Abstract
A microlocking implant prosthetic system has recently been developed to address the limitations of conventional screw- and cement-retained implant-supported fixed dental prostheses. This prosthesis system consists of a precision-machined abutment and an attachment that includes zirconia balls and a nickel-titanium spring, thus providing retrievability and constant retention of the prosthesis. In addition, screw-related complications are avoided because there is no retention screw. The occlusal access hole is of a smaller diameter than that of conventional screw-retained prostheses, which is beneficial for esthetics and occlusion. It also prevents common complications of cement-retained prostheses because residual cement around the prosthesis can be removed extraorally. This article presents a clinical treatment with this new prosthetic system.
Since the introduction of osseointegration, longitudinal studies have demonstrated the long-term success of osseointegrated implants. Implants have been reported to treat patients with partial edentulism successfully, and implants and implant-supported restorations have high success and survival rates. Implant-supported fixed dental prostheses (FDPs) may be retained either using a screw or cement.
Screw-retained FDPs have advantages including predictable retrievability, a minimal amount of interocclusal space needed, easy hygiene management, and easy removal upon repair or surgical intervention. However, screw-retained FDPs may be contraindicated and may require increased production time and cost. In addition, screw-retained FDPs require a high degree of accuracy to achieve a passive fit, and residual stress from screw tightening can occur between the multiunit FDPs and the implant if passive fit is not achieved. The most common complications to occur with screw-retained prostheses are prosthetic screw loosening, fracture, and prosthesis breakage.
Cement-retained FDPs also have advantages including compensation for inappropriately inclined implants, easier achievement of passive fit, the presence of an intact occlusal surface due to the absence of an occlusal access hole, and easier control of occlusion. However, cement-retained FDPs can be associated with retained cement, which may precipitate peri-implant diseases such as peri-implant mucositis and peri-implantitis. As early as 1997, Agar et al reported that an abutment may be scratched during the removal of excess cement from subgingival margins and that cement removal may be incomplete. Other authors have reported that the amount of undiscovered cement increases with margin depth.
To solve these complications, a microlocking implant prosthetic system has been developed to combine the advantages of predictable retrievability, good occlusion, and esthetics without complications associated with residual cement. Microlocking implant prosthetic systems consist of an assembly-type attachment and a precision-machined abutment to accommodate this attachment (EZ crown; Samwon DMP Co) ( Fig. 1 ). Retention of the microlocking implant prosthetic system is controlled by a ball and spring, which are the subcomponents of the attachment, and the retention groove in the abutment cylinder. The ball is composed of zirconia (ZrO 2 ), which has excellent biocompatibility, flexural strength, fracture strength, and wear resistance, and is a perfect sphere, with a 0.7-mm diameter. The springs located outside the zirconia balls are made of a nickel-titanium alloy. Nickel-titanium alloys can be subdivided into 3 types: nonsuperelastic or martensitic-stabilized, austenitic-active, and martensitic-active. The springs used in this system are martensitic-stabilized alloys, similar to the Nitinol (Unitek) product. Unlike austenitic-active and martensitic-active alloys, which exhibit a superelasticity effect or shape memory effects due to phase transformation caused by induced stress or temperature changes, martensitic-stabilized alloys do not undergo phase transformation because of their stable martensite structure. Thus, they do not have a superelasticity effect or shape memory effects but have a large working range and low modulus of elasticity. They are also much more elastic than other alloys such as stainless steel, cobalt-chromium, or beta-titanium, thus exhibiting excellent spring-back capacity.
Therefore, after the attachment is engaged in the abutment, the spring is able to provide a continuous and constant force on the ball, which is seated in the retention groove of the abutment. The stability of the microlocking implant prosthetic system is controlled by the abutment shape. The abutment cylinder has a height of 3.2 mm, which can indirectly resist lateral forces, and the upper hexagonal structure in the cylinder directly prevents rotation of the attachment. Also, the cylinder has a tapered external hexagon connection with a 25-degree convergence angle and compensates for the angle of the implant even when the implant is not positioned parallel. To fabricate the restoration, the overall protocol is similar to that of the combination screw–retained and cement-retained prostheses, but an occlusal access hole is not necessary in this new prosthetic system. If an occlusal access hole is formed according to the operator’s preference, the prosthesis can be easily removed using a dedicated removal driver.
This clinical report describes the restoration of an implant-supported single crown by using a microlocking implant prosthetic system.
Clinical Report
This treatment was conducted in patients who had a single osseointegrated implant in the posterior region and who agreed to a clinical feasibility study with the approval of the Pusan National University Dental Hospital Institutional Review Board (IRB no.: PNUDH-2017-035-MD).
Before impression making, the abutment was tightened to 35 Ncm according to the manufacturer’s protocol ( Fig. 2 ). Then, the attachment was inserted to the abutment by using a dedicated tool ( Fig. 3 ). Impressions were made with the closed mouth technique by using a silicone impression material (Imprint II VPS Impression Material; 3M ESPE).
A zirconia single crown fabricated with a computer-aided design and computer-aided manufacturing (CAD-CAM) system (exocad DentalCAD; exocad GmbH/Trione Z; DIO) was placed seated on the attachment. Interproximal contact areas, marginal fit, esthetics, and accessibility for oral hygiene were evaluated. The hole at the top of the attachment was sealed with Teflon tape, and the crown was cemented using self-adhesive resin luting cement (G-CEM LinkAce; GC America). Excess cement from the occlusal access hole was cleaned, and the crown was removed by using a removal driver ( Fig. 4 ). After removing excess cement around the cervical margin of the crown ( Fig. 5 ), the crown was reinserted on the abutment. The occlusal access hole was filled with a flowable composite resin (Filtek Z350 XT; 3M ESPE) ( Fig. 6 ).
