Abutment Screw Loosening Due to Dental Laboratory Error

The long-term success of a dental implant restoration is dependent on the permanence of its connection with the implant.Abutment screw loosening (ASL) is a frustrating complication for clinicians and patients. Literature reviews have reported the incidence of ASL to be between 7% and 11%.^1,2 Abutment micromotion and microleakage have been correlated with ASL.^3,4 Microleakage can permit contamination of the implant-abutment joint with lubricants such as saliva, blood, and a microbial extracellular matrix or slime layer.³ Abutment micromotion and microleakage are also concerns because they are considered risk factors for marginal bone loss and peri-implant disease.^5-7 A stable implant-abutment joint, therefore, is crucial.

ASL is not uncommon, yet comprehensive literature describing its exact mechanism is lacking.⁸ Factors that have been associated with ASL include the type of implant-abutment connection,^9,10 cement- versus screw-retained restoration,^11,12 screw material,¹³ occlusal forces,^14-16 and cantilevers.^8,17

There is also a scarcity of literature discussing the role of dental laboratory error as an etiology of ASL. Fabrication of custom abutments can be labor intensive and is heavily dependent on the skill of the laboratory technician. Poor workmanship that results in abutment imperfections can have detrimental effects on the stability of the implant-abutment joint. These defects potentially can lead to increased abutment micromotion and ASL.

The purpose of this article is to demonstrate and discuss how dental laboratory errors in fabrication of an implant restoration and mismanagement of the abutment surfaces can result in ASL. A method for examination of the abutment surface will be described.

Case 1: Improper Divestment

Custom castable abutments have been successfully used to restore implants for 30 years.^18-20 The premachined metal base (PMB) is manufactured with a highly polished connection surface.¹⁹ Its dimensions are designed to precisely fit the prosthetic platform of the implant (Figure 1 and Figure 2). Casting the alloy onto the PMB requires investing. During the devesting stage, the PMB must be protected from damage if an airborne-particle abrasion technique is used. Damage from airborne-particle abrasion can adversely affect the stability of the implant-abutment joint and ASL may result.^21,22

In Case 1, a 68-year-old male patient had an implant in site No. 30 (Tapered Plus, BioHorizons, biohorizons.com) that was restored with a screw-retained crown. An original internal hexed custom cast abutment (BioHorizons) was used in the fabrication of the crown. The implant had an internal hex connection with an internal tapered connection and a 3.5 mm prosthetic platform. The prosthetic screw was torqued to the manufacturer's recommendation of 30 Ncm. All excursive contacts were eliminated from the crown. Bilateral second-molar occlusion was present. No parafunctional habits were reported, and no excessive occlusal wear patterns were noted. After approximately 6 months, the patient reported that the crown had loosened, and it came off about 1 month later.

Close inspection of the crown revealed that the abutment hex had completely sheared off the base of the abutment (Figure 3). The retrieved fractured hex still had the fractured abutment screw in it (Figure 4). Photographic evaluation revealed several defects. First, the abutment surfaces surrounding the fractured hex connection that should have appeared polished were pitted. The fractured hex surfaces were also pitted. Presumably, this was the result of airborne-particle abrasion. Second, the beveled perimeter of the abutment was damaged. The scratch pattern suggests it was the result of a bur or disk. Lastly, ceramic material appeared to impinge on the beveled perimeter of the abutment base.

Case 2: Improper Polishing

The contact point of the abutment connection is designed to intimately engage the prosthetic platform of the implant. The abutment base must be protected during fabrication of the restoration. Imprecise polishing can result in damage to the abutment connection. Instability of the implant-abutment joint and ASL can be consequences of damage that occurred during the polishing process.²²

In Case 2, a 53-year-old male patient had an implant placed in site No. 19 (Tapered Plus) that was restored with a screw-retained crown. An original internal hexed custom cast abutment (BioHorizons) was used in the fabrication of the crown. The implant had an internal hex connection with an internal tapered connection and a 4.5 mm prosthetic platform. The prosthetic screw was torqued to the manufacturer's recommendation of 30 Ncm. All excursive contacts were eliminated from the crown. Bilateral first-molar occlusion existed. No parafunctional habits were reported, and no excessive occlusal wear patterns were noted. After approximately 1 year, the patient reported the crown had become loose.

When the loose crown was removed, it was found that the abutment hex had sheared off of the abutment base (Figure 5). The unfractured abutment screw was still in place. Photographs of the abutment connection were taken, and close inspection revealed damage to the beveled perimeter of the abutment base. It appeared that excessive polishing was the cause of this defect. The damaged perimeter bevel was found to be wider than the untouched beveled surface.

Case 3: Ceramic Glaze Residue

Abutment connections that are not protected during the restoration fabrication process are at risk of being encrusted with ceramic material. The presence of this material on the abutment connection can prevent complete seating of the abutment to the implant prosthetic table. As a result, the stability of the implant-abutment joint may be compromised.

In Case 3, a 75-year-old female patient had an implant in site No. 30 (Tapered Plus) that was restored with a screw-retained crown. An original internal hexed custom cast abutment (BioHorizons) was used in the fabrication of the crown (Figure 6). The implant had an internal hex connection with an internal tapered connection and a 4.5 mm prosthetic platform. The prosthetic screw was torqued to the manufacturer's recommendation of 30 Ncm. All excursive contacts were eliminated from the crown. Bilateral first-molar occlusion existed. No parafunctional habits were reported, and no excessive occlusal wear patterns were noted. After approximately 4 months, the patient reported the crown was loose.

The crown was removed and photographs of the abutment connection were taken. Close inspection revealed several problems. First, the abutment connection surfaces that should have appeared polished were pitted. This may have been the result of airborne-particle abrasion. Second, ceramic material covered the beveled perimeter of the abutment base. Exposed metal portions closer to the hex appeared to be excessively polished. This may have been a result of an attempt to remove the excess ceramic material.

It is doubtful that small flaws in the abutment connection can be detected with the naked eye or using surgical loupes. All the cases described in this article were documented by photographing them with adequate light. Multiple images of the abutment base were taken from several directions. These images were then magnified on a computer screen. This type of documentation and examination was necessary to identify the imperfections on the abutment surface.

When abutment screw loosening occurs, one of the first concerns should be whether the correct torque was applied to the abutment screw. Different implant systems require different abutment screw torque. Preload is the amount of clamping force established when the abutment screw is tightened.²² It is the clamping force that secures the abutment to the implant prosthetic platform. Inadequate preload has been shown to result in ASL and excessive preload is associated with screw fracture.²² Clinicians need to tighten the abutment screw to the manufacturer's recommended torque.⁸ In all of the cases presented in this article, abutment screws were torqued to manufacturer recommendations.

It is thought that dental implants may be more prone to occlusal forces than natural teeth due to their lack of a periodontal ligament. Thus, occlusal overloading is a concern for ASL. Occlusal forces on the molar regions are greater than on premolar and anterior teeth.²³ The forces of this load are concentrated in the area of the implant-abutment joint.²⁴ Especially in molar sites, it is not surprising that damaged or fractured abutment connections would occur with ASL.

Another question in these cases is the role of the type of implant restoration. A systematic review reported that 12.7% of single crowns experienced ASL compared to 5.6% of implant-supported bridges.²⁴ Studies also indicate that screw-retained single crowns have a higher incidence of technical complications and ASL than cement-retained single crowns.²⁵

The use of non-original components has been reported to be a potential etiology for implant-abutment misfit and ASL. Clinicians may choose to use a non-original component for several reasons, including lower cost. Several studies found that original abutments were superior to non-original abutments in mechanical outcomes, microleakage, and marginal accuracy.^26,27

In the three cases described, the potential roles of inadequate abutment screw torque, excessive occlusal forces, and restoration types were considered. If they played a role, it was thought to be secondary to the abutment surface defects discovered. Without the abutment defects, the likelihood of ASL would have been diminished.

Laboratory error must be part of the differential diagnosis list when attempting to determine the etiology of screw loosening. It is not feasible to photograph and examine a magnified image of every restoration prior to delivery. It is reasonable, however, to perform this level of examination with restorations that have experienced ASL. Without being able to adequately visualize the abutment, defects in the abutment connection may go undetected. Excluding the fractured hex in Case 1, none of the abutment defects described were detected without magnification on a computer. In all cases, the restorative dentists had years of experience and confidence in their dental laboratories. While the laboratories may ordinarily perform exemplary workmanship, variability in work performance is always possible. Inevitably, some restorations will occasionally be substandard. Photographic documentation of the abutment surfaces is useful in discussing the case with the dental laboratory technician and avoiding future problems.

Dental laboratory error in the fabrication of an implant restoration and mismanagement of abutment surfaces should be a consideration when abutment screw loosening occurs. Photographic documentation and evaluation of the abutment surfaces is useful in determining the etiology of the ASL.

Douglas H. Mahn, DDS
Private Practice limited to Periodontics and Implantology, Manassas, Virginia