Annals of the Rheumatic Diseases 1994; 53: 561-563

CASE STUDIES IN DIAGNOSTIC IMAGING Series Editor: V N Cassar-Pullicino*

Teenage monoarthralgia

Department of ClinicalRadiology, UnitedBristol HealthcareNHS Trust, Bristol,United KingdomC J WakeleyDepartent ofRadiology, FrenchayHealthcare Trust,Bristol, UnitedKingdomM J CobbyCorrespondence to:Dr C J Wakeley,Directorate of ClinicalRadiology, MarlboroughStreet, Bristol BS2 8HW,United Kingdom*Department of DiagnosticImaging, The Robert Jonesand Agnes Hunt OrthopaedicHospital, Oswestry,Shropshire SYlO 7AG,United Kingdom.

Charles J Wakeley, Mark J Cobby

Clinical historyA 15 year old girl presented with a six monthhistory of increasing pain and stiffness in herleft hip. The pain was worse after exercise andshe walked with a limp. On examination therewas apparent shortening of the left lower limbwith a ten degree fixed flexion deformity.Passive and active movements of the left hipwere reduced in all directions and a positiveTrendelenburg's sign was elicited. The fullblood count, plasma viscosity, serumelectrolytes, plasma proteins and urinalysiswere all normal. Radiographs of the left hipwere obtained and reported as normal (fig 1).

Conventional radiographs and MRI of thehip were obtained soon after admission (figs 1and 2).

Radiological findingsThe radiograph of the left hip at presentationwas initially considered normal (fig 1). An MRIscan was then requested (fig 2), which clearlydemonstrated a large zone of bone marrowoedema and a substantial joint effusion.Review of the conventional radiograph showsa radiolucent ring with a central zone ofincreased density (fig 3) and the diagnosis ofan intra-articular osteoid osteoma wasconfirmed using thin section high resolutionCT (fig 4). This elegantly demonstrated theosteoid osteoma with a calcified nidus withinthe anterior cortex, surrounded by a lowattenuation uncalcified ring and a substantialjoint effusion. Periosteal new bone formationwas also evident extracapsularly along theanterior proximal femoral shaft.

Figure 1 Anteroposterior radiograph of the left hip on presentation.

Differential diagnosisThe MRI initially appear to be relatively non-specific, leaving a wide differential diagnosis tobe considered. This includes infection,infiltrative marrow disorders, an inflammatorysynovitis, algodystrophy, transient bonemarrow oedema and bone marrow contusion.Once the radiolucent ring is identified on the

conventional radiograph the diagnosis of anosteoid osteoma is virtually the only seriousconsideration and may be readily confirmed byradioisotope imaging or CT. The elegantdemonstration by CT of the precise anatomicallocalisation of the nidus makes this thetechnique of choice.

DiagnosisIntra-articular osteoid osteoma.

DiscussionThe majority of osteoid osteomas are easilydiagnosed from conventional radiographs andscintigraphy. However, intra-articular osteoidosteomas have an unusual radiologicalappearance that may lead to delay in theirdiagnosis and hence their treatment. This casestudy illustrates how such a diagnosis may beoverlooked and, with the more frequent andearly use ofMRI, how non-specific the findingswith this technique may be.

561

on February 19, 2020 by guest. P

rotected by copyright.http://ard.bm

j.com/

Ann R

heum D

is: first published as 10.1136/ard.53.9.561 on 1 Septem

ber 1994. Dow

nloaded from

Wakeley, Cobby

..de.

Figure 2 (A) Coronal Tl-weighted (TRITE 550/25) and (B) transaxial T2-weighted(TRITE 2000/100) MRI through the hips. Bone marrow oedema is demonstrated in the leftfemoral neck and intertrochanteric region (arrows). This has a subtle low signal intensity onthe Tl-weighted image and a high signal intensity on the T2-weighted image. The marginsto the abnormal area are poorly defined but do not involve the epiphysis. A substantialjointeffusion is evident (arrow heads). No discrete focal abnormality is shown.

Twelve per cent of all benign bone tumoursare osteoid osteomas.' Several authorsdescribed the features of this benign tumour inthe early 1930s, but it was Jaffe in 19352 whointroduced the term 'osteoid osteoma'. In 1966Edeiken3 classified three subtypes of osteoidosteoma, each with different radiologicalfeatures and named according to their site oforigin from the bone. The commonest subtypeis the 'cortical' osteoid osteoma that typicallyarises in the cortex of a long bone. The tumourconsists of a nidus of osteoid material (that maycontain intra-tumoural calcification), and issurrounded by reactive peri-tumoural sclerosis.The 'cancellous' and 'subperiosteal' subtypesare less common. They usually arise in anintra-articular or juxta-articular position. Theirradiological appearances are less typical andmore varied. Reactive osteosclerosis is oftenabsent in this latter group and secondaryfeatures such as synovitis, a joint effusion andperiosteal reaction may predominate. Evenwithin these subtypes radiological featureshave been shown to vary between sites.4

This case is an example of an intra-articularosteoid osteoma. This rarer subtype of osteoidosteoma is particularly difficult to diagnose. Intheir case of osteoid osteoma of the talus,Yeager et al5 failed to show any abnormality onthe conventional radiograph. Likewise in thiscase the conventional radiographic findingswere extremely subtle. It was the ability of theMRI to image the synovitis and altered marrowsignal that prompted review of the con-ventional radiograph and suggested the diag-nosis. However, MRI cannot yet distinguishbetween an inflammatory synovitis or infectivesynovitis from a synovitis secondary to an intra-articular osteoid osteoma. Indeed severalauthors warn of the danger in falselyinterpreting the bone marrow changes as beingsecondary to either infection or a malignantneoplasm.6A combination of factors may make the

identification of an osteoid osteoma by MRIdifficult. These include the relatively thicksections of the images compared with the sizeof the nidus, the frequent requirements of aslice interspace, and the poor ability ofMRI toshow small areas of calcification using con-ventional spin-echo techniques. In addition,peritumoural oedema or peritumoural sclerosismay be so overwhelming that it obscures theminute lesion (osteoid osteoma) causing it.Treatment of an osteoid osteoma relies upon

surgical excision of the nidus of the tumour.From reviewing previous reports it is evidentthat no single imaging modality will bediagnostic in every case of osteoid osteoma andit will often be necessary to use a combinationof techniques to confirm the diagnosis of theexact size of its nidus.

Conventional radiography will demonstratethe nidus in most instances, but a tomographictechnique may be required for accurateanatomical localisation. Kransdorf et al' foundthat the nidus was better demonstrated on CTthan MRI in five of seven cases of osteoidosteoma and equally well in two cases.However, in one report MRI has been shownto be invaluable when the osteoscleroticreaction was so great that the nidus could notbe seen on CT.7 Bone scintigraphy is readilyavailable in most departments and the avidfocal uptake on early images is characteristic.However, scintigraphy may be less specificwhen the lesion is intra-articular as the increasein activity can be generalised within the joint,due to associated synovitis, osteoporosis, andhyperaemia.8 Unfortunately, the clinicalpicture in patients with an intra-articularosteoid osteoma is also less typical. Theclassical nocturnal quality of pain found in80% of extra-articular osteoid osteomas isabsent and the pain is less responsive tosalicylates.8 9MRI signal characteristics are extremely

varied for osteoid osteomas.5 7 10-12 This is notsurprising when the various subtypes and theirdiffering plain radiographic features areconsidered. In general the osteoid nidus isdepicted by a decrease in signal intensity on T1weighted images and increased signal on T2weighted images. Confusion in signal intensity

562

on February 19, 2020 by guest. P

rotected by copyright.http://ard.bm

j.com/

Ann R

heum D

is: first published as 10.1136/ard.53.9.561 on 1 Septem

ber 1994. Dow

nloaded from

Teenage monoarthralgia

Figure 3 Same illustration asfig 1. The MRI examination prompted review of theconventional radiograph which was initially considered normal. A radiolucent ring,however, is evident in thefemoral neck (arrows).

of the nidus has probably arisen due to varyingamounts of intratumoural calcification whichmay alter these signal changes considerably.5 12Adjacent to the osteoid nidus, surroundingsclerosis demonstrates a decrease in signalintensity, whereas surrounding oedema willshow as an increase in signal intensity on T2weighted sequences. The resulting signalcharacteristics again will differ according to theproportion ofoedema and sclerosis. It may wellbe that a more specific diagnosis could havebeen revealed by obtaining thinner slices orusing intravenous gadolinium in combinationwith fat suppression techniques.The mean age of patients with osteoid

osteomas in the South West England BoneTumour Registry was 18-3 years (range 2-62years). In this young age group it is particularlyimportant to keep ionising radiation to aminimum. Partly for this reason, and partlybecause of the superb contrast resolution ofMRI, it is likely that MRI will become acommon primary or secondary imaging tech-nique for many clinical problems in patients ofthis age group. Furthermore, the MRIexaminations may well be performed and inter-preted without access to other investigations. Itis therefore important to be aware of thepossible pitfalls of MRI in the diagnosis of anapparent monoarthritis.

In conclusion, this case study shows how thediagnosis of an intra-articular osteoid osteomawas suggested from apparently non-specificMRI appearances. It must be stressed,however, that no single imaging modality isalways diagnostic in this condition and that theMRI appearances can actually be confusing,misleading, or may be falsely interpreted asinfection or a malignant neoplasm.

1 Dahlin D C, Unni K K. Bone tumors: general aspects and dataon 8542 cases, 4th ed. Springfield, ill: Thomas, 1987:88-101.

2 Jaffe H L. Osteoid osteoma: a benign osteoblastic tumorcomposed of osteoid and atypical bone. Arch Surg 1935;31: 709-28.

3 Edeiken J, DePalma A F, Hodges P J. Osteoid osteoma(roentgenographic emphasis). Clin Orthrop 1966; 49:201-6.

4 Moser R P, Kransdorf M J, Brower A C, et al. Osteoidosteoma of the elbow. Skeletal Radiol 1990; 19: 181-6.

5 Yeager B A, Schiebler M L, Wertheim S B, et al. Case report:MR imaging of osteoid osteoma. Jf Comput Assist Tomogr1987; 11: 916-7.

6 Assoun J, Richardi G, Railhac J J, Baunin C, BonnevialleP. Osteoid osteoma: MR imaging versus CT. RSNA 79thScientific assembly and annual meeting 1993.

7 KransdorfM J, Stull M A, Gilkey F W, Moser R P. Osteoidosteoma. Radiographics 1991; 11: 671-96.

8 Cassar-Pullicino V N, McCall I W, Wan S. Intra-articularosteoid osteoma. Clinical Radiology 1992; 45: 153-60.

9 Cohen M D, Harrington T M, Ginsburg W W. Osteoidosteoma: 95 cases and a review of the literature. Seminarsin arthritis and Rheumatism 1983; 12: 265-81.

10 Bell R S, O'Connor G D, Wadell J P. Importance ofmagnetic resonance imaging in osteoid osteoma: A casereport. Can JSurg 1989; 32(4): 276-8.

11 Glass R B J, Poznanski A K, Fisher M R, Shkolnik A, DiasL. Case report. MR imaging of osteoid osteoma. Jf ComputAssist Tomogr 1986; 10: 1065-67.

12 Houang B, Grenier N, Greselle J F, et al. Osteoid osteomaof the cervical spine: misleadingMR features about a caseinvolving the uncinate process. Neuroradiolgy 1990; 31:549-51.

Figure 4 Transaxial high resolution CTsection throughthe leftfemoral neck. A densely calcified nidus within theanterior cortex of the intra-articular portion of the femoralneck is demonstrated (arrow) and surrounded by a welldefined radiolucent ring. A substantialjoint effusion ispresent (arrow heads). The appearances are characteristicofan osteoid osteoma with a reactive synovitis.

563

on February 19, 2020 by guest. P

rotected by copyright.http://ard.bm

j.com/

Ann R

heum D

is: first published as 10.1136/ard.53.9.561 on 1 Septem

ber 1994. Dow

nloaded from