A strict relation was described between the sagittal pelvic anatomy and the sagittal shape of the spine, particularly the amount of lordosis needed for each individual. Therefore, angular parameters were recommended because they are usable disregarding the size of the subjects [
In the same way, the distinction was established a long time ago by morphologists and paleontologists between the “pelvis in tension” of the quadrupeds and the “pelvis in pressure” characterizing the bipedalism [
Angular and dimensional parameters were measured on 272 lateral radiographies including the pelvis, the femoral heads and the lumbar column, in standardized standing position [
A  B  B1  B2  C  D1  D2  

Normal  Painful cases  Low back pain cases  LBP + leg pain cases  Children  Spondylolysis grade 12  Spondylolysis grade 34  








Men 27  Women 34  Men 69  Women 78  Men 26  Women 30  Men 43  Women 48  Men 6  Women 9  Men 26  Women 14  Men 4  Women 5  
Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  
Age (years)  30.9  7.5  43.1  14.4  42.6  13.7  46.8  17.2  7.2  1.9  27  12  36.5  8.1 
Pelvic incidence (PI) (°)  48.3  10.1  49.6  9.8  52.8  9.2  47.3  10.5  40.4  7.8  60.5  11.5  70.6  9.3 
Jackson’s angle (PRS1)  36.9  9.1  35.8  8.9  32.5  9.3  38.1  9.9  44  7.7  25.9  10.5  17.2  8.6 
Length of the upper plate of S1 (mm)  36  4.2  36  4  37  4.1  36  4  24.5  3.1  34  3.1  28.3  3.8 
Mean diameter of the femoral heads (mm)  56.2  6.8  57  7.1  58  7.2  56  6.9  35.4  5.5  52  5.9  48  5.5 
Femoral heads diameter/length of S1 (%)  64  5.1  63.2  5.1  63.8  5.2  63.0  6.9  69.2  3  65.3  5.9  53.3  3.4 
Sagittal pelvic thickness (SPT) (mm)  132  28.1  132  27.5  130  26.4  134  27.9  84.2  5.8  116  29.5  108.7  24.7 
Ratio “SPT/length of S1”  3.33  0.38  3.32  0.31  3.29  0.33  3.41  0.32  3.54  0.32  3.33  0.41  3.79  0.48 
Ratio “SPT/femoral heads diameter”  2.12  0.26  2.11  0.26  2.10  0.30  2.16  0.29  2.38  0.31  2.19  0.33  2.29  0.29 
Mean values (expressed in degree) and standard deviation of the parameters observed for the entire normal group (column A), the painful cases (column B, B1 only low back pain, B2 with leg spreading), and the children group (column C), the low grade listhesis (column D), and the high grade listhesis spondylolysis group (column E).
The first group comprised 61 healthy voluntaries (column A). Data were obtained several years ago from for original orthopaedic studies [
The third group (column C) comprised 15 children Xrayed for pathologies other than vertebral, aged from 4 to 10 years. Previous studies emphasized sagittal pelvic parameters evolving until 10 years old.
The fourth group comprised 49 spondylolysis cases. Forty of them were with low grade listhesis (LGL) (Meyerling’s stage 1 or 2) and without any distortion of the upper sacral plate (column D1). The mean age was of 27 years (SD 12, range 15 to 42 years). The 9 other cases were with high grade listhesis (HGL) (column D2), Meyerling’s stage 3 (8 cases) and 4 (1 case).
The Chairman of our Ethics Committee attested that the data collection of all included patients and healthy subjects was in agreement with the recommendation of the Institutional Review board of the institution.
The angular pelvic morphological parameters were (Figure
The pelvic incidence (PI): value of the angle between the line perpendicular to the upper plate of the first sacral vertebra (S1) at its midpoint and the line connecting this point to the femoral heads axis.
The pelvic lordosis or pelvic radiusS1 angle (PRS1): value of the angle between the sacral upper plate and the line connecting the posterior point of sacral plate to the femoral heads axis.
Pelvic tilting (PT): value of the angle between the vertical and the line connecting the midpoint of S1 and the femoral heads (parameter here is only used for geometrical demonstration in Appendix
The angular pelvic morphological parameters: the pelvic incidence (PI) and the pelvic lordosis or pelvic radiusS1 angle (PRS1).
Both parameters were proposed for the analysis of the spinopelvic sagittal balance, PI by DuvalBeaupere and colleagues [
These angular values were reported in degree.
The dimensional parameters were (Figure
The sagittal pelvic thickness (SPT): the distance between the midpoint of the upper plate of S1 and the middle of the femoral heads axis.
The length of S1: the distance between the anterior and posterior edge of the upper plate of S1.
The diameter of the femoral heads: mean value of the diameters of two femoral heads.
The overhang of S1 on the femoral heads (OVS1): distance between the femoral heads and the projection of the midpoint of the upper sacral plate, here expressed relatively to the length of the upper plate of S1 (parameter here is only used for geometrical demonstration in Appendix
The dimensional parameters: the sagittal pelvic thickness (SPT), the length of S1 and the diameter of the femoral heads.
Positive angular and dimensional values was defined as posterior, and negative one as anterior.
The values of SPT were also expressed relatively both to the length of the upper plate of S1 and to the diameter of the femoral heads. They were so independent of the height and size of the subjects. Only these relative values were used for the comparisons and the correlations with the angular parameters.
In 4 of the 9 cases of high grade spondylolysis, the upper plate of S1 appeared rounded at its anterior part (really dome shaped sacral plates were not retained because inaccuracy of measurements). Its anterior part was extrapolated from the anterior edge of S1 and the posterior segment of the upper plate.
The student
The mean values and the standard deviations of the parameters were reported in Table
The comparisons between the values of the parameters in each group were reported in Table
A  B  C  D  E  F  G  

Normal/painful 
Painful 
Normal/ 
Normal/ 
Children/ 
Children/ 
Spondylolysis  








Pelvic incidence (PI)  0.852  NS 












Jackson’s angle (PRS1)  0.799  NS 












Length of the upper plate of S1  0.000  NS  1.449  NS 

++ 








Femoral heads diameter  0.762  NS  1.662  NS 








1.945  NS 
Femoral heads diam./Length S1  1.030  NS  0.798  NS  1.142  NS 








Sagittal pelvic thickness  0.000  NS  0.873  NS 








0.771  NS 
SPT/length of S1  0.182  NS 


0.000  NS 


0.244  NS  0.286  NS 


SPT/femoral heads diameter  0.253  NS  1.193  NS  1.131  NS  1.663  NS 


0.717  NS  0.910  NS 
Comparisons of the values of the normal, painful, and spondylolysis groups: between the normal and the total painful group (column A), between the painful cases with and without leg spreading (column B), the normal group and the low grade (column C) and high grade (column D) spondylolysis group, between the children and the low (column E) and high grade spondylolysis group (column F), and between the two spondylolysis groups (column G). + for
Obviously, the mean length of the upper plate of S1 was significantly smaller for children and for HGL spondylolysis group than for adults (
The mean values observed here of femoral head diameter were greater than the published by forensic anthropologists on anatomical specimens [
Expressed in millimeter, the values of the “sagittal pelvic thickness” were significantly greater for the adults than for the children (
The Spearman correlation tests between the Pelvic Incidence and the SPT expressed relatively to the femoral head diameter and the upper length of S1 were reported in Table
Normal  LGL spondylolysis  Children  










 
SPT/TF  0.237 

0.454 

0.077  N.S. 
SPT/S1  0.334 

0.635 

0.3 

Spearman’s coefficients of correlation of the relation between pelvic incidence (PI) and the sagittal pelvic thickness (SPT) expressed relating to the femoral heads diameter (SPT/TF) and to the sagittal length of the upper plate of S1 (SPT/S1) for the control, the low grade listhesis spondylolysis, and the children groups.
At the same time that the pelvic incidence was described as the key parameter for the analysis of the sagittal balance of the spinopelvic unit, the sagittal pelvic thickness was proposed by DuvalBeaupere and colleagues to define the sagittal anatomy of the pelvis [
The expression according to the diameter of the femoral heads appeared inappropriate because this diameter was reported not proportional to the stature of the subject and interfering with ethnical factors [
Sacral tilt and sagittal pelvic thickness for low (35°) medium (55°), and high (75°) values of PI.
Geometrical connection between sagittal pelvic thickness (SPT), pelvic incidence (PI), and pelvic radius (PRS1). (a) The angles in a pelvic schema. (b) The angles on rectangular triangles used for the demonstration.
Relations between pelvic incidence (PI), pelvic tilting (PT), overhang of S1 (OVS1), and sagittal pelvic thickness (SPT).
The close relation between “PI” (and “PRS1”) and “SPT” highlighted the ability of a subject to compensate a sagittal disturbance of the spinopelvic unit. The projection of the gravity of the body segment supported by the pelvic structures (the femoral heads and the sacroiliac joints) was reported to characterize the clinical condition of the sagittal balance of the spinopelvic unit. It was described to be almost vertical and usually posterior to the lumbar segment, the upper plate of S1, and the femoral heads [
This more or less ability of the pelvis to react against sagittal disturbance was also connected to a painful torque effect of the gravity on the sacroiliac joints. In upright steady state, the gravity was projecting vertically to the axis of the sacroiliac joints (located at the junction of the first and second sacral vertebra) [
As lumbar curvature was reported influencing spinal muscles [
The SPT is a reliable sagittal dimensional anatomical pelvic parameter when it is expressed in value relative to the sizes of the femoral heads. It expresses the capacities of a subject to manage the sources of imbalance at the same time by its spatial compensatory adaptability and the stabilizing aptitudes of the musculoligamentous structures. SPT is strongly connected to the traditional anatomical sagittal angular parameters for the evaluation of the sagittal balance of the spinopelvic unit. Moreover, it allowed a better evaluation of the lever arms of the muscles between pelvis, spine, and hips and the sacroiliac joints. This functional analysis makes it possible to clarify the potential points of actions in revalidation and even avoid useless surgical operations. Clinical treatment should be aimed at improving the stability of the spinopelvic unit taking the sagittal anatomy of the pelvis into account and reducing mechanical stresses.
Geometrical connection between PI, PRS1 and SPT (Figure
In Figure
The angle “pelvic incidence”: angle between the perpendicular to the upper plate of S1 at its midpoint and the line joining this point to the axis of the femoral heads.
The angle “
The angle “PRS1”: angle between the upper plate of S1 and the line joining the posterior edge of the plate of S1 and the axis of the femoral heads. It was named angle “
The length “
The sagittal pelvic thickness was wanted: the distance in millimeters between the midpoint of the upper plate of S1 and the axis of the femoral heads.
The perpendicular to the line prolonging the sacral plate was drawing. The distance between the axis of the femoral heads and the intersection of these lines (named point “p”) was named “
Figure
In this diagram, q1 figured the sought pelvic thickness. It was the hypotenuse of the rightangled triangle whose tops were the midpoint of the plate of S1, the femoral axis and the point p, q2 being the hypotenuse rightangled triangle whose tops were the posterior edge of the plate of S1, the femoral axis and the point p.
Development:
Tangent
As
As
Consequently “
The pelvic thickness became consequently calculated as
Spt^{2} =
Spt = square root of
Influence of SPT on the ability of the pelvis to compensate sagittal spinopelvic imbalance.
Data’s published is as follows.
The projection in the upright position of the gravity of the body segments supported by several structures was investigated [
The centre of gravity of the body segment supported by the pelvis was located at T9 level, 0 to 14 mm before the vertebral body if the thoracic kyphosis was lesser to 35°, from 20 to 32 mm before if the kyphosis was more than 35°.
The relationships [
SS = 0.5481 · PI + 12.07 (±6.39)
PI = SS + PT.
Development (Figure
PT = PI – SS = 0.4519 · PI − 12.07 (±6.39)
sin(PT) = OVS1/SPT OVS1 = sin(PT) · SPT.
Similarly, the projection of the gravity supported by the femoral heads was more posterior when “PI” value was high (or “PRS1” low). On the other hand, a sagittal rotation of the pelvis will proportionally more influence the value of “OVS1” in the event of low value of “PI” (or high value of “PRS1”), because higher value of “SPT/S1”. So, the squatter pelvises (with high value of “PI”, low value of “SPT/S1, long “OVS1”) were considered less prone to destabilisation than the vertically lengthened pelvises (with low value of “PI”, high value of “SPT/S1, short “OVS1”, easily disturbed by a forward motion of the gravity of the upper body because of short “OVS1”). Inversely, the corrective backwards pelvic rotation appeared less effective when the vertical lever arm of action (“SPT”) was short.