Treatment of cervical pseudarthrosis after Smith-Robinson procedure with Halifax clamp fixation

Treatment of pseudarthrosis of the cervical spine has been debated extensively with various solutions being proposed. This article reviews 18 cases of pseudarthrosis after attempted anterior cervical discectomy and fusion with tricortical iliac crest autograft using the Smith-Robinson method. All cases were subsequently treated with posterior fusion using cancellous iliac autograft and fixation with Halifax clamps. All cases showed radiographic union at the 6-month follow-up. There were no complications related to the application of the device. One device showed loss of fixation in the follow-up period. The Halifax clamp is technically simple to apply and can be done safely. High success rates in obtaining fusion after failed anterior discectomy and fusion in the cervical spine and ease of application make this method of posterior fixation and fusion an attractive alternative for dealing with pseudarthrosis.     

Anterior lumbar interbody fusion

This is a report of 85 patients who underwent anterior lumbar interbody fusion (ALIF) for treatment of painful disc disruption (PDD) or symptomatic pseudarthrosis. The fusion rate was 80% by disc. The pseudarthrosis rate increased from 16% at L5-S1 to 21% and 31% at L4-5 and L3-4, respectively. There was a significant increase in pseudarthrosis rate in patients who smoked more than one pack per day. There was no difference in the fusion rate whether autogenous or cadaveric iliac crest graft or dowel versus tricortical block graft was used. Sixty-eight percent of patients were "able to work" after ALIF. The complication rate was low and retrograde ejaculation occurred in only one patient.     

Anterior cervical allograft arthrodesis and instrumentation: Multilevel interbody grafting or strut graft reconstruction

This retrospective study evaluated a single surgeon's series of patients treated by multilevel cervical disc excision (two or three levels), allograft tricortical iliac crest arthrodesis, and anterior instrumentation. The objective of this retrospective study was to compare fusion success and clinical outcome between multilevel Smith-Robinson interbody grafting and tricortical iliac strut graft reconstruction, both supplemented with anterior instrumentation in the cervical spine. The incidence of nonunion for cervical discectomy and fusion varies widely depending on the number of disc levels involved, type of bone graft used, and whether the anterior grafting is supplemented with instrumentation. An alternative to multilevel interbody fusion is corpectomy and strut grafting, in which the incidence of nonunion has been reported to be 27% with autograft and 41% with allograft. Sixty-four consecutive patients who underwent allograft tricortical iliac crest reconstruction and anterior cervical plating were studied. The average follow-up was 39 months. There were 38 patients in the discectomy and interbody grafting group and 26 patients in the corpectomy and strut graft reconstruction group. Pseudoarthrosis occurred in 42% of the anterior cervical interbody fusion patients and 31% of the corpectomy patients. Nonunion in two-level interbody fusions occurred in 36% of the patients as compared to 10% for patients with one-level corpectomies; while 54% of patients with three-level interbody fusions and 44% of patients with two-level corpectomies were noted to have pseudoarthrosis. Higher percentages of nonunion were noted in multilevel interbody grafting than in corpectomy with strut grafting and when more vertebral levels were involved. These radiographic and clinical findings underscore the shortcomings of multilevel anterior cervical allograft reconstruction with plating. Corpectomy may be the preferred method when multiple disc levels are fused. In addition, anterior corpectomy affords decompression of significant osteophytes in a safer and quicker manner. In retrospective studies, there is a need for long-term follow-up before accurate statements can be made about the study population.     

Autograft versus allograft in degenerative cervical disease.

Cervical spondylosis, common among people older than 50 years, may produce symptoms of cervical radiculopathy, myelopathy, or both. When patients with degenerative cervical disease require surgery, an anterior decompression of the neural elements and reconstruction of the anterior column with allograft or autograft is common. Allograft incorporates more slowly and less completely than autogenous bone graft, although harvest of autograft from the anterior iliac crest or the middle 1/3 of the fibula may be associated with postoperative morbidity. Overall, higher rates of fusion have been reported with autograft than allograft. However, similar fusion rates have been reported among nonsmokers having single-level anterior cervical interbody fusions. For patients requiring multilevel reconstruction surgery, fusion rates of approximately 100% have been achieved using autogenous strut grafts from the iliac crest and fibula, with a lower rate of incorporation for fibula allograft. The potential for a patient with a nonunion requiring additional operative treatment must be weighed against the potential for graft-related complications when choosing between autograft and allograft for anterior cervical reconstruction.     

Threaded cages for degenerative cervical disease.

Multiple techniques are used to accomplish anterior interbody fusion in the cervical spine. These various options reflect the lack of any one approach to obtain superior results. Extrapolating from the lumbar spine experience, a cervical cage was proposed as an alternative to the status quo. A device evaluation study was done to determine the safety and efficacy of the cage with a comparison with current allograft or autograft fusion techniques. The study resulted in two publications. One study is a multicenter trial whereas the other is a one-author study with long-term results. A review of the data from both studies follows. Comparable outcomes were seen with the allograft and autograft control group and the experimental group in the multicenter trial. However, that study reported significantly better fusion rates for the patients with cervical cages compared with the control group. Both studies reported a high percentage of donor site morbidity in the patients in the control group who had autograft fusion. A trend toward better outcomes for patients who received a cervical cage was seen in the long-term followup study. The data presented suggest interbody fusion cages are a safe and effective treatment of degenerative cervical disc disorders and avoid iliac crest donor site morbidity.     

Accelerating lumbar fusions by combining rhBMP-2 with allograft bone: a prospective analysis of interbody fusion rates and clinical outcomes.

BACKGROUND CONTEXT: Recombinant human bone morphogenetic protein-2 (rhBMP-2) is an osteoinductive protein approved for use in the anterior lumbar interspace. High fusion rates with rhBMP-2 have been reported with threaded interbody allograft dowels. There may be a clinical benefit for the patient by adding rhBMP-2 to the allograft. PURPOSE: To compare the fusion rates and clinical outcomes of patients treated with allograft interbody fusions with and without the addition of rhBMP-2. STUDY DESIGN: Prospective consecutive patient enrollment with minimum 24-month follow-up. PATIENT SAMPLE: Seventy-five patients with lumbar interbody fusions at 1-3 spinal segments. OUTCOMES MEASURES: Clinical: Numerical Rating Scale (NRS) and Oswestry Disability Index (ODI). Radiographic: X-ray and computed tomographic scan analysis using the Molinari-Bridwell fusion scale. METHODS: Seventy-five patients scheduled for lumbar fusion were enrolled sequentially. Group 1: 30 patients had anterior interbody allografts alone. Group 2: 45 patients had anterior interbody allograft filled with rhBMP-2. All cases had posterior pedicle screw instrumentation. A total of 165 surgical levels (62 allograft alone/103 allograft+BMP) were included. Fusion data and clinical outcomes were collected for a minimum of 2 years after surgery. RESULTS: Statistically higher fusion rates were observed in the patients with BMP at all time points compared with allograft alone. Group 2 (+ BMP) fusion rates were 94%, 100%, and 100% at 6, 12, and 24 months after surgery. Group 1 (-BMP) fusion rates were 66%, 84%, and 89% at the same time intervals. Clinical outcomes were significantly improved in Group 2 compared with Group 1 at 6 months. There were no revisions (0%) in the BMP group and 4 revision fusion surgeries (13%) in the allograft group. No untoward effects were attributable to the rhBMP-2. CONCLUSIONS: Our study confirms the efficacy of an innovative lumbar fusion technique: an interbody femoral ring allograft, combined with an osteoinductive stimulant (rhBMP-2), protected by pedicle screws. This combination of a structural interbody allograft with rhBMP-2 eliminates the insult of iliac crest harvest, allows for reliable radiographic analysis, and results in successful fusion formation in 100% of the cases in this study.     

Posterior cervical fusion failure in three morbidly obese patients following circumferential surgery

BACKGROUND: The stability of multilevel anterior corpectomy with fusion (ACF) is often enhanced by simultaneous posterior fusion (PF) which provides a "posterior tension band." Three morbidly obese patients undergoing circumferential surgery had posterior fusions performed without autogenous iliac crest graft to avoid donor site morbidity. METHODS: Three morbidly obese patients (300-350 lbs.), averaging 48 years of age, presented with rapidly progressive moderate/severe myelopathies. Magnetic resonance imaging (MRI) and computed tomography (CT) studies demonstrated severe ventral ossification of the posterior longitudinal ligament. Two to four level plated ACFs were performed utilizing fibula strut allograft and plates. Posterior spinous process wiring/fusion from C2-T1 were completed with braided titanium cables, fibula strut allografts, Inductive Conductive Matrix (a form of demineralized bone matrix), and allograft bone to avoid iliac crest donor site morbidity in such morbidly obese patients. Halo devices were utilized until fusion was documented on postoperative X-ray and 2D-CT studies subsequently obtained 3, 6, and up to 12 months postoperatively. Patients were followed an average of 3 years. RESULTS: Postoperatively, all 3 patients demonstrated mild residual myelopathy (Nurick Grade 0-I). Nevertheless, all 3 exhibited posterior pseudarthroses accompanied by anterior strut/plate extrusion (1 patient), partial anterior graft pseudarthrosis (1 patient), and a delayed strut fracture (1 patient). The first 2 patients required secondary posterior fusions performed with autogenous iliac crest graft, while the third fused with 6 months of additional bracing. CONCLUSIONS: Following circumferential cervical procedures, posterior fusions failed in 3 morbidly obese patients where iliac crest autograft was omitted in an attempt to avoid donor site morbidity.     

Donor site pain from the ilium. A complication of lumbar spine fusion

Chronic pain at the donor site was reported by 25% of 290 patients who had undergone anterior lumbar spine fusion for low back pain. Donor site pain has characteristic clinical features, may be severely disabling and is stubbornly resistant to treatment. The highest prevalence was in patients who had a tricortical full thickness graft taken through a separate incision overlying the iliac crest. Patients with a clinically unsatisfactory result from the spine fusion also had a significantly higher prevalence of donor site pain.     

A modified technique for anterior multilevel cervical fusion

 Anterior cervical fusion with interbody bone graft and anterior plating is commonly performed. Unfortunately, the plate has been reported to shield the graft from loading, thus reducing fusion rates. Interbody fusion cages have been effective in the lumbar spine and have gained acceptance in the cervical spine. Twenty-five patients underwent anterior cervical fusion with this modified technique. All patients received anterior diskectomy and corpectomy, placement of an interbody fusion cage packed with corpectomy bone, and application of an anterior cervical plate. Fusion was defined by radiographic evidence of trabecular bone bridging through the cage. No external bracing was used except soft collars as needed. Pre- and postoperative pain scales were completed and statistically analyzed using paired t tests. There were no cases of pseudoarthrosis or major neurological, vascular, or wound complications. There was one case of mild dysphagia that remained unresolved. Mean operative time was comparable to standard instrumented multilevel cervical fusion surgeries. Visual analogue pain scales were significantly improved following surgery. The advantages of using interbody cages with anterior plating include immediate stability and support, elimination of donor site pain from iliac crest bone autograft, and a decrease in pseudoarthrosis by halving the number of fusion surfaces. Received: November 15, 2000 / Accepted: December 18, 2001     

Anterior column support in surgery for high-grade, isthmic spondylolisthesis.

The literature is confusing as to the need for anterior column fusion in the surgical treatment of patients with high-grade dysplastic spondylolisthesis. The current authors present an analysis of consecutive pediatric patients treated surgically for high-grade spondylolisthesis with and without anterior column structural support with emphasis on fusion rates, segmental kyphosis correction, and functional outcomes. Thirty-seven surgical procedures were done in 31 patients for Meyerding Grade 3 or Grade 4 isthmic dysplastic spondylolisthesis. Patients were separated into two groups based on whether they had structural anterior column support (tricortical autogenous iliac crest) in addition to posterior fusion surgery. Group 1 consisted of 18 patients treated only with posterior surgery without anterior structural support (11 patients were treated with L4-sacrum posterior in situ fusion and seven patients were treated with posterior instrumented reduction with decompression and posterior fusion), and Group 2 consisted of 19 patients who had a reduction and circumferential fusion including anterior structural support. All patients had new radiographs taken at the time of followup (average, 3.1 years, range, 2 years-10 years 1 month) and completed a functional outcome questionnaire. The incidence of pseudarthrosis was 39% (seven of 18 patients) in Group 1 and 0% (0 of 19) in Group 2. All seven patients who had pseudarthrosis achieved solid fusion with a second procedure involving circumferential fusion with anterior column structural grafting. Outcomes regarding pain after treatment, function, and satisfaction were high in those patients who achieved solid fusion regardless of surgical procedure.     

Outcome of the Graf ligamentoplasty procedure compared with anterior lumbar interbody fusion with the Hartshill horseshoe cage

The objective of our study was to assess the efficacy of Graf ligamentoplasty in comparison with rigid fixation and fusion with the Hartshill horseshoe cage for similar severity of disc degeneration. Although studies have been done on the Graf ligamentoplasty procedure and the Hartshill horseshoe cage, their efficacy has never been compared in any study. This study was done to decide whether retaining mobility and stabilizing the spine is best or stiffening the lumbar segment by fusion is preferable. Between 1995 and 1997, a prospective randomized study was performed comparing Graf ligament stabilization and anterior lumbar interbody fusion. Twenty-eight patients had single-level Graf ligaments inserted and 27 patients had single-level anterior lumbar interbody fusion (ALIF) with a Hartshill horseshoe cage and tricortical iliac crest autograft. The two groups were similar in age, sex, symptoms, severity of the disc degeneration, and duration of follow-up. The chi-square test and t-test were used to evaluate the outcome. At a minimum follow-up of 2.1 years, we found that 93% of patients who had undergone Graf ligamentoplasty had a satisfactory outcome (rated "excellent" or "better") compared to 77.8% of patients who had been treated with ALIF with Hartshill horseshoe cage stabilization and fusion, when measured on the Oswestry Disability Index (P<0.05). Retaining mobility in the lumbar segments gives better results after stabilisation with Graf ligaments than rigid fixation and fusion with the Hartshill horseshoe cage in the short term. We will be watching this cohort of patients over the next few years.     

The use of rhBMP-2 in interbody fusion cages. Definitive evidence of osteoinduction in humans: a preliminary report

STUDY DESIGN: A prospective randomized controlled human clinical pilot trial. OBJECTIVES: To determine the feasibility of using rhBMP-2/collagen as a substitute for autogenous bone graft inside interbody fusion cages to achieve arthrodesis in humans. SUMMARY OF BACKGROUND DATA: Preclinical studies have shown rhBMP-2 to be an effective substitute for autogenous bone graft, but there are no studies to date documenting such efficacy for human spine fusion. METHODS: Fourteen patients with single-level lumbar degenerative disc disease refractory to nonoperative management were randomized to receive lumbar interbody arthrodesis with a tapered cylindrical threaded fusion cage filled with rhBMP-2/collagen sponge or autogenous iliac crest bone. Patients were evaluated with radiographs, sagittally reformatted computed tomography scans, and Short Form-36 and Oswestry outcome questionnaires. RESULTS: All 11 patients who received rhBMP-2 were judged by three independent radiologists to have solid fusions (at 6, 12, and 24 months postimplantation), whereas only 2 of the 3 control patients, who received the standard treatment of autogenous iliac crest bone, were deemed to be fused. The Oswestry Disability Questionnaire scores of the rhBMP-2 group improved sooner (after 3 months) than those of the autograft group, with both groups demonstrating similar improvement at 6 months. Short Form 36 scores continued to improve up to 24 months. CONCLUSION: The arthrodesis was found to occur more reliably in patients treated with rhBMP-2-filled fusion cages than in controls treated with autogenous bone graft, although the sample size was limited. There were no adverse events related to the rhBMP-2 treatment. This study is one of the first to show consistent and unequivocal osteoinduction by a recombinant growth factor in-humans.     

Biodegradierbarer Cage

Bioabsorbable implants are commonplace in knee and shoulder surgery. Bioabsorbable poly(l-lactide-co-D,L-lactide) (PLDLLA) cage devices have potential benefits over autologous tricortical iliac crest bone graft and metallic cages for cervical spine interbody fusion. The purpose of this study was to compare interbody fusion of an autologous tricortical iliac crest bone graft with that of a bioabsorbable cage using a sheep cervical spine interbody fusion model. This study was designed to determine differences in (1) the ability to preserve postoperative distraction, (2) biomechanical stability, and (3) histological characteristics of intervertebral bone matrix formation. Sixteen full-grown Merino sheep underwent C3/4 discectomy and fusion. After 12 weeks, there was no significant difference between the results with the bioabsorbable PLDLLA cages and tricortical bone grafts. The cage also did not show advanced interbody fusion but did, however, show large osteolysis, which allows skepticism regarding the value of this bioabsorbable implant.     

The use of 'hybrid' allografts in the treatment of fractures of the thoracolumbar spine: first experience

Harvesting autogenous bone grafts of the iliac crest carries complications and lengthens operative times. Allografts are preferred to avoid these problems. Fusion after using allogenic bone grafts has been well studied, by examining trabeculations and remodelling on anteroposterior and lateral radiographs. However, the question remains whether one can rely on radiographs alone to determine fusion. 'Hybrid' fresh-frozen allografts from the femur or tibia were used in 11 adult patients with a mean age of 56.4 years (range: 30-78 years) to stabilize the thoracolumbar spine after anterior decompression for trauma. In one case two adjacent levels were fractured, in another case two fractures occurred at different levels. Fresh-frozen allografts of the femur (in ten cases) and tibia (one case), filled with autogenous cancellous bone graft or pieces of rib, were used to reconstruct the anterior column of the spine. Stabilization was performed by means of a Kaneda device. Anteroposterior and lateral radiographs and, additionally, computed tomography (CT) examinations with reconstructions were used to study fusion. One patient died 1 month after surgery. At follow-up in ten patients, after a mean time of 30.2 months (range: 18-42 months), ten allografts showed a grade I fusion and one a grade III fusion. Additional data from the CT examination with reconstructions, however, showed cross-trabeculations in all cases, and a partially united allograft in the patient with a grade III fusion. Cross-trabeculations between the allograft and vertebral body was observed at 6 months, with remodelling occurring at approximately 2 years. Mean loss of correction was minimal, at 3.6 degrees (range 0 degree-16 degrees). Fresh-frozen femoral or tibial allografts worked effectively to maintain correction after trauma when combined with anterior instrumentation. CT examinations with sagittal and coronal reconstructions were more effective for evaluation of fusion compared with anteroposterior and lateral radiographs. The high fusion rate and the low morbidity achieved using allografts in this way supports the exclusive use of allografts in the anterior thoracic and lumbar spine in the future.     

两种前路减压融合术治疗颈椎间盘突出症的比较

目的对前路颈椎间盘切除减压的碳纤维椎间融合器(cage)植入术与自体髂骨块植骨加钛板固定术治疗单节段颈椎间盘突出症(CDH)进行临床疗效及影像学评估。方法需要手术治疗的单节段CDH患者60例,其中前路减压cage植入术31例,自体髂骨块植骨加钛板固定术29例。术前、术后及随访期间摄X线片,比较两组病例的手术时间、术中出血量、术后并发症、症状改善率、融合时间、融合率、融合节段椎体间高度的维持情况。结果cage植入术较自体髂骨块植骨加钛板固定术手术时间短,出血少(P<0.01)。两组融合节段椎体间高度较术前均明显改善(P<0.01)。两组间症状改善率、融合率无统计学意义(P>0.05)。cage植入术较自体髂骨块植骨加钛板固定术并发症少。结论cage植入术和自体髂骨块植骨加钛板固定术都是前路减压治疗单节段CDH的重要而有效的方法。而cage植入术在减小手术创伤的同时,可以获得同样的临床效果,是治疗单节段CDH理想的术式之一。     

A prospective clinical comparison of rectangular titanium cages and iliac crest autografts in anterior cervical discectomy and fusion

The complications of autogenous bone grafting cause spinal surgeons to seek alternative methods for cervical spinal fusion. This prospective study was conducted to evaluate the safety and efficacy of rectangular titanium cages as compared to the widely performed iliac crest autograft fusion. Thirty-six patients with cervical disc disease in whom an anterior cervical approach was indicated for discectomy were included in a prospective controlled study protocol with 1-year follow-up. The first 18 consecutive patients received iliac crest autograft, while the next 18 received rectangular titanium cages. According to Odom's criteria, 15 of 18 (83%) patients in both groups experienced good to excellent functional recovery. According to the patient satisfaction index, 17 of 18 (94%) in both groups were satisfied. There were no significant differences in neck or arm pain. Fusion was present after 1 year in 16 of 18 (89%) patients in the iliac crest autograft group and 15 of 18 (83%) in the rectangular titanium cage group. In the autograft group, one case of pseudarthrosis was present, and marked hip pain was observed in four patients. There were no implant-related complications in the cage group. The authors conclude that titanium cages in anterior cervical discectomy constitute a safe and efficient alternative to iliac crest bone autograft.     

Anterior cervical reconstruction using titanium cages with anterior plating.

STUDY DESIGN: A preliminary outcome assessment study of titanium cage implants with anterior cervical plating in anterior cervical reconstruction. OBJECTIVES: To evaluate the efficacy and safety of using titanium cage implants and anterior plating in cervical reconstruction. SUMMARY OF BACKGROUND DATA: Anterior decompression and interbody fusion is a widely accepted surgical treatment for patients with cervical spondylosis. Tricortical iliac crest autograft has been the gold standard but is associated with morbidity at the bone graft donor site, whereas allograft fibula is associated with pseudarthrosis. Problems such as pseudarthrosis, graft collapse, and extrusion still persist with the accepted method of harvesting and implanting bone autografts. METHODS: Thirty-four patients were treated by channel corpectomy followed by placement of a titanium cage packed with autogenous bone graft from the vertebral bodies to reconstruct the anterior column. An anterior cervical plate was added in 30 of 34 cases that involved decompression of two or more levels. The follow-up period ranged from 24 to 56 months, with an average follow-up period of 32 months, and included examination and radiography. RESULTS: Six months after surgery, there was radiographic evidence of fusion in 97% of the patients. Eighty-eight percent of the patients (30 of 34) did not experience any complications (neither cage dislodgment nor hardware failure). Four patients had complications that included pseudarthrosis (1), extruded cage (1), cage in kyphosis (1), and radiculopathy (1). CONCLUSIONS: Titanium cages provide immediate strong anterior column support with minimum hardware complications and avoid bone graft-site morbidity. Titanium cages, with concomitant use of anterior plating, offer an effective and safe alternative to bone autografts.     

Iliac crest reconstruction with a bioactive ceramic spacer

This study aimed to investigate the long-term clinical results of the apatite wollastonite-containing glass ceramic (AWGC) iliac spacer and to discuss its efficacy in reconstruction of the bone graft donor site at the iliac crest. Thirty-one patients were studied for more than 10 years. All patients underwent anterior spinal fusion using autogenous tricortical iliac bone graft. After harvest of tricortical iliac bone graft, an AWGC iliac spacer ranging from 15 mm to 70 mm in length was press-fitted into the gap. Long-term clinical results were obtained from radiological and blood examinations. Thirty patients (97%) were satisfied with the spacer. There was new bone formation around the spacer on the radiograph. There was no abnormal silicon concentration in blood examinations. AWGC iliac spacer appears to be useful in the reconstruction of harvested iliac crest. New bone formation occurs, reducing the defect size.     

The use of recombinant human bone morphogenetic protein 2 (rhBMP-2) to promote spinal fusion in a nonhuman primate anterior interbody fusion model

STUDY DESIGN: A study on the efficacy of recombinant human bone morphogenetic protein 2 (rhBMP-2) in a nonhuman primate anterior interbody fusion model. OBJECTIVES: To investigate the efficacy of rhBMP-2 with an absorbable collagen sponge carrier to promote spinal fusion in a nonhuman primate anterior interbody fusion model. SUMMARY OF BACKGROUND DATA: RhBMP-2 is an osteoinductive growth factor capable of inducing new bone formation in vivo. Although dosage studies using rhBMP-2 have been performed on species of lower phylogenetic level, they cannot be extrapolated to the primate. Dosage studies on nonhuman primates are essential before proceeding with human primate application. METHODS: Six female adult Macaca mulatta (rhesus macaque) monkeys underwent an anterior L7-S1 interbody lumbar fusion. All six sites were assigned randomly to one of two fusion methods: 1) autogenous bone graft within a single freeze-dried smooth cortical dowel allograft cylinder (control) or 2) rhBMP-2-soaked absorbable collagen sponges within a single freeze-dried smooth cortical dowel allograft cylinder also soaked in rhBMP-2. The animals underwent a baseline computed tomography scan followed by 3- and 6-month postoperation scans. Anteroposterior and lateral radiographs of the lumbosacral spine were performed monthly. After the monkeys were killed, the lumbar spine fusion sites were evaluated. Histologic evaluation of all fusion sites was performed. RESULTS: The three monkeys receiving rhBMP-2-soaked collagen sponges with a freeze-dried allograft demonstrated radiographic signs of fusion as early as 8 weeks. The control animals were slower to reveal new bone formation. The computed tomography scans revealed extensive fusion of the L7-S1 lumbar vertebrae in the group with rhBMP-2. A pseudarthrosis was present in two of the control animals. CONCLUSIONS: This study was able to document the efficacy of rhBMP-2 with an absorbable collagen sponge carrier and a cortical dowel allograft to promote anterior interbody fusion in a nonhuman primate model at a dose of 0.4 mg per implant site (1.5 mg/mL concentration). The late of new bone formation and fusion with the use of rhBMP-2 and cortical dowel allograft appears to be far superior to that of autogenous cancellous iliac crest graft with cortical dowel allograft.     

The biomechanical and histomorphometric properties of anterior lumbar fusions: a canine model

An in vivo model was developed to compare the biomechanical stability, incidence of radiographic union, bone formation rate, and bone graft remodeling parameters of anterior interbody fusions. Eighteen 1-year-old beagles underwent anterior and posterior spinal destabilization procedures at L5-L6 to produce a reproducible amount of spinal instability--resection of the anterior longitudinal ligament, L5-L6 intervertebral disk, L5 and L6 lamina, spinous processes, zygoopophyseal joints, and ligamentum flavum. Group I (N = 6) were surgically destabilized controls; Group II (N = 6) underwent anterior L5-L6 interbody fusion with iliac crest bone graft; and Group III (N = 6) underwent anterior stabilization with a longitudinal fibular strut graft in addition to the same operative procedure as Group II. Six months postoperatively the group with the highest incidence of successful radiographic L5-L6 arthrodesis was Group III, anterior interbody fusion and fibular stabilization (p less than .10). The rank order of biomechanical stability was the same for the three groups for both torsional and axial compressive stiffness, with Group I (destabilized controls) being the least rigid, then Group II (anterior fusion with iliac crest grafting only), and the most rigid to both torsion and axial compressive loading was Group III (anterior fusion with fibular stabilization and iliac crest bone graft). The bone formation rate [mm3/(mm3 x year) x 10(3)], which was derived from the distance between fluorochrome markers, revealed that the more stable the individual spinal construct, the lower the bone formation rate. In summary, the beagle provided a successful model for studying in vivo the response of anterior bone grafts over a 6-month interval and provided comparative biomechanical and histomorphometric data on spinal interbody fusion techniques.