Posterolateral Fusion Mass Research Articles

231. The use of fusion mass screws in revision spinal deformity surgery: preliminary results and analysis

BACKGROUND CONTEXT Revision spinal deformity surgery in a previously fused spine often presents a number of challenges. Conventional landmarks for pedicle screw placement may be obscured or nonexistent, increasing the potential for inadequate fixation and spinal cord or nerve root injury. In these situations, screws through an existing fusion mass in patients with prior spinal fusion represent an alternative to pedicle screw fixation. This may provide stable fixation while reducing the risk of complications related to pedicle screw placement. PURPOSE To further elucidate the safety and efficacy of fusion mass screws (FMS) in revision spinal deformity surgery as an alternative and/or adjunct to pedicle screw fixation. STUDY DESIGN/SETTING Case series. PATIENT SAMPLE Adult patients undergoing revision spinal deformity surgery. OUTCOME MEASURES BMD of fusion mass, AP thickness of fusion mass, presence and degree of canal breach, screw loosening, screw failure, pseudarthrosis. METHODS Fourteen freehand FMSs were placed in six cases of revision adult spinal deformity surgery between 2016 and 2018. All screws were 5 mm in diameter. FMSs were used to save distal levels from fusion, assist in closing a three-column osteotomy, stabilize a Charcot spine with severe degeneration, and achieve fixation in the setting of pedicular dysplasia associated with NF-1. The bone mineral density (BMD) and thickness of the posterolateral fusion mass was assessed on preoperative CT scans at the level of each planned FMS when available. The accuracy of FMS placement and presence of pseudarthrosis were evaluated on postoperative CT scans. RESULTS Fourteen freehand FMSs were placed in six cases of adult spinal deformity. In all cases, FMSs were combined with pedicle screws, at times at the same levels. There were no neurophysiologic alerts observed during FMS placement and no neurologic deficits or complications postoperatively. On postoperative CT, 1/14 screws had a low-grade breach into the canal ( CONCLUSIONS A mature fusion mass consists of a dense, vacant surface for screw fixation. FMSs may be used to achieve stable fixation in revision spinal deformity cases when pedicles are dysplastic and/or normal anatomical landmarks are obscured by an extensive fusion mass. Additionally, FMSs may provide an additional anchor at a level where two pedicle screws are already placed to increase the strength and rigidity of the construct, assist in osteotomy closure, and potentially shorten the construct. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Arthrodesis Radiologically Mimicking Ankylosing Spondylitis

Figure 1. (A) T-spine X-ray, lateral. (B) Spinemagnetic resonance imaging: Newly appeared compression fracture at T9 and segmental cord signal change at T9 level (white arrow head). Posterolateral fusion mass on T7-L1 that was misinterpreted as bony bridge were shown (white arrows). Additionally, bulging disc and facet arthrosis were shown at multiple level of spine. Journal of Rheumatic Diseases Vol. 22, No. 3, June, 2015 http://dx.doi.org/10.4078/jrd.2015.22.3.209

Proposed alternative revision strategy for broken S1 pedicle screw: radiological study, review of the literature, and case reports

Background contextThere have been many reports outlining differing methods for managing a broken S1 screw. To the authors' best knowledge, the technique used in the present study has not been described previously. It involves insertion of a second pedicle screw without removing the broken screw shaft. Study designRadiological study, literature review, and two case reports of the surgical technique. PurposeTo report a proposed new surgical technique for management of broken S1 pedicle screws. MethodsComputed tomography (CT) scans of 50 patients with a total of 100 S1 pedicles were analyzed. There were 25 male and 25 female patients with an average age of 51 years ranging from 36 to 68 years. The cephalad-caudal length, medial-lateral width, and cross-sectional area of the S1 pedicle were measured and compared with the diameter of a pedicle screw to illustrate the possibility of inserting a second screw in S1 pedicle without removal of the broken screw shaft. Two case reports of the proposed technique are presented. ResultsThe left and right S1 pedicle cross-sectional area in female measured 456.00±4.00 and 457.00±3.00 mm2, respectively. The left and right S1 pedicle cross-section area in male measured 638.00±2.00 and 639.00±1.00 mm2, respectively. There were statistically significant differences when comparing male and female S1 pedicle length, width, and cross-sectional area (p<.05). At 2-year follow-up, the two case reports of the proposed technique showed resolution of low back pain and radicular pain. Plain radiograph and CT scan showed posterolateral fusion mass and hardware in good position with no evidence of screw loosening. ConclusionsThe S1 pedicle dimensions measured on CT scan reviewed in the present study showed that it may be anatomically feasible to place a second screw through the S1 pedicle without the removal of the broken screw shaft. This treatment method will reduce the complications associated with other described revision strategies for broken S1 screws.

A prospective comparative study of radiological outcomes after instrumented posterolateral fusion mass using autologous local bone or a mixture of beta-tcp and autologous local bone in the same patient

We previously reported a retrospective analysis of radiographic changes in instrumented posterolateral fusion mass (PLF) established using a mixture of local autologous bone and beta tricalcium phosphate (b-TCP) in lumbar spinal fusion surgery. Here, we report a prospective study to compare the use of local bone and a mixture of local bone and b-TCP in PLF in degenerative spinal surgery. Radiological changes in the PLF mass in 42 patients were analyzed for 12 months. All patients had degenerative lumbar spinal disease and underwent instrumented fusion. Local autologous bone was used for PLF on the left side, and a mixture of local autologous bone and b-TCP was used for PLF on the right side. Lumbar spinal anterior-posterior (AP) images were performed immediately postoperative and at 1, 3, 6, and 12 months. Computed tomography (CT) was also done immediately postoperative and at 12 months. Fusion rate, radiodensity, and the dimensions of the PLF mass bilaterally on the AP X-ray were compared. The change in volume in both fusion bridges in the CT image was also compared. The overall fusion rates were 31/42 (73.8 %) and 24/42 (57.1 %) on the left and right sides, respectively. The decrease in radiodensity at 1 month postoperative was significantly greater on the left side than on the right side (from 0.79 to 0.74 versus from 0.81 to 0.78; p = 0.002). The mean immediate postoperative volume on the left side measured with CT was 5.1 cc (95 % CI, 4.94-5.34) and on the right side was 5.2 cc (95 % CI, 4.97-5.37). The mean volume at 12 months had decreased to 2.2 cc (95 % CI, 1.85-2.64) on the left side and 1.87 cc (95 % CI, 1.48-2.67) on the right side. The volume decrease on the right side was statistically greater than on the left side (p = 0.048). Based on the changes in radiodensity and fusion rate during follow-up, local bone seems to undergo earlier resorption and stabilization than the mixture of local bone and b-TCP..

The presacral retroperitoneal approach for axial lumbar interbody fusion

The presacral retroperitoneal approach for axial lumbar interbody fusion (presacral ALIF) is not widely reported, particularly with regard to the mid-term outcome. This prospective study describes the clinical outcomes, complications and rates of fusion at a follow-up of two years for 26 patients who underwent this minimally invasive technique along with further stabilisation using pedicle screws. The fusion was single-level at the L5-S1 spinal segment in 17 patients and two-level at L4-5 and L5-S1 in the other nine. The visual analogue scale for pain and Oswestry Disability Index scores were recorded pre-operatively and during the 24-month study period. The evaluation of fusion was by thin-cut CT scans at six and 12 months, and flexion-extension plain radiographs at six, 12 and 24 months. Significant reductions in pain and disability occurred as early as three weeks postoperatively and were maintained. Fusion was achieved in 22 of 24 patients (92%) at 12 months and in 23 patients (96%) at 24 months. One patient (4%) with a pseudarthrosis underwent successful revision by augmentation of the posterolateral fusion mass through a standard open midline approach. There were no severe adverse events associated with presacral ALIF, which in this series demonstrated clinical outcomes and fusion rates comparable with those of reports of other methods of interbody fusion.

Radiographic Analysis of Instrumented Posterolateral Fusion Mass Using Mixture of Local Autologous Bone and b-TCP (PolyBone®) in a Lumbar Spinal Fusion Surgery

Although iliac crest autograft is the gold standard for lumbar fusion, the morbidity of donor site leads us to find an alternatives to replace autologous bone graft. Ceramic-based synthetic bone grafts such as hydroxyapatite (HA) and b-tricalcium phosphate (b-TCP) provide scaffolds similar to those of autologous bone, are plentiful and inexpensive, and are not associated with donor morbidity. The present report describes the use of Polybone® (Kyungwon Medical, Korea), a beta-tricalcium phosphate, for lumbar posterolateral fusion and assesses clinical and radiological efficacy as a graft material. This study retrospectively analyzed data from 32 patients (11 men, 21 women) who underwent posterolateral fusion (PLF) using PolyBone® from January to August, 2008. Back and leg pain were assessed using a Numeric Rating Scale (NRS), and clinical outcome was assessed using the Oswestry Disability Index (ODI). Serial radiological X-ray follow up were done at 1, 3, 6 12 month. A computed tomography (CT) scan was done in 12 month. Radiological fusion was assessed using simple anterior-posterior (AP) X-rays and computed tomography (CT). The changes of radiodensity of fusion mass showed on the X-ray image were analyzed into 4 stages to assess PLF status. The mean NRS scores for leg pain and back pain decreased over 12 months postoperatively, from 8.0 to 1.0 and from 6.7 to 1.7, respectively. The mean ODI score also decreased from 60.5 to 17.7. X-rays and CT showed that 25 cases had stage IV fusion bridges at 12 months postoperatively (83.3% success). The radiodensity of fusion mass on X-ray AP image significantly changed at 1 and 6 months. The present results indicate that the use of a mixture of local autologous bone and PolyBone® results in fusion rates comparable to those using autologous bone and has the advantage of reduced morbidity. In addition, the graft radiodensity ratio significantly changed at postoperative 1 and 6 months, possibly reflecting the inflammatory response and stabilization.

SF-36 Outcomes and Fusion Rates After Multilevel Laminectomies and 1 and 2-level Instrumented Posterolateral Fusions Using Lamina Autograft and Demineralized Bone Matrix

Short-Form 36 (SF-36) outcomes and fusion rates were assessed after multilevel laminectomies and 1 (95 patients) or 2-level (45 patients) instrumented fusions. The posterolateral fusion mass consisted of lamina autograft and demineralized bone matrix (Osteofil/ICM: Sofamor Danek, Memphis, TN) in a 50:50 mix. SF-36 questionnaires were administered preoperatively and 3, 6, and 12 months postoperatively. Two independent neuroradiologists (blinded to study design) separately documented fusion on both dynamic x-rays and two-dimensional-computed tomography (2D-CT) studies performed 3, 6, and up to 12 months postoperatively until arthrodesis was demonstrated. Patients were followed an average of 3 years in both fusion series (minimum of 1.5 y). One-year postoperatively, comparably improved outcomes were observed for both groups on 6 of 8 Health Scales of the SF-36. 2D-CT studies documented 1-level fusion an average of 5.2 months (Standard Deviation 1.8) postoperatively in 88 (92.6%) of 95 cases, whereas dynamic x-rays confirmed fusion in 93 (98%) patients. For 2 patients undergoing 1-level fusions, both 2D-CT and dynamic x-ray documented pseudarthrosis/instability; both patients required secondary surgery an average of 8 months postoperatively. 2D-CT documented 2-level fusion an average of 6.1 months (Standard Deviation 1.9) postoperatively in 41 of 45 (91.2%) patients, whereas dynamic x-rays confirmed fusion in 43 (96%) patients. For 2 patients undergoing 2-level fusions, both 2D-CT and dynamic x-rays documented pseudarthrosis/instability; both patients required secondary fusion an average of 10 months postoperatively. High fusion rates and adequate outcomes were observed after multilevel laminectomies and 1 and 2-level instrumented posterolateral fusions performed using lamina autograft and demineralized bone matrix: Osteofil/ICM.

Fluoroscopic Computer-Assisted Pedicle Screw Placement Through a Mature Fusion Mass

Observational matched cohort study with computed tomography (CT) analysis of pedicle screw placement. Assess the clinical accuracy of computer-assisted fluoroscopy for the placement of thoracolumbar pedicle screws through a mature posterolateral fusion mass. Pedicle screw placement through an amorphous posterolateral fusion mass intuitively carries a higher risk of pedicle wall violation. Postoperative CT scans of 102 pedicle screws placed through a mature posterolateral fusion mass (n = 10 [T10-T12]; n = 92 [L1-S1]) were independently assessed and compared with a matched control (nonobscured anatomy) group. All screws were placed before any decompression using the FluoroNav system. In the fusion mass group, overall 81.4% of screws were completely within the pedicle. Seven medial and 12 lateral pedicle breaches occurred. Relative to the total number of screws, pedicle breaches were graded II (<2 mm) in 13.5%, III (2-4 mm) in 2.9%, and IV (>4 mm) in 2.0% of screws. The number and direction of pedicle breaches were not significantly different when compared with the control group. There were no clinically significant screw misplacements in either group. The use of computer-assisted fluoroscopy is safe and effective for the placement of thoracolumbar (T10-S1) pedicle screws through a posterolateral fusion mass without performing laminoforaminotomies.

Paralysis after transforaminal epidural injection and previous spinal surgery

This case discusses a rare but devastating complication of transforaminal epidural injection. Elements in the patient's history that may be risk factors are discussed. A 64-year-old man was evaluated for chronic low-back pain after multiple spine surgeries. After the most recent surgery, he suffered transient cauda equina symptoms. Because conservative therapy was not helpful for spinal stenosis and neuroclaudication, a left L2 transforaminal epidural injection was attempted, but a posterolateral fusion mass made this procedure impossible. A left L1 transforaminal approach was successful, and 1 mL of iopamidol (Isovue) contrast was injected, followed by 5 mL of a solution of 0.125% bupivacaine and 40 mg of triamcinolone. Approximately 1 to 2 minutes after injection, the patient described discomfort in the lower abdomen, and 1 minute later, he was unable to move his lower extremities. An MRI showed T2 signal change in the conus medullaris gray matter at T11-12, consistent with an acute vascular infarct. Spinal shock protocol with high-dose methylprednisolone was begun without change. More than 4 years later, the patient continues to be troubled by persistent paraparesis and chronic pain. This case report is part of a new and growing body of literature that demonstrates the potential risk of transforaminal injection. Further study is necessary to ensure that spinal vascular injuries can be kept to an acceptably rare level.

Spinal implant debris-induced osteolysis.

Generally, implant-induced osteolysis is a manifestation of an adverse cellular response to phagocytosable particulate wear and corrosion debris. Initially termed "cement disease," particle-induced loosening was recognized by Charnley in the early 1960s. Despite the plethora of information gained over the last 40 years on the basic science of periprosthetic bone loss, much remains unanswered. The effect of unintended debris resulting from wear and corrosion (e.g., micromotion between the interconnection mechanisms in spinal implants) remains a clinical concern. The current study highlights what is known of particle-induced osteolysis and how the presence of spinal implant particulate debris deleteriously influences osseointegration of posterolateral bone graft or disrupts an established posterolateral fusion mass. Tissue explant, animal, and cell culture studies have revealed the complexity of cellular reactivity involved in aseptic particle-induced osteolysis. The objectives of this study are twofold: 1) to highlight the dominant cellular participants in total joint arthroplasty particle induced osteolysis, which are purportedly the macrophage, osteoblast, fibroblast, and osteoclast and several of the dominant chemical mediators have been identified as well, which include prostaglandin E2, tumor necrosis factor-alpha, interleukin-1, and interleukin-6; and 2) to demonstrate the potential deleterious effects of spinal implant debris using animal models and analysis of soft tissue surrounding spinal implants in symptomatic patients. There are a growing number of proinflammatory and anti-inflammatory cytokines, prostenoids, and enzymes that have been shown to play important roles in the pathology of particle-induced osteolysis. Reports that aseptic granulomatous inflammation typical of that associated with corrosion debris appear to correlate with the complexity of the implant. Titanium particulate material was used to induce effects in 34 New Zealand White rabbits where analysis included serological quantification of systemic cytokines. Postmortem microradiographic, immunocytochemical, and histopathologic assessment of the intertransverse fusion mass quantified the extent of osteolysis, local proinflammatory cytokines, osteoclasts and inflammatory infiltrates. Clinical analysis of 12 patients more than 0.4 years after spinal implants (mean 4.03, range 0.4 to 11 years) presented with late operative site pain. Currently the etiology of this inflammation around spinal implants resembles particle-induced osteolysis around joint arthroplasties where there typically is a self-perpetuating fibroinflammatory zone adjacent to the implant, where macrophage exhaustion, reactive oxygen intermediates, and pro-inflammatory cytokines affect a host of local cell types and induce a widening zone of soft tissue damage and inflammation. Animal model analysis indicated increased levels of local inflammatory cytokines typically associated with osteolysis-tumor necrosis factor-alpha. Osteoclast cell counts and regions of osteolytic resorption lacunas were higher in the titanium-treated versus autograft-alone groups (P < 0.05), and the extent of cellular apoptosis was markedly higher in the titanium-treated sites at both time intervals. Electron microscopy indicated definitive evidence of phagocytized titanium particles and foci of local, chronic inflammatory changes in the titanium-treated sites. 11 of 12 clinical cases demonstrated elevated tumor necrosis factor-alpha levels and an increased osteoclastic response in the vicinity of wear debris caused by dry frictional wear particles of titanium or stainless steel. Resection of the wear debris and surrounding fibroinflammatory zone resolved clinical symptoms in all 12 cases. More basic science and clinical research is needed to develop novel strategies for gaining knowledge, and developing effective evaluation and treatment of patients with implant debris related osteolysis. Titanium debris simulating that produced by spinal implants introduced at the level of a spinal arthrodesis elicits an inflammatory cytokine mediated particulate-induced response through increased expression of intracellular TNF-alpha, increased osteoclastic activity and cellular apoptosis. This study highlighted the association between spinal implants particulate wear debris and increased potential for osteolysis. Aseptic osteolysis is among the primary reasons for failure of orthopedic implants. Increased awareness of this destructive process is becoming more important with the growing popularity of total disc arthroplasty and highly modular spinal implants.

The effect of spinal instrumentation particulate wear debris: an in vivo rabbit model and applied clinical study of retrieved instrumentation cases

Study design: The current study was undertaken to determine if the presence of spinal instrumentation wear particulate debris deleteriously influences early osseointegration of posterolateral bone graft or disrupts an established posterolateral fusion mass. Objectives: Using an in vivo animal model, the first phase (basic science) of this study was to evaluate the effect(s) of titanium wear particulate on a posterolateral spinal arthrodesis based on serological, histological and immunocytochemical analyses. The second phase (clinical) was to perform the same analysis of soft tissue surrounding spinal instrumentation in 12 symptomatic clinical patients. Summary of background data: The effect of unintended wear particulate resulting from micromotion between the interconnection mechanisms in spinal instrumentation remains a clinical concern. Methods: Thirty-four New Zealand White rabbits were randomized into two groups based on postoperative time periods of 2 months (Group 1, n=14) and 4 months (Group II, n=20). Group I underwent a posterolateral arthrodesis (PLF) at L5–L6 using tricortical iliac autograft or tricortical iliac autograft plus titanium particulate. Group 2 all received iliac autograft at the initial surgery and were reoperated on after 8 weeks and treated with PLF exposure alone or titanium particulate. Postoperative analysis included serological quantification of systemic cytokines. Postmortem microradiographic, immunocytochemical and histopathological assessment of the intertransverse fusion mass quantified the extent of osteolysis, local proinflammatory cytokines, osteoclasts and inflammatory infiltrates. Clinical aspect of study: Over the last 2 years, 12 patients more than 0.4 years after spinal instrumentation presented with painful paraspinal inflammation. At surgical exploration, the cultures were negative for infection and the surrounding soft tissue was examined for cytokine reactions. There was loosening of implants and osteolysis in the location of the wear debris in 8 of 12 patients. Results: Basic science phase: serological analysis of systemic cytokines indicated no significant differences in cytokine levels (p>.05) between the titanium or autograft treatments. Immunocytochemistry indicated increased levels of local cytokines: TNF-α at the titanium-treated PLF sites at both time periods (p<.05). Osteoclast cell counts and regions of osteolytic resorption lacunae were higher in the titanium-treated versus autograft-alone groups (p<.05), and the extent of cellular apoptosis was markedly higher in the titanium-treated sites at both time intervals. Electron microscopy indicated definitive evidence of phagocytized titanium particles and foci of local, chronic inflammatory changes in the titanium-treated sites. Clinical aspect: Eleven of 12 clinical cases demonstrated elevated TNF-α levels and an increased osteoclastic response in the vicinity of wear debris caused by dry frictional wear particles of titanium or stainless steel. Osteolysis most commonly involved loose transverse connectors. Resection of the wear debris and surrounding fibroinflammatory glycocalyx resulted in resolution of clinical symptoms in all 12 cases. Conclusions: Titanium particulate debris introduced at the level of a spinal arthrodesis elicits a cytokine-mediated particulate-induced response favoring proinflammatory infiltrates, increased expression of intracellular TNF-α, increased osteoclastic activity and cellular apoptosis. This is the first basic scientific study and the first clinical study demonstrating associations of spinal instrumentation particulates wear debris and increased cytokines and increased osteoclastic activity. Osteolysis is the number one cause of failure of orthopedic implants in the appendicular skeleton. Spinal surgeons need to increase their awareness of this destructive process.

Resorbable posterolateral graft containment in a rabbit spinal fusion model.

The authors studied the effect of a resorbable graft containment device in a rabbit posterolateral lumbar spinal fusion model. Twenty rabbits were divided into four groups: autologous bone graft (ABG), ABG with the MacroPore containment device (ABG + MP), demineralized bone matrix (DBM), and DBM with the containment device (DBM + MP). Fusion mass was assessed at 6 weeks with high-resolution radiography and volumetric computerized tomography. The graft containment device was associated with alteration of the fusion mass structure and significant enhancement of fusion mass volume (ABG versus ABG + MP, p = 0.027; DBM versus DBM + MP, p = 0.043). A bioabsorbable protective graft containment device successfully enhanced posterolateral spinal fusion mass volume.

The effect of titanium particulate on development and maintenance of a posterolateral spinal arthrodesis: an in vivo rabbit model.

The current study was undertaken to determine if the presence of titanium wear particulate deleteriously influences early osseointegration of posterolateral bone graft or disrupts an established posterolateral fusion mass. Using an in vivo animal model to evaluate the effect(s) of titanium wear particulate on a posterolateral spinal arthrodesis based on serologic, histologic, and immunocytochemical analyses. The effect of unintended wear particulate resulting from micromotion between the interconnection mechanisms in spinal instrumentation remains a clinical concern. Thirty-four New Zealand White rabbits were randomized into two groups based on postoperative time periods of 2 months (Group 1, n = 14) and 4 months (Group 2, n = 20). Group 1 underwent a posterolateral arthrodesis at L5-L6 using tricortical iliac autograft or tricortical iliac autograft + titanium particulate. Group 2 received iliac autograft at the initial surgery and were reoperated on after 8 weeks and treated with posterolateral arthrodesis exposure alone or titanium particulate. Postoperative analysis included serologic quantification of systemic cytokines. Postmortem microradiographic, immunocytochemical, and histopathologic assessment of the intertransverse fusion mass quantified the extent of osteolysis, local pro-inflammatory cytokines, osteoclasts, and inflammatory infiltrates. Serologic analysis of systemic cytokines indicated no significant differences in cytokine levels (P > 0.05) between the titanium or autograft treatments. Immunocytochemistry indicated increased levels of local cytokines (tumor necrosis factor-alpha) at the titanium-treated posterolateral arthrodesis sites at both time periods (P < 0.05). Osteoclast cell counts and regions of osteolytic resorption lacunas were higher in the titanium-treated versus autograft-alone groups (P < 0.05), and the extent of cellular apoptosis was markedly higher in the titanium-treated sites at both time intervals. Electron microscopy indicated definitive evidence of phagocytized titanium particles and foci of local, chronic, inflammatory changes in the titanium-treated sites. Titanium particulate debris introduced at the level of a spinal arthrodesis elicits a cytokine-mediated particulate-induced response favoring pro-inflammatory infiltrates, increased expression of intracellular tumor necrosis factor-alpha, increased osteoclastic activity, and cellular apoptosis. The presence of titanium particulate debris, secondary to motion between spinal implants, may serve as the impetus for late-onset inflammatory-infectious complications and long-term osteolysis of an established posterolateral fusion mass in the clinical setting.

Evaluation of Posterolateral Fusion Mass at Lumbosacral Junction Using Standard AP and Ferguson Radiographs

Evaluation of Posterolateral Fusion Mass at Lumbosacral Junction Using Standard AP and Ferguson Radiographs

Maturation of the Posterolateral Spinal Fusion and Its Effect on Load-Sharing of Spinal Instrumentation

We investigated the temporal relationship among the biomechanical, radiographic, and histological properties of a posterolateral spinal fusion mass to elucidate the changes in load-sharing of the spinal instrumentation and that of the fusion mass throughout the healing process. Destabilization of the posterior spinal column and transpedicular screw fixation at the segments between the third and fourth and the fifth and sixth lumbar vertebrae was performed in twenty-four sheep. A posterolateral spinal arthrodesis with use of autologous corticocancellous bone graft was done randomly at one of the two segments; the other segment (without bone graft) served as the instrumented control. Six animals each were killed at four, eight, twelve, and sixteen weeks postoperatively. Biomechanical testing showed that the posterolateral fusion mass had increased mechanical stiffness after the fourth week. The strain on the hardware, measured with use of rods instrumented with strain-gauges, decreased significantly (p < 0.01) beginning at eight weeks. Radiographically, three independent observations of each of the six animals at each time-period showed that, although all of the fusion masses were considered solid unions at sixteen weeks, bridging of trabecular bone was noted during only ten of eighteen observations at twelve weeks, three of eighteen observations at eight weeks, and none of eighteen observations at four weeks. Computerized tomography and histomorphometric analyses demonstrated that mineralization in the fusion mass increased in a linear fashion even after eight weeks. Histologically, the fusion mass consisted predominantly of woven bone at eight weeks; thereafter, it was gradually trabeculated. We found a great discrepancy between biomechanical stability and histological maturation of the posterolateral fusion mass. The biomechanical properties of a stable spinal fusion preceded the radiographic appearance of a solid fusion by at least eight weeks, suggesting that immature woven bone provided substantial stiffness to the fusion mass. The spinal instrumentation was subjected predominantly to bending stress rather than to axial stress, and the load-sharing of the spinal instrumentation decreased concurrently with the development of the spinal fusion.

The role of spinal instrumentation in augmenting lumbar posterolateral fusion.

Using a sheep model, clinically practical posterolateral intertransverse process fusion was successfully achieved and biomechanically tested to determine the load-sharing environment provided by spinal instrumentation and posterolateral fusion mass following solid arthrodesis. To quantify the in vivo load-sharing capacity of spinal instrumentation on augmenting the posterolateral intertransverse fusion. The hypothesis was that transpedicular screw fixation maintains the biomechanical contribution to the posterolateral fusion stability even after successful arthrodesis because of its providing anterior and middle column support. Although many previous studies have documented the biological and biomechanical advantages of posterolateral fusion, it is known that posterolateral fusion without spinal instrumentation allowed significant remaining motion at the fused segment even after the solid arthrodesis. Whether spinal instrumentation, especially transpedicular screw fixation, augments in vivo posterolateral fusion stability after solid arthrodesis has not been previously investigated. Radiographic, macroscopic, and biomechanical analyses of a posterolateral intertransverse process fusion model were performed on 18 sheep at 4 months postoperatively. The load-sharing contribution of the spinal instrumentation was calculated based on the stability with or without spinal instrumentation tested in five loading modalities. Histomorphometry of the vertebral body spanned by spinal instrumentation provided the information regarding the biological effect of the load-sharing capacity of spinal instrumentation on bone remodeling. All sheep who received posterolateral intertransverse process fusion demonstrated successful solid arthrodesis and high biomechanical quality of the posterolateral fusion mass when compared to previous posterolateral fusion models. The significant difference in stiffness between fixation and subsequent fixation removal was observed in flexion, despite maintaining high lateral bending stiffness equivalent to the fixation (with instrumentation) level. This significant load-sharing contribution of spinal instrumentation detected in flexion corresponded to 27% when compared to the fixation level. The qualitative and quantitative bone histology showed 64% of the volumetric density of bone in the fixation group when compared to that of the sham group as well as narrow trabeculae and reduced connection of trabeculae. The continuance in support offered by transpedicular screw fixation was assured in vivo after the solid posterolateral intertransverse process fusion. This was clearly demonstrated under eccentric loads in a sagittal plane, suggesting that transpedicular screw fixation was able to provide anterior and middle column support and resist eccentric loads.

The Effect of Spinal Implant Rigidity on Vertebral Bone Density A Canine Mode

An animal model of anterior and posterior column instability was developed to allow in vivo observation of bone remodeling and arthrodesis following spinal instrumentation. After an initial anterior and posterior destabilizing lesion was created at the L5-L6 vertebral levels in 63 adult beagles, various spinal reconstructive surgical procedures were performed--with or without bilateral posterolateral bone grafting, with or without bilateral oophorectomies, and with or without spinal instrumentation (Harrington distraction, Luque rectangular, Cotrel-Dubousset pedicular, or Steffee pedicular implants). Observation 6 months after surgery revealed a significantly improved probability of achieving a spinal fusion if spinal instrumentation had been used (X2 = 5.84, P = .016). Nondestructive mechanical testing after removal of all metal instrumentation in torsion, axial compression, and flexion revealed that the fusions performed in conjunction with spinal instrumentation were more rigid (P less than .05). Quantitative histomorphometry showed that the volumetric density of bone was significantly lower (ie, device-related osteoporosis occurred) for fused versus unfused spines. In addition, a linear correlation occurred between decreasing volumetric density of bone and increasing rigidity of the spinal implant (r = .778); ie, device-related osteoporosis occurred secondary to Harrington, Cotrel-Dubousset, and Steffee pedicular instrumentation. Oophorectomized dogs became more osteoporotic than their surgically matched controls (posterolateral bone grafting alone, Cotrel-Dubousset pedicular instrumentation, and Steffee pedicular instrumentation); device-related osteoporosis added to the degree of hormonally induced osteoporosis (t = 5.0, P less than .0001). This is the first study to date documenting the occurrence of stress shielding in the spine secondary to spinal instrumentation.(ABSTRACT TRUNCATED AT 250 WORDS)