Edgar G. Dawson

Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey

Neurologic deficits were compared to somatosensory evoked potential (SEP) spinal cord monitoring in a survey of spinal orthopedic surgeons. Experienced SEP spinal cord monitoring teams had fewer than one-half as many neurologic deficits per 100 cases compared to teams with relatively little monitoring experience. Experienced SEP monitoring teams also had fewer neurologic deficits than were seen in previous surveys of this group. Definite neurologic deficits, despite stable SEPs (false negative monitoring), occurred during surgery in only 0.063% of patients. Factors independently associated with fewer neurologic deficits also included the surgeons years of experience in orthopedic surgery and the use of the wake-up test. Other technical survey results are also presented here. These results confirm the clinical efficacy of experienced SEP spinal cord monitoring for prevention of neurologic deficits during spinal surgery such as for scoliosis.

Adjacent segment degeneration in the lumbar spine.

BACKGROUND A primary concern after posterior lumbar spine arthrodesis is the potential for adjacent segment degeneration cephalad or caudad to the fusion segment. There is controversy regarding the subsequent degeneration of adjacent segments, and we are aware of no long-term studies that have analyzed both cephalad and caudad degeneration following posterior arthrodesis. A retrospective investigation was performed to determine the rates of degeneration and survival of the motion segments adjacent to the site of a posterior lumbar fusion. METHODS Two hundred and fifteen patients who had undergone posterior lumbar arthrodesis were included in this study. The study group included 126 female patients and eighty-nine male patients. The average duration of follow-up was 6.7 years. Radiographs were analyzed with regard to arthritic degeneration at the adjacent levels both preoperatively and at the time of the last follow-up visit. Disc spaces were graded on a 4-point arthritic degeneration scale. Correlation analysis was used to determine the contribution of independent variables to the rate of degeneration. Survivorship analysis was performed to describe the degeneration of the adjacent motion segments. RESULTS Fifty-nine (27.4%) of the 215 patients had evidence of degeneration at the adjacent levels and elected to have an additional decompression (fifteen patients) or arthrodesis (forty-four patients). Kaplan-Meier analysis predicted a disease-free survival rate of 83.5% (95% confidence interval, 77.5% to 89.5%) at five years and of 63.9% (95% confidence interval, 54.0% to 73.8%) at ten years after the index operation. Although there was a trend toward progression of the arthritic grade at the adjacent disc levels, there was no significant correlation, with the numbers available, between the preoperative arthritic grade and the need for additional surgery. CONCLUSIONS The rate of symptomatic degeneration at an adjacent segment warranting either decompression or arthrodesis was predicted to be 16.5% at five years and 36.1% at ten years. There appeared to be no correlation with the length of fusion or the preoperative arthritic degeneration of the adjacent segment.

Spinal Cord Monitoring: Results of the Scoliosis Research Society and the European Spinal Deformity Society Survey

The Scoliosis Research Society (SRS) and the European Spinal Deformity Society (ESDS) membership was surveyed regarding the use of intraoperative monitoring of somatosensory evoked potentials in spinal surgery. A total of 242 people responded, with 188 using intraoperative monitoring. A second survey was distributed detailing the technical aspects of monitoring, of which 71 were returned. A total of 342 neurologic deficits were reported to have occurred with monitoring in place. Two hundred forty-six (72%) were accurately detected, and 96 (28%) were not detected by sensory cord evoked potentials (SCEP). There were 1,003 false-positive cases reported. The incidence of false-negative cases was related to those not monitoring both latency and amplitude, to using fewer recording electrodes, and with those surgeons doing more kyphosis corrections.

Effective doses of recombinant human bone morphogenetic protein-2 in experimental spinal fusion.

Study Design Nineteen dogs underwent L4‐L5 intertransverse process fusions with either 58 μg, 115 μg, 230 μg, 460 μg, or 920 μg of recombinant human bone morphogenetic protein‐2 carried by a polylactic acid polymer. A previous study (12 dogs) compared 2300 μg of recombinant human bone morphogenetic protein‐2, autogenous iliac bone, and carrier alone in this model. All fusions subsequently were compared. Objectives To characterize the dose‐response relationship of recombinant human bone morphogenetic protein‐2 in a spinal fusion model. Summary of Background Data Recombinant osteoinductive morphogens, such as recombinant human bone morphogenetic protein‐2, are effective in vertebrate diaphyseal defect and spinal fusion models. It is hypothesized that the quality of spinal fusion produced with recombinant human bone morphogenetic protein‐2, above a threshold dose, does not change with increasing amounts of inductive protein. Methods After decortication of the posterior elements, the designated implants were placed along the intertransverse process space bilaterally. The fusion sites were evaluated after 3 months by computed tomography imaging, high‐resolution radiography, manual testing, mechanical testing, and histologic analysis. Results As in the study using 2300 μg of recombinant human bone morphogenetic protein‐2, implantation of 58–920 μg of recombinant human bone morphogenetic protein‐2 successfully resulted in intertransverse process fusion in the dog by 3 months. This had not occurred in animals containing autograft or carrier alone. The cross‐sectional area of the fusion mass and mechanical stiffness of the L4‐L5 intersegment were not dose‐dependent. Histologic findings varied but were not related to rhBMP‐2 dose. Inflammatory reaction to the composite implant was proportional inversely to the volume of the fusion mass. Conclusions No mechanical, radiographic, or histologic differences in the quality of intertransverse process fusion resulted from a 40‐fold variation in dose of recombinant human bone morphogenetic protein‐2.

Evaluation of rhBMP-2 with an OPLA carrier in a canine posterolateral (transverse process) spinal fusion model.

Study Design Posterolateral L4-L5 transverse process fusions were done on 14 adult beagles. Six were implanted with recombinant human bone morphogenetic protein-2 carried by open-cell polylactic acid polymer delivery vehicle. Six received autogenous iliac bone graft. Two received carrier alone. Eleven were killed 3 months after implantation. One in each group was maintained for 8 months. Objectives. To compare recombinant human bone morphogenetic protein-2 and open-cell polylactic acid polymer with autogenous iliac bone for inducing transverse process fusion in the canine by 3 months and to determine whether transverse process decortication and implantation of carrier alone causes spontaneous transverse process fusion in the canine. Summary of Background Data Recombinant human bone morphogenetic proteins have healed segmental long bone defects in several models. They have induced interlaminar and facet fusions in canines. Interlaminar and facet fusions have occurred after sham decortications in canines. Recombinant human bone morphogenetic protein-2 has not been evaluated for transverse process fusion in canines. Transverse process fusion is a preferred clinical method for achieving posterior lumbar fusion. Methods Fusion sites were evaluated by serial computed tomography scans. After the dogs were killed, explanted spines were subjected to manual testing, mechanical testing, high resolution radiography, and histologic analysis. Results One hundred percent of recombinant human bone morphogenetic protein-2-implanted sites had solid transverse process fusion by 3 months according to all measures. No autografted sites were fused at this interval. Osseous bridging of posterolateral gutters occurred in the recombinant human bone morphogenetic protein-2-implanted sites after 2 months, the earliest radiographic measure. None of the carrier-only sites showed bone formation. Conclusions Recombinant bone morphogenetic protein-2 carried by open-cell polylactic acid polymer is superior to autogenous iliac bone for producing radiographically and mechanically solid transverse process fusions in canines by 3 months. Spontaneous transverse process fusion does not occur in canines after decortication and open-cell polylactic acid polymer implantation.

Experimental spinal fusion with recombinant human bone morphogenetic protein-2 without decortication of osseous elements

Study Design. L4-L5 intertransverse process fusions were produced with 58 μg, 230 μg, or 920 μg of recombinant human bone morphogenetic protein-2 in 20 dogs. Eleven had traditional decortication of posterior elements before insertion of the implant. Nine were left undecorticated. All animals were evaluated 3 months after surgery. Objectives. To determine whether decortication is a prerequisite for successful fusion in the presence of osteoinductive proteins such as bone morphogenetic protein-2. Summary of Background Data. Recombinant osteoinductive proteins can induce de novo bone in ectopic soft-tissue sites in the absence of bone marrow elements. Traditional methods for achieving spinal fusion rely on exposure of bone marrow through decortication to facilitate osteogenesis. It is hypothesized that the presence of an implanted osteoinductive protein obviates the need for exposure and release of host inductive factors. Methods. Recombinant human bone morphogenetic protein-2-induced intertransverse process fusions were performed with and without decortication. Fusion sites were evaluated by computed tomography imaging, high-resolution radiography, manual testing, mechanical testing, and histologic analysis. Results. One hundred percent of decorticated spines and 89% of undecorticated spines were clinically fused by 3 months. Ninety-one percent of decorticated spines and 78% of undecorticated specimens exhibited bilateral transverse process osseous bridging. The only spines that failed to achieve solid bilateral arthrodesis were in the lowest dose group. With the higher two doses, there was histologic evidence of osseous continuity between the fusion mass and undecorticated transverse processes. Conclusions. There were no statistical differences in clinical and radiographic fusion rates between decorticated and undecorticated sites. With higher doses of recombinant human bone morphogenetic protein-2, there was little histologic distinction between fusions in decorticated versus undecorticated spines.

Graft resorption with the use of bone morphogenetic protein: lessons from anterior lumbar interbody fusion using femoral ring allografts and recombinant human bone morphogenetic protein-2.

Study Design. This is a prospective cohort study examining the results and radiographic characteristics of anterior lumbar interbody fusion (ALIF) using femoral ring allografts (FRAs) and recombinant human bone morphogenetic protein-2 (rhBMP-2). This was compared to a historical control ALIF using FRAs with autologous iliac crest bone graft (ICBG). Objective. To determine whether the use of rhBMP-2 can enhance fusion ALIF with stand-alone FRAs. Summary of Background Data. ALIF is a well-accepted procedure in reconstructive spine surgery. Advances in spinal surgery have produced a multitude of anterior interbody implants. The rhBMP-2 has promoted fusion in patients undergoing ALIF with cages and threaded allograft dowels. The FRA still remains a traditional alternative for anterior support. However, as a stand-alone device, the FRA has fallen into disfavor because of high rates of pseudarthrosis. With the advent of rhBMP-2, the FRA may be more attractive because of its simplicity and remodeling potential. It is important to understand the implications when rhBMP-2 is used with such structural allografts. Methods. A total of 36 consecutive patients who underwent ALIF with stand-alone FRAs by a single surgeon (E.G.D.) at 1 institute were included. A cohort of 9 consecutive patients who received FRAs filled with rhBMP-2 was followed prospectively. After noticing suboptimal results, the senior author terminated this method of lumbar fusion. A total of 27 prior consecutive patients who received FRAs filled with autogenous ICBG were used for comparison. Analyzing sequential radiographs, flexion-extension radiographs, and computerized tomography with multiplanar reconstructions determined nonunions. Minimum follow-up was 24 months. Results. Pseudarthrosis was identified in 10 of 27 (36%) patients who underwent stand-alone ALIF with FRAs and ICBG. Nonunion rate was higher among patients who received FRAs with rhBMP-2 (i.e., 5 of 9 [56%]). Statistical significance was not established because of the early termination of the treatment group (P > 0.3). Of interest, radiographs and computerized tomography revealed early and aggressive resorption of the FRAs when used with rhBMP-2. This preceded graft fracture and even disintegration, resulting in instability and eventual nonunion. Conclusion. The use of rhBMP-2 did not enhance the fusion rate in stand-alone ALIF with FRAs. In fact, the trend was toward a higher nonunion rate with rhBMP-2, although this was not significant with the numbers available. This result appears to be caused by the aggressive resorptive phase of allograft incorporation, which occurs before the osteoinduction phase.

Distractive properties of a threaded interbody fusion device. An in vivo model.

Study Design. Twenty sheep underwent anterior lumbar interbody fusions with either a threaded titanium interbody fusion device (cage, n = 8), autogenous iliac crest dowel graft (autograft, n = 6), or interbody decortication only (sham, n = 6). Two sheep had misplaced cages and were excluded. Sheep were killed after 6 months. Objectives. To determine whether this model is useful for examining the distractive and fixation properties of interbody fusion cages. Summary of Background Data. Interbody fusion cages are used in anterior lumbar interbody fusion procedures to provide immediate intersegmental fixation and to distract and preserve interbody height. The process of physiologic anchorage by bone ingrowth into such devices is under investigation. Methods. Sheep were radiographed immediately after surgery and 2, 4, and 6 months after surgery. Interbody distraction and angulation were measured with a digital photo image analyzer at each time point. After the sheep were killed, stiffness to flexion, extension, and lateral bending moments were measured. Twelve untreated cadaver spines were also tested for comparison. Results. After surgery, interbody distraction successfully occurred in cage and autograft‐implanted sites. Loss of interbody height ensued, however, in all groups during the first 2 months. Percentage loss of height was lowest in cage sites. By 6 months, only cage sites remained distracted beyond normal. Fusions in all groups were stiffer than untreated spines. Autograft sites were stiffer than cage sites to lateral bending. Sham sites were stiffer than cage and autograft sites to flexion, but this likely resulted from complete intervertebral collapse. Conclusion. Despite early subsidence, interbody fusion cages successfully distracted and preserved interbody spaces. This model is useful for investigating methods of improving distraction and stabilization.

Recombinant Human Bone Morphogenetic Protein-2 on an Absorbable Collagen Sponge with an Osteoconductive Bulking Agent in Posterolateral Arthrodesis with Instrumentation: A Prospective Randomized Trial

BACKGROUND Recombinant human bone morphogenetic protein-2 (rhBMP-2) on an absorbable collagen sponge has been shown to be a safe and effective replacement for iliac crest bone graft when used with a threaded fusion device in anterior lumbar interbody arthrodesis. Use of rhBMP-2 on an absorbable collagen sponge in posterolateral lumbar arthrodesis requires the addition of a bulking agent to provide resistance against compression and to serve as an osteoconductive scaffold for new bone formation. METHODS We performed a prospective, randomized, multicenter pilot study to investigate the use of rhBMP-2 on an absorbable collagen sponge combined with a ceramic-granule bulking agent as a replacement for autogenous iliac crest bone graft in single-level posterolateral lumbar arthrodesis with instrumentation. The investigational group (twenty-five patients) was treated with a 1.5 mg/mL solution of rhBMP-2 on two strips of absorbable collagen sponge (total dose of rhBMP-2, 12 mg) combined with 10 cm(3) of ceramic granules. The control group (twenty-one patients) received iliac crest bone graft. Clinical outcomes were assessed with use of well-established instruments. Radiographs were reviewed to assess consolidation of fusion. RESULTS Eighty-eight percent (twenty-two) of the twenty-five patients in the investigational group and 86% (eighteen) of the twenty-one patients in the control group were considered to have completed the twenty-four-month follow-up. At all follow-up intervals, there were significant improvements in the clinical outcome measures, including the Oswestry Disability Index (ODI) scores, Short Form-36 scores, and back and leg pain scores, in both groups. At twenty-four months, the improvement in the mean ODI score, as compared with the preoperative score, was 28.2 points in the investigational group and 23.0 points in the control group. By twenty-four months, 95% (eighteen) of nineteen patients in the investigational group compared with 70% (fourteen) of twenty in the control group had a radiographically documented fusion. The overall success rate was 81% (seventeen of twenty-one) in the investigational group and 55% (eleven of twenty) in the control group (p = 0.345). CONCLUSIONS Compared with an iliac crest bone graft, the combination of an absorbable collagen sponge soaked with rhBMP-2 and ceramic granules resulted in trends toward improvements in clinical outcomes and toward a higher rate of radiographic fusion. This combination of an osteoinductive agent with an osteoconductive matrix may be an effective replacement for autograft in single-level posterolateral lumbar arthrodeses with instrumentation.

A prospective, randomized controlled clinical trial of anterior lumbar interbody fusion using a titanium cylindrical threaded fusion device.

Study Design. A prospective, randomized, controlled clinical trial comparing a cylindrical threaded titanium cage to a femoral ring allograft control for anterior lumbar interbody fusion. Objective. To compare these two implants with regard to arthrodesis. Secondary outcome measures included pain relief, neurological status, and general health status. Summary of Background Data. Anterior lumbar interbody fusion is a well-accepted procedure using trapezoidal femoral ring allografts or cylindrical titanium cages. Clinical and biomechanical studies evaluating these two distinct constructs are numerous; however, no prospective, randomized study comparing them has been done. Methods. A multicenter trial of 140 patients: 78 were randomized to the cylindrical threaded titanium cage device treatment arm and 62 patients randomized into the control group. All had autogenous iliac crest bone graft packed into the device. All patients had a single-level stand-alone anterior lumbar interbody fusion at either the L4–L5 or L5–S1 interspace for symptomatic degenerative disc disease. Radiographic fusion data were collected as well as multiple types of outcome data, including pain/disability scores, neurologic status, and overall health. Results. At 12 months, 97% of the cylindrical threaded titanium cage device group and 40% of the control group demonstrated radiographic fusion. At 24 months, 97% of the cylindrical threaded titanium cage group and 52% of the control group showed radiographic fusion. These fusion rate differences are statistically significant (P < 0.001). The Oswestry and neurologic scores were not significantly different between groups. Discussion. This is the first prospective, randomized, multicenter clinical trial that compares fusion cage results to control data. Conclusion. Cylindrical threaded titanium cages have a higher fusion rate, comparable improvements in clinical outcome (Oswestry, Low Back Pain Questionnaire, SF-36), and fewer secondary supplemental fixation procedures compared to the femoral ring allograft control.