Nowadays, two-piece implants with internal connection occupy a large space in the market of dental implant systems. Cover screw was designed to prevent growing of hard and soft tissues into the intra-implant cavity during bone healing process. The implant systems have different internal morphologies, and the cover screws differ from one another in shape and diameter. The primary objective of this study was to identify the possible risk factors of CSL, assuming that the implant site (maxilla/mandible), the thickness of gingiva and different implant designs might be associated with increased frequency of CSL. The results in the present study showed that the implants in the posterior region of the maxilla are more prone to CSL than those present in the posterior region of the mandible (p = 0.038). The location of the maxilla was shown to be a significant risk factor for early failure of dental implants(12), and more peri-implant MBL was observed in the maxilla.(13) Theoretically, cover screws higher than the implant platform are more susceptible to external forces. The diameter of a larger screw occupies increased area between the screw and the fixture, thus providing greater frictional retention force.(14) Also conical connection that is originated from the concept of Morse taper ensures a more reliable retention.(15) However, none of the factors related to cover screw configurations presented in this study showed association with higher frequency of CSL. It is more like a random low-probability event that has nothing to do with the design of the implant system.
But how could a cover screw become loose by itself under the cover of the gums? And this might be perhaps due to not completely tightened from the beginning because of the clinician’s improper operations. For example, the implant body might rotate along with the cover screw when the primary stability remains poor due to over-preparation of implant bed(16), so that the tightening torque is not enough to hold the screw joint together. The cover screw cannot be fully seated, and small particles such as bone graft materials get struck in the way accidentally, or it is inserted in a wrong direction, so that the threads are mismatched. Once the CSL occurs, then the peri-implant marginal bone resorption is greatly increased. Our results showed a statistically significant difference in MBL between the implants with CSL and normal implants (P < 0.001). Implants with CSL had a more severe MBL of 0.82 mm in just 3–4 months, which is a sign of worse prognosis from a clinical point of view. Physiological bone remodeling within the first year after implant placement is an adaptive process to surgical trauma and biological width establishment.(17) The etiologies of early MBL around dental implants have not been well understood yet. The risk factors associated with MBL could be mainly divided into mechanical factors and biological factors.(18)
In this study, increased early MBL related to CSL during bone healing of the canals can be explained from these two aspects. A microgap between implant and screw threads allows the passage of fluids containing bacteria and metabolic products to the implant cavity. With the presence of implant-screw microgap, additional micromovement under external forces causes thread slippage, eventually leading to screw loosening.(19, 20) Despite strict preoperative disinfection, oral cavity is still regarded as a bacterial environment. Bacterial contamination can be found in the intra-implant cavity both in the implants with healthy and diseased tissue conditions.(21) The microbial leakage that is associated with micromotion in two-piece implants might have a connection with significant inflammatory cell infiltration, leading to MBL.(22, 23) Simultaneously, the titanium debris and the particles that are discharged from the mechanical wear of implant-screw interface could leak out through the microgap and activate the immune responses, showing association of biomedical mediators with bone resorption.(24) Fibrous tissue encapsulation contributes to excessive micromotion(25), and might produce a combined effect on destruction of osseointegration. The granulation tissue formation was observed between the moving implants and new bone in a mouse model of implant micromotion system.(26) The presence of granulation tissue around the loose cover screws in our study is consistent with the results of their study.
The main limitations of this study were the inclusion of small number of patients. Many patients with pre-existing bone deficiency, periodontitis and smoking habits were excluded in order to eliminate other factors that affect the peri-implant MBL. Coupled with low incidence of CSL, only 90 patients were included in the two years. Also the retrospective study design makes it impossible to supplement some data and details that were not recorded. It is worth mentioning that the marginal bone resorption is a very complex process that is influenced by some host-related factors. Unfortunately, the bias caused by individual differences cannot be completely excluded. To further confirm the surgeon-related factors associated with CSL, a prospective clinical trial should be conducted.