Fixed Prosthodontics Complications

FIG 16.1 (A) When stress is plotted on the y-axis and cycles to failure are plotted on the x-axis, the fatigue curve of a material may be established. Any stress condition above the endurance limit will eventually cause fracture when enough cycles are applied. (B–C) Force-related fractured implant (i.e., cantilevered prosthesis and small diameter implant). 

Screw Loosening

Abutment screw loosening has been shown to be associated with an overall average of 6% of implant prostheses fabricated.3 Screw loosening is the most common implant prosthetic complication, accounting for approximately 33% of all postimplant prosthodontic complications.4 More recent studies indicate this complication occurs in approximately 8% of single crowns, 5% of multiple-unit fixed prostheses, and 3% of implant overdentures. De Boever has shown that 12% of prostheses exhibit loosening within 3 years,5 while Chaar has shown an incidence of 4.3% within 5 years and approximately 10% long term (5–10 years).6

Screw loosening may cause considerable complications. A loose screw may contribute to crestal bone loss because bacteria are able to colonize and harbor in the open interface. When an abutment screw becomes loose on a cemented crown, the crown may need to be cut off the abutment to gain access to the abutment screw, which results in patient disappointment and unproductive clinician time. If a loose abutment screw is not treated appropriately, fracture of the prosthesis, implant components, or the implant body may occur.

Etiology

External Force Factors (Box 16.1).

External forces that act on a screw joint greatly increase the risk of screw loosening. These forces may be called joint-separating forces when related to screw loosening; however, they are the same forces that are risk factors for implant failure, crestal bone loss, and component fracture. When the external joint-separating forces are greater than the force holding the screws together (called clamping forces), the screw will become loose. The external forces from parafunction, crown height, masticatory dynamics, position in the dental arch, and opposing dentition are factors that can dramatically increase the stress to the implant and the screw joint. In addition, conditions that magnify or increase these factors are cantilevers, angled loads, and poor occlusal designs.

 

Box 16.1

External Forces That Increase Screw Loosening

1. Parafunction

2. Crown height space

3. Masticatory musculature dynamics

4. Arch position (anterior, middle, posterior)

5. Opposing arch

6. Cantilevers

7. Angled loads

8. Poor occlusal designs

9. Lack of key implant positions

a. Canine

b. First molar

10. Inadequate implant number

a. No three adjacent pontics

11. Nonpassive prostheses

12. Inadequate Screw Torque

13. Excessive Screw Torque

14. Improper Prosthesis Insertion Technique

External forces applied to the joint system are important to account for when the aim is to decrease the incidence of screw loosening. The endurance limit of a material is the amount of force required to fracture the object when enough cycles are applied. The greater the force, the fewer cycles required before fracture occurs. It is the combination and relationship of both the amount of force and the number of cycles that is the cause of the screw loosening complication.

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

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FIG 16.2 (A) Excessive crown height space comparing an FP-3 to a FP-1, which leads to a vertical cantilever to any angle load. (B) Apical placement of implant results in greater force to the prosthesis and abutment screw leading to an increased incidence of screw loosening. (C) Poor implant positioning (e.g., implant placement too far posterior) leading to excessive resultant force from cantilever effect and greater stress on the screw system. Note the significant cusp height and opposing cusp concavity, which increases the shear component of force on the implant system. ([A] From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
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).

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FIG 16.3 (A) Nonpassive or improperly seated screw-retained restorations may be distorted when seated into position when the prosthetic screw is threaded. The distortion of the superstructure causes stresses that are concentrated at the crestal bone level and may result in bone loss. (B) Radiographic image depicting incomplete seating of abutment, which predisposes prosthesis to screw loosening. ([A] From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
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).

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FIG 16.4 (A) Insufficient torque applied to the screw leads to a greater incidence of screw loosening. If the screw is not torqued sufficiently or overtorque occurs, insufficient preload will result, which will most likely result in screw loosening. (B) The proper torque wrench and technique should be used according to the manufacturers’ specifications as implant systems have various recommended torque values. ([A] From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby. [B] Courtesy BioHorizons Implant Systems, Inc.)
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).

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FIG 16.5 Screw sizes vary according to manufacturers. In general, the larger the screw diameter, the greater preload that may be applied. Care should be exercised to not place too high of a torque as this will lead to screw deformation and screw loosening. 
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).

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FIG 16.6 Variation of abutment screw threads. (A) The more threads present on an abutment screw the less chance of screw loosening. (B) Material. Ideally, screw material should be titanium alloy, as gold alloy screws have a higher incidence of loosening and fracturing. (Courtesy Bio­Horizons Implant Systems, Inc.)
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).

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FIG 16.7 The rotational fit of the abutment-to-implant body is variable from one manufacturer to another. The greater the rotational misfit (red line and dotted line), the more force is applied to the abutment screw. (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)

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).

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FIG 16.8 (A) Plastic castable vs. metal coping abutments. The plastic castable abutments often lead to significant discrepancies in fit after casting. (B) Nonpassive casting resulting from incomplete seating of prosthesis. 
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.

TABLE 16.1

Failure Loads of Various Implant Types

Implant Type Static Failure Load (N)
1.0 mm external hexagon, 4 mm 966
1.0 mm external hexagon, 5 mm 1955
0.7 mm external hexagon 756
0.6 mm internal octagon 587
1.7 mm internal hexagon 814

From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.

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).

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FIG 16.9 The higher (or deeper) the antirotational hexagon component (x component on the graph), the less the force applied to the abutment screw (Fs) on the y-axis. A 0.7-mm hexagon height is standard in the industry and was used first by Nobel Biocare. A 1-mm hexagon height has less risk of screw loosening because the force on the screw is decreased. (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)

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

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FIG 16.10 To reduce forces on the abutment screw, the platform diameter of the implant is more important than the hexagon height. The larger the diameter (x-axis), the less the force applied to the screw (y-axis). (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
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).

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FIG 16.11 When a screw-retained restoration (yellow) is fixated in place, compressive, tensile, and shear forces are present with the screw components (blue). (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
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FIG 16.12 When the prosthesis is not passive (yellow), the compressive tensile and shear forces are directly applied to the implant system (i.e., Red Arrows: abutment, implant body, abutment screw, marginal bone, implant-bone interface). (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
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FIG 16.13 An improper fit between the abutment and the implant will lead to stresses that are applied throughout the implant concentrated at the crestal bone level, which may cause bone loss. In this example, a 50-µm misfit may force the implant body to move approximately 200 µm at the apex. (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
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

Prevention

Decreased Force.

Because of the directional relationship between force and screw loosening, the evaluation, diagnosis, and modification of treatment plans related to stress conditions are of considerable importance. After the clinician has identified the source of excessive force on the implant system, the treatment plan is altered in an attempt to minimize the negative impact on the longevity of the implant, bone, and final restoration.

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:

1. Light finger tighten with driver (~10 N/cm)

2. Maximum finger tighten with driver (~20 N/cm)

3. Implant screw should be torqued to the manufacturer specifications

4. After 5–10 minutes, the screw should be retorqued to the same manufacturer specifications.

Note: For cases of expected increased force, the implant screw may be retorqued a third time after 30–60 days.

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).

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FIG 16.14 Screw tightening sequence. (A) Incorrect tightening sequence for bar, leading to uneven distribution of stress to the screw system. (B) Correct sequence, which should be completed with light finger tightening, then maximum finger tightening, and lastly final torque wrench. (C) Incorrect tightening sequence for a fixed prosthesis. (D) Correct sequence, which should be completed with light finger tightening, then maximum finger tightening, and lastly final torque wrench. 
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

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FIG 16.15 (A) Final ideal torque technique (after hand tightening). Phase 1 final torque, then, after 5–10 minutes, phase 2 final torque. (B) Settling effect of the abutment screw occurs after the initial preload. A final torque is needed to maintain the initial preload. (Courtesy BioHorizons Implant Systems, Inc.)
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

Treatment

When confronted with a mobile prosthesis, it is important to determine if the mobility is a result of screw loosening or the actual implant being mobile (implant failure). Box 16.2 shows a technique to determine the etiology of the prosthesis movement (Fig. 16.16).

 

Box 16.2

Technique to Determine the Etiology of the Prosthesis Movement (see Fig. 16.16)

Step 1: Movement of crown. With the use of two mirror handles placed on the buccal and lingual aspect of the crown, a buccal-lingual force is applied to evaluate the mobility of the prosthesis. This will result in a subjective response from the patient experiencing either pain or no pain.

No pain: if no pain is present, then it is most likely the abutment screw is loose.

Pain: if pain is present, it may be the result of either the abutment loosening or implant failure. With respect to the abutment screw, the pain will originate from the movement of the crown, most likely resulting from tissue impingement. In some situations, this can be verified by a radiograph showing incomplete seating of the prosthesis with the abutment. If the pain originates from the implant, this will usually indicate failure of the implant.

Step 2: Palpation of buccal/lingual cortical plates. A second test is to palpate the buccal and lingual cortical plates over the implant body. If no pain is present, then it is most likely the abutment. If pain is present, it is usually a sign the implant has failed.

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FIG 16.16 (A–B) Checking buccal-lingual mobility of prosthesis with a mirror handle. (C–D) Palpation of buccal and lingual cortical plates to assess the presence of pain. 
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).

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FIG 16.17 Implant body movement. When buccal-lingual movement of the implant body is present, usually radiographic signs will be present confirming implant failure (i.e., circumferential radiolucency). 
Abutment Screw Movement
Option 1.

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).

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FIG 16.18 Technique for removing a screw. (A) Occlusal opening to obtain access to abutment screw with #8 diamond round bur. (B) Hex driver engages abutment screw. (C) Reverse torque removes the screw. (D) Implant-retained crown is removed along with screw. (E) Implant crown with abutment retained within the crown. (F) A new screw should always replace the old screw. The old screw is most likely deformed and will have a high incidence of chronic loosening. Screw is discarded; new screw is used to fixate implant crown. (G) PTFE tape is used to protect the screw head. (H) Opaque composite used to occlude access hole. 

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.

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FIG 16.19 Screw access. (A) Ideally, the screw will be able to be removed without compromising esthetics (i.e., anterior teeth—lingual, posterior teeth—central fossa). (B) Anterior crown where screw has to be removed via the facial surface, resulting in the need to remake crown. 
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FIG 16.20 (A–B) Ideally, the crown should not be sectioned or “cut-off.” This often leads to trauma to the abutment screw because of the very thin cement margin. 

Screw Fracture

Etiology

The etiologic factor most likely to cause screw fractures is biomechanical stress to the implant system. The biomechanical stress leads to partially unretained restorations or fatigue, which is directly related to an increased amount of force. Prosthesis screw fracture has been shown to occur with a mean incidence of 4% with a range of 0% to 19%. Abutment screw fracture is directly related to the screw diameter, with larger-diameter screws fracturing less often, with a mean incidence of 2% and a range of 0.2% to 8%3 (Fig. 16.21).

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FIG 16.21 Abutment screw fractures. (A) Fractured screw; note the large facial overcontouring resulting in a shear load to the screw. (B) Implant with screw fractured; space can be identified between the screw and the threads of the implant body, which result from loss of preload. (C) Prosthesis fracture leading to multiple fractured screws. (D) In some instances, the screw will fracture with the retained preload, which makes removal very difficult. 

The etiology of abutment screw fracture is the same for screw loosening (see above).

Prevention

Immediate Treatment of Loose Screw.

If an abutment screw is determined to be mobile, immediate treatment is recommended. The longer the time period that force is applied to a mobile prosthesis, the greater the chance the abutment screw will be deformed and possibly fracture. The loose screw follows a fatigue curve that is related to the number of cycles and the intensity of the repeated forces.

Treatment

Explorer.

The easiest method to remove a screw is to rotate the screw counterclockwise with a sharp explorer tip. Because a loose screw has no preload, the fractured component remains passive in the implant body. If the screw has been deformed or debris has been introduced between the screw and implant body, this technique may not be successful (Fig. 16.22).

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FIG 16.22 Explorer technique to remove screw. (A) Screw has lost its preload, can easily be unscrewed (counterclockwise). (B) Screw removal. 
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).

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FIG 16.23 Round bur (205LN). (A) A fractured abutment screw in a bruxing patient. (B) A very small round bur is used in a slow handpiece. (C) The bur is positioned in the seam of the screw and implant and as it rotates clockwise, friction unthreads the screw. (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
Inverted Cone Bur (≈image 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).

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FIG 16.24 Inverted cone (image carbide bur) with handpiece. (A) Screw fractures off within the abutment for screw. (B) image inverted cone. (C) The fractured screw is lightly tapped in the center of the screw, care must be exercised to not touch the internal walls/threads of the implant. 
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 image 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).

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FIG 16.25 (A) Slot the top of screw. A slot is made in the screw with a high-speed handpiece and narrow fissure bar. A screwdriver is then used to unthread the screw. (B) Use of an ultrasonic scaler to loosen debris between the screw and the internal threads of the implant. (From Misch CE: Dental implant prosthetics, ed 2, St Louis, 2015, Mosby.)
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).

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FIG 16.26 Salvin Implant Rescue Kit. (A) Place guide on implant and hold with stabilizing handle. Insert drill into implant handpiece. Set motor to REVERSE at 1000–1250RPM and 50–70 N-cm torque. (B) Drill in REVERSE using “up and down” motion to prepare 1- to 2-mm deep dimple into top of broken screw. (C) Insert tap into implant handpiece set motor to REVERSE at 70–80 rpm and 50–70 N-cm torque. (D) Insert tap into the 1- to 2-mm dimple in the top of the broken screw. Use the tap in REVERSE to remove broken screw. (E) Screw removed. (Courtesy Salvin Dental Specialties, Inc., Charlotte, NC.)
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Jan 12, 2018 | Posted by in Implantology | Comments Off on Fixed Prosthodontics Complications
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