Effects of rigidity of an internal fixation device: A comprehensive biomechanical investigation

Internal fixation with instrumentation often accompanies surgical fusion to augment spinal stability, provide tem-porary fixation while the surgical fusion mass unites, and enhance postoperative mobilization of a patient. Some surgeons, however, feel that the existing plate-screw desighs are too rigid and are the primary cause of “iatro-genic” adverse effects clinically observed. A three-part study, involving in vitro experimental protocol, analytical.finite-element-based models, and an in vivo canine inves-tigation, was undertaken to study the role of decreasing rigidity of a device on the biomechanical response of the stabilized segments. Two alternatives—the use of one variable screw placement (Steffee plate [unilateral, 1VSP model]) as opposed to two VSP plates (bilateral, 2VSP model) and two VSP plates with polymer washers placed in between the integral nut and plate (2MVSP model)—were considered for achieving a reduction in the rigidity of the conventional VSP system. The load-displacement data obtained from the in vitro experiments and the stress distributions within the stabilized and intact models predicted by the finite-element models revealed that the unilateral VSP system is less rigid and is likely to reduce stress shielding of the vertebral bodies compared with the 2VSP model. The undesirable effects associated with the use of the 1 VSP plate system are the presence of coupled motions due to the inherent asymmetry and the likely inability to provide enough rigidity for decompression procedures requiring a complete excision of the disc. The use of two MVSP plates overcomes these deficiencies. The presence of piolymer washers decreases the stress-shielding effects (by about 20%) compared with the 2VSP model immediately following surgery and by an additional 4% over a 30-day period due to the stress-relaxation phenomena associated with polymer washers. The in vivo canine investigation (Part III of the study), in which the quality of the bone within the stabilized segment in 2MVSP and 2VSP models was assessed at 6 months following surgery, revealed a similar trend. The use of pojymer washers in the 2MVSP canine group increased bone growth around screws, and the bone porosity was reduced compared with the 2VSP model. Thus, it appears that the new concept of using viscoelastic washers can be to a patient’s advantage to reduce the effects of a rigid device on the bone quality (eg, stress-induced osteopenia) until the surgical fusion mass unites.