Current Concepts in Acetabular Positioning in Total Hip Arthroplasty

Abstract

Being one of the most successful surgeries in the history of medicine, the indications for total hip arthroplasty have widened and are increasingly being offered to younger and fitter patients. This has also led to high expectations for longevity and outcomes. Acetabular cup position has a significant impact on the results of hip arthroplasty as it affects dislocation, abductor muscle strength, gait, limb lengths, impingement, noise generation, range of motion (ROM), wear, loosening, and cup failure. The variables in cup position are depth, height, and angular position (anteversion and inclination). The implications of change in depth of center of rotation (COR) are medialized versus anatomical positioning. As opposed to traditional medialization with beneficial effects on joint reaction force, the advantages of an anatomical position are increasingly recognized. The maintained acetabular offset offers advantages in terms of ROM, impingement, cortical rim press fit, and maintaining medial bone stock. The height of COR influences muscle activity and limb lengths and available bone stock for cup support. On the other hand, ideal angular position remains a matter of much debate and reliably achieving a target angular position remains elusive. This is not helped by variations in the way we describe angular position, with operative, radiologic, or anatomic definitions being used variably to describe anteversion and inclination. Furthermore, pelvic tilt plays a major role in functional positions of the acetabulum. In addition, commonly used techniques of positioning often do not inform us of the real orientation of the pelvis on operating table, with possibility of significant adduction, flexion, and external rotation of the pelvis being possibilities. This review article brings together the evidence on cup positioning and aims to provide a systematic and pragmatic approach in achieving the best position in individual cases.

Keywords: Acetabular cup position; Acetabulum; angular position; anteversion; arthroplasty; biomechanics; depth; height; hip; inclination; mediolateral; pelvic tilt; replacement; superoinferior.

Figure 1

X-ray pelvis 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

Figure 2

Schematic diagram showing surgeon position to assess radiographic and operative inclination. Position 1 assesses the projection of the operative inclination on the coronal plane, and therefore, the radiographic angle. Position 2 perpendicular to the vertical plane passing through inserter handle assesses operative inclination

Figure 3

Schematic representation on saw bones demonstrating pelvic tilt - the difference between the anterior pelvic plane and the coronal plane

Figure 4

X-ray pelvis with hip joints and proximal 1/3^rd of femur anteroposterior view showing templating for an uncemented total hip arthroplasty

Figure 5

X-ray pelvis with both hip joints and proximal 1/3^rd of femur anteroposterior view showing the difference in position between teardrop and inferior extent of posterior acetabular rim. To size the acetabulum accurately in templating as well as to understand cup positioning, it's important to recognize the distance between the two

Figure 6

X-ray pelvis with both hip joints and proximal half of femur, anteroposterior view in this patient, an anatomical position has been chosen for the cup. Reducing the acetabular offset would mean that a femoral stem with greater offset would have been required. Risk of impingement and reduced range of movement could result from medialization

Figure 7

Preoperative and postoperative radiographs of pelvis with both hips and proximal femur anteroposterior view in a patient with shallow acetabulum where the medialized position has been chosen. Choosing the anatomic position in this patient would have resulted in lateral uncoverage of the cup

Figure 8

Peroperative photograph showing a Judd pin is inserted at the supraacetabular level, and the suture with a knot is used to mark the distance to a fixed point on the greater trochanter before dislocation. This suture can be used to assess leg length and offset intraoperatively

Figure 9

X-ray (L) hip joint anteroposterior view and peroperative photograph showing measuring the lateral overhang of template and reproducing it intraoperatively to achieve correct cup inclination