Enhancing Long-Term Success With Screw-Retained Implants

The long-term success and survival rates of dental implants are higher than 95%.¹ Despite these favorable rates, biological and mechanical complications are common. Early identification and correction of these complications can help ensure long-term survival and success of the prosthesis and implant. Compliance with peri-implant maintenance therapy allows for this early detection and effective management of complications.

Biological Complications

All implants — regardless of restoration type — are susceptible to the biological complication of peri-implant disease. Three proven risk factors have been noted for the development of peri-implant disease:

1. History of periodontitis
2. Plaque accumulation
3. Lack of compliance with supportive peri-implant maintenance.^2–4

Other risk factors with less conclusive evidence but clinically relevant include smoking, retained cement, overcontoured restorations, open contacts, and inadequate keratinized tissue and tissue thickness around an implant.^5-11

Peri-Implant Diseases

Peri-implant diseases are inflammatory conditions involving the peri-implant tissue caused by a plaque biofilm. The two types are peri-implant mucositis and peri-implantitis.

Peri-implant mucositis is the presence of inflammation confined to the peri-implant soft tissue with no signs of progressive bone loss following the initial bone remodeling.¹² Clinical signs of inflammation include bleeding on probing, erythema, edema and/​or suppuration, and an increase in probing depths.¹³ If peri-implant mucositis is left untreated, it will progress to peri-implantitis.

Peri-implantitis is when inflammation has progressed to the peri-implant soft tissue with progressive bone loss.^14,15 The criteria for peri-implantitis are probing depths ≥ 6 mm in the presence of inflammation with ≥ 3 mm of bone loss.

Currently, no standard therapeutic guidelines for treating peri-implantitis are available. As a result, preventing peri-implant disease is critical with peri-implant maintenance therapy serving as a vital component. Patients who are noncompliant with peri-implant maintenance therapy are three times more likely to develop peri-implantitis compared to compliant patients.¹⁶ Peri-implant maintenance therapy allows for early identification and management of complications.

Mechanical Complications

Excessive occlusal forces and parafunctional habits are etiologic factors that contribute to the common mechanical complications of screw loosening and loss of screw integrity.

Functional loading of an implant is concentrated at the abutment-implant level, which also transfers to the crestal bone.¹⁷ Proper occlusion on a dental implant is critical for avoiding overloading forces. Patients with parafunctional habits (eg, bruxism) are at greater risk of occlusal overload. Excessive occlusal forces have been associated with peri-implant bone loss, prosthetic complications (eg, porcelain fracture), and fracture of the implant body.^18,19

Consistent occlusal re-evaluation and periodic occlusal adjustments are necessary parts of peri-implant maintenance therapy. Numerous implant occlusal schemes have been proposed in the literature. Despite this, most clinicians would agree that light contact in maximum intercuspation (MIP) and no excursive contacts are ideal for controlling stresses placed on the implant and prosthetic components.

Screw Loosening and Loss of Screw Integrity

Screw loosening is a common complication associated with screw-retained restorations. The reported incidence varies within the literature with some finding the incidence as high as 60%.¹⁷ Screw loosening may cause irreversible damage to the prosthesis or fracture of the implant body due to unstable off-axis loading. Although screw loosening is mainly a mechanical complication, it can lead to the biological complication of microleakage between the implant and abutment, causing crestal bone loss.²⁰ Furthermore, the implant connection plays a large role in the frequency of screw loosening, with external-connection implants demonstrating a higher frequency than internal-connection implants.¹⁷

The incidence of screw loosening and loss of screw integrity (stripping and fracture) is higher among restorations requiring an angled abutment or angled screw channel. Furthermore, the occurrence of screw loosening and failure increases as abutment angulation increases.^21,22 This is because greater stress is applied to the prosthetic components of an angled abutment, compared to a straight abutment.²²

Angled screw channel abutments are a newer prosthetic design that allow for malposed implants to be restored with a screw-retained restoration that would otherwise need to be restored as a cement-retained restoration. The largest advantage to this approach is that it eliminates the biological risk of residual excess cement associated with cement-retained restorations.

Angled screw channel abutments are associated with a higher risk of screw-head stripping.²³ The abutment screw head is designed with a hexalobular shape that is engaged with a sphere-shaped screwdriver at various angles.²⁴ The benefit of this design is also the weak point of the design. Tightening of the abutment screw can be done at an angle off-center from the long-axis of the implant, which is advantageous, however, it is also the reason why there is a greater risk of screw-head stripping.²⁴

Tightening of a loose abutment screw can easily be done with a screw-retained restoration. It is always best to confirm that the abutment screw is intact prior to retightening and retorquing it. Furthermore, proper torquing of the abutment screw, which includes a preload torque and final torque, can minimize the incidence of screw loosening.¹⁷

Preloading is the initial tightening force placed on the abutment screw that results in screw stretching. The force applied secures the implant-abutment connection which reduces microleakage at that junction.²⁰

A few measures can be taken to minimize screw stripping such as using a manufacturer-specified driver and torque wrench to tighten the screw and verify the screw head is free of debris so that the driver can fully seat and engage the screw head. Removal of a stripped or fractured screw usually requires a screw removal kit. The process is typically very tedious and time consuming; therefore, preventing screw stripping and fracture is important.

Peri-Implant Maintenance Therapy

Peri-implant maintenance therapy includes assessment of prosthetic stability, occlusion, presence of interproximal contact, examination of peri-implant hard and soft tissues, professional debridement, and review of oral hygiene instructions.

Prosthetic stability is directly related to the tightness and integrity of the abutment and abutment screw. Verification of prosthesis stability can be done by applying lateral and vertical pressure to the restoration and confirming no movement is present. Confirmation of a protective occlusal scheme can be done using a piece of articulating paper and shimstock.

Evaluation of interproximal contact can be done using dental floss. Interproximal contact loss (ICL) between an implant restoration and an adjacent tooth is quite common, with a prevalence of ~50% and occurring most frequently on the mesial surface.^8,9,25 ICL may be caused by the natural mesial tooth migration compensating for wear, progressive growth of the dento-alveolar complex, and the location and size of the contact itself.²⁶ ICL leads to food impaction, which is associated with worsened periodontal parameters (higher plaque index, gingival index, and probing depths) and caries formation.^9,25 As a result, peri-implant disease is more common at sites with an open interproximal contact.

Clinical examination of the peri-implant soft tissues includes assessment of the amount and quality of tissue, presence of inflammation, and gentle probing. Several studies show that inadequate keratinized tissue and thin peri-implant tissue result in higher bleeding on probing, plaque accumulation, gingival recession, and possibly crestal bone loss over time.^10,11 Probing around implants is recommended at each maintenance appointment to determine if probing depths have increased. Early detection and treatment of peri-implant mucositis are important to prevent peri-implantitis.

Radiographic examination of peri-implant hard tissues includes a periapical and vertical bitewing taken annually. This aids with assessing whether progressive bone loss has occurred.

Nonsurgical debridement may include the use of ultrasonic devices, curets, air-abrasive devices, lasers, and local antimicrobial agents. Reviewing oral hygiene instruction with the patient is critical and should be done at each maintenance appointment.

Conclusion

Screw-retained restorations have the advantage of retrievability; however, mechanical complications are common and they are not immune to the biological complication of peri-implant disease. Early detection and correction of these complications directly affect the long-term success and survival of the restoration and implant.

References

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From Decisions in Dentistry. August/September 2024; 10(5):10; 13-14