Paul A. Anderson

Mechanism of the burst fracture in the thoracolumbar spine. The effect of loading rate.

Study Design Calf lumbar spine motion segments were randomly assigned to two groups. After insertion of a transducer capable of measururing transient occlusion of the spinal canal during impact, a low rate axial impact was applied in one group and a high rate load in the other.Post -injury computed tomography to determine the effect of rate of load application on occlusion of the spinal canal. Objectives This study was designed to determine if for the same direction of impact and total energy delivered, occlusion of the spinal canal postvertebral fracture was related to the rate at which the impact was delivered (time from zero to peak load). Summary of Background Data Several reports based on clinical observation have hypothesized that axial burstfractures, which displace bone fragments into the canal. occur because of internal pressurization and explosion of the vertebral body.The extent of bursting of the body, which could be related to the rate at which the load is applied. Method Using calf lumbar spines, a transducer was placed within the spinal canal, after removal of the cord, to measure canal occlusion during impact. One group received axial compressive impacts at a mean loading rate of 400 msec (zero to peak load) using a materials-tesing machine. The energy of failure was determined and used to select a drop weight and distance for the high loading rate tests, which would yield equivalent impact energy. The second group received impacts at a loading rate of 20 msec.The pos-injury radiographs and canal occlusion measurements were compared. Results The same mean energy of impact was used in the fractures for both groups. Post-injury radiographs of the low loading rate group showed compressive fractures with a mean canal occlusion of 6.84%, whereas the high loading rate group had burst fractures with mean canal encroachment of 47.6% (P=0.0007> Conclusions For the same energy and direction of impact, a high impact loading rate produces fractures with significant canal encroachment, whereas minimal encoachment is seen for fractures produced at a low loading rate.

Flexion distraction and chance injuries to the thoracolumbar spine

Summary Seat belt legislation has resulted in increased restraint use by passengers in automobiles in Washington State. At Harborview Medical Center in Seattle, we have observed an increased incidence of seat belt-related injuries. Twenty cases of Chance-type thoracolumbar flexion-distraction fractures were reviewed retrospectively. Thirteen patients (65%) had associated life-threatening intra-abdominal trauma. Twelve of these patients had bowel wall injury. Ninety percent of the children had combined abdominal and spinal injury. Operative treatment of the spinal injury resulted in correction of lumbar kyphosis and lower incidence of back pain than nonoperatively managed cases. We recommend careful physical and radiographic examination of all patients with significant abrasion or bruising about the pelvis or abdomen related to seat belts. Victims of automobile crashes who are treated for bowel injury require thoracolumbar radiographs. Similarly, patients with Chance-type fractures should undergo diagnostic peritoneal lavage or computerized abdominal tomography.

Failure of halo vest to prevent in vivo motion in patients with injured cervical spines

Forty-two patients with cervical spine injuries immobilized in halo vests were studied prospectively to determine in vivo vertebral segmental motion. Lateral radiographs taken in the supine and upright positions within 5 days of injury demonstrated intervertebral motion. At noninjured levels, the positional change accounted for an average 3.9 degrees of angulation with the greatest motion occurring between the occiput and C1 (8.0 degrees). At the injured levels, sagittal plane angulation averaged 7.0 degrees and translation averaged 1.7 mm between the two positions. Fracture site motion did not correlate with either the fracture type or the injury level. Fracture site motion greater than 3 degrees of angulation or 1 mm of translation was observed at 35 (77%) of 45 injured levels. When treating patients who have unstable cervical injuries with halo vests, supine and upright radiographs should be obtained. If excessive motion is present, alternative methods of treatment should be considered.

Early stabilization and decompression for incomplete paraplegia due to a thoracic-level spinal cord injury

All patients treated between 1985 and 1990 for acute incomplete spinal cord injury between T2 and T11 were retrospectively studied. This level was chosen for study because by excluding cervical cord, conus, and cauda equina injuries, neurologic improvement could be attributed to improvement of spinal cord function. Only 14 patients with incomplete thoracic level paraplegia were identified, representing 1.2% of all spinal injuries. All 14 patients were treated by early operative reduction, stabilization, or decompression. Tweive patients had surgery within 24 hours of neurologic injury, one at 36 hours, and one at 5 days. Twelve patients had initial posterior instrumentation and fusion, one of whom subsequently had an anterior decompression. Two patients had initial anterior decompression and fusion. Both later had posterior instrumentation and fusion to treat progressive deformity. Follow-up averaged 20 months (range, 9–65 months). Neural function before surgery and at follow-up was given a Frankel grade and lower extremity motor index score. Of 13 surviving patients, seven were initially Frankel B and six Frankel C. Of the seven patients initially Frankel B, four recovered to Frankel E, two improved to Frankel D, and one remained Frankel B. Of the six patients originally Frankel C, five recovered to Frankel E and one improved to Frankel D. Average neurologic improvement was 2.2 Frankel grades per patient, lower extremity motor index improved from an average of 7 to 44. Early surgical reduction, stabilization, and decompression is safe and improves neurologic recovery in comparison to historical controls treated by postural reduction or late surgical intervention.

Biomechanics of indirect reduction of bone retropulsed into the spinal canal in vertebral fracture.

The biomechanics of indirect reduction of bone fragments retropulsed into the spinal canal in a burst fracture were investigated. In this model, tunnels were created in vertesbrae L1 and C5 oriented anterior-to-posterior, allowing access to the posterior longitudinal ligament. A probe containing a load-sensing tip was passed through the tunnel. Both the location of the tip and the load acting on it by posterior deflection of tho posterior longitudinal ligament were measured. In the lumbar spine, distraction was applied by spinal instrumentation that also permitted Independent kyphotic-lordotic alignment of the vertebrae, In the cervical spine, axial traction was applied through direct loading. Several clinically relevant observations were made. It was not possible to produce an anteriorly directed force in the posterior longitudinal ligament at less then 35% canal occlusion, partly because the posterior longitudinal ligament stands away form the midbody of the vertebra. Distractive forces of up to 150 N were applied in the limbar spine, which were nearly equal to the tensile breaking strength of the relative sagittal plane angulation of the vertebrae, distraction was the governing factor in generating force in the posterior longitudinal ligament. Because positioning the vertebrae in lordosis before applying distraction significantly slackens the posterior longitudinal ligament, it is suggested that distraction be applied before angular positioning of the vertebrae is performed.

Geometric changes in the cervical spinal canal during impact.

Summary of Background Data Although the extant of injury after cervical spine fracture can be visualized by imaging, the deformations that occur in the spinal canal during injury are unknown. Study Design This study compared spinal canal occlusion and axial length changes occurring during a simulated compressive burst frecture with the residual deformations after the injury. Methods Canal occlusion was measured from changes in pressure in a flexible tube with fluid flowing through it, placed in the canal space after removal of the cord in cadaver specimens. To measure canal axial length, cables were fixed in C1 and led through the foramen transversarium from C2-T1, then out through the base, where they were connected to the core rods of linearly variable differential transformers (LVDT). Axial compressive burst fractures were created in each of ten cadaveric cervical spine specimens using a drop-weight, while force, distraction, and occlusion were monitored throughout theinjury event. Pre- and postinjury radiographs and computed tomography scans compared transient and post-injury spinal canal geometry changes. Results In all cases, severe compressive injuries were produced. Three had an extension component in addition to compression of the vertabra and retropulsion of bone into the canal. The mean post-injury axial height loss measured from radiographs was only 35% of that measured transiently (3.1 mm post-injury, compared with 8.9 mm measured transiently), indicating significant recovery of axial height after impact. Post-injury and transient height loss were not significantly correlated (r2 = 0.230, P = 0.16) demonstrating that it is not a good measure of the extent of injury. Similarly, mean post injry canal area was 139% of the minimum area measured during impact indicating recovery of canal space, and post-injury and transient values were not significantly correlated (r2 = 0.272, P = 0.12). Mean post-injury midsagittal diameter was 269% of the minimum transient diameter and showed a weak but significant correlation (r2 = 0.481, P = 0.03). Conclusions Two potential spinal cord injury-causing mechanisms in axial bursting injuries of the cervical splne are occlusion and shortening of the canal. Post-injury radiographic measurements significantly underestimate the actual transient injury that occurs during impact.

Posterior stabilization of the lower cervical spine with lateral mass plates and screws

Abstract Posterior cervical fixation using lateral mass plates and screws is becoming increasingly used and accepted.Advantages include increased rigidity, ability to be used in cases where the lamina or spinous processes are deficient or missing, use across the occipito-cervical or cervico-thoracic junction, and need for less postoperative bracing. Safe placement of lateral mass screws requires complete exposure and identification of the boundaries of the lateral masses. The starting point for screw placement is 1 to 2 mm medial to the center of lateral mass. The screws are angulated outward 10 to 20 degrees and cranially 20 to 30 degrees to be parallel to the facet joints. An adjustable drill guide facilitates safe drilling and tapping techniques. All 102 patients with unstable cervical spines treated with AO reconstruction plates and autogenous bone graft had healed fusions based on flexion-extension radiographs. The reductions achieved postoperatively were maintained at follow-up. Two patients had transient radiculopathies secondary to screw placement. The indications for lateral mass fixation include cases where the lamina or spinal processes are deficient or missing, multilevel or rotational instabilities, when extension to the thoracic spine or occiput is required or when decreased bracing is beneficial.

3-D Display Of Magnetic Resonance Imaging Of The Spine

The original data is produced through standard magnetic resonance imaging (MRI) procedures with a surface coil applied to the lower back of a normal human subject. The 3-D spine image data consists of twenty-six contiguous slices with 256 x 256 pixels per slice. Two methods for visualization of the 3-D spine are explored. One method utilizes a verifocal mirror system which creates a true 3-D virtual picture of the object. Another method uses a standard high resolution monitor to simultaneously show the three orthogonal sections which intersect at any user-selected point within the object volume. We discuss the application of these systems in assessment of low back pain.

Failure of Halo-vest to Prevent In-vivo Motion in Patients with Injured Cervical Spines

Forty-two patients with cervical spine injuries immobilized in halo vests were studied prospectively to determine In vivo vertebral segmental motion. Lateral radiographs taken in the supine and upright positions within 5 days of injury demonstrated intervertebral motion. At noninjured levels, the positional change accounted for an average 3.9° of angulation with the greatest motion occurring between the occiput and C1 (8.0°). At the injured levels, sagittal plane angulation averaged 7.0° and translation averaged 1.7 mm between the two positions. Fracture site motion did not correlate with either the fracture type or the injury level. Fracture site motion greater than 3° of angulation or 1 mm of translation was observed at 35 (77%) of 45 injured levels. When treating patients who have unstable cervical injuries with halo vests, supine and upright radiographs should be obtained. If excessive motion is present, alternative methods of treatment should be considered.