Positioning of the acetabular component: Key elements
Total hip arthroplasty (THA) aims to return joint function and improve joint stability for patients. If surgeons do not account for the unique biomechanics of each patient, outcomes can be negatively impacted—leg length discrepancy, gait dysfunction, impingement, wear, and loosening are all issues that may arise. Accurate positioning of the acetabular cup is one element of a THA that can alleviate many of these concerns. There are five aspects of cup positioning that influence the success of THA: medialization, depth, height, angular positioning, and pelvic tilt. How do these considerations allow surgeons to accommodate patient-specific characteristics?
It will be of no surprise to AO Recon members to read, that medicine is moving away from a generalized, one-size-fits-all approach towards treatment strategies that take into account an individuals’ unique physiology and/or genetic composition, among other considerations [1, 2]. While customized care based on the unique needs of each patient has been forefront for quite some time, as our understanding of genetics and acknowledgement of physiological differences grows, so too do our treatment options.
Indeed, precision or personalized medicine, the tailoring of health care based on genetics, lifestyle, and environment, is gaining traction around the world—in 2015, US President Obama announced the launch of the National Health Institute’s Precision Medicine Initiative. The Initiative has the stated mission of enabling “a new era of medicine through research, technology, and policies that empower patients, researchers, and providers to work together toward development of individualized care.” 
In OECD countries, THA is expected to grow at a rate of 1.2% per year, with approximately 2.8 million annual hip replacements in 2050 ; another study on THA trends in the US predicts that by 2060, there will be 1.23 million primary THA’s a year in that country alone .
Orthopedic surgeons are seeing the introduction of technology such as 3D printing of device components and instrumentation that facilitate treatment customized to specific patients [6, 7]. While new technology and understanding certainly play a role in improved outcomes, successful THA is increasingly being seen as more reliant on achieving biomechanics optimized for each individual patient and less on achieving generic intraoperative targets.
Goals of THA
For a THA to be considered successful from a surgeon’s perspective, three interdependent elements must be satisfied. Firstly, functionality of the hip joint must be delivered so mobility can be returned to a patient. Secondly, stability within the joint is integral to meeting the goal of mobility and this must be achieved surgically. Thirdly, maximizing the lifespan of the implant is associated with realization of the previous two elements (see Figure 1). Revision THA for any reason brings increased risks to patients such as infection and re-do revisions . Malpositioning of the acetabular cup has been called “the most common cause of THA dislocation” .
Figure 1. Meeting the primary goals of total hip arthroplasty (THA) is integral to patient satisfaction and lower revision risk.
What we want to avoid: dislocation, pain, revision
Several issues may present themselves if stability and function are not restored during THA (see Figure 3). Limb-length discrepancy (LLD) is one of the most common complications of THA . Seen in up to 25% of cases, LLD generally only impacts function if greater than 10 mm . Whether post-operative LLD is directly or indirectly related to component malpositioning, atypical anatomy can complicate a surgeons mitigative options .
Gait is certainly impacted by LLD—limb length changes alter biomechanics. It has been reported that despite improvement after THA, patients displaying pre-operative abnormal gait patterns are unlikely to achieve a normal gait postoperatively [13, 14]. There is also indication that gait recovery patterns differ by gender . However, gait patterns also influence implant wear rates. Ardestani et al. found that gait was more influential on wear than component positioning in metal-on-polyethylene bearings . While rehabilitation exercises targeting hip extension and external rotation have been suggested as a way to address gait dysfunction , patient positioning during the THR procedure has also been highlighted as a factor that influences post-operative gait .
The mean age for THA decreased significantly in the US between 2000-2014 . In 2002, the average age for THR was 66.3, by 2015 it was age 64.9 . Younger patients are generally more active and expect more from their prostheses. This translates into accelerated wear for this demographic as well as active patients in other age groups. In a 2017 population-based cohort study, up to 35% of males and 20% of females who had THA in their early 50s were found to require revision within their lifetimes. If a patient was younger than age 60, the median time to revision was reported to be 4.4 years .
Additionally, wear and secondary loosening in younger patients is common . Survival of the acetabular component in this demographic has been reported to be 71% at 10 years and 54% 20 years postoperatively . Normal function of the acetabular cup in all ages of patient can also be impeded by larger abduction angles which produces a deeper depth of wear and may cause the THA to fail .
Instability and dislocation are other common complications in THA patients ; dislocation is reported in roughly 2% of all patients within a year of their procedure . If the range of movement after THA is inadequate, impingement of the prosthesis neck on the acetabular cup may occur (see Figure 2.). Subluxation and dislocation become real risks in this situation . Additionally, dislocation risk was found to be 10 times higher in patients with higher pre-operative American Society of Anesthesiologists (ASA) scores .
Brooks points out that “patient education cannot be over-emphasized, as most dislocations occur early, and are preventable with proper instructions”. However, dislocations also have surgical origins, such as “ [a surgeon’s] annual quantity of procedures and experience, the surgical approach, adequate restoration of femoral offset and leg length, component position, and soft-tissue or bony impingement… the design of the [implant] head and neck region, and so-called skirts on longer neck lengths” .
Figure 2. Simplified schematic of a patient's native hip range of motion and predicted complications when placement of THA components do not overlap with this. A surgeon’s goal is to position the implant as completely as possible within the range of motion of the native hip. Parcells B. Cup Placements: THA Technique. Available at: https://hipandkneebook.com/tha-chapters/2017/3/1/basic-hip-biomechanics. Reproduced with permission.
Complications are most often multi-factorial in their causes. Yet many of these above-mentioned problems can be addressed intra-operatively through a combination of measures. This three-part article series examines the role of, and how to achieve, accurate, patient-tailored acetabular cup positioning.
Figure 3. These potential THA complicationsare multi-factorial in their causes but accurate placement of the acetabular cup can decrease their chances of occuring.
Defining acetabular cup positioning
When we speak about cup positioning it is helpful to visualize it as a relationship between two parameters. Scheerlinck describes it as “the spatial relation between the hip rotation centre and the pelvis and, as the cup orientation around the rotation centre” . Rivière et al. reminds us that when seeking to restore the native anatomical function of a hip, consideration of “combined femoro-acetabular anteversion and the hip’s centre of rotation, and occasionally adjusting the cup position and design based on the assessment of the individual spine-hip relation” is required .
Acetabular cup positioning is an aspect of THA solely within the surgeon’s control and judgement [30, 31]. Its accurate orientation is one of the most important factors in achieving stability in the joint and preventing complications . Surgeons are trying to match the native range of motion with that of a given implant’s by adjusting the positioning of the THA components (see Figure 2) . It is a tricky endeavor since the hip’s center of rotation (COR) within the acetabular cup is influenced by its anteversion and inclination, height and offset .
This video illustrates the three-dimensional nature of cup orientation and the angles that describe: 1) inclination as rotation around the sagittal axis in the coronal plane; 2) version as the angle rotating around the longitudinal axis in the transverse plane; and 3) tilt as the angle rotating around the transverse axis in the sagittal plane .
Joint reaction force
The net force generated within a joint due to the forces that act upon it is known as joint reaction force (JRF) . JRF is a function of abductor force/tension and the lever arms of body weight (see Figure 4). Harris et al. notes that “the magnitude and direction of hip JRF influences patterns of stress observed at the cartilage and labrum” . The JRF is important because if component placement increases the forces in the hip joint it may cause squeaking in ceramic-on-ceramic hips or increased wear in other bearing surfaces , and lead to loosening and/or revision.
As a measurement of “the center of the femoral head and the true floor of the acetabulum”, acetabular offset (AO) varies widely within the population and between genders, dictating an individual’s particular biomechanics . The position of the acetabular cup and the amount it may or may not be offset influences the calculation of JFR of a hip .
Figure 4. Pelvis X-ray with both hip joints anteroposterior view showing importance of mediolateral position in determining joint reaction force. Medialization reduces body weight lever arm and increases abductor lever arm reducing joint reaction force which is calculated as JRF = BWxB – AbxA. Right side shows femoral offset, acetabular offset, and their contribution to global offset. BW – Body weight, Ab – Abductor force, A – Abductor moment arm, B – Body weight moment arm, JRF – Joint reaction force, AO – Acetabular offset, FO – Femoral offset. Bhaskar D, Rajpura A, Board T.Current Concepts in Acetabular Positioning in Total Hip Arthroplasty. Indian J Orthop. 2017 Jul;51(4):386-396. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5525519/. Permission to use under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.
JRF, as an estimate of the loading forces in the joint , is affected when the acetabular component is medialized during THA. The more the acetabular cup is medialized, the less it supports global offset and the femoral component must then be offset more than the pre-existing amount to compensate, however this compensation has reportedly delivered good outcomes in many cases [38, 41]. Conversely, it has also been reported that a reduction in femoral offset resulted in “inferior functional outcome scores” for patients .
It is challenging to place the acetabular cup since positioning must be optimized in terms of JRF while simultaneously balanced with rotation within the cup . Medialization refers to the cup position along the horizontal plane .
There are two schools of thought around medialization of the acetabular cup. On one hand, the Charnley approach, seen as conventional, classic, or traditional [45, 46], promotes universal medialization of the acetabular component to the medial acetabular wall. The anatomical global offset is maintained by increasing the femoral offset by the same distance that the acetabular offset is decreased. The thought is that the more medial the COR the better the moment arms [see Figure 4] . This approach moves the COR away from the native anatomic position.
The other philosophy is a more patient-specific consideration that seeks to reconstruct anatomic AO and re-establish each patient’s unique COR (see Figure 5) . Restoring the hip’s native COR, along with a small increase in femoral offset and slight inferomedial cup positioning was found by Asayama et al. to optimize abductor function .
Conventional versus anatomical approach
There are several studies that support the use of either a conventional or anatomical approach for acetabular cup placement. The discussion of medialization versus restoration of anatomical (kinematic) biomechanics is complicated by unknowns. Terrier et al. ask in particular, if there are “pelvic and femoral geometries [that are] more susceptible or resistant to biomechanical effects of cup medialization”. They also note a lack of agreed-upon anatomical parameters to guide surgical decision-making when considering the suitability of cup medialization for a patient .
Figure 5 highlights the differences between these approaches. With an anatomical (kinematic) approach, there are additional considerations that require analysis. There are some instances when one approach may be indicated over the other (see Table 1).
Figure 5. Comparison of anatomical (or kinematic alignment (KA)) THA and conventional THA. KA involves restoring the constitutional hip anatomy (proximal femur anatomy and acetabular centre of rotation) and taking into account the individual sagittal lumbopelvic kinematics in order to plan the implant design (cup and head size), the acetabular cup orientation (using the TAL) and the need for spinal surgery to correct a severe sagittal imbalance. SHR=spine-hip relation. TAL=transverse acetabular ligament. Rivière C, Lazic S, Villet L, et al. Kinematic alignment technique for total hip and knee arthroplasty: The personalized implant positioning surgery. EFORT Open Rev. 2018;3(3):98-105. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5890135/. Used with permission under Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) licence.
Table 1 is a non-exhaustive listing of some of the arguments and indications that may prompt a surgeon to select either a conventional or anatomic cup placement.
Table 1. Selected pros and cons for conventional and anatomic approaches to acetabular cup medialization
Another variable for consideration when determining the placement of the acetabular cup is the depth at which to situate the cup within the acetabulum. Component containment is critical for THA success and this ultimately influences decisions around the depth of the cup .
The type of cup being used (eg, cemented vs. non-cemented) [38, 53] and a patient’s preoperative anatomy (native acetabular floor depth) both contribute to the discussion as sufficient space must be created to house the implant without altering the biomechanics or removing too much bone stock.
The superoinferior position (or height) of the acetabular cup impacts JRF as well as the length of the limb . Relative to the anatomical COR, JRF were found to increase by 0.7% or decrease by 0.1% for every millimeter of either lateral or proximal displacement of the cup . However, it has been noted that more research is needed to better understand the relationship and impact of small changes in moment arms on JRF .
Limb length discrepancy (LLD) is one of the most common errors in THA . While Pathek et al. state that shortening is most common , others report lengthening to be more likely [55, 57]. Despite natural LLD being prevalent in up to 90% of the population , a postoperative LLD of more than ±10 mm is more likely to be associated with lower back pain, joint instability, gait disorders, and paresthesia [55, 59].
Component malpositioning can result in LLD . Archbold et al. suggest evaluating acetabular and femoral height independently of each other and they have presented good results in terms of LLD (94% had <6 mm difference compared to the normal side) by using the transverse acetabular ligament (TAL) to control the vertical height of the acetabular component .
Angular positioning: anteversion and inclination
Orientation of the acetabulum, whether native or associated with prosthetic cup placement, is described by its version (sagittal plane) and inclination (coronal plane) angles [33, 61]. Native acetabular angles are larger in females (21.3° ± 7.1°) than males (18.5° ± 5.8°) .
Stem et al. tell us that “acetabular anteversion is measured by the angle formed when a line is drawn between the anterior and posterior acetabular ridge and a reference line drawn perpendicular to a line between the posterior pelvic margins at the level of the sciatic notch” . Alternately, anteversion is defined on a lateral radiographic view by the angle between the acetabular axis and the coronal plane (see Figure 6) . Ideally, surgeons aim for slight anteversion of the cup with a slight anterior orientation to facilitate increased hip adduction .
Figure 6. Lateral x-ray. The acetabular anteversion is defined by the angle between the acetabular axis (line I) and the coronal plane (line J). In this patient, the angle measures approximately 25° (normal range between 5°–25°). Vanrusselt J, Vansevenant M, Vanderschueren G, et al. Postoperative radiograph of the hip arthroplasty: what the radiologist should know. Insights Imaging. 2015;6(6):591-600. https://doi.org/10.1007/s13244-015-0438-5. Used with permission under Creative Commons Attribution 4.0 International License.
Acetabular inclination is the angle between the articular side of the acetabular cup and the transverse axis (see Figure 7) . Inclination impacts contact forces along the surface of the cup dome; too much inclination results in edge loading and not enough inclination restricts the range of motion .
Figure 7. The acetabular inclination is measured by drawing a line through the medial and lateral margins of the cup (line E) and measuring the angle with the transverse pelvic axis (line D). The femoral stem positioning should be aligned with the longitudinal axis of the shaft (line F = normal, longitudinal axis of the shaft). Vanrusselt J, Vansevenant M, Vanderschueren G, et al. Postoperative radiograph of the hip arthroplasty: what the radiologist should know. Insights Imaging. 2015;6(6):591-600. https://doi.org/10.1007/s13244-015-0438-5. Used with permission under Creative Commons Attribution 4.0 International License.
Dislocation risk has been correlated to angular positioning, with Jolles et al. reporting that if total anteversion is not between 40° and 60° patients have a 6.9 times greater risk of joint dislocation . Lewinnek et al. showed an increased dislocation rate if cup orientation was outside of an anteversion window of 5° to 25° and outside of a lateral window of 30° to 50° .
Variability in defining “safe zone” angles
Cup positioning has been suggested to be flexible within a certain generic “safe zone”. However, there remains disagreement about what an adequate safe zone might be (see Table 2). McCollum and Gray put forward a position of 40° ± 10° abduction and 30° ± 10° flexion as being ideal to avoid dislocation and impingement . In 1978, Lewinnek suggested a cup inclination of 40° ± 10° and anteversion of 15° ± 10° [65, 67]. Sometimes dislocations still occur even when the cup is placed within the Lewinnek and other recommended safe zones and this phenomenon has been observed by numerous researchers [38, 67, 68, 69]. Abdel et al. notes that for some patients the best placement would fall outside of the Lewinnek safe zone and the characteristics of this patient group have yet to be defined .
Variability in the recommended angles for cup placement, or “safe zones”, highlights a problem that continues to be emphasized by researchers: the lack of a universal standard has resulted in variability in how surgeons measure the “safe zone” angles. The existence of multiple, at times conflicting, recommended safe zones, makes cross-study comparisons difficult and hampers evidence-based, definitive decisions on the matter [31, 67, 70].
Different ways to measure and describe the “safe zone”
Surgeons use three different reference systems, alongside the acetabular axis, to concretely determine and evaluate acetabular cup placement . Operative, radiographic, and anatomical reference systems are employed during planning, as well as intra- and postoperatively. Figure 8 shows the differences between each of these, indicating the planes considered. Each reference system refers to anteversion and inclination but how this is measured is unique to the difference systems.
Figure 8. Comparison of operative (A), radiographic (B), and anatomical (C) reference systems. (δ)=Inclination angle; (ϕ)=Recommended operative anteversion angle; (α)=Radiographic anteversion angle; (Ɵ)=Radiographic inclination angle; (β)=Anatomical inclination; (γ)=Anatomical anteversion. Harrison CL, Thomson AI, Cutts S, et al. Research Synthesis of Recommended Acetabular Cup Orientations for Total Hip Arthroplasty. J Arthroplasty. 2014 Feb;29(2):377-382. https://www.sciencedirect.com/science/article/pii/S0883540313004397#f0005. Used with permission under Creative Commons License Attribution 3.0 Unported (CC BY 3.0).
The operative reference system is defined by intraoperative patient positioning on the table, ideally with the patient in the lateral decubitus orientation. The radiographic reference system uses measurements obtained from x-rays that are taken before (for planning) and after (for evaluating) the procedure. If the THA was performed with the patient in the supine position, then radiographic definitions would also be used intraoperatively .
Harrison et al.’s study mathematically converted manufacturers instructions for cup orientation, textbook recommendations, and literature-recommended safe zones and techniques into a comparable framework that allowed cross-evaluation of these different guidelines .
Table 2 is a summary of selected information that illustrates some of their findings. The authors point out that the “majority of the recommended implant orientations are contained within Lewinnek's definition of the safe zone. However, Harris is on the edge of the safe zone, Calandruccio Campbell's Operative Orthopaedics [is] partially overlapping the safe zone and Charnley is not at all contained within the Lewinnek safe zone.”
Table 2. Acetabular cup inclination and anteversion angles as originally recommended in safety guidelines, academic textbooks, and published literature alongside their corresponding converted angles in operative, radiographic, and anatomical reference systems.
Note: Table based on information extracted from: Harrison CL, Thomson AI, Cutts S, et al. Research Synthesis of Recommended Acetabular Cup Orientations for Total Hip Arthroplasty. J Arthroplasty. 2014 Feb;29(2):377-382. Restructured and used with permission under Creative Commons License Attribution 3.0 Unported (CC BY 3.0).
Yoon et al. states that a reference system, and its related acetabular cup orientation angles, has “significant effect on the target orientation” of the cup. They emphasize the necessity for cup placement recommendations to be accompanied by a clear explanation of which reference frames and angle definitions are used. They go on to report that their analysis of nine published articles on safe zone recommendations revealed five different methods to describe cup orientation and two different methods to define the reference frame . After analysis, the authors recommended radiographic angles of 41° inclination and 16° anteversion and operative angles of 39° inclination and 21° anteversion as “safe zone” targets.
The final aspect of acetabular cup positioning that we will look at is pelvic tilt, which refers to the angle of the pelvis and is defined as the angle between the patient’s coronal plane and the anterior pelvic plane [31, 38]. It is a dynamic parameter that changes as the position of the body changes . THA doesn’t change a person’s pelvic tilt but the position of the pelvis affects acetabular cup placement as well as post-operative radiographic evaluation [55, 73]. Natural anterior and posterior pelvic tilt can vary between individuals by as much as 30°  and is not neutral in the majority of people .
If the pelvis is used as an intraoperative landmark there is an increased risk that errors will be introduced . It has been shown that the pelvic position during THA fluctuates in unpredictable ways right from initial positioning and throughout surgery and intraoperative range of motion tests . Additionally, significant differences in pelvic tilt between the supine and upright positions means that if cup placement is solely considered in the supine position then a risk of dislocation and instability are associated with standing .
Pelvic tilt directly influences the acetabular anteversion angle . Lembeck et al. found that pelvic tilt of 1° led to functional anteversion of the cup by approximately 0.7° . Zhu et al. reported the relationship to be 1° of anterior or posterior tilt to cause 0.8°of cup anteversion .
Depending on the tilt and rotation of the pelvis, postoperative x-rays of the acetabular cup position could be misinterpreted on anteroposterior pelvic x-rays. Mathematical corrections are recommended to bypass pelvis malpositioning in these cases .
Placing the acetabular cup is a challenging aspect of THA that requires surgeons to be cognizant of the component’s multi-factorial relationship to the body in three-dimensions. Now that we have presented the key elements for consideration, the next articles in this series discuss what is needed to customize a THA to a patient’s unique anatomy during planning (Part II) and execution (Part III).
This series of articles was created with the support of the following specialists (in alphabetical order):
This issue was created by Word+Vision Media Productions, Switzerland
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