Biomechanical stability of five stand-alone anterior lumbar interbody fusion constructs

Anterior lumbar interbody fusion (ALIF) cages are expected to reduce segmen tal mobility. Current ALIF cages have different designs, suggesting differe nces in initial stability. The objective of this study was to compare the e ffect of different stand-alone ALIF cage constructs and cage-related featur es on initial segmental stability. Human multisegmental specimens were test ed intact and with an instrumented L3/4 disc level. Five different ALIF cag es (I/F, BAK, TIS, SynCage, and ScrewCage) were tested non-destructively in axial rotation, flexion/extension and lateral bending. A cage 'pull-out' c oncluded testing, Changes in neutral zone (NZ) and range of motion (ROM) we re analyzed. Cage-related measurements normalized to vertebral dimensions w ere used to predict NZ and ROM. No cage construct managed to reduce NZ. The BAK and TIS cages had the largest NZ increase in flexion/extension and lat eral bending, respectively. Cages did reduce ROM in all loading directions. The TIS cage was the least effective in reducing the ROM in lateral bendin g. Cages with sharp teeth had higher 'pull-out' forces. Antero-posterior an d mediolateral cage dimensions, cage height and wedge angle were round to i nfluence initial stability. The performance of stand-alone ALIF cage constr ucts generally increased the NZ in any loading direction, suggesting potent ial directions of initial segmental instability that may lead to permanent deformity. Differences between cages in flexion/extension and lateral bendi ng NZ are attributed to the severity of geometrical cage-endplate sur face mismatch. Stand-alone cage constructs reduced ROM effectively, but the resi dual ROM present indicates the presence of micromotion at the cage-endplate interface.