For nearly thirty years the conventional screw retained denture or hybrid has been the standard of care for many dentists and patients. In many practices this is still a treatment of choice for converting denture patients to fixed implant supported restorations. In the mid 1980′s Dr George Zarb developed this full arch screw retained prosthesis that, for the times, was a great innovation that provided tens (if not hundreds) of thousands of patients fixed restorations to replace dentures. In the 1980′s this was state of the art for fixed implant supported restorations.
This hybrid restoration is fabricated with conventional denture teeth, processed to a cast or milled metal bar using denture base material. The prosthesis is subsequently a screw retainer to the implants or interim abutments facilitating a fixed prosthesis. This was an amazing upgrade for denture patients. Eliminating dentures and the pallet, reintroducing the concept of taste and having a fixed restoration can be life changing. However, the hybrid restoration has not been adequately predictable or trouble free. Many patients experience prosthetic failure, breakage of the acrylic and/or teeth coming loose from the restoration. Often this can happen in less than a year or a month or less, after receiving a new hybrid prosthesis.
Through material and functional analysis it becomes obvious why the hybrid restorations are so problematic. Denture teeth and denture base materials were developed for dentures. Dentures tend to be loose fitting. Patients applying excessive lateral loads to conventional dentures will simply dislodge the appliance. Through initial and continued function with dentures, patients adapt to denture functionality and learn quickly not to exceed functional limits. The functional load limits placed on a denture are well within the strength of denture materials, being 80-100 MPa flexural strength.
When utilizing the same denture materials to a screw retained prosthesis the element of dislodging the appliance is removed form the equation. Patients can easily place loads of over 400 MPa on fixed restorations, hence the problem of under engineering is evident. In addition, implant patients lose proprioception, resulting in the inability to gently guide or control functional movements, resulting in frequent high load unpredictable maxillary and mandibular tooth collision.
Through the advent of material science and CAD CAM we finally have a solution, Izir and Pretau bridges. These restorations are fabricated from full contour zirconia. Having a flexural strength of 1100 MPa the material far exceeds loads placed on screw retained restoration. The Izir restoration follows a protocol that is predictable and cost effective. Due to strength, it is built to withstand long term function and provide years of trouble free use. In addition, digital manufacturing of this restoration means we own the design file forever. In the event of an unforeseen problem, a remake is as simple as pulling up the original design file and milling a new prosthesis. Just a single appointment is needed to screw out the old and place an all new Izir. This is insuring for both the patient and restorative dentist.
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