Segmental stability and compressive strength of posterior lumbar interbody fusion implants.

STUDY DESIGN: Human cadaveric study on initial segmental stability and compressive strength of posterior lumbar interbody fusion implants. OBJECTIVES: To compare the initial segmental stability and compressive strength of a posterior lumbar interbody fusion construct using a new cortical bone spacer machined from allograft to that of titanium threaded and nonthreaded posterior lumbar interbody fusion cages, tested as stand-alone and with supplemental pedicle screw fixation. SUMMARY OF BACKGROUND DATA: Cages were introduced to overcome the limitations of conventional allografts. Radiodense cage materials impede radiographic assessment of the fusion, however, and may cause stress shielding of the graft. METHODS: Multisegmental specimens were tested intact, with posterior lumbar interbody fusion implants inserted into the L4/L5 interbody space and with supplemental pedicle screw fixation. Three posterior lumbar interbody fusion implant constructs (Ray Threaded Fusion Cage, Contact Fusion Cage, and PLIF Allograft Spacer) were tested nondestructively in axial rotation, flexion-extension, and lateral bending. The implant-specimen constructs then were isolated and compressed to failure. Changes in the neutral zone, range of motion, yield strength, and ultimate compressive strength were analyzed. RESULTS: None of the stand-alone implant constructs reduced the neutral zone. Supplemental pedicle screw fixation decreased the neutral zone in flexion-extension and lateral bending. Stand-alone implant constructs decreased the range of motion in flexion and lateral bending. Differences in the range of motion between stand-alone cage constructs were found in flexion and extension (marginally significant). Supplemental posterior fixation further decreased the range of motion in all loading directions with no differences between implant constructs. The Contact Fusion Cage and PLIF Allograft Spacer constructs had a higher ultimate compressive strength than the Ray Threaded Fusion Cage. CONCLUSIONS: The biomechanical data did not suggest any implant construct to behave superiorly either as a stand-alone or with supplemental posterior fixation. The PLIF Allograph Spacer is biomechanically equivalent to titanium cages but is devoid of the deficiencies associated with other cage technologies. Therefore, the PLIF Allograft Spacer is a valid alternative to conventional cages.

A surgical approach to the ventral aspect of the lumbar vertebrae in the sheep model.

The sheep model is frequently used for pre-clinical trials of spinal implants and vertebral interbody fusion devices. The lack of a well-documented multisegmental approach to the ventral aspect of the lumbar vertebrae has limited these trials to a posterior approach to the spine. A retroperitoneal approach to the sheep lumbar vertebrae was established and tested. One hundred and five sheep underwent the surgery. No major complications are reported. Major anatomical differences between sheep and humans were observed and documented. Anatomical variations in the sheep segmental vessel anatomy were also observed. Comprehensive knowledge of the retroperitoneal approach in sheep will facilitate pre-clinical studies testing ventral spinal implants or fusion techniques.

The efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar fusion in sheep.

STUDY DESIGN: An animal study was performed to evaluate lumbar spinal fusion radiologically and mechanically. OBJECTIVES: To assess the efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar arthrodesis in sheep. SUMMARY OF BACKGROUND DATA: Posterolateral spinal arthrodesis with autologous bone graft is the gold standard procedure for lumbar fusion. The procedure for harvesting bone from the iliac crest increases morbidity. Interconnected porous hydroxyapatite has been used effectively as an alternative to cancellous bone graft material in metaphyseal bone defects. Little is known about the efficacy of interconnected porous hydroxyapatite in achieving lumbar spinal fusion. METHODS: Four groups of seven sheep underwent bisegmental posterolateral lumbar fusion with instrumentation using different intertransverse graft material. In group 1, no graft material was used. In group 2, autologous bone was used. Group 3 had interconnected porous hydroxyapatite. Group 4 had an equip of interconnected porous hydroxyapatite and autologous bone. The animals were killed at 20 weeks after surgery. Radiographs and computed tomography images were obtained. The fusion masses were graded for bone resorption and trabecular connectivity on the computed tomography images. Mechanical testing of the specimens was performed, and the three-dimensional segmental motion was measured in flexion/extension, axial rotation, and lateral bending. RESULTS: The radiographic images were difficult to interpret because of the radiodense interconnected porous hydroxyapatite granules. According to mechanical stability criteria, the fusion rate for the different groups was as follows: 100% (14/14) for the autologous bone group, 72% (10/14) for the bone/interconnected porous hydroxyapatite group, 50% (7/14) for the pure interconnected porous hydroxyapatite group, and 15% (2/14) for the sham group. CONCLUSIONS: Spinal arthrodesis using interconnected porous hydroxyapatite alone or mixed with bone as graft material reduced segmental motion. It was not, however, as effective as autologous bone graft material in achieving spinal arthrodesis. The sheep model using autologous bone achieved a 100% fusion rate. Because the nonunion rate for a single level in humans may be as high as 40%, the fusion rate with bone/interconnected porous hydroxyapatite in humans may be lower than the 72% found in the sheep model. The little resorption of the radiodense interconnected porous hydroxyapatite granules made the radiologic evaluation of the fusion masses difficult.

Motor conduction alterations in patients with lumbar spinal stenosis following the onset of neurogenic claudication.

The pathogenesis of neurogenic claudication is thought to lie in relative ischemia of cauda equina roots during exercise. In this study we will evaluate the effect of the transient ischemia brought on by exercise on motor conduction in patients suffering from lumbar spinal stenosis (LSS). We will also evaluate the sensitivity of motor evoked potentials (MEPs) in detecting motor conduction abnormalities before and after the onset of neurogenic claudication. Thirty patients with LSS and 19 healthy volunteers were enrolled in the study. All LSS patients had a history of neurogenic claudication and the diagnosis was confirmed with a CT myelogram. Both groups underwent a complete electrophysiological evaluation of the lower extremities. The motor evoked potential latency time (MEPLT) and the peripheral motor conduction time (PMCT) were measured. The subjects were asked to walk on a flat surface until their symptoms were reproduced. A new set of electrophysiological tests was then performed. Exercise did not produce claudication in any of the control group subjects. Twenty-seven patients did have claudication. The pre-exercise MEPLT and nerve conduction studies in the control group fell within the normal range. In the patient group, 19 patients had increased baseline values for MEPLT to at least one muscle. There was a significant difference between the MEPLT and the PMCT values measured before and after exercise in the patients with signs of neurological deficit. This difference was not found to be significant in patients without neurological deficits (t-test P < 0. 05). It may be concluded that exercise increases the sensitivity of MEPs in detecting the roots under functional compression in LSS.

A new technique for measuring lumbar segmental motion in vivo. Method, accuracy, and preliminary results.

STUDY DESIGN: A direct method for three-dimensional in vivo spine kinematic studies was developed and used to measure segmental motion patterns in healthy subjects. OBJECTIVES: To validate the new method, and to study the L3-L4 segmental motion patterns for complex dynamic movements. SUMMARY OF BACKGROUND DATA: Conventional two-dimensional and three-dimensional radiographic methods have been used in the past to study spine kinematics. Few studies provided a direct approach to study segmental kinematics. No dynamic recordings of three-dimensional segmental motion patterns have been reported previously. METHODS: In 16 healthy men, Kirschner wires were inserted in the spinous processes of L3 and L4. Electromagnetic tracking sensors were attached to the pins. Motion data recorded during ranging exercises were used with biplanar radiographs to calculate L3-L4 segmental motion patterns. Errors resulting from pin deformation and the dynamic accuracy of the tracking system were investigated thoroughly. RESULTS: The average range of motion for flexion-extension was 16.9 degrees, for one side lateral bending 6.3 degrees and for one side axial rotation 1.1 degrees. Large intersubject variation was found in flexion-extension with values ranging from 7.1 to 29.9 degrees. Coupled motion patterns were found to be consistent among subjects in active lateral bending and inconsistent for active axial rotation. CONCLUSIONS: This new method offers dynamic recording capabilities and a measurement error comparable with stereo radiographic methods. Repetitive ranging experiments are highly reproducible. The range of motion for axial rotation seems overestimated in previous cadaveric studies. Coupling patterns show large variations between individuals.