Effect of Interbody Cage Geometry and Porosity in the Success of Interbody Spinal Fusion

Purpose: Using a non-human primate model, this study examined if variations in interbody cage hole geometry and porosity influence the success of spinal arthrodesis. Comparisons between three types of interbody cages were based on post-mortem computed tomography, plain film radiographs, biomechanical and histomorphometric analyses of each functional spinal unit (FSU).

Methods: Eight mature baboons (n=8, Papio cynocephalus) underwent an anterior transperitoneal approach with two midline destabilization sites of equal cylindrical dimension in the L3-L4 and L5-L6 disc spaces. Three different threaded cylindrical titanium cages (Types A, B and C) were randomized among the 16 arthodesis sites. The total porous surface area (39.3 mm2) in contact with the vertebral endplates was identical for each cage type, however, the circumferential hole patterns making up the porous areas differed: Type A Cage (n=6): Twelve small holes (1mm diameter with 1mm spacing); Type B Cage (n=4): Two large holes (5mm diameter with 3mm spacing); and Type C Cage (n=6): Eight holes (2.5mm diameter with 1mm spacing). Cages were packed with autogenous iliac crest. Following a 4-month survival period, fusion status was evaluated using flexion-extension radiographs, CT, quantitative intra-cage densitometry, biomechanical testing and histomorphometry.

Results: All baboons survived surgery without incidence of complication. Plain film radiographs and CT demonstrated fusions in 1/6 (17%) for Type A, 3/4 (75%) for Type B and 3/6 (50%) in Type C. Fifty percent (50%) of the endplates (6/12) from Type A cages indicated a radiolucent border greater than 20% on CT, whereas 25% of the endplates from both Type B and Type C cages (2/8 and 3/12, respectively) showed similar extent of radiolucency. There was no statistical differences in intra-cage CT densitometry (p=0.342). Biomechanical testing of the FSU’s demonstrated no statistical differences in range-ofmotion between the cage types for any testing modality (p>0.05), although Type B cages demonstrated the lowest range-ofmotion.

Discussion: Interbody cages with smaller fenestrations result in a reduced fusion rate, increased radiolucencies and inhibition of surface osseointegration. Conversely, larger holes appear to be less sensitive to interpositional fibrous tissue at the implant interface - enabling successful bony through-growth. Cage porosity and geometry can have a profound effect on the biological outcome of successful bone growth, although this may not be apparent in biomechanical stability. This is the first investigation to evaluate the biologic response of implant fenestration size on the success of interbody spinal arthrodesis.