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].
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- Positioning may not be what you think it is
- Pelvic movement happens
- Pelvic movement and imaging
- What is an acceptable cup depth?
- Reaming: proceed carefully
- Anatomic or medialized placement?
- Cup height influences LLD and JRF
- Anatomical landmarks are independent of positioning
- Is there an ideal angular position?
- Harnessing technology for angular positioning
- Spinopelvic considerations utilizing dual mobility implants
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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