**3.3 Disturbances can come from atypical morphotypes**

The angle of incidence is the morphologic parameter determinant for the adaptation of sagittal spinopelvic balance. Two types of population require specific attention relative to those with a normal incidence. In subjects with a high angle of pelvic incidence, theoretical lumbar lordosis is greater as the range of adaptation of the sacral slope may also be, according to the formula I= SS +PT. The femoral heads are projected forward relative to the sacrum and the acetabular anterior opening is less marked. The coxofemoral joints of these subjects have a greater theoretical available extension and therefore a better ability to adapt. The range of pelvic tilt and sacral slope adaptation is higher. On the other hand, these patients are risky for spine surgeons in cases of spinal arthrodesis: the surgical achievement of adequate lordosis is technically challenging and tolerance of residual flat back is very poor.

Inversely, in subjects with a low pelvic incidence angle, there is less theoretical lumbar lordosis and the adaptability of the sacral slope and pelvic tilt may be more limited. The femoral heads are embedded under the sacrum and the anterior opening of the acetabulum is more marked. Theoretically, these subjects have less available hip extension and a weaker capacity to adapt to sagittal imbalance **(Fig. 15).** In standing position, their hips are naturally in extension, which means they cannot increase the posterior range of motion significantly. Due to potential posterior impingements, these patients can experience anterior dislocation or subluxation in standing position. Compensatory mechanisms may involve hips and knee flexion (Vialle et al., 2005). Accordingly, these subjects are more easily subject to a global and unstable imbalance because of their small margin for adaptation.

Fig. 14. Excess anterior tilt of the pelvis in a seated position can cause anterior impingement;

The angle of incidence is the morphologic parameter determinant for the adaptation of sagittal spinopelvic balance. Two types of population require specific attention relative to those with a normal incidence. In subjects with a high angle of pelvic incidence, theoretical lumbar lordosis is greater as the range of adaptation of the sacral slope may also be, according to the formula I= SS +PT. The femoral heads are projected forward relative to the sacrum and the acetabular anterior opening is less marked. The coxofemoral joints of these subjects have a greater theoretical available extension and therefore a better ability to adapt. The range of pelvic tilt and sacral slope adaptation is higher. On the other hand, these patients are risky for spine surgeons in cases of spinal arthrodesis: the surgical achievement of adequate lordosis is

Inversely, in subjects with a low pelvic incidence angle, there is less theoretical lumbar lordosis and the adaptability of the sacral slope and pelvic tilt may be more limited. The femoral heads are embedded under the sacrum and the anterior opening of the acetabulum is more marked. Theoretically, these subjects have less available hip extension and a weaker capacity to adapt to sagittal imbalance **(Fig. 15).** In standing position, their hips are naturally in extension, which means they cannot increase the posterior range of motion significantly. Due to potential posterior impingements, these patients can experience anterior dislocation or subluxation in standing position. Compensatory mechanisms may involve hips and knee flexion (Vialle et al., 2005). Accordingly, these subjects are more easily subject to a global

no impingement in standing position

**3.3 Disturbances can come from atypical morphotypes** 

technically challenging and tolerance of residual flat back is very poor.

and unstable imbalance because of their small margin for adaptation.

Fig. 15. In subjects with a high angle of pelvic incidence, lumbar lordosis is greater and the coxofemoral joints have a greater theoretical available extension and a better ability to adapt.In subjects with a low pelvic incidence angle, there is less lumbar lordosis and the adaptability of the sacral slope and pelvic tilt may be more limited
