Various Facets of Screw Loosening in Implants: A Review

Introduction

Osseointegrated implants have revolutionized the options for dentists across the globe for restoring an edentulous space. These implants are used to replace a single tooth, multiple teeth or the whole arch. Dental implants are proven to have a success rate of 97% at 10 years and 75% at 20 years.¹ It has always been a clinician’s endeavor to achieve the highest success rate in the treatment provided to patients.

But just like any other treatment, complications and failures are bound to arise. Implant failures can be grouped into early (<3 months) and late (>3 months) based on occurrence. These can further be sub grouped into failures caused due to surgical errors and those due to prosthetic errors. Early failures include loss of primary stability, infection, failures due to immediate loading and late failures include screw loosening, fractures, etc.² (Refer Flow chart 1)

The junction between the implant and prosthetic abutment is one of the crucial areas of the implant prosthesis system that determines the amount of load on the implants. Among the above mentioned failures, some of the most commonly encountered implant failures are screw loosening and fracture of the implant or its components, which occur at this junction.³ Any misfit in this region can lead to unnecessary load or improper direction of load that can further lead to implant failures.1 Screw loosening of the abutments has been reported to occur in 6% to 48% of restorations.^{4, 5}

Many studies also reported that ‘loss of preload’ can be a plausible cause for screw loosening. According to Glossary of Prosthodontic Terms- ninth edition (GPT9), preload is the tension created in a screw, especially the threadings, when tightened. It is an engineering term used in dentistry to describe the degree of tightness or clamping force of a screw, usually in implant prosthodontics.⁶

The screw that connects the transmucosal abutment to the implant fixture is responsible for the clamping force. The design permits the screw to loosen or break in case of improper load before the fracture of the implant fixture. This fail-safe nature is due to their smaller size, composition, and the biomechanical parameters of the screw joint assembly.⁷

Branemark et al. suggested that the misfit in the implant abutment junction should not exceed 150 micrometer, but this is not practically achievable in most of the usual dental implant systems regardless of the torque applied.⁸ Therefore, the relationship between torque and preload depends on several factors such as size and shape of the screw, surface texture and characteristics, lubrication, rate of tightening, composition and their properties, to name a few.⁹

This review article aimed to discuss the various causes for loss of preload and the effects of this loss on the success of dental implants.

Factors responsible for loss of preload The factors affecting the loss of preload can be classified into: (Refer Flow chart 2)

Mechanical factors

Mechanical factors depend on the roughness of surfaces, the friction between them and resulting wear that occurs. Adhesion between two sliding surfaces leads to a form of wear known as galling. When a material undergoes galling, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces. Galling results from a combination of friction and adhesion. Galling can also occur due to inappropriate tribological loading, lack of proper lubrication and high contact pressure of about 1000 kpa.

Asperity, on the other hand, can be defined as "unevenness of the surface or roughness and ruggedness". Surfaces that are polished to a mirror finish are not truly smooth on a microscopic scale. They are rough with sharp and rugged projections, termed as asperities. When two surfaces that appear to be smooth macroscopically come in contact with each other, initially they only touch at these asperity points. When the surfaces are subjected to a compressive load, the asperities deform through elastic and plastic modes, leading to wear.¹⁰

Galling and asperity can lead to severe plastic deformation of the material (8000-10000 microstrain units), diffusion of materials between surfaces, fatigue wear and surface roughness. In case of implant and the abutment screws, galling and wear due to asperity can occur, for example, while screwing and unscrewing, which can further lead to loss of preload and further screw loosening. This can affect the longevity and economy of the parts of the implant system and thereby failure of implants.^11,12

Courtesy: Secatto FBS, Elias CN, Segundo AS, Cosenza HB, Cosenza FR, Guerra FLB (2017) The morphology of collected dental implant prosthesis screws surface after six months to twenty years in chewing.

Type of material

Retaining screws are usually fabricated from gold or titanium alloys. Gold is used in places where flexibility is required in the implant assembly as it has a high modulus of elasticity. This property enables it to show a certain amount of micro movement and even distribution of forces. Gold screws are indicated in long span edentulous cases or in case of a cantilever prosthesis. Nevertheless, this makes gold screws the “weakest link” in the implant complex as it fractures first in case of excessive overload. Titanium screws are much stronger in this regard, but their high frictional resistance is their main drawback as they tend to undergo galling more.

A study conducted by R Doolabh et al. evaluated the preload generated in gold and in titanium retaining screws and the effect of repeated torque on this preload and deduced that gold screws generate higher preload values than titanium.^7,13 Tsuge and Hagiwara measured the reverse torque values before and after loading both internal and external hex implant made of gold and titanium and concluded that titanium alloy screws were less prone to loosening than the Gold-Tite® screws, regardless of the type of connection.¹⁴ According to a study by Stuker et al., gold should be the material of choice for abutment fixation screws, since it produced the highest preload values and Titanium screws presented the highest torque removal values, followed by gold screws, and surface-treated titanium screws.¹⁵

Type of implant abutment connection

Based on the geometry, there are internal hex and external hex connections. Internal hex can be further divided into conical, clinical fit and combined connections. Clinical fit connection is where the two parts have parallel walls with geometric “index” to prevent rotation. In general, based on configuration, there are hexagonal, conical, spline, octagonal and cylindrical types of connections.¹⁶

In a study conducted by G Setia et al., a cantilever implantsupported prosthesis was used to examine the effects of occlusal loading on the stability of the abutment screw 50N on the pontic and 150N on all three units in cycles of 50,000, 100,000, 250,000, and 500,000. The external hexed implant performed equally well when compared with the internal connection implant.¹⁷ Another study indicated that the internal connection, together with the morse taper, best resists cyclic loading in terms of screw loosening.¹⁸ Other studies stated that the design of connection type was not significant in affecting the preload values.^4,14

Tightening torque

As the screw is tightened into the implant, both the speed of tightening and the force of tightening play an important role in maintaining the preload. Preload force increases linearly with an increase in tightening torque.^19-22 Techniques such as use of torque wrench or pausing and retightening while tightening can help maintain the screw stability and preload.

A study by Behnaz Ebadian et al. tested the effect of torque technique on preload. The techniques were grouped into: (Group 1) one time 30 Ncm torque, (Group 2) three times 30 Ncm torques with five-minute intervals, (Group 3) one time 30 Ncm torque, opening the screw followed by 30 Ncm retorquing and (Group 4) one time 35 Ncm torque. The maximum mean detorque values of the abutment screws were seen in group 4, 1, 3 and was least in group 2. The torque loss grew as the torque value increased. The detorque value in group 4 had the smallest difference, though.²⁰ In contrast to these findings, a study conducted by A Bacchi et al., concluded that the tightening technique had no significant influence on the loss of preload.²³ G Siamos et al., concluded that retightening abutment screws 10 minutes after the initial torque applications should be routinely performed to prevent loss of preload.²²

Number of threads

Studies show that preload of a single implant is independent of the number of threads.¹⁹ Although, in case of multiple units, thread number can have an effect on the preload force when it undergoes wear eventually. Further studies are needed, especially for short implants to evaluate the minimum number of threads that have to be present to maintain optimum preload force.¹⁹

Screw head angle

Technical complications, which is one of the common and late complications of implant prosthesis, can occur due to the friction that exists between the screw head and screw head counterbore. Therefore, various studies have shown that a relationship exists between screw head angle and the preload force. Zipprich et al., concluded that preload force increases linearly with increase in screw head angle for the same amount of tightening torque.¹⁹

Effect of contamination

During the process of implant placement and prosthesis delivery, the implant and its components come in contact with a lot of fluids. Some of these are saliva, blood, saline, chlorhexidine, silicone sealant and so on. These agents tend to get locked in between various implant components. For example, when these agents are present around the screw inside the implant system, they can increase the preload significantly due to reduced friction.

A study conducted by Jalali H et al., assessed the effect of blood and saliva contamination at the implant-abutment interface on preload and it was observed that the presence of blood or saliva at the abutment-fixture interface had no discernible impact on the detorque value following cyclic loading, although the detorque value of abutment screw significantly decreased after cyclic loading.^24,25 Florian Rathe et al. concluded in a study that none of the tested agents (saliva, blood, chlorhexidine gel, and special sealing silicone) resulted in significantly higher preload forces compared to the dry control.²⁶ However, increasing the torque and decreasing the coefficient of friction can be done to increase the preload developed.

Angular misfits

From a biomechanical point of view, failure of the retention screw is considered as a failure of the implant prosthesis itself. This is because, once a proper fit is obtained between the various components of the implant system, the masticatory load will be maintained by the preload. Although the etiology for failure of implant prosthesis is multifactorial, the lack of proper fit between components could be one of the main causes as per few reports. WG Assuncao concluded in his study that the unilateral angular misfit influenced the preload maintenance only before mechanical cycling.⁴

Type of prosthesis

All-on-four techniques are commonly used to rehabilitate edentulous arches. The two commonly used anchoring systems are the multi-unit abutments, which consists of straight or angulated components that convert the internal hex connection of an implant into a conical one and overdenture bridge. Santo Catapano et al. concluded that Multi Unit Abutment system (MUA) and OT-Bridge are reliable prosthetic anchoring systems which can tolerate occlusal loads on distal cantilever in all-on-four rehabilitation without any significant loss of preload in screw tightening.²⁷

Both the CAD-cast and three dimensionally printed (3DP) produce frameworks with clinically acceptable misfit. 3DP might not be the technique of choice for maintaining screw’s preload stability under an aggressive loading situation.²⁸

Discussion

The dental implant system consists of an implant fixture to which an abutment is connected. The implant prosthesis is either screwed or cemented to this abutment or a combination of both is used. The abutment is fixed to the fixture with the help of a screw. When a torque is applied to the screw, preload is the initial load.

The preload is a contributing factor for the stability of screw connection parts, and is affected by various mechanical factors. Retaining screws is being extensively studied and being constantly improved as screw loosening is a recognized problem in dental implant therapy. Screw loosening is the result of the screw being repeatedly deformed by the cyclic forces of mastication.

When the mating threads start to loosen, a process known as the critical bending moment, the loss of preload reaches a critical level at which any vibration will cause the screw to back out. The settling effect occurs due to microroughness on the two contact surfaces so that when initial torquing of the screw is applied, the rough areas collapse and lead to screw loosening. Thus, it is necessary to maintain preload to stop the joints from separating. 10% of the initial preload may be lost to smooth contact surfaces (embedment relaxation), rather than elongation stresses.

Several factors may cause reduction or loss of the retention screw’s preload in an implant supported prosthesis, such as the fit, inaccuracy, and rigidity of the superstructure, number and position of implants, cantilever length, occlusal morphology and insertion torque, occlusal overload, presence and type of lubricant, physical properties of materials, and screw settling.³

Another factor related to loss of preload in the retention screws is the mating surface of the components when the screw is tightened. The microroughness on these surfaces increases the microscopic distance between the components and the clamping force that keeps the joint stable is reduced. A recent advance in this regard is the friction-fit abutment (Zimmer dental). It is an internal hex type of connection wherein the abutment forms a cold weld connection with the implant when it is fully seated. Rotational freedom (misfit) for the implant/ abutment systems is 0 degrees when fully tightened to 30 Ncm. This essentially eliminates micromovement, tipping and the effects of vibration and thus screw loosening virtually disappears (Zimmer Dental Friction fit Abutments Brochure).

Conclusion

A dental clinician aims to provide the best treatment to the patients in order to achieve the best possible outcome. The above mentioned factors can be kept in mind while planning dental implants for edentulous patients. However, there are particularly simple ways by which clinicians can prevent the loss of preload.

In order to maintain adequate preload at the implant abutment connection:

(1) Use the screw that is packed with the prosthetic component

(2) Use calibrated torque-wrench

(3) Follow manufacturer’s instructions on torque values

(4) Make sure implant internal screw thread area is clear of debris and tissues

(5) Make sure the implant is sufficiently stable

(6) Use precise fitting screws of the same implant system

(7) Prosthetic screws are to be given to the labs

These measures can help in achieving good stability of implants and thereby improving the success of the implant treatment.

Conflict of Interest

None