The effect of cage stiffness on the rate of lumbar interbody fusion - An in vivo model using poly(L-lactic acid) and titanium cages

Study Design. A goat interbody fusion model using poly-(L-lactic acid) and titanium cages was designed to evaluate the effect of cage stiffness on lumbar interbody fusion. Objective. To investigate the effect of cage stiffness on the rate of interbody fusion. Summary of Background Data. Various types of cages considerably exceed the stiffness of vertebral bone, which ultimately may lead to postoperative complications. To avoid these complications, poly-(L-lactic acid) cages with limited stiffness have been designed. The mechanical integrity of the cages remains intact for at least 6 months. Methods. Interbody fusions were performed at L3-L4 of 15 Dutch milk goats, and one of three cages was randomly implanted: 1) a titanium cage (n = 3), 2) a stiff poly-(L-lactic acid) cage (n = 6), or 3) a flexible poly-(L-lactic acid) cage (n = 6). Interbody fusion was assessed radiographically by three independent observers 3 and 6 months after surgery. Results. At 3 months, all the poly-(L-lactic acid) specimens showed ingrowth of new bone, but with radiolucency in the fusion mass. At 6 months, solid arthrodesis was observed in four of six poly-(L-lactic acid) specimens, advanced ingrowth in one specimen, and infection in one specimen. Titanium cages showed ingrowth of bone, but with radiolucency in the fusion mass. Interbody fusion using poly-(L-lactic acid) cages showed a significantly higher rate statistically (P = 0.016) and more complete fusion than titanium cages of the same design. Conclusions. The reduced stiffness of poly-(L-lactic acid) cages showed enhanced interbody fusion, as compared with titanium cages after 6 months. Bioabsorbable poly-(L-lactic acid) cages thus may be a viable alternative to current interbody cage devices, thereby avoiding the concomitant problems related to their excessive stiffness. However, the bioabsorbability of the poly-(L-lactic acid) cages awaits investigation in a long-term study currently underway.