Mark Lorenz

The Biomechanical Effect of Postoperative Hypolordosis in Instrumented Lumbar Fusion on Instrumented and Adjacent Spinal Segments

STUDY DESIGN Change in lumbar lordosis was measured in patients that had undergone posterolateral lumbar fusions using transpedicular instrumentation. The biomechanical effects of postoperative lumbar malalignment were measured in cadaveric specimens. OBJECTIVES To determine the extent of postoperative lumbar sagittal malalignment caused by an intraoperative kneeling position with 90 degrees of hip and knee flexion, and to assess its effect on the mechanical loading of the instrumented and adjacent segments. SUMMARY OF BACKGROUND DATA The importance of maintaining the baseline lumbar lordosis after surgery has been stressed in the literature. However, there are few objective data to evaluate whether postoperative hypolordosis in the instrumented segments can increase the likelihood of junctional breakdown. METHODS Segmental lordosis was measured on preoperative standing, intraoperative prone, and postoperative standing radiographs. In human cadaveric spines, a lordosis loss of up to 8 degrees was created across L4-S1 using calibrated transpedicular devices. Specimens were tested in extension and under axial loading in the upright posture. RESULTS In patients who underwent L4-S1 fusions, the lordosis within the fusion decreased by 10 degrees intraoperatively and after surgery. Postoperative lordosis in the proximal (L2-L3 and L3-L4) segments increased by 2 degrees each, as compared with the preoperative measures. Hypolordosis in the instrumented segments increased the load across the posterior transpedicular devices, the posterior shear force, and the lamina strain at the adjacent level. CONCLUSIONS Hypolordosis in the instrumented segments caused increased loading of the posterior column of the adjacent segments. These biomechanical effects may explain the degenerative changes at the junctional level that have been observed as long-term consequences of lumbar fusion.

Load-bearing characteristics of lumbar facets in normal and surgically altered spinal segments

An in-vitro experimental study was carried out to measure the induced loading on human lumbar facets due to varying amounts of compressive axial load. Testing was done on L2-L3 and L4-L5 spinal motion segments obtained from cadavers at autopsy. The compressive loading was applied to the spinal specimens in first a neutral position and then in an extended position. The motion segments were tested in a normal state and after facetectomy of the left facet. Contact pressures were quantified using pressure-sensitive film. This film was inserted between the articulating surfaces of the two facet joints prior to placing axial loads on the motion segments. In addition to recording pressure, the film was also used to quantify contact area. This provided sufficient information to compute the total facet loadings for all cases studied. Results of this study show how facet loads, peak pressures and contact areas change with respect to the various loads, positions and states studied. In particular this study shows that the absolute facet loads remain relatively constant with increasing segmental compressive loads such that the facet load expressed as a percent of load applied to the segment decreases with increasing axial loads. It also shows that the contact area moves cranially at L2-L3 and caudally at L4-L5 with increasing loads in extension. Furthermore, this study indicates that the load on the remaining facet is reduced substantially after a facetectomy although peak pressure increases. Finally, this study demonstrates the substantial difference in facet loadings between L2-L3 segments and L4-L5 segments.

A comparison of single-level fusions with and without hardware

During a 47-month period, 68 patients were studied prospectively to compare single-level lumbar fusion with and without adjunctive pedicular fixation. Fusion indications were disabling back pain for a minimum of 6 months, inability to work, and failed conservative care. Twenty-nine patients fused without hardware and 39 fused with VSP fixation in identical procedures. All patients were evaluated according to fusion success, perceived pain, and return to work. Pseudarthrosis was demonstrated in 58.6% of the noninstrumented group. No pseudarthroses were noted in instrumented patients. Pain improvement in the nonhardware group was 41.4%, and in the hardware group, 76.9%. Return to work was 31% and 72% in each group, respectively. The fusion rate for patients exhibiting single-level disc disease improves with spinal fixation.

Pedicle morphology of the immature thoracolumbar spine.

Study Design. Human vertebral morphologic data were compiled from anatomic skeletal collections from three museums. Objectives. To quantify the morphometric characteristics of the pedicles of the immature thoracolumbar spine. Summary of Background Data. Little is known of pedicle morphology of the immature spine as related to pedicle screw fixation. Methods. A total of 75 anatomic skeletal specimens were acquired from C1 to L5 in the age range of 3 to 19 years. The data were collected and analyzed using a computerized video analysis system. Each vertebral pedicle was measured in the axial and sagittal planes. The measurements included the minimum pedicle width, the pedicle angle, the distance to anterior cortex, and anteroposterior and interpedicular spinal canal diameters. Results Wide variation in pedicle morphology between specimens at each vertebral level was found in the young population. In general, compared with the average adult data, a younger spine demonstrated a near uniform reduction in the linear pedicle dimensions at each vertebral level. Pedicles from the lower lumbar vertebrae attained dimensions acceptable for standard screw sizes at an earlier age than in the thoracic vertebrae. Conclusions. The data in this study indicates that pedicle screws may be used in the adolescent spine. However, care should taken to accurately ascertain pedicle size before surgery so that improper use of screws can be avoided. Growth of the pedicles in relation to the spinal canal indicates that the increase in pedicle size is lateral to the spinal canal.

Instability of the lumbar burst fracture and limitations of transpedicular instrumentation

Study Design This study analyzed the changes in the load-displacement behavior of lumbar spine segments caused by burst fractures that were experimentally produced in fresh human cadaveric spines. The effect of three transpedicular surgical constructs on stability was investigated in each specimen. Objectives To quantify the loss of mechanical stiffness caused by the injury, and to evaluate the stiffness of three transpedicular surgical constructs. Summary of Background Data Although various investigators have studied the biomechanical characteristics of the burst fracture and surgical stabilization techniques, few have reported quantitative data on the three-dimensional biomechanical instability of these fractures. Methods Load-displacement data were acquired in flexion, lateral bending, and axial rotation for intact specimens, after the L1 burst fracture was created and after the T12-L2 segments were stabilized using Luque plates, VSP plates, and Isola rods with one transverse connector. Results Spines with burst fractures showed a bilinear load-displacement behavior with significant instability (loss of stiffness relative to intact) at low loads (up to 3 N.m) in flexion, lateral bending, and axial rotation. The loss of stiffness was greatest in axial rotation over the entire load range (up to 10 N.m). If posterior element injury also was present, a significantly larger loss of stiffness was obsarved in flexion and axial rotation. The three transpedicular constructs improved the stability of the injured spine beyond that of the intact spine in flexion and lateral bending at low loads. At high loads, they restored the stiffness to intact levels. However, in axial rotation they did not restore the stiffness to pre-injury level, particularly when the posterior column was disrupted. Conclusions Reduction of the burst fracture returns the spine to its position of greatest inherent instability, essentially requiring the transpedicular instrumentation to be load bearing. To enhance mechanical stability, it may be necessary to augment the transpedicular construct, particularly when the posterior column is disrupted.

Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae

OBJECT There is limited data on the pullout strength of spinal fixation devices in the thoracic spine among individuals with different bone quality. An in vitro biomechanical study on the thoracic spine was performed to compare the pullout strength and the mechanism of failure of 4 posterior fixation thoracic constructs in relation to bone mineral density (BMD). METHODS A total of 80 vertebrae from 11 fresh-frozen thoracic spines (T2-12) were used. Based on the results from peripheral quantitative CT, specimens were divided into 2 groups (normal and osteopenic) according to their BMD. They were then randomly assigned to 1 of 4 different instrumentation systems (sublaminar wires, pedicle screws, lamina claw hooks, or pedicle screws with wires). The construct was completed with 2 titanium rods and 2 transverse connectors, creating a stable frame. The pullout force to failure perpendicular to the rods as well as the pattern of fixation failure was recorded. RESULTS Mean pullout force in the osteopenic Group A (36 vertebrae) was 473.2 ± 179.2 N and in the normal BMD Group B (44 vertebrae) was 1414.5 ± 554.8 N. In Group A, no significant difference in pullout strength was encountered among the different implants (p = 0.96). In Group B, the hook system failed because of dislocation with significantly less force than the other 3 constructs (931.9 ± 345.1 N vs an average of 1538.6 ± 532.7 N; p = 0.02). In the osteopenic group, larger screws demonstrated greater resistance to pullout (p = 0.011). The most common failure mechanism in both groups was through pedicle base fracture. CONCLUSIONS Bone quality is an important factor that influences stability of posterior thoracic implants. Fixation strength in the osteopenic group was one-fourth of the value measured in vertebrae with good bone quality, irrespective of the instrumentation used. However, in normal bone quality vertebrae, the lamina hook claw system dislocated with significantly less force when compared with other spinal implants. Further studies are needed to investigate the impact of different transpedicular screw designs on the pullout strength in normal and osteopenic thoracic spines.

Extradural synovial thoracic cyst

SUMMARY OF BACKGROUND DATA. Case studies documenting the incidence of thoracic intraspinal, extradural synovial cysts are limited. The occurrence of synovial cysts is associated with varied symptoms that differ among cervical, thoracic, and lumbar regions. The clinical appearance may be similar to other spinal diseases. METHODS. This report describes symptoms exhibited by and care provided for a patient with extradural synovial thoracic cyst.

An evidence-based medicine approach in determining factors that may affect outcome in lumbar total disc replacement.

Study Design. Literature research. Objective. To analyze the available evidence about a variety of factors that might affect outcome of lumbar artificial disc replacement. Summary of Background Data. Evaluating the scientific merit of new technology is important for a clinician considering incorporating these techniques. An evidence-based medicine approach can aid in this decision-making process. Methods. Eleven questions were asked about patient selection issues, surgical accuracy of placement, and evidence that motion preservation alters the natural history of degeneration. Studies where answers were found were ranked according to their level of evidence. Results. The majority of studies found were level IV, with only limited numbers of higher level studies. Only lower level studies with conflicting results assess the effect on outcomes of single versus multilevel surgery, L4–L5 versus L5–S1 implantations, patient’s age, and history of previous surgery. One lower level study suggests that mild-to-moderate facet degeneration does not influence outcomes. The extent of preoperative facet degeneration that can be accepted remains unclear, as level IV studies report degradation of facet degeneration after implantation. Higher level studies support the importance of surgical precision on clinical outcome and lower level studies give mixed results on the same issue. A level III prognostic study suggests that higher range of motion of the implanted segment may be associated with better outcomes, whereas 2 level IV therapeutic studies provide conflicting results. The incidence of adjacent level degeneration in lower level studies ranges between 17% and 28.6%, and can require additional surgery in 2% to 3% of patients. Two level IV studies suggest that preservation of motion may have a prophylactic effect on adjacent discs. Conclusion. Existing evidence does not provide definite conclusions in the majority of the questions regarding indications and factors that may affect outcomes. Where feasible, conclusions are mainly drawn from lower level, least reliable evidence. Highest quality data are short-term whereas longer-term data are of lower quality and in many instances conflicting. More high level studies with long-term follow-up are necessary to shed light to important clinical issues.

What is the correlation of in vivo wear and damage patterns with in vitro TDR motion response

Study Design. This study combined the evaluation of retrieved total disc replacements (TDRs) with a biomechanical study using human lumbar spines. Thirty-eight CHARITÉ TDRs were retrieved from 32 patients after 7.3 years average implantation. All implants were removed because of intractable back pain and/or facet degeneration. In parallel, 20 new implants were evaluated at L4–L5 and L5–S1 in an in vitro lumbar spine model. Objective. The purpose of this study was to correlate wear and damage patterns in retrieved TDRs with motion patterns observed in an in vitro lumbar spine model. We also sought to determine whether one-sided wear and motion patterns were associated with greater in vivo wear. Summary of Background Data. The comparison of polyethylene wear in TDRs after long-term implantation to those tested using an in vitro model had not yet been investigated. Methods. The wear patterns of each retrieved PE core was analyzed at the rim and dome. Thirty-five cores were further analyzed using MicroCT to determine the penetration symmetry. For the in vitro study the implants were tested under physiologic loads using a validated cadaveric model. Motion patterns of the in vitro-tested implants were tracked using sequential video-fluoroscopy. Results. Fifteen of 35 retrieved cores (43%) displayed one-sided wear patterns. Significant correlations were observed between implantation time and penetration and penetration rate. In the in vitro study, there was evidence of motion at both articulations, motion at both articulation but predominantly at the top articulation, and solelyat the top articulation. Core entrapment and pinching was observed and associated with visual evidence of core bending or deformation. Conclusion. This is the first study to directly compare the long-term PE wear and damage mechanisms in TDR retrievals with the motion patterns generated by a validated in vitro cadaveric testing model. The retrievals exhibited wear patterns consistent with the in vitro testing.

Symptomatic spondylolisthesis in adults: four decades later.

An average follow-up of 40 years was obtained for 12 patients with spondylolisthesis treated at Hines Veterans Administration Hospital between 1944 and 1951. In each case, the slip had been Grade 1 and at the L5-S1 level. Five had been treated conservatively and seven surgically with a Hibbs fusion from L4 to S1. Of the conservatively managed patients, all functioned well during their working years, although one did have chronic, nondisabling, low-back pain. This same patient demonstrated radiographic evidence of progression to a Grade 2 spondylolisthesis. Among those undergoing surgery, the poor results were confined to those patients whose fusion attempts failed. Management for low-grade spondylolisthesis should be conservative where possible. When the low-back pain is disabling and surgery becomes necessary, failure to obtain a fusion portends a poor clinical result.