaphysis. Occasionally, the lesion might extend into the

adjacent epiphysis across the barrier of the growth plate in

childhood. Most cases are due to primary haematogenous

seeding of the epiphysis by vessels penetrating the growth

plate in the neonate or through the vessels supplying the epiph-

ysis (Hunter’s circle) in older children.

1,2

Regardless, primary

osteomyelitis of the epiphysis is a rare disease with reports of

only a few cases in the literature. When it does occur, there is

a characteristic appearance on plain radiography, scintigraphy

and cross-sectional imaging with CT. In this setting, careful dis-

tinction must be made from chondroblastoma and epiphyseal

osteoid osteoma.

2

Epiphyseal bone abscess has been well

described on plain radiography and in a series of eight children

by CT.

2

We present two rare cases in which the epiphyseal bone

abscess was not apparent on plain radiography and was

confused with synovitis of the knee on the initial interpretation of

the scintigraphic studies in the first case. The second case,

which presented within 2 weeks of the first, went straight to MRI

and CT as a result of the incorrect scintigraphic report with the

first case.

CASE 1

A six year old boy presented with pain and swelling of the right

knee. At the time of presentation, his plain X-ray was thought

to be normal. Biochemical markers were abnormal with an

erythrocyte sedimentation rate (ESR) of 52 mm/h and leuco-

cyte count of 13.2

×

10

9

/L (Normal <11.0

×

10

9

/L). Bone

scintigraphy was reported as being consistent with synovitis

(Fig. 1). The scan was later reviewed and thought to be consist-

ent with epiphyseal osteomyelitis due to the focal epiphyseal

hyperaemia and uptake. Subsequent MRI and CT scanning

confirmed epiphyseal abscess formation (Figs 2,3). Drainage

of the abscess was performed under CT control with isolation of

Staphlococcus aureus

. The patient improved with appropriate

antibiotic therapy.

M Solomon

MB BS, FRACS, FAOrthA;

M Stening

MB BS, FRACS;

S Macdessi

MB BS;

C Shearman

FRACR;

J Pereira

MB BS, FRACR;

CC Hiew

MB BS, FRANZCR;

H Van der Wall

MB BS, PhD, FRACP.

Correspondence: Dr Hans Van der Wall, Department of Nuclear Medicine, Concord Hospital, Concord, New South Wales 2139, Australia.

Email: hansv@nmrf.org.au

Submitted 14 November 2001; resubmitted 15 July 2002; accepted 3 October 2002.

182

M SOLOMON

ET AL

.

CASE 2

A 15-month-old previously well boy presented with failure to

weight-bear on his right leg. Clinical examination raised the

possibility of either discitis or an irritable hip. The possibility of

infection was heightened by a raised ESR of 64 mm/h and a

c-reactive protein level of 26 mg/L (Normal <9 mg/L). Leuko-

cyte count was normal. He was commenced on antibiotic

therapy and within 48 h developed a hot, swollen right knee,

aspiration of which showed turbid fluid. Plain radiography of the

knee at this time was reported as normal apart from soft-tissue

swelling. No organisms were isolated from the fluid. The patient

continued to spike fevers and the swelling in the knee

increased. Both MRI and CT scans confirmed an epiphyseal

bone abscess (Figs 3,4). The antibiotic regimen was increased

with the addition of a second agent and the patient slowly

improved without further surgical intervention.

Fig. 3.

Case 2. (a) Axial CT demonstrating a well-defined lytic lesion in

the distal femoral epiphysis. (b) Inversion recovery coronal MR image

showing oedema in the distal femoral epiphysis, a small joint effusion

and extensive periarticular oedema.

Fig. 4.

Schematic representation of the anatomy and blood supply of

the epiphyseal/growth-plate complex in childhood. The growth plate acts

as a barrier between the epiphysis and metaphysis. Blood supply to the

epiphysis is by a separate system of arterial vessels to the metaphysis.

Fig. 1.

Case 1. (a) Anteroposterior radiograph showing decreased

density in the medial aspect of the distal femoral epiphysis. This was ini-

tially interpreted as a normal study. (b) Axial CT showing well-defined

lytic lesion of the distal femoral epiphysis. (c,d) Inversion recovery

coronal and sagittal MR images demonstrating corresponding hyper-

intense lesion in the distal femoral epiphysis. Note the extensive

periosteal and muscle oedema and the joint effusion.

Fig. 2.

Case 1. (a) The blood pool phase of the study after injection of

500 MBq of 99m Technetium methylene diphosphonate showing

intense hyperaemia of the distal femoral epiphysis on the right

(arrowhead). Note that the hyperaemia is confined to the epiphysis and

does not involve the tibial component, as occurs in synovitis. (b) The

delayed phase demonstrates increased uptake in the right epiphysis

(arrowhead).

SCINTIGRAPHY OF EPIPHYSEAL BONE ABSCESS

183

DISCUSSION

Epiphyseal osteomyelitis and/or bone abscess has been most

commonly reported around the knee, with the hip and distal

tibia being the next most common sites of involvement. In most

reported cases, the patients were more than 18 months of age,

suggesting that the vascular anatomy of the epiphysis might

play an important role in the development of such infections.

Brooks

3

and Trueta and Morgan

1

described the changing

anatomy between the child less than 1 year old and older

children. In children less than 1 year of age, vascular channels

penetrate the growth plate from the metaphysis and supply the

epiphysis. This changes in the older child with the growth plate

forming a barrier between the metaphysis and the epiphysis by

18 months of age. The blood supply to the epiphysis is then

derived from a large encircling artery (Hunter’s circle) that

drains into an interconnected subchondral venous system with

relatively sluggish blood flow in the epiphyseal sinusoids. This

system has similar haemodynamics to the venous channels in

the metaphyseal region. The arteries and capillaries of the

metaphysis loop sharply at the interface with the growth plate

with resultant fall in the velocity of blood flow, explaining the

predisposition to infection. It is likely that there is secondary

thrombosis of these vessels during the inflammatory response

to bacteria, leading to infarction and necrosis of the bony

tissues and subsequent bone abscess formation. These cases

span the acute presentation and the more subacute course of

bone abscess formation. It is conceivable that the plain radio-

graphs did not demonstrate the abscess in either case as it

might have been too early in the evolution of the infection. Alter-

natively, the degree of cortical damage might not have provided

enough contrast for resolution by plane radiography compared

with CT and MRI.

Initial interpretation of the scintigraphic study in case 1 as

synovitis was most likely based on the delayed images that

showed minimal alteration in uptake with some blurring of the

growth plate. However, the dynamic and blood pool images

showed a clear-cut area of focal hyperemia corresponding to

the distal femur, reflecting the surrounding soft-tissue altera-

tions in response to the infection. More intense delayed uptake

in the epiphysis might not have been apparent if there was com-

promise to the blood supply of the epiphysis, as has been

reported previously.

4

Although this case is atypical for epiphy-

seal osteomyelitis in terms of the scintigraphic appearance,

it underlines the importance of obtaining magnified or pinhole

images of the region of concern, especially in small children.

The differential diagnosis of such a lesion in the epiphysis

must include osteoblastoma and osteoid osteoma, with more

rare reports of epiphyseal osteosarcoma, chondromyxoid

fibroma and eosinophilic granuloma.

2

However, CT scanning

might disclose a number of features that are diagnostic of a

bone abscess. These include the presence of a sequestrum in

the abscess cavity, sinus tracts, associated soft-tissue abscess

and cortical destruction.

2

Additionally, the presence of a peri-

osteal reaction has high specificity for a chondroblastoma

because none of the other causes of lucent epiphyseal lesions

have been associated with this appearance.

2

Little systematic research has been published to date on the

defining MRI features of epiphyseal bone abscesses. However,

a characteristic target pattern of enhancement has been des-

cribed with Brodie’s abscess.

5

There is also some evidence that

gadolinium-enhanced studies might help define sequestrae.

6

CONCLUSION

These cases demonstrate several unusual features of epiphy-

seal osteomyelitis that have not been recognized previously.

The delay in recognition might reflect the natural history of the

disease as it progressed from the less-easily detected acute

to the better-characterized subacute form. It highlights the

atypical appearance that might be seen in scintigraphic studies.

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