Postmortem MRI Study On The Normal Developmental Pattern Of Sacrococcygeal Primary Ossification Centers In The Fetal Spine

The sacrum is a wedge-shaped bone that is wider above at its base and narrower below at its apex.The sacrum acts as the central axis of the pelvic girdle as it articulates with the last lumbar vertebra superiorly at the lumbosacral angle,and with the ilium laterally at the sacroiliac joints,as well as with the first coccygeal segment inferiorly.The development of the vertebral column can be divided into three successive phases:the blastemal stage,the chondrogenesis stage,and the osteogenesis stage.During the early embryonic period,a sacral vertebra appears to resemble a lumbar vertebra in all essential particulars.Only after the development of the blastema for bilateral costal elements can a sacral vertebra be distinguished from a lumbar vertebra.At Carnegie stage 19,separate chondrification centers develop in each of the sacral costal elements.At stage 21,the extremities of the upper three sacral costal element chondrification centers have fused with one another and given rise to a continuous cartilaginous auricular surface.The ossification pattern in the sacrum is complex,as the sacrum develops from approximately 21 separate primary ossification centers.Each of the five sacral segments is represented by the usual three primary ossification centers seen in all pre-sacral vertebrae-one ventral center for the centrum and two dorsal-lateral centers for each side of the neural arch.In addition to these,the first to third,and sometimes fourth,sacral segments also incorporate paired costal(lateral)element ossification centers,which located superolateral to their corresponding anterior sacral foramina.However,to date,little is known regarding the normal developmental pattern of sacral primary ossification centers during the fetal period.Moreover,there is a lack of information about the ossification of the coccyx during the fetal period.The caudal neural tube is the last to close,thus,the lumbosacral region is the most common region affected by neural tube defects and congenital spinal anomalies,e.g.,sacral agenesis,spina bifida,hemivertebra,butterfly vertebra,and osteochondral dysplasia.A wide range of manifestations of caudal regression syndrome has been described,from mild sacral dysplasia to sirenomelia('mermaid-like').However,the milder forms are much more common and are more difficult to be detected with prenatal imaging,especially in the first and early second trimester.Spina bifida refers to the defective development of meninges,posterior neural arches,paraspinal muscles,fascia,and/or skin due to derangement of the superficial ectoderm and neural ectoderm separation,which results from a failure in neural tube closure.According to whether neural tissue and/or meninges is exposed to the environment through a congenital bony defect,spina bifida is classically categorized as open and closed forms.Due to folate acid supplement and the application of effective antenatal screening programs,the birth rate of spinal bifida has decreased sharply.However,to date,spina bifida still account for a large portion of congenital anomalies.In-utero US is still the main imaging technique used in the prenatal diagnosis and monitoring of fetal spinal anomalies.However,as the development of MR faster imaging sequences including single-shot fast spin-echo(SSFSE),half Fourier acquisition single shot turbo spin-echo(HASTE),echo planar imaging(EPI),and susceptibility-weighted imaging(SWI),fetal spine MRI are increasingly being used in evaluating fetal spine with US suspicious spinal abnormalities or with indications for in utero spina bifida repairment.Excellent detail and soft tissue resolution can be obtained with fetal MRI,whereas fetal US may be more accurate in delineating bony structure.Furthermore,MRI can detect associated central nervous system(CNS)and non-CNS anomalies,especially Arnold-Chiari II malformation.Therefore,the combination use of these two techniques is conducive to a more accurate and comprehensive diagnosis of spinal abnormalities.Several studies,including histologic,radiographic,and US studies,have been performed to investigate the temporal and spatial developmental pattern for the centrum and neural arch ossification centers in the fetal spine.Centrum and neural arch ossification centers develop independently.On the one hand,the first spinal centrum ossification center(COC)appears in the lower thoracic-upper lumbar region,and then,further centers appear in both the cephalic and caudal directions.The development of COCs proceeds more slowly in the caudal than in the cranial direction;thus,the S5 COC is the last to appear.On the other hand,although literatures have not reached a consensus about the orderly craniocaudal sequence of appearance of neural arch ossification centers(NAOCs)in the fetal spine,especially within the nine most proximal vertebrae or even from the cervical to the upper lumbar spine,and at least three patterns have been postulated,it is generally believed that from the lower lumbar spine NAOC sequentially ossify downwards into the sacral spine.In general,compared with those in other spinal regions,sacral neural arch ossification centers,especially those of the S2-S5 vertebrae,develop later.However,there is still a lack of detailed information on the precise timing of appearance of individual sacral COCs and NAOCs,and also the existed data obtained from studies using different techniques vary significantly.To date,as far as the sacrum growth during the fetal period was concerned,only data on the sacral length and the morphometric parameters of the sacral COCs and NAOCs in fetuses at the gestational ages of 17-30 weeks,have been provided by previous researchers.Two previous postmortem MRI studies have demonstrated thatT2-weighted fast spin-echo and turbo spin-echo images can differentiate osseous from cartilaginous structures in the fetal lumbar spine.Other than the above two studies,no MRI study on the primary ossification centers of the fetal spine is available.Therefore,with the more widely use of in utero fetal spinal MRI,studies on the normal appearance time and morphology of the sacral COCs and NAOCs on MR images should be carried out to fill this gap.Three-dimensional dual-echo steady-state with water excitation(3D DESS with WE)sequences are primarily used for cartilage imaging,as these sequences have a high signal-to-noise ratio and the highest contrast-to-noise ratio for cartilage.On 3D DESS WE images,cartilage has intermediate signal intensity compared to osseous structures,which appear hypointense.As each fetal vertebra is composed of centralized ossification centers surrounded by cartilage,3D DESS WE sequence can be applied to fetal spinal imaging for studying the primary ossification centers.Moreover,the isotropic,high-resolution voxels obtained using this sequence enable 3D reformatting ofthe images into more convenient planes for visualizing the complex fetal sacral spine.After reviewing the literature on sacral costal elements,we found that the blastemal and chondrification stages in the developmental process of the costal elements of the sacral vertebrae have been studied to some extent and the origin of the chondrified costal elements within the sacral vertebrae is still under debate,that is,whether the structure located superolateral to their respective anterior sacral foramina in the sacral region and costal elements in the other spinal regions are same in origin.Since the ossification of the costal element occurs within the cartilaginous template,the origin of the sacral costal element ossification centers(CEOCs)has not yet been clarified either.To date,information about the ossification stage of development is still scant,and there is no consensus yet on the sacral levels at which CEOCs are normally present before birth.In addition,there are only limited reports available on the timeline of the appearance of the sacral CEOCs in the fetal period.Some authors described that the paired CEOCs of S1-S3 vertebrae appear at 6-8 months of fetal age,however,no accurate data for the timing of appearance of the CEOCs at each sacral level have been provided.Furthermore,no further quantitative study on the morphology and relative positions of the sacral CEOCs could be found,which may be useful for the prenatal diagnosis of skeletal dysplasias.Finally,investigation of the developmental aspects of the sacral CEOCs might shed light on the formation of the anterior sacral foramina and the sacroiliac joints.There is therefore a real need for detailed information on the normal development of the sacral CEOCs during the fetal period.When it comes to the ossification of the coccyx during the fetal period,there is very little information available.Classical anatomic books described that the ossification center for the first coccygeal vertebral body appears either towards the end of fetal life or certainly within the first year of life.A previous study found that a coccygeal ossification center was visible in 24(14.7%)newborn infants(mean GA,35 weeks),all of whom had five sacral COCs present.In view of these two ambiguous results,further investigations on the developmental pattern for the primary ossification centers of coccyx during the fetal period should be carried out.Exclusion of congenital spinal anomalies on US and MR images depends primarily on whether one normal ventral center for the centrum and two normal dorsal-lateral centers for each side of the neural arch can be visualized in the transverse plane.However,there is still a lack of anatomical basis for determing whether the development of a sacrococcygeal primary ossification center in fetus of a certain gestational age is normal or not.As a consequence,the temporal developmental pattern for individual sacrococcygeal primary ossification centers and the age-specific normative reference values for the morphometric parameters of individual sacral primary ossification centers are urgently needed.The aim of the present study was to describe the temporal pattern of appearance of individual sacrococcygeal primary ossification centers and provide the age-specific,normative MRI reference value for sacral length,the morphometric parameters of the S1-S5 COCs and NAOCs,as well as for the volume and the relative positions of the S1-S3 CEOCs,through the postmortem MR imaging of the entire spine in 17-42-week-old fetuses using the 3D DESS WE sequence.The data obtained from this study can supplement normal data on fetal sacrococcygeal vertebrae development and provide morphological basis for prenatal diagnosis and treatment of congenital spinal malformations and neural tube defects in the sacrococcygeal region.This study contains three parts,discussing the normal developmental pattern of the sacrococcygeal centrum ossification centers,the sacral neural arch ossification centers,and the sacral costal element ossification centers,separately,as follows:Part I.Normal developmental pattern of sacrococcygeal centrum ossification centersObjective:To describe the temporal pattern of the appearance of the S1-Cal centrum ossification centers(COCs)and provide reference data for the S1-S5 COCs and sacral length at various gestational ages(GAs).Materials and Methods:Postmortem magnetic resonance imaging(MRI)was performed on 71 fetuses(GA,17-42 weeks)using the 3D dual-echo steady-state with water excitation T2 sequence in the sagittal plane.To confirm the reliability of this sequence,the MRI data were compared with the CT and histologic data obtained from two fetuses(GAs,21 and 30 weeks).The presence or absence of each sacrococcygeal COC was recorded.Sacral length and S1-S5 COC height,sagittal diameter,transverse diameter,cross-sectional area,and volume were measured.Results:All fetuses showed S1-S3 COCs by 17 weeks,S4 COCs by 19 weeks,and S5 COCs by 28 weeks.The S4,S5,and Co-1 COCs were visualized in 70(98.59%),51(71.83%),and 21(29.58%)fetuses,respectively.Sacral length,height,sagittal and transverse diameters increased linearly,while cross-sectional area and volume increased exponentially with advancing GA.Mean growth rates of the sagittal and transverse diameters,cross-sectional area,and volume,but not of height,significantly differed among the S1-S5 vertebrae.Conclusions:We have presented the timing of appearance of individual sacrococcygeal COCs and the age-specific,normative MRI reference values for sacral length and the morphometric parameters of the sacral COCs,which are of clinical importance in the diagnosis of congenital sacral abnormalities and skeletal dysplasia.Part ?.Normal developmental pattern of sacral neural arch ossification centersObjective:This postmortem magnetic resonance imaging(MRI)study of the fetal spine aimed to describe the timing of appearance of the S1-S5 neural arch ossification centers(NAOCs)and provide reference data for the size of the S1-S5 NAOCs at various gestational ages(GAs).Materials and Methods:We obtained sagittal 3D dual-echo steady-state with water excitation T2 images of the entire spine in 71 fetuses(GAs,17-42 weeks).To confirm the reliability of this sequence,the MRI data were compared with the CT and histologic data obtained from two fetuses(GAs,21 and 30 weeks).The presence/absence of each sacral NAOC was recorded according to the GA.Length,width,cross-sectional area,and volume of S1-S5 NAOCs were measured.Results:No sex or right-left differences were found in the percentages of detection and morphometric parameters of the S1-S5 NAOCs.S1-S3 NAOCs first appeared at 17 weeks of GA,while S4 and S5 NAOCs first appeared at 18 weeks and 23 weeks of GA,respectively.The GAs from which all studied fetuses with detected NAOCs,by vertebral level,were as follows:S1,17 weeks;S2,19 weeks;S3,22 weeks;S4,23 weeks;and S5,39 weeks.S1-S4 NAOCs increased linearly in length and width,whereas exponentially in cross-sectional area and volume with advancing GA.There were significant pairwise differences between all sacral vertebral levels in the least-squares means of length,width,cross-sectional area,and volume(p<0.05),except between S4 and S5 in width(p = 0.188).Mean growth rates for length,width,cross-sectional area,and volume significantly declined from S1 to S5,with that for width in S3 slightly greater than in S2 as an exception.Conclusions:We have presented the timing of appearance of each sacral NAOC and the age-specific,normative MRI reference values for the morphometric parameters of the S1-S5 NAOCs,which are essential for the prenatal diagnosis of spina bifida and skeletal dysplasias.Part III.Normal developmental pattern of sacral costal element ossification centersObjective:This postmortem magnetic resonance imaging(MRI)study of the fetal spine aimed to describe the timing of appearance,shape,volume,and relative positions of the S1-S3 costal element ossification centers(CEOCs).Materials and Methods:We obtained sagittal 3D dual-echo steady-state with water excitation T2 images of the entire spine in 71 fetuses(gestational ages[GAs],17-42 weeks).Computed tomography and histological examinations were performed on two fetal specimens(GAs,21 and 30 weeks)to validate the MR images.The presence/absence of each sacral CEOC was recorded according to the GA.CEOC volume was measured.The relative positions of the CEOCs were assessed using distance A,distance B,and the BA ratio.Horizontal distance A was measured between the CEOC midpoint and the midpoint of the vertebral column at the same horizontal level.We drew a line that was perpendicular to the ipsilateral ilial articular surface and passed through the CEOC midpoint.The vertical distance B was measured between the CEOC midpoint and the point at which the perpendicular line intersected the ilial articular surface.BA ratio was calculated.Results:No sex or right-left differences were found in the percentage of detection,volume.and relative positions of the S1-S3 CEOCs.The S1,S2,and S3 CEOCs first appeared at 23,22,and 29 weeks,respectively.The S1 and S2 CEOCs could be detected in all fetuses with GAs of>30 weeks and>35 weeks,respectively,while the S3 CEOCs were variably present until term.The percentages of detection of the S1 and S2 CEOCs were significantly greater than that of the S3 CEOCs at each GA.At S1 and S2,CEOC volume increased exponentially with GA,while distances A and B increased linearly.The BA ratios at SI-S3 remained mostly constant.For both sides,the BA ratio was significantly greater for S1 than for S2(P<0.05)or S3(P<0.05).Conclusions:We have described the timeline of appearance as well as the volume and position of the S1,S2,and S3 CEOCs in the fetal spine on postmortem MRI according to GA.