In vitro biomechanical investigation of the stability and stress-shieldingeffect of lumbar interbody fusion devices

Object. Interbody fusion devices are rapidly gaining acceptance as a method of ensuring lumbar interbody arthrodesis. Although different types of devi ces have been developed, the comparative reconstruction stability remains c ontroversial. It also remains unclear how different stress-shielded environ ments are created within the devices. Using a calf spine model, this study was designed to compare the construct stiffness afforded by II differently designed lumbar interbody fusion devices and to quantify their stress-shiel ding effects by measuring pressure within the devices. Methods. Sixty-six lumbar specimens obtained from calves were subjected to anterior interbody reconstruction at L4-5 by using one of the following int erbody fusion devices: four different threaded fusion cages (BAK device, BA K Proximity, Ray TFC, and Danek TIBFD), five different nonthreaded fusion d evices (oval and circular Harms cages, Brantigan PLIF and ALIF cages, and I nFix device); two different types of allograft (femoral ring and bone dowel ) were used. Construct stiffness was evaluated in axial compression, torsio n, flexion, and lateral bending. Prior to testing, a silicon elastomer was injected into the cages and intracage pressures were measured using pressur e needle transducers. Conclusions. No statistical differences were observed in construct stiffnes s among the threaded cages and nonthreaded devices in most of the testing m odalities. Threaded fusion cages demonstrated significantly lower intracage pressures compared with nonthreaded cages and structural allografts. Compa red with nonthreaded cages and structural allografts, threaded fusion cages afforded equivalent reconstruction stiffness but provided more stress-shie lded environment within the devices.