Complete sacralization of L5 vertebrae: traits, dimensions, and load bearing in the involved sacra.

Background Context: Spines with sacralized fifth lumbar vertebrae are quite commonly encountered. Sacralizations represent a transitional state at lumbosacral junctions and are more susceptible to degenerative changes resulting from the altered load-bearing patterns at these regions.

Purpose: The present study is an attempt to analyze the morphology of sacra with complete and partially fused (sacralized) L5 vertebrae and compare it with those observed in normal sacra.

Study Design/setting: Measurements of dimensions and articular surface areas were recorded in samples bearing sacralized L5 vertebrae and in normal sacra. Relative contributions of dimensions and surface areas on the bone were worked out by calculating indices. Comparison with the normal sacra was expected to yield significant differences.

Outcome Measures: Means of all linear parameters and indices were compared between sacralized and normal samples and between the fused and unfused sides of unilaterally sacralized specimens. Unpaired Student t test was applied to assess the differences in the values.

Methods: Seven linear dimensions and five surface areas were measured in 330 sacra. Nine indices were formulated from them. Sixteen (4.8%) sacra presented with bilateral completely fused L5 vertebrae with the first sacral segments. Four (1.2%) showed unilateral complete unification of sacra with the transverse process of the L5 vertebra above. Parameters were compared between the normal and sacralized samples and also between the two varieties of sacralized specimen.

Results: Sacra with fused L5 possessed significantly smaller heights than the normal ones if the fused L5 vertebra was excluded from the measurements. On inclusion of the L5, these sacra presented a grossly reduced distance between the zygapophyseal facets, a greater distance of the coronal plane of the facet joints from the posterior aspect of the L5, a narrower interauricular distance, slightly increased body width at the top of the sacrum (now L5), a comparable auricular surface area (with inclusion of the fusion of L5 transverse process at the lateral mass) to the normal ones, attenuated facet area, and occasionally, small intervertebral space between L5 and S1. The unilaterally fused variety exhibited overall smaller dimensions. None of these sacra showed accessory articulations at their ala. The auricular surfaces in these sacra spanned from mid-L5 to mid-S2 segments. The fusion of L5 increased the sacral height, width, and auricular surface. The auricular surfaces appeared to be situated "low-down" in context of the six (including L5) sacral segments.

Conclusions: Sacralization of L5 vertebra entails morphological alterations in the sacrum. The remnants of the original sacra in these specimens presented grossly diminished parameters. Sacralization of L5 possibly represents a structural and biomechanical adjustment to compensate for reduced joint interfaces associated with smaller sacra. It may correspond to one end of the transitional "spectrum," the other end being defined by lumbarization of the S1 sacral segment.