Mandibular angle fractures represent 23–42% of all types of mandibular fractures , the most frequent causes being physical assault, motor vehicle accidents, falls and sports-related accidents . These injuries are currently treated by plate/screw osteosynthesis and, depending on the case, the bone segments are secured by one-miniplate fixation, two-miniplate fixation or by a single rigid plate. Owing to the biomechanics of this region, the treatment of fractures in the mandibular angle is associated with high complication rates .

Some studies have tried to establish an adequate protocol with the least possible morbidity for the treatment of mandibular fractures in the angle region and basically two types of osteosynthesis have been proposed . The first, known as rigid fixation, is based on the use of plates and large-diameter screws to provide sufficient rigidity and resist displacement of the bone fragments during mandibular function. The second, known as stable fixation, is based on Champy’s method, according to which mandibular osteosynthesis is achieved by malleable miniature plates (usually a single, non-compressive plate) fixed with monocortical screws via a buccal approach . In vitro studies and clinical trials have shown good results using miniplates according to Champy’s ideal osteosynthesis line principles for mandibular angle fracture treatment.

K roon et al. reported that one-miniplate osteosynthesis on the buccal side of the fracture or along the external oblique line is sufficient to withstand masticatory forces, but fixation in these regions did not resist lateral mandibular movements. They also observed that during movements simulating mastication, mainly in the molar region, there was a displacement of the mandibular base region (basilar line), which could not resist the forces with only one plate installed in the region of tension (alveolar process).

C hoi et al. and S chierle et al. have demonstrated that the installation of a second miniplate in the inferior border of the mandible could be more effective for fracture stabilization. E llis & W alker did not achieve satisfactory results using two non-compressive miniplates in the mandibular angle region. They argued that this procedure would require an extraoral or transcutaneous approach. Despite being adequate in specific situations, this treatment has the disadvantage of not stabilizing all mandibular movements associated with angle fractures.

Another disadvantage attributed to Champy’s mandibular osteosynthesis is the difficulty of a passive miniplate adaptation along the external oblique line, which, according to K roon et al. causes a tension effect that may lead to osteosynthesis failure or compromise its stability. A previous study of miniplate osteosynthesis of mandibular angle fractures using photoelastic models found that bone stress occurs exactly around the screws, which can loosen from the bone plate and compromise osteosynthesis. These effects are accentuated if the miniplates are not well adapted to the underlying bone .

The introduction of locking plate/screw reconstruction systems for the treatment of mandibular fractures has offered advantages over other plating systems. These plates function as internal fixators, achieving stability by locking the screw to the plate. A unique advantage of locking screw/plate systems is that it is unnecessary for the plate to have intimate contact with the underlying bone, making plate adaptation easier, preventing plate compression on the bone and minimizing bone traction against the plate .

A previous in vitro study demonstrated that the 2.0 mm locking plate/screw system provided greater fracture stability than conventional miniplates. These findings have stimulated the use of locking plate/screw systems in the treatment of facial fractures, especially in mandibular fractures, with good success rates . In vitro studies have shown, by evaluating the adaptation of straight plates to the fractured bovine ribs, that the number of screws per bone fragment may influence osteosynthesis stability. The effect of the number of screws when the miniplates are placed in tortuous regions and need to be bent, such as in the mandibular oblique line region, is yet to be clarified.

The purpose of this study was to evaluate in vitro the influence of the type of miniplate (conventional or locking) and the number of screws installed in the proximal and distal segments on the stability and resistance of Champy’s osteosynthesis in mandibular angle fractures.

Materials and methods

Sixty polyurethane hemimandibles (Synbone, model CHF 43.75, Malans, Switzerland) with bone-like consistency, having a medullar and a cortical portion, were randomly assigned to four groups ( n = 15). The hemimandibles were sectioned in the mandibular angle region guided by an acrylic resin template to simulate fracture. The bone segments were fixed by different osteosynthesis methods using 2.0 mm miniplates and 2.0 mm diameter × 6 mm long monocortical screws, forming the following groups: in group 1, two conventional screws were installed in each bone segment using a conventional straight miniplate ( Fig. 1 ); in group 2, two locking screws were installed in each bone segment using a locking straight miniplate ( Fig. 1 ); in group 3, three conventional screws were installed in the proximal segment and four conventional screws were installed in the distal segment, using a seven-hole conventional miniplate ( Fig. 1 ); in group 4, three locking screws were installed in the proximal segment and four locking screws were installed in the distal segment, using a seven-hole locking miniplate ( Fig. 1 ).

Hemimandibles representing the experimental groups prior to compressive strength test.

In all groups, in order to avoid poor adaption, the miniplates were first bent, adjusted and screwed to the hemimandibles. The screws were removed from the distal segment and a cut was made in the mandibular angle with a diamond disc to simulate a fracture, leaving 1 mm maximum space between the bone segments. The screws were replaced in the distal segment in the pre-drilled sites.

The polyurethane models were submitted to a compression test in a universal testing machine (Model 4202, Instron, Norwood, MA, USA) using a methodology similar to that designed by A rmstrong et al. , which simulates the forces applied by the masticatory muscles. The models were positioned in such a way to allow testing the resistance of the osteosynthesis methods during simulated mastication. The models were stabilized on an acrylic resin base developed for this study. The resistance produced by the mandibular condyle on the posterior region of the mandible during mastication was transmitted to the model (point A, Fig. 2 ). The load cell adapted to the testing machine simulated the resistance produced by the food bolus during mastication (point B, Fig. 2 ). The anterior shaft of the supporting base of the mandible simulated the resultant of the forces recorded during mastication (point C, Fig. 2 ). The resultants of the forces of the masseter, medial pterygoid, lateral pterygoid and temporal muscles were obtained from the compression applied on the occlusal region of the second molar.

Polyurethane mandible model fixed to the supporting base for the three-point biomechanical test. Point A, resistance produced by the mandibular condyle; point B, resistance produced by the food bolus; point C, resultant of the mastication forces. * Notches made on the base of the bone segments and surgical compass used to measure the vertical and horizontal displacement before the test.

Increasing compressive loads were applied on the models by the Instron machine until a 4 mm displacement was observed between the fractured segments either vertically or horizontally. The load at this moment was recorded. The displacements between the bone segments were measured close to the mandibular base region both vertically and horizontally, to obtain a 3D evaluation of the biomechanical test. For an accurate measurement of the 4 mm distance between the fractured bone segments, a notch was made with a bur ( Fig. 2 ) on the mandibular base region of each segment. The distance between these notches was measured prior to each test with a millimetre surgical compass. Four millimetre was added to this distance, that is, the compression test was considered completed when a 4 mm displacement was obtained between the segments ( Fig. 3 ).

Measuring the distance between the bone segments at the end of the test after a 4 mm vertical and horizontal displacement.

The means of compression loads in the four groups were analysed statistically by analysis of variance and Tukey’s multiple-comparison test. Significance level was set at 1%.