The relative influence of vertebral body and intervertebral disc shape on thoracic kyphosis

Clinical Biomechanics 14 (1999) 439±448

The relative in¯uence of vertebral body and intervertebral disc shape on thoracic kyphosis S. Goh a, R.I. Price b, P.J. Leedman c, K.P. Singer a,d,* a

School of Physiotherapy, Curtin University of Technology, Selby Street, Shenton Park, Western Australia 6008, Australia b Medical Technology and Physics, Sir Charles Gairdner Hospital, Western Australia c University Department of Medicine, The University of Western Australia, Western Australia d Department of Imaging Services, Royal Perth Hospital, Western Australia Received 15 September 1998; accepted 25 November 1998

Abstract Objective. The aim of this study was to quantify the morphology or shape of thoracic vertebral bodies and intervertebral discs, and to examine the ex vivo association of thoracic kyphosis with these shape parameters. Design. A quantitative, retrospective study design was applied to de®ne vertebral body and disc in¯uences on thoracic kyphosis. Background. Age-related progression of thoracic kyphosis is a well-de®ned process that is in¯uenced by the morphology of vertebral bodies. However, little is known about the contribution of intervertebral disc shape to the thoracic curvature. Methods. Vertebral and disc morphology, as represented by antero-posterior height ratios, were quanti®ed in 93 lateral spine radiographs and midsagittal computed tomography ®lms of ex vivo spines. Kyphosis was indicated by the Cobb angle. Linear and stepwise regression were applied to examine relationships for cumulative (T1±T12) and regional (T4±T9) analyses. Results. Vertebral morphology was highly predictive of thoracic curvature, while a poorer association was noted for disc morphology. The combined in¯uence of both accounted for >85% of the variability in kyphosis. There was a trend for a more pronounced anterior wedge con®guration of the midthoracic vertebral bodies and discs. Higher associations between variables were also noted in this region. Conclusions. The normal kyphosis of the thoracic spine re¯ects the morphological adaptation of both the vertebral bodies and intervertebral discs. Relevance This study contributes new data on the thoracic spine, particularly the characteristics of thoracic discs and their contribution to kyphosis genesis. Future directions for morphology studies should encompass more detailed examination of the thoracic discs and greater emphasis on the midthoracic segments, considering the prevalence of osteoporosis related fractures and subsequent deformity at these levels. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Thoracic; Kyphosis; Sagittal curvature; Vertebral body; Disc; Morphology

1. Introduction Evolution of the human spine with its characteristic S-shaped sagittal curve is seen as an adaptation necessary for maintenance of upright posture. Due to the physiological kyphosis of the thoracic column, the line of gravity typically lies anterior to the vertebrae, imposing mechanical loads on the anterior aspect of the vertebral bodies. The cumulative e€ects of these loads across the lifespan result in progression of kyphotic *

Corresponding author. E-mail: [email protected]

deformity [1±3]. In individuals with osteoporosis, this e€ect is accentuated [4], and often results in vertebral fracture, associated morbidity [5] and substantial economic costs [6]. Despite this, relatively less research effort has been directed towards the thoracic vertebral column, where the majority of spinal osteoporotic fractures occur. Thoracic spinal curvature is largely a function of vertebral body morphology [7±10]. This is attributed to the naturally inclined con®guration of the vertebral bodies which accounts for the anterior concavity of the thoracic curve [7]. In senile osteoporosis, the tendency for collapse of vertebral trabecular bone results in ac-

0268-0033/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 8 - 0 0 3 3 ( 9 8 ) 0 0 1 0 5 - 3

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centuated anterior wedging of the bony elements accompanied by increased kyphotic deformity [4]. Commonly there is associated vertebral fracture, particularly of the midthoracic vertebrae [4,11,12]. It is dicult however, to ascertain the nature of the contribution of the thoracic intervertebral discs in kyphosis genesis, due to the paucity of information on these discs. Little is known about their patterns of degeneration, structural change, or how they di€er across segments. However, preliminary evidence demonstrates greater change in the midthoracic discs, and gender di€erences in the prevalence of degeneration and herniation [13,14]. Furthermore, speculation suggests that greater torque in the midthoracic discs may in¯uence the disc degenerative process [15]. One recent X-ray study involving 100 asymptomatic female volunteers provided preliminary evidence that age-related kyphosis is a function of disc integrity as much as it is of vertebral morphology [8]. These ®ndings agree with hallmark pathological studies by Schmorl and Junghanns [9]. The authors proposed that progression of age-related kyphosis is based on typical changes in the intervertebral discs associated with loss of anterior height, particularly in males without osteoporosis. These observations prompt the consideration of further investigations of the thoracic discs, their potential role in determining curvature, and the in¯uence of gender within these relationships. Much literature concerns the deformation of the thoracic vertebral bodies, particularly as a means of documenting the prevalence and progression of spinal osteoporotic fractures [8,11,12,16]. However, standardisation of measurement protocols adopted for the quanti®cation of shape parameters has yet to be de®ned. There remains a lack of consistency in de®ning geometric landmarks within vertebrae and also technical limitations associated with in vivo radiographic techniques involving the thoracic spine. Distortion of the radiographic image may arise from the in¯uence of magni®cation, o€-centre imaging, and lateral tilt and rotation of the spine due to subject positioning [17]. In the upper thoracic region, osteopenia of vertebral bodies may result in poor image contrast in lateral view radiographs. The diverse nature of ®ndings presented in spinal morphometry studies presents considerable diculties in attempts to draw valid conclusions and limits the application of these results across a number of studies. Recently, a more accurate means of quantifying measurement of vertebral and disc morphology based on selection of corner landmarks was proposed [17,18]. Application of these principles may enable improved precision, objectivity, and reliability of spinal morphometry studies. Furthermore, in view of the challenges posed by in vivo techniques, the bene®ts a€orded by ex vivo studies may be worth considering. This approach has been adopted in recent research by Edmondston

et al., who examined various aspects of the thoracic spine from an ex vivo perspective [10,19±22]. Their investigations have generated additional insight into various aspects of thoracic curvature, vertebral morphology, vertebral compressive strength, and thoracic bone mineral content (BMC) and bone mineral density (BMD). This study investigated in greater detail, the ex vivo characteristics of thoracic sagittal curvature and morphology features of the thoracic vertebral bodies and intervertebral discs. The association of thoracic curvature with morphology and the in¯uence of gender within these relationships were examined. Segmental trends within the thoracic column were also investigated in a quantitative manner. 2. Methods 2.1. Study sample, inclusion/exclusion criteria Ex vivo cases from a database of sagittal computed tomography (CT) ®lms and lateral spine radiographs were considered for the study. Thoracolumbar vertebral columns with evidence of marked vertebral body pathology, severe osteophytic formation, scoliosis, spinal fracture or poorly de®ned vertebral margins were excluded. Postmortem records were reviewed to exclude cases with a history of metabolic disease, neoplasm or trauma. A total of 93 ex vivo cases (35 females, 58 males) were available for the study, comprising lateral view contact radiographs of 51 hemisected cadaveric spines, midsagittal thoracic CT scans of 12 cases obtained from a previous study [10], and lateral view contact radiographs of 30 whole vertebral columns, of which para-sagittal bone sections were available in 17 of these cases for direct comparison of morphology measurements (Fig. 1[A]) [19]. 2.2. Measurement and instrumentation Geometric measures were derived from corner landmarks located on superior and inferior endplate rim contours for each vertebral body (Fig. 1[B] and [C]) [18]. These were marked on tracing paper superimposed on the radiographic ®lm. For whole spine radiographs, mean dorsal vertebral height was derived from four dorsal corners [17], while two distinguishable corners were marked dorsally for the hemisected and CT series radiographs. The extent of the anteriorly wedged con®guration for discs and vertebral bodies was expressed as anterior to posterior height ratios. For the purpose of this study, the term `wedging' represents the morphological con®guration of `unfractured' vertebrae. Height variables were measured using an electronic digital caliper (NSK, Max-Cal, Japan Micrometer MFG, Osaka,

S. Goh et al. / Clinical Biomechanics 14 (1999) 439±448

Fig. 1. [A] Photograph of a para±sagittal thoracic vertebral bone section from T5. [B] Lateral radiograph of a hemisected spine from which morphology measurements were derived. [C] Anterior and posterior disc and vertebral body heights were measured from corner landmarks (indicated by arrows). The inferior dorsal corner is marked at the periphery of the vertebral silhouette. The superior landmark lies at the junction of the pedicle and the end plate, and is marked slightly inferiorly to exclude the `uncinate-like' process [18]. Osteophytes were excluded by selecting the intersection of the anterior vertebral border with superior and inferior end plates.

Japan). The extent of thoracic kyphosis, represented by the modi®ed Cobb angle method (Fig. 2) [23], was measured for both cumulative (T1±T12) and regional (T4±T9) thoracic regions. 2.3. Data reduction and statistical analyses Descriptive statistics were used to evaluate trends in measured parameters. Least squares regression analysis was performed to examine the magnitude of association between variables, while the extent to which morphology parameters were predictive of kyphotic deformity was evaluated using stepwise regression. Analyses were performed on pooled data, and then separately for males and females. Lines of best ®t for both genders were compared using analysis of covariance (A N C O V A ) [24]. Both cumulative (T1±T12) and regional (T4±T9)

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Fig. 2. Illustration of anterior wedge deformity of thoracic vertebrae and the accompanying progression in kyphosis. The nature of axial loading forces imposed on the anterior concavity results in a tendency for accentuation of deformity in the midthoracic region. Thoracic sagittal curvature was derived from perpendiculars extended from lines drawn through superior landmark markings of the selected cranial vertebrae and inferior markings of the selected caudal vertebrae (T1± T12 and T4±T9, respectively for cumulative and regional analyses). The resulting Cobb angle was measured from the intersection of the two perpendiculars.

data were analysed. A probability level of P < 0.05 (twotailed) was adopted as the criterion for accepting statistical di€erences. 2.4. Intraexaminer reliability Height measurements were repeated on two further occasions on four radiographs from the hemisected series, and four from the whole spine series. Separate tracings were performed on each occasion. In each case, measurements were taken from six adjacent vertebral levels. Accuracy in deriving landmarks from in vivo chest radiographs and para-sagittal histomorphometric bone sections were also examined. Reliability indices were derived from intraclass correlation coecients (ICC) calculated from repeated measures analysis of variance (A N O V A ). 2.5. In vivo vs ex vivo comparison Where available, ex vivo spine radiographs were matched with their corresponding in vivo radiographs to compare morphology and curvature measurements. Segmental levels on radiograph pairs were matched by unique bony characteristics of vertebrae. A comparative

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analysis of ex vivo radiographic and direct measurements from the histomorphometric bone slice samples was also performed. Data were analysed using least squares regression. 2.6. Radiographic distortion To examine for potential errors due to o€-centre positioning of ex vivo vertebrae from the X-ray beam, contact radiographs of a hemisected column were performed with a nominated vertebral body ®rst at the centre of the beam, then at 10 and 20 cm from the central ray of the X-ray beam. Ten repeated tracings of corner landmarks from the selected vertebral body were performed at varying intervals for each of the three images. Anterior and posterior vertebral body heights were measured as described previously. Analysis of variance was performed to examine for possible height di€erences between the various imaging positions, while coecient of variation (CV) was calculated for repeated measurements. 3. Results 3.1. Demographic data All 93 cases (35 females, 58 males) were available for regional (T4±T9) analyses. Mean age for female cases was 65.6 yr (SD, 20.9; range 20±95), and 54.7 yr for males (SD, 21.1; range 15±94). Radiographs of the 30 whole vertebral columns were excluded from cumulative analyses (T1±T12) due to poor radiographic de®nition of upper and lower thoracic segments. Cumulative data were therefore obtained from 63 cases (22 females, 41 males). Mean age for this subset of female cases was 58.8 yr (SD, 21.0; range 20±84), and 54.6 yr for males (SD, 20.9; range 18±94). 3.2. Reliability Repeatability of anterior vertebral height measurements was consistently high across all series (ICC 0.95±

0.99). In general, precision of repeated height measurements was high in the para-sagittal bone sections and ex vivo series (ICC 0.87±0.99), with a decreasing trend in the in vivo chest ®lm series (ICC 0.72±0.98). Disc height measurements were more accurately reproduced in the ex vivo series (ICC 0.87±0.95) than in the in vivo chest series (ICC 0.72±0.82). 3.3. In vivo vs ex vivo comparison Curvature measurements derived from the hemisected series were strongly correlated with those measured from whole spine and chest radiographs (r ˆ 0.96 and r ˆ 0.94, respectively) (Table 1). There was no systematic di€erence between ex vivo and in vivo measurements. Ex vivo measurements of disc morphology were poorly correlated with in vivo measurements (r ˆ 0.55±0.65). 3.4. Magni®cation e€ects There were no signi®cant di€erences between vertebral heights measured from the three image positions (P ˆ 0.65 and P ˆ 0.57 for anterior and posterior heights, respectively). CV values for analysis of repeated height tracings from the same radiograph ranged from 0.7% to 1.3%. 3.5. Descriptive 3.5.1. Mean morphology and curvature The extent of anterior vertebral wedging was greater in females, though not signi®cantly di€erent from males. Mean anterior wedging for all discs (cumulative) was higher in males, while the extent of wedging in midthoracic discs (regional) was greater in females. Gender di€erences in disc morphology were non-signi®cant. A higher mean curvature was noted for females, with a signi®cant gender di€erence evident for regional curvature (P ˆ 0.007). 3.5.2. Segmental level analysis At the segmental level, there appeared to be a trend for greater anterior wedging of vertebral bodies in the

Table 1 Correlation coecient values describing relationships between measurements derived from sagittal histomorphological bone sections, hemisected column radiographs, whole column radiographs and in vivo chest ®lms (CXR) Comparison

n

Bone sections vs. whole column X-rays Hemisected vs. whole column X-rays CXR vs. whole column X-rays CXR vs. hemisected column X-rays *

P < 0.05;



P < 0.01;



P < 0.001.

17 16 15 9

Measured parameter Vertebral morphology

Disc morphology

Cobb angle

0.83 0.79 0.86 0.80

± 0.91 0.65 0.55

± 0.96 0.83 0.94

S. Goh et al. / Clinical Biomechanics 14 (1999) 439±448

mid and lower thoracic segments (Fig. 3[A]). At most levels (T3±T11), greater wedging was evident in females, with a signi®cant gender di€erence at T6 and T7 (P < 0.05). The intervertebral discs demonstrated greater anterior wedging in the upper and midthoracic segmental levels (Fig. 3[B]). No gender di€erence in anterior disc wedging was noted at any segmental level.

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3.6. Regression 3.6.1. Age e€ects Age was signi®cantly correlated with all measured variables except vertebral morphology (Table 2). Figure 4 illustrates the nature of these relationships. A signi®cantly higher correlation between age and disc morphology was noted in males (Fig. 4[A] and [B]), while the association with thoracic curvature was signi®cantly higher in females (Fig. 4[C] and [D]). 3.6.2. Curvature vs morphology Signi®cant relationships were noted between thoracic curvature and measured variables, except regional disc morphology in males (Table 3). In general, there was a trend for higher correlation coecient values in females, with signi®cant gender di€erences for curvature versus vertebral morphology (Fig. 5[A] and [B]) and cumulative curvature versus disc morphology (Fig. 5[C]). Regional comparison of vertebral morphology with thoracic curvature resulted in higher correlation coecient values than cumulative comparison. 3.6.3. Stepwise regression model Thoracic curvature (Cobb angle) was designated as the dependent variable, while the independent variables were vertebral morphology, disc morphology, and age. Regression equations were produced following stepwise addition of independent variables into the model (Table 4). In both genders, the combined in¯uences of thoracic vertebral and disc morphology accounted for a large variance in thoracic sagittal curvature. In general, there was a trend for stronger predictive models in females and for analyses involving parameters in the midthoracic segmental levels (regional). Age had no e€ect on the models. 4. Discussion

Fig. 3. [A] Mean vertebral body morphology (antero±posterior height ratio) for T1±T12 segments. The extent of wedging was signi®cantly greater in females at vertebral levels T6 and T7. [B] Mean intervertebral disc morphology for segments from T1/2 to T11/12.

The combined in¯uences of mechanical loading on the thoracic vertebral column and age-related alterations in bone mass inherently result in morphological adaptation of the thoracic elements and an accompanying increase in kyphotic deformity. While the shape characteristics of the thoracic vertebral bodies and their role in determining kyphotic curvature have received considerable focus, the contribution of the intervertebral discs is less clear. Recent ex vivo investigations have enabled a more accurate study of the morphological characteristics of the thoracic spine [20±22], while addressing some of the limitations associated with previous in vivo approaches. The present ex vivo study examined in detail the morphology of the thoracic vertebral bodies and intervertebral discs, with reference to age-related

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Table 2 Correlation coecients describing ex vivo relationships between age and various thoracic spine related parameters for cumulative (T1±T12) (22 females, 41 males) and regional (T4±T9) (35 females, 58 males) analyses (gender comparisons of regression lines were performed using A N C O V A ) Variable Vertebral morphology Disc morphology Curvature (Cobb angle)

Cumulative Regional Cumulative Regional Cumulative Regional

Pooled

Female

Male

Gender comparison (A N C O V A )

ÿ0.17 ÿ0.16 ÿ0.62 ÿ0.52 0.62 0.47

ÿ0.41a ÿ0.17 ÿ0.53 ÿ0.50 0.68a 0.45

ÿ0.05 ÿ0.09 ÿ0.69 ÿ0.54 0.58 0.43

±b ±b    

(slope)

a

n ˆ 21. Gender comparison not performed. * P < 0.05;  P < 0.01;  P < 0.001. b

Fig. 4. Scattergrams depicting signi®cant linear relationships between age and [A] cumulative (T1±T12) disc morphology or antero±posterior height ratio (Females: n ˆ 22, r ˆ ÿ0.53, P < 0.05; Males: n ˆ 41, r ˆ ÿ0.69, P < 0.001). [B] regional (T4±T9) disc morphology (Females: n ˆ 35, r ˆ ÿ0.50, P < 0.01; Males: n ˆ 58, r ˆ ÿ0.54, P < 0.001). [C] cumulative Cobb angle (Females: n ˆ 21, r ˆ 0.68, P < 0.001; Males: n ˆ 41, r ˆ 0.58, P < 0.001). [D] regional Cobb angle (Females: n ˆ 35, r ˆ 0.45, P < 0.01; Males: n ˆ 58, r ˆ 0.43, P < 0.01).

changes and their contributions to sagittal curvature. In addition, regional trends were examined, with particular reference to patterns of morphological change in the midthoracic segments, given the tendency for osteoporotic fractures within this region. Examination of vertebral body morphology demonstrated a bimodal distribution of anterior wedging in both genders. Notably, this was seen in the midthoracic region, and to a lesser extent the T11 and T12 vertebral bodies. Similar trends are reported in previous in vivo [11,12,16,25] and cadaveric studies [10]. These ®ndings re¯ect the natural morphological adaptation of the thoracic vertebral bodies in response to

greater compressive loading at the apex of the kyphotic curve, which typically lies within the midthoracic region [2]. In osteoporosis, a metabolic imbalance between bone resorption and formation results in bone loss, leading to shape changes in the bony architecture of the vertebral bodies, which along with increased physiological loading at the apex of kyphosis (Fig. 2), predisposes the midthoracic vertebral segments to further morphological deformity or wedge fractures [4,11,12]. The midthoracic segmental levels also represent sites of frequent osteophytic formation, which may be attributed to bony reactive changes precipitated by disc degeneration and biome-

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Table 3 Correlation coecients describing ex vivo relationships between the Cobb angle and various thoracic spine related parameters for cumulative (T1±T12) (22 females, 41 males) and regional (T4±T9) (35 females, 58 males) analyses (gender comparisons of regression lines were performed using ANCOVA)

Comparison Cobb vs. vertebral morphology Cobb vs. disc morphology * a

Cumulative Regional Cumulative Regional

Pooled

Female

Male

Gender comparison (A N C O V A )

ÿ0.64 ÿ0.80 ÿ0.50 ÿ0.22

ÿ0.69 ÿ0.79 ÿ0.65 ÿ0.36

ÿ0.59 ÿ0.79 ÿ0.37 ÿ0.05

   (slope) ±a

P < 0.05;  P < 0.001. Gender comparison not performed.

Fig. 5. Scattergrams depicting signi®cant linear relationships between [A] cumulative (T1±T12) curvature and vertebral morphology or antero± osterior height ratio (Females: n ˆ 22, r ˆ ÿ0.69, P < 0.001; Males: n ˆ 41, r ˆ ÿ0.59, P < 0.001). [B] regional (T4±T9) curvature and vertebral morphology (Females: n ˆ 35, r ˆ ÿ0.79, P < 0.001; Males: n ˆ 53, r ˆ ÿ0.79, P < 0.001). [C] cumulative curvature and disc morphology (Females: n ˆ 22, r ˆ ÿ0.65, P < 0.01; Males: n ˆ 41, r ˆ ÿ0.37, P < 0.05). [D] regional curvature and disc morphology in females (n ˆ 35, r ˆ ÿ0.36, P < 0.05). No association was evident in males (n ˆ 58; r ˆ ÿ0.05; P > 0.05).

chanical loading within the thoracic region [26]. In females, the observed trend for greater vertebral wedge deformity was most notable at the 5th±8th segmental levels, as illustrated in Fig. 3[A]. In the absence of osteoporosis, it is likely that this gender di€erence represents a normal physiological gender bias in the morphological adaptation of the vertebral bodies. Several in¯uencing factors may be implicated, such as the predisposition for lower BMD values in females. In the lower thoracic region, the tendency to develop wedging of the vertebral segments may relate to the compressive loading forces on these lower thoracic

vertebrae at the thoracolumbar junction, as sites of increased ¯exion and mobility without the stabilisation of the rib cage [27]. An interesting ®nding from this study was the lack of association between age and vertebral morphology. Unfortunately, the con¯icting nature of results from other studies presents diculty in attempts to generate any valid conclusions [3,8,11,25], particularly with the variable nature of methodologies employed, subject characteristics, and lack of uniformity in de®ning measured variables. Furthermore, results from the present study may re¯ect limitation of the cross-sec-

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Table 4 Stepwise regression equations describing the association of thoracic curvature (dependent variable) with the independent variables vertebral morphology (VBM), disc morphology (IVDM) and age for [A] cumulative (T1±T12) analysis of 63 ex vivo cases (22 females, 41 males) [B] regional (T4± T9) analysis of 93 ex vivo cases (35 females, 58 males) Regression model

r2

[A] Cumulative: Pooled Female Male

Curvature ˆ 521.0±425.8 VBM±92.6 IVDM Curvature ˆ 524.4±422.3 VBM±98.4 IVDM Curvature ˆ 504.9±414.5 VBM±87.1 IVDM

0.86 0.92 0.80

[B] Regional: Pooled Female Male

Curvature ˆ 258.5±218.1 VBM±37.5 IVDM Curvature ˆ 252.4±211.6 VBM±36.2 IVDM Curvature ˆ 259.6±219.2 VBM±38.2 IVDM

0.93 0.94 0.91

Note: Age variable not in model. * P < 0.0001.

tional nature of the study, or the morphology parameters adapted for this study. However, it is suggested that a reduction in posterior vertebral height with the aging process renders the antero±posterior height ratio, as an index of wedging, less sensitive [11]. It is further proposed that vertebral dimensions remain stationary from the time peak adult stature is attained despite reductions in bone mass [25]. The nature of ®ndings reported thus far suggests a need for large scale longitudinal studies and standardisation of measurement criteria, to ensure an improved understanding of the e€ects of advancing age on vertebral shape, if the relationship does indeed exist. A similar pattern of anterior wedging was evident in the midthoracic intervertebral discs. This may be attributed to greater rotational forces at these segments, resulting in torsional stresses to the anuli of discs [15]. Gregerson and Lucas further suggest that the normal disc degenerative process in the thoracic spine tends to occur more anteriorly, resulting in concentric anular tears. In a postmortem examination of 16 thoracic columns, a higher incidence of thoracic disc degeneration was noted in transverse sections of the midthoracic segments, with a more noticeable trend in male cases [14]. A greater prevalence of asymptomatic anular tears, disc degeneration, bulge and herniation in males was reported in an MRI study of 90 healthy volunteers [13]. The authors suggested the role of greater repetitive trauma exposure in males as a predisposing factor. Supporting evidence for the signi®cantly greater agerelated progression in anterior disc wedging in males is also provided [9], where it was suggested that in males, loading stresses imposed on the thoracic spine were more likely to result in compression of disc tissue with subsequent anular degeneration than collapse of vertebral bodies. Furthermore, in older males without advanced osteoporosis, there is a greater tendency for degeneration of thoracic discs [9]. While conclusions cannot be drawn about the nature of the relationship

between anterior disc height and disc degeneration, it is possible that age-related progression in wedge con®guration of male thoracic discs may re¯ect underlying pathological changes. Consistent with previous studies, progression of the normal kyphosis was associated signi®cantly with age. Factors suggested to contribute to increasing kyphosis include loss of muscle tone [1], the e€ects of occupation and habitual posture [1], changes in vertebral body shape [3], and osteopenia [4]. Furthermore, the e€ect was more evident in females, which is in agreement with ®ndings by Milne and Lauder [1]. A number of possible causes have been suggested, such as reductions in physical activity in females relative to males, greater loss of muscle and ligamentous tone, head±forward postures, and the e€ects of dependant breasts in older females [1]. Recognising the limitations associated with cross-sectional data, these trends nevertheless re¯ect the normal age±related physiological changes in thoracic sagittal curvature. As presented earlier, shape characteristics of both the vertebral bodies and intervertebral discs are thought to contribute to sagittal curvature of the thoracic spine [8,9,28]. The combined in¯uence of vertebral and disc morphology accounted for 86% of the variability in Cobb angle measured along the whole length of the thoracic spine, and 93% for regional analysis of the midthoracic levels (Table 4). These values are substantially higher than those reported in one study [28], where the index of vertebral and disc wedging accounted for 42% and 48% of kyphosis in males and females, respectively. Considering the vastly di€erent measurement protocols, the variability in outcome is hardly surprising. The in vivo study of Milne and Lauder utilised a ¯exicurve for curvature measurement of the whole thoracic spine, while only the T7±T12 vertebral bodies were considered for morphology measurements derived from lateral chest radiographs. Furthermore, a limited age cohort (62±90 yr) was involved [28].

S. Goh et al. / Clinical Biomechanics 14 (1999) 439±448

Comparison of cumulative and regional analyses con®rmed the more in¯uential role of morphology on thoracic curvature in the midsegmental levels. In particular, the vertebral bodies accounted for over 60% of the variability in regional kyphosis (Fig. 5[B]), which is unsurprising in view of the previously discussed biomechanical in¯uences on the apex of thoracic concavity. These di€erences may also be due to greater in¯uences of the posterior spinal musculature in determining the whole kyphotic curvature. Overall, the extent of kyphosis was associated with vertebral morphology to a greater extent than the discs. However, the role of the thoracic discs in determining sagittal curvature is important, given the outcome of stepwise regression analyses and available evidence [8,9]. Addition of both vertebral and disc morphology variables to the stepwise regression model provided strong evidence that the geometry of thoracic sagittal curvature was largely de®ned by the combined in¯uences of both morphology indicators. The role of non±skeletal factors in the development of hyper-kyphosis has been discussed by various authors. [3,4]. Furthermore, it was proposed that the development of sagittal angulation in juvenile kyphosis was partly attributed to a loss in anterior disc height associated with herniation of the intervertebral discs through the vertebral endplates [9]. Indeed, changes in disc morphology may represent part of the spectrum of the normal aging process, and to some extent, age-related changes in disc shape may account for progression of thoracic kyphosis across the lifespan. Conversely, the lack of an association between age and vertebral morphology may suggest that the in¯uence of vertebral shape on sagittal curvature, while an important one, remains largely unchanged across the lifespan as a result of the more stable nature of vertebral body morphology during adulthood [25]. It also appears that both vertebral and disc morphology play more signi®cant roles in determining sagittal curvature in females, which may suggest greater importance of other extraskeletal factors in the maintenance of spinal curvature in males, such as ligaments and supporting musculature. The ex vivo approach of this study has enabled more detailed examination of shape parameters of the thoracic spine, obviating some of the limitations associated with in vivo techniques. Absence of overlying bony and soft tissue structures a€ords greater accuracy in selection of measurement landmarks, which allows reliable measurement of both vertebral and disc heights. Similarly high values have been documented in studies employing selection of corner landmarks [11,12,17,29]. The measurement protocol undertaken in the present study may be applied cautiously for investigation of in vivo lateral thoracic radiographs, with some anticipated compromise in measurement accuracy. In particular, subjective extrapolation of the superior posterior vertebral land-

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marks may be required, as the location is frequently obscured by rib shadows. The enhanced accuracy a€orded by ex vivo techniques was particularly evident in the hemisected spine series. Distortion e€ects were negligible, largely due to less overlapping of structures and reduced depth of the radiographic image. The validity of ex vivo measurements was further evidenced in the present study [21], particularly for curvature and vertebral morphology parameters. However, the noted associations between ex vivo and in vivo disc morphology values were lower (Table 1). It may be suggested that any motion of the ex vivo spines in the sagittal plane may have altered the shape con®guration of the discs, though this possibility is unlikely, given that ex vivo kyphosis measurements remained essentially unchanged compared to their corresponding in vivo values. Alternatively, the relatively poorer precision in landmark localisation for the in vivo ®lms, particularly for the discs, may have confounded the nature of the associations. It is therefore assumed that both curvature and morphology indicators remained unaltered in the ex vivo series. While it may be debatable that hemisection of whole spines may alter the con®guration of underlying structures and hence curvature properties of the spine, our present ®ndings show no evidence of this. Cobb angles measured from hemisected spines agreed strongly with those derived from ex vivo whole spines (r ˆ 0.96) and their corresponding in vivo lateral chest radiographs (r ˆ 0.94). 5. Conclusions These ®ndings con®rm that the normal spinal curvature of the thoracic column is dependent on the structural morphology of both the vertebral bodies and intervertebral discs. The nature of these relationships is stronger in females. Greater anterior wedging of the thoracic vertebral bodies and intervertebral discs was noted at the midthoracic segmental levels. Further morphologic investigation of the midthoracic segments is merited, given the prevalence of osteoporotic fractures in this region. Acknowledgements The authors wish to acknowledge Professor B. Kakulas, Department of Neuropathology, and Professor T.H.M. Chakera, Department of Imaging Studies, Royal Perth Hospital, for providing access to departmental resources, and Dr. J. Sommer, Curtin University of Technology, for statistical advice. The authors thank Professor Paul Brinckmann for helpful criticism on an early version of this manuscript. This work was funded through the NH & MRC (970244).

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