Positioning of the acetabular component: Intraoperative considerations
Total hip arthroplasty (THA) is a technically demanding procedure. Re-establishing a patient’s native biomechanics, such as center of rotation, is critical and can reduce the risk of complications and increase patient satisfaction. What should surgeons pay specific attention to when placing the acetabular cup? Accurate placement of this component is multi-factorial. In the final part of this article series we look at intraoperative considerations that will ensure patient-customized acetabular cup positioning. In addition, expert surgeons share insight on patients with protrusion and dysplasia and their views on the role of navigation in acetabular cup placement.
For many years conventional total hip arthroplasty (THA) was focused on medializing the prosthetic’s center of rotation (COR) compared to the native COR (see Part I). This was accomplished by medializing the acetabular component and using femoral offset to compensate for the increase. However, soft tissue around the hip will restrict the joint to its pre-existing range of mobility  and if the pre- and postoperative ranges are incongruent? Well, impingement and instability can be a problem .
In what has been referred to as a “kinematic revolution”, THA has evolved to include the consideration of a patient’s individual biomechanics when making decisions for implant component placement. Figure 1 illustrates the device components of a THA. Surgeons have moved away from a blanket approach to placement, towards one which seeks to restore a hip’s pre-existing range of mobility . Each patient presents unique acetabular orientation ; the thought is that native anteversion should be respected.
Figure 1. (Left) The individual components used in a primary total hip replacement. (Center) The components merged into an implant. (Right) The implant as it fits into the hip. AAOS. Revision Total Hip Replacement. Used with permission. Available at: https://orthoinfo.aaos.org/en/treatment/revision-total-hip-replacement/. Accessed March 7, 2019.
Let’s look at specific intra-operative actions surgeons can take to ensure an accurate, patient-specific acetabular cup placement. Part II of this article series addressed the planning phase, reiterating the need for thorough patient assessment, proper imaging, careful templating, and thoughtful preparation of a surgical strategy, including continency plans.
While supine positioning simplifies the assessment of the pelvis’s position and limb length during THA , more than 75% of surgeons perform THA with the patient in the lateral decubitus position  and the majority of these use a posterior approach . A direct anterior (minimally invasive) approach is also employed by some surgeons for THA; it has produced acceptable outcomes comparable to traditional THA in terms of cup placement [7, 8]. As we discuss further below, lateral decubitus positioning is associated with variation in pelvic tilt and despite assumptions that the pelvis is aligned with the coronal plane on the OR table, the alignment is in fact unknown.
With lateral decubitus positioning, the patient’s involved leg is in the "home position" (45° to 60° flexion, 20° to 30° internal rotation, and slight adduction), which is achieved by elevating the foot on a padded Mayo stand .
Adequate bolstering and support is of upmost importance and is intended to secure the patient in a known position throughout the procedure  – this, in theory, allows for straight-across comparison to preoperative x-rays and should facilitate execution of the acetabular cup’s planned anteversion and inclination.
However, research is showing this is not necessarily the case and undetected movement is common. Over a third of UK surgeons surveyed reported issues with the supports they use; a mere 31% considered their supports to be completely rigid. There is an identified need for further investigation and awareness around essential elements of patient support design . In particular, better tools to stabilize and specifically hold the pelvis in place are needed [11, 12].
Positioning may not be what you think it is
Schwarzkopf et al. found most intraoperative motion to be under 4°, however, they observed significant pelvic roll of over 10° in a large proportion of patients.  Malpositioning was determined to be “common” in a study by Lambers et al. and more likely if a patient had a higher Body Mass Index (BMI) . If movement is not compensated for then cup placement is compromised, with the inadvertent installation of the component outside of the planned safe zone a legitimate risk .
A preoperative strategy to help surgeons control for pelvic obliquity has been suggested by Beverland et al. Preoperatively, with the patient in the seated position, use a spirit level and draw parallel lines at the lumbosacral region. And while it may not be effective in patients with a high BMI, it will help illustrate pelvic adduction intra-operatively .
Pelvic movement happens
If the pelvis alters position during THA it will affect the orientation of the acetabulum and subsequently, the component’s orientation. Grammatopouloset al. found initial pelvis positioning to vary between surgeons. Movement between initial positioning and cup implantation depended on the approach (more with posterior approach vs. lateral) and type of support used. If patients are operated on in the supine position versus the lateral decubitus position, errors related to pelvic tilt are decreased .
In a study of changes in pelvic positioning during supine THA surgery with a direct anterior approach, between the start of the procedure and the time of cup implantation, 19/22 patients displayed anterior pelvic tilt (average of 3.1°; range: 1°-6°) on the sagittal plane. Changes in relation to the transverse plane revealed pelvic roll to varying degrees in 20/22 patients. In just over a third of patients the pelvic changes caused the cup version to change more than 5° .
Similar findings have been reported for THA patients in the lateral decubitus position, with pelvic tilt and intraoperative movement altering the cup’s orientation . However, measuring pelvic tilt with the assistance of computer navigation and applying Lembecket al. ’s  mathematical conversion factor of 0.7 improved cup position accuracy. It also allowed anteversion on the coronal plane to be determined .
Pelvic tilt changes during THA have been identified as at least a partial source of the variation in post-operative orientations of acetabular components. One intra-operative method of assessing pelvic tilt that has been recommended involves determining the “relative position of the symphysis pubis and the [anterior and contralateral superior iliac spine] ASISs” .
When operating with the patent in the supine position, it is recommended to minimize retractor traction right before cup implantation. As the pelvis is not fixed to the table, pelvic movement means surgeons should take “into account that anteversion of the cup implant in relation to the table plane is systematically higher than in relation to the pelvic entry plane . Leg twisting associated with the posterior approach also reorients the pelvis .
Pelvic movement and imaging
Images taken intra-operatively can be used to confirm the accuracy of component placement. If the pelvis is tilted during imaging the perceived position of the acetabular cup will be incorrect. Even if a surgeon attempts to compensate, their efforts are unlikely to be accurate . It has been suggested that only 30% of intra-operative AP pelvic images are “perfect”, resulting in a “slight” underestimation of the cup abduction angle measurement. This angle is more likely to be underestimated on lateral decubitus images compared to supine images .
James et al. “recommend positioning the C-arm so that the size and shape of the obturator foramen matches the standing preoperative anteroposterior pelvis image…this will allow for the native standing pelvic tilt to be accounted for intraoperatively and will result in the least variation in intraoperative and postoperative standing acetabular component orientation.” 
What is an acceptable cup depth?
In The Well-Cemented Total Hip Arthroplasty by Breusch et al. , the authors point out that the “cardinal rule” in terms of containment is “that the acetabular component (cup) should be completely…under the roof of the acetabulum”. Adequate coverage is needed for optimal and long-term fixation.
There is natural variation in the anteversion, retroversion, and abduction parameters of natural acetabular openings; these are also influenced by gender [23, 24]. The native cup is likely to be shallow in dysplasia patients and deeper in those with femoral protrusion. In all hips the goal is to balance containment parameters with restoration of the native center of rotation. How this is accomplished will differ depending on the patient’s anatomy. Widmer’s viewpoint is that the “most effective compromise is to use a prosthesis that has a large safe zone, realized by a high head-to-neck ratio, and orienting the cup such that a good containment is achieved and the safe zone is respected.” 
However, it’s been recommended that the edges of the cup should sit against Kohler’s line (ilioischial line) . This can require greater medialization to ensure proper containment. Press-fit uncemented components need a rim of peripheral cortical bone for stability and any medialization beyond this rim raises the risk of loosening .
Reaming: proceed carefully
Depth is increased through manual reaming up to bleeding cancellous bone  and is influenced by the diameter of the reaming instruments, the technique used, and the elasticity of the pelvis . If the site is over or under-reamed there is a risk of not achieving good fixation and/or altering native biomechanics, regardless of the type of cup used .
Alexander et al. noted that during their testing, the acetabular floor and notch were most likely to not be spherical by up to 4 mm after reaming, which may impact fit of the cup . If a surgeon assumes “the center of the acetabulum corresponds to the center of the acetabular fossa [they] risk eccentric reaming, possibly damaging the anterior wall.”  Reaming right to the floor was shown by Meermans et al. to significantly displace the COR both medially and superiorly and using high offset stems did not always compensate for this .
Anatomical or medialized placement?
As discussed in Part I, there is somewhat of a disagreement between Charnley’s  recommendation to universally medialize the cup, which prioritizes a reduction in joint reaction forces (JRF), and anatomical placement of the acetabular component with femoral offset to compensate . However, one of the goals common to both philosophies is the avoidance of both osseous and component impingement .
Considering the growing trend of preserving native biomechanics and awareness of the role of acetabular offset (AO), Bonninet al. compared the amount of COR displacement in conventional (medialization) and anatomical cup placement after THA. They found no COR displacement with anatomical placement yet over 5 mm of displacement in over 44% of conventionally positioned acetabular cups. They encourage conservative preparation of the acetabulum to restore native hip kinematics through as close to native AO as possible .
In patients with congenital hip disease, such as osteoarthritis, restoration of a patient’s native COR has been identified as particularly important. These cases are “not…straight forward” and surgically “demanding” ─ proper planning is paramount. Reestablishing the anatomical hip center, versus medialization, will “restore abductor lever arm and pelvic imbalances thus eliminating abnormal contact stresses on the hip joint” .
Worth mentioning is the finding that cup medialization with the same stem offset negatively impacts a hip’s postoperative range of motion (in flexion and internal rotation) yet increases the ROM in external rotation. This study also identified “negative correlations among flexion and laterally large and steep anterior inferior iliac spine[s].” This further supports consideration of a patient’s native anatomy as the success of medialization depends on morphology .
Cup height influences LLD and JRF
The height, or superoinferior position, of the acetabular cup holds ramifications for limb length and JRF. If the component’s placement alters the functional height of the acetabular cup, the limb is shortened or lengthened, depending on if the resulting COR is raised or lowered, respectively .
Raising the COR increases the JRF , heightens the risk of complications, and if raised more than 3 mm, makes restoration to within 5 mm challenging to accomplish [28, 34]. However, in dysplastic patients with poor native acetabular bone stock, creating a high hip center is sometimes unavoidable .
Archbold et al. put forward a technique that uses the transverse acetabular ligament (TAL) (see Figure 1) to control the acetabular height independently from femoral height. This was shown to produce good results in terms of minimizing limb length discrepancy (LLD) to 6 mm or less .
Special considerations for protrusion and dysplasia patients
Generalized recommendations for THA patients with protrusion, dysplasia, or degenerative disorders are difficult to make; each case is unique and requires measured consideration based on a surgeon’s previous experience and patient specific parameters. However, these expert surgeons share some tips for what should be on your radar for these types of patients.
Dislocation of the hip joint might be difficult in high-grade protrusia; in such cases the surgeon should consider osteotomy of the femoral neck in situ. Attempt to restore the hip joint’s COR to normal, taking into account the distance from the dome of the acetabular cup to the ilioischial (Kohler’s line); general rule is 2 to 3 mm distance. Extra care should be taken during acetabular reaming in the presence of deficient or absent medial floor to avoid damage to the interior structures like vessels, bladder, or bowels in extreme cases.
Prior templating indicates if special implants or augments for the acetabulum are needed. Maintain the integrity of the neurovascular bundle, especially the sciatic nerve; no more than 4 cm of limb lengthening is considered safe - femoral shortening might be required. Identify the true acetabulum through resection of the redundant capsule and formal visualization of TAL. Be careful with the acetabulum’s existing bone stock; slowly, sequentially ream the acetabulum’s floor and the walls. Avoid a high hip center as this increases implant failure and decreases the longevity of the implant.
An in situ osteotomy may be necessary to avoid a forceful dislocation that will predispose to a femoral fracture. Next, the reaming of the socket should be very gentle, with the floor being reamed minimally and only to freshen the bone. Rim-reaming is then performed in order to allow the cup to fix in an anatomic position. Any remaining defect is filled with autograft from the reamings. Fixation of the cup with screws is advisable. Over-medializing the cup will result in incorrect biomechanics and will increase the risk of impingement and dislocation.
In a dysplastic cup, it is important to place the cup as low as possible and as medial as possible to achieve fixation in an anatomic position. In most cases, superior-posterior grafting with the femoral head is not needed. On occasion, especially in Crowe III hips, this may be necessary. In Crowe IV hips, a small cup in an anatomical position will suffice, but a femoral shortening osteotomy will likely be required.
Anatomical landmarks are independent of positioning
Controlling the height, depth, and version of the acetabular component by referencing the TAL and labrum (see Figure 1) is a generally accepted technique. Using the TAL as a reference is thought to assist in the restoration of native biomechanics and effectively control version while compensating for any underlying bony abnormality, such as in dysplastic hips . Beverland et al. explains:
“In the normal hip the TAL and labrum extend beyond the equator of the femoral head and therefore, if the definitive acetabular component is positioned such that it is cradled by and just deep to the plane of the TAL and labrum, and is no more than 4 mm larger than the original femoral head, the centre of the hip should be restored. If the face of the component is positioned parallel to the TAL and psoas groove the patient specific version should be restored.” 
Figure 2. Anatomy of the acetabulum. Dhollander A. (Thesis) Repair of cartilage and fibrocartilage in large synovial joints. 2012. Available at: https://www.researchgate.net/publication/292333994_Repair_of_cartilage_and_fibrocartilage_in_large_synovial_joints. Accessed March 7, 2019.
Beverland notes that the TAL should not be used to determine inclination during cup placement. Archbold et al. described their use of the TAL to control acetabular depth, height, and version and used the residual labrum to define inclination. They provide specific instructions for orienting the reamer to re-establish natural version for a patient. They also reported a 0.6% (6/1000) dislocation rate at follow-up .
However, not everyone agrees that the TAL provides accurate guidance. Archbold et al. may have asserted that the TAL, even if requiring some surgical exposure, was almost always identified (99.7%), but Epstein et al. reported that surgeons have difficulty finding it. In their study, the TAL was only intraoperatively identified in 30/64 hips (47%) and was less likely to be identified if the patient had inferior acetabular osteophytes. They concluded that the TAL was not a dependable reference .
The acetabular teardrop is also used to guide component placement during THA. Aligning the lower acetabular component aspect with the lower teardrop edge, while not precisely replicating the anatomic hip center height (the difference was not clinically significant), was determined to be a method with “high simplicity, reliability, and stability” .
When asked about anatomical landmarks for cup placement, Mohamad Allami, MD, and Senior Consultant Orthopaedic Surgeon at the Alarabi Hospital for Surgical Specialty, Baghdad, Iraq, suggested referencing three bony and one soft tissue landmark to help achieve desired inclination and anteversion of the cup.
“Consider the ilium, superior pubic ramus, and superior acetabulum for bony landmarks. The transverse acetabular ligament (TAL) is a soft tissue landmark consistent in patients and is very useful, particularly for cup anteversion. However, you must be able to see the entire TAL to use it effectively. Care should be taken if the TAL is calcified or over-covered by osteophytes. In cases with a retroverted acetabulum, the TAL must be considered in tandem with other bony landmarks or the component will be placed in a retroverted positioned.”
Referencing internal landmarks eliminates the uncertainty around pelvic positioning. Figure 1 illustrates how the superolateral most point of the bony acetabulum and the lateral extent of the templated acetabular cup can used as references. During templating a measurement between these two points is taken and used to guide inclination intra-operatively .
Figure 3. X-ray (L) hip joint anteroposterior view and intraoperative photograph (R) showing measuring of the lateral overhang of template and reproducing it intraoperatively to achieve correct cup inclination. 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/. Used with permission under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.
Another anatomical guide that has been suggested to control cup version is the acetabular notch or psoas groove. Bhaskar et al. remind us that “especially in anatomical position of [the] cup, care should be exercised that there [is] adequate anteversion so that the [cup’s] anterior margin is deep to the notch to prevent psoas irritation.” 
Bassam Masri, MD, and Head of Orthopaedics at the University of British Columbia, Canada, shared his preferred approach to determining cup version:
“The most important landmark for determining the cup ante-version if the transverse acetabular ligament (TAL). In most hips, the TAL is well-preserved and if the inferior aspect of the cup is placed parallel to the TAL, the correct anteversion is typically selected. In the absence of a TAL, I estimate its position by drawing a liner along the axis of the fovea centralis, which would be perpendicular to the position of the TAL.”
Is there an ideal angular position?
Currently, there is no standardized measurement method or agreed upon ideal orientation for acetabular placement . Figure 2 illustrates how surgeons must simultaneously manage two planes since the inclination and anteversion angles are measured independently. As discussed in Part I of this article series, there is quite a bit of variation between identified “safe zones” for cup orientation (different reference frames and angular definitions), yet dislocations still occur when the cup is within these zones .
Figure 4. Surgical definition of anteversion (A) and inclination (B) angles. 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 CCLicense Attribution 3.0 Unported (CC BY 3.0).
In a comparison of nine papers, and after performing calculations to allow standardized cross-evaluation, Yoon et al. reasoned that the adjusted “safe zone” target should be 41° inclination and 16° anteversion for radiographic angles (operative angles of 39° inclination and 21° anteversion) . Beverland et al. “recommend 35° of apparent operative inclination (AOI) during surgery, as opposed to the traditional 45°” . Note that radiographic measurements are always lower than operative ones.
After comparing numerous “safe zones” and device manufacturers guidelines, Harrison et al. put forward a suggested safe zone they refer to as the ‘surgical reference’ that combines the common practicing of referencing inclination angle from a radiographic perspective and anteversion from an operative angle. Their recommendation is “restricting anteversions to no less than 5° and no more than 30°, [as] a new safety zone based on the Lewinnick zone… [with] approximately 40° surgical inclination and 17–18° surgical anteversion.” 
Harnessing technology for angular positioning
Aside from using computer assisted navigation during THA to solve problems around placement issues (see sidebar), surgeons are also innovating new tools to support more accurate positioning.
For example, Meermans et al. showed that when a digital protractor was used to adjust the cups operative inclination angle, and the hip’s circumference was taken into account, significant improvement in the cups radiographic inclination angle was achieved without impacting operative time . Additionally, a simple inclinometer used to measure inclination of the cup component improved its placement within a predetermined ‘safe zone’ (30° to 50°) .
Another group of surgeons reported combing two iPhone applications that used the accelerometer and camera functions (level indicator and protractor apps) to quickly and accurately improve acetabular cup placement .
Based on statistically significant differences between the cup inclination angles achieved by a group of junior surgeons using an iPhone app versus those who used a “free-hand” method for placement, Tay et al. suggest there is “a potential role for iPhone applications [for] junior surgeons…overcoming the steep learning curve.” 
Is there a role for computer assisted navigation in THA?
The use of computer assisted navigation tools during THA depend on a surgeon’s access to, training on, and opinions of navigation. Some researchers have shown this technology to improve the precision of acetabular cup placement [45, 46, 47, 48, 49]. We asked two expert surgeons for their opinion on the role of navigation in THA.
Bassam Masri, MD, Professor and Head of University of British Columbia’s Department of Orthopaedics, Head of Department of Orthopaedics at Vancouver Acute, Canada, replied, “There is no role for navigation in my practice,” which highlights the fact that adoption of this technology is not wide-spread, nor essential for good outcomes.
Mohamad Allami, MD, Consultant Orthopedic Surgeon, Hip & Knee Joint Replacement Surgery & Sport Medicine, UAE & Oman, noted that navigation may prove useful in certain situations. “While I don’t have personal significant experience in navigated hip surgery, I can identify its usefulness in the following conditions: 1) Multiple revision surgery, especially in cases where there is significant bone stock loss and alteration of anatomical landmarks; 2) Complex acetabular and pelvis fractures; and 3) Complex primary hip replacement surgery, including ankylosed hip, CDH, stiff/fused spine, and protrosio acetabulum, among others.”
Spinopelvic considerations utilizing dual mobility implants
An understanding of biomechanics and how the spine, pelvis and hip work together is increasingly being recognized as important in planning a successful THA. This is particularly true in patients with spine disease, long spine fusion, and/or deformity. Complications from THA, such as dislocation, pain, and impingement, are more prevalent in these populations as spinopelvic mobility can be changed [50, 51, 52, 53].
Phan et al. proposed four preoperative categories for patients with spinal deformity which could be used to guide an optimized placement of the acetabular component for each group. This approach advocates calculated adjustments based on a patient’s spinal flexibility and sagittal balance .
Dual-mobility cups have been used successfully to reduce instability in patients with spine-related issues, including those with spine degeneration . Dual-mobility cups are thought to help compensate for the difficult-to-determine best cup position when spinopelvic motion is already compromised. Dagneaux et al. remind surgeons to pre-operatively screen for abnormal spine-hip relations so appropriate planning can take place. However, it should be noted that while dislocation rates with dual-mobility cups are lower, revision rates due to infection can be elevated .
Lower back pain was found to be present in between 21.2% and 60.4% of patients who were candidates for a THA. In looking at the impact of spinopelvic alignment on acetabular positioning, Yeganeh et al. concluded that orthopedic and spine surgeons should work together to determine the best course of action, in combination with patient counseling .
Accurately placing the acetabular cup during THA is reliant on many factors. Surgeons are advised to carefully plan each procedure with an awareness of each patient’s biomechanics and spinopelvic relationship. In the words of Dr Allami, the take-home message of this issue of Surgical Insights is: “Appropriate templating is key for successful total hip replacement.”
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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