Biomechanics of implant abutment connection: A review

Headnote

Abstract

Modifications in implant placement at the initial surgery can pose significant rehabilitation problems at second-stage surgery, and the selection of an inappropriate abutment at this time can further complicate the situation. In spite of the significant evolution of a number of implant systems, implant design and features, those related to the mechanical behavior of implant-supported prostheses should be given utmost importance From an engineering point of view, the internal hexed implant with a Morse taper is definitely a better connection than an external hex implant that absolutely depends on the screw to hold the abutment in place. Morse taper connection has shown promising results in terms of stronger connection, better load transfer and reduced micro-movement.

Key words: Implant abutment connection, morse taper, abutment screw, preload.

Introduction

The development of titanium fixtures has brought several benefits for the rehabilitation of partial and complete edentulous patients. When biological and mechanical principles are respected, this treatment modality may successfully restore the functional and esthetic impairments cause-d by tooth loss.

The misfit at abutment-implant interface and the missing of a passive adaptation between the prosthesis and the abutment can lead prosthesis components, abutment screw or implants to fracture.1 Functionally, this misfit can cause overload at the abutment and distribute non-axial load along implant and marginal bone. The gap generated by this misfit can also be a trap for bacteria colonization, which might cause inflammatory reactions in the peri-implant soft tissues.2

Implant abutment interface

There are atleast 20 different implant/abutment interface variations on dental implants that are cleared for marketing byFDA(Figurel).

The implant abutment interface determines joint strength, stability andlateralandrotational stability.3

External hex connection

The advantages of external hex are its suitability for the two stage method, an anti-rotation mechanism, retrievability and compatibility among different systems.4

Possible disadvantages of the external hex are its micro- movements because of the size of the hex, higher center of rotation that leads to lower resistance for rotational and lateral movements and a micro gap leading to bone resorption.4

Thomas et al., have stated that implants featuring an external-hex, butt-joint interface have shown an incidence of abutment screw loosening of up to 38%.5 To overcome some of the inherent problems, solutions such as torque drivers, screw surface technology, platform size, andmate-rials have been investigated to obtain a set preload and greater clamping force.

Internal hex connection

One of the first internally hexed implants was designed with a 1.7-mm-deep hex below a 0.5-mm wide, 45 degree bevel. Its features were intended to distribute intraoral forces deeper within the implant to protect the retention screw from excess loading and to reduce the potential of microleakage. Internally connected implants also provide superior strength for the implant/abutment connection.3

Advocates of the internal hex connection have stated that, such configuration reduces vertical height from the implant platform to the top of the abutment, distributes lateral loading deep within the implant leading to a better-shielded abutment screw, engages long internal wall that creates a stiff, unified body to resist joint opening (micro-movement) and incorporates an audible and tactile "click-when the components are properly seated.6 This unique feature eases placement for the clinician and may reduce the need for radiographs following placement of the restorative components.

Morse taper connection

Since the introduction of the internal connection concept, further design enhancements have been made in an attempt to enhance the implant/abutment connection. Included in such efforts is the "Morse" taper, wherein a tapered abutment post is inserted into the non-threaded shaft of a dental implant with the same taper.3 Sutter and co-workers proposed an 8-degree internal taper connection, known as the Morse taper, between the implant and abutment.5

Advantages of Morse taper connection are its 8-degree taper with predictable vertical positioning and self-locking characteristics that dramatically enhances its ability to resist bending forces, abutment loosening, micro-movement andmicropump formation.5

When the tapered-interference fits are used, the abutment loosening seems to be less of a problem. "Precision-fit" internal morse taper design is such that an essentially "cold weld" is produced when the precision fit tapered abutment is seated to defined torque values

The tapered interference fit relies on the large contact pressure and resulting frictional resistance in the mating region of the implant abutment interface, to provide a secure connection (Table l).7

Abutment screw mechanics

The mechanics of the tapered interference fit, use two types ofconnectionmethods:7

(a) a screw and

(b) a tapered interference fit (also called Morse taper)

A common problem associated with the prosthetic application of dental implants is loosening and fracturing of screws that retain the abutment to the implant. Screw loosening occurs when the joint-separating forces acting on the screw joint are greater than the clamping forces holding the screw unit together.

Georgios et al., have stated that fracture of abutment screws is more prevalent than fracture of prosthesis-retaining screws.8"9

Joint clamping forces (preload) v/s joint seperating forces

Joint clamping forces or preload

McGlumphy et al., defined the screw joint as 2 parts tightened together by a screw, such as an abutment and implant being held together by a screw9 A screw is tightened by applying torque. The applied torque develops a force within the screw called the preload. As a screw is tightened, the screwelongates,producingtension.Elasticrecoveryofthe screw pulls the two parts together, creating a clamping force.10

Hence, established preload is directly proportional to applied torque. Too little torque leads to separation of the joint, screw loosening and failure of connection and too large torque results in stripping of screw threads and screw fatigue.

To be effective, the level of preload must be less than the elastic limit or proof load, i.e the maximum load at which no permanent deformation occurs. Depending on the stiffness of the screw material and whether or not the screw is being reused, preloads of 75% to 90% of the material elastic limit may be required to prevent loosening under moderate lateral loads.11

Joint Separating forces

Opposing the clamping force is a joint-separating force, which attempts to separate the screw joint. Screw loosening occurs when the joint-separating forces acting on the screw joint are greater than the clamping forces holding the screw unit together. Excessive forces cause slippage between threads of the screw and threads of the bore, resulting in a loss of preload.

Such intraoral separating forces may include, off-axis occlusal contacts, lateral excursive contacts, interproximal contacts between natural teeth and implant restorations, protrusive contacts, parafunctional forces and non-passive frameworks that attach to the implants (Table 2,3).12

Settling effect/ embedment relaxation

A significant mechanism that results in screw loosening of implant-supported restorations is the settling effect. No matter how carefully machined an implant surface is, it is slightly rough when viewed microscopically. Settling occurs as the rough spots flatten under load, since they are the only contacting surfaces when the initial tightening torque is applied.12

When the total settling effect is greater than the elastic elongation of the screw, the screw works loose because there are no longer contact forces to hold it in place.13

To reduce the settling effect, implant screws should be retightened 10 minutes after the initial torque application. This technique should be used as a routine clinical procedure.1012

Although loosening of screws is not a serious complication in itself, it is very inconvenient for the patient and the dentist (table 4).21 Usually simple tightening or replacing of the gold retaining or abutment screws is required, but occasionally, more extensive repair is indicated.1

Discussion

The success rate of osseointegrated implants is reported to be above 90%. Despite of the knowledge of the mechanisms of osseointegration, some faults still occur at implants systems. Recent researches assign those faults mainly to patient's prosthetic rehabilitation. The misfit at abutment-implant interface and the missing of a passive adaptation between the prosthesis and the abutment can lead prosthesis components, abutment screw or implants to fracture. New interface designs such as internal connection and Morse taper connection are now being applied on a variety of implants to improve the original external hex implant/ abutment interface. The goals of new designs are to improve connection stability throughout function and placement, and simplify the armamentarium necessary for the clinician to complete the restoration.

One of the most serious and prevalent problems associated with the restorative aspect of dental implants is loosening and fracturing of the screws that attach the prosthesis to the implant. An important mechanical factor that prevents abutment screw loosening and fracture is screw joint preload. Optimal preload for a screw is generated when the screw is elongated until it reaches but does not exceed its yield strength.

As supported by various studies, the internal hexed implant withaMorsetaperappearstobeabetterconnectionthanan external hex implants that absolutely depends on the screw to hold the abutment in place. As for screw loosening, the problem can be minimized by following manufacturers defined torque value, re-tightening the screw after 10 mins ofinitialtorqueandbyminimizingjointseparatingforces.

References

References

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Disclosure: The authors report no conflicts of interest.