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©1991 British Editorial Society ofBone and Joint Surgery
0301-620X/9l/3 139 $2.00
JBoneJoint Surg[Br] 1991 ; 73-B :423-9.
VOL. 73-B, No. 3, MAY 1991 423
THE ACETABULAR RIM SYNDROME
A CLINICAL PRESENTATION OF DYSPLASIA OF THE HIP
K. KLAUE, C. W. DURNIN, R. GANZ
From the Inselspital, Berne
The acetabular rim syndrome is a pathological entity which we illustrate by reference to 29 cases. The
syndrome is a precursor of osteoarthritis of the hip secondary to acetabular dysplasia. The symptoms are
pain and impaired function.
All our cases were treated by operation which consisted in most instances of re-orientation of the
acetabulum by peri-acetabular osteotomy and arthrotomy of the hip.
In all cases, the limbus was found to be detached from the bony rim of the acetabulum. In several
instances there was a separated bone fragment, or ‘os acetabuli’ as well.
In acetabular dysplasia, the acetabular rim is subject to abnormal stress which may cause the limbus to
rupture, and a fragment of bone to separate from the adjacent bone margin. Dysplastic acetabuli may be
classified into two radiological types. In type I there is an incongruent shallow acetabulum. In type II the
acetabulum is congruent but the coverage of the femoral head is deficient.
The term ‘hip dysplasia’ means an abnormality of shape
or size of the acetabulum or femoral head, or of their
proportions or alignment one to the other. Most often, it
is the acetabulum which is dysplastic and its dispropor-
tion with the femoral head gives rise to symptoms. We
will describe what we call the acetabular rim syndrome.
The nature of bone fragments about the acetabular
rim has long been a subject for discussion. Anatomists
have regarded these structures as remnants of secondary
centres of ossification and a normal part of development
of the acetabulum (Lilienthal 1909). They were described
first by Albinus in 1737 and later, in 1876, by Krause who
proposed the name ‘os acetabuli’. The fragments, which
are of varying sizes, can appear as early as six years of
age, and may disappear before 20 years (Perna 1922 ; de
Cuveland and Heuck 1957; Ponseti 1978). Persistence
after this age has been explained on the hypothesis of
‘hormonal disturbance’ (Freedman 1934).
Radiologists and clinicians have described the
K. Klaue, MD
C. W. Durnin, MD, Clinical Research Fellow
R. Ganz, MD, Professor of Orthopaedics
Department of Orthopaedic Surgery, Inselspital, CH-30l0 Berne,
Switzerland.
Correspondence should be sent to Dr K. Klaue.
fragments as non-specific osteochondritis comparable
with Perthes’ disease of the femoral head (Kargus 1933).
Dysplasia of the acetabulum appearing at puberty has
been attributed to secondary ‘absorption’ of the bony
acetabulum (Niethard 1984). The appearance of frag-
ments at later ages and their persistence have been
observed following trauma (Waschulewski 1967 ; Caudle
and Crawford 1988), rickets (Fromme 1921), osteomye-
litis, tuberculosis, and osteochondritis dissecans (Ruhle
1921 ; Schinz 1922). Fragments have been ascribed to
overloading of the acetabular rim in a dysplastic joint
causing fracture and separation of a segment of the rim
(Fiedler 1951). They are sometimes associated with cysts
in the acetabular roof (Outland and Flood 1936). In a
study of 1 1 1 1 pelvic radiographs from an asymptomatic
population, bone fragments were found in 2% to 3%
(Arho 1940).
Tears of the limbus, with or without an associated
bony fragment, are known to occur after traumatic
dislocation of the hip (Paterson 1957; Dameron 1959;
Rashleigh-Belcher and Cannon 1986 ; Shea, Kalamchi
and Thompson 1986). Limbus tears occurring without a
history of injury have been described only recently
(Altenberg 1977 ; Currier and Fitzerald 1988) and there
has been no explanation of their cause or their relation to
acetabular dysplasia.
Limbus tears have been diagnosed by arthroscopy
(Suzuki et al 1986; Ikeda et al 1988), conventional
arthrography (Dorrell and Catterall 1986) and CT scan
Fig. I
Fig. 2 Fig. 3
424 K. KLAUE, C. W. DURNIN, R. GANZ
THE JOURNAL OF BONE AND JOINT SURGERY
(Paterson 1987). There are no previous descriptions of
the use of the computerised arthrotomography, nor of
magnetic resonance imaging as we employ them.
CLINICAL MATERIAL
History. The presentation in all our cases was similar.
None were symptom-free. Most were young adults. They
complained of knife-sharp pain in the groin and a
sensation oflocking ofthe hip. These symptoms occurred
after a period of sitting, or sometimes after walking.
Some patients also described episodes of ‘giving way’
such as an inexplicable fall. Usually the pain could be
quickly relieved, often by shaking the limb, whereupon
normal walking could be resumed. Some activities which
predisposed to symptoms were rapid descent of stairs,
use of the breast-stroke when swimming, and sports such
as tennis or football. The common factor seemed to be
forced movements of adduction in combination with
rotation in either direction.
Questioning frequently discovered a long-forgotten
incident in which the hip had been stressed particularly
in rotation, causing sudden pain and the sensation of
‘dead leg’. Weight-bearing was painful and a period of
support with crutches had sometimes been necessary.
Two patients with these symptoms had undergone
inguinal or femoral herniorrhaphy ; another was sus-
pected of having entrapment of the lateral cutaneous
nerve of the thigh.
Clinical signs. Between painful episodes the affected hip
may be so normal to casual examination that an articular
origin for the pain may be doubted. When episodes of
locking occur frequently, residual pain may cause a slight
limp, possibly associated with weakness of the hip
abductors. However, in most cases pain can be elicited
by passive movement of the thigh into full flexion,
adduction and internal rotation (Fig. 1). This combina-
tion brings the proximal and anterior part of the femoral
neck into contact with the rim of the acetabulum
(Kapandji 1985), at exactly the point where the labrum
is likely to be damaged. The test exerts a shear force on
Clinical examination of the hip by passive flexion-
adduction-internal rotation. A sudden sharp pain is
elicited in the groin.
the limbus at its attachment to the acetabular bony
margin and if a tear is present, the dislocation of the
limbus may be palpable. Sometimes the opposite move-
ments, passive hyperextension with external rotation,
may also produce pain and a sensation of apprehension.
Imaging. It is not always possible to confirm the clinical
diagnosis of alabrum tear by imaging. A well-centred
anteroposterior radiograph, or the ‘faux-profil’ view of
Lequesne and de Seze (1961) may demonstrate a
congruent but short acetabular roof and an os acetabuli
(Fig. 2). On either of these views the radius of the
acetabulum may be seen to be greater than that of the
Figure 2 -Radiograph to show a well centred
femoral head. The acetabular rim has a loose
fragment (arrow) and the weight-bearing area
is therefore reduced. Figure 3 -Radiograph to
show incongruency between the femoral head
and the acetabulum. No free bony fragment is
visible in this instance.
Fig. 4
Fig. 5..r::i
.1. .r.
-‘- ,22’i3.88 .
,-
THE ACETABULAR RIM SYNDROME 425
VOL. 73-B, No. 3, MAY 1991
Right hip seen from the front. The limbus is detached from the bony
rim over one-fifth of its circumference.
CT scans. Figure 5a -Cysts in the anterior wall of the acetabulum
communicate both with the joint space and with a cyst on the anterior
quadrant of the capsule. Figure Sb -6 mm below the first cut, there is a
free fragment at the anterosuperior rim of the acetabulum. Operation
revealed an extra-osseous ganglion, communicating with an intra-
osseous cyst. The acetabular labrum was found to be detached at the
same site.
femoral head (Fig. 3). On the lateral view the anterior
centre-edge angle ofLequesne and de Seze (1961) and on
the anteroposterior view the lateral centre-edge angle of
Wiberg (1939) are reduced. Often the femoral head has
migrated laterally or anteriorly or in both directions, so
distorting the normal spherical shape of the acetabular
mouth into an oval.
In the few cases which we have investigated by
conventional arthrography and computerised arthroto-
mography we have been unable to demonstrate unequi-
vocally any tears of the limbus attachment, though we
were able to show hypertrophy, displacement and
truncation of the edge of the structure. Our own three-
dimensional adaptation oforthodox CT scanning (Klaue,
Wallin and Ganz 1988) allows a very accurate quantifi-
cation of the acetabular cover for the femoral head which
is not obtainable by conventional radiography. In six
cases this examination revealed the presence of cysts in
the soft tissues or intra-osseous ganglia at the edge of the
acetabular roof. In one case magnetic resonance imaging
suggested a limbus tear, which was confirmed at
operation.
Arthroscopy may permit visualisation of the limbus
but is difficult to perform and we have not used it.
Operative findings. In all our cases, the diagnosis of a torn
limbus made clinically was confirmed at operation. The
lesion was situated at the anterosuperior quadrant of the
acetabular rim and resembled a bucket-handle tear of a
knee meniscus. A hook could be passed between the body
of the limbus and the bone edge (Fig. 4).
Intra-osseous ganglia demonstrated by CT scans
(Fig. 5) were detectable from within the joint by use of
the same hook. These ganglia had a membranous lining
similar to that found in other intra-osseous ganglia.
Extra-osseous ganglia, when present, extruded from the
joint through the defect in the limbus to emerge on the
surface of the capsular structures, extending as much as
8 cm proximally.
Os acetabuli were easily identified. On their wider
surfaces we found true articular cartilage.
Treatment. In all our patients the primary problem was
deficiency of acetabular cover over the femoral head : the
principle of surgical management was to improve this by
re-orientating the acetabulum by means of a pen-
acetabular osteotomy (Ganz et al 1988, Fig. 6). In 12
Improved cover of the femoral head obtained by pen-
acetabular osteotomy. Re-orientation of the acetabulum
was achieved in three planes.
cases the tear in the limbus was repaired ; in 12 others the
torn limbus was resected ; in the remaining five the tear
wasjudged too small to need specific treatment.
Since our follow-up does not exceed four years it is
not yet possible to recommend a definitive management,
save to say that the slowest recovery was seen in those
cases in which the tear was repaired. Two of these
required revision to resect a re-ruptured limbus. In these
patients, and in those in whom the limbus was resected
primarily or left undisturbed the symptoms have all been
relieved.
RESULTS
Tables I, II an III summanise the clinical and radio-
graphic findings. The five hips in which there was
radiological evidence of degenerative change have been
excluded, because their radiographic measurements had
become unreliable. We still consider these cases to be
part of the series, since the history, examination, and
surgical findings confirm a common cause.
The data show that all cases with lesions of the
acetabular rim had reduced acetabular cover of the
femoral head. Bone lesions occurred most frequently in
hips which were congruent, that is those in which the
acetabular and femoral head centres closely corresponded
(Fig. 7). Soft-tissue lesions, not radiologically visible,
Spercentage cover of the femoral head assessed by computerised graphical
evaluation
te lat, distance from tear-drop to femoral head ;rf/ra, radius of the femoral
head divided by radius of the acetabulum, x 100 assessed on the
anteropostenior radiograph ;Cf-ca, distance between the centre of the
femoral head and the centre of the acetabulum, assessed on the anteropos-
tenor radiograph
Table II. Radiological results for 10 hips with bony
lesions of the acetabulum (six had one or more intra-
osseous ganglia; nine had a bony fragment at the
acetabular rim, four of them with an intra-osseous
ganglion)
Cf-ca (mm)
S Table I
Fig. 7
Table III. Radiological results for 14 hips with no
bony lesions visible on the acetabulum (all had a
limbus lesion, four with a soft-tissue ganglion)
426 K. KLAUE, C. W. DURNIN, R. GANZ
THE JOURNAL OF BONE AND JOINT SURGERY
Schematic diagram of the measurements employed to
estimate joint congruency and migration of the femoral
head.
Rf-ra is the difference in length between the femoral head
radius and the acetabulan radius.
Cf-ca is the distance between the centres of femoral head
and acetabulum (this method is also valuable on the ‘faux
profil’ image).
e lat is the distance between the vertical lateral plane of
the ‘tear drop’ and the medial vertical tangential line
touching the femoral head.
Table I. Radiographical results for 24 hips with no degenerative
changes
Measurement Mean ±SD Normal Source
Cover5 (per cent) 48 ± 2 70 Klaue et al
1988
LateralCEangle
(degrees) 6 ±3 >25 Wiberg 1939
Anterior CE angle
(degrees) -1 ± 4 >20 Lequesne and
de Seze 1961
Roofangle
(degrees) 23 ± 2 <10 T#{246}nnis1984
elat(mm)t 11.5±1
rf/ra (per cent)t 76 ± 3
Cf-ca(mm)t 7.5±1
Measurement Mean ± SE Normal
Cover (per cent) 50 ± 2 70
Lateral CE angle (degrees) I 1 ± 3 >25
Anterior CE angle (degrees) 4 ± 4 >20
Roofangle (degrees) 17.5 ± 2 <10
elat(mm) 11.1±1
rf/ra (percent) 85 ±3
3.4±1
Measurements Mean ± SE Normal
Cover(percent) 47 ±2 70
Lateral CE angle (degrees) 2 ± 5 >25
Anterior CE angle (degrees) -5 ± 5 >20
Roofangle (degrees) 27 ±4 <10
elat(mm) 11.9±1
rf/ra(percent) 70 ±3
Cf-ca(mm) 10.4±2
*see Table I
Fig. 8 Fig. 9
THE ACETABULAR RIM SYNDROME 427
VOL. 73-B, No. 3, MAY 1991
occurred in joints in which the radius of the acetabulum
was larger than that of the femoral head. The difference
between the measurements rf/ra and Cf-Ca in Tables I
and II is significant (p =0.05). The ANOVA test shows
the differences between the measurements rf/ra, Cf-Ca
and RA also to be significant (p =0.05).
DISCUSSION
Bedouelle (1984) described many shapes of acetabulum
as seen on conventional radiographs, but did not attempt
a classification. We recognise two groups of radiological
features associated with abnormal loading.
Type I: Hips in which the acetabulum is shallow, lies
more vertical than normal, and has a radius of curvature
greater than that of the femoral head. Such joints are
radiologically incongruent (Fig. 3).
Type II: Hips in which the acetabulum provides less than
normal cover for the femoral head (‘short roor), and has
a radius of curvature similar to that of the femoral head.
Suchjoints are congruent (Fig. 2).
A hip of type I is subject to abnormal shear force
and is potentially unstable. Often the labrum is hyper-
trophic and the capsule thickened (T#{246}nnis 1984). CT
scans show the acetabulum elongated in the craniocaudal
direction. The smallest diameter of this oval shape lies
transversely and matches the diameter of the femoral
head. Hypertrophy of the labrum and capsule can be
interpreted as a physiological response to the shear force
and an attempt to correct the bony mismatch of the joint.
Eventually the overstressed labrum may become de-
tached from the acetabular rim ; in these cases the rim
itself is unlikely to be overloaded (Fig. 8).
A hip of type II, which is congruent, is essentially
stable. However, the relative shortness of the acetabular
roof reduces the area of the loaded surface which is then
subject to increased pressure, particularly at that part of
the acetabular margin which lies against the upper pole
of the femoral head (Greenwald and O’Connor 1971;
Pauwels 1976 ; Bombelli 1983). The force evokes local
remodelling (Stadler et al 1982 ; Rubin 1984 ; Klaue and
Perren 1989) the result ofwhich may be a fatigue fracture
and separation of a rim fragment (Fig. 9). Often, there
are associated intra-osseous cysts in the acetabular roof
which communicate with the joint. These are concen-
trated at the overloaded roof edge. By their confluence
they may bring about fragmentation, and the appearance
of a so-called os acetabuli.
Reports of possible acetabular fatigue fractures are
few (Schmidt 1967; Maurer 1970; Kaps and Niethard
1986), and none of these authors commented on the
significance of the limbus. The associated hip dysplasia
is rarely mentioned (Fiedler 1951). Analysis of our own
cases suggests that fatigue fractures, with or without bone
cysts, only occur in congruous (type II) hips. In some of
our cases early radiographs were available in which no
bony fragments were visible. It is possible that some
fragments may represent persistent embryological rem-
nants which, in consequence ofoverload, fail to integrate.
Whatever the type ofdysplasia, a lesion at the acetabular
rim and the consequent shear forces generated are likely
eventually to cause instability and progressive degenera-
tion of the whole joint.
Ganglia or cysts around the hip joint have been
reported by many authors (Harrison, Schajowicz and
Trueta 1953 ; Eggers et al 1963 ; Samuelson, Ward and
Albo 197 1 ; Margreiter, Steiner and Mikuz 1 978 ; Garcia
and Chevalley 1985 ; Bergenudd et al 1987).
Cysts within soft tissues are more likely to occur in
the oval, unstable joints (type I) and partial detachment
ofthelimbus may be the initiating cause. Fetal dissections
have shown that a defect can occur between the labrum
and the rim at a very early stage of development : this
may constitute an area of potential vulnerability in the
adult (Walker 1981). A valvular pumping mechanism
from the joint to the cyst may play a part (Jayson and
Dixon 1970). Such a mechanism would fit in with our
proposition that ganglionic cysts arise through local
overloading secondary to reduced acetabular cover for
the femoral head. Cyst formation within the limbus may
be analogous to cystic degeneration in a knee meniscus
(Ueo and Hamabuchi 1984; Matsui, Ohzono and Saito
1988). No one has previously suggested that acetabular
dysplasia may be an essential precursor of cysts and
ganglia around the hip.
Cysts or ganglia within bone arise through splits in
Figure 8 -Pathophysiology of the acetabular rim in
the incongruent, type I hip. In this configuration, the
labrum is overloaded and may shear from the
acetabular bony rim. Figure 9 -Pathophysiology of
the acetabular rim in the congruent, type II hip. In
this configuration, the resultant force acts upon a
reduced articular surface. A ‘fatigue’ fracture may
occur causing a bony fragment or os acetabuli.
428 K. KLAUE, C. W. DURNIN, R. GANZ
THE JOURNAL OF BONE AND JOINT SURGERY
the acetabular cartilage (Landells 1953). These develop
after injury to the subarticular bone and multiple foci of
such damage may allow penetration of synovial fluid
deep to the subchondral bone plate to form cysts in the
cancellous bone (Freund 1940). Synovial fluid may also
penetrate bone through a defect between a damaged
limbus and the articular cartilage (Itoigawa, Azuma and
Kako 1980) which may be why such cysts were found in
hips with acetabular dysplasia of type II. Intra-osseous
ganglia are often visible on CT scans while remaining
undetectable on conventional radiographs despite their
occasionally large size. Our observations confirm the
report of McBeath and Neidhart (1976) that juxtalabral
intra-osseous ganglia may communicate with extra-
osseous ganglia.
In the past we have treated acetabular dysplasia by
the familiar techniques of shelf arthroplasty and Chiani
osteotomy. However, these techniques are extra-articular
and do not address the problem of the limbus tear. These
operations may eliminate pain if it is caused solely by
instability, but where the pain arises from the acetabular
rim syndrome the causative lesion may remain hidden
beneath such extra-articular cover, and may continue to
give rise to symptoms and to predispose to progressive
degeneration (Saito et al 1989). A better principle by
which to improve both femoral head cover and joint
congruency is to re-orientate the acetabulum (LeCoeur
1965 ;T#{246}nnis,Behrens and Tscharani 1981). Techniques
to achieve this are, however, both major and technically
demanding.
It is not clear what should be the surgical manage-
ment of the damaged limbus or the detached bone
fragment. The merit of refixing the limbus to the bony
rim may be questionable. Conversely, limbus resection
may impairjoint lubrication by removing the hydrostatic
function of the structure (Takechi, Nagashima and Ito
1982), and so hasten degenerative change (Butel et al
1973).
The acetabular rim syndrome is best recognised
through awareness of its existence, appreciation of its
symptoms and understanding of its aetiology.
We thank Dr C. Engel of the MEM Institute of Biomechanics, Berne,
for the statistical evaluation of our data and Mrs D. Hansen (Seattle,
Washington, USA) for preparation ofthe manuscript. Mr D. Reynolds,
FRCS, of London, had the patience to revise the whole manuscript and
we owe him our gratitude for his help.
No benefits in any form have been received or will be received
from a commercial party related directly or indirectly to the subject of
this article.
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