Abstract
Introduction
The safe zone of the acetabular cup for THA was discussed based on the AP X-ray films of hip joints. A supine position is still used to determine the cup position for CAOS such as navigation systems. There were few data about the implant positions after THA in standing positions. The EOS X-Ray Imaging Acquisition System (EOS system) (EOS imaging Inc, Paris, France) allows image acquisition with the patients in a standing or sitting position. We can obtain AP and lateral X-ray images with high-quality resolution and low dose radiation exposure. Recently, we have obtained the EOS system for the first time in Japan. We investigated 3D accuracy of the EOS system for implant measurements after THA.
Patients and Methods
We measured the implant angles of the 68 patients (59 females and 9 males, average age: 61y.o.) who underwent THA using the EOS system. The cup inclination and anteversion were measured in the anterior pelvic plane (APP) coordinate. The femoral stem antetorsion was defined as angles between the stem neck axis and the posterior condylar axis. These data were compared with the implant angles of the same patients measured by the post-operative CT scan images and the 3D image analysis using the ZedHip software (LEXI, Japan).
Results
The cup inclinations (average ±SE) measured by the EOS system and the CT scan were 40.6 ± 0.64° and 42.9 ± 0.53°, respectively. The cup anteversions were 22.9 ± 1.3° and 22.8 ± 1.0°, respectively. The stem antetorsions were 28.9 ± 1.3° and 29.8 ± 1.6°, respectively. The differences (average ± SE) between the EOS system and the CT scan in the cup inclination, the cup anteversion, and the stem antetorsion were −2.3 ± 0.38°, −0.09 ± 0.82°, and −0.90 ± 0.91°, respectively. There were strong correlations in measurement values between the EOS system and the CT scan (the Spearman's correlation coefficients of the cup inclination, the cup anteversion, and the stem antetorsion were 0.6521 [p<0.001], 0.7154 [p<0.001], and 0.8645 [p<0.001], respectively).
Discussion
The EOS system provides acceptable clinical accuracies in measuring acetabular cup and femoral stem angles after THA. The accuracy of the cup angles was accorded with that of the basic experimental data using a dry pelvis. Our data also demonstrated clinically acceptable accuracy in the measurement of stem antetorsion. This system can provide accurate snap shots of variable postures with high resolution. Using the EOS system, we may establish real optimum positions of THA implants by measuring the patients after THA in several postures including standing, squatting or sitting positions which required for Japanese ADL.
