Measurement of effective pedicle diameter in the human spine

Children with Spinal Muscular Atrophy Have Reduced Vertebral Body Height and Depth and Pedicle Size in Comparison to Age-Matched Healthy Controls

BACKGROUND

Most children with spinal muscular atrophy (SMA) develop spinal deformity, which may require surgical intervention. In addition to poor bone stock, vertebral body shape may hinder the placement of spinal implants resulting in complications and poor outcome. The aim of this study was to analyze whether vertebral body morphology of children and adolescents with SMA is altered in comparison to healthy age-matched controls.

METHODS

In this prospective cohort study, 17 children with SMA (mean age 8.7 ±1.0 years) and 13 adolescents with SMA (mean age 13.6 ±1.4 years), all with some degree of neuromuscular scoliosis, were analyzed by standardized radiographic measurements to evaluate vertebral body height and depth. Results were compared with age-matched healthy controls (n = 10 children; mean age 9.1 ± 1.6 years; n = 20 adolescents, mean age 13.1 ± 0.5 years). Computed tomography scans of 27 adolescents with SMA (13.5 ±1.2 years) and 25 healthy age-matched controls (13.8 ±2.0 years) were analyzed to define pedicle diameters.

RESULTS

All children and adolescents with SMA had decreased vertebral height and depth in comparison to age-matched healthy controls. In adolescents, reduced depth was more pronounced than height in the thoracic spine. Pedicle size was significantly reduced in the lower thoracic and lumbar area.

CONCLUSIONS

Reduced vertebral body height and depth and pedicle size in children and adolescents with SMA may influence surgical treatment of spinal deformity. Surgeons should be aware of anatomical differences and choose implant devices accordingly.

Micro-computed tomography analysis of the lumbar pedicle wall

Background

Although the pedicle is routinely used as a surgical fixation site, the pedicle wall bone area fraction (bone area per unit area) and its distribution at the isthmus of the pedicle remain unknown. The bone area fraction at the pedicle isthmus is an important factor contributing to the strength of pedicle screw constructs. This study investigates the lumbar pedicle wall microstructure based on micro-computed tomography.

Methods

Six fresh-frozen cadaveric lumbar spines were analyzed. Left and right pedicles of each vertebra from L1 to L5 were resected for micro-computed tomography scanning. Data was analyzed with custom-written software to determine regional variation in pedicle wall bone area fraction. The pedicular cross-section was divided into four regions: lateral, medial, cranial, and caudal. The mean bone area fraction values for each region were calculated for all lumbar spine levels.

Results

The lateral region showed lower bone area fraction than the medial region at all spinal levels. Bone area fraction in the medial region was the highest at all levels except for L4, and the median values were 99.8% (95.9–100%). There were significant differences between the lateral region and the caudal region at L1, L2 and L3, but none at L4 and L5. The bone area fraction in the lateral region was less than 64% at all spinal levels and that in the caudal region was less than 67% at the L4 and L5 levels.

Conclusions

This study provides initial detailed data on the lumbar pedicle wall microstructure based on micro-computed tomography. These findings may explain why there is a higher incidence of pedicle screw breach in the pedicle lateral and caudal walls.

Accuracy of 837 pedicle screw positions in degenerative lumbar spine with conventional open surgery evaluated by computed tomography

BACKGROUND

The spatial and directional accuracy of the positioning of pedicle screws in the lumbosacral spine with conventional open surgery assessed by computed tomography (CT) has been published in several studies, systematic reviews and meta-analyses with a short-term follow-up. Inaccurate pedicle screw insertion may cause neurologic symptoms and weakens the construct.

METHODS

The data of 147 patients operated on with transpedicular screw fixation based on anatomical landmarks, supported by fluoroscopy, by a senior neurosurgeon in our clinic between 2000 and 2010 were analyzed retrospectively. The accuracy of the pedicle screw position was assessed by using postoperative CT images and graded in 2-mm increments up to 6 mm by two independent surgeons and partly by an independent radiologist.

RESULTS

A total of 837 lumbosacral pedicle screws were inserted in 147 randomly selected patients by a senior neurosurgeon. A mean accuracy of 85.7% of the screws being inside the pedicles was identified by the surgeon observers, with 3.3% being perforated 4 mm or more outside the pedicles. Postoperative neurologic symptoms were observed on the side corresponding to the breach in an average of 25.9% of patients with pedicle perforations, and 89.2% of the misplaced screws were either medially or inferiorly inserted.

CONCLUSIONS

Screw application reached a mean accuracy of 85.7% based on anatomical landmarks supported by fluoroscopy, warranting computer-assisted navigation for increased accuracy. Our results of 24 patients (16.3%) with the breached screws indicate that the direction of the breach may be more important than the absolute deviation in causing new neurologic symptoms.

Pedicle screw reinsertion using previous pilot hole and trajectory does not reduce fixation strength

STUDY DESIGN

Fresh-frozen human cadaveric biomechanical study.

OBJECTIVE

To evaluate the biomechanical consequence of pedicle screw reinsertion in the thoracic spine.

SUMMARY OF BACKGROUND DATA

During pedicle screw instrumentation, abnormal appearance on fluoroscopic imaging or low current reading with intraoperatively evoked electromyographic stimulation of a pedicle screw warrants complete removal to reassess for pedicle wall violation or screw malposition. However, screw fixation strength has never been evaluated biomechanically after reinsertion using a previous pilot hole and trajectory.

METHODS

Thirty-one thoracic individual fresh-frozen human cadaveric vertebral levels were instrumented bilaterally with 5.5-mm titanium polyaxial pedicle screws, and insertional torque (IT) was measured with each revolution. A paired comparison was performed for each level. Screw reinsertion was performed by completely removing the pedicle screw, palpating the tract, and then reinserting along the same trajectory. Screws were tensile loaded to failure "in-line" with the screw axis.

RESULTS

There was no significant difference for pedicle screw pullout strength (POS) between reinserted and control screws (732 ± 307 N vs. 742 ± 320 N, respectively; P = 0.78). There was no significant difference in IT between initial insertion for the test group (INI) (0.82 ± 0.40 N·m) and control (0.87 ± 0.50 N·m) (P = 0.33). IT for reinserted screws (0.58 ± 0.47 N·m) had significantly decreased compared with INI and control screws (29% decrease, P = 0.00; 33% decrease, P = 0.00, respectively). The test group screws in the thoracic spine had significant correlations between initial IT and POS (r = 0.79, P = 0.00), and moderate correlations between reinsertion IT and POS in the thoracic spine (r = 0.56, P = 0.00).

CONCLUSION

Despite a significant reduction in pedicle screw IT, there was no significant difference in pedicle screw POS with reinsertion. Therefore, when surgeons must completely remove a pedicle screw for tract inspection, reinsertion along the same trajectory may be performed without significantly compromising fixation strength.

LEVEL OF EVIDENCE

N/A.

Thoracic pedicle classification determined by inner cortical width of pedicles on computed tomography images: its clinical significance for posterior vertebral column resection to treat rigid and severe spinal deformities-a retrospective review of cases

Background

Posterior vertebral column resection (PVCR) is an effective alternative for treating rigid and severe spinal deformities. Accurate placement of pedicle screws, especially apically, is crucial. As morphologic evaluations of thoracic pedicles have not provided objective criteria, we propose a thoracic pedicle classification for treating rigid and severe spinal deformities.

Methods

A consecutive series of 56 patients with severe and rigid spinal deformities who underwent PVCR at a single institution were reviewed retrospectively. Altogether, 1098 screws were inserted into thoracic pedicles at T2-T12. Based on the inner cortical width of the thoracic pedicles, the patients were divided into four groups: group 1 (0–1.0 mm), group 2 (1.1–2.0 mm), group 3 (2.1–3.0 mm), group 4 (≥3.1 mm). The proportion of screws accurately inserted in thoracic pedicles for each group was calculated. Statistical analysis was also performed regarding types of thoracic pedicles classified by Lenke et al. (SPINE 35:1836-1842, 2010) using a morphological method.

Results

There were statistically significant differences in the rates of screws inserted in thoracic pedicles between the groups (P < 0.008) except groups 3 and 4 (P > 0.008), which were then combined. The accuracies for the three new groups were 35.05%, 65.34%, and 88.32%, respectively, with statistically significant differences between the groups (P < 0.017). Rates of screws inserted in thoracic pedicles classified by Lenke et al. (SPINE 35:1836-1842, 2010) were 82.31%, 83.40%, 80.00%, and 30.28% for types A, B, C, and D, respectively. There was no statistically significant difference (P > 0.008) between these types except between type D and the other three types (P < 0.008).

Conclusions

The inner cortical width of thoracic pedicles is the sole factor crucial for accurate placement of thoracic pedicle screws. We propose a computed tomography-based classification of the pedicle’s inner cortical width: type I thoracic pedicle: absent channel, inner cortical width of 0–1 mm; type II: presence of a channel of which type IIa has an inner cortical width of 1.1–2.0 mm and type IIb a width of ≥2.1 mm. The proposed classification can help surgeons predict whether screws can be inserted into the thoracic pedicle, thus guiding instrumentation when PVCR is performed.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-278) contains supplementary material, which is available to authorized users.

Tapping insertional torque allows prediction for better pedicle screw fixation and optimal screw size selection

BACKGROUND CONTEXT

There is currently no reliable technique for intraoperative assessment of pedicle screw fixation strength and optimal screw size. Several studies have evaluated pedicle screw insertional torque (IT) and its direct correlation with pullout strength. However, there is limited clinical application with pedicle screw IT as it must be measured during screw placement and rarely causes the spine surgeon to change screw size. To date, no study has evaluated tapping IT, which precedes screw insertion, and its ability to predict pedicle screw pullout strength.

PURPOSE

The objective of this study was to investigate tapping IT and its ability to predict pedicle screw pullout strength and optimal screw size.

STUDY DESIGN

In vitro human cadaveric biomechanical analysis.

METHODS

Twenty fresh-frozen human cadaveric thoracic vertebral levels were prepared and dual-energy radiographic absorptiometry scanned for bone mineral density (BMD). All specimens were osteoporotic with a mean BMD of 0.60 ± 0.07 g/cm(2). Five specimens (n=10) were used to perform a pilot study, as there were no previously established values for optimal tapping IT. Each pedicle during the pilot study was measured using a digital caliper as well as computed tomography measurements, and the optimal screw size was determined to be equal to or the first size smaller than the pedicle diameter. The optimal tap size was then selected as the tap diameter 1 mm smaller than the optimal screw size. During optimal tap size insertion, all peak tapping IT values were found to be between 2 in-lbs and 3 in-lbs. Therefore, the threshold tapping IT value for optimal pedicle screw and tap size was determined to be 2.5 in-lbs, and a comparison tapping IT value of 1.5 in-lbs was selected. Next, 15 test specimens (n=30) were measured with digital calipers, probed, tapped, and instrumented using a paired comparison between the two threshold tapping IT values (Group 1: 1.5 in-lbs; Group 2: 2.5 in-lbs), randomly assigned to the left or right pedicle on each specimen. Each pedicle was incrementally tapped to increasing size (3.75, 4.00, 4.50, and 5.50 mm) until the threshold value was reached based on the assigned group. Pedicle screw size was determined by adding 1 mm to the tap size that crossed the threshold torque value. Torque measurements were recorded with each revolution during tap and pedicle screw insertion. Each specimen was then individually potted and pedicle screws pulled out "in-line" with the screw axis at a rate of 0.25 mm/sec. Peak pullout strength (POS) was measured in Newtons (N).

RESULTS

The peak tapping IT was significantly increased (50%) in Group 2 (3.23 ± 0.65 in-lbs) compared with Group 1 (2.15 ± 0.56 in-lbs) (p=.0005). The peak screw IT was also significantly increased (19%) in Group 2 (8.99 ± 2.27 in-lbs) compared with Group 1 (7.52 ± 2.96 in-lbs) (p=.02). The pedicle screw pullout strength was also significantly increased (23%) in Group 2 (877.9 ± 235.2 N) compared with Group 1 (712.3 ± 223.1 N) (p=.017). The mean pedicle screw diameter was significantly increased in Group 2 (5.70 ± 1.05 mm) compared with Group 1 (5.00 ± 0.80 mm) (p=.0002). There was also an increased rate of optimal pedicle screw size selection in Group 2 with 9 of 15 (60%) pedicle screws compared with Group 1 with 4 of 15 (26.7%) pedicle screws within 1 mm of the measured pedicle width. There was a moderate correlation for tapping IT with both screw IT (r=0.54; p=.002) and pedicle screw POS (r=0.55; p=.002).

CONCLUSIONS

Our findings suggest that tapping IT directly correlates with pedicle screw IT, pedicle screw pullout strength, and optimal pedicle screw size. Therefore, tapping IT may be used during thoracic pedicle screw instrumentation as an adjunct to preoperative imaging and clinical experience to maximize fixation strength and optimize pedicle "fit and fill" with the largest screw possible. However, further prospective, in vivo studies are necessary to evaluate the intraoperative use of tapping IT to predict screw loosening/complications.

Computed-tomography-based anatomical study to assess feasibility of pedicle screw placement in the lumbar and lower thoracic pediatric spine

PURPOSE

The anatomy of the pedicle is complex and three-dimensional; however, there are basic dimensions important for possible screw placement. There are relatively few studies examining the pedicle anatomy in children. This study was performed to evaluate the feasibility of pedicle screw placement in children aged 5-16, based on key anatomic dimensions. A case illustration is also provided.

METHODS

The CT scans of 102 consecutive children were studied. Patients with abnormal anatomy were excluded. The parameters of the pedicle isthmus width (W), estimation of screw length (L), and axial angle (A) were recorded for 1,632 pedicles from T10 through L5. Patients were divided into four age groups. Statistical analysis was performed evaluating the difference between males and females and of the particular anatomy at the thoracolumbar junction.

RESULTS

The pedicles increase in both L and W from T10-T12 and from L1-L5. L1 has a consistently smaller W compared to T12 in both genders over all age ranges. Estimating a W of 4.5 mm necessary for safe screw placement, we calculate that virtually all pedicles of T12 and L3-L5 are large enough for screw placement in both genders after age 8. L4 and L5 are large enough for screw placement in both genders in the youngest age range.

CONCLUSIONS

Most of the pedicles of the lower lumbar spine and T12 are large enough to house the smallest commercially available screw. Understanding of the anatomy at the thoracolumbar junction is important, as the W of L1 is consistently smaller than T12.

Pedicle screw placement with a free hand technique in thoracolumbar spine: is it safe?

STUDY DESIGN

Computerized tomography (CT) analysis of in vivo pedicle screw placement to determine their exact position in relation to the pedicle in thoracolumbar region (T10-L3).

OBJECTIVE

To evaluate the clinical accuracy of the placement of thoracolumbar pedicle screws with a free hand technique after reviewing preoperative imaging.

SUMMARY OF BACKGROUND DATA

CT scans have been used in research clinical settings to evaluate pedicle screw placement in thoracolumbar spine.

MATERIAL AND METHODS

Ninety-eight consecutive patients who underwent posterior stabilization using 640 transpedicular screws by 1 surgeon from T10 to L3 were analyzed. The mean age was 36.1+/-15.1 (13 to 73) years at the time of surgery. Pedicle screws were inserted using anatomic landmarks; specific entry sites, preoperative anteroposterior/lateral radiographs, and CT imaging were used to guide the surgeon. After preparation of entry point, a pedicle probe was carefully advanced free hand down the pedicle into the body. Careful palpation of all bony borders (flour and 4 pedicle walls) was performed before placement the screw. Postoperative CT scans were used to evaluate the position of all pedicle screws inserted. Screw cortical penetration was graded on the basis of anatomy (the wall penetrated) and distance of the penetration, with 2 mm of interval.

RESULTS

The number of screws inserted at each level were as follows: T10 (n=55), T11 (n=124), T12 (n=118), L1 (n=91), L2 (n=146), and L3 (n=106). Analysis of these pedicle screws using postoperative CT scans confirmed 37 (5.8%) violated screws, including 12 medial, 18 lateral, 2 superior, 1 inferior, and 4 anterolateral vertebral body penetrations. No neurologic, vascular, or pleural injuries occurred. No screws required postoperative repositioning.

CONCLUSIONS

Pedicle screw placement with a free hand technique after reviewing preoperative imaging seems to be accurate, reliable, and safe adjunct for the placement of thoracolumbar spine screws.

Thoracic pedicle screw placement: analysis using anatomical landmarks without image guidance

STUDY DESIGN

Prospective laboratory study analyzing the technique of pedicle screw placement in a cadaveric model.

OBJECTIVES

To determine whether a freehand technique without image guidance can be used to safely place pedicle screws in the thoracic spine.

SUMMARY OF BACKGROUND DATA

The use of thoracic pedicle screws for the treatment of spinal deformity has been gaining increased acceptance among surgeons. Although these implants improve deformity correction, there is still concern regarding the risks to neurological and vascular structures and regarding the experience level needed to use this implant. This study was designed to determine whether these implants could be placed safely without imaging modalities.

METHODS

Six fresh cadaveric specimens were instrumented from vertebral segments T4-T11. Ninety-six screws were placed along the anatomical axis of the pedicle. Pedicles were dissected to determine the wall violations, the position of neural structures, and the lateral coverage of the pedicle by the rib head.

RESULTS

Ninety-seven percent of screws had less than 1 mm of wall violation, with 84 screws (87.5%) fully contained within the pedicle. Four screws (4.16%) violated the medial cortex. No violations occurred superiorly, inferiorly, or anteriorly. Nerve roots were in contact with the inferior pedicle wall at all levels. The average distance from nerve to the superior pedicle ranged from 3.85 to 5.04 mm.

CONCLUSIONS

Placing pedicle screws along the anatomical axis without image guidance produced a low level of pedicle wall disruption. This technique uses a reproducible start point at each level, and the results are equal to or better than those of other cadaveric studies that have used guidance systems.

Is it safer to place pedicle screws in the lower thoracic spine than in the upper lumbar spine?

STUDY DESIGN

An anatomic study of 100 patients comparing the pedicle isthmic width of the lower thoracic spine and the upper lumbar spine using magnetic resonance imaging.

OBJECTIVES

To compare the lower thoracic pedicles and upper lumbar pedicles in nondeformity patients as a surrogate measure of safety of pedicle screw use.

SUMMARY OF BACKGROUND DATA

Pedicle isthmic width is the significant limiting factor in the safety and proper placement of transpedicular screws. The presumption in the past has been that the lumbar pedicles are larger than the thoracic pedicles. Few publications in the English-language literature specifically evaluate the association between the pedicle isthmic widths of the lower thoracic and upper lumbar.

METHODS

The study evaluates 100 patients, without coronal spinal deformities. MRIs were obtained of the pedicles from T10 to L2 and subsequently measured using the axial T2-weighted views. Lower thoracic and upper lumbar pedicle isthmus, the narrowest section of pedicle, was investigated and compared. The "medial pedicle to medial rib corridor" at T10-T12 was defined and measured as part of the methodology of the study. Statistical analysis included one-way analysis of variance with post hoc least significant difference pairwise comparisons.

RESULTS

The smallest pedicle isthmic width was at L1 (mean +/- SD, 6.0 +/- 1.6 mm), while T12 (mean +/- SD, 7.6 +/- 1.5 mm) had the largest pedicle width. Although smaller in diameter than T12, both T10 (mean +/- SD, 6.2 +/- 1.2 mm) and T11 (mean +/- SD, 7.5 +/- 1.6 mm) had larger pedicle width than L1 (P < 0.01). Pedicle widths were larger in males compared with females (P < 0.05).

CONCLUSIONS

The results show that the lower thoracic pedicles are larger than the upper lumbar pedicles. This may make it safer to place screws in the lower thoracic spine than in the upper lumbar spine. Upper lumbar may be so small (<5 mm) to preclude safe conventional screw placement.

Human osseous intervertebral foramen width

Alterations of the width of the human intervertebral foramen can play a pathophysiological role in low back pain. Osseous dimensions of the human intervertebral foramen are rarely recorded. Therefore, we present reference data obtained from skeletal samples of known lifestyle, population affinity, sex, and age at death. Cervical, thoracic, and lumbar vertebrae of functional transition zones of 71 macroscopically normal spines from early 19th century AD Swiss burial sites were selected. The intervertebral foramen widths (IFW) were analyzed with respect to possible lateralization and the impact of sex, individual age, and stature. Neither a significant side difference nor a correlation of IFW with individual age or stature could be found. Females show somewhat larger IFW than males, especially in the lumbar region. Data comparisons with earlier studies are limited due to methodological differences and possible interpopulational variations. Furthermore, the osseous intervertebral foramen only reveals a glimpse of the clinically relevant in vivo structure. Nevertheless, more focus on the osseous dimensions of the intervertebral foramen will provide baseline data of this important anatomical landmark. These data could also explore the peculiarities of the intervertebral foramen, such as its reverse sex dimorphism.

Effect of various tapping diameters on insertion of thoracic pedicle screws: a biomechanical analysis

STUDY DESIGN

A biomechanical cadaver study to assess the effect of various tapping diameters on thoracic pedicle screw insertional torque.

SUMMARY OF BACKGROUND DATA

Thoracic pedicle screws are now commonly used for deformity and nondeformity cases. The optimal insertion techniques, however, have not been determined.

PURPOSE

To investigate the effect of various tapping techniques before insertion of thoracic pedicle screws in terms of maximal insertional torque (MIT) or screw pullout.

MATERIALS AND METHODS

Thirty-four fresh cadaveric thoracic vertebrae were harvested and evaluated with dual-energy radiograph absorptiometry (DEXA) to assess bone mineral density (BMD). Twenty-three matched, fixed-head, 5.0-mm pedicle screws (group 1) were placed using the straight-forward (ST) trajectory (paralleling the endplate) at various thoracic levels after random side selection using either line-to-line tapping (5.0-mm tap) or 1-mm undertapping (4.0-mm tap) under direct and fluoroscopic visualization. After this, 11 matched 5.0-mm pedicle screws (group 2) were placed comparing undertapping by 0.5 mm (4.5-mm tap) with 1 mm undertapping (4.0-mm tap). MIT was recorded for each screw revolution with a digital torque wrench.

RESULTS

BMD averaged 0.732 g/cm2 (0.620-0.884 g/cm2) for group 1, and 614 g/cm2 (0.533-0.697 g/cm2) for group 2. In group 1, the average MIT was 0.153 +/- 0.009 (SE) Nm for line-to-line tapping and 0.295 +/- 0.021 (SE) Nm for 1-mm undertapping, a 93% increase in MIT (P < 0.0005). In group 2, the average MIT was 0.138 +/- 0.009 (SE) Nm for 0.5 mm undertapping and 0.202 +/- 0.018 (SE) Nm for undertapping by 1 mm, a 47% increase in MIT (P = 0.03). BMD correlated with undertapping by 1 mm in group 1 (P < 0.0005), but not with undertapping by 0.5 mm (P = 0.087), although there appeared to be a trend in osteoporotic specimens. There were no noted differences in MIT between thoracic regions/levels, despite small differences in thoracic pedicle widths (P = 0.193).

DISCUSSION AND CONCLUSION

Undertapping the thoracic pedicle by 1-mm increases MIT by 47% (P = 0.03) when compared with undertapping by 0.5 mm, and by 93% (P < 0.0005) when compared with tapping line-to-line.

Accuracy of thoracic vertebral body screw placement using standard fluoroscopy, fluoroscopic image guidance, and computed tomographic image guidance: a cadaver study

STUDY DESIGN

A surgical simulation study in human cadaver spine specimens was conducted to evaluate the accuracy of thoracic vertebral body screw placement using four different intraoperative imaging techniques.

OBJECTIVE

To compare standard fluoroscopy, fluoroscopy-based image guidance with two different referencing methods, and computed tomography-based image guidance by the measuring the time required for screw placement, the radiation exposure to specimen and surgeon, and the accuracy of screw position in the thoracic spine.

SUMMARY OF BACKGROUND DATA

Image guidance provides additional anatomic information to the surgeon and may improve safety of technically difficult surgical procedures. The placement of screws in the thoracic spine is a technically demanding procedure in which inaccurate screw positioning places the spinal cord, nerve roots, and paraspinal structures such as the aorta and pleural space at risk for injury. Image-guided surgery may improve the accuracy of thoracic screw placement.

METHODS

Using four different intraoperative imaging methods, two experienced surgeons placed 337 vertebral body screws through the pedicles of thoracic vertebrae in 20 human cadaver thoracic spine specimens. The specimens then were examined with radiographs, computed tomography, and anatomic dissection to determine screw position. Measurements included procedure setup and screw insertion time, radiation exposure to the specimen, the surgeon's hand, the surgeon's body, frequency, direction, and magnitude of screw perforation through the cortical margins of thoracic vertebrae.

RESULTS

As compared with surgery using standard fluoroscopy, fluoroscopy-based image guidance that uses multiple reference marks and computed tomography-based image guidance improves the accuracy of thoracic vertebral body screws, but increases the time required for screw placement and the specimen radiation exposure. Exposure to radiation is minimal at the surgeon's body level and dependent on surgical technique at the surgeon's hand level. Screw perforation occurs most frequently in the lateral direction.

CONCLUSIONS

Fluoroscopy-based image guidance that uses only a single reference marker for the entire thoracic spine is highly inaccurate and unsafe. Systems with registration based on the instrumented vertebrae provide more accurate placement of thoracic vertebral body screws than standard fluoroscopy, but expose the patient to more radiation and require more time for screw insertion.

Analysis of vertebral morphology in idiopathic scoliosis with use of magnetic resonance imaging and multiplanar reconstruction

BACKGROUND

Several studies have provided data on the vertebral morphology of normal spines, but there is a paucity of data on the vertebral morphology in patients with idiopathic scoliosis.

METHODS

The morphology of the pedicles and bodies of 307 vertebrae as well as the distance between the pedicles and the dural sac (the epidural space) in twenty-six patients with right-sided thoracic idiopathic scoliosis were analyzed with use of magnetic resonance imaging and multiplanar reconstruction.

RESULTS

A distinct vertebral asymmetry was found at the apical region of the thoracic curves, with significantly thinner pedicles on the concave side than on the convex side (p < 0.05). The degree of intravertebral deformity diminished farther away from the apex, with vertebral symmetry restored at the neutral level. In the thoracic spine, the transverse endosteal width of the apical pedicles measured between 2.3 mm and 3.2 mm on the concave side and between 3.9 mm and 4.4 mm on the convex side (p < 0.05). In the lumbar spine, the pedicle width measured between 4.6 mm at the cephalad part of the curve and 7.9 mm at the caudad part of the curve. The chord length and the pedicle length gradually increased from 34 mm and 18 mm, respectively, at the fourth thoracic vertebra to 51 mm and 25 mm, respectively, at the third lumbar vertebra. The transverse pedicle angle measured 15 in the cephalad aspect of the thoracic spine, decreased to 7 at the twelfth thoracic vertebra, and increased again to 16 at the fourth lumbar vertebra. The width of the epidural space was <1 mm at the thoracic apical vertebral levels and averaged 1 mm at the lumbar apical vertebral levels on the concave side, whereas it was between 3 mm and 5 mm on the convex side (p < 0.05).

CONCLUSION

Idiopathic scoliosis is associated with distinctive intravertebral deformity, with smaller pedicles on the concave side and a shift of the dural sac toward the concavity.

Pedicle morphology of the immature thoracolumbar spine

STUDY DESIGN

Human vertebral morphologic data were compiled from anatomic skeletal collections from three museums.

OBJECTIVES

To quantify the morphometric characteristics of the pedicles of the immature thoracolumbar spine.

SUMMARY OF BACKGROUND DATA

Little is known of pedicle morphology of the immature spine as related to pedicle screw fixation.

METHODS

A total of 75 anatomic skeletal specimens were acquired from C1 to L5 in the age range of 3 to 19 years. The data were collected and analyzed using a computerized video analysis system. Each vertebral pedicle was measured in the axial and sagittal planes. The measurements included the minimum pedicle width, the pedicle angle, the distance to anterior cortex, and anteroposterior and interpedicular spinal canal diameters.

RESULTS

Wide variation in pedicle morphology between specimens at each vertebral level was found in the young population. In general, compared with the average adult data, a younger spine demonstrated a near uniform reduction in the linear pedicle dimensions at each vertebral level. Pedicles from the lower lumbar vertebrae attained dimensions acceptable for standard screw sizes at an earlier age than in the thoracic vertebrae.

CONCLUSIONS

The data in this study indicates that pedicle screws may be used in the adolescent spine. However, care should taken to accurately ascertain pedicle size before surgery so that improper use of screws can be avoided. Growth of the pedicles in relation to the spinal canal indicates that the increase in pedicle size is lateral to the spinal canal.

Pedicle morphology in thoracic adolescent idiopathic scoliosis: is pedicle fixation an anatomically viable technique?

STUDY DESIGN

A radiographic study of thoracic pedicle anatomy in a group of adolescent idiopathic scoliosis (AIS) patients.

OBJECTIVE

To investigate the anatomic constraints of the thoracic pedicles and determine whether the local anatomy would routinely allow pedicle screw insertion at every level.

SUMMARY OF BACKGROUND DATA

In spite of the clinical successes reported with limited thoracic pedicle screw-rod constructs for thoracic AIS, controversy exists as to the safety of this technique.

MATERIAL AND METHODS

Twenty-nine patients with right thoracic AIS underwent preoperative thoracic CT scans and plain radiographs. Anatomic parameters were measured from T1 to T12.

RESULTS

Information on 512 pedicles was obtained. The transverse width of the pedicles from T1 through T12 ranged from 4.6-8.25 mm. The medial pedicle to lateral rib wall transverse width from T1 through T2 ranged from 12.6 to 17.9 mm. Measured dimensions from the CT scans showed the actual pedicle width to be 1-2 mm larger than would have been predicted from the plain radiographs. Age, Risser grade, curve magnitude, and the amount of segmental axial rotation did not correlate with the morphology or size of the thoracic pedicles investigated. In no case would pedicle morphology have precluded the passage of a pedicle screw.

CONCLUSION

Based on the data identified in this group of adolescent patients, it is reasonable to consider pedicle screw insertion at most levels and pedicle-rib fixation at all levels of the thoracic spine during the treatment of thoracic AIS.

Internal morphology of human cervical pedicles

STUDY DESIGN

The internal architecture of cervical spine pedicles was investigated by thin sectioning and digitization of radiographic images.

OBJECTIVES

To provide quantitative information on the internal dimensions and cortical shell thicknesses of the middle and lower cervical pedicles.

SUMMARY OF BACKGROUND DATA

Although there have been a number of studies presenting data on the external dimensions of the cervical pedicle, little is known regarding its internal architecture and cortical shell thickness along the pedicle axis.

METHODS

Twenty-five human cervical vertebrae (C3-C7) were secured to a thin-sectioning machine to produce three 0.7-mm-thick pedicle slices along its axis. Plain radiographs of the pedicle slices were scanned and digitized to facilitate measurement of the internal dimensions. Computer software was specifically developed to determine the external dimensions (i.e., pedicle height and width) and the internal dimensions (i.e., cortical shell thicknesses of the superior, inferior, lateral, and medial walls and the cancellous core height and width) of cervical pedicles.

RESULTS

Superior and inferior wall cortical thicknesses of pedicle thin slices were similar, whereas the lateral wall cortical thickness was significantly smaller than the medial wall thickness. The medial cortical shell (average value range: 1.2-2.0 mm) was measured to be 1.4 to 3.6 times as thick as the lateral cortical shell (average value range: 0.4-1.1 mm). When medial and lateral cortical thicknesses were normalized for external dimensions, the combined cortical shell thickness was thinnest at C7 (average value range: 18. 6-25.6% of the external width), and this result was statistically significant when compared with other vertebral levels.

CONCLUSIONS

The cervical pedicle is a complex, three-dimensional structure exhibiting extensive variability in internal morphology. Characteristics of the cervical pedicle at different spinal levels must be noted before transpedicular screw fixation.

Morphometric analysis of thoracic and lumbar vertebrae in idiopathic scoliosis

STUDY DESIGN

Prospective study on the morphometry of 337 pedicles in 29 patients with idiopathic scoliosis.

OBJECTIVES

To analyze by means of computed tomographic scans the vertebral morphometry in idiopathic scoliosis treated by pedicle screw instrumentation.

SUMMARY OF BACKGROUND DATA

Although several studies exist on the vertebrae's morphometry in normal spines, little is known concerning the morphometry of scoliotic vertebrae.

METHODS

The pedicles' morphometry between T5 and L4 was analyzed by computed tomographic scans in 29 surgically treated patients with idiopathic right thoracic scoliosis. Measurements included chord length, endosteal transverse pedicle width, transverse pedicle angle, and pedicle length.

RESULTS

The endosteal transverse pedicle width was significantly smaller (P < 0.05) on the concavity in the apical region of the thoracic spine and measured between 2.5 and 4.2 mm in the middle thoracic spine (T5-T9) and between 4.2 and 5.9 mm in the lower thoracic spine (T10-T12). In the lumbar spine, the width varied between 4.8 and 9.5 mm without significant differences between the concave and convex sides (P > 0.05). The chord length was shortest at T5, measuring 37 mm and increased gradually to 50 mm at L3 with significantly larger dimensions in male patients and on the concavity of the apical region in the thoracic spine (P < 0.05). The pedicle length varied minimally, with a range of between 20 and 22 mm, and was relatively consistent throughout the thoracic and lumbar spine. The transverse pedicle angle varied between 6 degrees in the lower thoracic spine and 12 degrees in the upper thoracic and lower lumbar spine.

CONCLUSION

The morphometry in scoliotic vertebrae is substantially different from that of vertebrae in normal spines, with an asymmetrical intravertebral deformity shown in scoliotic vertebrae. Pedicle screw instrumentation on the concavity in the apical region of thoracic curves appears critical because of the small endosteal pedicle width.

The radiologic anatomy of the lumbar and lumbosacral pedicles

STUDY DESIGN

An anatomic and radiologic study of lumbar and lumbosacral pedicle anatomy.

OBJECTIVES

To define the radiologic anatomy of the lumbar and first sacral pedicle in the coaxial projection.

SUMMARY OF BACKGROUND DATA

Fluoroscopic assistance for pedicle screw placement requires radiologic landmarks. The radiologic landmarks have previously been assumed. Detailed study of the correlation between anatomy and radiology is required.

METHODS

Lumbar vertebrae and sacra were marked with radiopaque material to demonstrate the pedicle cortical borders. The vertebrae were then imaged in the coaxial projection to determine the correlation between the pedicle cortex and the radiologic image. Pedicle dimensions were recorded.

RESULTS

Pedicle dimensions were consistent with known measurements, yet the long axis of the L4 and L5 pedicle ellipse was oblique to the vertical. Consequently, the minor diameter of the pedicle ellipse was considerably less than the measured pedicle width at L5. The radiologic pedicle image was consistently within the true pedicle cortex, by up to 3 mm, and probably represents the inner cortical border of the pedicle. The S1 pedicle has reliable anatomic landmarks, yet only the medial and superior borders were visualized.

CONCLUSIONS

The radiologic pedicle image in the lumbar and lumbosacral spine is a reliable guide to the true bony cortex of the pedicle. At S1 the pedicle image is less well correlated with the cortical borders of the pedicle, yet other reliable anatomic landmarks exist.

Comparison of the strengths of lumbosacral fixation achieved with techniques using one and two triangulated sacral screws

STUDY DESIGN

The strength of sacral screw fixation achieved with techniques using one sacral screw and two triangulated sacral screws with Chopin block was tested on 10 fresh human sacrum specimens.

OBJECTIVES

To compare the stiffness and the failure strength of single sacral screw fixation and those of two divergent triangulated sacral screw fixation under different loading conditions.

SUMMARY OF BACKGROUND DATA

Weak sacral screw fixation is the major cause of failure in instrumented lumbosacral fusion. Although previous studies have evaluated the strength of anteromedially and anterolaterally inserted sacral screws, the strength of two divergent triangulated sacral screws has not been evaluated biomechanically. It is assumed that increasing the number of sacral screws and using a triangulated insertion technique may increase the strength of purchase for lumbosacral fixation.

METHODS

Ten fresh human sacrum specimens were used in this study. Six specimens were from donors less than 30 years of age (younger group), and the other four were from donors more than 60 years of age (aged group). The specimens were assigned randomly to either one side one S1 pedicle screw inserted anteromedially or to the other side with two screws directed anteromedially and anterolaterally. All specimens were tested with an Instron material testing machine (Instron Limited, High Wycombe, England). The average stiffness under compression, tension, and torsion were calculated from the average slopes of the load deformation curves. The failure load was applied for the final testing.

RESULTS

The results showed significant differences in stiffness between fixation with one screw and fixation with two divergent triangulated screws. With one S1 pedicle screw fixation, the average stiffness was 203 N/mm for compression, 147 N/mm for tension, and 2 Nm/degree for torsion. With two-screw fixation the average stiffness increased to 255 N/mm (126%), 185 N/mm (126%) and 2.4 Nm/degree (120%), respectively. The average failure strength under tension loads was found to be 1450 N in the younger specimens and was reduced to 980 N in the aged specimens.

CONCLUSION

This study showed that fixation with two divergent triangulated screws to the sacrum was significantly stronger than one-screw fixation for lumbosacral fusion. The strength of fixation seems to have a negative correlation with aging.

Moments and forces during pedicle screw insertion. In vitro and in vivo measurements

STUDY DESIGN

Moments and forces during pedicle screw insertion were measured in vivo and in vitro and were correlated to several parameters of the screw-bone interface.

OBJECTIVES

To compare the in vitro and in vivo screw insertion loads and to relate these measurements to bone mineral density, pedicle size, and other screw parameters (material, diameter).

SUMMARY OF BACKGROUND DATA

The in vitro screw insertion torque has been correlated to the screw pullout forces and the number of cycles to ultimate interface failure. However, there are no comparable in vivo data.

METHODS

One hundred three pedicle screws were included in the study, 43 in vivo and 60 in vitro. Duel-energy x-ray absorptiometry boen mineral density data were available for 20 in vivo and 32 in vitro specimens. A custom-made sterilizable six-axis load cell was integrated into a torque wrench, enabling the recording of the applied moments and forces during screw insertion. Statistical analysis was performed to detect differences and correlations.

RESULTS

The mean in vivo insertion torque (1.29 Nm) was significantly greater than the in vitro value (0.67 Nm). The linear correlation between insertion torque and bone mineral density was significant for the in vitro data but not for the in vivo data. No correlation was observed between insertion torque and pedicle diameter. Two patterns of torque were observed during the insertion process.

CONCLUSIONS

There is a significant difference between the insertion loads measured in vivo and those measured in vitro. Additional research is needed to verify whether this method provides an indication of screw fixation quality.

Structural characteristics of the pedicle and its role in screw stability

STUDY DESIGN

Cross-sectional regional bone mineral density of the pedicle was measured by peripheral quantitative computed tomography. Biomechanical tests were performed to clarify the role of the pedicle in screw stability.

OBJECTIVES

To identify the structural characteristics of the pedicle that supports pedicle screw stability and the differences in these characteristics between normal and osteoporotic vertebrae.

SUMMARY OF BACKGROUND DATA

The pedicle screw is an essential component of many systems used to align the spine. The contribution of the pedicle to screw stability, however, has not been fully investigated.

METHODS

Trabecular, subcortical, and cortical bone mineral density and the area of the pedicle were measured by peripheral quantitative computed tomography. Bone mineral density also was recalculated in four circumferential layers. These parameters were compared between normal and osteoporotic individuals. The relative contribution of the pedicle to screw stability was evaluated by caudocephalad and pull-out loading in a vertebra with or without its body.

RESULTS

Inner trabecular, middle subcortical, and outer cortical bone mineral density and cortical bone area in the pedicle were significantly lower in osteoporotic vertebrae than those in normal vertebrae. In the pedicle, bone mineral density increased close to the outer layer. Bone mineral density not as thick even in the outer layer in osteoporotic subjects. Approximately 80% of the caudocephalad stiffness and 60% of the pullout strength of the pedicle screw depended on the pedicle rather than on the vertebral body.

CONCLUSION

Screw stability depends on the structural characteristics of the pedicle. The pedicle was denser in the subcortical bone, in which the threads of the screw engage, than in trabecular bone. In osteoporosis, bone mineral density was not as dense even in the outer layer, and the cortex was thinner than normal. A larger screw would not enhance screw stability and may break the thin cortex in osteoporotic vertebrae.

The value of percutaneous transpedicular vertebral bone biopsies for histologic examination. Results of an experimental histopathologic study comparing two biopsy needles

STUDY DESIGN

A 3.5-mm trephine was designed to overcome difficulties encountered in the histologic evaluation of vertebral bone samples obtained with a 2-mm trephine.

OBJECTIVES

To compare the 3.5-mm trephine with the 2-mm trephine.

SUMMARY OF BACKGROUND DATA

A review of results obtained with a 2-mm trephine showed that histologic examination of vertebral bone cores was disturbed by artifacts in 32 of 70 cases (46%). Although tissue diagnosis was possible from 61 samples, only 36 (51%) bone cores yielded a secure diagnosis.

METHODS

Transpedicular bone cores were obtained from the bodies of 54 fresh cadaver vertebrae with both trephines. In each vertebra, the 2-mm trephine was used on one side, and the 3.5-mm trephine was used on the other side. Longitudinal sections were prepared and examined macroscopically for length and breakages and microscopically for trabeculae, marrow, and artifacts. Each sample was graded for its value for histologic examination.

RESULTS

Significant differences were found between the two trephines for all criteria evaluated. Of 54 samples taken with the 2-mm trephine, 13 (24%) were graded "good," compared with 45 (83%) from the 3.5-mm trephine. Twelve (22%) "bad" samples were taken from the 2-mm trephine compared with three (6%) "bad" samples taken from the 3.5-mm trephine.

CONCLUSIONS

The 2-mm trephine does not provide suitable bone cores for histologic examination, whereas samples obtained with the 3.5-mm trephine are suitable.

The role of imaging and in situ biomechanical testing in assessing pedicle screw pull-out strength

STUDY DESIGN

This study determined the predictive ability of quantitative computed tomography, dual energy x-ray absorptiometry, pedicular geometry, and mechanical testing in assessing the strength of pedicle screw fixation in an in vitro mechanical test of intra-pedicular screw fixation in the human cadaveric lumbar spine.

OBJECTIVE

To test several hypotheses regarding the relative predictive value of densitometry, pedicular geometry, and mechanical testing in describing pedicle screw pull-out.

SUMMARY OF BACKGROUND DATA

Previous investigations have suggested that mechanical testing, geometry, and densitometry, determined by quantitative computed tomography or dual energy x-ray absorptiometry, predict the strength of the screw-bone system. However, no study has compared the relative predictive value of these techniques.

METHODS

Forty-nine pedicle screw cyclic-combined flexion-extension moment-axial pull-out tests were performed on human cadaveric lumbar vertebrae. The predictive ability of quantitative computed tomography, dual energy x-ray absorptiometry, insertional torque, in situ stiffness, and pedicular geometry was assessed using multiple regression.

RESULTS

Several variables correlated to force at failure. However, multiple regression analysis showed that bone mineral density of the pedicle determined by quantitative computed tomography, insertional torque, and in situ stiffness when used in combination resulted in the strongest prediction of pull-out force. No other measures provided additional predictive ability in the presence of these measures.

CONCLUSIONS

Pedicle density determined by quantitative computed tomography when used with insertional torque and in situ stiffness provides the strongest predictive ability of screw pull-out. Geometric measures of the pedicle and density determined by dual energy x-ray absorptiometry do not provide additional predictive ability in the presence of these measures.

Internal architecture of the thoracic pedicle. An anatomic study

STUDY DESIGN

In this study, data are presented that provide the surgeon with additional information about the internal structure of the thoracic pedicle, which is especially useful for pedicle screw fixation in the thoracic spine.

OBJECTIVES

To quantify the internal structure of the pedicle in the thoracic spine.

SUMMARY OF BACKGROUND DATA

There are many studies describing the external dimensions of the thoracic pedicle (i.e., pedicle height, pedicle width, and pedicle axis in the transverse and sagittal planes). However, there is little reliable information concerning the internal structure of the pedicle.

METHODS

Eighteen thoracic vertebrae were attached to a thin-sectioning machine and both pedicles were cut in six 1.0-mm thin slices. Slides of contact radiographs were rear-projected to a digitizer and the internal and external borders of the pedicle were digitized. Using special computer software, two external dimensions (i.e., pedicle height and pedicle width) and four internal dimensions (i.e., cortical thicknesses of the superior, inferior, medial, and lateral walls) were calculated.

RESULTS

The cancellous core was more than twice as large as the cortical shell, with a range from 65.6% to 78.6% with respect to the pedicle height, and 61.3% to 71.6% with respect to the pedicle width. The medial wall was between two and three times thicker than the lateral wall throughout all the pedicle slices and thoracic levels. These differences were highly significant (P < 0.001).

CONCLUSIONS

The thoracic pedicle is a complex three-dimensional structure that is mostly filled with cancellous bone. The medial wall is significantly thicker than the lateral wall, which could explain the fact that most of the pedicle fractures related to pedicle screw insertion occur laterally.