Pelvic Vertebra, the Cephalic Vertebra and the Concept of the Chain of Balance

Fig. 1

Oblique pelvis of poliomyelitis. The pelvis is displaced in the three dimensions of space: frontal, sagittal, horizontal


Fig. 2

The radiograph depicts the 3D reality as a “Chinese shadow”

The anatomical reality of the concept quickly appeared to me when both White and Panjabi [2] showed that the mobility of the sacroiliac joint was minimal (approximately 1.5° and up to 3.5° in peripartum women to facilitate natural delivery). In addition, the book by RJ and P. Ducroquet [3] on “la Marche et les Boiteries” (literally “the Walk and Limps”) demonstrated to me “the pelvic step” where the whole pelvis during walking is moved not only up and down but also rotated in the horizontal plane from right to left, as walking progresses. Finally, to comfort me in this concept, I frequently visited the Anatomical Museum, located in the slightly abandoned basement of the Parisian Baudelocque Maternity Hospital, in front of the Saint Vincent de Paul hospital where I worked. There I found a collection of pelvic and spinal skeletons where it could be noticed that many of them had considerable deformities of pelvic morphology which explained why they were there in this museum, for example, because of labour dystocia (obstructed labour) in parturients (females about to give birth) which often did not survive at that time.

So, it seemed appropriate to me to consider the pelvis as the last vertebra of the spinal skeleton and therefore to explain why, when one looked at a complete skeleton of the human body, how much this element played a role (interposed between trunk and lower limbs) in the erect posture of the human being. And it is with this same vision that appeared to me, around the same time, an identical but perhaps even more subtle role in the erect skeleton, of the “cephalic vertebra” (the whole head considered as a vertebra) where only the weight of the head at the other end of the articular chain played the role of the weight of a pendulum, provided that standing is regarded as the function of an “inverted pendulum “(Fig. 3).


Fig. 3

The erect human functions as an inverted pendulum from a Polygon of Support

Once I understood that, the remainder of my intentions would only serve to support and refine this concept according to multiple practical applications.

Anatomical Basis for the Pelvic Vertebra


The best studies come , as appropriate, from obstetric services, where very precise and complete descriptions relate to the different pelvic anatomy in women compared to men in the form of the outer pelvic ring (which is wider in females) at the opening of the iliac wings, antero-posteriorly and transversely. We know that pelvimetry is part of the armamentarium of the obstetrician. Moreover, recent work by Christophe Boulay [4] has shown a frequent prevalence of right–left asymmetry—if one refers to the respective size of the two iliac bones. Above all, the works of Duval-Beaupère et al. [5] have shown the individual variation of the angle of pelvic incidence (according to the name she gave to this angle), measured in sagittal projection between the perpendicular of the middle of the sacral endplate and the line uniting the centre of this endplate with the centre of the femoral head(s) (Fig. 4).


Fig. 4

From barycentrometric studies, Duval-Beaupère defined the (anatomical) angle of pelvic incidence (β) with its postural corollaries, sacral slope (α) and pelvic version (Y)

This angle is variable from one individual to another, it is generally lower for a newborn and increases steadily during the first 5 or 6 years to reach a plateau, but actually stabilizes only at the end of growth with an average around 50°.

It represents the spatial relationship of the two components of the sacroiliac joint. If we look at the evolution of species, we see that in quadrupeds, the angle of incidence is very low, is increased in some primates, then in the first hominids (for example Lucy, the 3.2-million-year-old skeleton) and finally in the human, as the erect standing position becomes dominant (Fig. 5).


Fig. 5

Pelvic incidence has changed with the evolution of the species, with concomitant changes in pelvic tilt

This is well understood when we examine pelvises of various kinds, dysplastic or not, where the anteroposterior distance, variable between the sacrum and the pubic symphysis, will be a determining factor in this angle. Similarly, the anteroposterior or lateral variation in the position of the centre of the acetabulum makes it possible to understand the individual variations of the angle of incidence.

On the other hand, when a lumbosacral fusion (L5S1 or L4L5S1) is performed, the starting point of this angle is prolongated upwards since the fused vertebrae form a block with the pelvis, and this automatically reduces this angle of incidence by creating a new pelvic vertebra (Fig. 6).


Fig. 6

Pelvic morphology changes the pelvic incidence. Lumbosacral fusion automatically decreases the angle of incidence

Intrapelvic Degrees of Freedom

The work of White and Panjabi [2] has perfectly demonstrated that the movements of the sacroiliac joint were minimal but real, of the order of 1.5° up to 3.5°. Due to the hollow rail shape of the lateral sacrum and its articulation with the solid rail shape of the medial ilium and the inverted L- or C-shaped articular surfaces of this joint, these so-called nutation or counter-change movements are relatively weak, locking together the unit at this point. The pelvic entity is created by the juxtaposition of the two right and left iliac bones to the sacrum posteriorly and which meet each other anteriorly at the pubic symphysis. The transverse axis of these movements of nutation (anterior tilting of the promontory, posterior tilt of the coccyx) and of counter-nutation (posterior tilt of the promontory and anterior of the coccyx) is located behind the articular surfaces (hollow rail/solid rail) at the level of insertion of the interosseous ligament or axillary ligament.

This explains why this movement combines sliding at the level of the hollow rail/solid rail with anteroposterior rotation around this transverse axis: it is the nutation against the wobble!

The pubic symphysis, in reality a diarthro-amphiarthrosis joint, has a particularly resistant structure with a vertical elliptical central zone. This looks like an intervertebral disc with an oval cleft in the centre (often virtual), but circumferentially surrounded by strong ligaments, which explains why its mobility is negligible in the normal state, and only during pregnancy can one observe some minimal mobility.

On the Other Hand, the Degrees of Freedom Around the Pelvic Vertebra Are Quite Considerable

At the level of the lumbosacral joint , there are six degrees of freedom in three planes of space (flexion/extension, right/left Inclination and right/left axial rotation), not to mention a minimal up/down axial mobility, according to the vertical axis coming from the compressive elasticity of the L5S1 disk.

Moreover, at the level of each hip joint , there are also six degrees of freedom. So, we understand how the pelvic vertebra, by these mobilities at the level of the upper and lower points of support of the elements which surround it, will be able to adapt to almost all spatial situations which it will meet during the various functions of human life, within the limits allowed by these degrees of freedom [6].

Plasticity of the Pelvic Vertebra

It must also be remembered that according to various pathologies, either congenital malformations or acquired paralytic or infectious origins, affecting each of the components of the pelvis (especially during a period of growth), we will be able to observe considerable morphological changes which will, of course, affect the function it represents in the equilibrium of the skeleton as well as in its function of transmission of supra- and sub-jacent forces and moments working on the joints.

The visit to the old obstetrical museum located in the basement of the Baudelocque Maternity Hospital was enlightening for me and made me discover not only the entity of the pelvic vertebra but also the formidable capacities of adaptation and compensation of the human machinery.

The Texture and the Bone Architecture of the Pelvis

By studying this point , it is quite easy to identify the best possible anchor points for inserting implants of instrumentation material [7, 8].

As for the sacral skeleton, it is certain that the solid zones are represented by the central body of the sacrum, whereas the wings (ala) are much more cancellous (spongy) and have low anchoring capacity (Fig. 7). Of course, the screws bearing on the two anterior and posterior cortices will have the best screw hold. Finally, the extreme solidity of the sacroiliac periarticular ligaments must be noted, including that of the iliolumbar ligament to understand that concomitant use of the two iliac wings with sacroiliac screw fixation considerably increases the lever arms and thus the rigidity of a lumbosacral assembly. Screws along this trajectory, between the inner and outer bony iliac cortices spanning as far as overlying the roofs of the acetabulii and aligned along the lines of force from the acetabulum to the sacroiliacs, create a very robust anchor.


Fig. 7

On this cross section of the sacrum (S1), note the lack of resistance of a possible osteosynthesis at the level of the sacral ala

Physiological, Pathophysiological and Mechanical Implications of the Pelvic Vertebrae

The Pelvic Vertebra as an “Intercalary Bone”

The pelvis will play this role between the skeleton of the trunk (essentially spine) and the skeleton of the lower limbs.

  1. 1.

    Here again is the study of the paralytic pelvic obliquity which has been decisive in the understanding of this role—the “3 planes of space” as mentioned, and also that in can be “deformed in its morphology”.

    It was originally caused by sub-pelvic causes (below the pelvis contractures or paralysis of the hips, knee, etc.), intrapelvic causes (congenital or infectious deformities, such as childhood sacroiliitis associated with paralysis, which if occurring early in infancy, would result in a torsional deformity of the pelvis +/− femoral head growth deformity and asymmetry of the peri-pelvic muscular elements) and finally, supra-pelvic causes, i.e. all spinal deformities, scoliosis and kyphosis (Fig. 8).

    Thus it was clear that a contracture of the hip in flexion-abduction forced the pelvis to move in the frontal plane with an inclination on the side of the hip anomaly, whereas if the hip was retracted in adduction or inversion, the pelvis was moving in the opposite direction and migrated superiorly in the frontal plane on the side of the hip anomaly.

    Similarly, in the sagittal plane, any “flexion contracture” of the hip (vicious attitude in flexion) caused a rocking of the pelvis forward or anteversion, while a deficit of flexion (by contracture of the gluteus for example) produces an automatic retroversion during tested sitting. Similarly, in this sitting or standing position, the anteversion of the pelvis was automatically associated with a greater lumbar lordosis as the pelvis tipped over while inversely its retroversion was always associated with a lumbar kyphosis; the causes of these disorders are secondary to peri-pelvic contractures or paralysis.

    In the horizontal plane, displacements in axial rotation were observed in the clockwise or counterclockwise direction as a function of the distribution of retractions or paralysis (Fig. 9).


  2. 2.

    So there were two phenomena that came together:

    1. (a)

      that of musculotendinous contractures or paralysis which forced the pelvis to move in space;


    2. (b)

      the effect of gravity which, according to the situations created by the first phenomenon, amplified the deformity in one direction or another.


    Each of these elements could be found in the deformities and the three-dimensional pelvic expression, both physiological and whatever the pathology involved: the pelvis was indeed an “intercalary bone” (Fig. 10).


  3. 3.

    Studies in the sagittal plane by many authors have reinforced this point of view

    1. (a)

      The work of Duval-Beaupère demonstrated that in standing there was a close relationship on a lateral spine radiograph, between the angle of pelvic incidence and the amount of lumbar lordosis required to have a normal erect posture (small angle of incidence = small lordosis, wide angle of incidence = large lordosis) (Fig. 11). This has provided a practical measure that is essential to the orthopaedic world by confirming this notion of “intercalary bone” and “pelvic vertebra” [4, 5].


    2. (b)

      The work of Roussouly on the normal population has come to demonstrate the reality of this notion again creating a practical classification of the sagittal balance of individuals, in four major categories that have now become standard [9].


    3. (c)

      The influence of sub-pelvic causes, in non-paralytic cases but for example in the so-called “normal” aging population, has been demonstrated by the works of Hovorka who describes the loss of the reserve of extension of the hip (Fig. 12), an initiating factor of lumbar kyphosis in the elderly [10].


    Finally, from a practical point of view, another proof: it is well known that when one has hip osteoarthritis associated with low back pain, the correction of the severe hip attitude by total hip arthroplasty often leads to the disappearance of low back pain.

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Apr 25, 2020 | Posted by in ORTHOPEDIC | Comments Off on Pelvic Vertebra, the Cephalic Vertebra and the Concept of the Chain of Balance
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