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Category Archives: Dental Implants

cemented-vs-screw-retained-crowns

Limitations to Hybridized Implant Restorations

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.

 

Results

implant loosen stats

 

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.

Assessing Implant Torque Device Reliability

Does Steam Sterilization affect Accuracy of Spring- style Implant Torque Devices?

When accounting for complications over a 5 year span, Mashid et al reported that “screw loosening has been stated as the most common complication in implant” dentistry. One leading cause could be torque driver reliability.

Torque drivers include “toggle-type or friction-style,” and “beam-type or spring-style.” Many believe spring-style offer more accuracy even though accuracy becomes suspect after steam sterilization. To test the effects of steam sterilization, Mashid’s team tested 5 each from Nobel Biocare, Straumann [ITI, photo at top], and Biomet 3i [3i]). The “peak torque” of each was measured before and after steam sterilization.

Materials and Methods

All devices were tested ten times at 35 Ncm prior to steam sterilization. Sterilization of each was managed according to manufacturer directions: “Nobel Biocare devices should be dismantled for disinfection, cleaning and drying and then the parts should be assembled before sterilization. In the ITI group, dismantling of devices is proposed. Each component should be disinfected, cleaned, and dried, but sterilized separately. For 3i samples, dismantling of the device is not proposed and disinfection of the outer surface is the only protocol to be considered.” The recommended protocols and sterilizations were repeated 100 times at 134°C for 18 min, and then measured 10 times for torque accuracy.

Results

Before steam sterilization, all the tested devices stayed within 10% of their targeted torque values. After 100 sterilization cycles, there were no significant difference in the Nobel Biocare and ITI devices. There was, however, an increase of error values in the 3i group, which showed more than a 10% difference with a maximum difference of 14% in 17% of torque measurements. The authors also reported that 3i torque devices had developed “corrosion of the spring in the handle” that may have contributed to its inaccuracy.

Different research by Santos et al, tested Biomet 3i, Nobel Biocare, Straumann, and Conexao at 20 Ncm and reported 62.5% were within 10% of the tested value. However, when tested at a 32 Ncm target, “only 33.3% of all values from each manufacturer were considered accurate,” with ITI being the most consistent in accuracy for both values. During the Santos testing, each torque device had been in clinical use for less than 2 years. With the amount of use and “sterilization protocols” unknown, their findings might better reflect the real world of clinical dentistry than would a highly controlled, laboratory testing protocol. The reports reinforce the need for manufacturer recalibration with frequency matched to use.

implant screws

Preventing Implant Screw Loosening

Screw Loosening, a Frequent Problem?

Implant screw loosening is a major concern for both dentists and dental technicians. Unfortunately, the literature indicates it is a frequent problem, with loosening rates as high as 12.5%. Today, there are new ways to decrease its frequency.

Forces keeping screws tight include the “friction between the threads, between the head of the screw and the abutment, and between the implant and the abutment. The force that clamps two screw-tightened components together is called the preload and it depends on the composition of the materials, the texture of their surface and their degree of lubrication.”

Some screws have special surface treatments that “reduce the friction coefficient” and increase the preload to keep screws tighter, longer. One example is TorqTite® (Nobel Biocare Holding AG, Balsberg, Kloten, Switzerland), which uses a “diamond-like, carbon” lubricant.

Using the two types of screws described above, Saliba et al, tested the amount of torque required for screw removal that would simulate how and why an entire implant system would loosen, clinically.

Materials and Methods

Testing was performed on 20 Neodent titanium implants (Osteointegráveis, Curitiba, Paraná, Brazil), with 4.1 platforms, similar to the original Branemark design. Their hexagon bases were removed so abutments could be “rotated on the implant platform during the loosening of the fixation screw.”

Ten relied on an abutment held to the implant with non treated M2.0 titanium screws (Neodent). A different set of ten screws were covered with TorqTite® (Nobel Biocare). “The abutment was attached with a screw, first using a hexagonal 1.2 mm digital wrench (Neodent), followed by tightening with a prosthetic ratchet torque wrench (Neodent) to a torque of 32 Ncm.” Then, the screws were unthreaded to record the highest torque values “required to completely loosen the abutment.”

Results

Titanium screws covered with solid lubricants performed better than plain titanium screws in maintaining the prosthetic implant-abutment junction. The results showed that their unscrewing torque value was higher than the torque applied during seating.

Implant screw chart

Which Screw to Use

Which type of screw, titanium or gold, is another variable worth exploring. In trying to better address loosening, Farina, et al compared gold and titanium screws. Testing for loosening values after simulated mastication included 20 dentures with eight different groups representing passive and vertical misfits with gold or titanium screws. Their results were based on “(1) 6 months of use, torque loosening, re tightening, another 6 months of use, and loosening torque; and (2) 1 year of use followed by loosening torque.”

Their research findings concluded, “After 6 months and another 6 months of clinical use simulation, titanium screws showed higher loosening torque values than did gold screws for the same fit level (P <.05). After 1 year of clinical use simulation, titanium and gold screws in passively fit dentures showed higher loosening torque values than they did in misfit dentures (P <.05). The titanium screws presented a decrease in the loosening torque after 1 year in misfit dentures.”

Above chart and image at top of page:

Biomechanical considerations for the screw of implant prosthesis: A literature review; J Korean Acad Prosthodont. 2010 Jan;48(1):61-68. Korean; Authors, So-Min Im, et al