Oblique pelvis of poliomyelitis. The pelvis is displaced in the three dimensions of space: frontal, sagittal, horizontal
The anatomical reality of the concept quickly appeared to me when both White and Panjabi  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  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.
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
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°.
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.
Intrapelvic Degrees of Freedom
The work of White and Panjabi  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 .
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
Physiological, Pathophysiological and Mechanical Implications of the Pelvic Vertebrae
The Pelvic Vertebra as an “Intercalary Bone”
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).
So there were two phenomena that came together:
that of musculotendinous contractures or paralysis which forced the pelvis to move in space;
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).
Studies in the sagittal plane by many authors have reinforced this point of view
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].
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 .
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 .
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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