|Year : 2012 | Volume
| Issue : 2 | Page : 86-90
Importance of microstructure of lumbar pedicle in screw placement
Kunal Chawla1, Mahesh Sharma2, Avinash Abhaya3, Ratesh Kumar4, Jyotsna Singh1
1 Senior demonstrator, Gian Sagar Medical College, Ramnagar, Punjab, India
2 Associate Professor, Govt. Medical College, Chandigarh, India
3 Assitant Professor of Anatomy, Govt. Medical College, Chandigarh, India
4 Assitant Professor of Anatomy, Gian Sagar Medical College, Ramnagar, Punjab, India
|Date of Web Publication||23-Jan-2020|
Senior demonstrator, Department of Anatomy, Gian Sagar Medical College, Ramnagar, District Patiala, Punjab -140 601
Source of Support: None, Conflict of Interest: None
Background and objective: Any structural deviation of the pedicle may result in interference of the weight transmission mechanism and compression of neural structures. The aim of this study was to qualitatively investigate osteonal structures within the cortical bone and thickness of medial and lateral walls of lumbar pedicle. Materials and methods: Specimens consisted of L3 vertebrae from 20 adult cadavers (14 males and 6 females) belonging to North West Indian population. The pedicles of each specimen were cut off at the pedicle-vertebral body junction and processed for paraffin sections after decalcification. 8-10 |i thick transverse sections were cut with rotary microtome at middle of the pedicle. The sections were stained with hematoxylin and eosin. Results: The mean cortical thickness on superior, inferior, medial and lateral was 1.3, 1.6, 0.8 and 0.6 mm respectively in males and 1.1, 1.3, 0.9 and 0.6 mm respectively in females. Cortical thickness was more on medial side compared to lateral side of the pedicle. The cortex of the inferior side of the pedicle was the thickest of all margins. The core of pedicle was filled up with irregularly placed bony trabaculae and marrow spaces. Conclusions: The section of pedicle resembled a shell with an outer cortex surrounding inner cancellous bone. The thickness of the cortical bone of pedicle was not homogeneous, being thicker on medial side compared to lateral side and cortical bone of the inferior side was thickest of all margins.
Keywords: Pedicle, Vertebra, Lumbar, Spine, Histology
|How to cite this article:|
Chawla K, Sharma M, Abhaya A, Kumar R, Singh J. Importance of microstructure of lumbar pedicle in screw placement. Natl J Clin Anat 2012;1:86-90
|How to cite this URL:|
Chawla K, Sharma M, Abhaya A, Kumar R, Singh J. Importance of microstructure of lumbar pedicle in screw placement. Natl J Clin Anat [serial online] 2012 [cited 2021 Jan 22];1:86-90. Available from: http://www.njca.info/text.asp?2012/1/2/86/298012
| Introduction|| |
The advent of modern era has led to increase in accidents particularly in people dealing with heavy mechanical devices. High velocity car accidents, adventure sports and warfare injuries add to this list. A very common site of injury is the vertebral column and that too the lumbar region. The malignant tumors of the prostate and other pelvic organs rapidly metastasize to the lumbar vertebrae. They are almost entirely made of cortical bone with a small core of cancellous bone. Recent studies show that the pedicle bone behaves differently from other trabecular and cortical bones in the human body. It has been proved that the pedicle is the strongest part of the vertebra even in osteoporotic bones. This unique anatomy of the pedicles provide an excellent implantation site for reconstructive spinal surgeries. Pedicle screw fixation is used to maintain and restore stability in such patients. King (1944) was the first to try placing screws parallel to inferior border of lamina and across the facet joints for internal fixation of lumbosacral region, whereas Boucher in 1959 successfully initiated passing long screws through the lamina and pedicle into vertebral body below for spinal fusion with internal splinting by screw fixation and thus temporarily stabilizing L4 to L5 and L5 to SI. Screws inserted into the pedicle for the reduction and stabilization of spondylolisthesis has given good results, Luque in 1986, introduced another method of interpedicular segmental fixation using two flexible reshaped rods placed on either side of the spine. The rods were contoured or bent to conform to the curve, and wires were threaded through the spinal canal at each vertebral level. The wires were then twisted around the rods on either side of the spine. However, since the wires were passed through the spinal canal, this system possessed a greater risk of neurological damage than other systems.
The trabecular architecture within the pedicle is isotropic and plate-like and the thickness and number of the trabeculae were greater than those of vertebral trabeculae. After screw insertion (increasingly larger screws) the expansion of pedicle due to plastic deformity was maintained after screw removal and 72% of pedicle fractures occurred laterally. Cortical dimensions of the lumbar pedicle have been studied using CT scans ,, but the histological architecture of the pedicle has been studied rarely.
The purpose of this study was to qualitatively and quantitatively investigate osteonal structures within the cortical bone and thickness of cortical walls of lumbar pedicle.
| Materials and Methods|| |
The present study was performed on 20 adult cadavers (14 males and 6 females). The cadaveric specimens were provided by Department of Anatomy, Govt. Medical College and Hospital, Chandigarh. Fourteen cadavers were male and six females, ages ranging from 30 to 70 (average 50 years). The vertebrae used in this study did not present traumatic lesions, tumoral lesions, or congenital abnormalities.
Third lumbar vertebrae were selected for this study as it lies in the center of lumbar region and is classified as typical lumbar vertebrae.
Spines of vertebrae from level LI to L5 were palpated and thus defined. Thereafter cadavers were dissected using the midline posterior approach. Soft tissue and muscles surrounding vertebral column from LI to L5 were cleared. Intervertebral disc between L2 and L3 were severed. Similarly intervertebral disc between L3 and L4 were transversely cut. The specimens of the third lumbar vertebra were taken out. The pedicles of the specimen were cut off at the pedicle-vertebral body junction and the pedicle-facet junction by using a reciprocal hand saw and processed for paraffin sections after making an anteroposterior nick on the lateral surface of the pedicle to distinguish between its medial and lateral aspects. The tissues were fixed using 10% formal saline followed by decalcification using Acidic ethylenediamine-tetra-acetic acid (EDTA) decalcifying solution. Decalcified tissues were processed for paraffin sections. Transverse sections of 8 -10 |l thickness were cut with rotary microtome at middle of the pedicle. The sections were later stained with hematoxylin and eosin.
Each cortex was measured with ocular micrometer calibrated against a stage micrometer. Thickness between right & left, male & female, medial & lateral and superior & inferior were compared.
The statistical analysis was carried out using Statistical Package for Social Sciences (SPSS Inc., Chicago, IL, version 15.0 for Windows). For all quantitative variables mean and standard deviation were calculated. Means were compared using student’s t-test for two groups. Statistical tests were two-sided and performed at a significance level of a = 0.05
| Result|| |
Transverse paraffin sections of the decalcified pedicles of the lumbar vertebra L3 (as it lies in center of lumbar vertebral column and being typical lumbar vertebrae) were studied after hematoxylin and eosin stain. The matrix of decalcified bone was strongly eosinophilic because of high content of collagen. The pedicles consisted of cancellous bone covered all around by cortical bone. The superolateral side of the section was identified by the presence of nick given before preparation [Figure 1] and [Figure 2]. Osteons were seen through the presence of the Haversian canal, which had concentric alignment of lamellae and lacunae around it. Lamellar bone layer was absent at the surface of pedicle where it came in contact with periosteum. The core of pedicle was filled up with irregularly placed bony trabaculae separated by labyrinth and marrow spaces containing a large number of adipocytes (A) and haemopoetic cords separated by sinusoids [Figure 3]. Trabaculae (T) in pedicle was thin and composed of irregular lamellae of bone with lacunae containing osteocytes. They were isotropic and plate like. No loss of trabecular mass was noted in any of the cases.
|Figure 1: Photomicrograph of lumbar pedicle showing medial and lateral cortex with central core of cancellous bone (H & E; × 20)|
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|Figure 2: Photomicrograph of lumbar pedicle showing inferior cortex with trabecular bone and bone marrow (H & E; × 100)|
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|Figure 3: Photomicrograph showing trabeculae (T) and bone marrow with scattered adipocytes (A) in the lumbar pedicle (Hv& E; × 200)|
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Parameters were individually measured at the right and left sides of L3 vertebrae, detailed values for which are depicted in [Table 1] and [Table 2].
|Table 1: Values of superior (S), inferior (I), medial (M) and lateral (L) pedicular cortical wall thickness of L3 vertebrae in males.|
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|Table 2: Values of superior (S), inferior (I), medial (M) and lateral (L) pedicular cortical wall thickness of L3 vertebrae in females (in millimeter).|
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- Superior (S) cortex: Mean thickness was 1.3 mm ± 0.4. Minimum superior cortical thickness was 0.8 mm and maximum 2.9 mm.
- Inferior (I) cortex: Mean thickness was 1.6 mm ± 0.5. Minimum inferior cortical thickness was 0.9 and maximum 3.0 mm.
- Medial (M) cortex: Mean thickness was 0.8 mm ± 0.2. Minimum medial cortical thickness was 0.5 mm and maximum 1.3 mm.
- Lateral (L) cortex: Mean thickness was 0.6 mm ± 0.2. Minimum lateral cortical thickness was 0.4 mm and maximum 1.2 mm.
The cortex of pedicle was thicker on medial than on lateral wall. Pedicle cortex was thicker on inferior than on superior wall. The section of pedicle resembles a shell with an outer cortex surrounding inner cancellous bone.
| Discussion|| |
Extensive anatomic studies of lumbar spine have been carried out to provide surgeons with sufficient knowledge about external pedicle dimensions. However there was lack of data concerning the internal architecture of the pedicle. The present qualitative analysis of lumbar pedicle has been done to enlighten the importance of microstructure of pedicle.
Lateral and medial cortical walls are not similar in thickness, with medial wall presenting a thicker cortex bone. Kothe et al. also reported difference of thickness on vertebral pedicles’ cortices. According to these authors, the pedicle’s lateral cortical thickness ranged from 0.4 to 0.6 mm, and the medial cortical from 0.9 to 1.7 mm. Pedicle’s cortical bone layer has been described as having distinct characteristics from the cortical bone layer that coats the vertebral body.
Cortical dimensions of the lumbar pedicle have been studied using CT scans, but the histological architecture of the pedicle has been studied rarely. It was reported that the cortex of the pedicle in each human lumbar vertebra had an osteonal structure with haversian canals laid down mainly in the anteroposterior (longitudinal) direction. The organization of osteons across the transverse cross-section was not homogeneous. The layer of lamellar bone that typically envelops cortical bone structures (such as in long bones) was not observed, and the lateral cortex was significantly thinner than the medial cortex. The cortical bone surrounding the pedicle differed from bone in other anatomical regions such as the anterior vertebral body and femur. The osteonal orientation and lack of a lamellar sheath may have accounted for the unique deformation characteristics of the pedicle cortex seen during pedicle screw placement.
In a study on morphological properties of eight (L3) cadaveric lumbar pedicles using high resolution Micro- CT system, it was observed that the structure of the pedicular cancellous bone was different from the vertebral body. The trabecular architecture within the pedicle was isotropic and plate-like and the thickness and number of the trabeculae were greater than those of vertebral trabeculae. Decrease in the bone volume with age is mainly by thinning of the trabeculae and increase in trabecular spacing, but not by loss of mass. This was true for all lumbar pedicles in our study.
The thoracic and lumbar pedicle diameter related to pedicle screw size was studied in six human cadaver spines using imaging techniques. Measurements were taken of outer cortical diameters of pedicle before and after screw insertion (increasingly larger screws). It was observed that expansion of pedicle was due to plastic deformity as increased diameter was maintained after screw removal and 72% of pedicle fractures occurred laterally. On sectioning it was observed that pedicle is a thin shell of cortical bone filled with cancellous bone.
Our observation in this study is consistent with those from previous studies described in literature. While working on cortical dimension of pedicle width using CT scan of lumbar spine in 41 patients of Chinese origin, it was found that pedicle cortex was thicker on medial aspect than on lateral aspect at all levels (L1-L5). Medial cortical thickness gradually increased from LI (1.5 ± 0.3 mm) to L5 (2.2 ± 0.7 mm). Lateral cortical thickness increased from LI (1.0 ± 0.3 mm) to L5 (1.8 ± 0.5 mm). Pedicle cortex was thicker on superior than on inferior wall at L3, L4, L5 levels. Minimum superior cortical thickness was at LI (3.2 ±1.0 mm) and maximum at L4 (4.1 ± 1.2 mm). Minimum inferior cortical thickness was at L5 (2.0 ± 0.9 mm) and maximum at L2 (3.8 ± 0.9 mm).
The values in the present study are comparatively lower than the other studies. The difference in values could be because of difference in race, techniques used and the observer’s bias cannot be ruled out.
The difference between right and left side was statistically insignificant (p < 0.05) similarly difference between male and female was also statistically insignificant (p < 0.05).
There was statistical difference between superior and inferior cortex (p value 0.0282) and so was case between medial and lateral cortex as ‘P’ value was less than 0.0001, by conventional criteria, this difference is considered to be extremely statistically significant.
In the histological examination of pedicle in the current study the thickness of cortex was found to be more on medial side compared to the lateral side of the pedicle. The cortical bone of the inferior side of the pedicle was the thickest of all margins. The core of pedicle was filled up with irregularly placed bony trabeculae and marrow spaces. The present findings were in accordance with those reported by Inceoglu et al. It is a common clinical finding that most of pedicle fractures related to pedicle screw occur at the lateral wall of pedicle. This could be explained by the findings of current study, that the lateral wall is the thinnest.
| Conclusion|| |
The section of pedicle resembles a shell with an outer cortex surrounding inner cancellous bone. The thickness of the cortical bone is not homogeneous, being thicker on medial side compared to lateral side of the pedicle and cortical bone of the inferior side was thickest of all margins. The cortical thickness when compared to Chinese population was found to be much thinner for North West Indian population. The present study provides useful data for Indian surgeons while using pedicular screw fixation.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]