Changes in Center of Rotation and Offsets With Teardrop-Landmark Acetabular Cup Positioning: A Computed Tomography...

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering
• Radiology

Background:

• Restoring the hip's center of rotation (COR) is crucial in total hip arthroplasty (THA).
• The teardrop landmark is commonly used for acetabular cup placement, but its impact on hip offsets is not fully understood.
• Understanding these effects is vital for optimizing surgical outcomes and biomechanical restoration.

Purpose of the Study:

• To evaluate the effect of teardrop-based acetabular cup positioning on hip offsets and COR in simulated THA.
• To compare these changes between patients with osteoarthritis (OA) and osteonecrosis of the femoral head (ONFH).
• To determine the clinical implications for biomechanical restoration after THA.

Main Methods:

• A computed tomography (CT)-based simulation study involving 60 hips (30 OA, 30 ONFH) with contralateral unaffected hips as controls.
• Acetabular cup positioning was simulated lateral to the teardrop, aligned with the acetabular floor using preoperative templating software.
• Acetabular (AO), femoral (FO), and vertical (VO) offsets were measured, and COR horizontal and vertical displacements were calculated.

Main Results:

• Teardrop-based positioning significantly decreased AO and FO while increasing VO (p < 0.01) compared to the unaffected side.
• The COR shifted medially (2.7-2.8 mm) and superiorly (2.0-2.8 mm) in both OA and ONFH groups.
• In OA patients, global offset (GO) decreased by 4.7 mm, primarily due to a significant reduction in FO.

Conclusions:

• Teardrop-based acetabular cup positioning in THA leads to medial and superior COR displacement.
• This positioning reduces AO and GO, potentially compromising biomechanical restoration, particularly in OA patients with decreased FO.
• Femoral component adjustment is essential for optimal biomechanical restoration following teardrop-based cup placement.