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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.
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 is an anatomical feature and is fixed in the mature skeleton. It determines the further individual structure of the spine (Figures 3+4).
According to this design principle, four anatomical types can be classified according to Roussouly (see Table 1).
| Roussouly type | Pelvic incidence | Middle lumbar lordosis | Lordosis vertex | Inflexion point |
|---|---|---|---|---|
| 1 | <45° | 45° | L5 | L3 |
| 2 | <45° | 50° | L4/5 | L2 |
| 3 | 45°-60° | 55° | L4 | L1 |
| 4 | >60° | 65° | L3/4 | Th12 |
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:
- Disc degeneration is more common in types 1 and 2 1Association Between Roussouly Classification and Characteristics of Lumbar Degeneration
- Facet arthroses are more frequently found in types 3 and 4 2Association Between Roussouly Classification and Characteristics of Lumbar Degeneration
- Above Roussouly type 4, overloading of the dorsal structures occurs. Therefore, spondylolistheses is more common here. 3Spondylolisthesis, pelvic incidence, and spinopelvic balance: a correlation study.
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.
