PLIF with Allograft Bone: Preliminary Results Comparing Cylindrical Dowels and Impacted Wedges

Methods: We retrospectively reviewed our series of patients that underwent posterior lumbar interbody fusions with allograft bone and supplemental pedicle screw fixation. Outcomes, fusion rates, and complications were evaluated between a group of patients that had cylindrical dowels compared to a similar group of patients that underwent placement of rectangular impacted wedges. Charts were reviewed and data were collected regarding outcomes, fusion rates, and complications between the two groups. Outcomes were assessed using a modified Prolo outcome scale. Osseous union was determined by the absence of lucency around the interbody implant, an increase in subchondral endplate sclerosis, and presence of bridging bone incorporating the graft as demonstrated on static lumbar radiographs and/or computed tomography. Stability was also determined by an absence of movement on dynamic radiographs.

Results: A total of 71 patients underwent PLIF with supplemental pedicle screw fixation with allograft bone (33 with cylindrical dowels, 38 with rectangular impacted wedges). There were no significant differences in the demographics between the two groups. In the 33 patients undergoing PLIF with cylindrical dowels the mean follow up was 12 months. The overall fusion rate was 95%, with an overall good or excellent outcome in 70% of the patients. Complications included three permanent nerve root injuries [9% of the group (two partial, one at L4, one at L5) and one complete (L5)], and two post operative wound infections. In the 38 patients that underwent PLIF with rectangular impacted wedges the mean follow up was 15 weeks. All patients in this group demonstrated early signs of osseous union, although follow up was too short to determine actual fusion rates. Seventy two percent of patients had excellent or good preliminary outcomes. Complications included two patients with transient lower extremity paresthesias that resolved by the time of early follow up. Four patients had wound infections and one patient developed a pneumonia two weeks after surgery. There were no permanent nerve root injuries in the impacted wedge group. All patients in both groups demonstrated stability on dynamic lumbar flexion extension radiographs. In comparison of the two groups, there was a significantly higher incidence of nerve root injuries in the group undergoing placement of cylindrical dowels. (p<0.05, ANOVA) There were no statistically significant differences in other complications or outcomes between the two groups.

Conclusions: Preliminary outcomes in both groups of patients demonstrated approximately 70% excellent or good outcomes with high fusion rates and 100% stability. There were no permanent nerve root injuries in the group of patients that underwent placement of rectangular impacted wedges compared to a 9% incidence of permanent nerve root injury in the group of patients undergoing placement of cylindrical dowels. The decreased incidence of nerve root injury in patients undergoing PLIF with impacted rectangular wedges is likely due to the diminished need for nerve root retraction when implanting rectangular impacted wedges compared to cylindrical dowels.