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Published online before print May 30, 2002, 10.1148/radiol.2241011202
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(Radiology 2002;224:120-123.)
© RSNA, 2002

Pediatric Imaging

"Missing" Sternal Ossification Center: Potential Mimicker of Disease in Young Children1

William J. Rush, BS, Lane F. Donnelly, MD, Alan S. Brody, MD, Christopher G. Anton, MD and Stacy A. Poe, MS

1 From the Departments of Radiology (W.J.R., L.F.D., A.S.B., C.G.A.) and Pediatrics (L.F.D., A.S.B., C.G.A., S.A.P.), Children’s Hospital Medical Center, 3333 Burnet Ave, Cincinnati, OH 45229-3039. Received July 16, 2001; revision requested September 10; revision received November 15; accepted January 8, 2002. Address correspondence to L.F.D. (e-mail: Lane.donnelly@chmcc.org).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To evaluate the frequency of "missing" sternal ossification center (asynchronous nonossification) in young children.

MATERIALS AND METHODS: Lateral chest radiographs obtained in 229 children (mean age, 3.7 years) were retrospectively evaluated for sternal ossification. Four superior sternal segments were considered normal if they were ossified to a similar degree. A segment was considered asynchronous if decreased ossification, as compared with the remaining sternal segments, was demonstrated or if ossification was absent. Asynchronous ossification of inferior sternal segment 5 was recorded separately. Logistic regression analysis was applied to determine if there was a statistically significant relationship (P < .05) between age or sex and pattern of sternal ossification (normal vs asynchronous).

RESULTS: Of the 916 superior four sternal segments (four segments in each of 229 patients) evaluated, 32 (3.5%) showed asynchronously decreased or absent ossification. Locations of these 32 segments follow: segment 1, two (0.2%) instances; segment 2, 14 (1.5%) instances; segment 3, two (0.2%) instances; and segment 4, 14 (1.5%) instances. Inferior segment 5 was not ossified in 73 (31.9%) patients. There was a statistically significant relationship between decreased age and increased likelihood of occurrence of asynchronous ossification of one of the sternal ossification centers 1–4 (P > .003) and of occurrence of asynchronous ossification at sternal segment 2 (P < .018).

CONCLUSION: Missing sternal ossification centers occur most commonly at segments 2 and 4. Such asynchronous nonossifications become less common in older children.

© RSNA, 2002

Index terms: Bones, growth and development, 472.81 • Sternum, abnormalities, 472.81 • Thorax, MR, 472.121411


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In pediatric imaging, a lack of knowledge of the order of ossification can lead the radiologist to misinterpret the absence or presence of an ossified structure as an abnormality. The sternum is anatomically divided into three parts: the manubrium (sternal segment 1), the mesosternum (sternal segments 2–4), and the xiphoid (sternal segment 5) (1–9). Many investigators (1–9) have stated that sternal segments 1–4 demonstrate ossification centers at birth. The timing of ossification of the xiphoid, or sternal segment 5, varies much more, and the xiphoid may remain nonossified for years (1–9). Some researchers (1–9) think that asynchronous ossification of one of the superior four ossification sites may be suggestive of a number of disease processes.

We recently examined a 2-year-old boy in whom a chest radiograph was obtained because of cough. The chest radiograph revealed no ossification of sternal segment 2 and suggestion of soft-tissue fullness in this area (Fig 1). The asynchronous absence of ossification led us to suspect the presence of a destructive process, such as osteomyelitis or Langerhans cell histiocytosis. However, MR imaging demonstrated no mass and normal nonossified cartilage (Fig 1). At long-term follow-up, no further abnormalities were disclosed. Soon after this patient was examined, several other radiographs obtained in young children were evaluated, and on these images, asynchronous nonossified sternal segments were present. This series of events led us to suspect that asynchronous nonossification of sternal segments may be present much more commonly than findings in previous publications suggest. Thus, the purpose of our study was to evaluate the frequency of the "missing" sternal ossification center (asynchronous nonossification) in young children.



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  		Figure 1a. Radiographs obtained because of cough in a 2-year-old boy show missing sternal ossification center. (a) Lateral radiograph shows markedly decreased ossification of sternal segment 2 (arrows). The absence of ossification and the absence of soft-tissue prominence in the region were interpreted as suggestive of a destructive process. Numbers denote sternal segments. The region of sternal segment 5 (arrowhead) is also nonossified. Findings of a radiographic skeletal survey were otherwise normal. 1, 3, and 4 = sternal segments 1, 3, and 4. (b) Sagittal fast spin-echo magnetic resonance (MR) image (repetitition time msec/echo time msec, 3,000/15) with intermediate weighting shows no abnormality in the region of sternal segment 2 (arrows). (c) Sagittal fast spin-echo MR image (3,000/80) with T2 weighting shows high signal intensity in ossified sternal segments 1, 3, and 4. Sternal segments 2 (arrows) and 5 (arrowheads) demonstrate low signal intensity, which is consistent with asynchronous ossification.

 


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  		Figure 1b. Radiographs obtained because of cough in a 2-year-old boy show missing sternal ossification center. (a) Lateral radiograph shows markedly decreased ossification of sternal segment 2 (arrows). The absence of ossification and the absence of soft-tissue prominence in the region were interpreted as suggestive of a destructive process. Numbers denote sternal segments. The region of sternal segment 5 (arrowhead) is also nonossified. Findings of a radiographic skeletal survey were otherwise normal. 1, 3, and 4 = sternal segments 1, 3, and 4. (b) Sagittal fast spin-echo magnetic resonance (MR) image (repetitition time msec/echo time msec, 3,000/15) with intermediate weighting shows no abnormality in the region of sternal segment 2 (arrows). (c) Sagittal fast spin-echo MR image (3,000/80) with T2 weighting shows high signal intensity in ossified sternal segments 1, 3, and 4. Sternal segments 2 (arrows) and 5 (arrowheads) demonstrate low signal intensity, which is consistent with asynchronous ossification.

 


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  		Figure 1c. Radiographs obtained because of cough in a 2-year-old boy show missing sternal ossification center. (a) Lateral radiograph shows markedly decreased ossification of sternal segment 2 (arrows). The absence of ossification and the absence of soft-tissue prominence in the region were interpreted as suggestive of a destructive process. Numbers denote sternal segments. The region of sternal segment 5 (arrowhead) is also nonossified. Findings of a radiographic skeletal survey were otherwise normal. 1, 3, and 4 = sternal segments 1, 3, and 4. (b) Sagittal fast spin-echo magnetic resonance (MR) image (repetitition time msec/echo time msec, 3,000/15) with intermediate weighting shows no abnormality in the region of sternal segment 2 (arrows). (c) Sagittal fast spin-echo MR image (3,000/80) with T2 weighting shows high signal intensity in ossified sternal segments 1, 3, and 4. Sternal segments 2 (arrows) and 5 (arrowheads) demonstrate low signal intensity, which is consistent with asynchronous ossification.

 

   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The lateral chest radiographs of 229 young children (125 boys, 104 girls; mean age, 3.7 years; age range, 5 months to 10 years) were retrospectively evaluated for sternal ossification. All chest radiographs were obtained at one of our outpatient satellite imaging centers to as closely as possible simulate a normal population. These centers, as opposed to our central hospital, were chosen because a large percentage of patients who are evaluated with chest radiographs at our outpatient centers are otherwise healthy children who are being evaluated for respiratory tract infection. Patients at the central hospital tend to have more complex medical problems.

All consecutive lateral radiographs obtained over a 2-month period in children 10 years of age or younger were considered for the study. The lateral radiographs that were evaluated were obtained in patients in whom both frontal and lateral radiographs were obtained. Conventional radiographic technique was used. If the lateral radiograph was rotated to the extent that the reviewer could not evaluate the sternal segments, the subject was not included in the study. Patients who had an underlying condition that may have predisposed them to abnormalities of sternal ossification were excluded from the study. These included patients with congenital heart disease (4,10), patients who had previously undergone sternotomy or thoracotomy, patients with pectus excavatum or carinatum (4,8,9), or patients with underlying bone dysplasia (11–14).

The sternal segments were numbered 1 through 5: The manubrium was designated sternal segment 1, the mesosternum was designated sternal segments 2–4, and the xiphoid was designated sternal segment 5. The four superior sternal segments (sternal segments 1–4) were considered normal if they were ossified to a similar degree. A segment was considered asynchronous if ossification was either decreased, as compared with the appearance of the remaining sternal segments, or absent. If an ossification center was small in relation to a double ossification at that level, this was not considered asynchronous. Asynchronous ossification of the inferior fifth sternal segment (sternal segment 5) was also recorded. Chest radiographs were graded by one of three pediatric radiologists (L.F.D., A.S.B., C.G.A). The retrospective review of this material was approved by our hospital internal review board without informed consent from the parents or guardians of subjects in whom the chest radiographs were obtained.

Logistic regression analysis was applied to determine if there was a statistically significant relationship (P < .05) between age or sex and pattern of sternal ossification (normal vs asynchronous) for each of five different sternal ossification centers as well as for a combination of sternal sites. Two patterns of sternal asynchronous ossification were evaluated: (a) at least one asynchronous ossification in sternal segments 1–5, and (b) at least one asynchronous ossification in sternal segments 1–4. Sex could not be used as a factor in the regression analysis for sternal segment 3 because both subjects with asynchronous ossification were female.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Of the 916 superior four sternal segments (four segments in each of 229 subjects) evaluated, 32 (3.5%) showed asynchronously decreased or absent ossification. Locations included the following: sternal segment 1 (manubrium), two (0.2%); sternal segment 2, 14 (1.5%); sternal segment 3, two (0.2%); and sternal segment 4, 14 (1.5%). The inferior fifth segment was not ossified in 73 (31.9%) patients. Findings are illustrated in Figures 2–4.



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  		Figure 2. Lateral radiograph obtained in a 1-year-old boy shows nonossification of sternal segment 2 (2). Sternal segments 1, 3, 4, and 5 (1, 3, 4, and 5) are ossified.

 


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  		Figure 3. Lateral radiograph obtained in a 10-month-old girl shows lack of ossification of sternal segment 5. Sternal segments 1-4 (1-4) are ossified.

 


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  		Figure 4. Lateral radiograph shows lack of ossification of sternal segments 2, 4, and 5 (arrows) in a 1-year-old girl. Sternal segments 1 (1) and 3 (3) are ossified.

 
There was a statistically significant relationship between decreased age and increased likelihood of having asynchronous ossification of one of sternal segments 1–4 (P > .003). There was a statistically significant relationship between decreased age and increased likelihood of occurrence of asynchronous ossification at the second sternal segment (P < .018). There was no statistically significant relationship between age and frequency of asynchronous ossification of sternal segment 1 (P < .771), sternal segment 3 (P > .247), sternal segment 4 (P < .125), or sternal segment 5 (P > .630) or for segments 1–5 combined (P = .644).

There were no statistically significant relationships between sex and frequency of asynchronous ossification at any of the individual sternal segments or for combinations of sternal segments 1–4 or 1–5 (P > .1, for all comparisons).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In some articles (15–18), attention has been called to imaging the chest wall to depict chest wall deformities, malignant and inflammatory. Attention also has been called to normal variations in the configuration of the anterior chest wall that, when detected at physical examination, may lead the radiologist to perform cross-sectional imaging to exclude a mass (15–18). Findings in these articles suggest that a nonossified sternal segment is abnormal and is an indication for further imaging. Few investigators in recent articles have addressed the issue of timing of ossification of the sternal segments.

In many anatomic regions in young children, bone structures ossify in a predictable fashion. A lack of knowledge of the order of ossification can lead the radiologist to misinterpret the absence or presence of an ossified structure as an abnormality. One of the anatomic regions for which the importance of knowing the order of ossification for interpretation of radiographs is the elbow (19,20).

Ossification of the sternal segments has been reported to occur in the manubrium and mesosternum during fetal life (1–9). The manubrium is reported to ossify first, in the 6th fetal month (1–9). The mesosternum is reported to ossify in a craniocaudal direction, with ossification occurring first in segment 2 and last in segment 4 (1–9). Ossification is reported to be present at birth (1–9). In this series, asynchronous nonossified sternal segments were observed in 3.5% of subjects, occurring most commonly at the second (superior mesosternal) and fourth (inferior mesosternal) segments. The recognition that sternal segments may show asynchronous ossification has several clinically relevant implications.

First, normal variation should be considered when a missing segment is observed on a lateral chest radiograph obtained in a child. The normal variations in sternal ossification should not be confused with a destructive process involving the sternum. The diagnostic work-up that was performed in the child with the missing sternal ossification center we examined was based on the previously published data that the superior sternal segments should be equally ossified and present at birth. In a child without signs or symptoms of chest wall pathologic findings, a nonossified sternal segment can most likely be considered a normal variant, and appropriate follow-up can be performed. This is particularly true if the nonossified sternal segment is the second or fourth sternal segment, which in our series demonstrated nonossification most commonly. Decreased or absent ossification of the first or third sternal segment can occur as a normal variant but is much less common than asynchronous ossification of the second, fourth, and fifth segments. Therefore, decreased or absent ossification of the first or third sternal segment should be viewed with greater clinical suspicion that an abnormality is present.

Second, age was shown to have a statistically significant relationship to asynchronous ossification. It is not surprising that asynchronous ossification becomes less common with increasing age, since ossification is reported to typically occur during fetal life, and sternal segments are typically ossified completely by adulthood. When an asynchronous ossified sternal segment is observed in older patients, there should be greater suspicion that an underlying abnormality is present.

Third, there was no relationship between sex and asynchronous ossification.

Last, findings in a number of previous publications have suggested that certain diagnoses should be considered when a nonossified sternal segment is observed. Delayed sternal ossification has been reported to occur in infants with congenital heart disease (4,10) and in subjects with several of the bone dysplasias (11–14). Dysplasias associated with delayed sternal ossification include camptomelic dysplasia (14), Noonan syndrome, and trisomy 17–18 (12,13). One report (4) suggested that a delayed sternal ossification detected on a lateral chest radiograph in an infant with no other clinical abnormalities should be considered suggestive of congenital heart disease. We suggest that the possibility of a normal variation in sternal ossification should be considered first in such cases. The diagnostic accuracy of a missing sternal ossification center must be tempered with the possibility of normal variation.

Findings in reports (1–9) concerning the ossification of the xiphoid, the fifth sternal segment, vary. Findings in most reports show agreement that the fifth sternal segment is not typically ossified at birth. Some researchers (4) report that the xiphoid may remain cartilaginous for life, and others (5) suggest that the xiphoid is typically ossified at 6 years of age. In this series of children with a mean age of 3.7 years, the xiphoid was not ossified in 31.9% of cases. This suggests that the timing of ossification of the xiphoid varies much more than that of ossification of the more superior four sternal segments but that the xiphoid often ossifies during childhood. Also, in this study there was no statistically significant relationship between age and ossification of sternal segment 5 or ossification of one of the first through fifth segments. These data also challenge the presence of a straightforward relationship between age and ossification of sternal segment 5.

In conclusion, nonossified or "missing" sternal ossification centers may be normal variants on lateral chest radiographs obtained in children and should not be mistaken for destructive bone lesions or as a sign of underlying congenital heart disease or bone dysplasia. On the basis of findings in our study group, we conclude that asynchronous ossified segments are more commonly observed in younger subjects.


   FOOTNOTES
 
Author contributions: Guarantor of integrity of entire study, L.F.D.; study concepts and design, all authors; literature research, A.S.B., L.F.D.; clinical studies, all authors; data acquisition and analysis/interpretation, all authors; statistical analysis, S.A.P.; manuscript preparation, L.F.D.; manuscript definition of intellectual content, editing, and revision/review, all authors; manuscript final version approval, L.F.D.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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  15. Faro SH, Mahboubi S, Ortega W. CT diagnosis of rib anomalies, tumors, and infection in children. Clin Imaging 1993; 17:1-7.[CrossRef][Medline]
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  17. Donnelly LF, Frush DP, Foss JN, O’Hara SM, Bisset GS, III. Anterior chest wall: frequency of anatomic variations in children. Radiology 1999; 212:837-840.[Abstract/Free Full Text]
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