Imaging in Hip Arthroplasty Management-Part 1: Templating: Past, Present and Future

Abstract

Hip arthroplasty is a frequently used procedure with high success rates. Its main indications are primary or secondary advanced osteoarthritis, due to acute fracture, osteonecrosis of the femoral head, and hip dysplasia. The goals of HA are to reduce pain and restore normal hip biomechanics, allowing a return to the patient's normal activities. To reach those goals, the size of implants must suit, and their positioning must meet, quality criteria, which can be determined by preoperative imaging. Moreover, mechanical complications can be influenced by implant size and position, and could be avoided by precise preoperative templating. Templating used to rely on standard radiographs, but recently the use of EOS^® imaging and CT has been growing, given the 3D approach provided by these methods. However, there is no consensus on the optimal imaging work-up, which may have an impact on the outcomes of the procedure. This article reviews the current principles of templating, the various imaging techniques used for it, as well as their advantages and drawbacks, and their expected results.

Keywords: 3D parameters; CT; arthroplasty; hip; radiographs.

Figure 1

Preoperative measurements. Those values can be measured on (a) an anteroposterior pelvic radiograph and on (b) a coronal CT image, in the plane established by the femoral neck axis and the femoral midshaft. AO corresponds to the acetabular offset (white line: distance between the femoral head center (blue circle) and the acetabular floor), FO to the femoral offset (little-dotted line: distance between the femoral head center and the femoral midshaft axis (mild transparent white line)), CDA to the cervico-diaphyseal angle (large-dotted white line). A measurement of AO from the pelvic midline is shown on (c) an AP pelvic view [21], as it is more suitable in case of hip prosthesis, especially in case of cup protrusion.

Figure 2

Femoral neck antetorsion measurement. Three axial CT slices must be selected: one shown in (a) at the femoral head center, one in (b) at the femoral neck to measure the femoral neck axis (yellow line), then one in (c) at the level of the roman arch to determine the intercondylar axis (green line). In (d), a global illumination reformat is shown to illustrate the 3D rendering of this measure, corresponding to the angle between the yellow and green lines.

Figure 3

Representation of the flexion/extension of the femoral stem, using 3D CT-scan reformat, adapted from Abe et al. [23]. The dotted white line represents the retrocondylar axis, the white line the sagittal femoral tilt, and the colored line the stem axis. A theoretical neutral position is shown in (a) with the orange line; a negative value superior to −3° between the femoral tilt and the sagittal stem tilt is defined as flexion in (b) with the yellow line, which is the actual axis of this prosthesis; and a positive value superior to 3° is defined as an extension in (c) with the red line.

Figure 4

Hip deformities. Five types of hip deformities are shown, adapted from Kase et al. [11]. On each scheme, the femoral head is colored in grey, the acetabulum in dark grey, and the acetabular cavity in transparent grey. The blue lines correspond to the acetabular center and the green dotted line to the vertical and horizontal axis of the femoral head. In (a), a centered hip is shown as both axes are superimposed; in (b), a medialized (medialization of the vertical axis of the femoral head with respect to the acetabular one); in (c) a lateralized (lateralization of the vertical axis of the femoral head with respect to the acetabular one); in (d) a proximalized (cephalic displacement of the horizontal axis of the femoral head with respect to the acetabular one); and in (e) a proximo-lateralized (cephalic displacement of the horizontal axis of the femoral head with respect to the acetabular one, and lateral displacement of the vertical axis of the femoral head with respect to the acetabular one). An arbitrary cut-off of 3 mm was used by the authors to consider a displacement in each plane.

Figure 5

Inter-teardrop axis. The inter-teardrop axis shown on an anteroposterior pelvic radiograph (white line), the teardrops being represented by the dotted lines.

Figure 6

Cortical index calculation. Cortical index is calculated by measuring the ratio between the diaphyseal diameter between the cortices at the level (orange double-headed arrow) and the inner canal diaphyseal diameter 10 cm below the lesser trochanter (yellow double-headed arrow).

Figure 7

EOS^® imaging example showing pelvic and lower limbs measurements. In this example, pelvic parameters are shown (PS: sacral slope, IP: pelvic incidence, VP: pelvic version), and multiple lower limbs measurements are available (femoral and tibial length, femoral head diameter, femoral neck length, cervico-diaphyseal angle, femoral offset, femoral and tibial version/torsion, knee valgus/varus, hip-knee shaft angle, femoral flessum/recurvatum).

Figure 8

EOS^® imaging example showing spine parameters and their relationship with pelvic parameters. Spinal kyphosis and lordosis angles are provided along with the sagittal vertical axis measurement, as well as pelvic parameters, to provide a global appreciation of the spinopelvic complex, considered well balanced in this example.