Cages: designs and concepts

Many new interbody fusion cages have been recently developed, but clinical studies analyzing fusion outcome are still scarce. Radiological methods to assess fusion are not standardized and are often unreliable. Cages have bee n stated to provide good Segmental distraction, provide axial load support and reduce segmental mobility, but there have been reports of failed fusion s because of implant failure. This paper presents a critical opinion on cur rent cage designs, stressing their clinical and biomechanical implications. Threaded cage designs compromise endplate integrity, and when placed in pa irs have inherent limitations for distraction. Non-threaded cage designs us ually preserve endplate integrity, but geometry may be inadequate to provid e a good surface match to the endplate. The concept of an open frame type c age is believed to have biological advantages, because large graft volumes inside the cage can be in direct contact with host bone. Cadaveric tests su ggest that open frame constructs have compressive strength similar to that of full surface contact cages. Restoration of segmental height, sagittal ba lance and increased neuroforaminal clearance are all functions of disc spac e distraction. The effect of cage instrumentation on axial load distributio n, however, is not well understood. Biomechanical experiments strongly sugg est supplementing cage instrumentation with posterior fixation, to achieve a marked increase in initial segmental stability. In the absence of gross s egmental instability, micromotion at the host graft interface may still exi st. As a result, fusion will never occur, instead a pseudoarthrosis will de velop. For monitoring fusion, the use of non-metallic cages has distinct ad vantages, because no metal artifacts will disturb radiological assessment.