[Primary stability of 2 PLIF (posterior lumbar interbody fusion)--a biomechanical in vitro comparison]

The purpose of this biomechanical in-vitro-study was to compare two different PLIF-techniques with two types of implants on human lumbar spine: PLIF with threaded cages, (Bagby and Kuslich, Spinetech, Minneapolis, USA) and PLIF with the Moss-Miami-implants, (DePuy International Limited, Leeds, Great Britain).

Six cadaveric human lumbar spine segments L2-5 were explanted, frozen at -20 degrees C and thawed before preparation. They were cut in two parts by discectomie and arthrotomie L3/4, so six specimen L2/3 and six specimen L4/5 were obtained and used in a crossover-trial. Analysis included testing in a tension-torsion-machine under axial compression with 600 N, rotation (left-right) with 25 Nm and shearing forces with 250 N without preload. This was first done in the intact and then in the fused specimen.

Stiffness before treatment was comparable in both groups irrespective of location. Posttreatment stiffness was higher with MOSS-MIAMI-implants as compared to PLIF with BAK-cages. Average relative superiority (and 95%-confidence intervall) were 1.98 (1.01-3.69) for compression, 2.30 (0.85-6.24) for rotation and 1.73 (0.78-3.84) for shearing. Statistical comparison of log posttreatment stiffness was significant for compression but not for rotation and shearing (2-sided independent crossover t-test).

This biomechanical in-vitro-study demonstrates the higher initial stability of PLIF with titanium surgical mesh and posterior instrumentation when compared to PLIF with threaded cages alone.