Spondylotic Spondylolisthesis: An Archeological Study of Pelvic and Lumbosacral Parameters of Possible Etiologic Effect in Two Distinct Racial Groups of High Occurrence

Methods: Skeletons from these groups were studied at the Smithsonian Institute NMNH. All sufficiently intact remains(n=29) from an isolated Aleut group with an adult occurrence rate of 27%(n=48), were studied radiographically. A second group(n=250) was a random sample of a large Dakota Arikara group with a 7.9% adult occurrence rate. They were measured and/or radiographed if intact n=140). Only three parameters could be accurately measured:

• Lumbar Index[LI] of L5.
• Pelvic Incidence[PI] (Duval-Beaupere et al)(1) was measured by X-ray.
• Sacral Table Angle[STA] (Inoue et al, the angle subtended by the S1 endplate and S1 posterior body cortex) (2). STA was measured by X-ray or a protractor modification. The data obtained from both groups were analyzed and compared to published Japanese data (Inoue).

Findings:

1. PI varies inversely with STA when SS is present(Pearson correlation-0.92p-value=.003)
2. PI is not associated with the presence or absence of SS in the Aleut. There is an association in the Arikara(p=.002). This losses significance when corrected for secondary decrease in STA that occurs with SS.
3. STA is associated with SS occurrence in the Alute(p=.01) and Arikara(p=.001)
4. STA significantly decreases in the presence of pars defects within each group; Aleut(p=<.01), Arikara(p=<.001), Japanese(=<.0001)
5. STA in normal and abnormal children is not significantly different from adults without SS but is significantly greater than adults with SS(p=<.001)

Discussion: The higher the STA, the more horizontal the S1cartilagous growth plate is. This may be a genetic characteristic that accounts for the different occurrence rate among groups. As pars fracture and then slippage occur, the axial stress on the growth plates changes differentially from posterior to anterior on L5-S1, causing less growth anteriorly on S1 and on the posterior-inferior growth plate of L5. Diminution of the STA and the LI are obligatory (lowering the anterior portion of S1 and narrowing the posterior height of L5). PI is defined as the sagittal plane angle measured from a perpendicular to the mid-point of the sacral table to the bi-femoral axis. As the sacral table tilts anteriorly with the occurrence of deformity, PI must decrease(p-value=.002). STA is significantly associated with SS pars defects, while PI is not. The sagittal contour change of the L-S area in SS is more likely due to the differential response of the cartilage growth plates to pressure before epiphyseal closure and changes from compressive and shear stresses after closure. Primary change in the entire pelvic shape and location of the bifemoral axis(PI) is less likely. STA appears possibly racially determined.

References:

1. Legaye J, Duval-Beaupere C, Hecquet J, Marty C. (1998). Eur Spine J, 7:99-103
2. Inoue H, Ohmori K, Miyasaka K. (2002). Spine, 27(8):831-838