The best source detailing the intended mechanism of the Synex implant is the manufacturer’s instruction guide to the performing surgeon (taken from Synex.com) and the following description is a condescend account of this material (does this pass as referencing?). Please note the specific guide referenced refers to the generic Synex system rather than the type II model specifically, though the core mechanism remains the same.
The intended mechanism of the Synex 2 is as such. After the surgeon has performed the corpectomy, they will measure the height of the gap created in the spine and use this information to select which specific size of implant is to be used. The implant chosen is then packed with bone graft to aid osseointegration (the central chamber of the device is hollow, referred to in the guide as the ‘cage’). It is then inserted into the spine, ideally centred on the vertebral endplate with a number of millimetres room between the device and the bone structure. The height of the device is adjustable on account of the central chamber being retractable and expandable. Once lodged the surgeon uses customised forceps to extend the central chamber of the implant, which is held in place by ratchets. This is essentially a simple mechanism, not far removed from that of a carjack, for instance. Once the correct calibration has been set, the device is locked into place by turning a key which would prevent movement of the ratchets, preventing retraction of the device upon assertion of pressure.
The implant is then packed with further bone graft before a supplementary support system is implanted to give additional support (please note that while the vertebral body device system (the Synex) is dependent upon this supplemental system for safe, correct function, the supplementary system is a stand-alone device).
As stated above, this procedure is taken from a generic Synex guide, and not specifically that of the Synex 2. But the unique feature of the Synex 2 was its improved kinematics, a feature not directly related to its specific mechanism of failure (this is discussed below/in section X.x)
Mechanism of Failure
Neither the FDA nor the Synthes statement goes far in explaining the mechanism of failure. In addition, although the Synex 2 is referred to in journal papers, its failure does not seem to be covered. This would be logical; the defect only became apparent in late 2009, and as such any paper encompassing this would most likely be yet to reach review. In addition, of the papers found with references to the Synex 2, all but one dated back to 2007 or before (comfortably pre-dating the recall) and the remaining paper was submitted 7 months after recall.
In light of this, the key resource in reviewing the exact mechanism of failure has been the Manufacturer and User Facility Device Experience (MAUDE). This service, provided by the FDA, effectively serves as a compendium of reports citing complaints against medical devices. Compiling these...