The anatomy of balance

Pelvic incidence

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For the understanding of degenerative diseases of the musculoskeletal system and especially of the spine, it is essential to deal with the special anatomical challenges of balancing in bipeds. While the balancing game in the periphery (ankle, knee) is characterized by dynamics (muscle actions for balancing, see previous chapter), near the body’s center of gravity the stability of stance and gait is predetermined by the individual static anatomy.

points for upright posture
Figure 1: The anatomy of the upright position is essentially determined by the positional relationship of the hip joint center and the posterior pelvic anatomy (sacrum).

As can be seen in Figure 1, it is only the positional relationships of the connection points of the upper end of the lower extremity (hip) and the lower end of the spine (cover plate of the sacrum[Sakrum]) that determine the individual anatomical implementation of upright gait.

Pelvic incidence
Figure 2: The ratio of sacrum-coverplate perpendicular to hip center can be expressed as pelvic incidence (PI).

Pelvic incidence is an anatomical feature and is fixed in the mature skeleton. It determines the further individual structure of the spine (Figures 3+4).

relationship between pelvic incidence and lumbar lordosis
Figure 3: Lumbar lordosis (LL) is complementary to pelvic incidence and equal in a range of up to +10°.
Figure 4: The position of the lower lordosis apex (L3-5) and the inversion point (=inflexion point, L3-Th12) are important reference points in spinal surgery.

According to this design principle, four anatomical types can be classified according to Roussouly (see Table 1).

Roussouly typePelvic incidenceMiddle lumbar lordosisLordosis vertexInflexion point
Table 1: Roussouly types and anatomical characteristics

These characteristics represent an ideal-typical individual pattern for the shaping of the upright gait. The transgression of these anatomical norms to one or the other direction can have different pathogenetic consequences for different diseases:

For the spine surgeon, an understanding of the parameters is important in order to be able to assess the load conditions on the intervertebral discs and facet joints and, on the other hand, to recognize the need for corrections or to be able to plan corrective operations correctly.

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