Abstract
Introduction
Previous research defines the existence of a “safe zone” (SZ) pertaining to acetabular cup implantation during total hip arthroplasty (THA). It is believed that if the cup is implanted at 40°±10° inclination and 15°±10° anteversion, risk of dislocation is reduced. However, recent studies have documented that even when the acetabular cup is placed within the SZ, high incidence dislocation and instability remains due to the combination of patient-specific configuration, cup diameter, head size, and surgical approach. The SZ only investigates the angular orientation of the cup, ignoring translational location. Translational location of the cup can cause a mismatch between anatomical hip center and implanted cup center, which has not been widely explored.
Objective
The objective of this study is to define a zone within which the implanted joint center can be altered with respect to the anatomical joint center but will not increase the likelihood of post-operative hip separation or dislocation.
Methods
A theoretical forward solution hip model, previously validated by telemetric devices and fluoroscopy data of existing implants, was used for analysis. The model allows for modifications of implant geometries/placement and soft tissue resection to simulate various surgical conditions. For the baseline simulation, the cup center was matched to the anatomical hip joint center, calculated as the center of the best fit sphere mapping the acetabulum, and the orientation of the cup was 40°/15° (inclination/anteversion). Keeping cup orientation the same, the location of the cup was moved in 1 mm increments in all directions to identify the region where a mismatch between the two centers did not lead to separation or instability in the joint.
Results
During both swing and stance phase, when the acetabular cup was placed within the optimal conic with a slant height of 5±1 mm, no hip instability or dislocation risk occurred. As the acetabular cup was translated to the boundary of the optimal conic, hip instability increased. When the acetabular cup was placed at the boundary of the optimal conic, up to 2 mm of hip separation in the lateral direction occurred during swing phase, resulting in a decrease in contact area and an increase in contact stress. As the cup was placed outside the optimal conic, severe edge loading and hip separation up to 3.5 mm occurred during swing phase. In general, this resulted in large increases in cup stress, resulting in increased risk of wear leading to early complications.
Discussion
This study introduces the concept of an optimal conic in the hip joint space to reduce the incidence of dislocation and hip instability after THA. Placing the cup center within the optimal conic reduces hip instability. Moving the cup further from the anatomical hip center increases the occurrence of hip instability. Cup placement within the optimal conic and angular SZ can lead to better postoperative outcomes.
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