A Case of Fusion of the Third and Fourth Lumbar Vertebræ

Abstract

Donaldson (1), in listing ten congenital abnormalities, gives the three most rare as (a) intercalation of a whole or portion of a vertebra, (b) non-development of one side of the centrum, (c) fusion of two vertebræ without formation of a disc. Evans (2) stated that the majority of anomalies of the vertebræ were those of intercalation as a result of imperfect segmentation in the ossification centers. From this there may be a block vertebra, due to the absence of intervertebral discs, which are more often at the cervical level. There may be a fusion of two bodies unilaterally, or complete, or there may be cross-fusion. Wise (3) reported a case in which the bodies of the vertebræ were tilted and the third lumbar vertebra on the right was united to the fourth on the left. He stated that there were two centers of chondrifications in each body and one in each half of the arch. At the eighth week of the embryo ossification begins in a single center in the body and in each half of the arch. That these abnormalities are not frequent is shown by Willis' study of 1,400 skeletons, in which he found the abnormalities to be confined to the lumbosacral area (4). In the embryo the first anlage of the spine is a longitudinal column of ectodermal cells called the notochord, which forms beneath the neural groove. Paralleling this notochord are longitudinal groups of mesodermal cells and paraxial mesoblasts. These two paraxial masses undergo segmentations, producing 37 somites. These somites become sclerotomes which grow toward each other around the notochord. The sclerotomes divide into two parts, the less dense cranial portion becoming the anlage of the intervertebral disc and the more dense caudal portion the scleromere or primitive vertebral body. The scleromeres become transformed into dense cartilage, there being at first two centers of chondrification (5). In the newborn the vertebral body shows three zones of osseus tissue: a middle transverse zone of lessened density which forms cancellous bone and two layers, one superior and one inferior, which are more dense and represent ossifying cartilage. The central, more vascular layer has large canals for blood vessels which are evident on the roentgenograms of all infants and which may persist to adult life. The central layer is smaller than the others, giving the appearance of a depression when observed in profile. The center for the anterior portion of the body unites with the posterior at about the fifth or sixth year. Secondary centers of ossification or epiphyses appear in the chondral superior and inferior portions of the discs between the sixth and seventeenth years and complete fusion of these centers with the main body occurs at about the twentieth year of life.