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Unicameral bone cysts have beenrecognized for many years as benign lesions; however, they remain trou-blesome in regard to their diagnosis and treatment.. bling the type A and B cells se

Unicameral bone cysts have been

recognized for many years as benign

lesions; however, they remain

trou-blesome in regard to their diagnosis

and treatment Virchow originally

described these lesions in 1891 as

“cystic structures,” which he

hypoth-esized occurred as abnormalities in

the local circulation Unicameral

bone cysts are also known as

soli-tary, or simple, bone cysts

Multi-loculated bone cysts are usually

included in this category Since the

first description of these cysts, many

authors have presented series

detail-ing the clinical characteristics and

diagnostic features of this disease

entity and discussing treatment

modalities

Unicameral bone cysts are

be-nign, fluid-filled cavities that tend to

expand and weaken the local area

They are generally seen in the

metaph-yseal areas of long bones in

skeletal-ly immature persons The etiology of

these lesions, which represent about 3% of biopsied bone tumors, is elu-sive They usually present with a pathologic fracture (Fig 1) Such fractures occur through thin, weak-ened bone and are generally not grossly displaced, nor are they diffi-cult to treat The cysts have a pre-dilection to occur in males more fre-quently than in females (2.5:1) Most patients (reportedly as many as 85%) are under 20 years of age.1

Once diagnosed, unicameral bone cysts continue to be a dilemma for the clinician because the natural history and management remain controversial Historically, recur-rence rates have ranged from 20%

to 50% after the various forms of treatment.2 Classification of indi-vidual lesions in a way that pre-dicts their natural history has been difficult Various authors have at-tempted to determine prognosis on the basis of patient age, site, size, or

history of previous fracture, but for any single cyst, these factors are unreliable

Pathogenesis

Various mechanisms have been proposed for the pathogenesis of unicameral bone cysts On the basis of electron-microscopic find-ings, Mirra et al3 considered them

to be intraosseous synovial cysts Jaffe and Lichtenstein4 observed dysplastic areas, which they be-lieved developed in response to trauma Cohen5,6 hypothesized that the cyst forms as a response to venous occlusion in the intramed-ullary space

Histologic examination of these lesions has been relatively unre-warding in regard to their patho-genesis Generally, the cyst walls are lined with a fibrous membrane, with occasional giant cells There is

no evidence of endothelial lining.1

It has been proposed that there are synovial cells in the lining,

resem-Dr Wilkins is Assistant Clinical Professor of Orthopaedics, University of Colorado School of Medicine, Denver.

Reprint requests: Dr Wilkins, Institute for Limb Preservation, Denver Orthopedic Specialists, PC, Suite 5000, 1601 East 19th Avenue, Denver, CO 80218

Copyright 2000 by the American Academy of Orthopaedic Surgeons.

Abstract

Unicameral, or solitary, bone cysts are unusual tumors seen in the ends of long

bones in skeletally immature persons The etiology of these lesions is poorly

understood Various hypotheses have included dysplastic processes, synovial

cysts, and abnormalities in the local circulation Most patients present with a

nondisplaced pathologic fracture, but occasionally cysts are found incidentally.

Plain radiographs typically show a symmetric lesion with cortical thinning and

expansion of the cortical boundaries Once diagnosed, unicameral bone cysts

con-tinue to be a treatment dilemma Traditional methods, such as prednisolone

ther-apy, usually involve multiple anesthetics and injections and are associated with

high recurrence rates Major surgical procedures, such as wide exposure,

curet-tage, and bone grafting, may be somewhat more effective, but still carry with them

significant morbidity and recurrence rates Newer techniques involving

percuta-neous grafting with allograft or bone substitutes or a combination of the two are

promising in light of their low complication rate and lower reoperation rate.

J Am Acad Orthop Surg 2000;8:217-224

Ross M Wilkins, MD, MS

bling the type A and B cells seen in

synovial tissue.3 The fluid within

the cyst has been analyzed and

shown to contain high levels of

oxygen-free-radical scavengers and

prostaglandins (prostaglandin E2,

interleukin-1, and proteolytic

en-zymes).7 These substances, which

cause bone resorption, may play a

role in the formation and growth of

cysts The fluid appears to be

inter-stitial fluid transudate or exudate

The cyst fluid has a lower total

pro-tein content than serum but higher

levels of protein-bound

hydroxy-proline, lactate, and alkaline

phos-phatase.8,9 Komiya et al7

demon-strated the role that these factors may have by showing that injection

of cyst fluid into mouse bone caused bone resorption

More recent research supports the theory that a vascular occlusion phenomenon occurs within the cyst.10 The pressures within a cyst are elevated above venous pres-sures It appears that if radiopaque dye is injected into the cyst with enough pressure, the dye can be extruded into the venous system of the limb Reestablishing these out-flow channels may assist in the involution of the cyst.10 Others have proposed that simply

lower-ing the interstitial pressure by mul-tiple perforations may cause cyst involution.11

Clinical Features

The symptoms of unicameral bone cysts are most often brought on by trauma On examination, the area is slightly warm and swollen Radio-graphs usually reveal a nondis-placed or minimally disnondis-placed frac-ture through an area of very thin, expanded cortical bone Occasion-ally, a fragment of the cyst wall has fractured and fallen into the fluid cavity This is evidenced by the radiographic “fallen leaf sign”12

(Fig 2)

The differential diagnosis in-cludes aneurysmal bone cyst and fibrous dysplasia When additional studies are warranted, magnetic resonance imaging most accurately delineates the central fluid collec-tion If a pathologic fracture has occurred, a fluid level may be visu-alized, mimicking the appearance

of an aneurysmal bone cyst There

is no convincing evidence, however, that a unicameral bone cyst will convert to an aneurysmal bone cyst

or other bone lesion

Unicameral bone cysts usually occur in younger patients The most common site is the femur, fol-lowed by the proximal humerus

At diagnosis, many cysts are imme-diately adjacent to, and appear to involve, the epiphyseal growth plate, which supports the theory that this is a growth disturbance rather than a true tumorous pro-cess Minor growth disturbances occasionally occur (10% of cases in one study13) When fractures do become evident, they rarely in-volve the growth plate itself

It is the opinion of many authors that the proximity of a cyst to the growth plate and its size at diagno-sis are directly related to the prog-nosis.2 Diaphyseal cysts do occur

Figure 1 A, A 9-year-old boy complained of pain in his right arm subsequent to a fall

while running A pathologic fracture through the cystic area in his proximal humerus was

barely discernible on this anteroposterior radiograph The patient was immobilized, and

healing was allowed to occur The patient subsequently underwent percutaneous injection,

by means of a two-needle technique, of a mixture of demineralized bone matrix and

autolo-gous marrow aspirate from his iliac crest Three weeks postoperatively, healing of the

frac-ture and some opacification of the cystic areas were demonstrated The superior cortical

perforation was made with a 5-mm cannulated needle device; the inferior perforation was

made with a bone-marrow aspiration needle B, Cortical hypertrophy and further

opacifi-cation of the cystic area were noted 8 weeks postoperatively The patient was released to

normal activities C, Radiolucency persisted at 3 years postoperatively; however, the

thick-ness of the cortical margins was maintained, and a slightly increased diameter of the

diaph-ysis was evident The patient was asymptomatic and participated in contact sports.

Admission 8 wk postop 3 yr postop

and can fracture Others have

pro-posed that patient age and

patho-logic fracture are directly related to

the future problems that may be

encountered.14,15 It is evident that

these cysts progress from active to

quiescent to an involutional stage

in the course of their natural

his-tory The difficulty for the clinician

is to assess the current stage of the

cyst at the time of diagnosis

Treat-ment of an active cyst may be

unsuccessful, whereas treatment of

a quiescent or involutional cyst

may be successful but unnecessary

The cyst usually progressively

shrinks as the patient approaches

skeletal maturity and may heal

spontaneously after growth is

com-pleted.16

Treatment Indications

There are two basic scenarios in which a unicameral bone cyst is diagnosed Occasionally, the cyst is discovered incidentally during investigation for another complaint

Usually, however, the cyst is symp-tomatic or is associated with a pathologic fracture (68% of cases in one study15) (Fig 1)

In the first situation, it is often difficult to decide whether the cyst

is in the active, latent, or involutional stage The mere size of the cyst it-self is probably of less importance than the structural properties of the area The strength of a cylinder is proportional to the square of its diameter Therefore, unless there is

a tremendous amount of cortical thinning, there may not be a compa-rable decrease in strength as a cyst expands the cortical margins If a cyst is discovered incidentally in an asymptomatic patient, it may be reasonable to choose close observa-tion rather than a surgical proce-dure If the cyst is active and obvi-ously enlarging during observation (3 to 6 months), treatment may be appropriate If, however, a cyst remains asymptomatic and the patient is able to maintain normal activities, continued observation is warranted, because the cyst may eventually resolve on its own One exception to this guideline is when a large cyst involves the subtrochan-teric region of the femur Early treatment may be needed to avoid fracture due to the high forces to which that area is normally subjected

A cyst that is symptomatic has

an incompetent osseous structure and has undergone either an obvi-ous or an undetected pathologic fracture Some authors have sug-gested that such a cyst will then undergo an involutional process and heal (Fig 3) However, in closely observed series, this occurs less than 10% of the time17(Fig 4)

In light of this statistic, the consen-sus is that the surgeon should allow the cyst to heal before proceeding with treatment By waiting, inter-nal fixation can usually be avoided The exception is when the fracture

is in a high-stress weight-bearing area, such as the femur

Surgical treatment of UBCs re-mains controversial Suggestions range from a hemicylindrical subto-tal resection18to a saline injection.10

Interpreting reported clinical series presents a dilemma because it is virtually impossible to ascertain whether the cysts in these studies were active, latent, or involutional when they were treated Few stud-ies have reported a clear and pre-cise set of criteria for treatment.15

Figure 2 A 9-year-old boy fractured his

proximal humerus playing baseball The

“fallen leaf sign” (arrow) is occasionally

seen in fractures through unicameral bone

cysts The cortical fragment becomes

dis-lodged from the margin at the time of

frac-ture and literally floats to the bottom of the

cystic structure.

Figure 3 A, Anteroposterior view of a

uni-cameral bone cyst in the proximal fibula in

an 8-year-old boy who had sustained a

pathologic fracture while running B,

Cortical thickening and partial obliteration

of the space was seen at 3 months The patient returned to normal activities, and observation was continued He was asymp-tomatic 3 years after the fracture.

Injection Techniques

In an early paper, Scaglietti19

described the technique of injecting

methylprednisolone into unicameral

bone cysts He reported successful

healing of the cyst with much less

morbidity than with resection or

curettage plus bone grafting The

procedure involves injecting

meth-ylprednisolone into the cyst under

fluoroscopic control while using

radiopaque dye to confirm entry

into the cyst Aspiration of the cyst

is done prior to injection The

return of clear, straw-colored fluid

is confirmatory of the diagnosis If

grossly bloody fluid is encountered,

a formal biopsy is advised to

ascer-tain whether the lesion is an

aneu-rysmal bone cyst or another type of

lesion The cyst is then flushed with

saline, and methylprednisolone is

injected with either a one-needle or

a needle technique The two-needle technique allows efflux of the saline and excess fluid through

an outflow needle in another region

of the cyst

Although in principle this proce-dure would seem to be advanta-geous by decreasing the morbidity due to a major surgical procedure, unfortunately it has not proved to

be very effective and usually in-volves multiple injections and anes-thetics.19 Overall, a review of the literature revealed recurrence rates

of 15% to 88% after an average of three injections.20-22 It is unclear what effect the methylprednisolone actually has on the local anatomy

Some authors advocate using only normal saline, because they feel that the mechanical disturbance of the injection is the important factor, rather than the agent itself.10 Other

authors also recommend that multi-ple percutaneous perforations be made in the cyst to normalize its local circulation and disrupt any venous obstruction.11

Surgical Techniques

Resection or curettage plus bone grafting has been employed as the definitive treatment for unicameral bone cysts However, in published series, the recurrence rates have ranged as high as 45%.2,18 The high rates of recurrence may be due to the fact that surgically treated cysts are more active, more aggressive, and likely to be recurrent Active uni-cameral bone cysts tend to be treated more aggressively from the outset Most published series are reports from tertiary referral centers, where the patient populations have cysts with a more difficult clinical course

Figure 4 A, A unicameral bone cyst in a 14-year-old boy who sustained a pathologic fracture while throwing a baseball B, At 6 months,

the cyst appeared to be active, and healing was delayed at the fracture site The patient had continued symptoms C, Fluoroscopic view shows a pituitary rongeur being inserted to obtain a percutaneous biopsy specimen by means of a cannula system D, With the diagnosis

of unicameral bone cyst confirmed on frozen section, the area was grafted percutaneously with calcium sulfate pellets and demineralized

bone matrix E, The area of the cyst and fracture showed cortical hypertrophy at 3 months postoperatively, and the calcium sulfate pellets

had been resorbed The patient resumed his activities in competitive baseball as a pitcher and remained asymptomatic.

The technique of resection or

curettage is relatively

straightfor-ward Once an approach has been

made to the bone, a cortical

win-dow is made, which allows access

to the entire contents of the cavity

(Fig 5) The clear fluid should be

removed, and the fibrous

mem-brane curetted from the cyst wall

If the cyst is immediately adjacent to

or involves the epiphyseal growth

plate, care must be taken to avoid

injury to the plate It is not

neces-sary to remove structural bone from

the outer cyst wall It is also not

ne-cessary to use adjunctive materials,

such as phenol or liquid nitrogen,

to perform this procedure Such

materials have secondary

complica-tions and may interfere with graft

and bone healing.15 Furthermore,

to employ these toxic and damag-ing agents is to imply that the local cells are the primary etiologic fac-tors There is no evidence to sup-port this hypothesis Neer et al2

found no difference in the rate of re-currence when phenol was used, and Schreuder et al15reported a 5%

rate of complications related to the use of liquid nitrogen

The choice of autologous bone graft or a substitute is dependent on the orthopaedic surgeon’s prefer-ence Autologous bone marrow, allograft, demineralized bone matrix (DBM), and other bone substitute materials have been used success-fully, thus sparing the patient the morbidity of an autograft harvesting site.23,24 Allograft bone chips have proved effective in the treatment of

cysts15,25,26(Fig 5) Calcium sulfate

in the form of plaster of paris has been used with a good success rate and a low recurrence rate (11%).23

The injection of autologous bone marrow is also effective, but this procedure requires several injec-tions14,27(Figs 1 and 6) Deminera-lized bone matrix, an osteoinductive material, can be injected percuta-neously into cysts28(Figs 1 and 6)

In the author’s series of 11 patients who received one percutaneous injection of DBM, no further treat-ment was required in any case after

2 years of follow-up, and 9 of the 11 cysts were entirely obliterated

An alternative technique in-volves the use of a combination of DBM (AlloGro, AlloSource, Den-ver) and calcium sulfate (OsteoSet,

Figure 5 A, Anteroposterior radiograph

of a 21-year-old man with a stress fracture through a cystic lesion in his left femoral

neck B, Computed tomogram through the lesion C, Photomicrograph of a

biop-sy specimen of a unicameral bone cyst (original magnification × 225) Note the bland-appearing fibrous tissue and

occa-sional giant cells D, Anteroposterior

view of the hip after the area was curetted and grafted with demineralized bone matrix and cancellous chips Cannulated screws were placed to stabilize the area

and prevent fracture E, Follow-up film at

6 months shows opacification in the area

of the lesion The patient had resumed all activities, including sports and running.

Wright Medical Technology,

Ar-lington, Tenn).29 The technique

in-volves perforation of the cyst wall

with a specially designed trocar

sys-tem (Wright Medical Technology)

(Fig 4) A biopsy specimen is

ob-tained, and the cyst is then irrigated

with normal saline under

fluoro-scopic control and packed through

the trocar with a 50:50 mixture (by

volume) of DBM and calcium

sul-fate pellets (Fig 7) This effectively

obliterates the cyst space and allows

rapid bone growth stimulation As

the calcium sulfate dissolves over 6

to 8 weeks, the DBM stimulates

local bone to grow into the vacated

space

In one series,29 11 patients with

unicameral bone cysts were treated

by using this technique Seven cysts

occurred in skeletally immature

patients After an average

follow-up period of 12 months (range, 4 to

20 months), no patient required additional treatment or sustained a fracture All patients subsequently returned to normal activities There has been only one recurrence, in a young patient who has remained asymptomatic and has not required further surgery The remaining 10 patients healed uneventfully after one procedure and had no subse-quent fractures

Complications

While the rate of complications fol-lowing injection or surgery is low, the relative morbidity of each treat-ment modality should be assessed

The most common complication of treating unicameral bone cysts is recurrence of the lesion after treat-ment and developtreat-ment of a subse-quent fracture In active and

other-wise healthy children, the initial fracture through a unicameral bone cyst can usually be weathered with-out much difficulty However, with repeated fractures, the psycho-logical consequences for young patients and their families can be significant The children become tentative in their daily activities, because of either their own fear of fracture or that of their parents Occasionally, family members be-come obsessed with the “fragility”

of the child, which may drastically alter the young patient’s lifestyle and attitudes Whichever treatment modality is utilized, it should be the procedure that will return the pa-tient to early normal activities and sufficiently heal the area so that there is no further concern about fracture with minimal trauma

Summary

Unicameral bone cysts are

relative-ly rare tumors, which are usualrelative-ly found in children The etiology of these cysts is unclear, although the-ories range from dysplastic pro-cesses to venous occlusion The ac-tivity of any one individual cyst is difficult to predict If the cyst is found incidentally and the patient

is asymptomatic, observation for several months may help determine whether the cyst is active, quies-cent, or involutional If the cyst is quite large and obviously active or

is associated with a pathologic frac-ture, treatment should be consid-ered Few cysts become completely obliterated subsequent to a patho-logic fracture, but the cyst may have sufficient cortical thickening

to provide a stable construct In this situation, continued observa-tion may be appropriate, and surgi-cal treatment may be unnecessary However, if it is evident that the cyst is still active after appropriate fracture healing, more aggressive treatment should be considered It

Figure 6 A, Lateral view of the leg of an 11-year-old girl in whom a fracture through a cyst

occurred while she was doing cartwheels She was treated conservatively with cast

immo-bilization B, Cortical thickening and remodeling were demonstrated 8 months after the

fracture, but the cyst persisted and appeared to be enlarging The patient was

asympto-matic, and observation was continued She was allowed to maintain normal activities

C, At 28 months after the original fracture, the patient was having pain once again during

cheerleading activities Radiograph shows partial involution of the proximal cystic area but

continued growth of the distal area Percutaneous grafting was performed with the use of a

mixture of DBM and autologous marrow aspirated from her iliac crest D, Radiograph

obtained 2 years postoperatively shows cortical thickening and involution of the cystic

areas The patient continued her cheerleading activities without pain or problems.

appears that more recent

percuta-neous treatment techniques are as

effective as prednisolone injection

and formal curettage with bone

grafting and usually require only

one anesthetic and procedure

Accurate determination of the stage of activity of the cyst at pre-sentation, coupled with appropri-ate care, is the key to successful treatment Surgical treatment may not be necessary for quiescent or

involutional cysts However, ac-tive, progressive cysts require intervention The ideal surgical procedure is predictable and suc-cessful in obliterating the cyst in one attempt

Figure 7 A, Lateral radiograph of a 12-year-old boy who had sustained three previous fractures of his distal tibia shows a multiloculated

bone cyst of the distal tibial metaphysis The patient had become very tentative in his daily activities and refused to participate in any

physical education classes or sports B, Film obtained 1 week after the lesion was curetted and percutaneously grafted with a mixture of calcium sulfate (OsteoSet) and DBM (AlloGro) Notice that the anterior portion of the cyst was not grafted C, Postoperative film

obtained at 8 weeks shows corticalization of the boundaries of the grafted areas A small anterior cyst remnant persists The patient had

resumed all athletic activities and was asymptomatic D, Additional remodeling in the intramedullary area was demonstrated at 6

months The patient had maintained a normal active lifestyle, including downhill skiing and soccer.

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