Direct real-time measurement of in vivo forces in the lumbar spine.

BACKGROUND CONTEXT: Accurate knowledge of the mechanical loads in the lumbar spine is critical to understanding the causes of degenerative disc disease and to developing suitable treatment options and functional disc replacements. To date, only indirect methods have been used to measure the forces developed in the spine in vivo. These methods are fraught with error, and results have never been validated using direct experimental measurements. PURPOSE: The major aims of this study were to develop a methodology to directly measure, in real time, the in vivo loading in the lumbar spine, to determine if the forces developed in the lumbar spine are dependent on activity and/or posture and to assess the baboon as an animal model for human lumbar spine research based on in vivo mechanical loading. STUDY DESIGN: Real-time telemetered data were collected from sensor-imbedded implants that were placed in the interbody space of the lumbar spines of two baboons. METHODS: An interbody spinal implant was designed and instrumented with strain gauges to be used as a load cell. The implant was placed anteriorly in the lumbar spine of the baboon. Strain data were collected in vivo during normal activities and transmitted by means of a telemetry system to a receiver. The forces transmitted through the implant were calculated from the measured strain based on precalibration of the load cell. Measured forces were correlated to videotaped activities to elucidate trends in force level as a function of activity and posture over a 6-week period. The procedure was repeated in a second baboon, and data were recorded for similar activities. RESULTS: Implants measured in vivo forces developed in the lumbar spine with less than 10% error. Loads in the lumbar spine are dependent on activity and posture. The maximum loads developed in the lumbar spine during normal (baboon) activities exceeded four times body weight and were recorded while animals were sitting flexed. Force data indicate similar trends between the human lumbar spine and the baboon lumbar spine. CONCLUSIONS: It is possible to monitor the real-time forces present in the lumbar spine. Force data correlate well to trends previously reported for in vivo pressure data. Results also indicate that the baboon may be an appropriate animal model for study of the human lumbar spine.

Biomechanical evaluation of a novel lumbosacral axial fixation device.

BACKGROUND: Interbody arthrodesis is employed in the lumbar spine to eliminate painful motion and achieve stability through bony fusion. Bone grafts, metal cages, composite spacers, and growth factors are available and can be placed through traditional open techniques or minimally invasively. Whether placed anteriorly, posteriorly, or laterally, insertion of these implants necessitates compromise of the anulus--an inherently destabilizing procedure. A new axial percutaneous approach to the lumbosacral spine has been described. Using this technique, vertical access to the lumbosacral spine is achieved percutaneously via the presacral space. An implant that can be placed across a motion segment without compromise to the anulus avoids surgical destabilization and may be advantageous for interbody arthrodesis. The purpose of this study was to evaluate the in vitro biomechanical performance of the axial fixation rod, an anulus sparing, centrally placed interbody fusion implant for motion segment stabilization. METHOD OF APPROACH: Twenty-four bovine lumbar motion segments were mechanically tested using an unconstrainedflexibility protocol in sagittal and lateral bending, and torsion. Motion segments were also tested in axial compression. Each specimen was tested in an intact state, then drilled (simulating a transaxial approach to the lumbosacral spine), then with one of two axial fixation rods placed in the spine for stabilization. The range of motion, bending stiffness, and axial compressive stiffness were determined for each test condition. Results were compared to those previously reported for femoral ring allografts, bone dowels, BAK and BAK Proximity cages, Ray TFC, Brantigan ALIF and TLIF implants, the InFix Device, Danek TIBFD, single and double Harms cages, and Kaneda, Isola, and University plating systems. RESULTS: While axial drilling of specimens had little effect on stiffness and range of motion, specimens implanted with the axial fixation rod exhibited significant increases in stiffness and decreases in range of motion relative to intact state. When compared to existing anterior, posterior, and interbody instrumentation, lateral and sagittal bending stiffness of the axial fixation rod exceeded that of all other interbody devices, while stiffness in extension and axial compression were comparable to plate and rod constructs. Torsional stiffness was comparable to other interbody constructs and slightly lower than plate and rod constructs. CONCLUSIONS: For stabilization of the L5-S1 motion segment, axial placement of implants offers potential benefits relative to traditional exposures. The preliminary biomechanical data from this study indicate that the axial fixation rod compares favorably to other devices and may be suitable to reduce pathologic motion at L5-S1, thus promoting bony fusion.

A pilot study to evaluate the effectiveness of small intestinal submucosa used to repair spinal ligaments in the goat.

BACKGROUND CONTEXT: Destabilization of the lumbar spine results from sacrifice of the anterior longitudinal ligament and disc when removed for graft or cage placement. In a similar fashion, transection of the interspinous ligament during surgical approaches to the posterior spine may result in segmental instability. Such instability can cause abnormal motion or implant migration resulting in a higher incidence of pseudarthrosis. Small intestinal submucosa (SIS) is a naturally occurring extracellular collagen-based matrix, which is derived from porcine small intestine. SIS contains cytokines and growth factors and has been shown to act as a resorbable scaffold in vivo that promotes host soft tissue regeneration with little scar tissue formation. SIS can be manufactured in laminated sheets of various sizes and thicknesses for different indications. Successful applications of SIS in animals have included dural substitution, rotator cuff repair, vessel repair, abdominal and bladder wall repair, and others. However, SIS has not been investigated to determine its ability to facilitate regeneration of spinal ligaments. PURPOSE: The purpose of this pilot study was to evaluate the efficacy of SIS as a barrier to prevent interbody device migration, and to act as a scaffold for regeneration of the anterior longitudinal ligament (ALL) and posterior interspinous ligament (PISL) in a goat model. STUDY DESIGN/SETTING: The thoracolumbar spine of the goat was exposed surgically. After resection and removal of the ALL or PISL at alternating levels, either SIS was placed or no treatment was administered. New ligament formation and SIS resorption were monitored over a 12-week period. OUTCOME MEASURES: Plain film radiographs and histomorphometry were used to assess the progress of healing over a 12-week time period. METHODS: Four skeletally mature nubian-alpine crossbred goats were used in this study. Under general anesthesia, each T10 to L5 motion segment was exposed surgically. Both anterolateral and posterior approaches were performed simultaneously at each level. Anteriorly, alternating levels received either 1) anterior discectomy, sacrifice of ALL and placement of SIS (SIS group); 2) anterior discectomy, sacrifice of ALL and no SIS (surgical control group) or 3) no surgical intervention (nonoperative group). A solid interbody spacer was placed into the disc space after discectomy to deter spontaneous anterior interbody fusion. Posteriorly, alternating levels were treated with either 1) sacrifice of the PISL with placement of SIS (SIS group); 2) sacrifice of PISL and no SIS (surgical control group) or 3) no surgical intervention (nonoperative group). The SIS was secured to the adjacent superior and inferior spinous processes to create a tension-band effect. Animals were radiographed immediately postoperatively to confirm placement of interbody spacers and anchors and to serve as a baseline for monitoring interbody spacer positioning. After surgery, all animals were allowed unrestricted motion for 12 weeks. At the end of the 12-week period, animals were radiographed and euthanized. The lumbar spine was harvested en bloc and processed for decalcified histologic evaluation. The dorsal and ventral aspects of each motion segment were analyzed for signs of inflammation and scar tissue formation, residual SIS and regenerated ALL or PISL. RESULTS: All animals tolerated the surgical procedure well, and there were no intraoperative or anesthesia-related complications. Twelve-week radiographs showed some evidence of ventral migration of the interbody spacers in several animals. Fifty percent (two of four) of spacers in surgical control group levels had migrated more than 10 mm (resulting in complete migration out of the disc space), whereas no spacers migrated completely out of levels with SIS placed. Gross analysis at necropsy indicated iatrogenic scar formation at operated levels, the degree of which was not different from surgical control group to SIS levels. Histologic evaluation of areas where the ALL had been removed indicated formation of organized fibrilar collagenous tissue that spanned the disc space at some levels where the SIS was placed. In some cases, the newly formed tissue was approximately the thickness of the ALL at the nonoperative group levels. The newly formed collagenous tissue was accompanied by sparse focal areas of inflammation, with small fragments of residual SIS at some levels. At surgical control group levels, there was a varying degree of connective tissue that ranged from moderately organized to randomly oriented with no significant signs of inflammation. Similarly, histologic analysis of some levels where SIS was placed posteriorly showed formation of organized collagenous tissues where the PISL had been removed. CONCLUSIONS: In this model, the SIS patch was sufficient to prevent acute ventral migration of interbody spacers from the disc space. The extent of long-term healing and new tissue formation in the SIS group indicates that it may be efficacious as a reparative intervention for transected ligaments in the spine. Most SIS specimens showed formation of organized collagenous tissue, indicating a long-term potential for ligament formation. However, in this model, 12 weeks of postoperative healing is insufficient to assess the full potential of SIS as a spinal ligament repair. Further research that follows the healing process to a longer time point postoperatively may be necessary to fully understand the potential of SIS as a resorbable scaffold for tissue replacement.