Morphological traits in sacra associated with complete and partial lumbarization of first sacral segment

Background context: Transitional states of vertebrae at the lumbosacral junction are associated with low back pain. Lumbarization may present as partial (incomplete separation of S1-S2 bodies and their neural arches) or complete separation of the S1 from the S2 segment in a sacrum. Separation of S1 entails alteration of the anatomy at the lumbosacral junction that affects load bearing at the region.

Purposes: To 1) identify characteristic structural features in sacra associated with lumbarization of the S1 sacral element; 2) compare linear dimensions and surface areas in such sacra with that of the normal samples; 3) calculate relative contributions of different dimensions in the variant sacral corpuses by working out indices; and 4) analyze load bearing at these sacra in light of their altered morphology.

Study design: Three hundred thirty-two dried human sacra were measured for specific dimensions and surface areas. They were screened for partial or complete lumbarization of the S1 segment.

Outcome measures: All parameters and indices of the lumbarized specimen were compared with those in the normal sacra.

Methods: Seven linear dimensions and five surface areas were measured in 332 sacra. Nine indices were formulated from them. Sacra with partial or complete lumbarization of S1 were analyzed for all the parameters and evaluated for their morphological variations and traits.

Results: A total of 3.9% of sacra presented lumbarization, seven (2.1%) with partial and six (1.8%) with complete separation of the S1. Sacrum with partial lumbarization presented parameters similar to the normal variety. Auricular surfaces in these sacra were situated lower than normal with negligible contribution from the S1 segments. Parameters in sacral samples showing complete separation of S1 (with remaining S2-S5 segments) were smaller than normal dimensions except for some. These "baseline" (S2-S5) sacra transmitted load to the sacroiliac joints via the S2 and S3 elements. Four of six of these samples demonstrated unilateral or bilateral accessory facets between the transverse elements of the free S1 segment and the ala of the remaining sacrum (formed by S2). Significant load passed through the accessory articulations that abutted near the sacroiliac joints. A good number of these sacra showed highly attenuated laminae and at times presented incomplete fusion at the spines (spina bifida).

Conclusions: Lumbarization involved distinct anatomical alterations at the S1-S2 junctions. Partially lumbarized samples showed 1) gap (mean 1.43 mm) between S1 and S2 bodies; 2) incomplete separation between S1 and S2 neural arch elements with the S1 arches resembling lumbar laminae; 3) well-developed "baseline" (S2-S5) segments; and 4) "low-down" (positioned at S2-S3) auricular surfaces. Complete separation of S1 involved 1) smaller overall dimensions with diminished facet depth, sacral height, and facet area at S1; 2) auricular surface areas comparable to the normal and greater than those found in the partially lumbarized specimen; 3) large "baseline" (S2-S5) sacra with "high-up" auricular surfaces (positioned at S2-S3); 4) occasional S1-S2 accessory articulations; and 5) small zygapophyseal joints and attenuated sacral laminae with high sacral hiatuses associated at times, with "spina bifida."