J Korean Assoc Oral Maxillofac Surg.  2016 Dec;42(6):352-357. 10.5125/jkaoms.2016.42.6.352.

Stability of the prosthetic screws of three types of craniofacial prostheses retention systems

  • 1Division of Oral and Maxillofacial Surgery, Department of Oral Diagnosis, Piracicaba Dental School, University of Campinas, Piracicaba, Brazil. edersiguaodont@gmail.com
  • 2Department of Oral and Maxillofacial Surgery, Universidad de La Frontera, Temuco, Chile.
  • 3Center for Biomedical Research, Universidad Autónoma de Chile, Temuco, Chile.


This study aimed to evaluate the stability of prosthetic screws from three types of craniofacial prostheses retention systems (bar-clip, ball/O-ring, and magnet) when submitted to mechanical cycling.
Twelve models of acrylic resin were used with implants placed 20 mm from each other and separated into three groups: (1) bar-clip (Sistema INP, São Paulo, Brazil), (2) ball/O-ring (Sistema INP), and (3) magnet (Metalmag, São Paulo, Brazil), with four samples in each group. Each sample underwent a mechanical cycling removal and insertion test (f=0.5 Hz) to determine the torque and the detorque values of the retention screws. A servo-hydraulic MTS machine (810-Flextest 40; MTS Systems, Eden Prairie, MN, USA) was used to perform the cycling with 2.5 mm and a displacement of 10 mm/s. The screws of the retention systems received an initial torque of 30 Ncm and the torque values required for loosening the screw values were obtained in three cycles (1,080, 2,160, and 3,240). The screws were retorqued to 30 Ncm before each new cycle.
The sample was composed of 24 screws grouped as follows: bar-clip (n=8), ball/O-ring (n=8), and magnet (n=8). There were significant differences between the groups, with greater detorque values observed in the ball/O-ring group when compared to the bar-clip and magnet groups for the first cycle. However, the detorque value was greater in the bar-clip group for the second cycle.
The results of this study indicate that all prosthetic screws will loosen slightly after an initial tightening torque, also the bar-clip retention system demonstrated greater loosening of the screws when compared with ball/O-ring and magnet retention systems.


Maxillofacial prosthesis; Prosthesis retention; Prosthesis failure; Torque

MeSH Terms

Maxillofacial Prosthesis
Prostheses and Implants*
Prosthesis Failure
Prosthesis Retention


  • Fig. 1 Prostheses retention systems. A, B. Bar-clip retention system. C, D. Ball/O-ring retention system. E, F. Magnets retention system.

  • Fig. 2 Detorque reading using a torque meter.


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