British Journal ofOphthalmology 1993; 77: 100-102

Orbital volume measurement in the management ofpure blowout fractures of the orbital floor

D G Charteris, C-H Chan, RW Whitehouse, J L Noble

AbstractWith the recent advent of accurate orbitalvolume assessment by computed tomography,a retrospective analysis was made of 31patients with 'pure' blowout fracture of theorbital floor, managed either surgicaily orconservatively, to determine whether orbitalvolume measurement could provide anadditional parameter ofuse in the managementof such fractures. There was a significantdifference in orbital volume discrepancybetween patients managed surgicaily orconservatively suggesting that this investi-gation may be of use in decision making onsurgical intervention in patients with orbitalblowout fractures.(BrJ7 Ophthalmol 1993; 77: 100-102)

Manchester Royal EyeHospital, Oxford Road,Manchester M13 9WHD G CharterisC-H ChanJ L Noble

Department ofDiagnostic Radiology,The Stopford MedicalSchool, Oxford Road,Manchester M13 9PTRW WhitehouseCorrespondence to:Mrs J L Noble.Accepted for publication3 November 1992

The term 'pure blowout' is used to refer to afracture of the floor or medial wall of the orbitwithout a fracture of the orbital rim. Thesefractures may cause entrapment of orbital softtissue structures with resulting enophthalmosand limitation of vertical or horizontal ocularmotility. This type of fracture must be dis-tinguished from an 'impure' blowout fracturewhere the orbital rim and adjacent facial bonesare involved. '

Diagnosis of a blowout fracture is made on thebasis of the characteristic clinical findings oflimitation of vertical (and horizontal) ocularmovements causing diplopia, enophthalmos,and, in some cases, decreased sensation in thedistribution of the infraorbital nerve. Themechanism of these findings is considered to bebuckling of the orbital rim and increased intra-orbital pressure causing expansion in the volume

of the bony orbit and entrapment of the orbitalsoft tissue structures in the fracture with orwithout injury to the infraorbital nerve as itpasses through the infraorbital groove. Theclinical manifestations may be masked in theinitial period following the injury by orbital andperiorbital bruising and oedema and swelling ofthe inferior and medial rectus muscles. Theclinical diagnosis can be supported by radio-logical investigations. Conventional plain x raysmay show the characteristic 'tear-drop'appearance in the maxillary antrum (althoughthis may be masked by blood in the antrum).Orbital tomograms provide a more sensitiveinvestigation for defining orbital floor fractures.Because of the complexity in the anatomy of theorbital and ethmoidal regions computed tomo-graphy (CT) is now recognised to be the bestimaging technique for diagnosing fractures.2The management of orbital blowout fractures

is aimed towards the prevention of long termdiplopia (due to restriction of ocular movements)and enophthalmos. Some cases will recover fullywith conservative management while othersrequire surgical intervention to release entrappedstructures from the fracture site and, onoccasions, to provide orbital volume replace-ment. The indications for, and timing of,surgical intervention remain controversial.3-6The rationale for early surgical intervention torelease entrapped extraocular muscles and pre-vent secondary muscle scarring and contracturehas been questioned as a significant numbers ofcases will continue to have resolution of diplopiain the weeks following injury, and such anapproach would therefore lead to unnecessarysurgery in a number of cases.7 It is, however,accepted that early surgery produces more satis-

Figure I (A) Transaxialimage to show the linesdrawn to outline the left orbitfor volume measurement(arrows). '1' is the region ofinterest, area=5-84 cm2. Nofracture is visible in thisimage. (B) Same patient as(A), section through theorbitalfloor to showdistortion caused by blowoutfracture (arrow). (C) Samepatient, oblique sagittalreformation along the line ofthe inferior rectus muscle.Thefracture has produceddownwards displacement ofthe muscle which is swollenin the midpart (arrow). AFig lB

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Orbital volume measurement in the management ofpure blowoutfractures ofthe orbitalfloor

Fig IC

factory results.489 The early identification ofcases likely to have significant long term sequelaeis therefore essential to the proper selection ofcases for surgical intervention.CT scanning has been demonstrated to be

useful in the assessment of cases of blowoutfracture which are at risk of developing lateenophthalmos and troublesome diplopia'° andcan therefore help in decidingwhich cases requiresurgical intervention. Furthermore it has beenshown that orbital volume measurements fromCT scans are significantly increased in post-traumatic enophthalmos" and are reversed byreconstruction ofthe bony orbit. 12 This study hasanalysed retrospectively orbital CT scans inpatients with orbital blowout fractures to investi-gate the potential oforbital volume measurementto provide a quantitative measure of use in theassessment of the need for surgical intervention.

Materials and methods

PATIENTSThirty one patients with a pure blowout fractureof the orbital floor without medial wall involve-ment were included in the study. All patients hadthe diagnosis confirmed by CT scan. Initialassessment and subsequent clinic follow up ofthe patients routinely included Hess chart andassessment of field of binocular single vision.'3Indications for surgical exploration were as

follows: (a) failure of the field of binocular singlevision to improve and increasing diplopia as

indicated in serial orthoptic assessment, (b)diplopia for more than 6 weeks with no or

minimal signs of improvement, especially if thediplopiawas in the primary ordowngaze position,(c) symptoms which developed late after an

initial symptom-free period, (d) presence ofenophthalmos, especially during the early stage,

(e) CT appearance of an extensive area ofcomminuted and depressed fracture of theorbital floor or entrapment of inferior rectusmuscle. The decision on surgical exploration wasmade after consideration of all these indications.Sixteen patients were managed conservativelyand 15 had surgical exploration. A standardlower lid blepharoplasty incision was made;care was taken to avoid the lower lid retractorsand the incision was deepened inferiorly to thelevel of the inferior orbital margin. The peri-osteum was divided below the inferior orbitalmargin and elevated, exposing the orbital floor.The fracture site was identified, any trappedtissue freed, and a silastic implant placed sub-periosteally along the floor of the orbit behindthe inferior orbital margin. The periosteum wascarefully closed and the wound sutured.

CT SCANNINGCT scanning was carried out using a low dosetechnique previously described.'4 Examinationswere performed on an IGE 9800 scanner.Contiguous 3mm axial sections were taken oftheentire orbit using a dynamic scan protocol,120 kVp, 40 mA, 2 second scan time andreformation in the coronal plane and an obliquesagittal plane parallel to the inferior rectusmuscle was routinely employed. The skin entrydose for this examination was 11 mGy,'I resultingin an estimated lens dose of less than 10 mSv.

Orbital volume was measured by using acursor to trace the orbital walls and the anteriororbital boundary as defined by a line joining thezygomaticofrontal process of each orbit (Fig 1).The volume was then calculated by summatingthe area of the orbit on each section and multi-plying by section thickness. 16 The measurementof orbital volume was carried out retrospectivelyand the discrepancy between the two orbits wasnot taken into account in the decision on surgicalintervention. Statistical analysis was carried outusing the non-parametric Mann Whitney U testto assess the relationship between the orbitalvolume discrepancy in the surgical and non-surgical groups.

ResultsOf the 31 patients included in the study 15 hadsurgical exploration and repair and 16 weremanaged conservatively. In the group ofpatientstreated surgically two had persistent and trouble-some diplopia, seven others had mild residualdiplopia in extreme upgaze which did not causeany significant problems. Residual enophthal-mos was present in two patients treatedsurgically.None of the patients in the group managed

conservatively had troublesome diplopia attri-butable to the fracture, although seven had mildresidual diplopia on extreme upgaze. Twopatients in this group had residual enophthalmos.The range oforbital volume discrepancy in the

surgical group was from 0-2 to 8-2 ml with amean of 4-01 ml (SD 2-22). In the non-surgicalgroup the range of orbital volume discrepancywas from 0-7 to 4-3 ml with a mean of 2-22 ml(SD 1-16). There is a significant difference in

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Charteris, Chan, Whitehouse, Noble

orbital volume discrepancy between the surgicaland non-surgical groups (p<0005).

DiscussionBoth surgical and non-surgical approaches havea place in the management of orbital blowoutfractures.' It is generally agreed that if surgicalintervention is indicated, results are better wherethis is undertaken as early as possible after theinjury to prevent secondary fibrosis and con-

tracture of orbital fascia and ligaments andsecondary involvement of extraocular muscles inthe fracture.489 In the initial period followinginjury selection of patients for surgical manage-ment depends on a combination of factorsincluding impaired ocular motility, enophthal-mos, and size of fracture. Assessment of patientsin this period is often hampered by orbitalhaemorrhage and oedema which can produceimpaired motility which subsequently resolves,and can also mask enophthalmos which may

develop at a later stage. Investigations which aiddecision making on surgical intervention at an

early stage are therefore likely to be of use in themanagement of patients with orbital blowoutfractures. Orbital CT scanning has provided a

means of reliably assessing the position andextent of fractures and the involvement of orbitaltissues in the fracture. "' In this study we havedemonstrated that accurate orbital volumemeasurement is possible on low dose CT scans ofpatients with blowout fractures and that there isa statistically significant difference in the orbitalvolume discrepancy between groups of patientsmanaged surgically and conservatively. Theadditional refinement of volume measurementon orbital CT scans has the potential to helpclinicians in deciding on surgical intervention inblowout fractures. There is, however, overlap inthe ranges oforbital volume discrepancy betweenthe two groups and no absolute value of volumediscrepancy can be given as an indication forsurgery. The potential of volume discrepancymeasurement is to provide an additionalparameter by which decisions on patientmanagement can be made.Orbital volume measurement can, in addition

to aiding in the assessment of patients for earlysurgical intervention, provide information use-

ful in planning volume replacement in latesurgery for orbital trauma." 12

It is notable that a low radiation dose wasemployed in the CT scans used in this study.This technique has previously been demon-strated to be as effective as higher dose techniquesin imaging orbital trauma'4 and to be capable ofproviding orbital volume measurements. 16Multiplanar, high dose CT scanning protocolsincur a high radiation dose to the globe,'7 how-ever the low dose protocol used in this studyresults in a surface radiation dose of only 25% ofthat of conventional axial images.I

This study suggests that orbital volumemeasurement from CT scans has a potentialapplication in the assessment of patients withorbital blowout fractures. Further prospectiveanalysis will be necessary to determine theclinical role of orbital volume assessment inpatient management.

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