Anatomic considerations for uncovertebral involvement in cervical spondylosis

Morphometric analysis of 54 dry cervical spines from C3 to C7 (a total of 270 cervical vertebrae) and bilateral dissection of 10 anatomic specimen cervical spines were performed. The uncinate processes were significantly higher at C4 to C6 (5.8 +/- 1.1 mm to 6.1 +/- 1.3 mm) levels than at C3 or C7 levels. The anteroposterior diameter of the intervertebral foramina was smaller at the C4, C5, and C6 levels compared with that at the C3 or C7 levels. The length of nerve root between the lateral border of dural tube and medial border of vertebral artery gradually increased from C3 (3.3 +/- 1.1 mm) to C7 (8.1 +/- 2.1 mm). A combination of higher uncinate process, smaller anteroposterior diameter of intervertebral foramina, and longer course of nerve roots in close proximity of the uncovertebral joints at the C4 to C6 levels may explain the predilection of nerve root compression by uncovertebral osteophytes at these levels. The distance from apex of the uncinate process to medial border of the transverse foramen gradually increased from C3 (1.7 +/- 0.8 mm) to C7 (3.3 +/- 1.0 mm), which may predispose the midcervical level to compression of the vertebral artery by laterally projecting uncovertebral osteophytes.

Components of the posterior calcaneal facet: anatomic and radiologic evaluation

The calcaneal facet of the posterior subtalar joint can be divided into two portions: an anterolateral portion and a posteromedial portion. The importance of this consideration is that the two portions of the facet do not lie in the same plane. This should be taken into consideration when interpreting radiographs of the subtalar joint. Measurements from 50 dry-bone specimens and radiographs of 10 cadaver specimens were taken. The radiographs were obtained after marking the posterior subtalar joint to demonstrate the corresponding site of each facet portion on radiographs. The posteromedial portion lies almost in the transverse plane, making an angle of approximately 40 degrees with the anterolateral portion. Each radiographic projection was explained by demonstrating the orientation of markings applied to the specimens and correlated to the data derived from the angular measurements. A protocol of sequential radiographs that can be used to evaluate the posterior subtalar joint is proposed.

Radiological evaluation of S1 dorsal screw placement

Twelve bony pelves were used in this study. S1 dorsal screws were inserted in the anteromedial, anterior, anterolateral, and anteroinferior directions. When the screws were inserted within 5-10 mm beyond the anterior sacral cortex, radiographs were obtained in the anteroposterior, modified inlet, modified outlet, and lateral projections to evaluate the position of the screws and penetration of the anterior cortex. In addition, 30 dried sacral were obtained for anatomic evaluation focused on the anterior and superior aspect of the sacral alae and its relationship to the anterior aspect of the sacrum. The results showed that a screw penetration of the anterior cortex of the S1 vertebral body or ala is best detected in the modified inlet view. A misdirected screw into the S1 anterior foramen is best seen in the modified outlet view. The lateral view also is useful in determining the extraosseous penetration. The average distance from the anteriormost limit of the ala to the anterior sacral cortex was approximately 11 mm. The average sagittal angle between the superior surface of the ala and S1 dorsal aspect was approximately 51 degrees. This study suggested that the modified inlet and lateral radiograph views are most useful for detecting screw penetration of the anterior cortex of the sacrum. The modified outlet projection is the best for determination of a screw violating the S1 anterior foramen. Also, the modified inlet projection will show the screw orientation relative to the mediolateral plane (the sacral canal and sacroiliac joint), and the lateral view will show the screw direction relative to the superoinferior plane.

Anatomic considerations for posterior approach to the sacroiliac joint

STUDY DESIGN

This anatomic study describes a new intraosseous, posterior approach to the sacroiliac joint.

OBJECTIVES

To define a transosseous approach to the sacroiliac joint in which a triangular bony window is raised on the posterosuperior aspect of the ilium that provides improved access to the sacroiliac joint for posterior fusion.

SUMMARY OF BACKGROUND DATA

A posterior approach to the sacroiliac joint has been widely used for debridement of infectious diseases and for fusion. Most conventional approaches to the sacroiliac joint are interosseous, and there is a relative lack of information on transiliac approaches.

METHODS

The projection of the sacroiliac joint on the outer table of the ilium and the thickness of the posterior ilium forming part of the sacroiliac joint were determined in 15 cadaveric pelves. A right angle, triangular bony window was raised from the posterior ilium to investigate the suitability of a transiliac approach in performing sacroiliac debridement and arthrodesis. A horizontal reference line 3-3.5 cm in length was drawn between a point 1 cm anterosuperior to the posteroinferior iliac spine and a point 1.5 cm superior to the superior border of the greater sciatic notch. A vertical reference line was extended superiorly for 2-2.5 cm perpendicular to and beginning at the anterior end of the horizontal reference line. The oblique arm of the right triangle was created by joining the superior end of the vertical reference line to the posterior end of the horizontal line.

RESULTS

Thirty percent to fifty percent of the articular surface of the iliac bone was removed with this triangular segment of bone, and a corresponding area of the sacral articular surface was visualized directly. It was possible to remove the rest of the articular cartilage with angled curettes in all specimens.

CONCLUSIONS

This approach facilities improved access to the sacroiliac joint for debridement and arthrodesis with minimal soft tissue dissection and iliac bone resection.

Anatomic considerations for safe placement of calcaneal pins

Placement of a Steinmann pin in the calcaneus is indicated in various orthopaedic conditions. Planning the point of entry and the direction of transcalcaneal pin insertion is crucial for avoidance of neurovascular injury, tendon injury, and subtalar joint violation. Fifteen cadaveric feet were studied in which transfixing calcaneal pins were inserted in posteromedial and anteromedial sites. The posteromedial site was at a point 3/4 the distance between the palpable tip of the medial malleolus and the heel, with the pin inserted transversely. The anteromedial site was at the sustentaculum tali with the pin inserted transversely angled 25 degrees to 30 degrees inferolaterally. Radiographs were then taken and the specimens were dissected to determine the path of each pin and the safe and danger zones for transcalcaneal pin placement. It was concluded that the posteromedial calcaneal pin site is safer and easier to determine.

Anatomic consideration of transpedicular screw placement in the cervical spine. An analysis of two approaches

STUDY DESIGN

This study compared the effectiveness of two transpedicular screw placement techniques: blind screw placement versus screw placement after direct determination of the superior, medial, and inferior borders of the pedicle through the opening of a "window" by the partial laminectomy and tapping technique.

OBJECTIVES

To determine if the incidence and severity of pedicle violations resulting from transpedicular screw placement could be reduced by direct determination of the superior, medial, and inferior borders of the pedicle through the opening of a "window" by partial laminectomy.

SUMMARY OF BACKGROUND DATA

Several studies regarding transpedicular screw fixation for unstable cervical spine injuries have been reported, but none has addressed the effectiveness in lowering the incidence of pedicle violation by opening a "window" by partial laminectomy for direct determination of the superior, medial, and inferior borders of the pedicle and using the tapping technique before and in planning for screw placement.

METHODS

Eight adult cadaveric cervical spines (40 vertebrae from C3 to C7) were used for this study. Two groups were formed according to screw placement techniques. The first group was composed of 38 blinded transpedicular screw placements. The second group was composed of 40 screw placements using the partial laminectomy and tapping technique. After transpedicular screw placement, all specimens were evaluated radiographically and visually for violation of the pedicle.

RESULTS

A decrease in the incidence and severity of pedicle violation was seen in the second group with opening of the lamina and tapping technique compared with the blind screw placement group. However, the percentage of screws found to violate the pedicle with the opening of the lamina and tapping technique still was relatively high.

CONCLUSIONS

Transpedicular screw placement in the cervical spine is difficult, and a high percentage of violations of the pedicle wall occur. This technique should not be used routinely.

Anatomic considerations of an anterior approach to the sacroiliac joint

This anatomic study was performed on 25 disarticulated pelves and 10 embalmed cadavers to evaluate an anterior approach to the sacroiliac joint. The angle of inclination of the superior limb of the sacroiliac joint was 15 degrees superolateral to inferomedial in the sagittal plane, and the inferior limb was parallel to the sagittal plane. The cranial lateral part of the ala overlapped the joint space and the adjacent part of the ilium. Elevation of a rectangular bone block on the cranial part of the ala removed the joint space from the superior limb and provided direct access to the joint space of the inferior limb.

Extensile medial approach to the femur

A proposed extensile medial approach to the medial surface of the shaft of the femur was studied on 30 cadaver thighs. The incision is made along a line extending from the mid inguinal point to a point one-third the distance from the adductor tubercle to the medial side of the patella. After mobilizing the sartorius muscle posteromedially, the medial femur is exposed by a three-step technique. Step I involves bluntly defining the internervous plane distally between the vastus medialis muscle and the adductor magnus tendon until limited proximally by the distal end of the vastoadductor membrane at an average of 9 cm from the adductor tubercle. In step II blunt dissection between the vastus medialis and the adductor longus muscles is carried from proximal to distal until limited by the proximal end of the vastoadductor membrane. Now the well-defined vastoadductor membrane area ("danger zone") measuring 6 cm in length on average is safely dealt with (step III) and the medial femur is exposed. Distal extension can be made to expose the knee joint. The approach can be extended proximally to the lesser trochanter between the vastus medialis and both the adductor brevis and pectineus muscles. Anatomic measurements in relation to the adductor tubercle and cross-sections of the thigh were made to better describe anatomic constants and variables in this rather unfamiliar medial thigh area.

The quantitative anatomy of the iliac vessels and their relation to anterior lumbosacral approach

Forty embalmed cadavers (24 men and 16 women) were obtained to evaluate the anatomic features of iliac vessels with respect to the anterior approach to the lumbosacral junction. Through a midline longitudinal incision over the linea aspera and retraction of the intraperitoneal contents, exposure of the abdominal aorta, inferior vena cava, and common iliac vessels was done. Direct measurements regarding the relations of these greater vessels to the sacral promontory were then performed. The results showed the average width of the trigone, measured between the medial edges of the left common iliac vein and the right common iliac artery, was 55.9 mm for male and 55.3 mm for female subjects, respectively. The average height of the trigone, measured from its apex to the sacral promontory, was 36.9 mm for male and 35.2 mm for female subjects separately. The width of the uncovered left common iliac vein averaged 7.2 mm for male and 6.3 mm for female subjects, respectively, with a range of 3-11 mm for both male and female specimens. One should be always aware of the medially placed left common iliac vein and an approach medial to the right common iliac artery to the L5-S1 disk is recommended if anterior surgery is desired.

Anatomic considerations for the anterior exposure of the proximal portion of the radius

The rate of posterior interosseous nerve injury is still of major concern during surgical exposure of the proximal portion of the radius. The objective of this study was to find the best way to protect the important neurovascular structures during anterior exposure of the proximal radius and to define the safest anatomic orientation for plate and screw placement during open reduction and internal fixation of the proximal radius. In 30 cadaveric upper limbs, the proximal portion of the radius was exposed through a modified anterior Henry approach. The important anatomic structures were localized and demonstrated on radiographs. Plates and screws were applied anterolaterally (in five specimens) and laterally (in another five specimens), and the locations of the safe and danger zones were noted. Lateral placement of the plate is preferred over the more commonly used anterolateral plating, because it carries less risk of injuring the posterior interosseous nerve during screw application and it does not impinge on the biceps tendon and block pronation.

Vulnerability of great medullary artery

STUDY DESIGN

The present study describes anatomic observations on great medullary artery and intercostal arteries pertinent to thoracolumbar spinal surgery.

OBJECTIVES

This study reveals the vulnerable course of the great medullary artery and its relationship to the lateral or posterolateral approach to thoracic spine.

SUMMARY OF BACKGROUND DATA

There are no previous anatomic data on the length of the great medullary artery, its intradural course, its relationship with the anterior spinal artery, and the distance between two adjacent intercostal arteries.

METHODS

The location of the intercostal arteries was defined, and the distance between two adjacent arteries was measured at a point on the lateral surface of the vertebra midway between its anteroposterior diameter. The intradural length of the great medullary artery and the angle it formed with the anterior spinal artery at the point of anastomosis were also measured.

RESULTS

The mean intradural length of the great medullary artery was 3.6 cm (range, 1.7-8.1 cm), and it passed over 1-3 disc spaces before joining the anterior spinal artery at a mean angle of 20.1 degrees (range, 12-28 degrees). The average distance between two adjacent intercostal arteries from T6 to L2 was 3.6 cm (range, 2.8-4.0 cm), which provides a safe window through which a herniated thoracic disc may be approached if surgery is indicated.

CONCLUSIONS

The acute angle between the great medullary artery and anterior spinal artery indicates that these two arteries are in close proximity for considerable length and are liable to be compressed together with the intervening vascular collaterals by a space-occupying lesion, such as disc herniation or a fractured fragment. The longer the intradural course of the great medullary artery, the more vulnerable it is to compression by disc herniation or fracture. The intercostal and lumbar arteries are located at the midportion of the lateral aspect of the vertebral bodies rather than at the level of intervertebral discs. Discectomy or decompression of the anterior thoracic canal may be accomplished through a lateral or posterolateral extracavitary approach between two intercostal or lumbar arteries.

An anatomic study of the thickness of the occipital bone. Implications for occipitocervical instrumentation

STUDY DESIGN

The authors measured the thickness and quality of occipital bone regions to determine screw placement during occipitocervical fusion and described the projection of the posterior dural venous sinuses.

OBJECTIVE

This study provides anatomic data relevant to areas of screw placement into the occiput during occipitocervical fixation.

SUMMARY OF BACKGROUND DATA

Few reports exist regarding the morphometrics of the occipital bone and intracranial structures relevant to occipitocervical fusion.

METHOD

The thickness of the posterior inferior occipital bone was measured relative to a 10 x 5 cm grid. Sections were evaluated grossly and histologically. The projections of the posterior dural venous sinuses were determined by direct measurements.

RESULTS

The maximum thickness of the occipital bone, which ranged from 11.5 to 15.1 mm in males and from 9.7 to 12.0 mm in females, was at the level of the external occipital protuberance. The occipital bone was thicker than 8 mm in an area extending laterally from the external occipital protuberance for 23 mm and consisted of dense cortical bone with little or no diploic bone. The projection of most of the torcula on the external surface of the occipital bone was located superior to the center of the external occipital protuberance (mean, 12.6 mm superior and 4.7 mm inferior to external occipital protuberance), whereas that of the transverse sinus was distributed more evenly above and below the external occipital protuberance (mean, 7.3 mm superior and 6.5 mm inferior).

CONCLUSIONS

Screws that are 8-mm long may be inserted in the region of the superior nuchal line (Level 0) extending 2 cm laterally from the center of the external occipital protuberance, 1 cm from the midline at a level 1 cm inferior to the external occipital protuberance (Level 1), and 0.5 cm from the midline at a level 2 cm inferior to the external occipital protuberance (Level 2). The major dural venous sinuses are situated immediately beneath the thickest regions of the occiput and are at risk of penetrative injury during screw placement.

A modified dorsal approach to the wrist for arthrodesis of the non-rheumatoid wrist. An anatomical study

Fourteen cadaveric wrists were dissected to investigate a modified dorsal approach that involved osteotomy of the small and dorsal tubercles without opening the third compartment. This approach could be safely made with good exposure of the dorsum of the wrist. The mean normal angle formed by the extensor pollicis longus tendon at the level of the dorsal tubercle was 144 degrees. An approach that involves division of the third compartment may lead to effective lengthening of the extensor pollicis longus musculotendinous unit by 8 to 17 mm with corresponding decrease in the tension generated by its contraction. The modified approach permits restoration of the normal alignment of the extensor pollicis longus tendon, and may be useful for performing arthrodesis of the non-rheumatoid wrist in young manual workers.

The quantitative anatomy of the cervical nerve root groove and the intervertebral foramen

STUDY DESIGN

The present study evaluated the cervical nerve groove and intervertebral foramen using dried vertebrae and cadaveric cervical spine.

OBJECTIVES

To measure the cervical nerve groove in eight linear and one angular dimensions and the intervertebral foramen in two linear diameters.

SUMMARY OF BACKGROUND DATA

Several anatomic studies of the cervical spine exist, but very little quantitative data have been reported on the cervical nerve groove.

METHODS

Dried cervical vertebrae, C3-C7, from 41 complete vertebral sets (205 vertebrae) and 14 cadaveric cervical spine were obtained for the present study. Anatomic evaluation focused on the cervical nerve groove for dry specimens and intervertebral foramen for cadaveric specimens. Ten linear and one angular measurements were made bilaterally. The mean, range, and standard deviation were calculated for all of the specimens and for male and female specimens separately.

RESULTS

Differences in dimensions of male and female specimens were not found to be statistically significant. The average lengths of the medial zone and distances from the midline of the vertebral body to the anterior border of the medial zone for male and female specimens consistently increased from C3 to C7. The width of the medial zone was larger in C3 than that of C4, C5, and C6 in male and female specimens. The minimum width for all levels ranged 1-2 mm. The medial zone depths gradually increased from C3 (3.2 mm for male and 2.3 mm for female specimens) to C7 (4.9 mm for male and 4.4 mm for female specimens). The smallest anteroposterior distances from the posterior midpoint of the lateral mass to the posterior border of the nerve groove were found in C7 (6.7 mm for male and 6.1 mm for female specimens). The general trend of the foraminal height and width increased from the cephalad to caudal except at C2-C3.

CONCLUSIONS

These data may enhance understanding of the important bony elements associated with the cervical spinal nerves and roots as they pass through the cervical nerve groove and the intervertebral foramen.

Anatomic and X-ray film studies of the distal articular surface of the radius

Knowledge of the surgical and topographic anatomy of the distal articular surface of the radius is valuable during diagnostic and operative procedures involving the distal radius. The aim of this study is to assist the surgeon in determining the location, displacement, and angulation of acute or healed distal radius articular fractures. Measurements were taken of the distal articular surface of 50 dry radii. Also, the margins and surfaces of the distal articular surface of 12 adult cadaver radii were marked by solder and radiopaque dye, respectively. X-ray films were then taken to define the margins and bony landmarks. The lunate facet surface area (53%) was found to be slightly larger than the scaphoid facet surface area (47%). Both the palmar tilt and the radial inclination were demonstrated on the marked x-ray films. Using the 30 degrees cephalad angled anteroposterior projection of the distal radius can help assess the dorsomedial fragment of the lunate fossa in a die-punch fracture.

Projection of the lumbar pedicle and its morphometric analysis

STUDY DESIGN

This study defined the projection point of the lumbar pedicle on its posterior aspect and its relation to a reliable landmark and reported pedicle dimensions based on 50 lumbar spines.

OBJECTIVES

To establish the best starting point for a pedicle screw for passing the screw down the center (axis) of the pedicle; to describe quantitatively the relations of the pedicle projection point to a reliable landmark; and to evaluate the linear and angular dimensions of the lumbar pedicle.

SUMMARY OF BACKGROUND DATA

Posterior transpedicular screw fixation has been most widely used for management of the unstable lumbar spine. Several studies of pedicular anatomy exist, but little quantitative data regarding the location of the lumbar pedicle axis for each level have been reported.

METHODS

Fifty dry lumbar specimens (250 lumbar vertebrae) were obtained for study of the lumbar pedicle. Anatomic evaluation focused on determination of the projection point of the lumbar pedicle axis on the junction of the superior facet and the transverse process and measured the distance from the projection point to the midline of the transverse process for each level of the lumbar vertebrae. Pedicle dimensions, including linear and angular, also were measured.

RESULTS

Differences in dimensions between men and women were not found to be statistically significant. The average distance from the projection point to the midline of the transverse process consistently changed from L1 to L5. Above L4, the projection point for men and women averaged 3.9 mm for L1, 2.8 mm for L2, and 1.4 mm for L3 superior to the midline of the transverse process, respectively. At L4, the projection point was close to the midline of the transverse process (0.5 mm inferior). At L5, the projection point was an average of 1.5 mm inferior to the midline of the transverse process.

CONCLUSIONS

The average distance from the projection point of the lumbar pedicle axis to the midline of the transverse process consistently varied at different levels. This information may prove helpful in the placement of screws into the lumbar pedicle.

The location of the vertebral artery foramen and its relation to posterior lateral mass screw fixation

STUDY DESIGN

This study evaluated the anatomic relationship between the vertebral artery foramen and the posterior midpoint of the cervical lateral mass using cervical spine specimens.

OBJECTIVES

To determine quantitatively the location of the vertebral artery foramens from C3 to C6 and their relationship to the posterior midpoints of the lateral masses.

SUMMARY OF BACKGROUND DATA

Anatomic studies of the cervical nerve root and facet relative to lateral mass screw placement have been addressed. It is necessary to know the correct location of the vertebral artery foramen during lateral mass screw placement to minimize the risk of injury to the vertebral artery.

METHODS

Forty-three cervical spines from C3 to C6 were directly evaluated for this study. Anatomic evaluation included the dimension of the vertebral artery foramen and its projection on the posterior aspect of the lateral mass. The vertical distance from the posterior midpoint of the lateral mass to the posterior border of the vertebral artery foramen, and the angle between the parasagittal plane and the line connecting the posterior midpoint of the lateral mass with the lateral limit of the vertebral artery foramen, were also measured.

RESULTS

The vertical distances from the posterior midpoint of the lateral mass to the vertebral artery foramens at C3-C6 averaged from 9.3 to 12.2 mm for male and female specimens. The average angles medial to the sagittal plane, between the parasagittal plane and the line connecting the posterior midpoint of the lateral mass with the lateral limit of the vertebral artery foramen, from C3 to C5, were found to range from 6.0 degrees to 6.3 degrees for male specimens and from 5.3 degrees to 5.5 degrees for female specimens. At C6, the average angles lateral to the sagittal plane, between the parasagittal plane and the line connecting the posterior midpoint of the lateral mass with the lateral limit of the vertebral artery foramen, were 6.4 degrees for male specimens and 5.4 degrees for female specimens.

CONCLUSIONS

The present study indicated that there is no risk of damaging the vertebral artery if a screw is directed perpendicular to the posterior aspect of the lateral mass at C3-C5 and 10 degrees lateral to the sagittal plane at C6 starting at the midpoint of the lateral mass.

Anatomic considerations for posterior iliac bone harvesting

STUDY DESIGN

This study analyzed bony features of the posterior ilium and relevant vital structures using cadavers and dry ilium specimens.

OBJECTIVES

To determine quantitatively the safely zone of the posterior ilium and relevant vital structures with regard to bone graft harvesting.

SUMMARY OF BACKGROUND DATA

The most frequently used site for bone graft harvesting is the posterior ilium. However, complications related to posterior iliac bone harvesting, such as donor site pain, neurovascular injury, instability of the sacroiliac joint, and herniation of abdominal contents, are still major concerns. Very little research with regard to the quantitative study of the posterior ilium has been reported.

METHODS

Six cadavers (four male, two female) were used for the first part of this study. The posterior superior iliac spine was determined as a reference landmark. The distances from the posterior superior iliac spine to the superior cluneal nerves, the gluteal line, and the superior gluteal vessels were measured. The second part of the study involved 30 adult, dry iliac bony specimens. The posterior iliac region (extra-articular portion) was divided into three zones, and the corresponding dimensions of these zones were measured.

RESULTS

The average distances from the posterior superior iliac spine to the superior cluneal nerves, gluteal line, and superior gluteal vessels were 68.8, 26.6, and 62.4 mm, respectively. The average width, height, and maximum thickness for Zone 1 were 34, 27.8, and 17.1 mm, respectively; the measurements for Zone 2 were 16.5, 31.8, and 14.2 mm, respectively. The average height for Zone 3 was 20.4 mm, and the average maximum thickness was 16.8mm.

CONCLUSION

The ideal area of the posterior ilium for bone graft harvesting was found in Zone 1. Zones 2 or 3 may be considered it a greater quantity of cancellous bone graft is required; however, the risk of injury to the sacroiliac joint and superior gluteal vessels in these zones is increased.

Anatomic considerations of plate-screw fixation of the anterior column of the acetabulum

Fifteen cadaveric adult bony hemipelvis specimens and 30 adult dry bone specimens were obtained to evaluate the configuration of the anterior column of the acetabulum and to develop a safe path for screw placement into it. Each cadaveric specimen was sectioned at 1-cm intervals, beginning at the level of the inferior border of the acetabulum (junction between the anteroinferior edge of the acetabulum and the most anterolateral edge of the superior ramus of the pubic bone). The plane of the cross-section was perpendicular to the anterior column. The projection of the medial acetabular boundary on the anterior column was determined by analysis of each cross-section. Results showed that the average width of the anterior column at 1.0, 2.0, and 3.0 cm superior to the inferior acetabular boundary is 31.0 +/- 4.7, 34.2 +/- 5.1, and 39.4 +/- 6.2 mm, respectively. At 1.0 cm superior to the inferior margin of the acetabulum, the average medial angulation for 0.5-, 1.0-, and 1.5-cm entry points lateral to the pelvic brim were 24.9 +/- 4.4 degrees, 35.5 +/- 5.2 degrees, and 44.4 +/- 6.6 degrees, respectively. At 2.0 cm superior to the inferior acetabular margin, the corresponding average medial angulation for 0.5-, 1.0-, 1.5-cm entry points were determined to be 29.2 +/- 5.5 degrees, 38.6 +/- 5.9 degrees, and 48.1 +/- 5.7 degrees, respectively. At 3.0 cm superior to the inferior acetabular margin, these angles were found to be 20.7 +/- 4.3 degrees, 29.4 +/- 6.0 degrees, and 39.3 +/- 5.9 degrees, respectively. All of the above mentioned angles are with respect to the perpendicular of the longitudinal axis of the anterior column without violation of the hip joint. Screws placed 1.0 cm lateral to the pelvic brim at the levels of 1.0, 2.0, 3.0, and 4.0 cm superior to the inferior acetabular margin and directed perpendicular to the anterior column penetrated the hip joint.

Projection of the S2 pedicle onto the posterolateral surface of the ilium. A technique for lag screw fixation of sacral fractures or sacroiliac joint dislocations

STUDY DESIGN

This study analyzed the sacroiliac articulation at the level of the second sacral vertebra (S2). Anthropometric measurements were performed on 20 cadaveric pelves to determine the optimal starting point for lag screw fixation of the sacroiliac joint at S2.

OBJECTIVES

The measurements were utilized to identify a region on the outer table of the posterior ilium which will provide a starting point for consistent safe placement of a lag screw across the sacroiliac joint into the ala of S2.

SUMMARY OF BACKGROUND DATA

Previous studies have defined the optimal starting point on the outer table of the ilium for the projection of lag screws into the ala of S1. No data are available for lag screw fixation of the sacroiliac joint at S2.

METHODS

Twenty human cadaveric pelves, disarticulated at the sacroiliac joint and fixed in a holding frame designed to maintain the sacrum and ilium in anatomic reduction, were utilized to identify a point on the outer table of the posterior ilium at which an interfragmentary screw could be inserted into the center of the pedicle of the second sacral vertebra.

RESULTS

The starting point on the posterolateral ilium for screw insertion into the center of the S2 pedicle was found to exist 1.5 +/- 0.31 cm superior and 2.5 +/- 0.3 cm posterior to the apex of the greater sciatic notch only when the screw or guide pin was advanced at an angle perpendicular to the long axis of the sacrum.

CONCLUSION

During lag screw fixation of posterior pelvic ring disruptions, aberrant screw placement may impose considerable risk to adjacent vascular, visceral, or neural structures. After anatomic reduction of the sacroiliac joint, safe and accurate screw fixation can be achieved by utilizing the starting point and insertion trajectory described in this paper.

Optimal technique of screw placement in the ischial tuberosity for posterior acetabular fractures

Thirty dry adult bony specimens and eight embalmed cadavers were used to report on the morphological data of the ischial tuberosity and to determine the most optimal technique for ischial tuberosity screw placement for open reduction and internal fixation of posterior acetabular fractures. The average width, height, and depth of the ischial tuberosity were 27.0 mm, 32.2 mm, and 32.4 mm, respectively. The average angles between the posterior and medial aspects and between the posterior and lateral aspects of the ischial tuberosities were 79.5 degrees, and 111.5 degrees, respectively. The risk to the internal pudendal neurovascular bundle increases with either a more medially placed screw or a laterally placed screw that is angled medially. The tendinous origin of the hamstrings becomes quite substantial (7-10 mm thick) at a point 2 cm distal to the inferior acetabular margin. The exposure of the ischial tuberosity should therefore be restricted to this level. The entry point of the screws should be 5 mm or 10 mm medial to the lateral margin of the ischial tuberosity, and the screws should be directed 35-40 degrees, 45-50 degrees, and 50-55 degrees caudally at the level of the inferior acetabular margin and 1 cm and 2 cm below it, respectively, to obtain the most favorable bony purchase.

The projection of the lateral sacral mass on the outer table of the posterior ilium

STUDY DESIGN

This study was designed to construct the projection of the lateral sacral mass on the posterior ilium using cadaveric pelves and to measure the dimensions of the projection.

OBJECTIVES

The present study was undertaken to determine quantitatively the location of the lateral sacral mass on the outer table of the ilium.

SUMMARY OF BACKGROUND DATA

Anatomic studies relative to instrumentation of the posterior iliosacral region are few. No previous anatomic studies with regard to the projection of the lateral sacral mass have been reported.

METHODS

Twelve cadaveric pelves were used for this study. To determine the projection of the lateral sacral mass on the outer table of the posterior ilium, several Kirschner wires were drilled along the outermost peripheral edge of the lateral sacral mass from the inner table of the ilium through the outer table. A triangle projection on the outer table of the posterior ilium was then constructed according to the placed Kirschner wires. The widths and height of the projection, and the distances from the axis of the projection to posterior superior iliac spine and posterior inferior iliac spine were measured bilaterally.

RESULTS

The average height of the projection of the lateral sacral mass was 61.4 mm. The average base width of the projection was 56.8 mm. The average distances from posterior superior iliac spine and posterior interior iliac spine to the longitudinal axis of the projection of the lateral mass were 30 mm and 27.4 mm, respectively.

CONCLUSIONS

This study reported the average location of the lateral sacral mass on the outer table of the ilium. The superior area of the projection may be an ideal zone for transiliosacral screw placement.

Evaluation of cervical posterior lateral mass screw placement by oblique radiographs

STUDY DESIGN

The present study analyzed the two-dimensional representation of cervical lateral mass screws by oblique radiographs compared with cadaveric placement. This was accomplished by posterior and lateral cervical dissection of the lateral masses and intervertebral foramina, keeping the emerging nerve roots intact. The intervertebral foramina were divided into two zones for the study.

OBJECTIVES

To identify and describe the value of oblique radiographs in evaluating posterior lateral mass screw placement in the cervical spine.

SUMMARY OF BACKGROUND DATA

Posterior plate-screw fixation is an effective method of stabilizing the traumatized cervical spine. Because of the surrounding anatomy, precise placement of screws must be attained to avoid iatrogenic injury to the nerve roots, and incorrectly placed screws must be identified quickly to minimize the neurologic complication. No previous radiologic study regarding evaluation of the lateral mass screw placement has been reported.

METHODS

Six cervical spines were removed from embalmed cadavers. Posterior and lateral removal of soft tissue ensued until the lateral masses and spinal nerves were clearly and completely exposed. Two specimens and 20 screws were used for each of the following methods: Roy-Camille, zone 1 placement, and zone 2 placement. Zone 1 was defined as the area between pedicles of adjacent vertebrae. Zone 2 was defined as the area between transverse processes of adjacent vertebrae. Forty-five degrees oblique left and right, anteroposterior, and lateral radiographs were taken.

RESULTS

All screws placed by the Roy-Camille technique and 19 of 20 screws intentionally placed in zone 1 were represented accurately by oblique radiographs. Nineteen of 20 screws placed in zone 1 were well appreciated in the foramen in oblique view. However, 13 of 20 screws placed in zone 2 and approximating the nerve root were inaccurately represented or ambiguous in oblique radiographs.

CONCLUSIONS

Oblique radiographs are valuable to view the relationship between screw placement and foramina. Screws crossing the line connecting the posterior borders of the intervertebral foramina and appearing in the pedicle actually exit the bone and may risk damaging the nerve root.

Radiologic evaluation of iliosacral screw placement

STUDY DESIGN

This study analyzed anteroposterior, lateral, inlet, and outlet radiographic representations of different iliosacral screw orientations and evaluated anatomic features of the superior aspects of the sacral alae.

OBJECTIVES

The purpose of this anatomical and radiologic study was to assess the value of anteroposterior, inlet, outlet, and lateral views with regard to the planning of iliosacral screw placement, to determine if screws penetrating the sacral surfaces and foramina can be detected during or after operation, and to evaluate the anatomy of the superior aspects of the sacral ala quantitatively.

SUMMARY OF BACKGROUND DATA

Direct iliosacral screw fixation has recently become popular because it provides stable fixation using reasonably small implants and is biomechanically equal or superior to other techniques of internal fixation. However, misinterpretation of the relationship of pelvic radiographs and the position of a screw may result in incorrect screw placement during surgery or misdiagnosis of postoperative neurologic complications. The morphology of the sacrum is complex. No previous data relative to the superior aspect of the sacral alae are available.

METHODS

Four bony pelves were used to model the different iliosacral screw orientations possible during iliosacral reconstruction. A drill bit was inserted laterally from the posterior ilium through the sacroiliac joint and into the S1 vertebra. Radiographs were taken from anteroposterior, lateral, inlet, and outlet views for evaluation of placement. Twenty-two dry sacra were obtained for anatomic evaluation of the superior aspects of the sacral alae. All symmetrical structures were measured bilaterally. Measurements included three angular and two linear parameters.

RESULTS

The results showed that a misdirected drill bit penetrating the anterior aspect of the ala is best appreciated by the inlet view. A misdirected drill bit penetrating the superior aspect of the ala or the S1 foramen is best represented in the outlet view. The average angle between the coronal plane of the S1 vertebra and the anterior aspect of the ala was 27.1 degrees; between the superior aspect of the S1 vertebral body and superior edge of the ala, 36.9 degrees; and between the superior aspect of the S1 vertebral body and posterosuperior edge of the ala, 24.5 degrees.

CONCLUSIONS

The inlet view shows the orientation of screws relative to the coronal plane and extraosseus screws extending anterior to the ala, whereas the outlet view elucidates the placement of screws relative to the transverse plane and extraosseus screw tips extending into the sacral foramina or superior to the ala. Evaluation of preoperative pelvic computed tomography scans may be helpful in understanding the unique morphology of each individual patient and enhancing the safety of iliosacral screw placement.

Radiologic and anatomic evaluation of the anterior sacral foramens and nerve grooves

STUDY DESIGN

The present study evaluated the anterior sacral foramen using plain radiographs and projected the positions of S1-S3 anterior sacral foramen and corresponding nerve root groove on the posterior aspect of the sacrum.

OBJECTIVES

To evaluate the plain radiographs of anteroposterior, inlet, and outlet views regarding the sacral foramen, and to determine quantitatively the location of the anterior sacral foramens on the posterior aspect of the sacrum.

SUMMARY OF BACKGROUND DATA

Injury to the sacral nerve roots associated with posterior sacral screw placement remains a potential hazard. Few studies regarding the evaluation of the anterior sacral foramen and its projection on the posterior sacral surface are available.

METHODS

Six bony pelves were harvested from preserved cadavers. The superior aspects of the sacral alae, the openings of the anterior and posterior foramens of S1-S2, were marked by outlining them with K-wires. Anteroposterior, inlet, and outlet plain radiographs were taken. The bony sacra were further disarticulated from the above six pelvic specimens. K-wires were drilled through the sacra to project the dimensions of the anterior foramens and nerve grooves of S1-S3 onto the posterior sacral surface. The dimensions between the perimeter of the projection and the corresponding posterior foramen were measured.

RESULTS

The plain radiographs show that the shape and relative position between the anterior and posterior foramens vary with different projections. It was believed that outlet projection is the best view of plain and radiographs in the evaluation of the sacral foramens and corresponding pedicles. The approximate boundaries of the anterior sacral foramens' projections were 6 mm superior, 10 mm lateral, 3 mm inferior, and 3 mm medial to the corresponding margins of the posterior foramens.

CONCLUSIONS

The outlet projection is the most useful view in plain radiographs for the evaluation of sacral foramens and pedicles. Quantitative data of the anterior sacral foramen's anatomic position on the dorsal aspect of the sacrum may be helpful in the sacral pedicle screw placement.