Implant prosthetics (Part 3) – Angulated screw channels for occlusal screw fixation

Implant prosthetics (Part 3) – Angulated screw channels for occlusal screw fixation

652
0
Implantatprothetik

In implant prosthetics, occlusally screw-retained restorations may reach functional and esthetic limits, for example, if screw channel openings are located in the buccal surfaces or between teeth. In edentulous fixed implant restorations, openings of screw channels may also occur in areas of the reconstructed gingiva.

Andreas Kunz, Dr. Insa Herklotz

In Parts 1 and 2 of the series of articles on implant prosthetics, we reported in detail on the decision factors of screw-retained and cemented implant-supported fixed dental prostheses. For occlusal screw-retained restorations, a key statement referred to an ideally positioned implant axis in order to optimally design the occlusal screw exit point in the tooth surface. The optimal position of the screw exit point is in the center of the tooth for posterior teeth and in the palatal region for anterior teeth (Fig. 1). Angulation of the screw channel – independent of the implant axis – allows new variations of screw-retained restorations (Fig. 2). Anuglated screw channels offer flexible treatment options for the best possible esthetic and functional results.

Fig.1: Occlusal openings of implant screws should not be located in functionally and esthetically relevant areas. To avoid fractures of the ceramic, it is important to ensure sufficient material thickness around the screw channel.
Implantatprothetik

Fig. 2: Angulated screw channels (ASC) can be created by changing the angle between the implant axis and the screw channel.

Implant prosthetics and angulated screw channels

Angulated screw channels should not be confused with angulated abutments. Here, different designations of system components of individual implant system providers can often be confusing. Angulated abutments have an angled portion. The screw channel is at the same angle as the implant axis. Another variant is angulated bridges or bar abutments, generally also described as intermediate abutments. This angulated abutment is currently popular for restorations according to the MALO CLINIC® protocol. The abutment consists of an angulated (often 17° or 30°) abutment onto which another abutment can be screwed (Fig. 3).

Implant prosthetics

Fig. 3: Variations of angulated abutments (from left to right): Angulated abutment, angulated bar/bridge abutment, titanium base for angulated screw channel with crown.

 

For abutments or crowns and bridges with angulated screw channels, the degree of angulation of the screw channel varies depending on the implant system. The angulation between the screw channel and the implant axis creates an angular bend. The basic prerequisite for angulation is a special screw and a screwdriver designed for this purpose (Fig. 4). The screw head and screwdriver with ball head must have special degrees of freedom for the angle of angulation to ensure a reliable force transmission of the required insertion torque (Fig. 5). In addition to the screw and screwdriver, the abutment (e.g. the titanium adhesive abutment) must have a special recess to adjust the angle of the screwdriver (Figs. 6 and 7). Numerous implant manufacturers offer solutions with different angulations for different implant systems.

Implant prosthetics

Fig. 4: Comparison of screwdrivers for angulated  or straight (standard) screw channels.

Implantatprothetik

Fig. 5: The screw head and the ball head of the screwdriver must be matched to each other. The screw has an inwardly lowered edge, which is important for the degrees of freedom of the ball head (screwdriver). The illustration on the right shows angulated system components from different suppliers.

An exemplarily selection of angulated screw channel solutions:

up to max. 30°, Straumann (Freiburg)

up to max. 30°, Ariston Dental (Athens, Greece)

up to max. 20°, Heraeus Kulzer (Hanau)

up to max. 25°, Medentika (Hügelsheim)

up to max. 25°, NobelBiocare (Kloten, Switzerland)

up to max. 25°, Dental Smart Solutions (Barcelona, Spain)

up to max. 30°, (Lleida, Spain)

up to max. 30°, Dentsply (Charlotte, USA)

Implant prosthetics

Fig. 6: The angle between the implant axis and the angulated screwdriver position can be varied.

Implantatprothetik

Fig. 7: The bend between the implant axis (screw axis) and the screw channel often does not allow insertion of the screw after bonding. The possibility of inserting the screw depends on the angular bend on the one hand and on the diameter of the screw in relation to the canal diameter on the other.

Angulated screw channels (implant prosthetics) in the literature

In the literature (Pubmed), the first articles on angulated screw channels can be found from 2012 (Park et al, Laudensack et al, Gjelvold et al) [1, 2, 3]. The advantages of angulating the screw channel are as explained earlier evident. Anitua et al and Friberg et al found no statistical difference in implant loss or bone resorption at the implant between angulated and straight screw channels [4, 5]. An important topic in the current literature is the force transmission of the screwdriver to the screw head in relation to the torque of the implant system. Hu et al demonstrated the influence of force transmission of the insertion torque on the screw at different angulations of screw channels at 0°, 10°, and 20° [6]. In an investigation of different abutment systems with different screw channel angulations, Swamidass et al showed no significant difference between the 0° and 20° groups of an implant system. However, torque differences were found for implant systems with lower insertion torque [7]. In a comparative investigation by Opler et al, the influence of the screw angulation was shown in relation to the torque loss of angulated (ASC=Angulated Screw Channel) and straight (SCR = Screw Retained Crowns) screw channels. This showed influence in terms of screw head torque at 0° to 15° showed no significant difference. However, a reduced torque was observed at a screw angulation of 25° and 28°.

In summary: changes in the angulation between the screw head and screwdriver can affect screw torque and final preload. Extreme angulations above 15° may affect screw performance and be clinically relevant in areas of high occlusal stress [8].

Practical application of angulated screw channels in implant prosthetics

The various investigations show how important it is to use the screw head screwdrivers validated by the manufacturer with matching abutments. If screw channels are angled without special systems, the transmission of force between the screw head and the screwdriver cannot be optimally guaranteed. In the anterior region, the exit point of the screw opening should be designed palatal with sufficient distance (> 1 mm) to the incisal edge. In the posterior region, the screw opening should be placed centrally in the occlusal surface. The lower the angulation of the screw channel, the better the force transmission of the screwdriver to the screw head (Figs. 8 and 9). If the angulation of the screw channel is severe, the original screw must be inserted before bonding with the titanium base. The bend between the implant axis (screw axis) and the screw channel often does not allow insertion of the screw after bonding. The insertion possibility of the screw depends on the angular bend on the one hand and on the diameter of the screw in relation to the canal diameter on the other. Unfortunately, not all implant systems yet offer special components for the angulation of screw channels. This constraint is a prosthetic limitation for the optimal positioning of occlusal screw openings.

Implant prosthetics

Fig. 8: If the screw channel is highly angled, the original screw must be inserted before bonding with the titanium base. The screw channel must be made of the framework material circularly (for all-ceramics: lithium silicate or ziconia).

Implantatprothetik

Fig. 9: Invisible … an angulated screw channel is often difficult to recognize after fabrication. Here, teamwork is required to ensure that the wrong screwdriver is not used when inserting the prosthetic restoration.

Literature:

[1] Park JI, Yoon TH. A three-dimensional image-superimposition CAD/CAM technique to record the position and angulation of the implant abutment screw access channel. J Prosthet Dent. 2013 Jan;109(1):57-60

[2] Lautensack J, Weber V, Wolfart S. Template to determine the position and angulation of the abutment screw channel for implant-supported, cement-retained restorations. J Prosthet Dent. 2012 Feb;107(2):134-6

[3] Gjelvold B, Sohrabi MM, Chrcanovic BR. Angled Screw Channel: An Alternative to Cemented Single-Implant Restorations–Three Clinical Examples. Int J Prosthodont. 2016 Jan-Feb;29(1):74-6

[4] Anitua E, Fernández-de-Retana S, Alkhraisat MH. Survival and Marginal Bone Loss of Dental Implants Supporting Cad-Cam Angled Channel Restorations: A Split-Mouth Retrospective Study. Eur J Dent. 2020 Mar;14(2):194-199. doi: 10.1055/s-0040-1709895. Epub 2020 May 24.

[5] Friberg B, Ahmadzai M. A prospective study on single tooth reconstructions using parallel walled implants with internal connection (NobelParallel CC) and abutments with angulated screw channels (ASC). Clin Implant Dent Relat Res. 2019 Apr;21(2):226-231.

[6] Hu E, Petrich A, Imamura G, Hamlin C. Effect of Screw Channel Angulation on Reverse Torque Values of Dental Implant Abutment Screws. J Prosthodont. 2019 Dec;28(9):969-972

[7] Swamidass RS, Kan JYK, Kattadiyil MT, Goodacre CJ, Lozada J. Abutment screw torque changes with straight and angled screw-access channels. J Prosthet Dent. 2021 Apr;125(4):675-681

[8] Opler R, Wadhwani C, Chung KH. The effect of screwdriver angle variation on the off-axis implant abutment system and hexalobular screw. J Prosthet Dent. 2020 Mar;123(3):524-528.