Change in Pull-Out Force during Resorption of Magnesium Compression Screws for Osteosynthesis of Mandibular Condylar Fractures

Abstract

Background: Magnesium has been used as degradable fixation material for osteosynthesis, but it seems that mechanical strength is still a current issue in these fixations. The aim of this study was to evaluate the axial pull-out force of compression headless screws made of magnesium alloy during their resorption.

Methods: The tests included screws made for osteosynthesis of the mandible head: 2.2 mm diameter magnesium alloy MgYREZr (42 screws) and 2.5 mm diameter polylactic-co-glycolic acid (PLGA) (42 pieces, control). The screws were resorbed in Sørensen's buffer for 2, 4, 8, 12, and 16 weeks, and force was measured as the screw was pulled out from the polyurethane block.

Results: The force needed to pull the screw out was significantly higher for MgYREZr screws than for PLGA ones (p < 0.01). Within eight weeks, the pull-out force for MgYREZr significantly decreased to one third of its initial value (p < 0.01). The dynamics of this decrease were greater than those of the pull-out force for PLGA screws (p < 0.05). After these eight weeks, the values for metal and polymer screws equalized. It seems that the described reduction of force requires taking into account when using magnesium screws. This will provide more stable resorbable metallic osteosynthesis.

Keywords: condylar head fracture; fixation material; magnesium; mandible; polyglycolic acid; polylactide; pull-out force; screw.

Figure 1

Appearance of samples used in the resorption experiment: magnesium alloy screws on the left-hand and poly(lactic-co-glycolic acid) screw on the right-hand side. All screws had a length of 14 mm. Their length did not change during the experiment. The photographs were not scaled; hence the different lengths in the picture. Asterisks indicate statistical significance in the pull-out force.

Figure 2

Comparison of axial pull-out force of magnesium alloy screw (blue) versus control: poly(lactic-co-glycolic acid) screw (green). The confidence intervals (brackets), mean values (points), and the statistical difference between two groups for each time point (asterisks) were determined.

Figure 3

Mandibular head fracture fixation. The phenomenon of reduced mandibular ramus height appeared (asterisk). (A) Fracture site. (B) Osteosynthesis by two magnesium 1.7 mm × 14 mm screws. (C) Pre-op scan—the mandibular head is dislocated downward which shortens the mandibular ramus. (D) Fixation by compressive headless screws; visible gas bubbles are the air introduced into the wound during open reduction; height of mandible head is marked by an asterisk. (E) 6-month post-op follow-up—fixed bone remodeling, remnants of the produced hydrogen gas (arrow), shortening of the mandibular ramus as a result of the proximal fragment (mandible head) down-shifting along the fissure of the fracture (asterisk).