The Various Causes of Scalloped Vertebrae with Notes on Their Pathogenesis

Abstract

“Scalloping” is used to describe that change in the configuration of the vertebrae, in which there exists an exaggeration of the normal slight concavity of the dorsal surface of the body. Scalloping in association with a locally expanding mass in the spinal canal is well recognized. There are other less well known causes of this type of vertebral deformity. This paper will present a retrospective study in which examples of scalloping were collected from our files and from the literature and were classified according to the pathogenic mechanisms involved. Our aim is to bring a sense of order to a subject which is discussed in a rather fragmentary manner in the literature. Examples from the various groups will be illustrated, and hypotheses regarding the mechanisms of scalloping will be offered. In formulating a classification of scalloped vertebrae, we have taken into consideration the pathogenic mechanisms by which the configuration of bones may be altered. An abnormal configuration might result either primarily, from defects in the development and maintenance of bones subjected to normal extrinsic forces, or secondarily in response to exposure to abnormal external forces. The vertebrae share with the cranium the unique skeletal function of containing in a closed space a pulsatile, fluid-filled system comprised of the organs of the central nervous system, the cerebrospinal fluid, and the meninges. During the period of growth and development the vertebrae must adapt to the increasing volume of these structures. In disease processes the bony walls of the spinal canal may be deformed by pathologic expansion of intraspinal contents, by increased cerebrospinal fluid pressure, or, we hypothesize, by abnormalities in the tissues which serve to protect the vertebrae from the normal cerebrospinal fluid pressure and pulsations. The following classification groups the various causes of scalloped vertebrae according to the apparent mechanisms which ultimately influence the dynamic balance of osteoblastic and osteoclastic activity at the posterior vertebral body surface. Increased Intraspinal Pressure Tumors Causing Scalloping The earliest recognized and best known cause of localized widening of the spinal canal is a localized expanding lesion. This phenomenon was described by Elsberg and Dyke (5) who demonstrated that localized expanding intraspinal lesions might increase the interpediculate distance and deform the pedicles. Subsequent publications by other authors (8, 24) indicate that widening of the interpediculate distance and alteration in the configuration of the pedicles, in the presence of a local expanding lesion, precede scalloping. It has been proposed that the dorsal longitudinal and annular ligaments afford the dorsal surfaces of the vertebral bodies some protection from extrinsic pressure and cerebrospinal fluid pulsations (24).