Michael J. Schendel

The effects of immobilization of long segments of the spine on the adjacent and distal facet force and lumbosacral motion

Long levels of spinal instrumentation and fusion are common in surgery for spinal deformity. The effect on the remaining mobile segments is not well understood. The changes in lumbar facet loading and lumbosacral motion were evaluated as the number of immobilized levels increased. Four fresh canine cadaveric spines from T6 to sacrum were used. Lumbosacral motion was measured with an instrumented spatial linkage device, and facet loads were measured at L1, L4, and L7 using a strain gauge technique. Lumbosacral motion and facet loading were significantly increased (P < 0.05) after immobilization of proximal segments, and the amount of the increase was dependent on the number of immobilized segments (P < 0.05). This indicates that immobilization of long segments of the spine influences the remaining mobile segments by increasing the load and motion not only at the immediately adjacent segment but also at the distal segments.

Comparison of in vivo and in vitro adjacent segment motion after lumbar fusion.

Study Design. For in vitro studies, there is no basis for choosing a “load control study” over a “displacement control” study. This study qualitatively compared results from in vitro and in vivo tests, allowing the authors to address the experimental assumptions that in vitro testing contributes to the understanding of the in vivo condition. Objectives. To compare motion changes at segments adjacent to fusions for in vitro and in vivo tests. Summary of Background Data. Investigators have measured the effects of spinal fusions on the adjacent segment in a human cadaver model and found greater adjacent facet joint load after fusions. Others have found significant increases in motion and facet loads at segments adjacent to in vitro lumbosacral and long fusions, when the same range of motion was repeated before and after immobilization of lumbar segments. Methods. L2–L3 motion was measured in vitro by an instrumented spatial linkage under load and displacement control before and after immobilization of segments (L3–L7). In vivo, L2–L3 motion was measured while animals walked on a treadmill. L3–L7 was fused and the L2–L3 motion testing was repeated. The change in in vivo adjacent segment motion was qualitatively compared with the in vitro change under “load” and “displacement” control. Results. Under “load” control, in vitro facet motion did not significantly change after immobilization, whereas under “displacement” control, the facet motion significantly increased from 2.2 ± 0.4 mm to 4.1 ± 0.6 mm. Post-instrumentation, in vivo L2–L3 facet motion increased significantly. This change in vivo related better to the changes seen in the in vitro “displacement” control test than to the in vitro “load” control test.

Effect of immobilization and configuration on lumbar adjacent-segment biomechanics

The motion and facet load changes at the adjacent segment after lumbosacral immobilization (with different angles) were quantified in vitro using canine spines. Changes were examined under flexion, extension, and lateral bending. An increase (extension: 62%; flexion: 85%; left bend: 30%; right bend 26%) in motion at the adjacent segment was found for all motions after immobilization. Adjacent-segment facet load remained constant under any load state or immobilized configuration. For all configurations, the facet contact site impinged in extension, remained unchanged in left bending, and moved superiorly in right bending. This study has shown that after immobilization the facet contact patterns at the adjacent segment changed, load was unchanged, and segmental motion increased when the lumbar spine reproduced the same range of motion. The configuration of the immobilized segments had no effect on these changes.

Experimental measurement of ligament force, facet force, and segment motion in the human lumbar spine

Facet forces, longitudinal ligament loads, and vertebral body motion were experimentally measured in five fresh human lumbar spine segments, L1-L2. Strain gages on the bone surface were used to quantify facet loads. Buckle transducers were used to measure anterior and posterior longitudinal ligament loads. The three-dimensional motion of the motion segment was measured with an instrumented spatial linkage. The facets were found to carry no load in flexion, large loads during extension (205 N at a 10 Nm moment and a 190 N axial load), torsion (65 N at a 10 Nm moment and a 150 N axial load), and lateral bending (78 N at a 3 Nm moment and a 160 N axial load). The facet contact site on the inferior articular process of L1 was found to move inferiorly to a position of tip impingement near the lamina as extension moments increased. Impingement occurred in the range of 4-6 Nm extension. The posterior and anterior longitudinal ligaments were predominantly loaded in flexion and extension, respectively. No ligament loads occurred in lateral bending and torsion. A 1 cm strip of the anterior longitudinal ligament carried loads up to 130 N at the largest extension moment of 11.4 Nm. The posterior longitudinal ligament had a 60 N load at the largest flexion moment of 7.1 Nm. There was no pre-load in the ligament detectable with the buckle transducers (> 4 N). The facets and ligaments began carrying load immediately with applied load, without a lax region. The experimental technique developed and used provides a good tool for obtaining simultaneous facet joint loads, ligament loads, and vertebral body motion without altering the motion segment.

The natural history of asymptomatic thoracic disc herniations.

Study Design. Magnetic resonance imaging was used to determine the natural history of asymptomatic thoracic disc herniations. Objectives. To determine whether thoracic disc herniations change in size over time. Summary of Background Data. Based on previous work by the authors of the present study, the incidence of asymptomatic thoracic disc herniations is approximately 37%. The natural history of thoracic disc herniations is unknown. The natural history of lumbar and cervical disc herniations in symptomatic individuals who become asymptomatic has been shown in multiple studies frequently to result in a decrease in size of the herniation. Methods. Twenty patients with 48 asymptomatic thoracic herniations previously diagnosed with magnetic resonance imaging underwent repeat magnetic resonance imaging using sagittal T1‐weighted spine echo and axial multiplanar gradient refocused images at each thoracic disc level from T1 to T12 for a mean follow‐up period of 26 months. Midsagittal canal diameter was recorded, and disc herniation square area was measured using a computer‐assisted digitizing program. Disc herniations were categorized according to percentage of canal compromise. The change in size of the disc herniations over time was analyzed. Results. All patients remained asymptomatic during the follow‐up period. A total of 48 disc herniations were identified from the original magnetic resonance images. There were 21 small (0–10% canal compromise) disc herniations, 20 medium (>10–20%) disc herniations, and seven large (>20%) disc herniations. Of the 21 small disc herniations, 18 showed no significant change in size, whereas three showed a measurable increase in size. Of the 20 medium‐sized disc herniations, 16 showed either a small or no change in size, one showed a significant increase in size, and three showed a significant decrease in size. Of the seven large disc herniations, three demonstrated no change in size, and four demonstrated a significant decrease in size. In addition, five new disc herniations were detected in four patients; one was small, and four were moderate in size. Conclusions. Based on the results of this study, the authors believe that asymptomatic disc herniations may well exist in a state of relative flux, yet exhibit little change in size and remain asymptomatic. There was a trend, however, for small disc herniations either to remain unchanged or increase in size and for large disc herniations often to decrease in size.

Rotational changes of the vertebral-pelvic axis following Cotrel-Dubousset instrumentation.

Ten consecutive patients with adolescent idiopathic scoliosis and King-Moe curve Types II and III, scheduled consecutively for Cotrel-Dubousset instrumentation, underwent pre- and postoperative computed tomography scans with axial slices through each vertebra, and including the pelvis. Vertebral rotation was measured and referenced to the pelvis. Average derotation of the thoracic apex after surgery was 9%. King-Moe Type II curves tended to derotate more successfully (average 26% improvement), while Type III curves derotated very little, if at all (average 1.3% worsening of the rotational deformity). Type II curves often showed segmental rotational changes outside the levels of instrumentation, while Type III curves did not; more frequently the spinal-pelvic axis rotated en bloc. It appears, therefore, that Cotrel-Dubousset instrumentation does not consistently or predictably derotate the thoracic apex relative to the pelvis, and coronal plane correction may only be apparent, due to transmitted torque and rotation of the entire spinal-pelvic axis.

Effect of Patient Position on the Sagittal-Plane Profile of the Thoracolumbar Spine

Although the normal sagittal profile of the thoracolumbar spine has been described, this has been obtained primarily by using young individuals standing. We sought to describe the sagittal profile of the thoracolumbar spine in an older population in the supine cross-table lateral position compared with that standing. We enrolled 50 volunteers with no history of back pain or spine deformity and 50 matched subjects with mechanical back pain (LBP) only. Lateral radiographs of the thoracolumbar spine (T10-S1) in both standing and cross-table supine positions were obtained. Lordosis from L1 to S1, kyphosis from T10 to L1, and the changes seen moving from the supine position to standing were calculated. There were few differences comparing the two groups in either the standing or cross-table supine position, or when changing positions. Within each group, however, there were small, but significant, differences in the midlumbar and thoracolumbar spine when comparing supine versus standing. Both asymptomatic individuals and those with a history of LBP demonstrated similar small but statistically significant increases in lumbar lordosis and thoracolumbar kyphosis when standing versus supine. The clinical significance of these findings remains to be determined.

Biomechanical analysis of facet and graft loading in a Smith-Robinson type cervical spine model

Objective This study determined the effect of change in graft height on the forces across a smith-Robinson graft as well as across the posterior elements of the same motion segment. Study Design The study utilizes a strain gauge technique for the measurement of facat joint loading and a subminiature load cell for the measurement of graft loads. Summary of Background Data A number of cases of Smith-Robinson procedures have had some form of collapse of the interspace and graft material after surgery. Some patients with collapse of the graft go on to have prolonged sclerotomal-type pain or pseudarthrosis. The appropriate amount of distraction is not well defined in the literature and may affect the outcome. Methods Carvical spines (C5-C6) were instrumented by placing strain gauges bilaterally on the pedicles of C6 (to measure the forces across the posterior elements). A miniature load cell with matching metallic shims was used to measure the force across the graft site and to distract the segment. Forces across the posterior elements and the graft site were measured, during flexion loading, and compared as the disc space was distracted. Conclusions In spondylotic specimens (4–5mm disc heights) disc space distraction in excess of 3.0 mm from preoperative height caused a significant decrease in both the ratio of posterior element to graft loading and posterior element loads. These findings may help explain recent clinical reports of a limit of effective disc space distraction

Magnetic resonance imaging and biological changes in injured intervertebral discs under normal and increased mechanical demands.

Study Design An animal model was used to examine the short‐term tissue response to changes in the mechanical environment after the structure (disc) is mechanically injured. Objectives To observe changes in an injured intervertebral disc and the corresponding motion segment when the mechanical demands of the disc were increased by fusion of the adjacent motion segments. Summary of Background Data Disc degeneration has been modeled in animals by producing a tear in the anulus via laminectomy, laparotomy, or posterolaterally. Methods of altering and quantifying the mechanics of the intervertebral joint by use of internal fixation and fusion in the canine have been developed. Methods Eight dogs divided into two groups (a study and a control group) had anular stab wounds (L2‐L3). The study group was surgically instrumented posteriorly from L3 to L7. Magnetic resonance imaging studies were conducted for all animals before and periodically after the surgical procedures. At the end of the study, the segments were processed histologically and biochemically. Results Anular bulging was seen on magnetic resonance imaging in all control animals 4 months after injury and did not progress out until 6 months after injury. Similar changes were seen in study animals, but 75% were herniated by 6 months. Histologic changes correlated with magnetic resonance imaging changes. No significant difference in water or proteoglycan content of the disc tissue between groups was found. Conclusions Progression from the bulging of the anulus to herniation was not evident in damaged discs not subjected to adjacent fusions. No change in water or proteoglycan content as a function of altered mechanical state was found, suggesting the short‐term effect of the altered mechanics is on the mechanical structure and not on the cells or extracellular matrix.

Rotational changes of the vertebral pelvic axis after sublaminar instrumentation in adolescent idiopathic scoliosis.

Study Design The authors studied the rotational effect of sublaminar wiring on the spinal pelvic axis on 20 patients who were being treated for adolescent idiopathic scoliosis. Objectives To determine if sublaminar wiring effectively derotates the scoliotic spine. Summary of Background Data The correction of the rotational deformity in adolescent scoliosis via sublaminar wiring is not well quantified in the literature. The derotation maneuver of Cotrel‐Dubousset has been shown to produce variable and unpredictable amounts of axial derotation. Methods Twenty patients who underwent posterior spine fusion for adolescent idiopathic scoliosis were evaluated using computed tomography scans and plain radiography before and after surgery and at a subsequent follow‐up examination (average time of follow‐up examination, 35 months after surgery). The degree of angle of vertebral rotation about the sagittal plane and that relative to the pelvis were measured before and after surgery and at a follow‐up examination. Results The primary thoracic curves were not derotated significantly relative to the pelvis with sublaminar wiring. Primary thoracolumbar curves instrumented on the convexity with pedicle screws were derotated significantly relative to the pelvis (P =.001). The average initial correction was 57%. On final follow‐up examination, the correction was 24% (18 of 20 twenty individuals lost axial correction by an average of 34%). In nine of 20 patients the spine was more rotated, relative to the pelvis, than it had been before surgery. No coronal or sagittal decompensation was seen in any curve type. Conclusions Coronal and sagittal plane correction of scoliotic curves may be achieved with sublaminar instrumentation. The ability to derotate axially the scoliotic spine appears to be variable, however, and, in most cases, curve‐type dependent. Over time, much correction appears to be lost, and in many patients the scoliosis actually becomes worse than it was before surgery. Nonetheless, the apical derotation that takes place appears to be reasonably true: the percent correction of angle of rotation about the sagittal plane and the percent correction of angle of rotation about the sagittal plain relative to the pelvis were closely correlated. Derotation forces applied to the instrumented spine do not appear to be transmitted to more distal segments.