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Principles of Attachment Selection

Hamid Shafie

One of the most confusing issues for dentists is choosing the appropriate attachment assembly for implant overdenture cases. They usually ask themselves many questions when it comes to selecting the right attachment assembly. First, which attachment should one use? Would a bar or stud attachments be best? Depending on the answers to those questions, more considerations follow. For instance, which bar or stud would be best for this particular case?

Learning about the mechanical properties and the load distribution characteristics of different attachments is the easiest way to determine which one to use. Most available attachments demonstrate different levels of resiliency. Attachment resiliency is associated with the movement between the abutment and the prosthesis in a predetermined direction or directions. The more directions or planes in which the prosthesis can move, the less stress is placed on the implant, in turn transferring more forces to the residual ridge. That being said, the attachment is more resilient.

Various Movements Allowed by Resilient Attachments

• Vertical Movement: The prosthesis is allowed to move bodily toward the tissue. This type of movement results in even loading and support from the entire anterior-posterior length of the residual ridge. Typically, movement is stopped by the supporting structure of the residual ridge, meaning as soon as the prosthesis comes into contact with the residual ridge and passes the resiliency of the soft tissue, it stops.
• Hinge Movement: Hinge movement is that in which the prosthesis revolves around an axis that has been formed by the most posterior attachments on each side of the arch.
• Rotation Movement: Rotation movement allows the prosthesis to rotate around an axis that runs anterior-posteriorly. Anytime masticatory forces are applied to one side of the prosthesis, it rotates around the crest of the ridge, and the opposite side rotates up and across the arch.
• Translation and Spinning or Fishtailing: In this type of movement, the prosthesis moves in an anterior-posterior movement, or a bucco-lingual direction, without any rotation. The prosthesis, in turn, revolves around a vertical axis.
• Combination of the Above Movements

TYPES OF ATTACHMENTS BASED ON RESILIENCY

Rigid Non-Resilient Attachments

No movement occurs between the abutment and the implant. When utilizing a rigid non-resilient attachment assembly, the implant receives 100 percent of the chewing forces, providing no relief to the supporting implants.

This type of attachment is recommended when a sufficient number of implants are available. A screw-retained hybrid overdenture is an example of a rigid non-resilient attachment.

Restricted Vertical Resilient Attachments

This type of attachment provides 5–10 percent load relief to the supporting implants, and the prosthesis can move up and down with no lateral, tipping, or rotary movement. In other words, the attachment resists any lateral tipping or rotary movements.

Hinge Resilient Attachments

This type of attachment resists any lateral tipping, rotational, and skidding forces. Hinge resilient attachments provide almost 30–35 percent load relief to the supporting implant. Each time one utilizes an attachment that provides hinge resiliency, the vertical components of the masticatory forces are shared between the attachments and the posterior portions of the residual ridge—the buccal shelf and retro molar pad. A Hader bar or any other kind of round bar can provide hinge resiliency. (Refer to Figures 6.23 through 6.27.)

Combination Resilient Attachments

Attachments of this type allow unrestricted vertical and hinge movements. This attachment uniformly transfers the vertical component of masticatory forces to the entire length of the residual ridge. Anytime we utilize this type of attachment, we increase the tissue support of the prosthesis during mastication. No matter where the masticatory load is applied to the overdenture, the ridge receives the vertical component of the forces. This type of attachment offers 45–55 percent load relief to the supporting implants. The Dolder bar joint (egg shaped) is a combination resilient attachment (Figure 6.30).

Rotary Resilient Attachments

This type of attachment provides vertical hinge and rotation movements. We utilize these attachments so that the prosthesis can move vertically and hinge-wise and rotate around the sagittal plane. Rotary resilient attachments transfer both the vertical and horizontal components of masticatory forces to the residual ridge. Movements of the prosthesis are determined by the location, direction, and magnitude of the forces that have been applied to the prosthesis. Usually this type of attachment provides 75–85 percent load relief to the supporting implants. Some of the stud attachments (prefabricated individual attachments) provide rotary resiliency. (Refer to Chapter 5.)

Universal Resilient Attachments

These attachments provide vertical, hinge, translation, and rotation movements. Basically, you see all types of movement; the attachment provides resistance only to movements away from the tissue. This type of attachment offers 95 percent load relief to the supporting implants. Magnetic attachments are the best example of the universal resilient attachments.

ATTACHMENT SELECTION CRITERIA

• Available bone
• Patient’s prosthetic expectations
• Financial ability of the patient to cover treatment costs
• Personal choice and clinical expertise of the dentist
• Experience and technical knowledge of the lab technicians

Patients with advanced resorption of the alveolar ridge are good candidates for bar or telescopic attachment assemblies. These attachments offer a considerable amount of horizontal stability.

Patients with minimum alveolar ridge resorption are good candidates for studs or magnetic attachments assemblies. Magnets provide the least amount of retention compared to the other attachments, and they lose their initial retention capacity very soon. Studs are ideal for patients with a narrow ridge, because in these cases the bar would interfere with the tongue space.

DIFFERENT ATTACHMENT ASSEMBLIES

• Clips and bars
• Studs
• Magnets
• Telescopic copings (rigid or non-rigid)

Rigid telescopic copings transfer most of the masticatory forces to the supporting implants. This increases the risk for implant fatigue and eventual fracture of the implant or its components. Rigid or minimally resilient attachment assemblies transfer the minimum load to the posterior alveolar ridge; therefore, the patient experiences the least alveolar bone resorption.

FACTORS INFLUENCING THE DESIGN AND RESILIENCY LEVEL OF THE ATTACHMENT ASSEMBLY

• Shape of the arch
• Distribution of the implants in the arch
• Length of the implants and degree of implant bone interface
• Distance between the most anterior and the most posterior implants

BIOMECHANICAL CONSIDERATIONS

One hypothesis suggested that the bar connecting the implants should be parallel to the hinge axis; this rule was followed by many clinicians, but no studies have supported this claim. One long-term study (5–15 years) analyzed the influence of placing the bar parallel to the hinge axis on peri-implant parameters, including the clinical attachment level. The outcome of the type of retention, splinted versus unsplinted, was also assessed. No significant correlations were found. (Refer to Chapter 6.)

DISTAL EXTENSION TO THE BAR

Distal extensions provide a high level of stability against lateral forces, particularly in the mandible, and may protect the susceptible denture-bearing tissue from load forces. They should not extend beyond the positio/>

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