Does Bioresorbable Cage Material Influence Segment Stability in Spinal Interbody Fusion? Matthijs R. Krijnen, MD*  ; E. R. Valstar, PhD‡§; Theo H. Smit, PhD†  ; and Paul I. J. M. Wuisman, PhD*§  To reduce long term complications associated with nonre- sorbable interbody fusion cages, bioresorbable cages are be- ing developed. We investigated the influence of bioresorbable cage material on segment stability, intervertebral disc height and fusion in vivo using radiostereometric analysis compar- ing 70/30 poly(L-lactide-co-D,L-lactide) (PLDLLA) cages with titanium cages. Twenty-eight goats were randomized to receive PLDLLA (n = 21) or a titanium control (n = 7) cage at L3-L4. Range of motion for flexion and extension and change in intervertebral disc height were measured before and after surgery and at followup (3, 6, and 12 months). Fusion was graded with a validated radiographic score. Al- though the PLDLLA cage could not provide the optimal en- vironment for a successful high fusion rate, the range of motion of the PLDLLA segments gradually decreased in time and was similar to the titanium control group at 12 months. In addition the decrease of intervertebral disc height was similar for both PLDLLA (1.4 ± 0.8 mm) and titanium (1.3 ± 1.0 mm) specimens. Both results showed a bioresorb- able cage does not lead to less decrease of motion or more loss of intervertebral disc height in time compared to tita- nium. This study therefore supports further development of a bioresorbable cage concept. Lumbar spinal interbody fusion is performed to relieve symptoms by stabilizing the spine at one or more levels and sometimes realigning the spine and restoring interver- tebral disc (IVD) height. 34 To achieve these goals, many different cage designs and materials (typically nonresorb- able) have been developed. 23,34,46,49 At present, approxi- mately 151,000 lumbar interbody fusions are performed in the United States annually, 8 with good and excellent clini- cal success reported in studies with 2 to 4 years fol- lowup. 4,11,19,21,28 However, the outcome of long term in- teraction of the cage with the spinal segment is not known. Recent reports show an increasing number of failed cages. 24,35,37 The increase in complications arising after lumbar interbody fusion are ostensibly related to cage de- sign and material 34 and surgical procedure. 9 The disap- pointing results have provided the incentive to investigate bioresorbable materials 1,2,29 as suitable alternatives for in- terbody fusion cages. 12,15,41 Bioresorbable cages have several advantages as com- pared with nonresorbable (titanium, carbon, polyether- etherketone) cages. First, in contrast to metallic cages, bioresorbable cages are radiolucent, which facilitates ra- diographic analysis of the fusion zone. Second, bioresorb- able cages have mechanical properties similar to bone, theoretically facilitating bone graft remodeling, bone in- growth, and bone fusion. 42 Third, after bony fusion be- tween two vertebral bodies, the bioresorbable cage device degrades over time and resorbs completely, avoiding the complications inherent to permanent foreign bodies (eg, infection, migration, foreign body reaction). 24,35 However, a concern about a bioresorbable cage is the sometimes subtle balance between the need for stability during the fusion process and cage degradation rate: early mechanical failure of the cage could lead to instability and loss of IVD height, and possibly to pseudarthrosis. There- fore, appropriate cage material, design, and maintenance of IVD height and segment stability are vital to consider in the development of resorbable cages. We asked whether one bioresorbable cage design could maintain segment stability, IVD height, and fusion. From the *Department of Orthopaedic Surgery and the †Department of Physics and Medical Technology, VU University Medical Center, Amster- dam; the ‡Department of Orthopaedic Surgery, Leiden University Medical Center, Leiden; the §Department of Biomechanics, Faculty of Mechanical, Marine and Material Engineering, Delft University of Technology, Delft; the  Skeletal Tissue Engineering Group Amsterdam (STEGA). The STEGA has received funding from Medtronic Sofamor Danek. Each author certifies that the Animal Ethics Committee of the VU University Medical Center has approved the animal protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research. Correspondence to: M. R. Krijnen, MD, VU University Medical Center, Department of Orthopaedic Surgery, Postbox 7057, 1007 MB Amsterdam, The Netherlands. Phone: 31-20-4442355; Fax: 31-20-4442357; E-mail: MR.Krijnen@vumc.nl. DOI: 10.1097/01.blo.0000224015.42669.cc CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Number 448, pp. 33–38 © 2006 Lippincott Williams & Wilkins 33