Techniques for Optimizing Acetabular Component Positioning in THA
In this article, the authors review techniques and tools available to improve surgical competency and reduce the risk for component malpositioning, as well as provide expert recommendations for surgeons who perform total hip arthroplasty.
James E. Feng, MD; Afshin A. Anoushiravani, MD; Nima Eftekhary, MD; Jonathan Vigdorchik, MD; and Ran Schwarzkopf, MD, MSc
Ideal and accurate acetabular component positioning in total hip arthroplasty (THA) remains a challenge. It is estimated that newly fellowship-trained arthroplasty surgeon must perform 50 consecutive THAs to achieve a satisfactory level of surgical competency.  Even for the experienced surgeon, between 39% and 50% of acetabular cup placements fall outside the Lewinnek abduction and/or anteversion safe zones.[ 2–6]
Moreover, poor component positioning is the most significant risk factor for hip instability and excessive early liner wear.  Other complications of poor component positioning include: [5,8–13]
- Shell-liner dissociation
- Leg-length discrepancy
- Limited range of motion (ROM)
- Hardware squeaking in ceramic bearing hips
In this article, we review a variety of techniques and tools available to improve surgical competency and reduce the risk for component malpositioning. Key points include the following:
- For a near anatomic positioning, acetabular cup placement should have 40°±10° of abduction and 15°±10° of anteversion.
- Medialization of the acetabular component optimizes joint reaction forces, but it also increases the risk for reduced global offset, decreased ROM until impingement, and reduces acetabular bone stock for future revision surgery. Medialization should be tailored to each individual patient.
- Preoperative surgical templating should be performed for all patients using standardized digital radiographs with known magnification or a known reference marker.
- The transverse acetabular ligament is a reliable anatomic structure that can improve the surgical accuracy and consistency of acetabular cup depth, height and version.
- Computer-assisted navigation and robotic surgery have demonstrated improved component positioning, reduced surgical variability and may be a useful asset in complex THA.
Acetabular Component Positioning and Templating
Preoperative templating is regarded by many surgeons as an essential step in the preparation for THA (Figure 1).  Although no study to date has investigated the direct effects of templating on improved acetabular positioning versus not templating the hip, 80% to 97% of the implanted cups can be correctly estimated within a ±1 stem size when templating is used. [14,15] Thus, templating allows surgeons to preoperatively select an appropriately sized component inventory, which reduces surgical idle time. In addition, templating encourages the surgeon to think 3-dimensionally and anticipate complex anatomy and potential intraoperative complications.
Figure 1. Anteroposterior pelvis radiographs demonstrate presurgical templating of the hip and results at the 2-week post-surgery follow-up.
The major variables to be accounted for in the preoperative phase of THA acetabular cup placement include:
- Acetabular cup inclination and version
- Center of rotation (CoR)
- Leg length
- Shell size
In a landmark study by Lewinnek et al,  reduced hip dislocation rates were observed after placing acetabular cups between 30 ° and 50° of abduction and 5° and 25° of anteversion. Shortly after its publication, the study gained widespread acceptance among the orthopaedic community as a guideline for the acetabular cup “safe zone,” despite the fact that the study analyzed only 113 hip radiographs and 9 hip dislocations. 
More recent studies have called these findings into question. [16,17] Abdel et al  demonstrated that cup placement was within the Lewinnek abduction safe zone in 92% of patients who dislocated after undergoing a posterior THA and 65% of patients who dislocated after an anterolateral THA. Other studies have also shown that an inclination angle in excess of 45° accelerates the rate of polyethylene linear wear by 40% and may also be correlated with an increased risk of hip squeaking in patients with ceramic-on-ceramic THAs. [7,13] Similarly, metal-on-metal bearings with excessive inclination angles suffer from increased edge wear, leading to elevated serum metal ion levels and the risk for adverse reaction to metal debris such as local tissue necrosis, lymphocytic infiltration, and, in rare cases, pseudotumors. [10–12]
Acetabular anteversion has also been shown to affect polyethylene wear, but it is more commonly identified as playing a greater role in THA stability. [4,7,18] As with inclination, the existence of an anteversion safe zone is now being questioned. Abdel et al  reported that 69% of posterior and 33% of anterolateral THAs were complicated by subsequent hip dislocation despite cups being placed in the anteversion safe zone.
Although cup placement in most of the hips that dislocated fell within the Lewinnek safe zone,  cup placement outside the safe zone has been found to increase the relative risk of dislocation. In a study by Biedermann et al,  a U-shaped distribution was observed for anterior and posterior dislocations as acetabular anteversion angles deviated from 15° bi-directionally. Patients with less than 4° or more than 24° of anteversion were at 6-times greater risk for posterior and anterior dislocations, respectively.
Current studies are limited in assessing other complications related to anteversion, potentially due to the difficulties of evaluating anteversion using standard postoperative imaging. Even when evaluating 342 controls in the study by Biedermann et al,  only one third of the hips could be evaluated for cup anteversion using digital radiographs.
In summary, most dislocations following THA occur in patients whose acetabular cups were placed within the Lewinnek safe zone, although the relative risk for dislocation is greater in patients in whom acetabular cup placement falls outside the safe zone.  It is also evident that known and unknown patient factors, such as spinopelvic alignment and developmental dysplasia of the hip (DDH), are unaccounted for at this time and may affect the outcome of THA in a subset of patients. Current evidence on how to identify and surgically manage these patient factors is lacking, but with the advent of new technologies, such as full-body, 3-dimensional radiographs, new guidelines are likely to be developed in the near future.
- At this time, optimal cup placement is 40° of inclination, with a tolerance of 30° to 50°, and 15° of anteversion, with a range of 5° to 25°, in a normal primary THA. [19,20]
Center of Rotation and Sizing
Optimal acetabular cup placement also depends on anterior-posterior (AP), medial-lateral (depth) and superior-inferior (height) positioning. Changes in these axes alter the hip’s CoR, which in turn alters the joint reaction force (JRF), maximal hip abductor muscle forces (Fmax), and minimum required abductor muscle forces to maintain a stable pelvis during normal gait (Freq). [21,22]
In a biomechanical computer modeling study by Heller et al,  lateralization of the hip CoR resulted in the most drastic alteration in the JRF, with each 10-mm shift resulting in an 8% increase in the JRF, compared with a 1% increase in JRF with superior shifts. Moreover, superior and lateral shifts in the CoR may also lead to a postoperative abductor lurch or Trendelenburg sign due to prominent decreases in abductor muscle forces or required abductor muscle forces to maintain a stable pelvis. 
Although superior translation of the hip’s CoR is unfavorable, the increase in JRF and Freq is relatively small. The trade-off of anatomic superior-inferior acetabular positioning for satisfactory bony purchase is warranted in patients with significant acetabular dysplasia. In a study of hip dysplasia by Nawabi et al,  patient with Crowe type II and III hips had a 97% 12-year survivorship for all-cause revisions when the hip CoR was medialized and placed superiorly to improve bony purchase. This was no different from patients with Crowe type I dysplasia with anatomically placed CoR. Moreover, patient-reported outcomes, acetabular loosening, and wear rates were the same. 
Medialization of the acetabular cup has been demonstrated to reduce the rate of liner wear, and in turn, improve the survivability of the implant.  These benefits must be balanced against the increased risk for bony impingement, reduced ROM, and loss of bone stock for future potential revision surgeries. Medialization of the hip also requires restoration of the hip’s global offset using extended offset femoral heads or acetabular liners. Failure to do so may result in increased head-liner microseparation due to poor abductor tensioning, which could cause accelerated liner wear.  Furthermore, extended offset acetabular liners nullify the benefits afforded by acetabular medialization and increase the rate of aseptic loosening due to altered torsional forces at the bone-cup interface. [25,26]
- Using a standard supine AP radiograph of the pelvis, the acetabular component should be appropriately templated at 40° of inclination with respect to the horizontal tear drop line.
- The medial border of the cup is medialized towards the radiographic teardrop. The biochemical advantage of medializing the hip’s CoR and obtaining improved bony purchase must be balanced against the risk of acetabular protrusion and the need to preserve bone stock.
- The inferior border of the acetabular cup should be approximately in line to the inferior border of the radiographic teardrop line.
- The superior aspect of the acetabular cup should maximize bony coverage, but up to 33% of acetabular cup may be uncovered.
- Femoral neck osteotomy should also be accounted for to ensure proper restoration of the patient’s leg length discrepancy and global offset of the hip.
Perioperative and Intraoperative Optimizations
Patient setup on the operating table is integral to optimal acetabular positioning. Ideally, the sagittal plane of the pelvis should be parallel with the floor for patients in the lateral decubitus position, while the coronal and transverse planes should be parallel with the walls of the room. Deviations in any of these parameters can significantly alter the positioning of the acetabulum and can distort the surgeon’s sense of geometric orientation. With each 1° increase of anterior pelvic tilt, acetabular anteversion decreases on average by 0.74°, with a 0.2° to 0.5° increase in inclination. [27,28] Similarly, alterations in pelvic axial rotation can lead to changes in anteversion and inclination.  Rotation of the pelvis in the coronal plane alters acetabular inclination. 
Milone et al  assessed rigid pelvic positioners for their reliability in patients in the lateral decubitus position for computer-navigated THA. With the computer navigation system’s arm placed into the wound, true anteversion angles deviated by more than 5° in 41% of patients and more than 10° in 22% of patients compared with the surgeon’s predicted anteversion angle. Similarly, actual inclination angles also deviated from the surgeon’s expected inclination angles by more than 5° in 18% of patients and more than 10° in 2% of patients. These alterations in positioning are likely due to patient positioning and patient positioner type used.
Lower extremity manipulation and dislocation has also been shown to introduce additional pelvic pitch and roll. [31,32] In a study by Nishihara et al,  mean pelvic movement following hip dislocation resulted in 12°±4° of posterior pelvic tilt, 2°±4.4° of pelvic inclination, and 7°±5° of pelvic internal rotation. These studies demonstrate the need for a reliable technique to assess 3-dimensional pelvic orientation intraoperatively.
The transverse acetabular ligament (TAL), identified by Archbold et al,  is an anatomic structure that has been proposed to reliably re-approximate the anteversion, height, and depth of the natural hip (Figure 2). [8,33] Successful intraoperative identification of the TAL occurred in 99.7% of 1000 consecutive THAs, of which only 0.6% dislocated over an 8 to 41-month follow-up period.  In a randomized control study by Meermans et al,  acetabular component positioning using the TAL resulted in a significantly more accurate and less variable placement of the acetabular cup. More importantly, no acetabular components fell outside the Lewinnek safe zone when assessed by postoperative radiographs, compared with 20% of free-handed acetabular cups that did. Beverland et al  have further described identification of the TAL.
Figure 2. The TAL should be well exposed for optimal acetabular positioning. Subsequent cup placement should be placed parallel to the TAL for optimal anteversion and to assist in assessing the most appropriate depth and height of the cup.
Interobserver reliability in identifying the TAL is variable. [33–36] As originally described by Archbold et al,  soft-tissue and osteophytes must be removed in approximately 50% of surgical cases prior to properly exposing the TAL. Surgical experience, therefore, may be associated with consistent identification of the TAL. It should also be noted that a follow-up cadaveric study by Hiddema et al  demonstrated the TAL could also be utilized in gauging acetabular cup inclination. However, no other studies have been performed to evaluate this finding, and the original studies by Beverland and Archbold reported the TAL did not assist with inclination. [8,33]
With the growing popularity of direct anterior approach THA, some surgeons have proposed the use of intraoperative fluoroscopy to improve acetabular positioning, and several studies have demonstrated the benefits of fluoroscopic guidance when used with this approach. [37,38] When comparing the direct anterior approach with fluoroscopy with a standard posterior approach, the results become less clear.
Leucht et al  compared postoperative radiographs from direct anterior THAs with fluoroscopy with those from standard posterior approach THAs without fluoroscopy. The direct anterior approach demonstrated more anatomically appropriate levels of anteversion, but also significantly more variability: 25.9°±8.2° for the direct anterior approach versus 35.3°±7.1° for the posterior approach.  Nam et al  found similar outcomes for direct anterior approach THA with fluoroscopy and posterior approach THA without fluoroscopy, with 68.2% and 70% of acetabular cups falling within Lewinnek’s safe zone, respectively. However, computer-assisted navigation in posterior THA was superior to both, with 90.9% of acetabular components falling within the expected safe zone. 
It should be noted that previous studies have identified 5 major risk factors that increase the incidence for component malpositioning: [3,5]
- Minimally invasive surgery (MIS) (odds ratio [OR] 6.10)
- Surgeon experience (OR 2.30)
- Surgical volume (OR 2.07)
- Anterolateral versus direct lateral approach (OR 2.02)
- Obesity (OR 1.35)
Although many of these variables are modifiable, it may be impractical to modify some of them – for example, surgical approach and volume – in an established orthopaedic practice. Instead, these risk factors should serve as a reminder of situations in which more attention should be paid during acetabular cup positioning.
- Complete exposure of the acetabulum should be performed, with adequate removal of the overlying soft tissue and osteophytes covering the acetabular rim.
- Removal of soft tissue in the proximity of the inferior acetabulum should be performed to adequately expose and identify the TAL.
- Relative to the TAL, the acetabular cup should be placed parallel or slightly anteverted to the TAL. However, in patients with an abnormal TAL – as in the case in DDH, acetabular retroversion, ankylosing spondylitis, lumbosacral fusion, and other spinal deformities – the anteversion presented by the TAL may be inadequate for proper acetabular placement. Considerations should be made for advanced imaging, computer navigation and a discussion with a spinal surgeon.
Computer– and Robotic-Assisted THA
Computer navigation and robotic systems have been developed to assist surgeons when positioning THA components and to reduce the complications related to component malpositioning.  Acetabular component malpositioning is the single most important factor in determining the risk of dislocation, responsible for 22.5% of all THA revisions and 33% of acetabular revisions. [42,43] A cadaveric study by Jolles et al  compared cup placement with computer navigation versus a freehand method. Computer navigation resulted in a mean error in alignment of 1.5° in inclination and 2.5° in anteversion, compared with 10° and 3.5°, respectively, when using the freehand technique.  Similarly, a retrospective observational study evaluating computer navigation and a freehand technique found that 91% of acetabular components were within 40° ±10° and 15° ±10° for both acetabular inclination and anteversion, respectively, versus around 70% when using mechanically guided techniques. 
Although computer navigation and robotic-assisted surgery reduce the variability seen with component positioning and potentially increase ROM, no study to date has demonstrated clinically significant superiority. [31,41,45–47] The literature available on these systems only evaluates short-term outcomes. It is hypothesized that their perceived benefits may only be apparent with longer-term follow-up.
Despite offering less variability in component positioning, navigated systems have several limitations that may increase patient morbidity and the cost burden associated with THA:
- Learning curve. Navigated systems are associated with an initial learning curve, which may lead to increased surgical time, preventable systems errors, and in rare cases, component malpositioning. Preliminary studies of the knee demonstrate this learning curve may only be 16 to 20 cases, even in the hands of the general orthopaedic surgeon. [48,49]
- Additional imaging. Most computer navigation and robotic systems require advanced imaging prior to surgery, exposing patients to high radiation doses.
- Increased costs. Many systems require specific hardware, software, instrumentation, and training, which add substantially to the cost of THA without short-term clinical justification.
It should be recognized that computer navigation and robotic-assisted THA provide surgeons with reliable alternatives for the positioning of THA components, particularly in complex cases in which freehand alignment techniques and anatomic landmarks may be less reliable than in a native hip.
- Preoperative advanced imaging and 3-dimensional templating continue to be critical elements for appropriate placement of the acetabular cup in computer-navigated and robotic-assisted surgery (Figure 3).
- Poor patient positioning may cause suboptimal surgical exposure, reference plane acquisition, stability assessment, and anatomic landmark verification, and therefore should not be overlooked.
- Although computer-navigated and robotic-assisted technologies are useful in complex cases, surgeons should be familiar with their use in standard primary THAs prior to their use in more complex surgical cases.
- Intraoperative placement of the acetabular component by computer-assisted guidance should be confirmed through conventional THA principles.
Figure 3. Preoperative templating using 3-dimensional reconstructed CT images allows surgeons to better assess and plan surgical implantation of the acetabular component. All variables, including cup inclination, anteversion, and placement, may be optimized to the patient’s inherent factors.
Complex THAs are frequently less forgiving than routine primary procedures. This is particularly true in patients with spinal pathology (eg, scoliosis and lumbar fusions) or hip dysplasia and those requiring revision surgery. The presence of a significantly disrupted anatomy limits the surgeon’s ability to align and position the acetabular component due to the distorted or obliterated anatomic landmarks, such as the sourcil line, TAL, and ilioischial and iliopubic lines.
In a propensity score analysis, Perfetti et al  reported dislocation rates 7 times higher in patients with 4-level fusions than in patients with no fusion, demonstrating the need for careful preoperative planning and improved intraoperative techniques. As computer navigation and robotic systems become more reliable and affordable, their indications will likely broaden to incorporate more complex THAs, improving the accuracy and outcomes of these challenging surgeries.
Acetabular component positioning continues to be a challenge even for the experienced orthopaedic surgeon. Simple techniques, such as preoperative templating, identification of anatomic landmarks, and surgical experience, have been shown to improve acetabular cup positioning and reduce complication rates. New technologies, including navigated instrumentation, may enhance surgical outcomes.
James E. Feng, MD; Afshin A. Anoushiravani, MD; Nima Eftekhary, MD; Jonathan Vigdorchik, MD; and Ran Schwarzkopf, MD, MSc, are from NYU Langone Orthopedic Hospital, NYU Langone Health, New York, New York.
The authors have no disclosures relevant to this article.
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