The effect of cage stiffness on the rate of lumbar interbody fusion: an in vivo model using poly(l-lactic Acid) and titanium cages.
METHODS
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.


BACKGROUND
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.
van Dijk
Martijn
M
Skeletal Tissue Engineering Group Amsterdam (STEGA), Department of Orthopaedic Surgery, Vrije Universiteit Medical Center, Amsterdam, The Netherlands.
Smit
Theo H
TH
Sugihara
S
S
Burger
Elisabeth H
EH
Wuisman
Paul I
PI
D020341:Absorbable Implants; D000818:Animals; D015194:Diffusion Chambers, Culture; D004548:Elasticity; D006041:Goats; D007700:Kinetics; D019344:Lactic Acid; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D010012:Osteogenesis; D011091:Polyesters; D011108:Polymers; D011859:Radiography; D015203:Reproducibility of Results; D013123:Spinal Fusion / Q000295:instrumentation*; D014025:Titanium
D011091:Polyesters; D011108:Polymers; D019344:Lactic Acid; C033616:poly(lactide); D014025:Titanium