Abstract
Introduction
In total hip arthroplasty (THA), it is important to define the coordinate system of the pelvis and femur for standardization in measuring the implant alignment. A coronal plane of the pelvis (functional pelvic coordinates) in supine position has been recommended as the pelvic coordinates for cup orientation and an anatomical plane of the femur (posterior condylar plane: PCP) is widely used as the femoral coordinates to measure stem or femoral anteversion. It has been reported that the pelvic sagittal tilt in supine does not change a lot after THA. However, changes in the axial rotation of the posterior condylar plane after THA have not been well studied. If the horizontal tilt of PCP of the femur in a resting position changes a lot after THA, the combined anteversion theory cannot be functional. Therefore, we evaluated the angulation changes of the posterior condylar plane after THA and analyzed the related factors by using CT images.
Methods
Forty patients (5 men and 35 women, mean age 58 years) with hip osteoarthritis who had undergone THA were the subjects of this study. CT images used for measurements were taken preoperatively (preop-CT) and 3 weeks after THA (postop-CT), and more than 2 years after THA (2nd postop-CT).
Measurements were done on the reconstructed CT images using 3D viewer software. The axial rotation of the femur was measured as the angle between the posterior condylar line (PCL) and a line through the bilateral anterior superior iliac spines. To analyze the factors relating to the rotational change of the femur, change in femoral anteversion, leg length, and leg medialization after THA were also measured. Surgical approach (posterolateral: 32 cases, direct anterior: 8 cases) was also evaluated as a factor relating to the rotational change.
Results
PCL was externally rotated at an average angle of 3.3° at preop-CT, −10.4° at postop-CT, and −7.2° at 2nd postop-CT. There was a significant difference between preop-CT and postop-CT, preop-CT and 2nd postop-CT (p<0.01, respectively).
Femoral anteversion decreased 0.5°, the leg was lengthened 11.7mm, and was medialized 8.5mm after THA.
In the analysis of the related factors, only the leg length change and the amount of leg medialization significantly correlated with the rotational change between preop-CT and postop-CT (β=−0.367, −0.316, respectively). On the other hand, no factors correlated with the rotational change between preop-CT and 2nd postop-CT.
Discussion
PCL at a resting position internally rotated 13.7° after THA and remained 10.5° internally rotated from the preoperative position at more than 2 years after THA. This internal rotation after THA may have occurred by releasing the external contracture caused by osteoarthritis. Because the PCL at a resting position internally rotates approximately 10° after THA, we have to consider this change when we calculate the range of motion of the hip in the combined anteversion theory.
Conclusion
PCL at a resting position internally rotates approximately 10°after THA and we may need to consider this change in the combined anteversion theory.
