1. No category

n Feature Article
Etiology and Severity of Impingement
Injuries of the Acetabular Labrum:
What Is the Role of Femoral Morphology?
Christopher J. Dy, MD, MSPH; Steven J. Schroder, MD; Matthew T. Thompson, MS;
Jerry W. Alexander, BS; Philip C. Noble, PhD
abstract
Full article available online at Healio.com/Orthopedics. Search: 20120525-12
Injuries to the acetabular labrum have been seen in association with femoroacetabular
impingement, but recent studies have reported labral pathology in patients with normal hip morphology. The hypothesis of the current study was that labral lesions could
occur without femoroacetabular impingement but that labral pathology would occur
more commonly and more severely in hip joints that exhibit reduced head–neck offset.
The presence, location, and severity of labral injury were recorded in 22 cadaveric
specimens. Computed tomography was used to define the anatomic parameters of
proximal femoral morphology. Three-dimensional modeling was used to simulate hip
positions that typically cause labral impingement, including high flexion and internal
rotation. Femoral morphology was compared between specimens with and without
labral pathology using descriptive statistics. Labral pathology was seen in 15 of 22
specimens and was located in the anterosuperior portion of the labrum. No difference
existed in age, femoral neck shaft angle, anteversion, acetabular depth, head diameter,
alpha angle, or beta angle between specimens with and without labral pathology. The
severity of labral pathology correlated with the alpha angle of the proximal femur.
This study demonstrates that damage to the labrum may occur in hips with normal
proximal femur morphology. However, the findings also indicate that the presence of
morphologic features that increase the risk of impingement may predispose the hip
joint to a characteristic pattern or severity of labral pathology. The results confirm the
importance of considering both femoral morphology and athletic-type activities of the
hip when determining the mechanism responsible for injury of the acetabular labrum.
Drs Dy, Schroder, and Noble and Messrs Thompson and Alexander are from the Institute of
Orthopaedic Research and Education, Houston, Texas; Dr Dy is also from the Department of Orthopaedic
Surgery, Hospital for Special Surgery, New York, New York; Dr Schroder is also from the Department of
Orthopaedic Surgery, University of California Irvine, Irvine, California; and Dr Noble is also from the
Barnhart Department of Orthopaedic Surgery, Baylor College of Medicine, Houston, Texas.
Drs Dy, Schroder, and Noble and Messrs Thompson and Alexander have no relevant financial relationships to disclose.
This study was performed at the Institute of Orthopaedic Research and Education, Houston, Texas.
Correspondence should be addressed to: Christopher J. Dy, MD, MSPH, Department of Orthopaedic
Surgery, Hospital for Special Surgery, 535 E 70th St, New York, NY 10021 ([email protected]).
doi: 10.3928/01477447-20120525-12
e778
A
B
C
Figure: Photographs of stage 2 damage of the acetabular soft tissues with a defined labral tear and
intact acetabular cartilage (A), stage 3B damage
with a labral tear and focal adjacent chondral pathology (B), and stage 4 damage with diffuse labral
and chondral damage (C).
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Impingement Injuries of the Acetabular Labrum | Dy et al
T
he acetabular labrum is a fibrocartilagenous rim of tissue that
enhances the stability of the hip
joint. The labrum encases the femoral
head within the acetabulum and restricts
hip motion, particularly in the extremes
of activity. By providing a partial seal of
the hip joint, the labrum limits the biomechanical stress placed on the soft tissues of
the joint. Previous investigation by the authors showed labral damage to negatively
effect the biomechanical environment of
the hip joint, with subluxation of the femoral head occurring in extremes of joint
motion when the labral seal is disrupted.1
This finding, coupled with observational
studies depicting a significant association
between labral and chondral pathology,2
illustrates the potential for further irreversible joint degeneration after an initial
injury to the acetabular labrum.
Although the exact mechanism of damage to the labrum has not yet been delineated, recent investigations have identified
specific maneuvers of the hip and morphologic variants of the hip joint as factors involved in labral injury. McCarthy
et al2 hypothesized that labral injury may
occur from a mechanism in which the anterior labrum is subjected to substantial
mechanical demands during pivoting or
twisting of the hip in moderate amounts
of flexion or extension. This hypothesis
has been supported by the current authors’
work, which demonstrated the substantial
tensile strains that are developed within
the labrum during these activities3 and
by motion capture analysis showing impingement during extreme ranges of motion experienced by dancers with morphologically normal hips.4
Alternatively, Ganz et al5 implicated
morphologic variants of femoral and acetabular anatomy as causative of nontraumatic labral pathology. Cam-type
femoroacetabular impingement, in which
reduced head–neck offset of the femur
exists, is thought to be more damaging
to the labrum than pincer-type femoroacetabular impingement, in which excessive
JUNE 2012 | Volume 35 • Number 6
deepening of the acetabulum exists.5 In
cam-type femoroacetabular impingement,
labral injury occurs secondary to internal
rotation of the abnormal femur in flexion
angles of approximately 90°. The shearing forces created during internal rotation
are thought to damage the cartilage, separating it from the overlying labrum in the
anterosuperior segment of the acetabular
rim.
Although the mechanisms of labral
injury proposed by McCarthy et al2 and
Ganz et al5 appeared to be plausible and
could coexist, Ganz et al5 concluded that
nearly all nontraumatic labral lesions occur secondary to femoroacetabular impingement.5 The current authors’ previous
work does not completely agree with this
conclusion,1,3 and, for this reason, further
investigation of the relationship between
labral pathology and femoroacetabular
impingement is warranted. This investigation is of particular merit because of the
potentially damaging consequences of
labral damage and the more common and
successful use of surgical techniques for
early treatment of femoroacetabular impingement.6-8
The current study investigated the effect of femoral morphology on the occurrence, severity, and location of labral
pathology. The hypothesis was that labral
lesions could occur in the absence of femoroacetabular impingement but that labral
pathology would occur more commonly
and more severely in hip joints that exhibited reduced head–neck offset. In addition, the authors evaluated whether the
site of labral pathology was concordant
with the impingement location of the femur along the acetabular rim during high
deep flexion of the hip joint.
Materials and Methods
Twenty-two fresh-frozen cadaveric
specimens consisting of a hemipelvis and
hip joint with capsule and labrum intact
were obtained from willed body programs
for use in this study. Details of age and
sex were available for 16 of the 22 do-
nors (9 men and 7 women; average age,
74 years). The soft tissues surrounding the
femoroacetabular joint were sequentially
removed, and the joint was disarticulated in a controlled and careful manner to
avoid damage to the labral and chondral
surfaces of the acetabulum. The articular
surfaces of the acetabulum and femoral
head of each specimen were carefully inspected for pathologic changes.
The severity of labral pathology was
classified using the staging system described by McCarthy,9 with modifications for use with cadaveric specimens.
Specimens with no sign of labral or chondral pathology were classified as stage 0.
Specimens with a discrete tear of the labrum at its free margin were classified as
stage 1. Specimens with a defined labral
tear, intact acetabular cartilage, and focal
damage to the subjacent area of the femoral head were classified as stage 2 (Figure
1A). Specimens with a labral tear and an
adjacent focal acetabular chondral lesion
were classified as stage 3. Further subdivision of stage 3 was based on the size of the
chondral lesion: specimens with chondral
lesions ,1 cm were classified as stage 3A
and those with larger chondral lesions as
stage 3B (Figure 1B). Specimens with a
diffuse labral tear with associated diffuse
articular cartilage degeneration were classified as stage 4 (Figure 1C). The locations
of the labral and chondral lesions around
the acetabular margin were identified using a clockface system, with the middle of
the inferior acetabular notch designated as
6 o’clock.10,11
Evaluation of Femoral Morphology
The femur and hemipelvis of each
specimen were scanned by computed tomography scanned, and 3-dimensional
(3-D) computer models of each specimen were reconstructed using customized
software (Mimics 9.1; Materialise, Ann
Arbor, Michigan). Computer analysis was
used to describe the morphology of each
femur by representing each bone as an assembly of discrete anatomic objects cor-
e779
n Feature Article
1A
1B
1C
Figure 1: Photographs of stage 2 damage of the acetabular soft tissues with a defined labral tear and intact acetabular cartilage (A), stage 3B damage with a labral
tear and focal adjacent chondral pathology (B), and stage 4 damage with diffuse labral and chondral damage (C).
responding to the head, neck, and shaft of
the femur. The procedure for describing
the morphology of each femur consisted
of initially segmenting each 3-D model
of the femur into its anatomic objects
and then defining the reference origin,
anatomic axes, and surface shape of each
object. It was then possible to define, in
mathematical terms, the relative displacement of each object with respect to any
other (eg, posterior slip of the femoral
head on the neck), as well as dysmorphic
changes in the inherent size and shape of
each object itself (eg, coxa magna). The
following morphologic parameters were
derived using this method:
1. The diameter of the femoral head,
defined by the sphere of best fit
to the subchondral surface of the
head.
2. The femoral neck axis, defined by
the line of best fit to the centroids
of a series of 20 cross sections
through the femoral neck.
3. The anterior head–neck offset,
measured using the method described by Eijer et al12 by calculating the distance between 2
lines parallel to the neck axis,
the first along the anterior cortex
of the femoral neck and the second along the apex of the anterior
femoral head (Figure 2).
4. The normalized anterior head–
neck offset ratio, defined by Eijer
et al12,13 as the anterior offset
of the femoral head divided by
e780
2
Figure 2: Determination of the anterior head–neck offset and the alpha angle by segmenting 3-dimensional reconstructed computed tomography models and analyzing individual components of the femur.
the head diameter. A vector was
drawn between the center of the
femoral head and the point at
which the surface of the anterior
half of the femoral head deviated
from a sphere.
5. The extent of the spherical surface of the femoral head, as described by:
• The alpha angle, defined as
the angle between the femoral neck axis and a line connecting vector from the center of the femoral head to the
anterior boundary of sphericity. This angle was recorded
after positioning the femur
to simulate the Dunn view in
45° flexion, a view shown by
Meyer et al14 to be the most
sensitive projection for detecting a large alpha angle.
Recent analysis of asymptomatic individuals showed
the mean alpha angle to be
48°, with 1 SD being 8°.15
The specimens were stratified
according alpha angle cutoff
levels of 55° (1 SD above
mean) and 63° (2 SD above
mean)15 for further analysis.
• The beta angle, defined as the
angle between the posterior
interruption of sphericity and
the femoral neck vector.16
6. Femoral anteversion in specimens
with intact femoral condyles (16
of 22 specimens) using a simula-
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impingement and is hereafter referred to
as the impingement zone.
3A
3B
Figure 3: Anteroposterior (A) and posterolateral (B) 3-dimensional reconstructed computed tomography
views showing a specimen impinging while internally rotated in 90° of flexion during the virtual range-ofmotion simulation, during which the hip joint was internally rotated until the surface of the anterior femoral
neck impinged with the simulated surface of the labrum.
tion of the table-top examination
of the angle of the femoral head
and neck relative to the frontal
plane of the body.
7. The neck–shaft angle between the
femoral neck vector and the shaft
of the femur.
Computer Simulation of Labral
Impingement
Virtual simulation of the motion of
each hip joint to the point of labral impingement was performed with the computer model of each specimen using kinematic routines (Unigraphics 4.0; EDS,
Plano, Texas). The virtual model of each
pelvis was positioned such that a plane
tangent to the anterior surfaces of both
anterior superior iliac spines and the pubic symphysis was oriented vertically.
The horizontal axis of the pelvis was
defined by a vector joining the anterior
superior iliac spines. The femur was initially oriented in (1) neutral adduction,
defined by orienting the epicondylar axis
parallel to the horizontal plane; (2) neutral flexion/extension, defined by aligning the vertical axis and the tangent line
to the posterior cortex at the midlength
of the diaphysis; and (3) neutral axial rotation, defined by aligning the posterior
condyles and the coronal plane. To rep-
JUNE 2012 | Volume 35 • Number 6
resent the acetabular labrum, a curve fit
was made to the rim of the acetabulum
and extruded 5 mm, corresponding with
the spatial location of the predicted surface of the labral margin.
Because it is generally appreciated
that femoroacetabular impingement occurs in positions of high flexion with
internal rotation, each hip joint model
was placed in a range of flexion angles
from 70° to 110°. At each flexion angle,
the hip joint was internally rotated until
the surface of the anterior femoral neck
impinged with the simulated surface of
the labrum (Figure 3). This was done to
examine maneuvers that would be potentially painful in a clinical setting, as
previous work has described nociception in the labrum.17 Once this position
was recorded, additional internal rotation was added to the point of bone-onbone impingement, as simulated by other
authors.18 The internal rotation angle
and clockface segment at which the lesion passed underneath the labrum was
recorded for each case. The clock-face
location of impingement following internal rotation in 90° flexion was also determined for each specimen to specifically
investigate the mechanism proposed by
Ganz et al5 and Tannast et al19 for damage of the labrum in femoroacetabular
Statistical Analysis
Specimens were separated into 2
groups based on the presence or absence
of labral pathology. The morphologic parameters, predicted location of impingement between the femur and the acetabular margin in flexion/internal rotation
maneuvers, and staging and location of
labral pathology were compared between
the 2 groups using the Mann-Whitney U
test. The association of femoral morphology with the occurrence and severity of
labral pathology was evaluated individually using Spearman’s rank correlation
test. Fisher’s exact test was used to determine any difference in the probability of
labral lesions between the morphologically normal and abnormal cohorts. The
impingement zone and lesion location
were compared using the paired Wilcoxon
signed rank test (SPSS version 15.0
software; SPSS, Inc, Chicago, Illinois).
Nonparametric tests were used in the absence of a normal distribution within the
small sample size. Statistical significance
was defined as P,.05.
Results
Labral pathology was identified in 15
(68%) of the 22 hip joint specimens examined. Donors of specimens with labral
lesions were predominantly men (70%),
compared with donors of normal specimens, 33% of whom were men, but this
difference was not statistically significant
(P5.09). No significant difference existed
in age, femoral neck–shaft angle, femoral
anteversion, acetabular depth, acetabular
index, anterior offset ratio, alpha angle, or
beta angle between the 2 groups (Table).
No difference existed in the likelihood of
the specimens having labral pathology
when the femora were classified morphologically using alpha angle thresholds of
55° (1 SD above the mean) and 63° (2 SDs
above the mean).15
Some degree of labral pathology was
e781
n Feature Article
observed in 15 of the 22 specimens: 2
specimens had stage 1 pathology, 5 had
stage 2, 6 had stage 3 (1 stage 3A and 5
stage 3B), and 2 had stage 4. The most
common lesion consisted of a circumferential separation of the labrum and
the acetabular articular cartilage within
the anterior-superior quadrant. The average location of the largest labral lesion
in each specimen was at 2.460.3 o’clock
and ranged from 12:30 to 4 o’clock. No
significant difference existed between the
normal and abnormal femora in terms of
the prevalence of labral pathology, depth
of the acetabulum, femoral anteversion, or
neck–shaft angle (Table).
The amount of internal rotation permitted at each flexion angle was compared
between the morphologically normal and
abnormal cohorts and between the groups
with and without labral pathology. When
the specimens were compared according
to an alpha angle threshold of 55°, no significant difference existed between permitted internal rotation in 90° of flexion
in normal (a,55°) and abnormal femora
(a.55°) (16.4°63.4° vs 13.7°62.8°, respectively; range: 0°-38°; P5.54). When
the specimens were compared according
to the pathologic status of the labrum,
those with labral lesions permitted an
average of 8.3° more internal rotation at
90° of flexion than the normal specimens
(17.5°62.6° vs 9.2°62.8°, respectively;
P5.03) (Table).
The mean lesion location was approximately 2:30 on the clockface, and
the mean impingement zone location was
approximately 1.8 o’clock. In specimens
with labral pathology, the severity of the
lesion was significantly correlated with
the alpha angle (rho50.504; P5.02) but
was not significantly associated with any
other measurement of femoral morphology. Fisher’s exact test revealed no significant correlation between head–neck
morphology (alpha and beta angles) and
the location of the impingement zone.
However, the neck–shaft angle was directly and significantly correlated with the
e782
Table
Demographic and Femoral Morphology Characteristics
of Specimens With and Without Labral Pathology
Normal Labrum
(n57)
Labral Pathology
(n515)
P
5 (70)
5 (33)
.09
74.869.5
73.2610.3
.76
24.961.1
25.862.1
.33
0.51960.061
0.50360.055
.56
Mean head diameter, mm
46.363.2
49.064.6
.18
Mean neck–shaft angle, deg
Parameter
No. of male specimens (%)
Mean age, y
Acetabular morphology
Mean acetabular depth, mm
Mean acetabular index,
depth/diameter
Femoral morphology
130.464.6
128.967.4
.61
Mean femoral anteversion
angle, deg
9.268.0
11.165.9
.59
Mean anterior offset ratio
0.1360.03
0.15560.05
.19
Mean a angle, deg
63.5612.7
61.5613.1
.74
4/7
9/15
..99
46.768.5
44.566.6
.51
3
8
..99
9.267.5
17.5610.1
.03
No. a.55°
Mean b angle, deg
No. b.62°
Deg of permitted internal
rotation at 90° of flexion
Abbreviation: deg, degrees.
impingement zone location (rho50.442;
P5.049), indicating that impingement on
internal rotation in 90° of flexion occurs at
more anterior locations as the neck–shaft
angle increases.
Discussion
The identification of labral pathology
as an early event in the progression of
joint disease underscores the need for a
better understanding of the pathomechanical processes leading to injury. Several
factors, including hip joint morphology
and specific maneuvers of the hip, have
been proposed as being responsible for
labral damage.1-3,5,20 The current study
was designed to understand the femoral
morphologic factors associated with the
occurrence of labral lesions, the location
of the lesions, and the characteristics of
the maneuvers required for impingement.
The findings do not support the conclusion
that labral pathology is predominantly associated with patients whose hip anatomy
(such as those with cam or pincer lesions)
prevents motion beyond a threshold value
of range of motion. The results showed no
difference between the amount of permitted internal rotation at 90° of flexion between specimens with alpha angles ,55°
and .55°. This finding is contradictory to
that of Ganz et al5 that a value of ,20°
of internal rotation in end-range degrees
of flexion is indicative of femoroacetabular impingement and a high risk for labral
pathology.5
In the current study, specimens with
labral pathology exhibited significantly
higher amounts of permitted internal rotation at 90° of flexion than those without
labral pathology, despite no significant
difference in alpha angle or anterior offset
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ratio. This may be explained by the theory
that labral lesions occur in response to tensile overload of the labrum in certain positions, especially those involving rotation
or abduction in extension or modest flexion of the morphologically normal hip.1,3
Charbonnier et al4 performed dynamic
motion analysis and correlative magnetic
resonance imaging in professional ballet dancers with morphologically normal
hips and showed that impingement occurs during extreme ranges of motion.
The findings of McCarthy et al2 and of the
current study further support the hypothesis that labral pathology can occur from
forceful impaction of the femoral neck
into impinging positions during maximal
range of motion, even in morphologically
normal hips.
The current study’s findings do not refute the significance of cam lesions in contributing to labral pathology, given that the
alpha angle was directly correlated with
the severity of labral pathology. This finding supports the rationale for surgical osteochondroplasty to increase the anterior
offset and subsequently decrease impinging forces on the acetabular labrum.21 The
current study joins other recent studies in
showing engagement of cam lesions at the
anterosuperior labrum22 and the relationship between asphericity and the severity
of subsequent acetabular soft tissue injury.23-26 Although the importance of considering femoroacetabular impingement
in the evaluation of young patients with
hip pain is acknowledged, labral or chondral pathology can occur in patients with
morphologically normal hips. Conversely,
bony characteristics suggestive of femoroacetabular impingement may be present
in asymptomatic patients.25,27,28
The current study investigated the
effect of various femoral morphology
factors on the occurrence of labral lesions and related hip motion parameters.
The approach used had several distinct
strengths. The identification and testing
of specimens with labral lesions allowed
for a standardized and controlled evalua-
JUNE 2012 | Volume 35 • Number 6
tion of the hip joint. In addition, the use
of 3-D computer modeling permitted the
standardized measurement of femoral
morphology factors. The protocol used
for testing labral impingement allowed
efficient determination of the postural maneuvers required to cause impingement
at specific locations along the acetabular rim, a task that would be difficult to
accomplish in a clinical or surgical setting. Previous models have shown the
usefulness and validity of 3-D computed
tomgraphy–based methods to evaluate
hip range of motion.19,22,29 However, conducting joint motion tests with computed
tomgraphy–based model reconstructions
does not account for the dynamic stabilization effect of the surrounding soft tissues and the presence of intracapsular soft
tissues such as the labrum. This is a limitation acknowledged by Kubiak-Langer
et al,29 who justified this shortcoming by
referring to Ganz et al’s5 intraoperative
findings that motion of the hip joint with
femoroacetabular impingement is restricted by bony contact.
In addition, the maneuvers investigated
in the current study were limited to internal rotation of the flexed femur. This was
done to focus on a mechanism of impingement specifically described as related to
femoral morphology.5,19 The findings of
the current study are also limited by the
tests selected to evaluate morphology of
the femur. Other measurements or evaluations of femoral morphology may also
be useful, but current assessment protocol
was limited to measures used in the clinical setting and reported in the published
literature.12,13 Femoral version may influence the amount of internal rotation permitted prior to impingement, regardless
of whether a cam lesion is present. No
difference existed in the femoral version
between specimens with and without
labral pathology. Because the scope of the
current study was confined to the effect
of femoral morphology on chondral and
labral damage, evaluation of acetabular
morphology was limited to measurement
of each specimen’s acetabular depth and
index. This limitation was accepted because Ganz et al’s20 suggestion that camtype femoroacetabular impingement is
“clearly more dangerous” to the acetabular labrum than pincer-type emoroacetabular impingement.
Additional limitations of the current
study include those inherent to cadaveric
investigations, particularly the lack of a
clinical history of the presence or absence
of hip joint symptoms. Donor age of the
specimens was substantially greater than
the age of patients who generally present
with labral lesions. It is plausible that degenerative labral lesions may have been
present in these older specimens. This
limits the direct clinical applicability of
the findings because degenerative lesions
in older patients are not equivalent to acute
soft tissue trauma in younger, active patients often evaluated for femoroacetabular impingement. Ideally, this study would
be replicated in a series of younger cadaveric specimens to strengthen the potential
clinical correlations. However, obtaining
a sufficient number of specimens in a
younger age range is challenging. In addition, damage to the soft tissues of the hip
was classified with a staging system originally designed for use during arthroscopic
examination. This limitation was accepted
in exchange for the ability to standardize
and categorize the level of pathology present in the cadaveric specimens.
Despite these limitations, the current
study was an objective assessment of the
influence of femoral morphology on the
potential for lesions of the acetabular labrum. Future clinical studies of the effect
of femoral morphology factors on the occurrence and characteristics of labral lesions are warranted based on the findings
of the current study.
Conclusion
The importance of the acetabular labrum in maintaining joint integrity is emphasized by the recent increase in interest
in its normal function, the factors leading
e783
n Feature Article
to its injury, and the consequences of its
pathology. The current study demonstrated that damage to the labrum may occur in
hip joints with normal morphology of the
proximal femur. However, the findings
also indicate that decreased femoral head–
neck offset may predispose the hip joint to
a characteristic pattern or severity of
labral pathology. Although further investigation is needed to apply the findings of
the current study in the clinical setting, the
results confirm the importance of considering femoral morphology and athletictype activities of the hip when determining the mechanism responsible for injury
of the acetabular labrum.
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