COMPENDIUM OF ANATOMICAL VARIANTS

An Unusual Articulation at theLumbosacral Junction

DIANA McLAUGHLIN* AND EDWARD C. WEBER

Department of Anatomy and Cell Biology, Indiana University School of Medicine, Fort Wayne, Indiana

We describe here a vertebral column with a nor-mal number of vertebrae but an articulation betweenthe fourth lumbar and first sacral vertebrae.Although numerous variations of the lumbosacralregion have been described (Manners-Smith, 1909;Nutter, 1914; Willis, 1923, 1929, 1959; Taylor,1939), this is the first report of an L4/S1 articulation.

A 25-year-old male presented with low back pain.To determine the cause, a CT scan was performedthat revealed an anomaly of the lumbosacral junc-tion. The body and pedicles of the fifth lumbar verte-bra were normal. However, the pedicles sprouteddysplastic transverse and superior articular proc-

esses. Missing from L5 was the dorsal vertebral arch,including the inferior articular facets, laminae, andspinous process. More striking were the accompany-ing changes in the L4 vertebra. On the left side, thedysplastic L5 superior facet formed a pseudoarthro-sis with an elongated L4 pars interarticularis. Justposterior, the L4 inferior facet extended inferiorly tothe S1 superior facet as a normal appearing left L4/S1 facet joint (Fig. 1). The hypoplastic L5 superiorfacet of the right side, though, appeared to have asmall, but nearly normal articulation with a smallright L4 inferior facet. The bony continuation fromthe right L4/L5 articulation, which again appeared to

Fig. 1. Volume rendered display, lumbosacral CT scan.

*Correspondence to: Diana McLaughlin, Indiana University Schoolof Medicine, 2101 E. Coliseum Blvd., Fort Wayne, IN 46805, USA.E-mail: dmmclaug@iupui.edu

Received 10 September 2007; Accepted 7 October 2007

Published online 25 November 2007 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ca.20561

VVC 2007 Wiley-Liss, Inc.

Clinical Anatomy 21:775–776 (2008)

be an elongated L4 pars interarticularis, extendedinferiorly to an extremely small, but relatively normaljoint with the right S1 superior facet (Fig. 1).

Embryologically, the vertebrae begin developmentin the 4th week. Formation of six cartilaginousgrowth centers—two in each half of the vertebralarch and two in the vertebral body—occurs duringthe 6th week. The growth centers located in the ver-tebral arch normally fuse both with each other andthe body, extending dorsally as the laminae and ven-trally as the pedicles. In addition, extensions ofthese centers form the transverse and spinous proc-esses (Moore and Persaud, 1998). Our patient’sanomaly appears to be an error of both segmenta-tion and induction. The most dorsal pair of the fourvertebral arch growth centers of L5 failed to segmentfrom the corresponding growth centers of L4. Thisresulted in an elongated pars interarticularis of L4 oneach side, with inferior facets articulating with thesacral facets. The more ventral pair of the vertebralarch growth centers of L5 extended anteriorly toform pedicles and laterally to form transverse proc-esses. Failure of this tissue to form a left superior L5facet and the development of only a rudimentaryright superior L5 facet may have resulted from twofactors. First, some of the mesenchyme that wouldnormally have contributed to the superior L5 facetsmay have been incorporated into L4. Second, thelack of inferior facets of L4 in the normal positionmay have resulted in failure of induction of normalL5 superior facets.

The difference in appearance of the right and leftL5 superior facets can be explained by the degree offailed induction. On the left, failure of induction was

complete whereas the appearance of a relativelynormal L4/L5 facet joint on the right suggests thatfailure of induction on this side was incomplete. Re-markable, too, is the asymmetry of the L4/S1 facetjoints. Whereas the mesenchyme of the dorsalgrowth center failed to segment completely on theleft, the same mesenchyme did, to a minor extent,segment on the right.

The underlying skeletal abnormalities describedearlier most likely contributed to this patient’s lowback pain.

ACKNOWLEDGMENTS

We would like to express our sincere appreciationto Dr. Joel Vilensky for his advice and comments onthis manuscript.

REFERENCES

Manners-Smith T. 1909. The variability of the last lumbar. J AnatPhysiol 43:146–160.

Moore KL, Persaud TVN. 1998. The Skeletal System in The Develop-ing Human: Clinically Oriented Anatomy. 6th Ed. Philadelphia:Saunders. p 405–424.

Nutter JA. 1914. Congenital anomalies of the fifth lumbar vertebraand their consequences. J Anat Physiol 48:24–36.

Taylor RG. 1939. Anomalies of the lumbosacral articulations. JAMA113:463–465.

Willis TA. 1923. The lumbo-sacral vertebral column in man, its sta-bility of form and function. Am J Anat 32:95–123.

Willis TA. 1929. An analysis of vertebral anomalies. Am J Surg 6:163–168.

Willis TA. 1959. Lumbosacral anomalies. J Bone Joint Surg Am 41:935–938.

776 McLaughlin and Weber