Dentists restoring implants always want successful outcomes. One area of constant concern is the potential for peri implantitis that occasionally leads to implant failure. To help avoid this, many dentists have migrated to screw retained crowns and bridges, eliminating possible complications from cement. However, screw retained crowns also present risks for future complications. The answer might be a blending of the two.
Implant related cement sepsis is a known cause for peri implantitis. However, Korsch found, in 2014, this to be more cement type related that previously thought. Another complication for cemented abutment crowns is abutment screw loosening. Screw retained implant crowns eliminate complications from cement related risks. However, a problematic lightly cemented implant crown can be removed and repaired or temporarily replaced with an easily fashioned temporary crown holding its position. A problematic screw retained crown is far more difficult and expensive to repair or replace, and its space more complex and time consuming to temporize. This has led some to rely on hybridized screw retained crowns that are cemented and cleaned extra orally with a prefabricated, lab-placed screw access hole.
It is important to understand materials’ strengths and weaknesses before deciding upon a new application, such as a hybrid screw retained implant restoration. In the past, implant crowns have been primarily made from porcelain fused to metal (PFMs). In recent years, there has been a move away from PFMs to cleaner and more esthetic all-ceramic crowns made from lithium disilcate or zirconia. Some dentists have shown a preference for lithium disilicate in esthetically critical cases. However, little is known about the long-term performance of this material as an implant crown with a screw access hole.
Research by Biskri in 2013, noted the brittleness, low elasticity, and unidirectional crystals of lithium disilicate. But the material has also been widely reported to be more fatigue resistant than feldspathic porcelain. Despite its benefits over traditional porcelain, research by Dhima in 2014 showed far more predictable strength when lithium disilicate is at least 1.5 mm thick, occlusally.
Lassle, in his 2015 master’s thesis, described testing the viability of hybridized lithium disilicate screw retained crowns affixed to Nobel conical, 5.5 stock abutments with a 1.5 mm collar. The crowns were digitally designed, mandibular first pre molars, with 2 mm of occlusal thickness and axial walls ranging from .5 mm gingivally, to 1.5 mm near the occlusal table. Occlusal access holes were created in #1 prior to glazing in the “blue” state, #2 after glazing, both using copious amounts of water for cooling. The control had no access hole. The crowns were silanated and cemented with RelyX™ Unicem (3M Espe), and allowed to set 24 hours prior to testing. A control group followed the same protocol, but without an access hole.
It is clear from the results that placement of a screw access hole in lithium disilicate leads to a significant decrease in load bearing strength. According to Lassle’s findings, lithium disilicate would be contraindicated for this purpose.
Despite our potential for bias in selecting screw retained, cemented, or hybridized screw retained, some researchers believe there is inconclusive evidence of clinical significance between them, as reported by Sherif in 2014. Cement retained implant crowns are less expensive, seat passively, and are easier to work with. Screw retained implant crowns eliminate possible cement related complications, and offer retrievability after screw loosening. A deciding factor for the third option should be the material to be used.
The research conducted by Lassle was revealing. However, we should keep in mind that people don’t chew with a constantly increasing pressure of .1 mm per minute against a 3 mm steal ball. Yes, lithium disilicate is definitely weekend by a central fossa hole, as evidenced by the early fractures along the central groove. But that doesn’t mean they will always fail, clinically. However, if we are looking for greatest certainty when using hybridized screw retained implant crowns, zirconia would be a surer bet, according to testing by Hussien et al, in 2016, showing zirconia to be over 3 times stronger than lithium disilicate.