Cantilevers/Increased Crown Height Space.

One of the most common etiologic factors that results in screw loosening is excessive continuous occlusal forces. The most common example occurs in prostheses with improper occlusal contacts. The greater the stress applied to the prostheses, the greater the risk of abutment screw loosening. A nonideal prosthetic design may potentiate the force applied. Cantilevers increase the risk of screw loosening because they increase the magnitude of forces to the implant system: there is a direct relationship between the length of the cantilever and force applied to the prosthesis.⁷ Any of these external forces applied to a cantilever will further magnify the joint-separating forces. For example, cantilevers on prostheses lead to uneven occlusal loads. Uneven occlusal loads cause repeated cycles of compression and then tension and shear of implant components. Screws are especially vulnerable to tensile and shear forces. Both of these are dramatically increased with cantilever forces or angled loads. Because the screw is an inclined plane, the continued vibration causes it to unthread. The greater the range of external forces, the fewer the number of cycles necessary before screw loosening.

When an increased crown height space exists (poor crown-implant ratio), there is a resultant greater force applied to the screw. This usually results in a greater risk of screw loosening (or fracture). Boggan et al demonstrated that the force that is applied to the screw is directly related to the crown height. The crown height acts as a vertical cantilever, which magnifies the force on the abutment screw (Fig. 16.2).⁸

Parafunction.

Of all the external forces that cause screw loosening, the primary factor is parafunction related. A horizontal bruxing patient loads the implant crown with an angled force repeatedly. This increases the magnitude of force, cycles to fatigue failure, and the angle of the force that places shear on the interface. Abutment screw loosening can be expected in a patient with a severe bruxing habit. A parafunction patient increases the amount of force to the system while also increasing the number of cycles to the system. Hence, fractures of porcelain and cement seals and screw loosening or fracture are inevitable. When the adjacent natural teeth are mobile to lateral or angled forces, the rigid implant and implant crown may be overloaded. A heavy bite force occlusal adjustment, which allows the adjacent teeth to move before implant crown contact, is recommended to reduce the risk of overload.

Continuous occlusal loads can have a cumulative effect on the preload, and the screw material may undergo deformation.⁹ When the force exceeds the yield strength, plastic deformation occurs, and the screw begins to deform. This material deformation causes the screw to loosen and leads to potential failure of the prosthesis.

Screw loosening is also affected by the amount of the force and the number of cycles and is similar to fatigue. External methods to limit screw loosening include factors that reduce the biomechanical stress. These include key implant positions (i.e., to distribute forces evenly), sufficient number of implants (i.e., adequate surface area), passive frameworks, and adequate occlusal schemes.¹⁰

Splinted vs. Nonsplinted Crowns.

Screw loosening of abutment or prosthetic screws occurs more often on individual implant crowns than on crowns that are splinted together. For example, in a report for single molar replacement, the abutment screw-loosening rate was 40% during a 3-year period. When two splinted implants were used to replace the molar space, the screw loosening was reduced to 8%.¹¹ The stress distribution of splinted prosthetic units results in less force applied to the screw system. Studies have shown splinted implant-retained overdentures have far less screw loosening in comparison to fixed prostheses.¹²

Crown/Abutment Not Fully Seated.

If the abutment is not fully seated because of improper abutment placement, tissue impingement, or bone impingement, a poor distribution of force in the screw system will result, which leads to increased screw loosening. When the abutment is not seated fully and completely tightened, the prosthetic screw will be distorted, which leads to inadequate preload and subsequent screw loosening or fracture (Fig. 16.3).

Insufficient/Excessive Torqueing.

When improper preload via the torqueing process is applied to the abutment screw, screw loosening will often occur. This may be caused by either excessive or insufficient tightening of the abutment screw. An implant screw is similar to a bolt joint in engineering. There is a preload (tightening force) placed on the screw, which develops a force within the screw. As the screw is tightened, it elongates, producing tension, which results in the implant screw acting like a spring. The preload stretch of the screw is maintained by frictional force, and the tension between the screw and the implant/abutment is termed a clamping force. When insufficient preload is applied to the screw, there is insufficient clamping force, which ultimately leads to screw loosening, especially under occlusal loading. When excessive force is applied, the clamping force is easily released, and screw loosening will occur (Fig. 16.4).

Screw Diameter.

The diameter of the abutment screw may have a significant effect on the amount of preload applied to the system before deformation occurs. The greater the screw diameter, the higher the preload that may be applied, which results in a greater clamping force on the screw joint. However, the coping and prosthetic screws vary greatly according to the type, size, and material. The strength of the material increases by a power of four when the diameter of the screw doubles (a screw with twice the diameter is 16 times stronger). As a result, abutment screws loosen less often because they can take a higher preload compared with coping and prosthetic screws. Some companies offer similar diameters for abutment and prosthetic screws. As a result, a similar clamping force may be used for either component (Fig. 16.5).

Screw Material.

The composition of the screw is another factor that modifies its performance. The composition of the metal may influence the amount of strain in the screw from preload and the point of fracture, directly affecting the amount of preload that can be safely applied. Screw material and yield strength vary greatly when all other factors are similar (e.g., 12.4 N for a gold screw to 83.8 N for a titanium alloy screw fixation).¹³

The deformation or permanent distortion of the screw is the end point of the elastic modulus. Titanium alloy has four times the bending fracture resistance of grade 1 titanium. Abutment screws made of grade 1 titanium deform and fracture more easily than the alloy. Titanium alloy is 2.4 times stronger than grade 4 titanium. As such, a higher torque magnitude can be used on the titanium alloy abutment screw and female component (found within the implant body), less on grade 4 titanium, less on grade 1 titanium, and the least on gold screws.

The elongation of metal is related to the modulus of elasticity, which depends on the type of material, width, design, and the amount of stress applied. The material of which the screw is made (e.g., titanium alloy, titanium, or gold) has a specific modulus of elasticity. A prosthetic gold screw exhibits greater elongation than a screw made of titanium alloy but has a lower yield strength.

Although the strengths of titanium grades are dramatically different, the modulus of elasticity is similar for grade 1 to 4 titanium. Hence, the strain of the abutment screw is similar with each grade of titanium, but the safety load relative to fracture is different. Titanium alloy (grade 5) has a slightly higher modulus of elasticity. Although not clinically relevant to metal-bone osseointegration, the titanium alloy screw should have a slightly higher preload value. This is not a consequence relative to permanent deformation or fracture because it is more than twice as strong as the other grades of titanium.

The metal for the screwdriver used in the torque wrench is also important to consider. Stripping of the screw head prevents the clinician from tightening or removing the screw. Some manufacturers make the torque wrench driver out of titanium alloy, and the screw is made of gold or titanium. The concept is that the torque wrench will not deform the hexagon and will not strip, so the device lasts longer. However, this is not ideal. It is easier to replace the torque wrench driver than the abutment or prosthetic screw. Because of this, the torque wrench should be made of titanium and the screw of titanium alloy.

From a clinical standpoint the receptor site for the torque wrench is also a feature of the screw head to consider. The screw head has a rotation feature, most commonly a hexagonal design. The more sides to the rotation feature, the more often the head will strip. A slot or triangular feature will strip less than a hexagon (Fig. 16.6).

Component Fit.

In the science of machining metal components, there is a range of dimensions that manufacturers use. For instance, an implant 4 mm in diameter may actually range from 3.99 mm to 4.01 mm. Likewise, the abutment and prosthetic coping connection also has a range. As a result, if a smaller implant body hex dimension is mated with a larger abutment connection, the components may not ideally fit together. Most implant manufacturers allow for a misfit range that results in the abutment or coping being able to rotate ±10 degrees on the implant body. Components between the abutment and implant body may have a misfit of 10 degrees in a rotational dimension, and horizontal discrepancies have been reported up to 99 µm.^14,15 These ranges are different with respect to each implant system. The more accurate the component fit, the less force applied to the abutment or prosthetic screw (Fig. 16.7).

The incidence of screw loosening is also a function of the accuracy of fit of the flat-to-flat connection of the implant and abutment or prosthetic component. Implant abutment connections or prosthetic connections with an unstable mating interface place undue stress on the screw that connects the components. Mechanical testing has demonstrated a direct correlation between the tolerance of the flat-to-flat dimension of the external hexagon and the stability of the abutment or prosthetic screw. Binon showed that a mean flat-to-flat range of less than 0.005 mm exists on the hexagon, and a flat-to-flat range of less than 0.05 mm for the entire sample would result in a more stable screw joint.¹⁶ Studies have shown plastic castable patterns, which can be highly inaccurate, to have a vertical misfit as high as 66 µm.¹⁷

The same manufacturing conditions apply to impression transfer copings and analogs. Many manufacturers have a wider machining range (+ or − variance) for the prosthetic components to reduce the cost of manufacturing. When transfer copings and analogs are used in impressions and then to fabricate the prosthesis in the laboratory and the implants are splinted together, the prosthesis may not passively seat.

Many manufacturers recommend the use of plastic (non-metal) burn-out posts. Plastic burnout prosthetic copings cost less, but they exhibit much greater laboratory variance and poor fit because of irregularities and settling of the superstructure. Besides cost, another advantage of a plastic burnout pattern for a coping is that one type of metal is used for the coping and superstructure, lessening the risk of metal corrosion or separation between the coping and superstructure.

To reduce settling a machined coping may be used to fit the implant abutment more accurately. Some manufacturers suggest a titanium coping to reduce the risk of misfit. However, oxides form on the titanium-machined coping surface and impair metal adherence when the prosthesis or abutment metal work is cast to the coping. Mechanical retentive features on the coping improve this metal-to-metal attachment.

Laboratory studies demonstrate that an alloy-cylinder compatibility exists when noble metal alloys are used rather than titanium for a superior metal-to-metal connection. A machine coping connection is still present, so it is superior to the plastic components used to cast one metal.¹⁸ The risk of oxides forming between the coping and metal of the prosthesis is also reduced (Fig. 16.8).

Implant Design.

The type and design of the dental implant has a significant impact on screw loosening. As a general rule, most implant bodies have an antirotational feature for the abutment connection. The most common designs are an external hexagon, an internal hexagon, a Morse taper, and a Morse taper with threads.

Factors that affect the abutment screw connection and screw loosening include the height (or depth) of the hexagon and the platform diameter. Boggan et al studied the influence of design factors on the mechanical strength and quality of fit of the implant abutment interface. Whereas failure mode for static test samples was bending or deformation of the abutment screw, fracture of the abutment screw was the common failure mode for the fatigue test samples. The static failure load was greater for the external hex implants of 1 mm in height compared with implants with an internal hexagon of 1.7 mm. The larger-diameter implant had the greatest static load before failure (Table 16.1).⁸ As the hexagon height (or depth) increases, the load on the abutment screw decreases. Likewise, as the diameter of the implant platform increases, the force on the abutment screw decreases. Reduction of the lateral load (P) on the abutment screw is crucial to prevent the load on the screw to be beyond the yield strength of the material.

The height (or depth) of the antirotational hexagon is directly related to the force applied to the abutment screw with any lateral load. Because the crown is connected to the abutment and the abutment rests on the implant platform, a lateral force on the crown creates a tipping force on the abutment. This tipping force is resisted by the hexagon height or depth, the platform, and the abutment screw. When the arc of rotation is above the hexagon height, all of the force is applied to the abutment screw. For the hexagon height to be above the arc of tipping forces, the hexagon height must be at least 1 mm for a 4-mm-diameter implant. Yet many implant manufacturers feature a hexagon height of only 0.7 mm, so almost all of the force is directed to the abutment screw, increasing the occurrence of screw loosening and fracture (Fig. 16.9).

The difference between external vs. internal connections has been well documented. Studies have shown the incidence associated with external-connection (EC) implants was 18.3% at a mean of 5.3 years (217 of 1183 restorations; maximum, 59.9%).^19,20 The complication rate with internal-connection (IC) implants was 2.7% at a mean of 4.5 years (142 of 5235 restorations; maximum, 31.6%).^21,22 Other studies have shown the external hex to have a significantly higher incidence of screw loosening than internal hex (MA-EC, 15.1%; Zr-EC, 6.8%; MA-IC, 1.5%; Zr-IC, 0.9%).²³

The platform dimension upon which the abutment is seated is also an important factor in screw loosening. Larger-diameter implants, with associated larger platform dimensions, reduce the forces applied to an abutment screw and change the arc of displacement of the abutment on the crest module. For example, in a report by Cho et al, abutment screw loosening over a 3-year period was almost 15% for a 4-mm implant diameter but less than 6% for the 5-mm implant diameter (Fig. 16.10).²⁴

Screw vs. Cement Retained.

When evaluating the prosthesis type (cement vs. screw), studies have shown screw-retained (8.5%) had a much higher incidence of screw loosening in comparison to cement retained (3.1%). These complications have a greater incidence with screw-retained restorations compared with cement-retained restorations because cement-retained restorations are more passive and have less strain on the implant system.²⁵ Although a cement-retained restoration is more common, screw-retained restorations are indicated when low-profile retention is necessary on a short abutment or when the implant bodies are more than 30 degrees from each other and splinting is required to restore the patient. Additionally, a screw-retained prosthesis has the advantage of less chance of tissue irritation because of the high incidence of retained cement with a cement retained prosthesis.

Screw loosening and partially unretained restorations are common complications of nonpassive castings. The more passive the fit on the implant abutment for screw retention and the more controlled the occlusal forces, and the more secure the prosthesis. The repeated compressive and tensile forces from nonpassive castings under occlusal loads cause vibration and loosening of the screw components. Accuracy in design and fabrication of the metal superstructure are determining factors for the reduction of forces at the implant abutment and implant-bone interface.

Passive screw-retained restorations are more difficult to fabricate than passive cement-retained restorations. When the screw is threaded into position, the superstructure may distort, the implant may move within the bone, or the abutment screw may distort. The distortion of the superstructure and implant system may reach a level such that a 500-µm original gap may not be detectable.²⁶ As a result, the casting may appear to fit the implant abutment for screw retention. However, the superstructure, bone, and components do not bend beyond their elastic limit, and compression, tensile, and shear forces are placed on the bone-implant interface.²⁷ The bone must remodel to eliminate these forces. If the forces are beyond physiologic or ultimate strength limits, resorption of the bone-implant interface occurs. As a result, greater crestal bone loss has been associated with nonpassive castings. Creep (a constant force applied over time on a material) or fatigue also can contribute to fracture of the components over time because of a constant load or cyclic load frequency (Figs. 16.11 to 16.13).

Anatomic Location.

The location of the prosthesis in the oral cavity is also a significant factor in the incidence of screw loosening. Sadid-Zadeh showed a significant incidence difference with respect to anatomic locations; anterior (12.8%; 51 of 398 restorations) and posterior positioning (4.8%; 144 of 2972 restorations). However, when evaluating internal connection implants, they had an associated higher incidence of screw loosening in the posterior region (4.3%) than the anterior region (0.7%).²⁸

Prosthetic Design.

The prosthetic design may be altered to minimize the possibility of screw loosening. Ideal implant placement in the key implant positions should be adhered to. Cantilevers should be eliminated or reduced, especially when high occlusal forces are present. Additionally, implant protection principles should be adhered to including reduction of cuspal inclines of the prosthesis (decreased cusp height), decreased occlusal table, and no lateral contacts, especially in the posterior.

Ideal Preload.

The ideal torque force on an abutment screw varies by manufacturer and may range from 10–35 N/cm. This preload is determined by many variables including the screw material, screw head design, abutment material, abutment surface, and possible lubricant. To reduce the incidence of screw loosening, the abutment screw should be torqued by the following protocol:

Screw Tightening Sequence.

When screw tightening a multi-unit fixed implant prostheses, a proper sequence and technique is crucial to obtain the correct torque. The torque should be applied incrementally amongst all screws so that not one screw is tightened fully. This is based on the fact that a multi-unit prosthesis is unlikely to be “completely” passive. A nonideal tightening sequence will lead to either an insufficient or excessive amount of torque placed onto a specific screw thread. Undertorque will lead to insufficient clamping force and lack of ideal stretching of the screw. This will most often lead to screw loosening. Overtorque will lead to permanent deformation of the screw which may lead to screw fracture (Fig. 16.14).

Settling Effect.

Settling is a term used to describe the effect of various implant parts wearing and fitting closer together. Minor irregularities on or within a casting that incorporates the top of an abutment or screw can cause slight elevation of the casting or the screw head. Over time, micromovement wears down the irregularities, and the parts fit closer together. However, this settling relaxes the preload force on the prosthetic screw and is more likely to cause screw loosening. This embedment relaxation or loss of preload has been shown to be approximately 2% to 10 % of the initial preload within the first few seconds or minutes after tightening. This is the reasoning for the above protocol to include a second retorque after 5–10 min to regain the lost preload due to settling (Fig. 16.15).²⁹

Torque Under Moist Conditions.

Studies have shown when placing and torqueing abutment screws, more accurate torque values result under wet conditions vs. dry.³⁰ Saline may be used to lubricate the screw prior to placement of preload to maximize the accuracy of the preload.

Wider Implant Bodies.

The use of wider implant bodies results in decreased force on the screw. Graves has shown increasing implant size from 3.75 mm to 5.0 mm results in 20% greater strength, while increasing implant size from 3.75 mm to 6.0 mm to increases the strength by 33%.³¹

Implant Movement.

Mobility of the implant indicates failure of the implant and necessitates immediate removal. A radiograph may reveal a circumferential radiolucency. The site should then be reevaluated after adequate healing for the need of bone grafting, implant placement, or change in prosthetic treatment planning (Fig. 16.17).

Removing a cemented crown from a mobile abutment is very challenging with crown removal techniques (e.g., crown bumper). The impact force that is applied to the mobile crown is dissipated because of the loose screw. This may result in damage to the internal threads of the implant body. In addition, when an implant crown margin is subgingival, it is often difficult to obtain access for the crown remover. In poorer bone densities, overzealous use of a crown remover may result in loss of bone-implant interface.

Option 2.

The safest and most predictable treatment option to treat abutment movement is accomplished with making an occlusal access, and turning the cement-retained crown into a screw-retained crown (Fig. 16.18).

The following are the steps for completing this procedure:

1. Evaluate and determine the location and angulation of the implant abutment screw (buccal-lingually and mesial-distally). An intraoral radiograph is often helpful.

2. With a round diamond bur (≈#8 round), access is made through the occlusal surface to remove the abutment screw (i.e., central fossa: posterior teeth and lingual aspect of crown in anterior teeth).

3. After the screw is located, the screw is engaged with the appropriate hex driver, reverse-torqued, and the screw is removed.

4. Discard old screw and place new screw.

5. Torque to the manufacturer specifications with ideal protocol.

6. Cover access hole with filler (polytetrafluoroethylene [PTFE]) and opaque composite.

In situations where the access hole is through the facial aspect of the prosthesis (i.e., anterior crowns), the crown will need to be removed and a new crown fabricated. Care should be exercised when cutting the crown off because in most cases it is difficult to determine the cement location (Fig. 16.19). This may result in sectioning the crown too deep causing damage to the abutment, abutment screw, or implant body (Fig. 16.20). A safer method includes the above technique (access with screw removal) with fabrication of a new prosthesis. If the abutment remains fixated to the prosthesis, the prosthesis can be easily removed by gently heating the prosthesis with a Bunsen burner.

Ultrasonic/cavitron Device.

If debris is present between the threads, an ultrasonic or cavitron device may be used. The vibration (≈20,000–30,000 rpm) will usually dislodge the debris, and the screw can then be removed via the explorer method.

Round Bur (205LN).

A very small round bur or 205LN can be used in a slow-speed handpiece or AS123 screwdriver. The tip of the bur is placed at the seam of the fractured screw and abutment (implant). As the bur spins clockwise, the friction placed on the screw makes it turn counterclockwise, and the screw unthreads (Fig. 16.23).

Inverted Cone Bur (≈ Bur).

With an inverted cone bur in a high-speed handpiece (ideally electric handpiece in reverse), gently touch the top of the screw. This will usually result in the screw being extruded from the implant body. Care should be exercised to not touch the implant body with the bur because this will result in damage to the implant body threads. With this technique, always use a throat pack to prevent loss (Fig. 16.24).

Slot the Top of Screw.

A slot 1 mm deep is made through the center of the screw with a high-speed handpiece and a very narrow fissure bur (or bur). A small screwdriver is then used to unthread the screw. Be careful using this technique because the bur may inadvertently perforate the side of the implant body. There is no predictable method to repair the implant body if this occurs. The patient should be informed that implant failure may result as a consequence of this technique (Fig. 16.25).

Manufactured Retrieval Instruments.

There are multiple retrieval kits on the market that are used to remove fractured screws. These are usually specific for the type of implant body type (internal, external, trilobe, etc.) (Fig. 16.26).