The Neonatal Ilium—Metaphyseal Drivers and Vascular Passengers

Trabecular bone architecture is known to be influenced by mechanical forces, which act to maintain and remodel the internal structural composition of bone into a biomechanically optimal form, which is capable of withstanding prevailing loading conditions (Lanyon, 1974, 1984; Turner, 1998; Ehrlich and Lanyon, 2002). These mechanical forces, which act through bone functional adaptation, influence bone morphology throughout growth and development. The skeleton is remodeled in response to localized forces, ultimately resulting in a relatively static and characteristic adult morphology (Turner, 1998; Huiskes et al., 2000; Carter and Beaupre, 2001; Gosman and Ketcham, 2009). In particular, trabecular bone architecture is known to respond to its environment and in essence develop into a load bearing structure from the earliest developmental period (Mulder et al., 2007). In light of this and because the ilium is an important structural component of the locomotor apparatus, it is important to gain a detailed insight into the trabecular arrangement of this bone from as early in the developmental spectrum as possible.

The implied mode of growth and ossification in the ilium has been well documented in the literature with studies documenting that initial ossification is observed between the end of the second and beginning of the third intrauterine month in the perichondrium superior to the greater sciatic notch (Bardeen, 1905; Adair, 1918; Noback, 1944; Gardner and Gray, 1950; Noback and Robertson, 1951; Laurenson, 1964a; O'Rahilly and Gardner, 1975; Birkner, 1978). From this initial site of bone formation, ossification is said to radiate superiorly over the internal and external surfaces of the iliac blade in an appositional manner (Laurenson, 1965; Birkner, 1978; Delaere et al., 1992). Following initial appositional ossification of the cortex, nutrient invasion initiates endochondral ossification (Delaere et al., 1992; Scheuer and Black, 2000). Nutrient vessel distribution and endochondral progression, from the point of dominant arterial invasion, located in proximity to the trabecular chiasma, is proposed to radiate in a uniform concentric pattern throughout the disintegrating cartilage anlage towards peripheral regions (Fig. 1).

Very little work has been directed towards trabecular and cortical bone structure and variation throughout ontogeny. This is particularly true with regards to the structural composition of the human pelvic complex (Dalstra et al., 1993). Previous work on the neonatal ilium has involved the application of various imaging techniques for the nondestructive evaluation of both internal architectural and external morphological features (Cunningham, 2009; Cunningham and Black, 2009a, 2009b, 2009c). An initial study applied plain-plate radiography to elucidate the overall gross bone morphology of the ilium and demonstrated the presence of defined density trajectories in the fetal and neonatal representations (Cunningham and Black, 2009a). These density trajectories held strong parallels with the adult representation, which had previously been attributed to the forces associated with direct stance related bipedal transfer (Kapandji, 1987, Aiello and Dean, 1990; Scheuer and Black, 2000; Martinon-Torres, 2003). This precocious structural patterning was hypothesized to have arisen from a combination of genetic and early nonload bearing functional influences, which combined to produce an “adult-like” gross bone pattern (Cunningham and Black, 2009a). However, because of the unexpected early appearance of these density trajectories, in the absence of direct load bearing, further analysis was undertaken to quantify the internal trabecular characteristics within the neonatal ilium (Cunningham and Black, 2009b). This was achieved through the application of micro-computed tomography (microCT), allowing for the nondestructive visualization of internal structural composition. Model-independent stereological analysis applied to the microCT data confirmed earlier two-dimensional radiographic findings by demonstrating a well defined structural patterning within the trabecular architecture of the fetal and neonatal ilium. This finding led to primary hypotheses of potential nonweight bearing functional interactions and consideration of the implications that progressing endochondral ossification had on the early pattern (Cunningham and Black, 2009b). Most recently, a study investigating the cortical arrangement of the neonatal ilium has added data regarding early variation in cortical thickness. This study demonstrated a graded pattern of cortical bone thickening in line with the expected growth pattern achieved through normal appositional ossification and developing muscle mass, that is, more peripheral regions showed thinner cortices, whereas more central areas displayed a generally thicker cortex (Cunningham and Black, 2009c). This communication builds upon these previous studies by further investigating both the cortical and trabecular structure of the neonatal ilium by analyzing specific relationships between adjacent volumes of interest (VOI).

This study investigates a cross-section of the available trabecular and cortical bone parameters used for structural quantification. These include bone volume fraction (BV/TV), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), trabecular number (Tb.N), structural model index (SMI), degree of anisotropy (DA), and cortical thickness. Each of these parameters yields important information regarding structural composition allowing for an interpretation of the functional influences encountered by the ilium (Parfitt et al., 1987).

MATERIALS AND METHODS

Selection of Volumes and Regions of interest

Trabecular analysis.

Trabecular analysis involved the selection of twenty-three volumes of interest (VOI 1–23) based on a uniform grid, which was superimposed onto the iliac surface using specific anatomical points as detailed in a previous communication (Cunningham and Black, 2009b) (Fig. 2). To identify and select the analysis field within a particular VOI, an elliptic volume was selected and placed in the appropriate grid position within a biphasic bone/nonbone region and interpolated throughout the 3D data set for each grid square. Once the VOI was in an appropriate position, bone parameters were calculated.

Cortical analysis.

Cortical analysis involved placement of the same grid using consistent anatomical points and was positioned onto the pelvic and gluteal surfaces of the ilium. However, to distinguish cortical analysis from trabecular analysis the term region of interest (ROI) was used to describe the two dimensional analysis transects for the cortical surfaces. Ultimately, regions of interest (ROI's) are directly comparable with VOI's. This grid contained twenty-three regions of interest (ROI 1–23), which allowed for comprehensive coverage of the cortical shells on respective surfaces of the ilium (Fig. 3). True cortical thickness measurements were defined from a point on the endosteal surface, where no trabecular struts were observed to anchor, extending to a parallel point on the periosteal surface (Fig. 4). The method applied for thickness assessment is documented in a previous communication (Cunningham and Black, 2009c).

Consistent efforts were made to position the analysis grids so that comparable VOI could be analyzed between specimens. It is acknowledged that there may be some degree of variation between individuals, however, it is suggested that this variation is minimal because of all ilia occupying the same developmental group and as such having very similar size and shape.

RESULTS

Trabecular Analysis

Trabecular structural parameters were obtained from histomorphometric analysis of three-dimensional slice data produced from microCT. Analysis of variance highlighted an overall significant difference between VOI's for each structural parameter (Table 1). Further to this, pairwise multiple comparisons permitted investigation into the relationship between individual regions. Overall statistical significance between all VOI's for each trabecular parameter is summarized in Fig. 5. To investigate specific regions of trabecular patterning, statistical analysis was restricted to adjacent VOI. For levels of statistical significance between each adjacent VOI see Fig. 6. Adjacent VOI that demonstrate a statistically significant difference between values are summarized in Fig. 7. Overall statistical significance, where a significant difference is observed in one or more trabecular indices is summarized in Fig. 8. The results for trabecular volume statistical analysis revealed a structural patterning which compartmentalized the trabecular volume into well defined regions.

Table 1. One way analysis of variance of between volume measurements

Trabecular Parameter	Statistic	Degrees of Freedom	P	Significance
BV/TV	H	22	<0.001	***
Tb.Th	H	22	<0.001	***
Tb.Sp	H	22	<0.001	***
Tb.N	H	22	<0.001	***
SMI	F	22	<0.001	***
DA	H	22	<0.001	***

• H = Kruskal–Wallis one-way ANOVA on ranks for nonparametric data; F = parametric ANOVA.
• *** = very highly significant difference between volumes of interest.

Statistical differences between adjacent VOI for each trabecular parameter investigated are presented:

Bone volume fraction.

Adjacent VOI displayed a statistically significant difference between values of bone volume fraction included: 5v9, 7v8, 10v15, 13v14, 15v16, 15v19, 16v20, 17v21, 18v22, and 19v23 (Fig. 6 and 7). In the neonatal ilium, statistically significant differences in BV/TV separated the trabecular volume into a distinct acetabular component (VOI's 21–23), which was characterized by high values of BV/TV, an anterior inferior component (VOI's 19 and 20), which demonstrated reduced BV/TV and an iliac blade component, which in itself contained a general demarcation of anteriorly and posteriorly associated volumes. Each of these regions was distinct from the trabecular chiasma (VOI's 14 and 15), which demonstrated a low value of BV/TV (18.909% and 19.013%, respectively). Within the iliac blade trabecular volume, statistically significant differences were observed between the central ilium (VOI 9) (21.710%) and the superior body (VOI 5) (34.415%), the trabecular chiasma (VOI 15) (19.013%) and central body (VOI 10) (30.857%), the posterior auricular (VOI 7) (34.866%) and cranial auricular volumes (VOI 8) (26.734%), the caudal auricular (VOI 13) (31.953%) and trabecular chiasma volume (VOI 14) (18.909%), as well as the anterior superior perimeter (VOI 16) (41.044%) and trabecular chiasma volumes (VOI 15) (19.013%). This division of the iliac trabecular volume from early in the developmental period, evidenced via BV/TV, demonstrates not only a compartmentalization of progressing ossification but also highlights a modeling gradient.

Trabecular thickness.

From the early developmental period distinguishable differences in Tb.Th are observed across the ilium. Adjacent VOI displayed a statistically significant difference between values of Tb.Th included: 1v4, 4v5, 9v14, 10v15, 14v18, 15v16, 17v18, and 17v21 (Fig. 6 and 7). Statistically significant differences are observed between regions associated with the greater sciatic notch (VOI 17) (0.204mm) and surrounding volumes (VOI's 18 and 21) (0.166 mm and 0.151 mm). Additionally the trabecular chiasma (VOI 14 and 15) (0.209 mm and 0.186 mm) is demarked by statistical differences between VOI's 9, 10, 16, and 18 (range of 0.148–0.166mm). Differences in Tb.Th were also observed between the superior perimeter of the iliac crest (VOI 1) (0.146 mm) and the superior body (VOI 4) (0.169 mm). Finally, differences were observed between volumes constituting the superior body (VOI's 4 and 5) (0.169 mm and 0.148 mm, respectively).

Trabecular separation.

Adjacent VOI displayed a statistically significant difference between values of Tb.Sp included: 1v4, 5v9, 10v15, 14v18, 15v16, 15v19, 16v20, 17v21, 18v22, and 19v23 (Fig. 6 and 7). Statistically significant differences in Tb.Sp between volumes, divide the ilium into an acetabular component (VOI's 21–23) (range of 0.218–0.239mm), an inferior body component (VOI's 18–20) (range of 0.317–0.341 mm), and an iliac blade component. Within the iliac blade there are further statistical differences, which are considered to separate the blade into anterior and posterior portions. These differences occur between the trabecular chiasma (VOI 15) (0.509 mm) and the anterior superior perimeter (VOI 16) (0.225 mm), the trabecular chiasma (VOI 15) and the central body (VOI 10) (0.27 mm), the central body (VOI 9) (0.359 mm) and the superior body (VOI 5) (0.252 mm) and finally between the superior body (VOI 4) (0.304 mm) and the superior perimeter (VOI 1) (0.226 mm).

Trabecular number.

Adjacent VOI displayed a statistically significant difference between values of Tb.N included: 5v9, 9v10, 10v15, 14v18, 15v16, 15v19, 16v20, 17v21, 18v22, and 19v23 (Fig. 6 and 7). In the neonatal ilium, statistically significant differences between values of Tb.N follow a similar pattern to that for Tb.Sp in that a defined acetabular component (VOI's 21–23) (range of 2.583–2.818 mm⁻¹), an inferior body region (VOI's 18–20) (range of 1.546–1.816 mm⁻¹), and a blade component of the ilium are observed. Within the trabecular volume of the blade there are demarcations between the central body (VOI 9) (1.405 mm⁻¹) and the superior body (VOI 5) (2.335 mm⁻¹), the trabecular chiasma (VOI 15) (1.046 mm⁻¹) and the central body (VOI 10) (2.087 mm⁻¹) as well as between the trabecular chiasma (VOI 15) (1.046 mm⁻¹) and the anterior superior perimeter (VOI 16) (2.63 mm⁻¹).

Structural model index.

Adjacent VOI displayed a statistically significant difference between values of SMI, although still within the range of values for a plate-like model of bone architecture included: 4v8, 9v14, 10v15, 13v14, 14v18, 15v16, 15v19, and 16v20 (Fig. 6 and 7). These statistical differences illustrate that the SMI for the trabecular chiasma volumes (VOI's 14 and 15) (2.011 and 1.977, respectively) are significantly higher in value than all surrounding volumes. Additionally, differences are observed between the anterior superior perimeter (VOI 16) (1.254) and the anterior inferior body (VOI 20) (1.678), as well as the cranial auricular volume (VOI 8) (1.632) and the superior body (VOI 4) (1.249).

Degree of anisotropy.

Adjacent VOI displayed a statistically significant difference between values for the DA included: 7v8, 8v13, 13v14, 14v18, 15v19, 16v20 (Fig 6 and 7). The statistical differences observed divide the ilium into a superior blade component and an inferior body/acetabular component through differences observed between the trabecular chiasma volumes (VOI's 14 and 15) (0.818 and 0.815, respectively) and the inferior body (VOI's 18 and 19) (0.538 and 0.520, respectively), as well as the anterior superior perimeter (VOI 16) (0.839) and the anterior inferior body (VOI 20) (0.629). Additional areas of significant difference are located between the cranial auricular volume (VOI 8) (0.861) and the posterior auricular volume (VOI 7) (0.684), the cranial auricular volume (VOI 8) (0.861) and caudal auricular volume (VOI 13) (0.583), and finally between the caudal auricular/trabecular chiasma volume (VOI 13) (0.583) and the trabecular chiasma (VOI 14) (0.818).

Collectively, statistical differences between adjacent VOI's appear to divide the trabecular volume into six regions, which are relatively isolated on the basis of trabecular characteristics. This differentiation can be visualized using a 2D sagittal slice through the ilium (Fig. 9). Each of these regions, identified through quantification and gross visualization, focus from a central region, previously referred to as the trabecular chiasma (Macchiarelli et al., 1999; Rook et al., 1999). The positioning of this trabecular chiasma coincides approximately with the location of the primary centre of ossification and dominant nutrient artery invasion (Region 1 on Fig. 9). The regions observed to radiate from this central locus are directed postero-superiorly, antero-superiorly, and inferiorly. The postero-superior directed radiation first presents as a volume of trabecular bone, which is less mature than that of the chiasma (Region 2a), and then as a terminal region of most recently modeled trabecular bone (Region 4). In addition to the recently modeled architecture in region 4, there appears to be a distinctive trabecular pattern associated with growth towards the greater sciatic notch. The antero-superior directed radiation is observed to extend towards the iliac crest as a volume of recently modeled trabecular bone (Region 3a). Finally, the inferiorly directed radiation is observed to extend towards the acetabular metaphyseal region, first presenting as a volume of less mature trabecular bone than that of the chiasma (Region 2b), then as a volume of most recently modeled trabecular bone (Region 3b). Each of these trabecular growth trajectories are summarized in Fig. 10. It must be noted that there are no significant differences between regions 2a and 2b other than in the DA. This is also true between regions 3a and 3b where the only statistically significant differences observed are those for DA. Each of the compartmentalized regions outlined will be fully discussed by relating bony morphology to specific functional influences.

Cortical analysis.

In addition to trabecular structural data, cortical thicknesses of the pelvic and gluteal iliac shells were obtained via linear measurement of cross-sectional microCT slice data. Cortical analysis demonstrated that there is a differential thickening of the iliac cortex between pelvic and gluteal shells. Summarized statistical significance is displayed in Table 2. Analysis of variance revealed that this statistical difference occurs in all regions of interest except the peripheral metaphyseal regions. This further adds to the evidence of a distinctive patterning of bone formation in relation to “growing” metaphyseal and “nongrowing” nonmetaphyseal regions. Continued modeling at peripheral metaphyseal growth regions is proposed to produce a continued renewal of the growing cortex leading to similar values of cortical thickness for the pelvic and gluteal shells in these regions.

Table 2. ANOVA between values of pelvic and gluteal cortical thicknesses for each ROI produced the H statistic and P-value

ROI	H statistic	P
1	0.12	0.733
2	1.17	0.28
3	167.96	<0.001*
4	172.06	<0.001*
5	174.71	<0.001*
6	179.47	<0.001*
7	168.88	<0.001*
8	179.75	<0.001*
9	176.39	<0.001*
10	179.08	<0.001*
11	0.61	0.434
12	182.43	<0.001*
13	179.45	<0.001*
14	179.35	<0.001*
15	179.55	<0.001*
16	1.51	0.219
17	179.41	<0.001*
18	173.39	<0.001*
19	173.54	<0.001*
20	171.62	<0.001*
21	0.05	0.821
22	3.72	0.054
23	2.27	0.132

• * denotes a statistically significant difference between cortices.

DISCUSSION

Initial bone modeling in the ilium occurs at specific metaphyseal growth fronts, which are present at the iliac crest and acetabular component (Scheuer and Black, 2000). It is proposed that initial modeling in the ilium is partly responsible for the characteristic trabecular architecture and cortical morphology observed during the early developmental period. Furthermore, it is suggested that modeling is partially controlled by “metaphyseal drivers,” which cause a radiation of trabecular and cortical bone growth from specific metaphyseal growth fronts. The statistical analysis for this study indicated the presence of three growth regions directed by metaphyseal drivers at the iliac crest (anterior and posterior) and acetabular component, and three “restricted growth regions” located at the anterior inferior iliac spine region, the greater sciatic notch region and the caudal auricular region where there are no metaphyseal growth fronts.

The pattern of bone architecture in the neonatal ilium is proposed to be a composite of regions of bone modeling and regions of bone remodeling. Firstly, during early bone development in the ilium, before the influences of remodeling, a baseline template is proposed to result from the initial modeling of the primary spongiosa, which is intimately related to primary chondrocyte arrangement, initial mineralization, vascular invasion of the tissue, and subsequent calcification. Several studies have demonstrated that the structure of the cartilaginous growth plate matrix and the subsequent primary trabecular bone structure are closely associated (Byers et al., 2000; Olsen et al., 2000). Subsequently, after formation of the primary spongiosa, the structure undergoes initial remodeling in response to developing functional forces and associated anatomical interactions, which occur concomitant with ongoing modeling at growth fronts.

It is therefore suggested that initial and continued modeling may be partially directed by metaphyseal drivers, which cause the trabecular architecture to adopt a specific morphology, which radiates from the centre of ossification towards growth fronts at the iliac crest and acetabular component. These metaphyseal drivers direct the formation of initial immature primary spongiosa and early thin cortical bone production. Further to this, subsequent remodeling is proposed to alter regions of more mature bone formation and have an influence on regions of the ilium, which are not constantly being extended by modeling. Additionally, it is suggested that initial modeling and subsequent remodeling of the trabecular bone may be influenced by the presence of the dominant vascular arrangement within the ilium, which is necessary to support active growth. As the ilium grows in size, its vascularity also increases, therefore, as vascular elements are space occupying tissues, they will be a driver for remodeling causing Tb.N to decrease, Tb.Sp to increase, bone volume fraction to decrease and will result in a compensatory increase in Tb.Th. This vascular arrangement is proposed to cause primary bone formation and subsequent remodeling to proceed around its distribution, ultimately dictating where the trabeculae will be arranged in broad terms.

Vascular invasion is thought to have a definite and profound influence on the trabecular morphology of the iliac cancellous bone during the earliest stages of development. Initial consideration is given to the presence of the dominant osteogenetic nutrient artery of the ilium and its function as an organizer of bone formation (Trueta, 1963). It can be hypothesized that the close relationship between the positioning of the vascular network and timing of vascular invasion, which initiates endochondral ossification, may have an impact on the structural organization of the proceeding ossifying architecture. Further to this, it is well documented that angiogenesis is closely associated with bone resorption and formation mechanisms (Brandi and Collin-Osdoby, 2006; Eriksen et al., 2007), resulting in a direct remodeling relationship. The dominant nutrient artery of the ilium is observed to enter the cortical bone at a location superior-medial to the greater sciatic notch, perhaps in response to angiogenic signals from disintegrating cartilage cells (Alini et al., 1996; Carlevaro et al., 1997; Petersen et al., 2002; Ortega et al., 2004). This artery proceeds by invading the underlying cartilaginous anlage, initiating the process of trabecular bone formation in a position corresponding to the future trabecular chiasma. Once inside the cartilaginous body of the ilium, the nutrient artery appears to bifurcate into a superior and inferior major branch. These respective branches radiate in a fan-like orientation superiorly into the iliac body towards the iliac crest and inferiorly into the acetabular component, indicative of the driving forces of the opposing metaphyseal growth plates. Arterial injection and Spalteholz clearing of this vascular network has intimated the positioning of the nutrient artery and its resultant bifid diffuse fanning (Crock, 1996). The distribution of arterial invasion ranges from the most substantial dominant vessels, which first enter the bone, to the fine arteriole network in the superior blade and acetabular regions (Fig. 11). As the presence of nutrient vessels is observed before the formation of the first trabecular elements (Laurenson, 1964b; Scheuer and Black, 2000), growth and remodeling of trabeculae proceed around the invading, proliferating and growing vessels. Furthermore, with continued growth and angiogenesis of the nutrient vessels, trabeculae may become further remodeled (Trueta, 1963). As the trabeculae become separated by the vascular elements it is necessary for structural strength to be regained so as to prevent the trabecular structure from becoming weakened leading to functional failure. Functional failure in the neonate would most likely arise from muscle pull on weakened plates that could perhaps separate.

To describe the pattern in more detail and in relation to the positioning of nutrient invasion and subsequent endochondral ossification it is appropriate to first consider the most mature area of initial bone formation followed by the pattern of radiating endochondral growth. Initial consideration is therefore given to the trabecular chiasma (Region 1) where the observed trabecular morphology is considered to be heavily influenced by proximity to the primary centre of ossification and the concomitant point of dominant nutrient invasion. This is reflected in the fact that the trabecular characteristics of the trabecular chiasma are statistically different from surrounding VOI's in one or more trabecular parameters. Within this region the largest nutrient vessels for the neonatal ilium will be present as they pierce the periosteum and diffusely radiate throughout the trabecular network (Brookes, 1971; Crock, 1996). The presence of this comparatively large vascular network is considered to have implications for the surrounding trabecular network resulting in a low BV/TV, which consists of a low number of thickened, well spaced trabeculae. This region is likely to be most affected by the vascular demand of the rest of the bone. Combined with increasing vascular demands, the proximity of the ossification centre is also considered to further contribute to the observed morphology. As this is the site of initial bone formation, this region has had an extended time for advanced modeling and potential remodeling of trabeculae associated with developing functional forces. This advanced development may serve to explain the more robust trabecular characteristics in this region. This region also displays significant cortical thickness on both pelvic and gluteal cortices, which supports the hypothesis that this region is the locus of primary ossification and a site of significant muscle attachment and remodeling activity (Rosse and Gaddum-Rosse, 1997).

Each subsequent compartment of trabecular architecture is observed to diverge from the trabecular chiasma in a pattern of radiating endochondral and periosteal growth. This has been confirmed by statistically significant differences in trabecular morphology between the trabecular chiasma and surrounding VOI's. The first of these to be considered are the regions, which radiate superiorly and inferiorly from the trabecular chiasma region and are represented by a trabecular morphology, which is considered to be less mature than that of the trabecular chiasma. Region 2a is observed to radiate postero-superiorly from the trabecular chiasma and region 2b is evident radiating antero-inferiorly from the trabecular chiasma (Fig. 10). These areas encompass VOIs 8 and 9 and 18–20, respectively, and are both quantifiable by an increased BV/TV, comprising increased Tb.N as well as decreased Tb.Th and Tb.Sp. The altered trabecular characteristics may be primarily attributed to the fact that this trabecular volume is less mature than the trabecular chiasma and as such has undergone reduced remodeling resulting in the observed trabecular characteristics. Additionally, the space occupying vascular supply within these regions has finer branches, which have not forced full remodeling of the trabecular scaffold. The metaphyses are heavily influential over vascular morphology and probably dictate the directionality of the vascular network towards the growth fronts. This is reflected in the fact that vascular presence is directed towards metaphyseal growth plates and to a lesser extent towards nonmetaphyseal regions. In region 2b, the morphology of the anterior section may be attributable to the fact that this region is associated with a “restricted growth” region where the endochondral ossification is not progressing towards a growth plate but is instead juxtaposed between the trabecular chiasma and a nonmetaphyseal cortex at the anterior inferior iliac spine. Therefore, the trabecular morphology within this region is different to that observed in surrounding growth regions resulting in the statistically significant differences between VOI; 14–18, 15–19, 16–20, 17–18, 18–22, and 19–23. When considering the vasculature of region 2b, it is somewhat different to that observed in “growing” regions as there are no significant branches of arterial supply directed antero-inferiorly (Fig. 11) (Crock, 1996). As the passage of the nutrient artery and its branches are proposed to be primarily directed by the demands from areas of rapid growth it not surprising that the arterial supply does not extend fully into these VOI's because of the absence of a growth plate. This is considered to contribute to the characteristic restricted growth trabecular morphology observed towards the anterior inferior iliac spine. The altered morphology can be visualized on a 2D sagittal microCT slice (Fig. 9). The endochondral progression within this region of the ilium is considered to be different from growing regions as there is no continuous modeling front and instead trabeculae, which have been laid down are now beginning to be remodeled into a more mature form. The only difference between regions 2a and 2b are their respective values for DA, where region 2b tends towards a more organized morphology. The fact that all other parameters are not statistically different fits with the theory that these regions represent modeling fronts behind the most mature region at the ossification centre. However, the more isotropic trabecular conformation in region 2b may be explained by its potential to be influenced by hypothesized retrograde limb movement forces (Cunningham and Black, 2009a, 2009b), which impose a requirement for early force alignment.

The subsequent regions of distinct trabecular morphology are observed to radiate antero-superiorly towards the iliac crest (Region 3a) from region 2a and the trabecular chiasma as well as inferiorly into the acetabular component (Region 3b) from the inferior body. Both of these regions have been termed peripheral growing regions because of their trabecular morphology, which is characterized by a high bone volume fraction consisting of a high number of thin, tightly packed trabeculae. This morphology is explainable as these volumes are directed towards the iliac crest and acetabular metaphyseal growth plates respectively, which are modeling fronts for advancing endochondral growth. Because of the continuous modeling towards growth regions during the neonatal developmental period these regions can be classified as growing. The presence of antero-superiorly and inferiorly directed branches of the dominant nutrient vessels (Fig. 11) (Crock, 1996), contributes to evidence that these regions of the ilium are actively growing through continuous modeling. These regions display very similar trabecular morphology reflected in all trabecular indices except DA. As discussed previously the more isotropic trabecular alignment in the acetabular component suggests that potential retrograde forces, proposed to be transmitted across the cartilaginous acetabulum from reflexive limb movement, may induce a precocious alteration within the trabecular architecture making it distinct from other newly modeled peripheral trabecular volumes in the crest region.

The final region of demarked trabecular morphology is observed postero-superiorly (Region 4). This is a large region, which encompasses VOI from the greater sciatic notch through to the posterior iliac crest. In this region, gross visualization of trabecular distribution from 2D microCT slices (Fig. 9), highlighted a differentiation in trabecular characteristics within the trabecular volume, which were not reflected in the statistical differences between quantitative values. This is surprising as it would be expected that this volume of trabeculae would reflect multiple influences including the position of the auricular surface, the proximity of the sciatic nerve, the modeling front associated with the postero-superior metaphyseal driver and any potential forces induced from reflexive limb or axial movements. However, these individual influences are indistinguishable as reflected by statistical differences in the trabecular architecture. This may be because of the proposed multiple influences being superimposed and ultimately masking one another, leading to the composite trabecular pattern observed which cannot be defined by statistically significant differences. Specifically, within region 4, the trabecular architecture associated with the greater sciatic notch may be described as a further restricted growth region. As described previously for the antero-inferior region, it is not expected that this trabecular volume should have a significant contribution from the trabecular arterial system due to its endochondral progression being directed towards a nonmetaphyseal border. This is once again confirmed by analysis of the iliac vascular distribution, where a defined arterial void is situated in the trabecular volume associated with the greater sciatic notch (Fig. 11) (Crock, 1996). Additionally, within the postero-superior perimeter of region 4 the trabecular architecture reflects a growing front where continued modeling results in a trabecular morphology characterized by a high bone volume fraction, again consisting of numerous, thin, tightly packed trabeculae. This is a similar morphology to that observed in the anterior superior and acetabular trabecular volumes.

As outlined, when considering the modeling progression superiorly within the iliac blade, respective anterior and posterior metaphyseal drivers can be hypothesized to control the proceeding growth at the iliac crest. This dual focused modeling progression divides the superior peripheral iliac trabecular volume into a distinct anterior and posterior extension. Therefore, these two trabecular areas have been termed growing regions and may explain the trabecular differences which divide the iliac blade into anterior and posterior extensions. It has previously been considered that the iliac crest epiphysis ossified from a midline point on the iliac crest and proceeded to extend anteriorly and posteriorly (Francis, 1940). However, this view has been expelled by the recognition that the iliac crest epiphysis instead ossifies from two individual ossification centers, which will later combine into a single epiphysis before fusion (Scheuer and Black, 2000). The anterior iliac crest epiphysis forms the anterior part of the crest and the posterior iliac crest epiphysis forms the posterior part of the crest (Stevenson, 1924). The positioning of the future anterior and posterior epiphysis corresponds to the two different regions of trabecular arrangement (regions 3a and 4). It is proposed that the formation of this bipartite iliac crest epiphysis and the associated underlying trabecular rays may be partly directed by the attachment of musculature to the iliac crest. Muscle attachment sites along the iliac crest can be divided into defined muscle compartments and individual muscles, which reflect the two parts of the iliac crest. Anteriorly, the internal and external oblique's and transversus abdominus muscles take attachment, whereas posteriorly, gluteus maximus, latissimus dorsi, quadratus lumborum, and erector spinae muscles take attachment (Rosse and Gaddum-Rosse, 1997). The differing forces induced by these distinct muscle groups may act to partly induce the trabecular differences observed between the two regions.

The presence of two iliac crest epiphyses suggests the presence of two separate, but synchronized, growth fronts for which there must be an associated growth trajectory of ensuing endochondral ossification and an associated nutrient supply. The presence of a bidirectional growth trajectory from the trabecular chiasma and the presence of a vascular supply for each trajectory (Fig. 11) (Crock, 1996), may explain the trabecular differences between these regions. Differences in vascular distribution between regions 3a and 4 can be visualized in Fig. 11 where there is a defined increased vascular density associated with the posterior region. Additionally, gross visualization of this pattern can be observed on a sagittal microCT cross-section (Fig. 9). Within each of the anterior and posterior growth trajectories there is a defined modeling gradient, which although not reaching statistical significance between most VOI's, is evident from inspection of absolute values. This gradient is reflected in an increasing BV/TV and Tb.N towards the periphery combined with a gradual decrease in Tb.Th and Tb.Sp. This modeling gradient may be attributable to the pattern of trabecular formation mirroring the arrangement of mineralized cartilage columns at the metaphyseal growing front (Byers et al., 2000; Olsen et al., 2000). This results in peripheral regions maintaining a newly modeled trabecular appearance, which gradually becomes influenced by functional remodeling and advanced modeling towards the trabecular chiasma.

Further adding to the theory of multiple regions of growing metaphyseal and restricted growing nonmetaphyseal origin is the statistical significance observed between values of cortical thickness for pelvic and gluteal cortices. This analysis demonstrated that no statistical difference was observed for cortical values between pelvic and gluteal shells in growing perimeter regions (ROI's 1, 2, 11, 16, 21, 22, 23). This may suggest that there is a continuous modeling front laying down new bone, which has yet to experience any significant remodeling influences. Conversely, a statistically significant difference between pelvic and gluteal cortices in proposed restricted growth peripheral regions was observed. It is suggested that this differential thickening is because of restricted growth regions being subjected to remodeling forces, opposed to renew appositional modeling, because of their distance from an active growth zone.

The results of this study clearly do not take into account the effects (if any) on both the cortical and trabecular bone when the child commences locomotion, stance, and bipedality. This work will now be extended into the older age ranges to assess the effects of increasing mobility on the architecture of the ilium.

CONCLUSION

This study proposes a revised model for progressive endochondral ossification in the neonatal ilium. The presence of defined regions of distinguishable trabecular characteristics arising from a central trabecular chiasma suggests that there is a compartmentalized radial growth. This growth is observed to extend towards growth plates in growing trabecular trajectories and towards nonmetaphyseal peripheral regions in “nongrowing” trabecular rays. This pattern of growth has been related to the presence, positioning, and expansion of the vascular supply to the ilium of which dominant invading branches and subsequent radial fanning correspond well with the trabecular compartmentalization observed. Additionally, within growing compartments a modeling gradient is observed that is indicative of primary trabecular ossification, which mirrors the columnar alignment of the initial cartilaginous template in peripheral regions. This primary spongiosa is then gradually remodeled in response to temporal functional interactions ultimately converging towards the most advanced region of growth at the trabecular chiasma. Therefore, this study contributes towards an advanced understanding of the trabecular patterning and progressing ossification within the ilium during the neonatal developmental period.