Impingement with Total hip Replacement

Transcription

Dorr_CCR.fm Page 1832 Wednesday, July 11, 2007 2:05 PM
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COPYRIGHT © 2007
BY
THE JOURNAL
OF
BONE
AND JOINT
SURGERY, INCORPORATED
Current Concepts Review
Impingement with Total Hip Replacement
By Aamer Malik, MD, Aditya Maheshwari, MD, and Lawrence D. Dorr, MD
Investigation performed at The Arthritis Institute, Inglewood, California
➤
Impingement is a cause of poor outcomes of prosthetic hip arthroplasty; it can lead to instability, accelerated
wear, and unexplained pain.
➤
Impingement is influenced by prosthetic design, component position, biomechanical factors, and patient variables.
➤
Evidence linking impingement to dislocation and accelerated wear comes from implant retrieval studies.
➤
Operative principles that maximize an impingement-free range of motion include correct combined acetabular
and femoral anteversion and an optimal head-neck ratio.
➤
Operative techniques for preventing impingement include medialization of the cup to avoid component impingement and restoration of hip offset and length to avoid osseous impingement.
T
he principles regarding impingement in the natural
osseous (anatomic) hip put forth by Ganz et al.1-5 are
similar in concept to what can occur in the prosthetic
hip. To understand impingement, it is helpful to recognize
the common mechanisms that cause mechanical abutment
in both anatomic and prosthetic hips. In the anatomic hip
joint, impingement is a mechanical abutment conflict between the bone of the femur and the pelvis; in a total hip replacement, it is contact between the metal femoral neck and
the cup liner or bone-to-bone contact such as between the
greater trochanter and the pelvis3,6,7. The femoral head-neck
ratio, which is the relationship between the diameter of the
femoral head and the diameter of the femoral neck, influences impingement. Cam impingement is caused by a reduced femoral head-neck ratio. An example is the pistol-grip
deformity that is created by a decreased offset of the femoral
head-neck junction3 (Fig. 1). Cam impingement in a prosthetic hip is caused by any implant feature that reduces the
head-neck ratio. A skirt on the metal femoral head or a large
circular femoral neck can cause mechanical abutment in a
prosthetic hip through this mechanism8-10 (Fig. 1). Pincer
impingement in the anatomic hip is a mechanical abutment
caused by acetabular retroversion, protrusio, or coxa profunda. Pincer impingement in the prosthetic hip is caused by
hooded and constrained liners or by placement of a small
femoral head in a big acetabular cup11-13. Failure to remove
acetabular osteophytes so that the metal neck or the femoral
bone abuts on the osteophytes is another cause of pincer impingement (Fig. 1).
Because impingement is a dynamic process, it has been
difficult to identify it and to define its prevalence on the basis
of clinical evaluations or plain radiographs. In the clinical setting, some causes of failure such as wear or dislocation are
inferre to be related to impingement8-10,14,15, but a direct relationship with impingement has been difficult to document.
There are no radiographic techniques with which to validate
the occurrence of impingement. Retrieval studies are performed to examine implants that have failed16-20, but we are not
aware of any autopsy retrieval studies of well-functioning
prosthetic hips that have shown the true prevalence of impingement. The purpose of this review is to discuss the current understanding of the mechanisms of impingement in
total hip replacements, the clinical consequences of impingement, and new developments.
Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants
in excess of $10,000 from Zimmer. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (ORTHOsoft). No
commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other
charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
J Bone Joint Surg Am. 2007;89:1832-42 • doi:10.2106/JBJS.F.01313
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THE JOURNAL OF BONE & JOINT SURGER Y · JBJS.ORG
VO L U M E 89-A · N U M B E R 8 · A U G U S T 2007
IMPINGEMENT
WITH
TO T A L H I P R E P L A C E M E N T
Fig. 1
Biomechanics of impingement. Reduced clearance leads to repetitive abutment between the femur and the acetabular rim in
the anatomic hip or between the femoral component and the acetabulum in the prosthetic hip. A: A normal anatomic hip and
an ideal total hip replacement with a large femoral head and a high head-neck ratio. B: Cam-type impingement in the native
hip caused by a reduced femoral head-neck offset and similar impingement in a prosthetic hip with a small femoral head
and a skirted femoral neck. C: Pincer-type impingement can result from excessive overcoverage of the femoral head in the
native acetabulum or from inadequate removal of acetabular osteophytes in the prosthetic hip. D: A combination of the cam
and pincer types of impingement in the native hip as well as in a prosthetic hip with a small femoral head, a head-neck ratio
of <2.0, a large cup, and a polyethylene liner with no chamfers.
Mechanisms of Impingement
in Total Hip Replacements
mpingement in the prosthetic hip is both device and
surgeon-dependent21. The device-design factors are those
that influence the femoral head-neck ratio as well as features
of acetabular design. The surgeon controls the position of the
cup with regard to inclination and anteversion as well as to its
depth in the osseous acetabulum. Following placement of the
cup, the surgeon controls the level of the osseous femoral neck
cut and the placement of the femoral component for the biomechanical reconstruction of the hip length and offset, which
reduces the occurrence of impingement22,23.
A common implant design feature that causes cam-type
impingement is a reduced head-neck ratio15,19,21. The articulation of the prosthetic hip requires an acetabular component of
a certain thickness, thereby diminishing the size of the femoral
head compared with that of the osseous femoral head. A headneck ratio of <2.0 in a prosthetic hip seems to greatly increase
the risk of impingement19. The head-neck ratio is influenced
by the head size, the femoral neck geometry, and the use of a
skirt on the femoral head10,19,21 (Figs. 2-A and 2-B). Cam-type
impingement can occur with use of a small head on a large
circular taper8,9,15 or use of a skirted femoral head8,10,12, both of
which can result in a head-neck ratio of <2.019. A trapezoidalshaped neck designed to create a better head-neck ratio, particularly with small heads, is preferable7,21.
Features that increase acetabular impingement include
the chamfer geometry of the rim of the polyethylene21,24,25 and
the presence of an extended-rim (hooded) liner, particularly if
the hood is incorrectly positioned in the hip21,26,27. The surgeon
increases the risk of impingement by placing the cup in a lat-
I
eralized horizontal position28,29 or by failing to remove acetabular osteophytes that can impinge against the metal neck or
the femoral bone30,31. If a hood is used in an operation performed through a posterior approach, its apex should be
placed posteroinferiorly (in the 4 o’clock position in left hips
and in the 8 o’clock position in right hips), as the most frequent site of impingement is posterosuperior20,27.
Bone-on-bone impingement is surgeon-dependent, as
the surgeon controls implant position and the restoration of
limb length and offset6,22. A short hip length, or more commonly a short offset, places the hip at risk for the femoral
bone impinging against the pelvis at the extremes of motion.
Most commonly, the offset of the hip in a standard total hip
replacement should even be increased a few millimeters to
avoid impingement because the femoral head is smaller than
the osseous head22,32,33. The neck-shaft angle of the femoral
component used by the surgeon can influence the reconstruction of both limb length and offset7,23. The surgeon needs to be
aware of the neck-shaft angle and the level of the corresponding osseous neck cut for that implant (Figs. 3-A and 3-B).
Even in ideally reconstructed hips, two causes of impingement persist. Patients who are particularly flexible (usually women) have a risk of osseous impingement at the
extremes of motion3,30. The use of the largest femoral head size
possible will be of benefit in flexible patients34,35. A second
cause of impingement is the degree of pelvic tilt that occurs in
some patients as a result of the static pelvic position on the
operating table relative to the dynamic pelvic position during
activities5,36. Even with correct combined anteversion of the
implants and biomechanical reconstruction of the hip, the extremes of flexible pelvic positions change the component and
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Fig. 2-A
Figs. 2-A and 2-B Head size and neck geometry can influence the
head-neck ratio, which is the head diameter divided by the neck diameter. These illustrations show the relationship between the cup and liner
with different head and neck designs. Fig. 2-A The effect of increasing
the femoral head size on impingement. A larger head increases the impingement-free range of motion.
Fig. 2-B
A trapezoidal stem geometry favors an increase in the impingementfree range of motion compared with that associated with a circular
neck design. The inset picture illustrates the decrease in neck diameter with the trapezoidal design (darkly shaded area) compared with that
of a circular neck design (lightly shaded area).
bone relationships so that impingement can still occur5,36.
The variables causing impingement are additive so that
it is incumbent on the surgeon to understand the device being
used, its influence on biomechanical reconstruction, and its
limitations in the context of patient variables. A good example
of a patient who is at high risk for impingement would be a
flexible woman with a small skirted femoral head and a poorly
positioned elevated acetabular liner.
Finite-Element Analysis
he focus of finite-element studies has been to determine
component designs and positions that are least likely to
cause impingement throughout the range of motion24,25,37-44.
Computer modeling is attractive because it makes it possible
to study specific variables under well-controlled conditions.
T
IMPINGEMENT
WITH
The limitation of these studies is the inability to evaluate the
relationship of the three-element implant-soft tissue-bone
structure clinically6,45. Furthermore, the optimal component
position for an impingement-free range of motion in a finiteelement study may well have an adverse effect on wear and
therefore implant longevity. An example of this is the recommendation by D’Lima et al.24 that the ideal inclination of the
acetabular component to avoid impingement is 45° to 55°.
However, this same research group observed clinically that inclination exceeding 45° led to a 40% increase in mean linear
wear of the polyethylene41.
One weakness of finite-element studies is that the angles
of the cup are altered without any change in the center of rotation (depth) of the cup (Figs. 4-A, 4-B, and 4-C). When the
center of rotation is not moved medially and/or superiorly, adequate coverage of the cup by acetabular bone can be achieved
only with higher inclination. Clinically, surgeons have learned
that, by moving the cup medially and/or superiorly from the
original center of rotation, they can reduce the inclination to no
more than 45° and yet provide appropriate coverage of the cup
so that the cup is not lateralized28,29,46,47 (Figs. 4-A, 4-B, and 4-C).
Avoiding a lateralized cup decreases the likelihood of impingement of the metal neck against the rim of the cup.
A second error in computer modeling of the position of
the acetabular component has been the assumption that the anteversion of the femoral component is 10° to 15°21,37. The surgeon can control the amount of anteversion of a cemented stem
but not of a noncemented stem40,48. Because of the necessity to
obtain a press-fit of the implant into the bone, the anteversion
presented by the bone must be used for the implant40,48. In two
studies, computed tomography scans of implanted stems
showed a range of femoral anteversion of 30° of retroversion to
45° of anteversion, with a mean of 16.5° and 16.8°31,49.
Clinical Consequences of Impingement
ontact between a metal neck and a plastic liner can have a
number of potentially adverse consequences, including
limited motion and function; increased stress on the liner rim,
resulting in dislodgment of the modular liner or accelerated
loosening of the implant; liberation of metal debris from the
femoral neck; generation of rim wear, potentially increasing
the risk of osteolysis; and subluxation and dislocation21.
Dislocation is a frequent cause of implant failure that
occurs because of impingement8,9,15,21,36,50,51. When impingement occurs in the prosthetic hip, there is a sliding contact of
the femoral head within the polyethylene or a hard-on-hard
articular surface43. The polyethylene creates a resisted moment to the femoral head, which helps prevent it from sliding
out of the polyethylene bore43. If the external loading challenge
creating the impingement is great enough, the resisting moment cannot contain the femoral head and dislocation occurs.
Laboratory and clinical models have demonstrated the
direct relationship between impingement and dislocation.
Computer-aided-design studies have shown a correlation between specific implant variables and clinical outcomes with
regard to dislocation8,15. Barrack et al.8 found that, when the
C
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Fig. 3-A
IMPINGEMENT
WITH
Fig. 3-B
Fig. 3-A An osteoarthritic right hip with a varus femoral neck-shaft angle of 124°. A standard implant with a neck-shaft angle of 131° does not reproduce the correct offset. Fig. 3-B A high-offset implant with a neck-shaft angle of 121° allows correct reconstruction of offset.
femoral neck of an implant had a large circular cross-sectional
diameter, the clinical dislocation rate (eight of fifty-two; 15%)
was three times higher than the dislocation rate of implants
with a smaller trapezoidal neck (two of forty-six; 4%) (p =
0.07). With use of earlier computer modeling, they had verified an increase in impingement and a 46% decrease in the arc
of motion in association with the large femoral tapers8. Similarly, Padgett et al.15, with a computer model study, found that
small-diameter heads with a larger taper demonstrated impingement at <90° of flexion. In a clinical review of 254 primary hip prostheses with the same neck design, they found an
overall dislocation rate of 4.7% (twelve hips). As stratified by
head size, the dislocation rates were 3.6% for 28-mm bearings,
4.8% for 26-mm bearings, and 18.8% for 22-mm bearings.
Padgett et al. discontinued clinical use of the 22-mm head
with a large taper as a consequence of these results15.
Pain is a common consequence of impingement52,53.
When impingement on capsular or tendon soft tissues occurs in
a prosthetic hip, inflammation and swelling frequently result in
groin pain53,54. Three scenarios of pain resulting from soft-tissue
impingement are: (1) when a large acetabular component overhangs medially or the lesser trochanter abuts against the ischium, causing iliopsoas tendinitis; (2) when the capsule is
compressed between the metal neck and the cup; or (3) when
the capsule is compressed between the greater trochanter and
the ilium53,54. Pain can be relieved by local anesthetic infiltration
or surgical release of the iliopsoas tendon without the need for
revision of the acetabular component54,55. Patients who experience subluxation and pain may require computed tomography
scans to document osteophytes or component malposition that
creates the impingement causing the subluxation31.
Impingement between the metal neck of the femoral
component and the polyethylene rim of the cup can damage the
polyethylene both at the site where the neck contacts the rim
and the egress site where the femoral head escapes from the
polyethylene bore43. When the external load challenges are high,
the resistive moment within the polyethylene can exceed the
yield strength of the polyethylene and, with chronic impingement, can lead to polyethylene damage through increased
wear and/or cracking of the liner with subsequent implant
failure10,19,56,57. Oxidized liners are at highest risk for damage
and failure from impingement56,57. Birman et al.56 analyzed 120
metal-backed conventional polyethylene liners and found
seventy-one (59%) to have impingement damage secondary to
contact between the metal neck and the polyethylene, seventyeight (65%) to have oxidation damage, and forty-eight (40%)
to have cracks in the polyethylene. Cracks were always associated with some degree of impingement damage and oxidation.
Retrieval studies have shown impingement to be a contributing cause of increased wear. Yamaguchi et al.20 correlated impingement with linear wear, with the average wear
rate being 0.33 ± 0.28 mm/yr for liners with impingement
compared with 0.19 ± 0.14 mm/yr for liners without impingement (p = 0.009). Usrey et al.19, in a retrieval study of 113 cups,
correlated volumetric wear of liners with the degree of impingement; the average volumetric wear rate was 159 ± 42 mm3/yr
for liners with severe impingement compared with 70 ± 21
mm3/yr for liners with no or mild impingement (p = 0.02).
Kligman et al.58 found evidence of impingement in sixty-two
of eighty-six modular polyethylene liners and reported that
it was correlated with backside polyethylene wear and screwmetal shell corrosion and fretting. The superolateral (posterosuperior) area of the liner is the most common site of
impingement16,20,27. Yamaguchi et al. found that the most common site was 78° ± 20° posterosuperiorly. Shon et al.18 confirmed that posterior impingement is the most common but
found greater variation in impingement sites. It is possible
that this variation was a combination of the impingement and
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Fig. 4-A
An osseous anatomic osteoarthritic acetabulum that averages 55° of
inclination and 12° of anteversion is shown. Placing a cup in this position provides adequate osseous coverage but is unfavorable for wear
and stability.
the egress site in hips that had subluxated or dislocated. Both
posterosuperior and posteroinferior impingement can occur
in the extension phase of the gait cycle20.
Impingement has been implicated as a cause of loosening
of both femoral and acetabular components12,59,60. Bosco and
IMPINGEMENT
WITH
Benjamin59 and Kobayashi et al.61 reported a clinical correlation
of impingement with wear and subsequent osteolysis. Both
groups of authors reported loosening of a femoral component
resulting from wear debris generated almost exclusively by the
polyethylene damage at the neck-cup impingement site. Isaac et
al.17, in their retrieval study, found that one of four possible
mechanisms of cup loosening was increased shear and tensile
forces at the bone-cement interface resulting from increased
impingement. Murray27 also correlated impingement with loosening of the Charnley cup. Dobzyniak et al.62 examined the
causes of revisions done in the first five years following total hip
replacements; they reported that loosening was the most common cause for revisions performed from 1986 to 1991 and instability was a more common cause for those done from 1992 to
2001. Loosening occurs early when it is caused by mechanical
abutment between poorly fixed implants. The prevalence of instability in the second half of the study by Dobzyniak et al. may
have been caused by a change from cemented to noncemented
stems and a failure to recognize a change in the femoral anteversion of some stems from what had been anticipated. These authors desired a stem anteversion of 15° to 20°, which can be
controlled by the surgeon when a cemented stem is implanted
but is uncommonly achieved when a cementless stem is used.
Metal-on-metal and ceramic-on-ceramic impingement
each causes specific adverse outcomes leading to failure peculiar to the particular bearing surface. Recent reports seem to
indicate an increased risk of fracture and squeaking due to
surface abrasion of the ceramic-on-ceramic couple related to
component malposition and impingement63,64. Metal-onmetal implants have been shown to generate metallosis secondary to impingement52,65. Howie et al.66 found, in a retrieval
study, that nine (38%) of twenty-four McKee-Farrar prosthe-
Fig. 4-B
Lateralizing the cup also is unfavorable with regard to impingement and consequent wear and instability. If the cup is uncovered, there will be component-to-component impingement (arrows).
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IMPINGEMENT
WITH
Fig. 4-C
Medializing the cup maintains the cup in 40° of inclination and 25° of anteversion while obtaining
correct cup coverage, but it increases the risk of bone-to-bone impingement (arrows). This can be
avoided by adjusting the level of the neck cut or using a longer femoral head or a high-offset stem.
ses had impingement caused by a poor head-neck ratio.
Hip resurfacing provides large femoral head sizes that are
favorable in terms of the range of motion and stability, but an
ideal head-neck ratio can be difficult to achieve and there is
still a risk of implant failure caused by impingement67,68. Beaulé
et al.67 recognized the risk of impingement from malposition of
the implant or from a lack of correction of an underlying deformity resulting in a reduced head-neck ratio (such as a pistolgrip deformity). Two considerations regarding offset must be
kept in mind to avoid cam impingement postoperatively: first,
in the coronal plane, it is critical to maintain the anterosuperior offset of the femoral head on the femoral neck and to
Fig. 5
Palpation to detect possible impingement by assessing the relationship of the tip of the lesser trochanter to the tip of the ischium. At least one fingerbreadth of distance should be present. The same
test should be done to test for impingement of the greater trochanter against the ilium with the lower
limb in external rotation and abduction and to test for impingement of the greater trochanter against
the anterior inferior iliac spine with the lower limb in internal rotation, flexion, and adduction.
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avoid notching, and second, in the sagittal plane, the anterior
offset should be reconstructed. Beaulé et al. found that preoperatively 57% of sixty-three osteoarthritic hips had a femoral
head-neck offset ratio of ≤0.15, which requires correction at
surgery to decrease the risk of impingement.
Wiadrowski et al.69 found that the poor acetabular design of the Wagner metal-on-polyethylene hip resurfacing
prosthesis was a risk factor for impingement, with ninety-two
of 109 retrieved components showing peripheral damage
caused by impingement of the femoral neck on the polyethylene cup. Wiadrowski et al. attributed this observation to the
hemispherical (180°) sector of the cup and recommended that
future designs have less of a hemispherical shape. The new
generation of hip resurfacing implants, such as the Birmingham hip implant (Smith and Nephew, Memphis, Tennessee)
and the Durom hip implant (Zimmer, Warsaw, Indiana) have
sector angles of 159.2° and 165°, respectively. Impingement
can still be a problem with the new metal-on-metal design.
Amstutz et al.70 reported the need for a revision of a surface replacement in a dysplastic hip that had a poor femoral offset
that had resulted in trochanteric-ischial impingement.
Current Developments for Avoiding Impingement
Component Positioning
nteversion of cemented stems can be controlled by the surgeon because a stem with a diameter smaller than that of
the medullary canal of the femur can be used and can be manipulated into 10° to 15° of anteversion while being fixed with
the cement. Research on the position of the acetabular component has focused on a so-called safe zone, which was considered
safe for stability (and hopefully avoidance of impingement)
when used in combination with a stem in 10° to 15° of anteversion. For cemented total hip replacements, Charnley recommended that the cemented cup be positioned in little or no
anteversion71, whereas Müller72 and Coventry et al.73 suggested
an anteversion angle of 10° and Harris74 recommended an anteversion angle of 20°. Lewinnek et al.75 recommended a target
range of 5° to 25° of anteversion, and McCollum and Gray36
suggested 20° to 40° of anteversion with use of pelvic anatomic
landmarks for intraoperative cup placement. McCollum and
Gray discounted the importance of femoral stem positioning
because they used cemented stems, and they considered that
head coverage by the acetabulum changes very little with simple
internal and external rotation of the lower limb when femoral
anteversion is 10° to 15°. The clinical data on impingement
(discussed in the section on clinical consequences of impingement) were all derived on the basis of operations in which the
surgeon assumed a femoral anteversion of 10° to 15° while positioning the cup into a given target amount of anteversion.
It is now known that ≤45° of inclination is best for
achieving stability and preventing wear41,42. Forty degrees is
commonly referenced as the best target number because it provides a 5° margin of error21. Achieving 40° of inclination with
cup coverage at the time of the operation requires medialization or superior displacement of the acetabular center of rotation. The center can be medialized by as much as 7.5 mm or
A
IMPINGEMENT
WITH
displaced superiorly by as much as 13 mm without clinical
consequences46,48. Moving the center of rotation in and/or up
results in cup coverage that prevents metal neck-on-cup impingement, but it can reduce the length and offset of the hip,
causing bone-on-bone impingement. The solutions for these
problems are a higher osseous femoral neck cut, a longer modular head, a high-offset stem, or a combination of these21-23.
Recent research has increased our awareness of the surgeon’s inability to control anteversion of a cementless femoral
stem. The inflexibility of the position of the cementless stem
was suggested by D’Lima et al.40, in their finite-element study. In
fact, this makes intuitive sense because a cementless stem must
have a tight fit in the bone. The femur has variable anteversion
of the neck and variable anterior diaphyseal bowing, both of
which influence the anteversion of the prosthetic neck in relation to the femoral axis76. The wide range of femoral stem version was confirmed by Wines and McNicol49 with use of
postoperative computed tomography scans. They found a range
of 15° of retroversion to 45° of anteversion with a mean of 16.8°
of femoral anteversion. A similar mean of 16.5°, with a range of
30° of retroversion to 37° of anteversion, was observed by Pierchon et al.31, also with computed tomography scans, in their
study of cemented stems. These two studies emphasized that the
surgeon did not control the femoral stem, even when it was
fixed with cement, as well as had been thought. One of us
(L.D.D.) and colleagues77 used imageless computer navigation
and found a mean of 5° of anteversion of the femoral stem in
men, a mean of 9° in women, and a mean of 7° in the entire
group. This anteversion was close to the mean of 9.8° found by
Maruyama et al.76 when they measured intact cadaver femora.
A second factor that influences the anteversion of the
prosthetic stem relative to the femur is the anterior bow of the
femoral diaphysis, which can be as much as 10°76. The more
the femur is bowed anteriorly, the less the relative anteversion
of the prosthetic stem in relation to the femur. This has a
greater effect on cementless straight stems than on cementless
anatomic stems, which compensate somewhat by fitting the
anteversion of the metaphysis.
The concept of combined anteversion of the stem and
cup has been emphasized by Ranawat. He has taught a manual
combined anteversion test for total hip replacement since the
early 1990s78. With the cup and stem in place, the lower limb is
positioned in neutral (or slight hip flexion) and is internally rotated until the femoral head is symmetrically seated (coplanar)
in the cup. The amount of internal rotation in degrees needed
to produce a coplanar head and cup is the combined anteversion78. Ranawat and Maynard recommended a combined anteversion of approximately 45° in female patients and 20° to 30°
in male patients79. McKibbin defined the stability index for anatomic hips to be 30° to 40°, with a range of 20° to 35° for men
and 30° to 45° for women80. A combined anteversion of <20°
was defined as severe retroversion. Barrack21 indirectly addressed combined anteversion by recommending cup anteversion of 15° when stem anteversion is 15°, but he recommended
an increase in cup anteversion if stem anteversion is <15°. Widmer and Zurfluh44 stated that combined anteversion could be
1839
determined by the formula: cup anteversion + (0.7 × stem anteversion) = 37.3° (for example, femoral anteversion of 10° ×
0.7 = 7°, so cup anteversion should be 30°). Finally, Komeno et
al.81 used computed tomography scans to compare twenty dislocated hips with eighteen nondislocated hips in Japanese patients. The mean combined anteversion was 47.8° in the hips
without a dislocation, 27.4° in the hips with a posterior dislocation, and 72.2° in those with an anterior dislocation. These
numbers are higher than those reported for non-Japanese patients because femoral anteversion is greater in Japanese patients, in whom dysplasia is the most common reason for total
hip replacement. In both cases (posteriorly and anteriorly dislocated hips) the combined anteversion was significantly different
from that of the hips without dislocation (p = 0.0074 for the
posteriorly dislocated group and p = 0.0056 for the anteriorly
dislocated group). Komeno et al. concluded that the dislocation
rate is not affected by the positioning of either the cup or the
stem alone but is influenced by the combined anteversion.
Combined anteversion is also an important factor in
surface replacement. McMinn82 stated that excessive anteversion of the femoral neck, such as with developmental dysplasia
of the hip, can cause impingement on, and edge loading, in an
optimally positioned cup. His target was a combined anteversion of 45°, so that if the femoral neck is in 40° of anteversion
the cup is placed in 5° of anteversion. McMinn recommended
a derotation femoral osteotomy if the osseous femoral anteversion is ≥60°.
Biomechanical Hip Reconstruction
Correct hip length and offset are both necessary to avoid impingement. Femoral reconstruction controls the biomechanical reconstruction because the level of the femoral neck cut
and the head length that are used determine the hip length
and offset (and the resting length of the muscles). Charles et
al.22 studied the effect of soft-tissue balancing of the hip. If the
cup position does not change the hip center of rotation by >5
to 6 mm medially or superiorly, the templated femoral neck
cut will reestablish the limb length and offset. If the cup is excessively medialized or the angle between the osseous neck
and the shaft is ≤125°, an offset femoral stem is needed to reproduce offset without lengthening of the limb22,23.
The correctness of the reconstruction of the hip length
and offset can be anatomically evaluated intraoperatively (Fig.
5). The lesser trochanter should not touch the ischium with the
lower limb in full extension; it should be proximal to the tip of
the ischium by at least one fingerbreadth (the proper relationship can be determined from the preoperative radiograph if a
nearly normal contralateral hip is available for imaging). The
greater trochanter should not touch the ilium in external rotation and abduction, or in flexion, adduction, and internal rotation. All anterior acetabular osteophytes must be removed. The
metal femoral neck should not touch the rim of the cup at the
extremes of motion21,43. Impingement commonly occurs in hips
with low anteversion of a cementless femoral stem (≤5°) and
hips with a low offset, and the anterosuperior aspect of the capsule and even the distal side of the anterior inferior iliac spine
IMPINGEMENT
WITH
may need to be removed from these hips30,48. In flexible women
and in hips with low femoral anteversion, impingement may be
avoided only with the use of a large femoral head.
Femoral stems with a modular neck could allow optimization of limb length and offset as well as control of femoral
version and varus or valgus angulation of the neck. However,
to our knowledge, there are no published data to corroborate
these theoretical advantages. Furthermore, a new interface
may be a source of wear and dissociation between the neck
and the metaphyseal component, which was recently reported
after the use of one modular neck design83.
In hip resurfacing, there is little flexibility to adjust limb
length and offset if the cup is excessively medialized or placed
superiorly. Therefore, the cup center of rotation must be reproduced and the proximal femoral reconstruction should
maintain the prosthetic center of rotation as near to the anatomic femoral center of rotation as possible67. Anterior femoral neck osteophytes must be carefully resected to reduce the
risk of anterior cam impingement postoperatively. A trochanteric osteotomy with trochanteric advancement is another
method of increasing clearance.
Large Heads
Large femoral heads of ≥36 mm effectively solve the problem of
how to achieve a correct head-neck ratio. This head size provides a head-neck ratio of >2.0 even when a 14 to 16-mm taper
neck with a 16-mm thickness at the base of the taper is used.
However, the use of large heads has created a separate set of
technical limitations and concerns. For example, the cup position with hard-on-hard surfaces is even more important than
that with metal-on-polyethylene surfaces. With metal-on-metal
articulations, cup inclination of >50° can cause edge loading of
the femoral head on the cup and result in so-called runaway
wear; with ceramic-on-ceramic articulations, this cup position
can cause fracture or squeaking84. The use of metal-on-metal acetabular components for surface replacement or for conventional total hip replacement with a large head has reduced
acetabular sector angles to 159.2° (Birmingham implant; Smith
and Nephew) and 165° (ASR; DePuy, Warsaw, Indiana, and
Durom, Zimmer) and requires nearly complete coverage of the
cup, which can promote inclination in excess of 50°. Preparation of the acetabular bone must be adjusted for the 159° to
165° cups to allow coverage with inclination of <45°. Technically, this preparation is more difficult because the acetabular
preparation is done with reamers that have a 180° angle. Reaming medially must be adequate to achieve coverage and correct
inclination, but it must be limited to ensure Zone-2 contact for
these 159° to 165° cups with a flattened dome85.
Highly cross-linked polyethylene has allowed routine
implantation of femoral heads of ≥36 mm in diameter. Laboratory studies have shown no increase in wear86. Our unpublished results of use of a 38-mm cobalt-chromium head
articulating with Durasul highly cross-linked polyethylene
(Zimmer) showed a linear wear rate of 0.026 mm/yr at three
to four years postoperatively, which does not differ from our
published five-year linear wear rate of 0.029 mm/yr following
1840
the use of 28-mm cobalt-chromium heads articulating with
Durasul polyethylene87. Volumetric wear with 38 and 44-mm
heads (28.1 ± 19 mm3/yr) is greater than that with 32-mm
heads (18.2 ± 9.7 mm3/yr, p = 0.021). This volumetric wear remains well below the 87 mm3/yr threshold for osteolysis88. If
cups with highly crossed-linked polyethylene are abducted
>55°, there is the threat of breakage89.
Techniques for Avoiding Impingement
rthopaedic surgeons can agree about the benefits of the
implant design changes made in the last decade to avoid
impingement: elimination of skirts on femoral heads15, chamfering of the polyethylene rim37, narrow femoral necks8, large
femoral heads35, and perhaps modular femoral necks83. There
remains uncertainty about the best technical methods for
minimizing neck-on-cup and/or bone-on-bone impingement
in conventional total hip replacements.
The primary question for surgeons is what constitutes
the so-called safe zone for the cup. The most often used safe
zone is 5° to 25°, as described by Lewinnek et al.75. McCollum
and Gray36 recommended 20° to 40°. Surgeons who use the anterior approach for arthroplasty have always recommended
less cup anteversion. Charnley71 suggested little or no anteversion, and Müller72 and Coventry et al.73 suggested 10°. Surgeons who use the anterior approach, especially with the
patient in the supine position, may judge the position of the
cup with use of Murray’s anatomic plane, whereas those who
approach the hip posteriorly, with the patient in a lateral position, view the radiographic angle, which may explain some of
the differences between target values90.
The concept of using combined anteversion, rather than
target values, to determine the cup position when mating it
with the stem is becoming more prevalent 40,44,48,82,91. With a cemented stem in 10° to 15° of anteversion, the cup should be
placed within a “safe zone” of 25° ± 10° so that the combined
anteversion is 25° to 35° for men and 35° to 45° for women
(on the basis of Murray’s radiographic plane values)44,79,80. The
combined anteversion should be the same for both the anterior and the posterior approach, being that the same Murray
definition is used, because wear and durability are related to
avoidance of impingement18-20,52,56,58,90.
Total hip replacement with a cemented stem allows a safe
zone of targeted cup position because the position of the femoral stem is adjustable. However, the position of a cementless
femoral stem in a total hip replacement is nearly fixed, so the
cup position must be adjustable40,44,48,91. The surgeon must determine the femoral anteversion with a trial stem before positioning the cup, which means that the femoral preparation must be
completed prior to the acetabular preparation. The stem anteversion can be judged against the axis of the thigh by aligning
the thigh according to the epicondyles of the knee. This requires
that the surgeon accept a change in the sequence in which he or
she performs the operation. The advantage of this technique is
the wide range of femoral version that is possible (15° of retroversion to 45° of anteversion)49. In our experience with cementless femoral stems, the range has been 15° of retroversion to 30°
O
IMPINGEMENT
WITH
of anteversion and the surgeon, on the average, can correctly estimate the femoral version to within 5°. However, this margin of
error for the surgeon’s estimation of the stem version still results
in better precision for hip reconstruction than an assumption of
10° to 15° of femoral anteversion for every case.
The acetabular cup, when used with a cementless femoral stem, should be positioned in relation to the stem position
rather than on the basis of a target value. This is necessary because, if femoral anteversion is low (5° of anteversion to 5° of
retroversion), positioning a cup in 20° will increase the risk of
posterior dislocation; if the femoral anteversion is high (25° to
30° of anteversion), a cup positioned in 20° will be at risk for
anterior dislocation, especially if a posterior hood is used. Cup
anteversion needs to be correctly mated to the stem anteversion, and in our experience such cup anteversion ranges from
10° to 15° in women with hip dysplasia to 30° to 35° in men
with a pistol-grip deformity. A safe zone for a cup used with a
cementless stem is not a realistic concept because the stem anteversion cannot be controlled.
The safe zone for inclination is <45°. We consider the optimum cup inclination to be 40° as this allows a margin of error of
up to 5° for surgical placement, which would maximize cup inclination at 45°. The lower limit of inclination is commonly set at
30° because of the limitation of coverage of the superior metal
without excessive medialization or superior displacement, which
would decrease the offset of the hip and substantially increase
the risk of bone-on-bone impingement. If medialization or superior displacement does decrease the offset of the hip, the solution is to use a higher femoral neck cut, a longer modular head,
an offset stem, or a combination of these21-23.
The use of a large femoral head ensures an acceptable
head-neck ratio, even with a circular femoral neck. The large
head provides a margin of error of combined anteversion for
stability, but it may not reduce the margin of error for wear,
which requires inclination of ≤45° and is related to the combined anteversion (the higher the acetabular anteversion, the
less the wear)42.
The improvement in articulation surfaces has raised
confidence about increasing the durability of total hip replacement87. Impingement must still be avoided to fulfill these
expectations, so research has continued with increased interest3,15,18,19,21,35,37,39,44,56,63,64,67,68,81,92-96. Computer navigation has improved the accuracy of component positioning97,98. Navigation
provides a scientific method for total hip reconstruction with
numerical confirmation of the combined anteversion and cup
inclination of ≤45°. Studies involving computer navigation
have suggested the benefits of using a combined anteversion
technique, rather than target values, for determining cup position, since femoral stem anteversion is known48,77.
Aamer Malik, MD
Aditya Maheshwari, MD
Lawrence D. Dorr, MD
The Arthritis Institute, 501 East Hardy Street, 3rd Floor, Inglewood, CA
90301. E-mail address for L.D. Dorr: [email protected]
1841
IMPINGEMENT
WITH
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Impingement with Total hip Replacement

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