Surg Radiol Anat (2005) 27: 123–128 DOI 10.1007/s00276-004-0305-4

O R I GI N A L A R T IC L E

Kenya Nojiri Æ Morio Matsumoto Æ Kazuhiro Chiba Yoshiaki Toyama

Morphometric analysis of the thoracic and lumbar spine in Japanese on the use of pedicle screws

Received: 29 December 2003 / Accepted: 7 September 2004 / Published online: 12 January 2005
Springer-Verlag 2005

Abstract The pedicle screw and hook have become popular instruments in treating spinal deformity and disease. This study gathered morphological data on thoracic and lumbar spines in a Japanese population that should serve as useful reference for posterior instrumentation surgery. One hundred and three dry bones were used to investigate the morphology of pedicle and facet in thoracic and lumbar spines. Measurements included the diameter and axial length of pedicle from T8 to L5, height and width of facets and thickness of articular processes from T1 to T12, and axial angle of pedicle from T1 to L5. The diameter and axial length of pedicle were smallest at T8, diameter was largest at L5 and axial length was largest at L3. Height of facets and thickness of articular processes were largest at T12. Men tended to have larger pedicles and facets than women. Transverse angle of pedicle was smallest at T12. These precise data may provide useful information when performing posterior instrumentation surgery and when developing new spinal implant systems for Asians.

the anatomy and orientation of the posterior elements of the spine is essential for safe and accurate surgical procedures. Identifying anatomical landmarks of the spine is particularly important for accurately placing spinal instruments. Numerous studies have described the anatomy of the posterior elements of the spine, but most have used Western persons for reference [2, 3, 4, 6, 7, 8, 9, 14, 16, 17, 20, 21, 22, 23, 24, 26, 29]. In addition, many of these studies have limitations, such as a small sample size and lack of demographic data including race and age. In this study we conducted a morphometric analysis of the posterior elements of the spine in a relatively large number of cadaveric specimens obtained from Japanese persons whose sex and age at the time of death were precisely documented. This allowed us to adopt robust statistical analyses. These morphometric data should contribute to better results in posterior surgeries, especially in those involving posterior spinal instrumentation and may also serve as a basis for the development of new spinal implants for Asians.

Keywords Thoracic spine Æ Lumbar spine Æ Morphology Æ Japanese Æ Dry bone

Materials and methods Bone specimens

Introduction Posterior approaches have widely been used in the treatment for degenerative diseases, trauma, deformities, and tumors of the spine. A thorough understanding of

K. Nojiri Æ M. Matsumoto (&) Æ K. Chiba Æ Y. Toyama Department of Orthopaedic Surgery, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku, 160-8582 Tokyo, Japan E-mail: [email protected] Tel.: +81-3-53633812 Fax: +81-3-33536597

Bone specimens from 103 subjects (56 men, 47 women) were selected from the human dry bone collection of the Jikei University (Tokyo, Japan). Subjects were born between 1883 and 1946 and died between 1950 and 1992; their age at death ranged from 20 to 85 years (mean: men 55.6, women 57.5; Table 1). Specimens were from the thoracic and lumbar vertebrae and the femur; bones with substantial damages or obvious degenerative changes were excluded. The femurs were used to calculate subjects’ height by Fujii’s [10] equation: height (cm)=81.3+1.88·maximum length of the femoral bone (cm) in men, 72.8+1.945·maximum length of the femoral bone (cm) in women. Subjects’ height estimated in this way was 160.1±7.0 cm in men and 148.9±4.1 cm in women.

124 Table 1 Age distribution of subjects Age group (years)

Men

Women

20–29 30–39 40–49 50–59 60–69 70–79 80+ Total

3 5 10 17 11 7 3 56

3 3 5 13 12 9 2 47

Methods Dimensions of the posterior elements at both (left and right) sides of the vertebral body were measured with a resolution of 0.1 mm using electronic slide calipers (NSK, Tokyo, Japan). Measurements were made in triplicate and expressed as a mean value. For angulations the vertebral body was fixed with a special holder before photographs were taken at two directions with a digital camera (Cyber Shot, Sony, Tokyo, Japan) having a resolution of 3 million pixels from a distance of 100 cm. Pictures of the bottom of the vertebral body were taken while the superior surface of the vertebral body was held parallel to the ground, and pictures of the left side of the vertebral body were taken while the spinous process was held parallel to the ground. The obtained images were captured into a personal computer (Vaio, Sony) and analyzed using an image analysis software (Scion version 4, Scion, Md., USA). All parameters were measured excluding degenerative changes. The t tests were performed with the SPSS statistic software (SPSS; Chicago, Ill., USA) and P values under 0.05 were considered significant. Measurements The transverse diameter, sagittal diameter, and axial length of the pedicles were measured from T8 to T12 and L1 to L5 level. As described by Berry et al. [3], the transverse diameter was the minimum diameter, and the sagittal diameter was the maximum diameter of the pedicle isthmus (Figs. 1, 3). As proposed by

Fig. 1 Measurement of the thoracic and lumbar pedicle. TDP Transverse diameter of the pedicle; ALP axial length of the pedicle

Fig. 2 Measurement of the thoracic facet. HCR height of the cranial facet; HCA height of the caudal facet; WCR width of the cranial facet; WCA width of the caudal facet; TCR thickness of the cranial articular process; TCA thickness of the caudal articular process; DCAM distance between caudal facet and midline; asterisk midpoint of the facet

Zindrick et al. [29], the pedicle axis was defined as a line which was perpendicular to and bisecting the narrowest diameter of the pedicle. The axial length was determined as the distance from the posterior aspect of the laminar cortex to the anterior aspect of the cortex of the vertebral body along the pedicle axis, as recommended by Olsweski et al. [20] (Fig. 1). For height and width of the cranial/caudal facet and thickness of the articular process measurements were made from T1 to T12 level, according to the method proposed by Ebraheim et al. [9]. The midpoint of the facet was defined as the intersection of the maximum and the minimum diameters of the cranial/caudal articular surfaces. The height and width were defined as

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vertebral midline in the axial plane was defined as the transverse angle. The angle formed between the superior margin of the vertebral body and a line through the pedicle axis in the sagittal plane was defined as the sagittal angle. Angles in caudal directions were expressed as positive and those in cephalad directions as negative (Fig. 3). For angles of the caudal facet measurements were made from T1 to T12 levels. The angle of the caudal facet was formed between the direction of the caudal articular surface and a line of the vertebral midline in the axial plane (Fig. 3).

Results

Fig. 3 Angle of the thoracic and lumbar pedicle, thoracic facet and sagittal diameter of the pedicle. TAP Transverse angle of the pedicle; SAP sagittal angle of the pedicle; CFA angle of the caudal facet; SDP sagittal diameter of the pedicle

the length of vertical and horizontal lines that pass through the midpoint of the facets, respectively. The thickness was measured in the anteroposterior direction at the midpoint of the facets (Fig. 2). The shortest distance from the midline of the internal surface of the vertebral arch to the inner margin of the caudal facet was defined as the distance between the caudal facet and the midline (Fig. 2). For the transverse angle and sagittal angle of the pedicle measurements were made from T1 to L5 level. As prescribed by Olsweski et al. [20], the angle formed between a line through the pedicle axis and a line of the

Table 2 Measurement of the thoracic and lumbar pedicle (mm)

*P<0.05, **P<0.01 vs. men

T8 T9 T10 T11 T12 L1 L2 L3 L4 L5

The transverse diameter of the pedicle was smallest at T8 level and increased to T12, and after a decrease in the upper lumbar lesion it reached its maximum at L5 level (Table 2). The sagittal diameter of the pedicle was also smallest at the T8 level and increased from this level to the T12 level, and again after a slight decrease in the upper lumbar region, increased again to its maximum at L5 level (Table 2). The axial length of the pedicle was smallest at T8 level and remained almost unchanged until T11, then began to increase at T12 level to reach its maximum at L3 level and then decreased again in the lower lumbar region (Table 2). The height of the cranial facet tended to increase slightly at the T10–T12 level but was otherwise similar at all levels. The width of the cranial facet was maximum at T1 level; it then decreased progressively until the midthoracic region, increased slightly in the lower thoracic region, and then decreased again at T12. The thickness of the cranial articular process decreased gradually from the T1–T4 level in women and from the T1–T5 level in men, then increased progressively to reach its maximum at T12 (Table 3). The height of the caudal facet was smallest at T1, gradually increased until T5, then decreased again until T7 in women and T8 in men; it then started to increase again progressively, to reach its maximum at T12. The width of the caudal facet decreased from the T1–T6 level and increased thereafter to reach its maximum at T10. The thickness of the caudal articular process began

Transverse diameter

Sagittal diameter

Axial length

Men

Women

Men

Women

Men

Women

5.1±1.2 5.4±1.3 6.0±1.3 7.6±1.6 8.1±1.6 7.4±2.0 7.8±1.7 9.1±1.7 10.1±1.7 11.1±1.7

4.8±0.9* 5.0±1.0** 6.0±1.4 7.0±1.3** 7.7±1.5 6.9±1.5* 7.4±1.5 8.9±1.6 9.7±1.4* 10.6±1.5*

11.7±1.3 12.4±1.3 14.5±1.7 16.3±1.7 17.0±2.8 15.9±2.8 14.8±1.6 14.7±1.3 15.5±2.0 20.7±3.0

10.9±1.3** 12.1±1.4 14.1±1.9 16.0±1.7 16.6±1.5 15.2±1.4* 14.4±1.2* 14.2±1.1** 15.0±1.8 20.2±2.3

38.2±3.4 38.8±3.4 38.8±3.3 38.8±3.6 39.9±4.0 42.5±3.7 44.0±3.5 45.0±3.7 44.3±3.6 43.4±3.7

36.6±2.8** 36.7±3.0** 36.8±2.9** 36.7±2.9** 37.7±3.3** 40.6±3.3** 42.7±3.4** 43.6±3.4** 43.0±3.3** 41.2±5.5**

3.3±0.8 3.3±0.6 3.2±0.6 3.1±0.6 3.0±0.7 3.2±0.6 3.2±0.6 3.4±0.6 3.6±0.6 3.8±0.6 3.9±0.7 4.3±0.8

3.1±0.5* 2.8±0.6** 2.8±0.5** 2.7±0.5** 2.8±0.5* 2.9±0.5** 3.0±0.5** 3.2±0.5* 3.4±0.6 3.6±0.7 3.5±0.7** 4.2±1.2

9.2±1.6 9.4±1.3 9.6±1.6 10.0±1.5 10.0±1.6 9.7±1.2 9.7±1.4 9.6±1.3 10.0±1.4 10.3±1.5 10.4±1.7 11.1±1.9

8.4±1.3** 8.5±1.2** 8.8±1.2** 9.1±1.2** 9.4±1.4** 9.0±1.1** 8.9±1.1** 9.2±1.4* 9.3±1.3** 9.7±1.5** 9.7±1.7** 10.4±1.7**

10.3±1.4 10.2±1.2 10.0±1.5 9.6±1.5 9.4±1.3 9.1±1.2 9.3±1.1 9.8±1.3 10.5±1.5 10.8±1.5 9.7±1.3 8.8±1.2

9.8±1.5* 9.3±1.4** 9.2±1.2** 8.9±1.3** 8.6±1.2** 8.5±1.1** 8.7±1.1** 9.2±1.2** 9.6±1.4** 9.9±1.5** 9.2±1.4* 8.1±1.1**

4.5±0.7 4.2±0.7 4.3±0.7 4.3±0.8 4.2±1.1 4.0±0.7 4.0±0.7 4.1±0.7 4.3±0.7 4.3±0.9 4.5±1.0 5.0±0.9

3.8±0.6** 3.6±0.6** 3.6±0.7** 3.6±0.6** 3.5±0.6** 3.5±0.8** 3.5±0.6** 3.6±0.7** 3.8±0.7** 3.8±0.6** 4.5±1.0 4.8±0.9

7.9±1.1 6.3±1.1 5.4±0.9 4.9±1.0 4.9±0.9 5.1±0.9 5.2±0.8 5.2±0.9 5.3±0.8 5.4±1.0 6.9±1.5 8.8±1.5

7.4±1.1** 5.7±1.0** 4.8±0.8** 4.3±0.7** 4.3±0.7** 4.7±0.9** 4.6±0.8** 4.7±0.7** 4.9±0.9** 5.0±0.9* 6.6±1.4 8.4±1.3

to decrease at T1, was smallest at T6 and T7 and then increased progressively again to reach its maximum at T12 (Table 3). The height of the caudal facet was significantly greater than that of the cranial facet at T4, T5, T9–T12. The width of the caudal facet was significantly greater than that of the cranial facet at T9, T10; on the other hand, the cranial facet was significantly greater than the caudal facet at T1, T8, T12. The thickness of the caudal articular process was significantly greater than that of the cranial articular process through all thoracic levels. The distance between the caudal facet and the midline decreased from T1 to T4 level, gradually increased from T6 level, to reach its maximum at the level of T12 (Table 3). The transverse angle decreased caudally from the T1 to the T12 level. The transverse angle at T12 was negative, indicating that the pedicle axis was oriented outward at this level. In the lumbar region the transverse angle gradually increased caudally and reached its maximum at L5 (Fig. 4a). The sagittal angle was at its maximum at the level of T3 in women and T4 in men and then started to decrease to reach its minimum at L4 (Fig. 4b). The angle of the caudal facet was similar at all thoracic levels except for T11 and T12 (Fig. 5). Men tended to have larger pedicles and facets than women; on the other hand, no differences were observed in the angle of the pedicle and caudal facet between men and women. With regard to the correlations between the measured parameters and the age and estimated height of the individual subjects, a weak correlation was noted between the estimated height of the subjects and each

*P<0.05, **P<0.01 vs. men

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12

11.3±1.5 10.6±1.4 9.9±1.3 9.7±1.3 9.4±1.4 9.2±1.3 9.0±1.3 9.1±1.1 9.6±1.3 10.0±1.5 10.4±1.5 9.7±1.1 8.7±1.3* 8.5±1.2** 8.2±1.0** 8.2±1.0** 8.5±1.1** 8.6±1.1** 8.3±1.1** 8.3±1.1** 8.5±1.2** 8.8±1.3** 8.8±1.1** 9.2±1.8 9.2±1.5 9.2±1.3 9.2±1.3 9.1±1.2 9.3±1.3 9.3±1.4 9.3±1.4 9.3±1.4 9.3±1.3 9.5±1.5 9.7±1.4 9.7±2.0

10.4±1.7** 9.9±1.3** 9.1±1.2** 8.9±1.1** 8.7±1.3** 8.5±1.1** 8.3±1.2** 8.5±1.1** 9.0±1.3** 9.2±1.3** 9.5±1.1** 9.2±1.4**

Women Men Women Men Women Men Women Men Women Men Men Women Men

Women

Width of the cranial facet Height of the cranial facet

Table 3 Measurement of the thoracic facet (mm)

Thickness of the cranial articular process

Height of the caudal facet

Width of the caudal facet

Thickness of the caudal articular process

Distance between caudal facet and midline

126

Fig. 4 a Transverse angle of the pedicle (TAP). b Sagittal angle of the pedicle (SAP)

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Fig. 5 Angle of the caudal facet (CFA). *P<0.05, **P<0.01 men vs. women

measured parameters (r=0.215–0.428) except angles of the pedicle and caudal facet. No relationship was found between the estimated height of the subject and the angles. Age had no significant correlation with any of the measured parameters.

Discussion Pedicle screw and hook instrumentation have become increasingly popular in the treatment of spinal deformities and various diseases. In this context the morphometry of the pedicles and the facet joints have been described in many studies. Spinal morphometric parameters have been measured on radiography [24, 29] and computed tomography [1, 2, 5, 14, 29] by image analysis or directly measured with slide calipers and protractors [3, 4, 7, 8, 9, 11, 13, 16, 26, 27, 28] in cadavers, or sometimes using both modalities [12, 17, 18, 19, 20]. In this study we used dry bone specimens obtained from the bone collection at the Jikei University to directly examine the morphology of the posterior spinal elements since a large number of bone specimens derived from subjects over a wide age range could be investigated. In pedicle screw placement the transverse diameter and the axial length of the pedicle are crucial parameters for selecting appropriate screw size, while the transverse angle of the pedicle is a crucial parameter for the direction of screw insertion. In the present study it was found that the transverse diameter of the pedicle increases in the caudal direction in the thoracic spine region, with a slight decrease at the thoracolumbar junction, then increases again towards the lower lumbar region. This observation is consistent with the findings in previous studies using direct measurements [7, 8, 11]. The measured values of the transverse diameters of the pedicles were similar to or greater than those reported by Scoles et al. [26]. This is probably because their bone samples were acquired from subjects born between 1893 and 1938, and therefore constituted a much older collection than ours. Based on their computed tomography studies Krag et al. [14] showed that the pedicle diameters

below T10 level were 5 mm or greater, and even 7 mm or greater in the lower lumbar region in their bone specimens, with a few exceptions. In our study pedicles with a transverse diameter of less than 7 mm between L3 and L5 levels accounted for 5.3% (33/618) of the total bone specimens studied. Furthermore, no pedicles with a transverse diameter of less than 5 mm were observed at these levels. These findings suggest that placement of a 5 mm pedicle screws at L3 or lower would scarcely allow penetration of the pedicle wall. In contrast, pedicles whose transverse diameter at L1 was less than 5 mm or less than 7 mm accounted for 7.8% (16/206) and 50% (103/206) of the total specimens, respectively. Very precise screw size selection is thus necessary at this level. Transverse diameters of the pedicle at T8 and T9 levels that were less than 4 mm accounted for 17% (35/206) and 13.1% (27/206) of the specimens examined, respectively. In such cases there is a high probability that a 4-mm screw would penetrate the pedicle wall. For the sagittal diameter and axial length of the pedicle, our data are similar to those reported in previous studies. The sagittal diameter of the pedicle increased in the caudal direction except for a slight decrease around the midlumbar region. The axial length of the pedicle was at its maximum in the midlumbar region and decreased in the lower lumbar region. In this study pedicles whose axial length at L5 was less than 40 mm accounted for 23.7% (49/206) of the total bone specimens examined. Accordingly, insertion of a 40-mm pedicle screw in length, which is commonly used, may perforate the anterior surface of the vertebra. The transverse angle of the pedicle may be an important parameter for correct pedicle screw placement. Louis [15] and Roy-Camille et al. [25] recommend that a pedicle screw be inserted in the straight direction. In contrast, Krag et al. [14] and Zindrick et al. [29] believe that insertion of the pedicle along the medial trajectory is a safer technique. The results of our study reveal that the transverse angle of the pedicle at T12 is at its minimum, which supports the reports by Kim et al. [13] and Zindrick et al. [29]. The sagittal angle of the pedicle in the thoracic and upper lumbar regions was caudad in the sagittal plane and was cephalad at L3–L5 level. Therefore orientation of the pedicle in the sagittal plane is another important factor that must be taken into consideration during screw insertion. For posterior fixation, hook instrumentation is also popular, in addition to pedicle screw fixation. Although the pedicle morphology has been described by many researchers, few studies [6, 9, 23] have described the morphometry of the thoracic facets in detail, which surgeons should know about for hook procedures. In this study the height of the caudal facet tended to be greater than that of the cranial facet, but no marked difference between the two was observed with respect to width. The thickness of the caudal articular process was significantly greater than that of the cranial articular process at all thoracic levels. The distance between the caudal facet and the midline, an important parameter

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for selecting the sizes of hooks for vertebral arches, was greater in the upper and lower than in the middle thoracic region. The angles of the caudal facet from the T1– T11 level were almost similar, but at the T12 level the facet was closer to the sagittal plane. The results of the present study showed revealed that surgeons should take this into account when placing hooks on the facets or laminas. One of the limitations of this study is that the specimens were old and might not be comparable to the size of modern populations. However, because spinal diseases affect mostly elderly populations, the results of this study based on the measurements of old specimens have clinical relevance.

Conclusions We report the results of direct morphometric analysis of the posterior elements of the thoracic and lumbar spines using dry bone specimens of Japanese subjects whose sex and age at the time of death were known. These precise data may provide useful information when posterior instrumentation surgery and for the development of new spinal implant systems. Acknowledgements We sincerely appreciate Dr. S. Takeuchi and H. Yamashita at Department of Anatomy, Jikei University School of Medicine, for allowing our access to invaluable human skeletal collection and also for their kind advice.

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