COMPARISON OF MECHANICAL CHARACTERISTICS OF DEPLOYMENT FOR EXPANDABLE VS. FIXED INTERBODY CAGES: IMPLICATIONS FOR PREMATURE CLINICAL FAILURES

摘要

Both expandable and non-expandable interbody cage designs are available to surgeons for cases involving spinal reconstruction and stabilization following single level lumbar corpectomy [1]. Different expandable cage designs are on the market [2], but all involve a manual mechanism to intraoperatively adjust the compressive load applied by the cage against the vertebral body endplates. Non-expandable cages do not have such a mechanism, and surgeons rely on cutting the cage to the appropriate length to achieve a "press-fit." The clinical reasoning behind the expandable cage design is that the adjustability of the device will allow the surgeon to achieve the maximum contact area between the cage and the vertebral endplate without "overstuffing" the interbody space and thus causing a sagittal-plane deformity. Anecdotal evidence from previous clinical [3] and biomechanical [4] studies exists to suggest that the expandable cage provides for a more consistent and stable interface between the cage and the endplate; however, there are also several case reports of catastrophic adjacent-level vertebral body fractures, presumably due to aggressive expansion of these cages in patients with low bone quality [5]. This same study noted poor tactile feedback for expandable cage designs in sensing the amount of compressive load applied to the endplates and the authors cited this lack of device sensitivity as a potential cause of over-distraction of the cage. Little biomechanical evidence exists to explain these clinical findings.