Radiographic Markers in Spondyloptosis: Implications for Spondylolisthesis Progression

Rush Medical College, Chicago, Illinois, USA

Treatment algorithms for spondylolisthesis are well defined. Accepted protocols base treatment on age, symptomatology & degree of slippage. However, recent classification schemes focus on spinopelvic morphology & dysplasia as best determinants for progression. Several parameters of the pelvis & lumbo-sacral (LS) junction are fundamental in determining spinal lordosis & balance in relation to the gravity line. These parameters are fixed, independent of position, & determine lumbar to sacral relations. Frequency of common denominators of high-grade spondylolisthesis, such as the degree of dysplasia & spinopelvic morphology, is unknown.

OBJECTIVE: To assess pelvic morphology, sacral kyphosis & posterior element dysplasia in spondyloptosis as compared with historical controls of the normal & low-grade spondylolisthesis populations.

METHODS: We retrospectively reviewed only patients with spondyloptosis (slippage over 100% according to Wiltse) & excluded posttraumatic, neoplastic & iatrogenic cases. Bony hook/catch dysplasia at LS junction was graded on a relative point scale. Pelvic incidence (PI) & sacral kyphosis (S1 to S4) was measured.[1,2]

RESULTS: 53 cases (16 males & 37 females) were studied. Mean age was 22 years (range 11-55). Mean sacral kyphosis was 56.0° (SD +/- 15°). Posterior element dysplasia was present in 64% of cases. Mean Pelvic Incidence was 76° (SD +/- 10°).

CONCLUSIONS: Spino-pelvic morphology has been described in the normal & low-grade spondylolisthesis populations. Only a fraction of these patients, if untreated, will progress to spondyloptosis. Accepted prognostic factors, such as percent of slippage, only describe an ongoing process & do not add to understanding of etiology. They are useless in identifying those lower grade slips at risk for progression. Biomechanicaly, this progression is linked to increased gravity shear stress across LS junction, & inability to resist such stress. Increased stress is related to increased verticality of LS joint upon weight bearing, which is individually predetermined by pelvic morphology & sacral anatomy. A key pelvic parameter - pelvic incidence, is fundamental in determining saggital spine curvature required for economic spino-pelvic balance over the femoral heads. PI value becomes stable at age 4 & remains unchanged throughout adulthood. PI relates the sacral slope to the pelvic radius & is independent of pelvic orientation. It remains unchanged despite adaptive changes present in higher-grad spondylolisthesis. An increased PI results in increased verticality at the LS junction & therefore increase in shear stress. PI (76°) in our series is certainly higher than reported PI in the normal (48.2°- 53.2°) & low-grade isthmic spondylolisthesis (mean 64.5°) populations. Resistance to shear stress is provided by anterior & posterior spinal elements. Posterior element dysplasia decreases the mechanical resistance to such stress. Reported incidence of such dysplasia is as high as 42% in the higher grades of spondylolisthesis compared to 64% in our spondyloptosis series. The risk for significant progression of spondylolisthesis correlates with each individual's pelvic anatomy & can be assessed by measuring a reproducible fundamental pelvic parameter - pelvic incidence & by determining the patients posterior element dysplasia.

[1] Legaye, Duval-Beaupere, Hecquet, Marty; Pelvic incidence: a fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves; Eur. Spine J. 1998;7:99-103

[2] Antoniades, Hammerberg, DeWald; Sagittal Plane Configuration of the Sacrum in Spondylolisthesis; Spine 2000;25(9):1085-1091