Thierry Marnay MD, Montpellier France
Spondylolisthesis L5-S1 may be recognised as a complex deformity of the spine with this progressive dislocation of the superior part of the spine over the pelvic belt. The study of the sagittal balance acquired after the vertical position helps to understand some of its aspects.
Standing position in mankind
From Lucy Australopithecus to Homo sapiens sapiens, a lot of modifications may be described as evolution of the joint-squelettal area.
In fact, we will focus on pelvic and spine modifications, especially their aspects and consequences on the vertical position.
Five lumbar vertebras create the capability to develop a lumbar lordosis. (This allow the setting of the gravity centre as close as possible to the gravity line).
The pelvis height is reduced and the iliac becomes more sagittal (buttock muscles development).
If we compare the pelvis of a chimpanzee (long sacrum vertebra, only two lumbar vertebras) we understand that 5 lumbar vertebras increase the capability of the lumbar curve and reduce the height of the pelvis (Fig.1)
During the human development and growth, the evolution of the spine crosses several phases from one curve at birth (in kyphosis), development of cervical lordosis and when walking, development of the lumbar curve.
The pelvic positioning is one of the most important points of the stage.
When there is no real lumbar curve, the pelvis is maintained in a retro-version position with a flexion of hips and knees. To get to its final positioning it must turn and climb around the femoral heads (rotary ascension) (Fig.2). From the flexion to the straight positioning, the power of this rotation is performed by the ligaments of the hip (all of them limit the extension) (Fig.3) and the psoas muscle. With the tension band of those structures, the pelvis rotates around the hips and moves from retro to ante-version. The positioning almost get on line the gravity line, L5S1, centre of the hip. At the same time, with the ante-version of the hips, L5S1/ horizontal angle increase (inferior part of the lumbar lordodis), the gravity centre moves the gravity alignment (superior part of the lumbar curve) (Fig.4).
The area of balance is not fixed but it is in fact a "chimney" of equilibrium managed with ligaments and muscle activity linked to the equilibrium organs (Fig.5).
Lot of parameters have been described but we will limit their description to those we use in the spondylolisthesis analysis.
Sacral tilt, incidence and version allow the description of the pelvic anatomy and positioning on a sagittal point of view.
There are trigonometric relationships (version + sacral inclination = incidence) between those parameters (Fig.6).
On the lumbar pelvic femoral system, forces are applied as trunk weight on the promontory of S1, ground reaction on the centre of the hips. To maintain in balance such an unstable system, the ligaments and muscles develop their activity. Those two simplified forces create a moment of rotation whose centre is the centre of the hips. The result of this moment may be calculated as M= ï ï Wï ï ï ï AB ½ ½ sin ˆ (M = moment, W = weight of the trunk, AB = length between the centre of the hips, sin ˆ = sinus of the version). The forces to balance this moment are developed by all the soft tissue structures. When A and B are vertically aligned, sin ˆ is = 0 so the moment is = 0 (Fig.7 & 8).
The anatomical modifications of the pelvis demonstrate the use of this measure, as the reduction of the length AB when 5 lumbar vertebras, the increasing of the version angle when the pelvis is retroversed or when the incidence parameter is high.
Spondylolisthesis and balance
When the pelvis is maintained in retroversion when standing position is acquired and/or when lordosis development is limited (zygapophyses anatomy) the hips are maintained projected forwards (high version angle) with a progressive lengthening of the isthmic part (often followed by a fracture). When lordosis is limited, the isthmic fracture allows the extension L4L5. Those two associated mechanisms allow the slipping of L5 on S1, which reduces the unbalance of the lumbar pelvic femoral system and the global balance of the increasing lordosis.
Combined to Bradford's parameters that describes L5 dysplasy (association of congenital and growth deformities), version, incidence, pelvic length and moment allow the understanding of the mechanism of the deformity and its potential of evolution as risks factors (Fig.9).
The incidence angle must be measured too when surgery is indicated. The correction of the slipping percentage is not the most important criteria of the reduction. The correction of L5 S1 kyphosis and pelvic retroversion is more important. This capacity of reduction is limited in case of high incidence angle. The risk of the reduction is to maintain or recreate a desequilibrium of L5/pelvis/centre of hips system by reducing only the percentage of slipping (Fig.11). It appears that fractures in surgery are main part of the time the consequences of the non respect of the new balance created by the slipping itself or worse by increasing the retroversion of the pelvis (especially when reduction was performed by distraction between L4 and S1) (Fig.12&13).
Standing acquisition mechanism, anatomical analysis and pelvic parameters, especially the moment of balance allows to understand some of the complex mechanism of spondylolisthesis and to prevent when a surgical treatment is indicated, the misbalance spine/pelvis/femoral heads that create the conditions of a pseudoarthrosis and a recurrent slipping to recover a better equilibrium or a spondylolisthesis at the level above.