The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 89, Issue 8

Current Concepts Review | August 01, 2007

Impingement with Total Hip Replacement

Aamer Malik, MD¹; Aditya Maheshwari, MD¹; Lawrence D. Dorr, MD¹

¹ The Arthritis Institute, 501 East Hardy Street, 3rd Floor, Inglewood, CA 90301. E-mail address for L.D. Dorr: Patriciajpaul@yahoo.com

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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.

Investigation performed at The Arthritis Institute, Inglewood, California

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2007 Aug 01;89(8):1832-1842. doi: 10.2106/JBJS.F.01313

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Abstract

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.

Figures in this Article

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 pelvis^3,6,7. 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 mechanism^8-10 (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 cup^11-13. 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 impingement^8-10,14,15, 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 failed^16-20, 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.

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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

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 impingement^22,23.

A common implant design feature that causes cam-type impingement is a reduced head-neck ratio^15,19,21. 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 head^10,19,21 (Figs. 2-A and 2-B). Cam-type impingement can occur with use of a small head on a large circular taper^8,9,15 or use of a skirted femoral head^8,10,12, 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 preferable^7,21.

Features that increase acetabular impingement include the chamfer geometry of the rim of the polyethylene^21,24,25 and the presence of an extended-rim (hooded) liner, particularly if the hood is incorrectly positioned in the hip^21,26,27. The surgeon increases the risk of impingement by placing the cup in a lateralized horizontal position^28,29 or by failing to remove acetabular osteophytes that can impinge against the metal neck or the femoral bone^30,31. 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 posterosuperior^20,27.

Bone-on-bone impingement is surgeon-dependent, as the surgeon controls implant position and the restoration of limb length and offset^6,22. 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 head^22,32,33. The neck-shaft angle of the femoral component used by the surgeon can influence the reconstruction of both limb length and offset^7,23. 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 motion^3,30. The use of the largest femoral head size possible will be of benefit in flexible patients^34,35. 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 activities^5,36. 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 occur^5,36.

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Fig. 2-A: Figs. 2-A and2-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.

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Fig. 2-B: A trapezoidal stem geometry favors an increase in the impingement-free 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).

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

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 motion^24,25,37-44. 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 clinically^6,45. 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 lateralized^28,29,46,47 (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 stem^40,48. 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 implant^40,48. 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

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 impingement^{8,9,15,21,36,50,51}. 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 dislocation^8,15. 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.

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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.

Pain is a common consequence of impingement^52,53. When impingement on capsular or tendon soft tissues occurs in a prosthetic hip, inflammation and swelling frequently result in groin pain^53,54. 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 ilium^53,54. Pain can be relieved by local anesthetic infiltration or surgical release of the iliopsoas tendon without the need for revision of the acetabular component^54,55. 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 failure^10,19,56,57. Oxidized liners are at highest risk for damage and failure from impingement^56,57. 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 impingement^16,20,27. 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.

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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.

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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).

Impingement has been implicated as a cause of loosening of both femoral and acetabular components^12,59,60. 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 impingement^63,64. Metal-on-metal implants have been shown to generate metallosis secondary to impingement^52,65. 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.

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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.

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 impingement^67,68. 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 =0.15, which requires correction at surgery to decrease the risk of impingement.

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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.

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

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 =45° of inclination is best for achieving stability and preventing wear^41,42. 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 displaced superiorly by as much as 13 mm without clinical consequences^46,48. 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 these^21-23.

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 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 limb^22,23.

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 motion^21,43. 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 may need to be removed from these hips^30,48. 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 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

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 impingement^{18-20,52,56,58,90}.

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 adjustable^40,44,48,91. 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 these^21-23.

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 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 positioning^97,98. 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 known^48,77.

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Topics

neck ; dislocations ; femur ; head of femur ; neck of femur ; hip joint ; metals ; reconstructive surgical procedures ; hip replacement arthroplasty ; polyethylene

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Lawrence D. Dorr, M.D.

Posted on November 15, 2007

Dr. Dorr et al. respond to Mr. Nijhof.

The Arthritis Institute, Inglewood, CA

We thank Mr. Nijhof for his letter. We wrote this Current Concepts article with expectations that the reader would understand that the terms medialization and lateralization mean a change in the acetabular center of rotation. Perhaps Mr. Nijhof did not equate these terms to a change in center of rotation, because he did not discuss this concept in his letter.

Because the bony acetabulum in arthritic patients averages 55 degrees of inclination(1), the center of rotation must be moved superiorly or medially to obtain coverage of the cup with inclination of 45 degrees or less. Inclination should not exceed 45 degrees for limiting wear of the articulation(2). We never implied that changing the center of rotation medially or superiorly automatically changes inclination. The angle of inclination remains the surgeonâ€™s choice, but changing the center of rotation affords the surgeon the opportunity to get it correct.

Mr. Nijhof suggests that it is hard to understand how medialization reduces component impingement. We would respond by noting that the metal neck of the stem cannot impinge on the edge of the cup when the edge is covered by bone. Furthermore, his comment about lower edge loading is not clear to us. We have conducted numerous wear studies, including one that analyzes the vector directions of wear, and except for measurement errors with computer generated wear measurements, inferior loading does not occur(3).

If the center of rotation is lateralized with an identical inclination to a medialized cup, the metal edge of the cup is at risk of impinging with the metal neck of the stem. Correct coverage eliminates that. The elimination of bony impingement is a function of offset and leg length.

The figures used in the article are illustrations to represent a principle. The principles illustrated with these figures are that correct inclination and coverage of the cup occurs with change of the center of rotation as needed to achieve this, and correct offset and leg length will avoid bony impingement.

Mr. Nijhof is correct about references 2 and 3 which were used to confirm the benefit of not lateralizing the center of rotation. As he states, these references confirm the benefit.

The avoidance of impingement is critical for the most comfortable and most durable total hip replacements. The concept of correctly changing the center of rotation to avoid impingement is one that surgeons must understand and that is why it was emphasized in this article.

References:

1. Dorr LD: Hip arthroplasty. Minimally invasive techniques and computer navigation, 1st edition. Philadelphia, PA, Saunders Elsevier, 2006.

2. Patil S, Bergula A, Chen PC, Cowell CW, Jr., Dâ€™Lima DD: Polyethylene wear and acetabular component orientation. J Bone Joint Surg Am. 2003;85-A Suppl 4:56-63.

3. Wan Z, Boutary M, Dorr LD: Precision and limitation of measuring two-dimensional wear on clinical radiographs. Clin Orthop Relat Res. 2006 Aug;449:267-74.

Marc W Nijhof

Posted on November 01, 2007

Impingement with Total Hip Replacement

Endo-Klinik, Hamburg, GERMANY

To The Editor:

With interest we read the paper of Malik et al.(1). The statements on the effect of medialization/lateralization of the cup on component impingement are confusing. It is stated that medialization of the cup can avoid component impingement and that the surgeon increases the risk of impingement by placing the cup in a lateralized horizontal position. The authors also state that appropriate coverage of the cup (45 degrees or less of abduction) can be achieved by moving the cup medially and/or superiorly.

What is confusing to us is that the authors seem to imply that medialization/lateralization per se automatically results in a subsequent change in inclination of the cup.

It is also hard to understand why a medialized cup with reduced inclination (with subsequent lower edge loading) would give less component impingement than a lateralized cup with the same inclination.

The figures presented in this paper show that lateralization and uncoverage of the cup can result from an abducted cup, especially when the acetabulum is steep and shallow. However, in contrast to what the authors state, it is the effect of the reduced inclination, and not the mere lateralization or uncoverage that causes component-to-component impingement. Furthermore, one could also imagine that a surgeon, who places the cup too lateral, would be tempted to place the cup more vertically (to increase coverage), thereby reducing the change of metal neck-on-cup impingement.

Finally, two studies that are referenced (2,3) did not study the effect of (lateralized horizontal) cup placement on impingement. What was shown in these studies was that contact forces are lower when the cup is not lateralized with a possible beneficial effect on preventing prosthetic loosening.

The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. 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 author, or a member of his immediate family, is affiliated or associated .

References:

1. Malik A, Maheshwari A, Dorr D. Current Concepts Review: Impingement with Total Hip Replacement. J Bone Joint Surg Am 2007; 89: 1832-1842

2. Johnston RC, Brand RA, Crowninshield RD. Reconstruction of the hip. A mathematical approach to determine optimum geometric relationships. J Bone Joint Surg Am. 1979;61:639-52.

3. Yoder SA, Brand RA, Pedersen DR, Oâ€™Gorman TW. Total hip acetabular component position affects component loosening rates. Clin Orthop Relat Res. 1988;228:79-87.

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Aamer Malik, Aditya Maheshwari, Lawrence D. Dorr; Impingement with Total Hip Replacement. The Journal of Bone & Joint Surgery. 2007 Aug;89(8):1832-1842.

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