 |
 |
 |
|
| home | CME | help | feedback | subscriptions | archive |
|
The Journal of Bone and Joint Surgery (American). 2007;89:1832-1842. doi:10.2106/JBJS.F.01313 © 2007 The Journal of Bone and Joint Surgery, Inc.
Impingement with Total Hip Replacement1 The Arthritis Institute, 501 East Hardy Street, 3rd Floor, Inglewood, CA 90301. E-mail address for L.D. Dorr: Patriciajpaul{at}yahoo.com
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.
The 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.
Impingement 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 lateralized 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 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.
The 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. 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 finite-element 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.
Contact 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 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, seventy-eight (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 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 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-on-metal 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 prostheses 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 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
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.
Component Positioning Anteversion 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 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 determined by the formula: cup anteversion + (0.7 x stem anteversion) = 37.3° (for example, femoral anteversion of 10° x 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
Biomechanical Hip Reconstruction
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 ( 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
Highly cross-linked polyethylene has allowed routine implantation of
femoral heads of
Orthopaedic 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° 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
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 interest
3,15,18,19,21,35,37,39,44,56,63,64,67,68,81,92-96.
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
  CiteULike  Connotea  Del.icio.us  Facebook  Technorati  Twitter What's this?
This article has been cited by other articles:
Letters to the Editor:Read all Letters to the Editor
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Top
Abstract
Introduction
0.15, which requires correction at surgery to
decrease the risk of impingement. 
60°. 
| home | CME | help | feedback | subscriptions | archive | site index |
|
Stanford University Libraries' HighWire Press (TM) assists in the publication of JBJS Online. |