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CMESAM

OBJECTIVE. Anatomic variants and incomplete ossification and fusion of the develop-ing spine may result in an erroneous diagnosis of injury or disease. This article reviews some of the more common imaging findings that may present as pseudotrauma. Normal develop-ment of the spine is reviewed, including synchondroses and ossification centers. Imaging of common variants is presented, with a focus on CT.

CONCLUSION. Recognition of the normal developing spine and variants can prevent an incorrect diagnosis of injury and inappropriate treatment.

Carr et al.Pseudotrauma of the Spine

Integrative ImagingReview

age. The centrum and odontoid centers fuse be-tween 3 and 6 years of age across the subdental synchondrosis. A secondary ossification center known as the os terminale forms at the cranial tip of the odontoid by 3 years old and fuses with the odontoid by 12 years old.

The C3 through L5 vertebrae are similar in their patterns of development. These vertebrae each have three primary ossification centers: one centrum and two neural arches (Fig. 3), but the shapes of the ossification centers vary be-tween the cervical, thoracic, and lumbar spine. Fusion of the primary ossification centers is usually complete by 6 years old. It is important to note that the vertebral body is not analogous to the centrum; the vertebral body is comprised of the centrum as well as the ventromedial por-tions of both neural arches. Several secondary centers of ossification develop around 16 years old and fuse by 25 years old. Typically, there are five secondary ossification centers, located at the tip of the spinous process, the tips of the transverse processes, and the upper and lower margins of the vertebral body (ring apophyses).

Variants of the Atlas (C1)There are several ossification and fusion

variants of the atlas. The anterior arch usually develops from a single ossification center, but uncommonly there may be up to four small ossification centers that form the anterior arch [2]. When multiple, these centers are usually symmetric (Fig. 4). Nonfusion of the atlas os-sification centers is a normal finding in young children. However, in some cases fusion nev-er occurs, resulting in a cleft that persists into

Imaging of Trauma: Part 1, Pseudotrauma of the Spine—Osseous Variants That May Simulate Injury

Robert B. Carr1,2

Kathleen R. Tozer Fink1 Joel A. Gross1

Carr RB, Tozer Fink KR, Gross JA

1Department of Radiology, University of Washington, Harborview Medical Center, Seattle, WA.

2Present address: Department of Radiology, Massachu-setts General Hospital, 55 Fruit St, GRB273A, Boston, MA 02114. Address correspondence to R. B. Carr (rcarr2@u.washington.edu).

Integrat ive Imaging • Review

CME/SAM This article is available for CME/SAM credit.

AJR 2012; 199:1200–1206

0361–803X/12/1996–1200

© American Roentgen Ray Society

Injuries to the spinal column are common in patients who sustain blunt trauma, and up to 15% of such patients will have a fracture

of the thoracolumbar spine [1]. Radiography and CT are the mainstays for the initial eval-uation of these patients. Anatomic variations are common, and many variants may be mis-taken for acute spinal injury. Knowledge of the normal ossification patterns of the spine and the appearance of these variants will help prevent an incorrect diagnosis.

Normal DevelopmentThe atlas (C1) and axis (C2) show unique

patterns of vertebral ossification, different from the similar pattern of development of the remaining cervical, thoracic, and lumbar vertebrae. The atlas develops from three pri-mary ossification centers: one anterior arch and two neural arches (Fig. 1). The neural arches are usually ossified at birth, but the anterior arch may not ossify until the child is 1 year old. The ossification centers are sep-arated by two anterior synchondroses and a single posterior synchondrosis. The posterior synchondrosis typically fuses between 3 and 5 years of age, and the anterior synchondro-ses fuse between 5 and 8 years of age.

The axis develops from five primary ossifi-cation centers: two vertically oriented odontoid centers, two neural arches, and one centrum (Fig. 2). The two odontoid centers usually fuse before birth. The neural arches fuse posterior-ly by 3 years old, and they fuse to the odon-toid and centrum at between 3 and 6 years of

Keywords: pseudotrauma, spine development, spine variants

DOI:10.2214/AJR.12.9083

Received April 14, 2012; accepted after revision June 13, 2012.

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adulthood. A cleft shows smooth corticated margins, which helps distinguish it from a fracture. Atlantal clefts most commonly occur at the posterior synchondrosis but may occur anywhere within the C1 ring (Fig. 5).

In approximately 0.1% of the population, the anterior ossification center fails to form. This may result in overgrowth of the neural arches as they attempt to fuse anteriorly, re-sulting in an anterior midline cleft [3]. There is often associated hypertrophic change (Fig. 6). Alternatively, an anterior midline cleft could form from incomplete fusion of a bipartite or multipartite anterior ossification center.

A normal groove is present along the pos-terolateral aspect of the atlas on each side, which transmits the vertebral artery and suboc-cipital (C1) nerve. These structures are covered by the posterior atlantooccipital membrane. This membrane may become ossified, resulting in formation of a ponticulus posticus, meaning “little posterior bridge” in Latin. If ossification results in a complete osseous ring surrounding the vertebral artery, it forms the arcuate fora-men (Fig. 7). This variant has been described in association with neurologic symptoms, such as headache and neck pain. It has also been de-scribed as a complicating feature in surgical lateral mass fixation [4]. Occasionally, the at-lantooccipital membrane may be partially os-sified, resulting in an apparent bone fragment posterior to the superior articular process. This should not be confused with a fracture of the atlas or occipital condyle (Fig. 8).

Variants of the Axis (C2)The centrum and odontoid ossification

centers fuse across the subdental synchon-drosis. A remnant of this synchondrosis of-ten persists into adulthood. It appears as a sclerotic line surrounded by lucency and should not be mistaken for trauma (Fig. 9).

There are three entities that may result in an ossific density adjacent to the cranial aspect of the odontoid process: persistent os terminale, os odontoideum, and odontoid fracture. The os terminale is a normal secondary ossifica-tion center that usually fuses to the odontoid process by 12 years old [5]. However, an un-fused os terminale may persist into adulthood. It presents as a well-corticated ossicle that abuts the odontoid tip. It lies superior to the at-lantodental articulation and transverse atlantal ligament (Fig. 10).

An os odontoideum is larger than a persistent os terminale. It is a smooth well-corticated os-sicle located superior to a small odontoid pro-cess. An orthotopic os odontoideum is located

in the expected anatomic position posterior to the anterior arch of C1. A dystopic os odontoi-deum is located anywhere else, often near the foramen magnum and sometimes fused with the basion [6, 7]. The cause of os odontoideum has long been debated, with theories suggest-ing both congenital and acquired causes. Cur-rent evidence suggests that an os odontoideum may result from trauma during childhood [6].

A feature that helps distinguish an os odon-toideum from a type 2 odontoid fracture is the distance between the intact odontoid process and ossific fragment. A fracture will usually have a narrow zone of separation between the fracture fragments, and the overall size and shape of the odontoid process will be main-tained. An os odontoideum usually results in a larger gap between the os and the odontoid process, and it usually does not exhibit the ex-pected contour of the upper odontoid process [8]. Hypertrophy of the anterior arch of C1 is a common associated finding (Fig. 11).

Anterior pseudosubluxation of C2 on C3 is usually seen in children less than 8 years old. It is caused by ligamentous laxity and a more horizontal orientation of the facet joints in children compared with adults. Displacement is usually most pronounced on flexion radio-graphs, and it may resolve during extension. A line may be drawn along the posterior cor-tical margin of the spinal canal at C1 and C3 (sometimes referred to as the “spinolaminar line”). This line should pass within 2 mm of the equivalent location at C2 (Fig. 12).

Variants of the C3 Through L5 Vertebrae

Anterior wedging of the cervical vertebral bodies is a normal finding in young children. This is often most pronounced at the C3 level (Fig. 12). There may be a difference in height of up to 3 mm between the anterior and middle as-pects of the vertebral bodies, which should not be mistaken for a compression fracture [9]. This wedging will resolve as the child matures.

Occasionally, there will be a single linear osseous defect through the margins of a trans-verse foramen. This is not well described in the literature but likely represents a small de-velopmental cleft (Fig. 13). A ringlike struc-ture, such as a transverse foramen, usually fractures at two or more locations. Such frac-tures are often displaced. An isolated defect within the bony wall of the transverse fora-men is unlikely to represent a fracture.

Secondary ossification centers are a nor-mal finding between the ages of 16 and 25 years. Occasionally, these centers may re-

main unfused later into adulthood (Fig. 14). The margins will always be smooth and cor-ticated as opposed to acute fractures, which are not corticated and often are irregular [10].

Herniation of disk material between the ring apophysis and vertebral body may result in a limbus vertebra. A small triangular bone fragment with smooth corticated margins rep-resenting the ring apophysis will remain sep-arated from the adjacent vertebral body. This most commonly occurs in the lumbar spine and most commonly affects the anterior-supe-rior aspect of the vertebral body (Fig. 15).

A Schmorl node results from herniation of disk material through the vertebral body end-plate. This can occur anteriorly, posteriorly, or centrally [11]. Occasionally, a Schmorl node might be initially confused with an acute frac-ture, particularly if there is associated physio-logic wedging of the vertebral body. The char-acteristic appearance of a Schmorl node as a well-circumscribed rounded lucency in the vertebral body with associated endplate defect and a thin sclerotic rim should help differenti-ate the two entities.

Clefts may occur at several locations with-in the vertebrae (Fig. 16). The most common location is a cleft through the spinous pro-cess (spina bifida occulta), resulting from failed osseous fusion of the posterior syn-chondrosis. A cleft may also occur within the pars interarticularis (spondylolysis), pedicle (retrosomatic cleft), or lamina (retroisthmic cleft). The most common of these is spondy-lolysis, which most commonly occurs at L5, followed by L4, and less commonly at oth-er levels. Spondylolysis is likely multifacto-rial in cause, due to repetitive stress in people with a congenital predisposition [12].

Retrosomatic and retroisthmic clefts are far less common than spondylolysis. The cause of these clefts is unclear, but they are likely associated with repetitive stress. These clefts may result in symptoms of chronic in-stability but should be differentiated from acute fractures. The characteristic location, sclerotic margins, and associated degenera-tive changes are features that help differenti-ate them from acute fractures [13, 14].

Miscellaneous ConditionsMotion artifact may appear to represent an

acute injury, such as a fracture or facet sublux-ation, especially on reformatted images (Fig. 17). Abnormal displacement of structures, such as tubes and soft tissues, may confirm motion artifact, and multiplanar reformations are often helpful in the evaluation.

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two cases. J Orthop Traumatol 2010; 11:189–193

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9. Lustrin ES, Karakas SP, Ortiz AO, et al. Pediatric

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lanas S, Cozcolluela MR. MDCT of variations

A

A

Fig. 1—Normal ossification of atlas.A, 12-month-old girl with normal ossification pattern. Axial CT image shows single anterior arch (arrowhead) and paired neural arches (arrows).B, Axial CT image in 14-year-old girl shows expected appearance of atlas.

Fig. 2—Normal ossification of axis.A, 6-month-old girl with normal ossification pattern. Coronal CT image shows odontoid (arrowhead), paired neural arches (arrows), and centrum (asterisk).B, Coronal CT image in 21-year-old woman shows expected appearance of axis.

B

B Fig. 3—5-month-old boy with normal ossification pattern of C3 vertebra. Axial CT image shows centrum (asterisk) and paired neurocentral synchondroses (arrows).

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Fig. 5—Comparison of clefts and fractures.A, 24-year-old woman with neck trauma. Axial CT image shows nonfusion of right neurocentral synchondrosis (arrow) and posterior synchondrosis (arrowhead).B, 10-year-old boy with acute fractures of atlas. Axial CT image shows irregular margins and displacement of fracture near site of neurocentral synchondrosis (arrow) and second fracture more posteriorly (arrowhead).

Fig. 8—58-year-old man with cervical trauma.A and B, Axial (A) and sagittal (B) CT images show partial ossification of posterior atlantooccipital membrane (arrow), which may simulate small fracture fragment.

Fig. 4—5-month-old boy with bipartite anterior arch. Axial CT image shows anterior midline synchondrosis (white arrow) separating two components of anterior arch. Right anterior synchondrosis (arrowhead) and posterior synchondrosis (black arrow) are incompletely fused. Left anterior synchondrosis has fused.

Fig. 6—33-year-old man with congenital nondevelopment of anterior arch. Axial CT image shows focal hypertrophy and attempted anterior fusion of neural arches (black arrow). There is nonfusion posteriorly (white arrow).

Fig. 7—39-year-old woman with arcuate foramen. Sagittal CT image shows typical appearance of foramen (arrow), which transmits vertebral artery and suboccipital nerve.

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Fig. 10—Appearances of os terminale.A, 5-year-old girl with normal os terminale. Coronal CT image shows normal location of this secondary ossification center superior to odontoid (arrow).B, 38-year-old woman with nonfusion of os terminale. Sagittal CT image shows persistent os terminale that never fused to odontoid (arrow).

Fig. 9—25-year-old woman with normal appearance of subdental synchondrosis scar. Sagittal CT image shows linear density (white arrow) surrounded by lucency (black arrows).

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Fig. 11—Comparison of os odontoideum and chronic type 2 odontoid fracture.A, 26-year-old woman with os odontoideum. Sagittal CT image shows os odontoideum (arrow), hyperplastic anterior arch of C1 (arrowhead), and small odontoid (asterisk). Notice gap between odontoid and os odontoideum.B, 86-year-old man with chronic type 2 odontoid fracture. Sagittal CT image shows that fracture fragment retains expected shape of odontoid (asterisk) and there is small gap (arrow) separating fragment from normal-appearing odontoid base.

Fig. 12—Pseudosubluxation of C2 on C3.A, 12-month-old boy with acute trauma. Lateral radiograph shows anterior location of C2 relative to C3 but normal posterior spinolaminar line (black line), which helps confirm that this is normal finding. Note normal anterior wedging of C3 vertebral body (arrow).B and C, 7-year-old boy involved in motor-vehicle collision. Sagittal CT image (B) shows pseudosubluxation of C2 on C3 (arrow). Sagittal MR image (C) shows no evidence of ligamentous injury (arrow).

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Fig. 13—68-year-old man with acute trauma. Axial CT image shows small cleft in anterior aspect of right transverse foramen of C5 (arrow). No other areas of lucency were identified to suggest presence of fracture.

Fig. 15—21-year-old man involved in fall. Sagittal CT image shows limbus vertebra at anterior-superior aspect of L3 vertebral body (arrow). Note sclerotic margins and that triangular fragment does not match with adjacent vertebral body, distinguishing this from acute fracture.

Fig. 14—Comparison of fractures and secondary ossification centers.A, 22-year-old woman involved in motor-vehicle collision. Sagittal CT image shows acute fracture through L4 spinous process (black arrow). Note irregular nonsclerotic margins. Secondary ossification center is present at tip of L3 spinous process (white arrow). Note smooth sclerotic margins.B, 19-year-old woman involved in motor-vehicle collision. Axial CT image shows acute fracture of right transverse process of C7 (arrow).C, 17-year-old man involved in motor-vehicle collision. Axial CT image shows secondary ossification center at tip of right transverse process of T7 (arrow). Note smooth corticated margins and compare with B.

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Fig. 17—52-year-old man involved in motor-vehicle collision.A and B, Sagittal CT images show apparent fracture through inferior facet of C5 and anterior subluxation of C5 on C6 (arrow). However, note similar offset involving anterior soft tissues and endotracheal tube (arrowhead, B). These findings were caused by motion artifact.

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Fig. 16—Locations and appearances of clefts. Note irregular sclerotic margins of clefts in B, C, and D.A, 42-year-old man with normal anatomy. Axial CT image shows location of retrosomatic cleft (1), spondylolysis (2), retroisthmic cleft (3), and spina bifida occulta (4).B, 33-year-old man with lumbar pain. Axial CT image shows retrosomatic cleft involving pedicle (arrow).C, 29-year-old man with lumbar pain. Axial CT shows spondylolysis bilaterally (arrows).D, 38-year-old woman with lumbar pain. Axial CT shows left-sided retroisthmic cleft (arrow).

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F O R Y O U R I N F O R M A T I O N

This article is available for CME/SAM credit. Log onto www.arrs.org, click on AJR (in the blue Publications box), click on the article name, add the article to the cart, and proceed through the checkout process.

The reader’s attention is directed to part 2 accompanying this article, titled “Imaging of Trauma: Part 2, Abdominal Trauma and Pregnancy—A Radiologist’s Guide to Doing What Is Best for the Mother and Baby,” which begins on page 1207.

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