Tae-Hong Lim

The effect of disc degeneration and facet joint osteoarthritis on the segmental flexibility of the lumbar spine.

Study Design. A biomechanical and imaging study of human cadaveric spinal motion segments. Objective. To investigate the effect of both disc degeneration and facet joint osteoarthritis on lumbar segmental motion. Summary of Background Data. Spinal degeneration includes the osteoarthritic changes of the facet joint as well as disc degeneration. Disc degeneration has been reported to be associated with spinal motion. The association of facet joint osteoarthritis with lumbar segmental motion characteristics and the combined influence of disc degeneration and facet osteoarthritis has not yet been investigated. Methods. A total of 110 lumbar motion segments (52 female, 58 male) from 44 human lumbar spines were studied (mean age = 69 years). Magnetic resonance images were used to assess the disc degeneration from Grade I (normal) to Grade V (advanced) and the osteoarthritic changes in the facet joints in terms of cartilage degeneration, subchondral sclerosis, and osteophytes. Disc height, endplate size, and facet joint orientation and width also were measured from the computed tomographic images. Rotational movements of the motion segment in response to the flexion, extension, lateral bending, and axial rotational moments were measured using a three-dimensional motion analysis system. Results. Female motion segments showed significantly greater motion (lateral bending:P < 0.001, flexion:P < 0.01, extension:P < 0.05) and smaller endplate size (P < 0.001) than male ones. The segmental motion increased with increasing severity of disc degeneration up to Grade IV, but decreased in both genders when the disc degeneration advanced to Grade V. In male segments, the disc degeneration-related motion changes were significant in axial rotation (P < 0.001), lateral bending (P < 0.05), and flexion (P < 0.05), whereas female segments showed significant changes only in axial rotation (P < 0.001). With cartilage degeneration of the facet joints, the axial rotational motion increased, whereas the lateral bending and flexion motion decreased in female segments. In male segments, however, motion in all directions increased with Grade 3 cartilage degeneration and decreased with Grade 4 cartilage degeneration. Subchondral sclerosis significantly decreased the motion (female: axial rotation, P < 0.05; extension, P < 0.05 vs.— male:flexion,P < 0.05). Severity of osteophytes had no significant association with the segmental motion. Conclusion. Axial rotational motion was most affected by disc degeneration, and the effects of disc degeneration on the motion were similar between genders. Facet joint osteoarthritis also affected segmental motion, and the influence differed for male and female spines. Further studies are needed to clarify whether the degenerative process of facet joint osteoarthritis differs between genders and how facet joint osteoarthritis affects the stability of the spinal motion segment.

Anatomic changes of the spinal canal and intervertebral foramen associated with flexion-extension movement

Study Design A cadaveric study was done to analyze the dimensional changes in the spinal canal and intervertebral foramen of the lumbar spine with flexion and extension movements. Objectives To investigate the relationship between flexion and extension movements and morphologic changes in the spinal canal and the intervertebral foramen. Summary of Background Data Previous studies have reported that the dimensions of the spinal canal and the intervertebral foramen may change significantly with motion. The purpose of this study was to assess the quantitative changes in the spinal canal and the intervertebral foramen with segmental flexion‐extension movements. Methods Nineteen fresh cadaveric spines yielding 25 motion segments were used. The lumbar motion segments were frozen and then imaged in axial and sagittal projections by a computed tomography scanner. They were thawed then, and the motion segments were loaded to 5.7 Nm in flexion (13 motion segments) and in extension (12 motion segments) specimens. While in flexion or extension, the specimens again were frozen and imaged by computed tomography scan. The frozen specimens then were sliced using a cryomic‐rotome in the sagittal plane to study the dimensions of the intervertebral foramen. Eighteen other fresh cadaveric spines were sliced sagittally for study in the neutral position. Results The axial computed tomography scans showed that extension significantly decreased the canal area, midsagittal diameter, and subarticular sagittal diameter, whereas flexion had the opposite effects. The sagittal computed tomography scans showed that extension decreased all the foraminal dimensions significantly, whereas flexion increased all the foraminal dimensions significantly. The translational changes were associated with the bulging of the disc and the presence of traction spurs. The cryomicrotome sections showed the cross‐sectional area of the foramen to be 12% greater for the flexion group and 15% smaller for the extension group than the cross‐sectional area of the neutral group. Nerve root compression in the foramen was found to be 21.0% in neutral, 15.4% in flexion, and 33.3% in extension groups. Conclusions This study supports the concept of dynamic spinal stenosis. In addition to static anatomic changes, careful dynamic studies may be required to evaluate better the central canal and the foramen.

The relationship between disc degeneration, facet joint osteoarthritis, and stability of the degenerative lumbar spine

Degenerative processes in the disc and facet joints affect the stability of the motion segment. The exact relations among disc degeneration, facet joint osteoarthritis, and the kinematics of the motion segment are not well defined in the literature. Magnetic resonance imaging and functional radiography of the lumbar spine were analyzed to examine the relations among segmental instability, facet joint osteoarthritis, and disc degeneration in patients with degenerative disorders of the lumbar spine. Seventy consecutive patients (mean age, 46 years) had both magnetic resonance imaging and flexion and extension radiographs of the lumbar spine. The lumbar instability was classified into abnormal tilting on flexion, rotatory instability in the sagittal plane, and translatory instability. Translatory instability was subdivided into anterior, posterior, and anteroposterior translatory instability. Disc degeneration as seen on T2-weighted sagittal images was classified into five grades. Facet joint osteoarthritis as seen on axial T1-weighted images was divided into four grades. This study revealed that the kinematics of the lumbar motion segment are affected by disc degeneration and facet joint osteoarthritis. Abnormal tilting movement on flexion and anteroposterior translatory instability both had negative associations with facet joint osteoarthritis. However, anterior translatory instability was positively associated with disc degeneration and facet joint osteoarthritis. Rotatory instability in the sagittal plane and posterior translatory instability were not associated with disc degeneration and facet joint osteoarthritis.

The origin of chondrocytes in the nucleus pulposus and histologic findings associated with the transition of a notochordal nucleus pulposus to a fibrocartilaginous nucleus pulposus in intact rabbit intervertebral discs

Study Design. Intact rabbit lumbar intervertebral discs were examined histologically. Objectives. To demonstrate the origin of chondrocytes in the nucleus pulposus, and to document histologic findings associated with the transition of a notochordal to a fibrocartilaginous nucleus pulposus. Summary of Background Data. A human nucleus pulposus undergoes a chronological transition from a notochordal to a fibrocartilaginous nucleus pulposus. However, the origin of chondrocytes forming fibrocartilage in the nucleus pulposus and the mechanisms of transition remain unknown. Methods. Hematoxylin-eosin– and safranin O–stained slides obtained from 125 intact rabbit intervertebral discs were observed with light and polarized light microscopy. Results. Of the 125 intervertebral discs examined, 58 had a notochordal nucleus pulposus. The remaining intervertebral discs had a nucleus pulposus with fibrocartilage lamellas or fibrocartilage fibers. All forms of fibrocartilage lamellas and fibers found in the nucleus pulposus were formed by chondrocytes that had originated and migrated from the cartilage endplate. The origin of chondrocytes proceeded in a centripetal direction from the periphery toward the center of the cartilage endplate. The newly formed fibrocartilage lamellas and fibers, therefore, initially involved replacement of the peripheral regions of the nucleus pulposus, followed by replacement of the central region. This centripetal sequential replacement mechanism decreased the size of the notochordal tissue while increasing the lamellar structure of the intervertebral disc. Conclusions. Chondrocytes in the intact rabbit nucleus pulposus originated and migrated from the cartilage endplate. The chondrocytes changed notochordal nucleus pulposus into fibrocartilaginous nucleus pulposus by depositing fibrocartilage lamellas and fibers in a centripetal direction.

The relationship between disc degeneration and flexibility of the lumbar spine

BACKGROUND CONTEXT A relationship between degenerative changes of the intervertebral disc and biomechanical functions of the lumbar spine has been suggested. However, the exact relationship between the grade of disc degeneration and the flexibility of the motion segment is not known. PURPOSE To investigate the relationship between degenerative grades of the intervertebral disc and three-dimensional (3-D) biomechanical characteristics of the motion segment under multidirectional loading conditions. STUDY DESIGN/SETTING A biomechanical and imaging study of human cadaveric spinal motion segments. METHODS One hundred fourteen lumbar motion segments from T12-L1 to L5-S1 taken from 47 fresh cadaver spines (average age at death, 68 years; range, 39 to 87 years) were used in this study. The severity of degeneration (grades I to V according to Thomsons system) was determined using magnetic resonance (MR) images and cryomicrotome sections. Pure unconstrained moments with dead weights were applied to the motion segments in six load steps. The directions of loading included flexion, extension, right and left axial rotation, and right and left lateral bending. RESULTS When the MR images were graded, 2 segments had grade I disc degeneration; 45, grade II; 20, grade III; 26, grade IV; and 21, grade V. When the cryomicrotome sections were graded, 14 segments had grade I disc degeneration; 31, grade II; 22, grade III; 26, grade IV; and 21, grade V. Segments from the upper lumbar levels (T12-L1 to L3-4) tended to have greater rotational movement in flexion, extension, and axial rotation with disc degeneration up to grade IV, whereas the motion decreased when the disc degenerated to grade V. In the lower lumbar spine at L4-5 and L5-S1, motion in axial rotation and lateral bending was increased in grade III. CONCLUSIONS These results suggest that kinematic properties of the lumbar spine are related to disc degeneration. Greater motion generally was found with disc degeneration, particularly in grades III and IV, in which radial tears of the annulus fibrosus are found. Disc space collapse and osteophyte formation as found in grade V resulted in stabilization of the motion segments.

Morphologic Changes in the Lumbar Intervertebral Foramen Due to Flexion-extension, Lateral Bending, and Axial Rotation: An in Vitro Anatomic and Biomechanical Study

Study Design. A biomechanical and anatomic study with human cadaveric lumbar spine. Objectives. The purpose of this study is to examine the morphologic changes in the intervertebral foramen during flexion, extension, lateral bending, and axial rotation of the lumbar spine and to correlate these changes with the flexibility of the spinal motion segments. Summary of Background Data. Previous studies showed morphologic changes in the intervertebral foramen during flexion and extension; however, those changes during lateral bending and axial rotation were not well known. Methods. There were 81 motion segments obtained from 39 human cadaveric lumbar spines (mean age 69 years). The motion segments were imaged with CT scanner with 1-mm thick consecutive sections. For biomechanical testing each motion segment was applied with incremental pure moments of flexion, extension, lateral bending, and axial rotation. Rotational movements of the motion segment were measured using VICON cameras. After application of the last load, the specimens were frozen under load, and then CT was performed with the same technique described above. Six parameters of the intervertebral foramen were measured, including foraminal width (maximum and minimum), foraminal height, disc bulging, thickness of ligamentum flavum, and cross-sectional area of the foramen. Results. Flexion increased the foraminal width (maximum and minimum), height, and area significantly while significantly decreasing the disc bulging and thickness of ligamentum flavum (P < 0.05). However, extension decreased the foraminal width (maximum and minimum), height, and area significantly. Lateral bending significantly decreased the foraminal width (maximum and minimum), height, and area at the bending side, whereas lateral bending significantly increased the foraminal width (minimum), height, and area at the opposite side of bending. Likewise, axial rotation decreased the foraminal width (minimum) and area at the rotation side significantly while significantly increasing the foraminal height and foraminal area at the opposite side. The percent change in the foraminal area was found significantly correlated with the amount of segmental spinal motion except for the extension motion. Conclusions. This study showed that the intervertebral foramen of the lumbar spine changed significantly not only on flexion–extension but also on lateral bending and axial rotation. The percent change in cross-sectional foraminal area was correlated with the amount of segmental motion except for extension motions. Further studies are needed to assess the morphologic changes in the intervertebral foramen in vivo and to correlate clinically.

A Biomechanical Comparison of Modern Anterior and Posterior Plate Fixation of the Cervical Spine

Study Design. A biomechanical study was designed to assess relative rigidity provided by anterior, posterior, or combined cervical fixation using cadaveric cervical spine models for flexion–distraction injury and burst fracture. Objectives. To compare the construct stability provided by anterior plating with locked fixation screws, posterior plating with lateral mass screws, and combined anterior-posterior fixation in clinically simulated 3-column injury or corpectomy models. Summary of Background Data. Anterior plating with locked fixation screws is the most recent design and is found to provide better stability than the conventional unlocked anterior plating. However, there are few data on the direct comparison of biomechanical stability provided by anterior plating with locked fixation screws versus posterior plating with lateral mass screws. Biomechanical advantages of using combined anterior-posterior fixation compared with that of using either anterior or posterior fixation alone also have not been well investigated yet. Methods. Biomechanical flexibility tests were performed using cervical spines (C2–T1) obtained from 10 fresh human cadavers. In group I (5 specimens), one-level, 3-column injury was created at C4–C5 by removing the ligamentum flavum and bilateral facet capsules, the posterior longitudinal ligament, and the posterior half of the intervertebral disc. In group II (5 specimens), complete corpectomy of C5 was performed to simulate burst injury. In each specimen, the intact spine underwent flexibility tests, and the following constructs were tested: (1) posterior lateral mass screw fixation (Axis plate) after injury; (2) polymethylmethacrylate anterior fusion block plus posterior fixation; (3) polymethylmethacrylate block plus anterior (Orion plate) and posterior plate fixation; and (4) polymethylmethacrylate block plus anterior fixation. Rotational angles of the C4–C5 (or C4–C6) segment were measured and normalized by the corresponding angles of the intact specimen to study the overall stabilizing effects. Results. Posterior plating with an interbody graft showed effective stabilization of the unstable cervical segments in all loading modes in all cases. There was no significant stability improvement by the use of combined fixation compared with the posterior fixation with interbody grafting, although combined anterior-posterior fixation tended to provide greater stability than both anterior and posterior fixation alone. Anterior fixation alone was found to fail in stabilizing the cervical spine, particularly in the flexion–distraction injury model in which nocontribution of posterior ligaments is available. Anterior plating fixation provided much greater fixation in the corpectomy model than in the flexion–distraction injury model. This finding suggests that preservation of the posterior ligaments may be an important factor in anterior plating fixation. Conclusions. This study showed that the posterior plating with interbody grafting is biomechanically superior to anterior plating with locked fixation screws for stabilizing the one-level flexion–distraction injury or burst injury. More rigid postoperative external orthoses should be considered if the anterior plating is used alone for the treatment of unstable cervical injuries. It was also found that combined anterior and posterior fixation may not improve the stability significantly as compared with posterior grafting with lateral mass screws and interbody grafting.

Effect of endplate conditions and bone mineral density on the compressive strength of the graft-endplate interface in anterior cervical spine fusion

Study Design. Destructive compression tests and finite element analyses were conducted to investigate the biomechanical strength at the graft–endplate interface in anterior cervical fusion. Objectives. To investigate the effect of endplate thickness, endplate holes, and bone mineral density of the vertebral body on the biomechanical strength of the endplate–graft interface in an anterior interbody fusion of the cervical spine. Summary of Background. Subsidence of the graft into the vertebral body is a well-known complication in anterior cervical fusion. However, there is no information in the literature regarding the compressive strength of the graft–endplate interface in relation to the endplate thickness, holes in the endplate, and bone mineral density of the vertebral body. Methods. Biomechanical destructive compression tests and finite element analyses were performed in this study. Cervical vertebral bodies (C3–C7) isolated from seven cadaveric cervical spines (age at death 69–86 years, mean 79 years) were used for compression tests. Bone mineral density of each vertebral body was measured using a dual energy radiograph absorptiometry unit. Endplate thickness was measured using three coronal computed tomography images of the middle portion of the vertebral body obtained using a computer-assisted imaging analysis. Then each vertebral body was cut into halves through the horizontal plane. A total of 54 specimens, consisting of one endplate and half of the vertebral body, were obtained after excluding eight vertebrae with gross pathology on plain radiograph. Specimens were assigned to one of three groups with different endplate conditions (Group I, intact; Group II, partial removal; and Group III, complete removal) so that group mean bone mineral density became similar. Each endplate was slowly compressed until failure using an 8-mm-diameter metal indenter, and the load to failure was determined as a maximum force on a recorded force–displacement curve. The effect on the strength of the graft–endplate interface of various hole patterns in the endplate was studied using a finite element technique. The simulatedhole patterns included the following: one large central hole, two lateral holes, two holes in the anterior and posterior portion of the endplate, and four holes evenly distributed from the center of the endplate. Stress distribution in the endplate was predicted in response to an axial compressive force of 110 N, and the elements with von Mises stress greater than 4.0 MPa were determined as failed. Results. The endplate thickness and bone mineral density were similar at all cervical levels, and the superior and inferior endplates had similar thickness at all cervical levels. There was no significant association between bone mineral density and endplate thickness. Load to failure was found to have a significant association with bone mineral density but not with endplate thickness. However, load to failure tends to decrease with incremental removal of the endplate, and load to failure of the specimens with an intact endplate was significantly greater than that of the specimens with no endplate. Finite element model predictions showed significant influence of the hole pattern on the fraction of the upper endplate exposed to fracture stress. A large hole was predicted to be more effective than the other patterns at distributing a compressive load across the remaining area and thus minimizing the potential fracture area. Conclusion. Results of this study suggest that it is important to preserve the endplate as much as possible to prevent graft subsidence into the vertebral body, particularly in patients with poor bone quality. It is preferable to make one central hole rather than multiple smaller holes in the endplate for vascularity of the bone graft because it reduces the surface area exposed to fracture stresses.

Orientation and osteoarthritis of the lumbar facet joint.

Several studies have shown an association between sagittal orientation of the facet joint and degenerative spondylolisthesis. There is currently no information available on the association between orientation of the facet joint and osteoarthritis. This study examined the association between orientation and osteoarthritis of the lumbar facet joints. One hundred eleven consecutive patients underwent plain radiography and magnetic resonance imaging. These patients were divided into two groups: No Degenerative Spondylolisthesis Group (98 patients) and Degenerative Spondylolisthesis Group (13 patients). In the No Degenerative Spondylolisthesis Group, segments with higher grades of osteoarthritis showed more sagittal orientation of the facet joints at the L3–L4 and L4–L5 levels. The facet joint was oriented significantly more sagittally in the Degenerative Spondylolisthesis Group than in the No Degenerative Spondylolisthesis Group at the L4–L5 and L5–S1 levels. The severity of facet joint osteoarthritis was significantly higher in the Degenerative Spondylolisthesis Group than in the No Degenerative Spondylolisthesis Group at the L3–L4, L4–L5, and L5–S1 levels. A significant association was found between sagittal orientation and osteoarthritis of the lumbar facet joints, even in patients without degenerative spondylolisthesis. Facet joint osteoarthritis, rather than spondylolisthesis, is the pathoanatomic feature that is associated with sagittal orientation of the facet joints in patients with degenerative spondylolisthesis.

Biomechanical comparison of cervical spine reconstructive techniques after a multilevel corpectomy of the cervical spine.

Study Design. An in vitro biomechanical study of several reconstructive techniques after a two-level cervical corpectomy. Objectives. To evaluate, compare, and quantitate the stability of several reconstructive strategies (anterior, posterior, or anterior/posterior with or without instrumentation) after a multilevel cervical corpectomy. Summary of Background Data. Several clinical and biomechanical studies have questioned the stability of stand-alone long-segment anterior plate fixation after a multilevel (≥2) corpectomy. The large cantilever forces generated within the stabilized construct, particularly at the caudal screw–bone interface, have led to plate and screw dislodgement and the need for further surgical intervention. The addition of posterior segmental instrumentation has been shown to improve overall stability and decrease local stresses on the anterior fusion construct (graft and plate). Materials and Methods. Seven fresh-frozen cadaveric human cervical spines (C1-T1) were harvested. The C1–C2 and C7-T1 vertebral bodies were embedded in poly-methylmethacrylate (PMMA). Three VICON cameras tracked three-dimensional segmental motions at the ends of the fusion construct after a two-level corpectomy and placement of a strut graft with or without instrumentation. Pure moments (flexion/extension, lateral bending, and axial rotation) were applied to the C1 level of each specimen. The motion segments were loaded to a maximum of 2 Nm using dead weights. Testing was first performed on the intact specimens. Then, a two-level corpectomy at the C4 and C5 levels was performed. A PMMA strut graft was then placed into the corpectomy site. Biomechanical testing was then repeated among three different reconstruction techniques: 1) anterior cervical locking plate (PEAK; Depuy-Acromed, Raynham, MA) with dual unicortical screw fixation at C3 and C6; 2) posterior cervical instrumentation (Summit; Depuy-Acromed) using a 3.0-mm rod with segmental lateral mass screw fixation from C3 to C6; and 3) a combined anterior–posterior instrumentation using the anterior PEAK plate and posterior Summit rod system. Results. In all pure moments tested (flexion/extension/lateral bending/axial rotation) the combined anterior–posterior instrumentation reconstruction model and the posterior-only instrumentation model were significantly more rigid than the anterior-only instrumentation model (P < 0.05). Interestingly, no statistically significant difference was noted between the combined anterior plate/posterior instrumentation model and the posterior instrumentation-only model. Conclusion. The biomechanical results obtained suggest that posterior segmental instrumentation confers significant stability to a multilevel cervical corpectomy regardless of the presence or absence of anterior instrumentation. In cases in which the stability of a multilevel reconstruction procedure is tenuous, the surgeon should strongly consider the placement of segmental posterior instrumentation to significantly improve the overall stability of the fusion construct.