The Morphology of Collected Dental Implant Prosthesis Screws Surface after Six Months to Twenty Years in Chewing

Research Article

J Dent & Oral Disord. 2017; 3(3): 1063.

# The Morphology of Collected Dental Implant Prosthesis Screws Surface after Six Months to Twenty Years in Chewing

Secatto FBS¹, Elias CN²*, Segundo AS³, Cosenza HB4, Cosenza FR5 and Guerra FLB6

¹Dentistry Association of São José do Rio Preto, R Roma, Fernandópolis, SP, Brazil

²Instituto Militar de Engenharia, Pr Gen Tiburcio, Rio de Janeiro/RJ, Brazil

³Dentistry School. Cuiaba University, R Professora Azélia Mamoré de Melo, Cuiabá, MT, Brazil

4Private Clinic, R Ondina, 286, 15015-205, SJ do Rio Preto, SP, Brazil

5APCD Presidente Prudente, R Ondina, 286, SJ do Rio Preto, SP, Brazil

6Private Clinic, Av Francisco Chagas Oliveira, SJ do Rio Preto, SP, Brazil

*Corresponding author: Elias CN, Materials Science Department, Military Institute of Engineering, Pr Gen Tiburcio, 80, Rio de Janeiro, RJ, Brazil

Received: May 10, 2017; Accepted: June 14, 2017; Published: June 21, 2017

## Abstract

There is no consensus among manufacturers and clinicians about the effects of long-term use and the loosening mechanisms of prosthetic retaining screws in implant-supported prostheses. The purpose of this work is study the state of Ti-6Al-4V prosthetic retaining screws collected from patients after long-term use. The morphology, deformation, galling, wear, cracks and surface defects of prosthetic retaining screws collected (n=14) from patients after longterm use (6 months to 20 years) were analyzed using a scanning electron microscope. The reasons for removing were screw loosening and patient pain (n 12) or screw fracture (n 2). SEM images showed that the screws had plastic deformation due to the tightening and oral loads. Loosening of the screws may be attributed to the loss of preload due to plastic deformation, corrosion, grooves from manufacture processing, adherence of organic material to the surface and cyclic loading. Loosening of the screws is not correlated with time of use, but with plastic deformation and other wear processes. It is not possible to predict for how long a prosthetic screw can maintain the preload. Loosening or fracture of the screw is unpredictable and depends on loading conditions, patient care and the periodicity of retightening.

Keywords: Prosthetic screws; Screw loosening, Screw tighten; Screw torque

## Introduction

Dental implants are intensively used in rehabilitation of partially or totally toothless patients. The great acceptance by professionals and patients is related to the high levels of success and advantages shown by the implants in contrast to other types of rehabilitation. The success of dental implants depends on several issues, such as the knowledge of risk factors and a better understanding of the component biomechanics. As Goiato et al. [1] report, the survival rates of dental implants according to the bone density were: type I, 97.6%; type II, 96.2%; type III, 96.5%; and type IV, 88.8%.

In order to increase the success rate, the implants manufacturers seek to develop prosthesis connections that provide better mechanical stability of the implant and the superstructure. There are two kinds of restoration supported by dental implant system with external hexagon or internal connections: screw-retained and cement-retained implant crown. The advantage of screwed prostheses is that the prosthesis can be easily replaced without damage to the implant.

During tightening of the prosthetic screw, a torque is applied to keep the parts connected and steady. The screw joint stability is function of the preload tension achieved in the screw. Preload is the technical term for the stress caused by tightening the screw that holds the prosthesis to the implant. It is important to understand that the mechanical stability of the implant-prosthesis system depends on the intensity of the preload applied to the screw, the shape of the screw threads and the coefficient of friction between the parts. A higher tightening torque leads to a higher preload, and a higher preload increase the compressive force, the clamping force and the friction between the screw threads and the prosthetic component [2]. The balance between the preload and occlusal forces determines the absence of movement between the implant and the abutment. The preload induces an elastic elongation of the screw and a recovery tendency of the elastic distortion. During the masticatory functions, the threads must stay under tension to keep the screw tightened. In some situations, the condition of the screw preload may be lost. For instance, axial loading can plastically deform and flatten the surfaces of the screw threads, changing the roughness and the friction coefficient between the screw threads and the implant [3]. Besides, the vibrations due to occlusal forces cause both bending of the screw and plastic deformation of the screw threads, leading to the loss of preload and loosening of the prosthetic screw [4].

In the preload, the tensions in the screw are proportional to the applied torque. Low torque do not guarantee a good coupling, but if the torque is excessive, it may cause plastic deformation of the screw threads, compromising the mechanical stability of the implantprosthesis system. Therefore, to assure a good coupling it is necessary that the screw be correctly stressed. The tractive force must be higher than the masticatory load and lower than the yield stress of the screw material. One of the issues related to oral rehabilitation is the loosening of the screw that retains the prosthesis over the implants. In the process of screw loosening, the application of outside forces, such as those associated with mastication, cause the reduction of preload and surface erosion of the coupling parts. The stability of the coupling parts depends on the maintenance of preload. The friction among the prosthesis screw thread with the prosthesis component screw threads, the implant platform, the surface finishing, the dimensional tolerances, the tight fitting between the prosthesis screw and the coping, the use of adequate tightening/untightening sequences, the correct design of the implant/abutment interface and the use of the right type of screw material are all factor that contribute to avoid screw loosening.

Literature showed that several issues contribute to prosthetic screw loosening [5], among them an inappropriate tightening torque, wide occlusal tables, inclined cusps, maladaptive components, side loads, and parafunctional habits. There are reports of implant failure in the literature by loss of fixation of the screw to the abutments [6]. This may result from component mismatch, low friction coefficient between the parts, and the use of an inadequate tightening speed [7- 9].

The prevention of loosening or fracture of the screw begins with passive prosthesis and well balanced occlusion. It is also important to consider the quantity of bone resorption around the implant, the length and number of implants, the opposite jaw, the implant angulation, and parafunctional habits. Nowadays, improvements in the materials used and research on the mechanism of the implantprosthesis coupling have reduced the incidence of screw loosening [10].

The large number of implants available in the market brings a multiplicity of choices. It is necessary that the professionals be aware of the products available. Each screw needs, individually, a different tightening torque, according to the shape and material of the screw threads and of the abutments. Increasing the torque increases the coupling force and the stability of the prosthesis-implant system, but if the torque is increased above a critical value (the yield stress of the screw material), plastic deformation of the screw threads will reduce the coupling force of the components. Therefore, coupling failure may result from excessive of insufficient torque. The recommended tightening torque depends on the mechanical properties of the material and the friction coefficient between the parts. The main area of occurrence on stress concentration is located in the coupling between the screw head and the screw rod. This point favors the appearance of cracks that lead to material fracture.

Some dental implant manufacturers suggest the use of a coated abutment screw to prevent the displacement of dental prostheses. Coating the abutment screws decreases the friction coefficient and increases the preload for a given tightening torque [11]. However, this results in a lower untightening resistance that may have adverse effects on the stability of the implant-abutment system. Under cyclic loading, Ti screws without coating are more stable than TiN, TiCN, Teflon and Parylene coated screws. The literature suggests that one must be aware of the magnitude of the untightening torque when specifying a certain coating/preload combination of screw size, coating and screw material [11].

One clinical issue frequently discussed by dentists is when the abutment or prosthesis screw must be changed or retightened. Unfortunately, the manufacturers do not provide this information. Professionals must decide how frequently the screw should be tightened or replaced based on clinical experience. The number of tightening that the screw can stand is also controverting. There are suggestions that the screw be retightened periodically [2], whereas other authors recommend that retightening should be avoided [12]. The purpose of this work is to analyze prosthetic screws used in dental implants to evaluate the morphology of the screw surface and relate the results to the time of use, which varied from six months to twenty years.

## Materials and Methods

Figure 1 shows relevant parts of prosthetic retaining screws in dental implant-supported prostheses structure and some terminology used in the present work. The structure of Figure 1 has five parts: dental implant, abutment screw, abutment, coping and prosthetic retaining screw.

The sample of this study consisted of fourteen Ti-6Al-4V prosthetic retaining screws (#5 in Figure 1) collected from patients after different times of use. It is commercially available Microunit abutment and EstheticCone abutment model. In present work, 13 screws prosthesis were removed from abutment pieces Multiunit® (Nobel Biocare - Sweden) and one was of the EsthetiCone® (Nobel Biocare - Sweden) type used for single tooth prosthesis. Among these screws, two belonged to prostheses that rehabilitate the maxilla and 12 belonged to the lower jaw; five were in abutments over cone Morse taper implants and nine over external hexagon implants. All implants had a platform size of 4.1 mm. The relevant characteristics of the screws (prostheses, connection, intermediary, place and time of use) are shown in Table 1.