Pre- and postoperative offset and femoral neck version measurements and validation using 3D computed tomography in total hip arthroplasty

Abstract

Background: Restoration of a correct biomechanical situation after total hip arthroplasty is important.

Purpose: To evaluate proximal femoral symmetry of acetabular and femoral offset and femoral neck anteversion pre- and postoperatively in hip arthroplasty by semi-automated 3D-CT and to validate the software measurements by inter- and intraobserver agreement calculations.

Material and methods: In low-dose CT on 71 patients before and after unilateral total hip arthroplasty, two observers used a digital 3D templating software to measure acetabular offset, true and functional femoral offset, and femoral neck anteversion. Observer agreements were calculated using intraclass correlation. Hip measurements were compared in each patient and between pre- and postoperative measurements.

Results: Preoperatively, acetabular offset (2.4 mm), true (2.2 mm), and functional global offset (2.7 mm) were significantly larger on the osteoarthritic side without side-to-side differences for true and functional femoral offset or femoral neck anteversion. Postoperatively, acetabular offset was significantly smaller on the operated side (2.1 mm) with a concomitantly increased true (2.5 mm) and functional femoral offset (1.5 mm), resulting in symmetric true and functional global offsets. There were no differences in postoperative femoral neck anteversion. Inter- and intraobserver agreements were near-perfect, ranging between 0.92 and 0.98 with narrow confidence intervals (0.77-0.98 to 0.94-0.99).

Conclusion: Acetabular and concomitantly global offset are generally increased in hip osteoarthritis. Postoperative acetabular offset was reduced, and femoral offset increased to maintain global offset. 3D measurements were reproducible with near-perfect observer agreements. 3D data sets should be used for pre- and postoperative measurements in hip arthroplasty.

Keywords: 3D-measurements; Hip arthroplasty; acetabular offset; femoral neck anteversion; femoral offset; proximal femoral symmetry.

Fig. 1.

Illustration of the true femoral offset (TFO), which is the true perpendicular distance from the proximal femoral long axis and the rotational center of the femoral head, and the functional femoral offset (FFO), which is the horizontal distance from the proximal femoral long axis and the rotational center of the femoral head. FO: femoral offset.

Fig. 2.

Screen capture showing the best-fit circles defining the femoral head, the definition of the long axis of the proximal femur, acetabular offset, and femoral offset (FO). Functional FO was measured on the coronal reformation, true FO was measured in three dimensions. The external objects at the right hip are bone density phantoms.

Fig. 3.

Schematic 3D illustration of the definition of femoral neck anteversion (FNA) in the current study. The long axis of the femur is defined by two points in the proximal femur: (i) at the inferior border of the lesser trochanter and (ii) at a point about 6 cm distal in the femoral shaft. The FNA is the angle between a perpendicular line between the proximal femoral long axis and (iii) the femoral head and a line between the femoral condyles.

Fig. 4.

Bar chart showing the differences in measurements between the non-affected side and the side planned for total hip arthroplasty for femoral neck anteversion, acetabular offset, true femoral offset, true global offset, functional femoral offset, and functional global offset. Data for the two observers combined.

Fig. 5.

Bar chart showing the differences in measurements between the non-affected side and the postoperative values for the operated side for femoral neck anteversion, acetabular offset, true femoral offset, true global offset, functional femoral offset, and functional global offset. Data for the two observers combined.

Fig. 6.

Correlation plots for measurements of acetabular offset, femoral neck anteversion, true and functional femoral offset for two observers in 213 non-operated hips.

Fig. 7.

Femoral neck anteversion (FNA) is different depending on the choice of long axis of the femur. (a) In the current study, the proximal part of the femur was chosen for the long axis definition since that corresponds to the length of the femoral stem in hip arthroplasty. FNA is 35°. (b) In previous definitions, e.g. by Billing and Murphy et al., the long axis was defined by a point in the knees and a point in the proximal femur, giving a different measurement value of FNA, 20°.