Abstract

Nowadays implant therapy is an effective and reliable treatment with a high success rate, but various mechanical Complications can occur. Thus, the scope for prevention and management needs to be emphasized. The aim of this review article is to identify the different causes of dental implant failure due to fixture and or abutment screw fractures and to describe the management and the preventive options for these failures, so trying to assist the clinicians to plan accurately the implant-supported prosthesis treatment.

1. Introduction

The longevity and success of an implant-retained or implant-supported prosthesis depend primarily on biological and mechanical factors ¹. The implant failure can be classified into early or late failures. Early failures occur shortly after surgery and characterized by the lack of osseointegration. In contrast, late failures correspond to those implants that have been regarded as successful for some time, and they occur after prosthetic restoration has been made ^{2, 3}.

There are two main causes for late implant fracture:

(a) Mechanical problems, the most frequent cause, including Metal fatigue, due to biomechanical overloading leading to loosening /or fracture of abutment or prosthetic screws and the wear and fracture of the prosthesis or various components in the system.

(b) Biological problems, loss of supporting tissue secondary to infection or peri-implantitis; the prevalence of peri-implantitis is estimated to be 4-15% among the surviving implant population (i.e., implants still in the mouth) ^{4, 5}.

The present review identifies the different causes of dental implant failure due to fixture and or abutment screw fractures and also to describe the management and the preventive options for these failures. However, to solve these complications, one needs to understand the mechanics of such components.

2. Implant Fixture Fracture

a) Mechanical overloads; as a result of:

i. Metal fatigue when the resistance limit is exceeded.

ii. Patient's physiological alterations (e.g., para functional activity).

iii. Prosthetic problems, including inadequate occlusion, the presence of distal extensions or cantilevers in implant-supported prostheses, and a lack of prosthetic passive fit over the implants ^{6, 7, 8, 9, 10}.

b) Peri implant vertical bone loss; as a result of:

i. Chronic peri-implant inflammation.

ii. Occlusal trauma.

When vertical bone loss coincides with the apical limit of the screw, thereby joining trans-epithelial abutment to implant, the risk of implant fracture increases drastically ^{6, 7, 8, 9, 10, 11}.

c) Defects in implant design and manufacturing:

Metal fractures are related to microstructures that influence the material properties. They are related to the localized identification of chemical phases and segregations, frequently associated with failures at interfaces or components, which in turn can cause the fractures ^{12, 13}.

The incidence of dental implant fracture is between 0.16-1.5% as reported in the literature ^{14, 15, 16, 17, 18, 19}. One study reported that the majority of fractured implants are located in the regions of the molars and premolars ^{7, 11, 20} and that the fractures were evenly distributed between the maxilla and the mandible, ¹² whereas another study reported more fractures in the maxilla than in the mandible. ²⁰

Implants with small diameters such as 4 mm ²⁰ and 3.75 mm ^{12, 13} tend to fracture more easily than those with large diameters, especially when placed in a posterior location. A study has reported that an implant with a diameter of 5.0 mm is three times stronger than one with a diameter of 3.75 mm, whereas a 6.0-mm implant is six times stronger than a 3.75-mm implant ²⁰.

Coronal bone resorption observed in most implant fracture cases has been described as a primary cause of fracture ^{7, 21, 22, 23} Loss of supporting bone if extended to the level corresponding to the end of the abutment screw, the bending resistance in this region will be reduced. When such resorption extends apically beyond the third implant thread, it reaches a structurally weak zone coinciding with the end of the prosthetic screw and the border of the hybrid surface. This contributes to the fatigue at a point of low resistance to torque. The warning signs of such fatigue include loosening, torsion, or fracture of the post screws, and ceramic fractures of the prosthesis.

Patients may often report spontaneous bleeding and mobility. Exploration (manually or electronically) confirms increased mobility, increased pocket depth, and gingival indexes, and occasionally, also, plaque accumulation resulting from the patient's fear of the pain is triggered by brushing. Radiologically, separation of the fragments and bone loss may be seen. It has been reported that marginal bone reabsorption seems to be the most important risk factor indicating the start of an implant fracture, and may often extend beyond the actual fracture line ¹². Thus, an x-ray study is very useful.

For diagnostic purposes, the fracture risk factors have been grouped into three main categories [Table 1]. In the presence of more than three factors pertaining to one or more of these categories, the risk of fracture is high. ²⁴

There are three management options in the event of an implant fracture: ^{12, 25}

a) Complete removal of the fractured implant using explanation trephines ^{12, 25}. After the removal of the fractured implant, a new larger diameter implant can be installed at the same surgical bed or at another place to achieve primary stability ²² Their complete removal is considered to be the best solution to the problem

b) Removal of the coronal portion of the fractured implant with the purpose of placing a new prosthetic post ^{12, 25}. It is essential to radiologically confirm the absence of radio transparency and to electronically determine the mobility of the fragment. A prosthetic post should be considered only if there is still sufficient remaining internal thread retention ¹².

c) Removal of the coronal portion of the fractured implant, leaving the remaining apical part integrated in the bone ^{12, 25} and the missing tooth/teeth can be restored using conventional prosthodontic procedures like removable partial denture, fixed partial denture etc.

A recent study described "apicoectomy" as a suitable technique for removing fractured implants and inserting new implants at the same clinical session. This technique was based on opening a hole in the bone in order to improve the visualization of the apical fragments of the fractured implants and to remove those fragments through this hole. Afterward, a new implant is placed in a conventional manner, and the hole is closed by using the same bone that is removed from the patient ²⁶.

The following preventive measures have been suggested:

a) Avoid placing hybrid-type implant fixtures.

b) Carefully control occlusal forces: Eliminate all posterior contacts in mandibular eccentric movements

c) Perform staggered placement of implants: Avoid a straight-line configuration

d) Avoid or minimize posterior cantilevers and buccolingual offsets, particularly in partially edentulous patients

e) During the chronic loosening of gold or abutment screws or the fracture of components, critically reassess the prosthesis, retighten the abutment or use a gold screw, and check the fit.

f) Pronounced bruxers or clenchers who have experienced multiple implant fractures should be managed by the placement of additional implants fractures ²⁰.

g) Ensure perfect fit: Resolder and assess passive fit ²⁷.

Two types of screws are used in implant dentistry: abutment screws and prosthetic screws. Abutment and prosthetic screws come in various shapes and sizes and are made from a variety of materials, including gold, commercially pure titanium, and surface-treated or coated titanium alloys. Although seemingly simple, the mechanics of abutment and prosthetic screws are complex, a phenomenon described as screw joint mechanics ²⁸.

The misfit at the abutment-implant interface and the absence of a passive adaptation between the prosthesis and the abutment can lead to fracture and the most common technical complication is a fracture of the abutment screw.

The stability of the implant abutment connection is influenced by factors such as internal connection versus external connection, screw head design, screw geometry, materials, screw diameter, preload, joint separating force, settling effect, and overall screw mechanics.

Factors affecting the implant-abutment connection system are as follows:

The implant-abutment interface connection is generally described as an internal or external connection. Relative to external connections, the internal implant-abutment connections have been shown to present a higher stability and improved force distribution as a result of :

a) The internal implant-abutment connections have the ability to dissipate lateral loads deeply within the implant, there by shielding the abutment screw better from stress.

b) The longer internal wall engagement that creates a stiff, integrated body to resist joint opening (micro movement) ^{29, 30}.

c) After cyclic loading the internal abutment connection is stronger relative to external connections ^{31, 32}.

While mechanical strength is gained, the thinner lateral fixture wall at the connecting part of the internal connections may lead to a higher absolute strain value at the cervical area. Such stress shift has raised concerns regarding an increased risk of marginal bone resorption or fixture fracture ³³.

Although there are studies that show that the fracture strength in implant systems with an external fixation is equal or superior to those with internal fixation ^{34, 35}.

The literature supports a gold alloy screw with a flat head, internal hex or square, and a high tightening force (torque driver) as having the greatest ability to produce the best results ³⁶.

• To maximize the preload and minimize the loss of input torque to friction, the head of the screw should be wider than the thread diameter.

• An abutment head most often should be flat. A tapered head design reduces the clamping effect and reduces the tensile force in the threads of the screw. The tapered screw head distorts and aligns non passive components and gives a non passive casting the appearance of proper fit. However, the superstructure is not deformed permanently and leads to stress in the system. A flat-head screw distributes forces more evenly within the threads and the head of the screw, and is also less likely to distort a non passive casting. As such, the abutment head should also be flat on top to increase the clamping force in the screw head and the tensile force in the threads ³⁷.

The most commonly used retaining screws are either of gold or titanium. Gold screws are designed to be the most "flexible" portion of the implant assembly. Due to their higher modulus of elasticity than titanium, they permit an adequate micro movement to distribute force to the implant body ³⁸. Implant posts, that are retained with gold or gold coated screws, show a reduction of screw loosening and improved clamping force in comparison with titanium screws ^{39, 40}.

Titanium alloy has four times the bending fracture resistance of grade 1 titanium. Therefore, abutment screws made of grade 1 titanium will deform and fracture more easily than the alloy. A higher torque magnitude can be used on the titanium alloy abutment screw and implant body ³⁹. Titanium retaining screws are stronger than gold, but have a lower modulus of elasticity; metal fatigue will produce a gold screw fracture before the titanium retaining screw is affected. ⁴¹ The major disadvantage of titanium retaining screws is their tendency to cause galling, which results in excessive friction between the two mating surfaces thereby causing a localized welding with a further roughening of the mating surfaces. ⁴² Galling occurs in the following manner: Titanium of the retaining screw slides in contact with the titanium of the implant body, the coefficient of friction increases whereby titanium molecules transfer from the mating surfaces. ⁴³ This has been described as the adhesive wear mechanism ⁴⁴ which causes slight damage to both the implant body and the retaining screw threads.

The greater the diameter, the higher the preload that may be applied and the greater the clamping force on the screw joint. As a general rule, abutment screws loosen less often and can take a higher preload than coping screws ⁴⁵.

The screw joint has been described as two parts tightened together by a screw, such as an abutment and implant being held together by a screw. The initial clamping force, or preload, is the tensile force that develops when the screw is tightened into place. Preload depends on the torque applied, material, and design of the screw, and surface roughness ⁴⁶. Frictional forces that develop between the screw threads make the applied torque inversely proportional to the preload, so the more torque that is applied, the more preload is lost ^{47, 48}. If the torque delivered to a screw is not optimal, screw loosening may occur, making the system susceptible to fracture under occlusal load. As a screw is tightened, it elongates, thereby producing tension. Elastic recovery of the screw pulls the two parts together, creating a clamping force ⁴⁹. Excessive forces cause slippage between the threads of the screw and the threads of the bone, resulting in the loss of the preload ⁵⁰. Fractured abutment and prosthetic screws are a challenging and time-consuming complication in implant dentistry ⁵¹. When a screw fractures, the fragment inside the implant or abutment must be removed, or the implant must be modified. Otherwise, it may lose its original ability to retain prosthesis and could become useless ⁵⁰.

Intra oral separating forces may include off-axis occlusal contacts, lateral excursive contacts, interproximal contacts between natural teeth and implant restorations, protrusive contacts, para functional forces, and non passive frameworks that attach to the implants. Opposing the clamping force is a joint-separating force, which attempts to separate the screw joint. Screw loosening occurs when the joint-separating forces acting on the screw joint are greater than the clamping forces holding the screw unit together ⁴¹.

Settling occurs as the rough spot interfaces flatten under load/micro movements, since they are the only contacting surfaces when the initial tightening torque is applied ⁵¹. It has been reported that 2-10% of the initial preload is lost as a result of settling ⁵². This reduction effect is known as embedment relaxation ⁵³.

The process of screw loosening has been described in two stages ⁵⁴. Initially, external forces such as mastication applied to the screw joint causes slippage, contributing to the release of preload of the screw. The second stage of loosening involves continual preload reduction below critical level, allowing threads to turn and the loss of intended screw joint junction ⁵⁵.

A literature review of clinical complications of osseointegrated implants showed that screw loosening or screw fracture varied from 2 to 45% of the implant restorations, with the highest amount in single crowns ⁸. In one of the systematic reviews that evaluated the 5-year survival rates of implant-supported single crowns, it was observed that from the 26 clinical studies included, the cumulative incidence of abutment screw or abutment loosening was 7.3% after 5 years of clinical service in both external and internal connections ⁵⁶.

Posterior abutment screws loosen at a higher percentage than anterior implants. This supports the project of eliminating unnecessary occlusal and off-axial forces on implant-supported restorations ³⁶. One of the studies have reported a 38% loosening of single implant restorations in the posterior maxilla and mandible, but such studies do not take into account various important factors ⁵⁷.

a) It is recommended in clinical practice that in order to reduce the settling effect, the implant screws should be retightened 10 min after the initial torque application ^{58, 59, 60, 61}.

b) Mechanical torque gauges should be used instead of hand drivers to ensure a consistent tightening of the implant components to recommended torque values ⁵¹.

c) The use of sealers to fill in the gaps between these crews and implant threads and adhesives to increase the frictional resistance has been suggested to reduce screw loosening. (Ceka Bond, Preat, San Mateo, CA) is listed as an adhesive paste on the package insert. ⁶²

d) Cho et al 2004 ³⁶ supported the following clinical recommendations:

i. Ensure that implants are placed perpendicular to the occlusal plane.

ii. Frameworks should have minimal cantilever lengths.

iii. Use components with low tolerance levels for component misfit.

iv. Use components with anti rotational features for single tooth restorations.

Generally, treatment or intervention should always mitigate future risk and improve the prognosis ⁶². In certain situations, however, the intervention itself may be the biggest risk factor. In reviewing published studies, some techniques have been found to be clearly more likely to cause irreversible damage to the implant components and should therefore be considered higher risk procedures. Because a direct relationship exists between the risk of damage and degree of difficulty, the clinician must understand not only the technique itself but also the inherent risks and difficulty associated with each technique

The first step in managing any fractured screw is to obtain a detailed history and perform a thorough clinical examination. Every attempt should be made to determine the cause of the screw fracture to minimize the risk of subsequent complications. The next important step is to confirm that the screw is fractured and determine the location of the fracture. Generally, fractures that occur above the implant level are easier to manage than fractures occurring more apically. Screw fracture may be confirmed by direct visualization, radiographic examination, tactile sensation through the use of an instrument, comparison with an undamaged screw of the same system, or by using other undamaged components of the same system to see if an obstruction is prohibiting complete seating. If the restoration has been missing for an extended period, the peri-implant soft tissue may overgrow, making access to and visualization of the implant difficult. In this situation, a low-frequency diode laser may be used to trim the tissue and expose the implant platform. Use of an electrosurgery unit or scalpel should be avoided to avoid any heat transfer within the implant or scratching of the implant surface. Other techniques that may help to improve visualization include the use of dental loupes with a coaxial or light-emitting diode headlamp or a dental surgical microscope, especially in situations with deep screw fractures ^{63, 64}.

The application of risk-based decision-making plan, in relation to the various reported techniques for managing fractured screws may allow for a logical and structured approach in managing fractured abutment and prosthetic screws.

Once the appropriate examination and diagnostic steps are complete, a tentative plan should be established using the proposed decision making tree ^{1, 65, 66, 67, 68, 69}. A small rigid instrument such as a scaler, sickle explorer, or endodontic explorer was the initial instrument of choice for rotating the remaining apical portion counterclockwise and removing the fractured fragment. Oblique fractures may be easier to manage with this technique because a purchase point usually exists in which the instrument may be engaged. Although this is considered a low-risk choice, care should always be used in engaging the broken segment with any instrument because the instrument may fracture or damage the internal threads of the implant. Flexible instruments such as endodontic files or modified instruments should be used with caution because of the decreased strength of the instrument. If the fractured segment does not move with use of a hand instrument, an ultrasonic scaler can be used to loosen the screw ⁶⁷. This technique can be used in conjunction with standard instrumentation or in an alternating fashion, but care must always be taken not to damage the internal threads of the implant. In addition, this method should only be attempted a few times to ensure that the broken segment is not wedged further into the implant. Sometimes, even after the screw has been loosened, it is still difficult to grasp and remove. If a loose fragment is near the platform, a cotton tip applicator may be used as a driver, as the cotton threads can often grip the fragment, making removal easier ⁶⁹.

If the remaining screw fragment cannot be rotated with basic hand instruments, it may be rotated using rotary instruments ^{50, 70, 71, 72, 73, 74, 75, 76, 77, 78}. This can be done by introducing a bur, a drill, or a trephine mounted on a handpiece into the internal chamber of the implant until it contacts the screw fragment. Then, as it is rotated counterclockwise, it may unwind the screw fragment ^{71, 72}. If the screw fragment cannot be engaged with an instrument, a dimple or access point can be created using a ½ round bur mounted on a handpiece under copious irrigation; this dimple may be modified into a slot for better engagement ^{50, 75, 78, 80}. Furthermore, burs and instruments may also be modified to allow for better engagement of the fragment and to prevent damage to the implant ^{72, 75}. Aftermarket screw retrieval kits are available, including Fragment Fork (Astra Tech; Dentsply Sirona), ITI Dental Implant System (Institut Straumann AG), IMZ TwinPlus Implant System1 (Dentsply Sirona), Screw Removal Kit Replace (Nobel Biocare), and Certain-Screw Removal Kit (Biomet 3i). These kits may be helpful when low-risk techniques have been unsuccessful ^{74, 77, 80}. If all attempts at rotating the screw fragment fail, it may be drilled out completely, although this technique requires a high degree of attention and care to prevent damage. Specialized systems and drill guides are important in these situations, and a screw tap tool may become necessary ⁷⁹. An important consideration when removing a fractured screw is not to damage the internal threads of the implant, so care must always be taken whenever a rotary instrument is introduced into the internal chamber of the implant ^{80, 81}. The use of drill guides or sleeves can help stabilize the drill and subsequently reduce the risk of damage to the internal threads ^{50, 73, 76, 81}. Furthermore, there is a high risk of heat generation whenever screws are drilled, and for that reason, drilling should be performed intermittently at a reduced speed with copious irrigation ^{74, 78}.

In situations where removal of the screw fragments is not possible using a low- or moderate-risk technique, intentional modification of the implant may be necessary to keep the implant serviceable ^{82, 83, 84, 85, 86, 87, 88, 89, 90}. If the internal aspect of the implant is significantly damaged or the screw fragment is irretrievable, the clinician may choose to remove the implant and place a new one. However, this decision can be influenced by financial factors, patient experience, and patient willingness to undergo additional surgery. An implant in such situations may also be completely buried in soft tissue, an option that may require the fabrication of a new prosthesis, and may compromise function and/or esthetics ^{90, 91}. If the patient is unwilling to sacrifice an irreversibly damaged implant, attempts can be made to maintain the use of this implant by retapping the internal threads once the irretrievable screw fragment is drilled out. This can be accomplished with the use of specific retapping tools and may require alternative or modified parts to successfully restore the implant in question ^{81, 83, 86}.

If all attempts fail and the patient is still unwilling to sacrifice the implant, a custom cast post and core may be fabricated for the implant .These techniques generally start with the removal of remaining screw fragment, followed by removal of the internal threads of the implant by using a diamond rotary instrument or tungsten carbide bur in a high-speed handpiece under copious irrigation. An autopolymerizing acrylic resin pattern can either be made directly on the prepared implant or indirectly on a stone cast. The pattern is then cast using nickel- or cobalt-chromium alloys, although other types of metal alloys have also been reported ^{82, 83, 88, 89, 90}. Once the seating of the custom cast post and core has been confirmed, it can be cemented. The appropriate restoration can then be fabricated following conventional prosthodontic protocols.

3. Conclusion

In the present review, in order to present a complete picture, various clinical as well as in vitro studies on the mechanics of implant fixture fracture and abutment screw loosening/fracture of a single implant-abutment connection have been reviewed. The incidence of such complications is very small and the literature is still not conclusive concerning the choice of connection and/or crown-retaining system. However, understanding the mechanics of these failures will help the clinicians in formulating a more predictable treatment.

References