Abstract
INTRODUCTION
Dislocation is one of the most important complications in THA. Dual mobility cup (DMC) inserts reduce the risk for dislocation after total hip arthroplasty by increasing the oscillation angle. A lower rate of dislocation with use of a DMC insert has been reported in different studies. But there is no available research that clearly delineates the stability advantages of DMC inserts in primary THA. The aim of our study was to evaluate the area of the safe zone for a DMC insert, compared to a fixed insert for different anteversion angles of the femoral component.
Material and Methods
A model of the pelvis and femur were developed from computed tomography images. We defined the coordinate system of the pelvis relative to the anterior pelvic plane and the coordinate system of the femur relative to the posterior condylar plane. In our model, we simulated a positive anteversion position of the acetabular cup. The lower border for cup inclination is 50°. The safe zone was evaluated for the following range of motion of the implant: 120° of flexion, 90° of flexion 30° of internal rotation, 30° of extension, 40° of abduction, 40° of adduction, and 30° of external rotation. (Fig.1) The safe zone was calculated for both a fixed insert and a DMC insert over a pre-determined range of three-dimensional motion, and the effect of increasing the anteversion position of the femoral component from 5° to 35° quantified. The ratio of the safe zone for a DMC insert to a fixed insert was calculated.
Results
A wider safe zone was obtained for a DMC insert over all range of motion conditions. A DMC insert increased the stability of the implant between 10° and 15° along both anterior-posterior and vertical axes of the acetabular cup. (Fig.2) When stem anteversion were varied 5°, 10°, 15°, 20°, 25°, 30°, 35°, ratio of safe zone (a DMC insert / a fixed insert) were changed 8, 10.1, 6.3, 4.9, 5.2, 6.6, 10.6. (Fig.3)
Discussion
The safe zone of a DMC insert is always larger than a fixed type insert. In every stem anteversion patterns, safe zones were expanded to all direction with 10° to 15°. Under 15° of stem anteversion, area of both inserts are almost stable. Area ratio is lowest with 20° of stem anteversion. Over 25° of stem anteversion, both area decreased and area ratio increased gradually. Over 30° of stem anteverison, safe zone of a fixed type are very small area. It is difficult for us to set acetabular shell in that small area. But we will get a larger area by using DMC.
We performed a simulation analysis to evaluate the increase in area of the safe zone when using DMC inserts, compared to fixed inserts. To use of a DMC insert would bring in a 5–11-fold expanded area of the safe zone. In especially, DMC is a useful when stem anteversion is over 30°.
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