ASYMMETRY OF VERTEBRAL ARTICULAR PROCESSES AND FACETS CAROLINE WHITNEY Department of Anatomy, Washington University The finding of a s y m e t r y of the articular processes and facets of the eleventh and twelfth thoracic vertebrae in two of the skeletons of the anthropological collection a t Washington University led to the review of all available material with the object of determining the incidence and possibly, relations of this condition. This abnormality was reported by Struthers‘ in 1875 as one of the many variations which may occur in the transition region between the thoracic and lumbar spines. Barclay Smith2 in 1902, described a twelfth thoracic vertebra with asymmetrical twelfth superior facets, the right being of the normal thoracic type, flattened and directed posteriorly, the left being typically lumbar, deeply concave and directed medially. He briefly mentions four other examples with exactly the same appearance and concludes that the condition is very rare. Later, in 1911,3while reporting a case of multiple anomalies of the vertebral column, Barclay Smith again mentions the asymmetry of the articular processes of the eleventh and twelfth dorsal vertebrae; but this time he says that “the lack of symmetry of the opposed facets is not an uncommon condition,” and that “usually the flat facet is on the right side.” LeDouble* (1912) reports such a condition in the last thoracic vertebra of a horse. The material used for the present study, shown in Table 1, consists of spines prepared from the cadavers dissected in the anatomical laboratory, the race, sex and age of which are of record. TABLE 1. Race White .............................................. Negro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Totals .............................................. Males 139 69 208 Females Totals 20 19 39 159 88 247 The ages extend from 8 years to 82 years; 9 of the spines are from individuals of 20 years and less; 62 spines are of the decade 51-GO years, and this period is evidently the mode; 42 are 71 years and older. ‘Struthers, John. On variations of the vertebrae in man. J . Anat. 6 Physiol., 1875, IX, 2-96. *Smith, Barclay. Two rare vertebral anomalies. J . Anat. 6 Physiol., 1902, XXXIX, 372. %rnith, Barclay. Multiple Anomalies in a Vertebral Column. J . dnat. 6Pkyswl., 1911, XIV, 144-147. 4LeDoilble. Trait6 des variations de la colonne vertkbrale de l’homme et de leur signification au point de vue de l’anthropologiezoologique. Paris, 1912. AM. J. PHYS.ANPHROP.,1926,Vol. IX, No. 4. 451 CAROLINE WHITNEY 453 Four of the spines, as a whole, show a mild osteo-arthritis. One spine presenting the asymmetry of the articular processes occurs in a case of cleido-cranial dysostosis6in which there is scoliosis; the other spines in the series present no pathological changes. I n this series of spines thirteen show asymmetry of the articular processes and surfaces: the distribution of the variation is recorded in Table 2. TABLE 2. NO. Race Sex Age 55 56 White White White White White Negro White Male Male Male Male Male Male Male 50 50 30-40 46 72 70 53 64 81 168 171 197 No. 198 205 244 272 M.317 M.318 Race Sex White Negro Negro Negro White White kale Male Male Female Female Male Age 49 42 60 49 75 40 Two distinct types of the variation were found, one the mirror image of the other. In all cases with the exception of No. 244 and M. 318 the general scheme is as follows (Fig. 1):The superior articular processes of the eleventh thoracic vertebra are symmetrical and show the usual characteristics of the thoracic region; the inferior facets are asymmetrical, the left one being thoracic in type, flat, and directed anteriorly; the one on the right is lumbar in type, convex and directed laterally. The upper processes and facets of the twelfth correspond to the lower facets of the eleventh; i. e., the left facet is flat and directed posteriorly, the right is concave and directed medially. The lower facets of the twelfth are normal, that is typically lumbar in appearance and symmetrical. In No. 244 (Fig. 2), the asymmetry is the reverse of that described above. In No. M.318, the asymmetry is like that of No. 244, but it occurs between the last thoracic and the first lumbar vertebrae. This first lumbar vertebra has a costal facet on the right articulating with a thirteenth rib, but none on the left. The right superior articular process is thoracic in character, the left is lumbar. The sacrum (five segments) and coccyx (four segments) are regular. In regard t o the frequency of the variation the figures drawn from the samples of our small series are here recorded as possibly showing the tendencies but with no claim of their being accurate for a large collection of spines. It apppears that the variation is present in somewhat more than 5 per cent (5.66) of the Whites examined and in somewhat more than 4 per cent (4.54) of the Negroes. Again, in white males the &Terry,R. J. Rudimentary Clavicles and other Abnormalities of the Skeleton of a White Woman. J. dnat. b Physiol., 1899, XXXIII, 413-422. ASYM?VIETRY OF PROCESSES AND FACETS 453 percentage (5.75) is higher than in black males (4.34). More data for these comparisons of the variation are much to be desired for determining the status of variability in different races. The level at which the asymmetry of the articular processes was found is one of the three a t which changes in the form of the vertebrae occur and where there is a change in the type of motion allowed between the vertebrae. At C2-T1 there is a change from a rather free movement in all directions to limited movements, chiefly of rotation, flexion and extension; a t T12-L1 the change is from rotation chiefly to movements which are largely flexion and extension; and at L& the change is from angular motion to fixation. The limitation of movement between adjoining vertebrae is effected largely by the form and direction of the facets, by the ligaments and by the thinness of the intervertebral discs. At the junction of the lumbar and thoracic segments symmetrical variations in the types of facets have been frequently noted. Struthers,6 Ward,? LeDouble* and others have remarked the inconstancy of the level at which the change from dorsal to lumbar type Of articulation occurs; and they have also noted that the change from one type to the other is often a gradual process extending over three or four vertebrae, rather than a precise and complete change in passing from one vertebra to the next. Asymmetrical variations are less frequently noted. In the literature no record could be found of asymmetrical articular facets, excepting pathological cases, occurring in any region other than the thoraco-lumbar transition zone; and in the present series of cases the asymmetry was limited to that region. Scoliosis of any etiology is accompanied by asymmetry of the skeleton with definite changes in the vertebrae-rotation of the body of the vertebra to the convexity of the curve, lessening of the vertical depth of the body on the convexity of the curve, and, in the more advanced cases, a deepening of the articular facets in the convexity of the curve. The asymmetry of articulations in the present series, might then be a response to scoliosis. The skeleton of the dwarf with cleido-cranial dysostosis was the only one which showed any marked scoliosis. In this case the lumbar type of facet is on the left-the convexity of the curve where all the facets are somewhat deepened. However, the whole left side of the vertebra in question is lumbar in type, a condition not usually seen in scoliotic spines. Other examples of scoliotic spines, dried specimens from the pathological museum, were examined, but Struthers. LOG. cit. 'Ward. Osteolow. London. 1876. CAROLINE WHITNEY 454 none of these showed any vertebra with a lumbar type of facet on one side and a thoracic type of facet on the other. There has been described a “normal” scoliosis which, according to numerous observers, is to be found in almost every individual. If there were any relation between asymmetry of the articular processes and the “normal” scoliosis one would expect to find the incidence of the variation higher than 13 out of 247. A well known cause of asymmetry of the skeleton is that difference in stress which attends the unequal use of the right and left hands. It has been shown, in respect to the arms that in a right handed individual the bones of the right side are longer and sturdier, and the areas of muscular attachment are more marked. The clavicle of the right side is thicker, shorter and may show slightly greater curvature. In the spine, according to G a ~ p p the , ~ “normal” scoliosis of the thoracic region is toward the left in left-handed individuals. Regarding the skull, Elliott Smithla has shown that this also presents a definite asymmetry-the prominence of the occipital pole of the cerebrum on the left in a right-handed individual leaves a deeper impression, the fossa striata, on the skull than the less prominent pole on the right; and it is well known that the superior sagittal sinus, and its corresponding groove, is commonly deviated to the right. There is a reversal of these markings in left-handed individuals. Mooreheadll (1902) in reviewing the work of Harting, Bischoff, Gaupp, and Thiele, adds some observations of his own on the symmetry of the body of the newborn child. He comes to the conclusion that the asymmetry is a feature which is developed in later life in response t o the difference of muscular stress. Measurements were made of the arm bones and of the clavicles of the present series, and the measurements of the right and left side compared. The skulls were studied for those signs of right and lefthandedness which have been mentioned. It was found by the evidence of the limb and cranial criteria that twelve of the series were righthanded skeletons; of these, eleven possessed the flat facet on the left, one presented the flat facet on the right side. The thirteenth skeleton QGaupp. Die normalen Asymmetrien cles menschlichen Korpers. Jena. 1902. Wmith, G. Elliott. On asymmetry of the caudal poles of the cerebral hemispheres dnd its influence on the occipital bone. Anat. Ans., 1907, XXX, 574-578. Asymmetry of the brain and skull. J . Anal. & Physiol., 1906-07, XLI. 236. . Right Handedness. B. M . J., 1908, 11, 597. The London Lady. B. M. J.,1925, 11, 881. llMoorehead, &I. B. Relative weights of the right and left sides of the body in the foetus. J . Anat. & Physiol., 1902, XXXVI, 400-404. . ASYMMETRY OF PROCESSES AND FACETS 455 was left-handed according to the criteria mentioned and in this one the flat facet was on the right. The amount of material presented is insufficient to draw a definite conclusion as to the relation of asymmetry of facets to handedness. However, the fact that the skeleton of the left-handed individual presented an asymmetry the mirror image of that seen in all but one of the right-handed skeletons seems to indicate that such a relation may obtain, However, the presence in the right-handed series of one skeleton (No. M.318) with an asymmetry identical with that of the left-handed skeleton must be explained before such a relationship may be said actually to exist. No. M.318 does not fit in with the series in at least three other respects, viz. : (1) the asymmetry occurs a t a lower level (TI2-L1) than is the case in the other spines; (2) this first lumbar vertebra possesses a costal facet on the right side articulating with a thirteenth rib; and (3) there are 25 movable vertebrae. The twentieth vertebra, has been reckoned the first lumbar because in all respects, save in the possession of a costal facet, it is characteristically lumbar; there are therefore six segments in the lumbar spine of skeleton No. M.318. Summarizing the above observations it is apparent that asymmetry of articular processes is of relatively frequent occurrence in the series examined and is limited to the last two rib-bearing vertebrae. A larger series must be studied in order to determine the exact incidence as well as to compare the frequency in Whites and Negroes. In the majority of cases the asymmetry is of the type showing the flat facet on the right. If this condition were related to the right and left-handedness of the ordinary i,ndividualone would expect to find a much higher incidence. However, the fact that in the spine of the one left-handed skeleton in this series presenting asymmetrical facets the asymmetry is the mirror image of that evidenced in the great majority of right-handed skeletons suggests the possibility of there being a relation to an exaggerated handedness such as that brought about by certain occupations. There appears to be no relation between this variation and either the pathological scoliosis or the so-called “normal” scoliosis. My thanks are due to Professor R. J. Terry and Dr. H. A. Harris for their advice and criticism in the preparation of this paper. FIG.1. (left) Eleventh and twelfth thoracic vertebrae of skeleton No. 272, showing the asymmetry of articular processes and facets as they appear in all but two of the anomalous series. FIG.2. (right) Eleventh and twelfth thoracic vertebrae of skeleton No. 244, in which the asymmetry of the processes is inverted in respect to the type shown in No. 272.