Thoracic pedicle screw placement guided by computed tomographic measurements

Cadaveric pedicle screw placement guided by the measurements from axial computed tomography (CT) scans in the thoracic spine was assessed in this study. Axial CT scans were performed on four cadaveric thoracic spines, and the measurements included the pedicle transverse angle, inner pedicle width, and distance between the midline of the vertebra and the pedicle axis on the dorsal aspect of the lamina. With utilization of the data from CT scans, screws were directly placed into the thoracic pedicle from T1 to T10. Screw penetration of the pedicle was determined by gross examination. The results showed that the largest pedicle transverse angle was found at the levels of T1-2, and the smallest occurred at the T3 through T8 levels. The value of the pedicle inner width was quite different between specimens with a minimum of 3.0 mm at T4 and a maximum of 9.2 mm at T10. Gross examination of the pedicle showed that 13 (16.3%) of 80 screws penetrated the pedicle wall, with a Grade I penetration in 11 pedicles and a Grade II penetration in 2 pedicles. Screw penetration of the medial wall was found in four pedicles and penetration of the lateral wall was noted in nine pedicles. No screw penetration of the superior and inferior walls of the pedicle was identified in any of the four specimens. Thoracic pedicle screw placement guided by the measurements from axial CT scans significantly reduced the incidence of pedicle penetration. Axial CT measurements of the pedicle inner diameter and transverse angle as well as the starting point for screw insertion are recommended if pedicle screw fixation is intended in the thoracic spine.

The projection of the cervical disc and uncinate process on the posterior aspect of the cervical spine

BACKGROUND

Surgical techniques of foraminotomy for decompression of the cervical nerve have been well described in the literature. Excessive resection of the facet joint and laminae may decrease segmental stability and increase scar formation. How much bony resection is adequate to remove a soft or hard disc herniation is not known. No studies regarding this subject are available.

METHODS

Thirty-nine adult dry bone spines from C3 to C7 were used and four measurements on each vertebra were taken in this study. The first three measurements included the vertical distances between the superior borders of the lamina and the vertebral body measured at the midline of the laminae, the middle of the lamina, and the lamina-facet junction, respectively. The fourth was the horizontal distance between the medial most border of the superior facet and the tip of the uncinate process.

RESULTS

No significant differences between male and female specimens were found in any measurements in this study. The mean vertical distances from the superior border of the lamina inferior to the superior border of the vertebral body measured at the three points for all levels were approximately 1-3 mm, although the standard deviations for those were relatively high. The tip of the uncinate process was located from 2 mm at C3 to 1 mm at C6 medial to the medial most border of the superior facet, and then changed to be located 1 mm lateral to the medial most border of the superior facet.

CONCLUSIONS

This study suggests that a semicircular laminotomy placed on the inferior aspect of the lamina above may be adequate for a lateral soft disc herniation because the inferior border of the disc is higher than the superior border of the inferior lamina, whereas a traditional foraminotomy is needed for a hard disc pathology.

Anatomic relation between the cervical pedicle and the adjacent neural structures

STUDY DESIGN

An evaluation of the anatomic relation between the cervical pedicles and the adjacent neural structures.

OBJECTIVES

To determine quantitatively the antomic relation of the cervical pedicles to the adjacent nerve roots and dural sac.

SUMMARY OF BACKGROUND DATA

Transpedicular screw fixation in the cervical spine has been reported, but little quantitative data regarding the anatomic relation between the cervical pedicles and the surrounding neural structures are known.

METHODS

Twenty cadavers were used for dissection to observe the relations of the cervical pedicles to the adjacent dural sac and nerve roots. After removal of whole posterior bony elements including the spinous processes, laminas, and lateral masses, the isthmus of the pedicles, the dural sac, and the nerve roots of C3-C7 were exposed. Direct measurements included the distance from the pedicle to the superior and inferior nerve roots and the dura. Also, the pedicle height and width were measured at its isthmus.

RESULTS

No distance was found between the pedicle and the superior nerve root, nor between the pedicle and the dural sac at C3-C7 for all specimens. The mean distances between the pedicle and the inferior nerve roots for all specimens ranged from 1.4 to 1.6 mm. The mean pedicle heights and widths for all specimens at C3-C7 ranged from 6.0 to 6.5 mm and 4.7 to 5.3 mm, respectively. A significant difference between male and female specimens was noted in the pedicle heights for all levels measured (P = 0.001), and in the pedicle widths for the levels of C4 and C6 (P = 0.05).

CONCLUSIONS

This study suggests that the incidence of neurologic injuries may be higher in screw penetration of the medial or superior cortex of the pedicle than in screw penetration of the inferior cortex of the pedicle.

Extensile approach to the anterolateral surface of the humerus and the radial nerve

A proposed approach to the anterolateral surface of the humeral shaft that would allow for exploration of the radial nerve was studied in 30 cadaver arms. The incision starts proximally along the posterior border of the deltoid muscle and extends anteriorly and distally over the lateral border of the biceps muscle. A deep dissection is made in the internervous plane between the deltoid and the triceps muscles proximally and between the longitudinally split fibers of the brachialis muscle distally. The approach provides access to the anterolateral surface of the humerus up to the level of the axillary nerve and the posterior circumflex humeral vessels. The insertion of the deltoid muscle into the anterior border of the humerus is preserved and the radial nerve is protected by the triceps muscle proximally and by the retracted lateral portion of the brachialis muscle distally. The entire course of the radial nerve in the arm can be exposed. Proximally, the radial nerve can be exposed by elevating the lateral head of the triceps muscle from the humerus. Distally, the radial nerve can be exposed between the brachioradialis and the brachialis muscles. A plate can be applied on the anterolateral surface of the humerus without having to elevate the firmly attached anterior deltoid insertion.

Anatomic basis of the anterior surgery on the cervical spine: relationships between uncus-artery-root complex and vertebral artery injury

Vertebral artery injury is a serious complication during anterior surgery on the cervical spine. However, little information is available in the literature concerning the mechanism of vertebral artery laceration during the procedures of the anterior cervical decompression. In the current study twenty-eight cadavers were dissected to determine the location and relationships of the fibro-ligamentous tissues to the uncinate process, vertebral artery and nerve roots from the C3 to C6 levels. The vertebral artery and nerve root are encased by a fibro-ligamentous band at the level of the intertransverse space. This fibro-ligamentous band is attached to the lateral aspect of the uncinate process and uncovertebral joint, which combines the vertebral artery, nerve root and uncinate process to form a complex or unit. The fibro-ligamentous tissues between the uncovertebral joint and vertebral artery may explain the propensity to vertebral artery laceration during resection of the uncinate process or an osteophyte projecting from the uncovertebral joint. For this reason, the authors recommend that before resection of the uncinate process or uncovertebral joint is performed, it is necessary to thoroughly dissect the fibro-ligamentous tissues off the uncinate process.

Cartilage and synovium of the peroneocuboid joint: an anatomic and histological study

The surface area, thickness, and composition of the articular cartilage of the peroneocuboid articulation and the location of the synovium were investigated in 15 cadaver foot specimens. The articulations of the medial side of the peroneus longus tendon and lateral side of the cuboid were covered with fibrous and hyaline cartilages, respectively. On the lateral tuberosity of the cuboid, there is a facet that has 79.37+/-20.24 mm2 articular cartilage area with an oval shape to conform to that of the articular surface of the peroneus longus tendon (articular cartilage area, 67.35+/-28.53 mm2) with which it articulates. The mean thickness of the articular cartilage of the peroneus longus tendon and cuboid was 0.34+/-0.08 and 0.52+/-0.07 mm, respectively. The peroneocuboid joint has its own joint capsule. The synovial cavity does not communicate with the sheath of the peroneus longus tendon. Synovial membranes were attached to the margins of the articular surfaces of the cuboid immediately peripheral to the cartilage region.

The quantitative anatomy of the laminas of the spine

STUDY DESIGN

This study evaluated the dimensions of the laminas from C2 to L5 using adult spine specimens.

OBJECTIVES

To provide a set of quantitative data for the laminas from C2 to L5.

SUMMARY OF BACKGROUND DATA

Anatomic evaluation of the pedicle and facet joint in the spine has been extensively reported. No detailed studies of the laminas from the cervical to the lumbar spines exist.

METHODS

Thirty-seven spines from C2 to L5 were directly evaluated for this study. Anatomic evaluation of the laminas included the laminar height, width, thickness, and angulation.

RESULTS

In general, the measurements were greater in male specimens than in female specimens, although significant differences (P < 0.05) between male and female specimens were noted in only three measurements. The greatest laminar height was at T11 (25.1 +/- 2.5 mm), and the least was at C4 (10.4 +/- 1.1 mm). The greatest laminar width was at L5 (15.7 +/- 2.0 mm), and the least was at T4 (5.8 +/- 0.8 mm). Laminar widths in the cervical region were slightly more than those in the thoracic region. The greatest laminar thickness was at T2 (5.0 +/- 0.2 mm), whereas the least was at C5 (1.9 +/- 0.6 mm). Laminar thickness tended to increase in the upper thoracic region and to decrease slightly in the lower thoracic region. The mean laminar thickness of the lower cervical region was least in the whole spine. The widest angle was at C3 (116.1 +/- 8.8 degrees) and T7 (112.3 +/- 8.0 degrees) and the narrowest was at C2 (99.1 +/- 8.0 degrees) and L3 (99.9 +/- 6.3 degrees). The slope angles of the laminas varied from 97.8 +/- 3.0 degrees at T9 to 129.0 +/- 7.5 degrees at L3.

CONCLUSIONS

Surgical placement of sublaminar instruments may benefit from this quantitative study through the use of the provided anatomic parameters of the laminas.

Anatomic relationship of the cervical nerves to the lateral masses

Eight cervical specimens were transversely sectioned with slices approximately 2 mm to 3 mm in thickness to evaluate the anatomic relationship of the spinal nerves to the lateral masses. Results showed that the spinal nerve either does not appear or, when it does, is situated anteromedially to the superior facet on the cross sections through the upper portion of the superior facet. The anterolateral aspect of the superior facet is free from the spinal nerve. Cross sections through the lower pedicle of the vertebra showed that the spinal nerve rested on the transverse process anterolateral to the lateral mass. The mean distances between the posterior midline of the lateral mass and the posterior border of the spinal nerve measured 15 degrees in the lateral direction were 16.1+/-1.7 mm for C3, 16.5+/-1.8 mm for CA, 16.8+/-1.2 mm for C5, 16.3+/-2.0 mm for C6, and 8.5+/-0.9 mm for C7. This study suggests that the anterolateral corner of the superior facet and the anterior aspect of the lateral mass lateral to the origin of the transverse process would be safer zones for screw exit. Attention should therefore be paid to the screw orientation for the Magerl technique and to the screw length for the Roy-Camille technique. Care should be taken to insert the screw into the C7 lateral mass.

Quantitative anatomy of the transverse foramen and pedicle of the axis

Forty dry C2 cervical vertebrae were obtained to evaluate quantitatively the anatomy of the C2 transverse foramen and pedicle. Computed tomography (CT) scans and plain radiographs were also obtained of 20 specimens to evaluate the internal structure of the transverse foramen and pedicle. The results of the measurements showed that differences between male and female specimens were found to be significant for four of seven linear parameters and for one of the two angular parameters. With regard to the sides, differences were found to be significant for two of the seven linear parameters and both of the angular parameters between right and left measurements. The variation of the foramen size between sexes was more significant than that between sides, and a remarkable variation between sides and between sexes was found in the transverse foramen horizontal angle and the pedicle width. The inferior pedicle width was approximately 3 mm less than the superior pedicle width. CT scans revealed that the lateral wall of the pedicle is very thin compared with the medial wall. Considering the greater variation of the C2 foramen orientation, smaller inferior pedicle dimension, and thinner lateral pedicle wall, a screw placement as close as possible to the mediosuperior cortex is recommended to avoid violation of the transverse foramen if transpedicular screw fixation in C2 is intended.

Computed tomographic evaluation of the internal structure of the lateral mass of the lower cervical spine

Axial computed tomography scans with a slice thickness of 2 mm taken from 12 cervical spines were used to study the internal structure of the lateral mass. Images representing the middle of zone I (Heller's classification) and the top of zone III were analyzed. The measurement of zone I involved anterior cortex thickness (ACT) while measurements of zone III included ACT, lateral cortex thickness (LCT), posterior cortex thickness (PCT), lateral mass thickness (LMT), and lateral mass width (LMW). The percentage of the ACT and PCT with respect to the LMT (ACT/LMT and PCT/LMT) were calculated. Results showed the average ACT in zone I ranged from 1.6 to 1.8 mm. In zone III, the average LMT and LMW ranged from 8 to 9 mm and 13 to 15 mm, respectively. The smallest LMT was found at C7. The average ACT and PCT for all levels ranged from 1.8 to 2 mm. The ACT with respect to the LMT (ACT/LMT) was approximately 17% to 19% for C3 to C5 and C7, and 15% for C6 separately. The PCT with respect to the LMT (PCT/LMT) was approximately 16% to 18% for C3 to C6, and 20% for C7. These results show the ventral cortex of the lateral mass is relatively thicker and support the concept of bicortical screw purchase.

The vertebral body depths of the cervical spine and its relation to anterior plate-screw fixation

STUDY DESIGN

A study was performed to measure the vertebral body depths in different locations from C2 to C7.

OBJECTIVES

To measure the vertebral body depths in 10 linear dimension from C2 to C7.

SUMMARY OF BACKGROUND DATA

Anterior plate-screw fixation of the cervical spine has been the common surgical procedure for management of multilevel degenerative disc disease and fracture dislocation. However, injury to the spinal cord during drill or screw placement is the most feared complication of this procedure. It is beneficial for one to have a knowledge of the vertebral body depths in different locations of the vertebral body before anterior cervical plating.

METHODS

Twenty-seven cervical spines from C2 to C7 were evaluated directly for this study. Anatomic evaluation of the vertebral body included the anteroposterior midline sagittal depth and the anteroposterior parasagittal depth 5 mm lateral to midline on the superior and inferior endplates, as well as on the middle body. Measurements also were made of anteroposterior parasagittal vertebral depth with both medial and lateral inclination of 10 degrees, with respect to the parasagittal plane of the vertebral body.

RESULTS

In general, the measurements of male specimens were larger than those of female specimens. Significant differences were noted at 21 measurements over C3 through C7. The mean depths of the superior endplate for all male and female specimens increased consistently from C3 to C7. The mean depths of the inferior endplate varied but generally increased from C2 to C6, then decreased to C7. The mean sagittal and parasagittal middle vertebral body depths were both 14 mm.

CONCLUSIONS

This information, in conjunction with preoperative computed tomographic evaluation, may be helpful in determining proper screw length during anterior plating of the cervical spine.

Anterior tibial artery and its actual projection on the lateral aspect of the tibia: a cadaveric study

The anterior tibial artery (ATA) is at risk of injury during high tibial osteotomy, Ilizarov wire placement, pin placement in external fixation, or proximal locking screw insertion, as the artery is not visualized intraoperatively. The ATA is anchored to the oval foramen of the interosseous membrane on the proximal tibia by the deep fascia and recurrent genicular vascular branches. Segment 1 (from the bifurcation of the popliteal artery to the level of the interosseous foramen) and the proximal part of segment 2 (from the interosseous foramen to the level where the artery crosses the anterior border of the tibia) may be damaged when pin, wire or screw placement is directed posterolaterally at that level. Distally, a straight mediolateral pin or Ilizarov wires may lacerate the artery. Segment 2 of the ATA descends against the interosseous membrane in its proximal part, which is projected on the posterior third of the tibia relative to the sagittal plane; in its middle part, it runs close to the lateral cortex of the tibia, it is projected on the middle third of the tibia; in its distal part it runs gradually towards the anterior third of the tibia and contacts with the anterior third of the tibial cortical surface. This information may help reduce risk of injury to the ATA during high tibial osteotomy, external fixation and pin placement or insertion of locking screws.

Cervical uncinate process: an anatomic study for anterior decompression of the cervical spine

Morphometric evaluation of 54 dry cervical spines from C3 to C7 (a total of 270 cervical vertebrae) was performed to determine the bony boundaries of the uncinate process for resection of the uncinate process for access to posterolateral osteophytes or herniated disks at the time of anterior cervical diskectomy. The uncinate processes were significantly higher (p < 0.01) at the C4-C6 levels (5.8 +/- 1.1 mm to 6.1 +/- 1.3 mm) than at the C3 or C7 levels. The distance between the medial and lateral margins of the base of the uncinate process was significantly smaller (p < 0.01) at the C3 level (4.9 +/- 0.7 mm) than at the C7 level (6.3 +/- 0.7 mm). The anteroposterior diameter of the medial margin of the uncinate process decreased gradually from the C5 (12.5 +/- 1.5 mm) to C7 levels (11.6 +/- 1.3 mm) (p < 0.05). The inter-uncinate distance widened from the C3 (19.2 +/- 1.5 mm) to the C7 (24.6 +/- 2.1 mm) levels (p < 0.01). The mid-anteroposterior diameter of vertebral body increased gradually from the C3 (14.7 +/- 1.1 mm) to the C7 levels (16.1 +/- 1.5 mm) (p < 0.01). The width of the vertebra increased gradually from C3 to C7 (from 19.2 +/- 1.8 mm at C3 to 25.6 +/- 2.0 mm at C7) (p < 0.01). Knowledge of all the aforementioned data may be helpful during anterolateral cervical uncosectomy or uncoforaminotomy.

The reliability of the lateral radiograph in determination of the optimal transarticular C1-C2 screw length

STUDY DESIGN

This study assessed the value of using lateral radiographs in evaluating the optimal screw length in transarticular C1-C2 screw fixation.

OBJECTIVES

To assess the reliability of the lateral radiograph in determining the optimal transarticular C1-C2 screw length.

SUMMARY OF BACKGROUND DATA

Transarticular C1-C2 screw placement is usually performed using anatomic landmarks and fluoroscopy. A lateral fluoroscopic image is valuable when directing screws in the sagittal plane, but its exact role in determining screw length has not been investigated.

METHODS

Eight cervical spine specimens were used in this study. Screw placements were performed in each specimen, fixed in the exact lateral position and under direct visualization. After each placement, a lateral radiograph was taken. The odontoid process was divided into three equal portions. Another portion anterior to the odontoid process was called the anterior tubercle region. The number of screw tips appearing in each portion on the radiograph was then recorded for each placement. In addition, 30 C1 specimens were measured to evaluate the anterior part of C1.

RESULTS

The results showed that 12.5% of the screws placed 2 mm short of reaching the ventral cortex and 0 mm overpenetrating the ventral cortex of the lateral mass of C1 projected in the radiograph on the anterior tubercle region, 37.5% on the anterior region of the odontoid process, and 50% on the middle region of the odontoid process. Twenty-five percent of the screws that were placed to overpenetrate, by 2 or 4 mm, the ventral cortex of the lateral mass of C1 were projected on the anterior tubercle region in the radiograph, and 50% and 62.5% were projected on the anterior region of the odontoid process, respectively. The mean vertical distance between the anteriormost point of the anterior tubercle of the anterior ring and the middle of the ventral cortex of the lateral in all specimens was 5.6 +/- 1 mm, and the mean transverse angle of the anterior ring relative to the frontal plane was 21.1 +/- 3.5 degrees.

CONCLUSIONS

This results in this study indicate that a lateral radiograph may not be reliable in determining the optimal screw length, although it is valuable in directing accurate screw angle in the sagittal plane. Preoperative computed tomographic evaluation of the C1-C2 region may be helpful in estimating the location of the screw tip on the lateral radiograph during surgery.

The anatomic location of the dorsal ramus of the cervical nerve and its relation to the superior articular process of the lateral mass

STUDY DESIGN

This study analyzed the anatomic relation of the dorsal ramus of the cervical spinal nerve to the lateral mass.

OBJECTIVES

To determine the location of the dorsal rami of the cervical spinal nerves from C3 to C7 in relation to the superior articular processes of the lateral masses.

SUMMARY OF BACKGROUND DATA

The anatomic study of the cervical spinal nerve and its relations to adjacent bony structures have been addressed. No previous anatomic study with regard to the location of its dorsal ramus relative to the lateral mass has been reported.

METHODS

Twelve specimens were obtained for study of the dorsal rami of the cervical spinal nerves. All soft tissues surrounding the cervical spinal nerves from C3 to C7 were dissected from the intervertebral foramens until the dorsal rami were clearly exposed. The facet joints in the corresponding levels were then opened by removal of the capsules. Three measurements, including the height of the dorsal ramus, the distance between the dorsal ramus and the tip of the superior articular facet, and the angle of the dorsal ramus relative to the superior articular surface, were taken for each ramus.

RESULTS

The results showed that the mean height of the dorsal ramus for both sides decreased progressively from C3 (2.2 +/- 0.6 mm) to C7 (1.2 +/- 0.2 mm). The mean distance between the dorsal ramus and the tip of the superior facet showed an inconsistent change, with the maximum value seen at C5 (7.4 +/- 1.6 mm) and the minimum at C7 (5.5 +/- 2.9 mm). The mean angle of the dorsal ramus relative to the superior articular facet ranged from 23.3 degrees +/- 14.3 degrees to 29.8 degrees +/- 11.2 degrees.

CONCLUSIONS

The dorsal ramus of the cervical spinal nerve is closer to the anterolateral corner of the base of the superior articular processes. Lateral mass screws directed to the anterolateral corner of the base of the superior articular process should be avoided.

Vulnerability of long thoracic nerve: an anatomic study

The anatomic course of 40 long thoracic nerves was studied in relation to anatomic landmarks and reference lines, that is, the axillary lines and first 2 ribs. After its supraclavicular course, the nerve passes beneath the clavicle within the axillary sheath and then emerges from the axillary sheath. As it passes inferiorly and posteriorly from the point of emergence to the posterior angle of the second rib (that is, the attachments of serratus anterior muscle), it makes a posterior angle of 30.7 degrees +/- 4.3 degrees on average, relative to the anterior axillary line. It then continues to descend inferiorly between the middle and posterior axillary lines. When the arm is raised, the axillary neurovascular bundle moves superiorly with the movements of the arm. The long thoracic nerve is angulated and stretched at the point it passes out of the axillary sheath.

Ground reaction forces and EMG activity with ankle bracing during inversion stress

PURPOSE

The purpose of this investigation was to evaluate the effects of external ankle support on ground reaction forces and myoelectrical activity of selected lower extremity muscles during dynamic inversion stress.

METHODS

Twenty-four healthy males performed five trials of a lateral dynamic movement at a rate between 80-90% of their maximal speed under three ankle brace conditions (no brace--control, Aircast Sport-Stirrup, Active Ankle). Ground reaction forces along the mediolateral axis and EMG activity of the peroneus longus, tibialis anterior, and medial gastrocnemius were simultaneously recorded during force plate contact.

RESULTS

Ankle bracing did not affect peak impact force (P > 0.05), maximum loading force (P > 0.05), or peak propulsion force (P > 0.05) in the lateral direction compared with the control condition. Ankle bracing reduced the EMG activity of the peroneus longus during peak impact force compared with the control condition (P < 0.05), although no differences were noted between the two braces. Furthermore, peroneous longus activity during maximum loading force and peak propulsion remained unaffected (P < 0.05). Ankle bracing did not affect the EMG activity of the tibialis anterior and medial gastrocnemius at the point of peak impact force, maximum loading force (P > 0.05), and peak propulsion force (P > 0.05).

CONCLUSIONS

These data suggest that ankle bracing may not affect the forces experienced at the foot and ankle, but helps reduce the strain placed on the peroneus longus during peak impact force. Furthermore, ankle bracing does not alter the function of the tibialis anterior and medial gastrocnemius during dynamic inversion stress.

Safe lateral-mass screw lengths in the Roy-Camille and Magerl techniques. An anatomic study

STUDY DESIGN

Investigation of the mean safe lateral-mass screw lengths in the Roy-Camille and Magerl screw techniques in cadaveric cervical specimens.

OBJECTIVES

To report the mean screw path length and to evaluate the relation of the screw trajectory to the nerve root in the Roy-Camille and Magerl screw techniques.

SUMMARY OF BACKGROUND DATA

Potential injury to the cervical nerve root caused by too long a screw remains a major concern. Few studies regarding proper screw length and its relation to the adjacent nerve root are available.

METHODS

Fourteen cervical spines were used for this study. Each lateral mass from C3 to C7 was drilled according to the techniques described by Roy-Camille (right side) and Magerl (left side). The cervical spines were harvested from the cadavers, and the anterior aspect of the lateral mass and spinal nerve were exposed. The screw path length between the dorsal and ventral cortices of the lateral mass were measured. An additional measurement was taken from the ventral aspect of the lateral mass to the nerve root along the screw path.

RESULTS

The mean screw path length in the Roy-Camille technique decreased consistently from C3 (15.7 +/- 1.7 mm) to C7 (11.3 +/- 0.8 mm). The mean distance from the ventral cortex to the nerve root ranged from 1.2 to 2.3 mm, and the smallest value was at C7. The mean screw path length in the Magerl technique also decreased from cephelad to caudal, with a range of 15-16 mm at C3-C6 and a mean value of 13.8 mm at C7.

CONCLUSIONS

A safe screw length is 14-15 mm in the Roy-Camille technique and 15-16 mm in the Magerl technique at C3-C6. A short screw may be used at C7 if desired.

Translaminar facet screw placement: an anatomic study

Anatomic measurements for screw path length, caudal and lateral angles, and superior and inferior lamina border thicknesses from L1 to L5 were measured for translaminar facet screw fixation in 30 dried lumbar spines. All measured values were fairly constant from L1 to L5. The mean values of the length of the screw path and lateral angle gradually increased from L1-2 to L5-S1 levels (41 to 54 mm, 39 degrees to 60 degrees, respectively). The caudal angle of screw placement relative to transverse plane gradually decreased from L1-2 to L5-S1 levels (60 degrees to 38 degrees). The superior border of the lamina was relatively thinner, with the mean thickness ranging from a minimum of 1.3 mm at L1 to a maximum of 2.0 mm at L5. The thickness of the inferior border of the lamina increased from L1 to L5 (from 6.7 mm to 7.8 mm). This study confirmed that a translaminar facet screw, 40 mm to 50 mm long at L1 to L5 levels, 60 mm long at L5-S1 level, and 4.5 mm in diameter, should be inserted through the lumbar facet joint at an angle of 40 degrees to 50 degrees laterally at L1 to L5 levels, 50 degrees to 55 degrees laterally at L5-S1 level, 45 degrees to 60 degrees caudally at L1 to L5 levels, and 35 degrees to 40 degrees at L5-S1 level caudally.

Modified Magerl technique of lateral mass screw placement in the lower cervical spine: an anatomic study

Twelve adult embalmed cadaveric cervical spines were used to modify the traditional Magerl technique for screw placement in the lower cervical spine. The starting point for the drill bit was 2 mm inferior to the inferiormost edge of the superior facet and 1-2 mm medial to the posterior midline of the lateral mass. The orientation of the drill bit was parallel to the superior facet in the sagittal plane, and 25-30 degrees lateral in the transverse plane. After drilling, we made direct measurements of the screw path length between the dorsal and ventral cortexes of the lateral mass and screw-path angles in both the sagittal and transverse planes. The results showed the mean screw-path length for all levels ranged from 11 to 15 mm with the smallest value at C7. The mean sagittal and transverse angles of the screw path ranged from 41 to 43 degrees and from 26 to 28 degrees. Penetration of the superior facet was found in three (2.5%) cases. Possible spinal nerve violation was seen in 13 (10.8%) cases if the drill bit was excessively overpenetrated. This study suggested that our modified Magerl technique for lateral mass screw placement be as high as possible without impinging on the facet joint, and drilling be as cranial and lateral as possible to leave the superior articular process as high and lateral as possible. This may further decrease the incidence of spinal nerve injury. Careful drilling and tapping technique is recommended to avoid overpenetration.

Anatomic considerations of anterior transarticular screw fixation for atlantoaxial instability

STUDY DESIGN

Anatomic parameters of C1 and C2 were measured in 30 dried human cervical spines. Anterior transarticular C1-C2 screws were placed in 15 cadaveric spines.

OBJECTIVE

To provide anatomic data for anterior transarticular atlantoaxial screw or C1-C2 screw and plate fixation.

SUMMARY OF BACKGROUND DATA

A posterior approach to fixation in the atlantoaxial joint has been well described. Damage to the vertebral artery is documented as a rare complication of posterior atlantoaxial transarticular screw fixation. An anterior surgical approach to exposing the upper cervical spine for internal fixation and bone graft recently has been developed. No anatomic information regarding the anterior transarticular atlantoaxial screw or screw and plate fixation between C1 and C2 is available in the literature.

METHODS

Direct measurements using digital calipers and a goniometer were taken from 30 pairs of dried human C1 and C2 vertebrae. The anterior transarticular C1-C2 screw insertion point is at the junction of the lateral edge of the C2 vertebral body to 4 mm above the inferior edge of the C2 anterior arch. The parameters related to anterior transarticular atlantoaxial screw fixation or screw and plate fixation between the C1 lateral mass and the C2 vertebral body were measured. Fifteen embalmed cadavers were used for anterior C1-C2 transarticular screw placement. Longer screws (30-40 mm) were used to detect whether the screw tips violated the upper cervical canal or vertebral arteries.

RESULTS

In the anterior transarticular atlantoaxial screw placement, lateral angulation of the screw placement relative to sagittal plane ranged from 4.8 +/- 1.8 degrees to 25.3 +/- 2.6 degrees. The posterior angulation of the screw placement relative to the coronal plane ranged from 12.8 +/- 3.1 degrees to 22.6 +/- 3.2 degrees. The length of the medial screw path ranged from 14.7 +/- 1.5 mm to 25.4 +/- 2.8 mm. In the anterior screw and plate fixation, the anteroposterior diameter of the inferior facet articular surface ranged from 16.2 +/- 1.6 mm to 17.1 +/- 1.8 mm. The anteroposterior diameter of the C2 vertebral body ranged from 9.3 +/- 1 mm to 16.2 +/- 1.8 mm. The anterior prevascular retropharyngeal approach appropriately exposed the atlantoaxial joint for anterior transarticular C1-C2 screw placement. No screws violated the vertebral artery and cervical canal.

CONCLUSIONS

An anterior transarticular atlantoaxial screw 15-25 mm long can be inserted with a lateral angulation of 5-25 degrees relative to the sagittal plane and a posterior angulation of 10-25 degrees relative to the coronal plane. Additionally, in C1-C2 anterior plate fixation screws 15 mm long could be anchored in the inferior facet of the C1, and screws 9-15 mm long could be anchored in the C2 vertebral body.

Anatomic considerations of the vertebral artery: implications for anterior decompression of the cervical spine

Anatomic dissection and measurements of the vertebral artery relative to the medial margin of the longus colli and the anterior margin of the vertebral body from C6 to C3 were performed in this study. The average angle of the vertebral artery relative to the midline was 4.3 +/- 2.6 degrees overall. The average distance between the medial margin of the longus colli and medial margin of the vertebral artery gradually decreased from the C6 level (11.5 +/- 1.0 mm) to the C3 level (9.0 +/- 1.3 mm). The average distance between anterior margin of the vertebral body and anterior margin of the vertebral artery gradually increased from the C6 level (7.2 +/- 1.9 mm) to the C3 level (9.6 +/- 2.1 mm). The distance between medial borders of the longus colli muscles gradually decreased from the C6 level (13.8 +/- 2.2 mm) to the C3 level (7.9 +/- 2.2 mm). Relative to the medial margin of the longus colli muscle and the anterior margin of the vertebral body, the vertebral artery is more lateral and anterior at the C6 level but more medial and posterior at C3 level. This knowledge may facilitate effective decompression of the lateral cervical canal and improve the margin of safety of the surgeon.

The quantitative anatomy of the superior gluteal artery and its location

Twenty cadaveric posterior gluteal regions were dissected to quantitatively determine the location and dimensions of the superior gluteal artery and its branches on the posterior gluteal region. Two reference lines were defined: one (transverse) was drawn from the posterior inferior iliac spine (PIIS) across the ilium to the anterior superior iliac spine (ASIS), and another (vertical) was drawn orthogonal to the transverse line at a point 8 cm from the PIIS. Results showed that the average distances from the posterior superior iliac spine (PSIS) and PIIS to the superior gluteal artery origin at the sciatic notch were 60 mm and 42 mm, respectively. The deep superior branch traveled an average of 18 mm above the transverse line at the vertical line, and averaged 54 mm from the iliac crest at the same point. This same branch inserted in the medius an average of 29 mm from the ASIS, and 11 mm above the transverse line, while providing 4 to 7 perforators to the gluteus medius and 0 to 2 to the gluteus minimus. The deep inferior branch traveled an average of 23 mm below the transverse line along the vertical line and penetrated the minimus or medius 36 mm from the ASIS, while providing 3 to 8 perforators to the medius and 1 to 3 to the minimus. The average distance from the deep inferior branch to the superior edge of the acetabulum along the vertical line was 25 mm. These data may be helpful in preventing the injury of the superior gluteal artery during the posterior gluteal approach.

Anatomic considerations of pedicle screw placement in the thoracic spine. Roy-Camille technique versus open-lamina technique

STUDY DESIGN

In this cadaveric study, the outcomes of two techniques for pedicle screw placement in the thoracic spine were compared.

OBJECTIVES

To assess the Roy-Camille technique, and to determine whether pedicle screw placement, aided by partial laminectomy, could decrease the incidence of pedicle violations.

SUMMARY OF BACKGROUND DATA

Pedicle screw fixation in the thoracic spine remains technically challenging. The Roy-Camille method may be one of the leading techniques of thoracic pedicle screw placement. However, there are few studies evaluating this technique and determining methods to decrease the incidence of thoracic pedicle penetration with screw insertion.

METHODS

Ten cadaveric thoracic spines from T1 to T10 were used for pedicle screw placement. Two techniques of transpedicular screw placement were used, the Roy-Camille technique (screw placed on the right side; used in 95 screw placements) and the open-lamina technique screw placement with combined partial laminectomy (screw placed on the left side; used in 94 screw placements). After screw placement, all specimens were evaluated visually to determine violation of the pedicle.

RESULTS

The screw placement with the Roy-Camille technique had a higher percentage of pedicle violation (54.7%) than did that with the open-lamina technique (15.9%). No Grade III violation was seen in the screw placement with the open-lamina technique.

CONCLUSIONS

The Roy-Camille technique was associated with a high incidence of pedicle violation, whereas screw placement with a partial laminectomy significantly reduced the incidence of pedicle violation. Pedicle screw fixation in the thoracic spine remains a technical challenge and should not be used routinely. Screw placement with the open-lamina technique is recommended if pedicle screw fixation is strongly indicated in the thoracic spine.

Cartilage and synovium of the human atlanto-odontoid joint. An anatomic and histological study

The surface area, thickness and composition of articular cartilage of the atlanto-odontoid joints were investigated in twenty human cadaveric cervical spine specimens. The specimens were also examined grossly and by light microscopy to determine the location of the synovium. The anterior arch of the atlas and ventral and dorsal articular surfaces of the dens were covered with hyaline cartilage. The mean values of the articular surface areas on the ventral surface of the dens and anterior arch of the atlas were 55.10 and 58.24 mm2, respectively. The mean thickness of the articular cartilage of the anterior arch of the atlas, ventral and dorsal surfaces of the dens was 0.80, 0.81 and 0.82 mm, respectively. Synovial membranes were associated with the joint capsules and surrounding tissues of both anterior and posterior atlanto-odontoid joint spaces, where the synovial membranes were attached to the margins of the articular surfaces of the dens and anterior arch of the atlas anteriorly and the region of the cruciate ligament immediately peripheral to the cartilage region apposed to the dens and dens cartilage itself, posteriorly.