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 [1] and if the pre- and postoperative ranges are incongruent? Well, impingement and instability can be a problem [2].

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 [2]. Each patient presents unique acetabular orientation [3]; the thought is that native anteversion should be respected.

 

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.

 

Patient positioning

While supine positioning simplifies the assessment of the pelvis’s position and limb length during THA [4], more than 75% of surgeons perform THA with the patient in the lateral decubitus position [5] and the majority of these use a posterior approach [6]. 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 [9]. 

Adequate bolstering and support is of upmost importance and is intended to secure the patient in a known position throughout the procedure [10] – 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 [6]. In particular, better tools to stabilize and specifically hold the pelvis in place are needed [11, 12].

 

EXPERT COMMENTARY

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.

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 [5]. 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.” [5]

 

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 [36].

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 [37].

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” [38].

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 [4].

 

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.” [4]

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 [39]. 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 [4].

 

 

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) [40]. Beverland et al. “recommend 35° of apparent operative inclination (AOI) during surgery, as opposed to the traditional 45°”  [5]. 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.” [39] 

 

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 [41]. Additionally, a simple inclinometer used to measure inclination of the cup component improved its placement within a predetermined ‘safe zone’ (30° to 50°) [42].

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 [43].

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.” [44]

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 [54].

Dual-mobility cups have been used successfully to reduce instability in patients with spine-related issues, including those with spine degeneration [55]. 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 [56]. 

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 [57].

 

Conclusion

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.”