Autogenous cancellous bone is commonly reported as the most successful material for bone grafting. For successful fusion to occur, graft stabilization with rapid and reliable bone repair is necessary. However, surgical procedures requiring iliac crest bone graft (ICBG) harvesting introduce a variety of potential risks to patients including increased surgical time, increased blood loss, and increased postoperative trauma pain.
There is only a finite amount of ICBG available for harvest from any one site, and optimization of the harvested graft is of paramount importance. Therefore, the challenge to surgeons is to achieve successful bone fusion while using a limited supply of autogenous bone graft, thereby minimizing risk to patients. The use of a bone graft extender may enhance the success of the bone fusion by expanding the available ICBG.
This radiographic report details the short-term results of the use of ICBG with a commercially prepared allograft demineralized bone matrix (DBM) in patients undergoing lumbar spina! fusion.
MATERIALS AND METHODS
A retrospective clinical study of consecutive patients who underwent a fusion procedure between June 2000, and September 2001 was performed. AlloMatrix Injectable Putty (Wright Medical Technology, Ine, Arlington, Tenn) with ICBG was used for instrumented lumbar spinal fusion. Each fusion involved a similar technique with subperiosteal exposure, decompression, and graft bed preparation that included decortication of the posterior elements (transverse processes and excision of the facet joints).
AlloMatrix Injectable Putty is a bone graft material with both osteoinductive and osteoconductive properties. This bone putty is a combination of DBM (bioassayed, 86% by volume) with surgical-grade calcium sulfate (OsteoSet, Wright Medical Technology, Ine). This combination of materials enables rapid and reliable bone repair through the process of endochondral ossification. AlloMatrix Injectable Putty was prepared in a mixture ratio of 2:1 with the harvested autogenous bone. After ICBG was harvested and morselized, the resulting slurry was combined with an equal volume of AlloMatrix Injectable Putty to form a thick paste. The graft was then packed into the prepared graft bed. The instrumentation was placed, and excess graft was placed around the pedicle screws.
Figure 1: Radiographs of a single-level fusion demonstrate an instrumented fusion with increasing consolidation of the fusion mass at 3 months (A, B), 6 months (C, D), and 1 year (E, F).
Candidates for spinal fusion included skeletally mature patients undergoing spinal fusion procedures for the treatment of degenerative conditions such as spinal stenosis or spondylolisthesis. Clinical assessments were performed at all follow-up intervals (1, 3, 6, and 12 months).
Early postoperative (1 -month), and 3-, 6-, and 12-month radiographs were assessed by an independent spine surgeon for fusion. Radiographs included standing spot lateral radiographs and Ferguson anteroposterior (AP) or standing AP views of the lumbosacral junction.
The radiographic fusion patterns were categorized into one of the following four grades as described by Lenke: (1) definitely solid - solid, big trabeculated bilateral fusion masses; (2) possibly solid - unilateral large fusion mass with contralateral small fusion mass; (3) probably not solid - small, thin fusion masses bilaterally; and (4) definitely not solid - graft resorption bilaterally or fusion mass with obvious bilateral pseudarthrosis.
Of the 65 patients included for study, 59 (91%) patients were undergoing surgical intervention for lumbar spinal stenosis, three (5%) patients had lumbar spondylolisthesis, two (3%) patients had stenosis, and one (1%) patient had bilateral spondylolisthesis. There were 43 (64%) women and 22 (36%) men in the study. The average patient age was 56 years (20-85 years, SD= ±16). There was no statistical difference in age between men and women.
The early (1 month) assessment of bone fusion by average Lenke scores for this population was 3.7 (2-4, SD= ±0.6) and improved to an average of 3.2 (1-4, SD=0.8) at 3 months. At 6 months, the average Lenke score improved to 2.2 (1-4, SD= ±1.0) and at 12 months, 1.6 (1-4, SD= ±0.9). Figures 1 and 2 show the progression of graft incorporation at each time period.
Each subsequent follow-up evaluation revealed an increase in improvement in average Lenke score and was statistically significant between the early (1 -month) and recent (12-month) follow-ups.
There were statistically significant changes in Lenke score between 1 -month and 3-month follow-up (P<.01), between 3-month and 6-month follow-up (P<.001), and between 6-month and 12-month followup (P<.01). Also, there was no statistically significant change in Lenke scores between patients who smoked and those who did not.
Successful bone fusion requires a source of osteoprogenitor cells, induction of progenitor cells into an osteoblastic lineage for bone formation, an osteoconductive surface, and stability for bony consolidation.1
Autogenous cancellous bone is commonly reported as the most successful material for bone grafting. Osteogenic, osteoconductive, and osteoinductive properties, in conjunction with large trabecular surface areas, lead to a probability of successful autogenous graft incorporation.
Surgical procedures requiring ICBG harvesting introduce a variety of potential risks to patients, including increased surgical time, increased blood loss, and increased postoperative pain.2,1 From the increasing number of patients undergoing surgical procedures that use bone graft and the potential risks and complications, alternative substances have been introduced and the use of bone graft extenders studied.410
The goal of instrumentation for lumbar fusion procedures is to achieve a stable environment for successful bone fusion. Although the type of instrumentation for fusion stabilization varies, the results of these procedures with autogenous bone graft alone remain the standard. Alexander et al" showed the use of autogenous graft in the form of local bone with OsteoSet calcium sulfate pellets in achieving significantly successful results in spinal fusion.
Kelly et al12 demonstrated excellent results using calcium sulfate alone and also in conjunction with additional materials (demineralized bone, bone marrow aspirate, autograft) in a variety of applications. There was no significant difference in the radiographic evidence of graft incorporation. Of the overall complications (12%, 15 109), only four (3.6%) may have been attributed to the graft composite.
Figure 2: Radiographs of a multilevel fusion demonstrate an instrumented fusion with increasing consolidation of the fusion at 3 months (A, B), 6 months (C, D), and 1 year (E, F).
Results of studies both using calcium sulfate alone and with autogenous graft as an extender are encouraging. Cunningham et al13 showed both radiographic and mechanical superiority of calcium sulfate with autogenous graft for spinal arthrodesis in a bovine model. Similar results were achieved by Turner et al14,15 in a canine model using calcium sulfate with autogenous graft.
In each study, calcium sulfate was a viable supplement to the autogenous graft. Alexander et al11 compared calcium sulfate plus local/laminectomy (50:50 ratio) bone directly to ICBG alone and found identical increases in bone formation at both spinal fusion sites. Our study incorporates autogenous bone with calcium sulfate and allogenic DBM.
The composite material used may also enhance the structural integrity of the graft during the healing and incorporation process. Tay et al'6 hypothesized that calcium sulfate graft extenders provide consistent resorption profiles and mechanical properties.
Graft incorporation in instrumented lumbar fusion can be monitored radiographically and reveals the balance between graft resorption and new bone formation. Following the Lenke criteria for radiographic assessment of graft incorporation, we found a predictable, reliable, and statistically significant progression of radiographic evidence of progressive graft incorporation through 12 months. These results are equivalent to reported results of ICBG use only.
The use of AlloMatrix Injectable Putty as a bone graft extender may enhance the success of spinal fusion by maximizing the available ICBG and avoiding potential risk factors and possible reoperation due to graft failure or graft site complications.
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