DESIGN OF THE CONNECTING BAR
Elsyad MA et al. showed that a Hader bar (figure 2) provides overall more axial and non axial retention than a Dolder bar. These 2 types of bars offer poor retention in case of lateral forces (8). Dos Santos MBF. et al demonstrated that a round section bar exerted less tension on the « clips » and prosthetic screws than the Hader bar. However, the latter would exert less tension on the peri-implant tissues than a round section bar (9).
De la Rosa Castolo G. et al. compared connecting bars with different cross-sections : square, round, rectangular and L-shaped. The square-cut design would deliver the least amount of tension forces to the bone, implants and screws, as opposed to the rectangular cross-section which would deliver the most tension forces (6).
The greater the diameter of the bar, the less bone loss. The shorter the distal cantilever the less bone loss. Furthermore, in case of a distal cantilever, the stress transmitted to the bone is greater with 30°-tilted implants versus straight implants. However, there seems to be no significant difference between the cross-section design of the bars (round, square), and the stress exerted on the implants and on the bone (6). Weinländer M. et al. showed a significant difference between a machined bar and the prefabricated round section bar in terms of prosthetic maintenance (10) : connecting bars would need more maintenance and follow up. A CB-mucosa space of 1 or 2 mm allows better stress distribution on implants and peri-implant bone. Moreover, this minimum distance of 2 mm is recommended for the patient’s good oral hygiene maintenance (11). Joshi S. et al. showed that the higher the CB-mucosa distance, the greater the stress on the peri-implant bone. (12)
These studies were performed on 2-implant-retained prostheses. To our knowledge, no study has yet been conducted for 4 or more implants.