Abstract
The aim of the present study was to evaluate the effects of horizontally favourable and unfavourable mandibular fracture patterns on the fixation stability of titanium plates and screws by simulating chewing forces. Favourable and unfavourable mandibular fractures on 22 sheep hemimandibles were fixed with 4-hole straight titanium plates and 2.0 mm × 7 mm titanium screws according to the Champy technique. Hemimandibles were mounted with a fixation device in a servohydraulic testing unit for compressive testing. Displacement values under 20, 60, 100, 120, 150, 200 N, maximum displacements, and maximum forces the model could resist before breakage were recorded and compared. The authors found no statistically significant differences between the groups for the displacement values in the force range 60–200 N (60, 100, 120, 150 and 200 N). Statistically significant differences for maximum displacement values (displacement values at the breaking forces) between the groups were found ( P < 0.05). There was no evidence for the need to apply different treatment modalities to mandibular fractures regardless of whether the factures are favourable or not.
Fractures of the mandibular angle deserve particular attention because they represent the highest percentage of mandibular fractures, and have the highest postsurgical complication rate, making them the most challenging and unpredictable mandibular fractures to treat. Despite the evolution in the treatment of maxillofacial trauma, no single treatment modality has been revealed to be ideal for mandibular angle fractures. Size, shape, number and biomechanics of plate and screw systems with varying configurations and combinations used for the treatment of mandibular angle fractures are well documented and research continues. To the best of the authors’ knowledge, the direction of the fracture line, the muscular forces acting on the fragments and displacement of the fragments subsequent to muscle contraction, conceptually the ‘favourability’ or ‘unfavourability’ of a specific fracture, have not been studied. These concepts are discussed in basic text books; a ‘favourable’ fracture refers to a pattern in which the pull of the muscle attachments on the fractured segments reduces the fracture, whilst an ‘unfavourable’ fracture refers to a fracture line in which the muscle pull distracts the fracture segments. The principle of favourability or unfavourability of mandibular angle fractures is based on the direction of the fracture line, as viewed on radiographs in the horizontal or vertical plane. A horizontally favourable fracture resists the upward displacing forces (masseter and temporalis muscles) on the proximal fragment when viewed in the horizontal plane. The purpose of this biomechanical study was to analyse the effects of horizontally favourable and unfavourable mandibular fracture patterns on the fixation stability of titanium plates and screws.
Materials and methods
This study was approved by Baskent University Institutional Review Board (Project no: D-DA 08/04) and supported by Baskent University Research Fund. The mandibles of 11 healthy sheep of a similar age group (aged 9–11 months, all fed on a natural diet) were studied. All mandibles were stripped of their soft tissues and sectioned in the midline between the central incisors. The specimens were kept moist and refrigerated until all testing was performed. The models were sectioned with the osteotomies performed via a reciprocal saw; favourable fractures were created in left hemimandibles whilst unfavourable fractures were created in the right ones. All osteotomies were performed on a standard basis as follows: superior border of the osteotomy was set 1 cm anterior to the most concave point of ascending ramus. A tangent connecting the most posterior condylar and most posterior angular point was drawn ( Fig. 1 , line 1); and the osteotomy line towards the inferior border was set according to a pilot line parallel to this tangent ( Fig. 1 , line 2). Favourable fracture lines were created by forming a +15° angle with the pilot line whilst unfavourable fracture lines formed a −15° angle with the same pilot line ( Fig. 2 ). All hemimandibles were fixed with 4-hole straight titanium miniplates and 2.0 mm × 7 mm titanium screws which were inserted into the superior border in a standard fashion according to the principles of Champy (Figs 3 and 4 ). A custom-made biomechanical testing model that had been used in the authors’ previous studies was adapted to a servohydraulic testing unit (Instron 8874; Instron, Warwick, UK) and samples were fixed from the mandibular condyle and incisor regions.
Occlusal bite force was applied to the posterior mandible and each hemimandible was subjected to a continuous linear compression until plastic deformation was seen. During the test, load and displacement data were recorded. Maximum forces (in N) that the models could resist before breaking, maximum displacements (in mm) of the models, and displacement values (in mm) under 20, 60, 100, 120, 150 and 200 N were compared. For each continuous variable, normality was checked by the Shapiro–Wilks test. Mann–Whitney U -tests were applied for comparisons between two groups and the Friedman test was used for repeated measurements. Data were expressed as mean ± SD and median (min–max) and a P value <0.05 considered as significant. Statistical analysis was performed using the statistical package SPSS v 15.0 (SPSS, Inc., Chicago, IL, USA).
Results
None of the study models failed during testing and they met the criteria for this biomechanical study. Summary of the results and analysis is presented in Tables 1 and 2 . Comparison of displacement values of favourable and unfavourable fracture groups under given forces is shown in Table 1 . No statistically significant differences were found between the groups for the displacement values in the force range 60–200 N. The only difference between the groups was for an applied force of 20 N. The difference for the maximum displacement values at breaking forces was statistically significant, whilst it was non-significant for breaking forces between groups ( Table 2 .)
| Displacement (mm) | |||
|---|---|---|---|
| Force (N) | Favourable | Unfavourable | P value |
| 20 N | 0.054 | 0.022 | 0.046 * |
| 60 N | 0.182 | 0.121 | 0.070 |
| 100 N | 0.396 | 0.347 | 0.599 |
| 120 N | 0.526 | 0.480 | 0.974 |
| 150 N | 0.711 | 0.711 | 0.922 |
| 200 N | 1.150 | 1.123 | 0.974 |
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