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BIOMECHANICAL CONSIDERATIONS
In the past three decades, a variety of studies has contributed to the conceptualization of the biomechanical principles dictating mandibular behaviour during normal function. The two-dimensional models demonstrate tension at the level of the dentition and compression at the lower border of the mandible whereas the 3-dimensional approach includes forces of the musculature on the balancing side during mastication.11,12 Based on these principles, different methods of plate fixation have evolved to solve the problem of displaced fracture segments. Currently, controversy continues unabated about the use of one or two mini-plates in mono-cortical plating for the purpose of providing adequate support and stability to facilitate effective immediate function. In the conventional plating systems, stability is derived from tightening the screw perpendicular to the mini-plate and adjacent bone.
Anatomical constrains limit intra-oral access for bi-planar placement of the plate located on the lateral surface of the external oblique ridge. The difficult anatomical access necessitates compensation by drilling and screw application at an angle other than the required right angle.
This practice results in an inevitable acute placement angle of screw, screwdriver and screw to bone interface. Mechanical engineering theory states that, for identical placement loads, screws inserted perpendicular to the engagement surface, should provide twenty percent more resistance to displacement than other placement angulations.13 However, viewed from a biomechanical perspective, bi-cortical engaging screws inserted at an angle smaller than 90° have a longer surface area of interfacial cortical bone contact and this factor may eliminate the theoretical disadvantage of screw placement at 60° angulation to the bone surface.14 If this principle is applicable, it can be assumed that the 60° and the 90° configurations should exhibit similar biomechanical characteristics.
STATEMENT OF THE PROBLEM
The preceding résume of the literature indicates an awareness of the factors that influence the prognosis of mandibular fracture fixation. In general, these factors are related to three areas:
(i) Anatomical and surgical constraints
(ii) Analytical investigations of the biomechanical behaviour of the mandible
(iii) Biomechanical design and location of the fixation system While the first area has enjoyed extensive investigation, the second area which has to do with prediction of functional stability, is subject to complicated analytical methodology. The third area, which concerns the design and location of the fixation systems, is extensively but inconclusively reviewed. Very little attention has been given to:
(i) Development of less complicated methods for delineating the biomechanical behaviour of the mandible for functional stability determination of fracture fixation.
(ii) Addressing the problem of anatomical positioning of the plating system to ensure a minimal invasive surgical technique and cost effective operating time.
(iii) The relationship between biomechanical stability and the screw placement angle in mono-cortical fixation. Since the prognosis of mandibular angle fractures, osteosynthesis segments are dependent on post-operative stability of the displaced segments. There is a need for detailed consideration of the fixation characteristics of acute vector angled screw mini-plate designs.
FABRICATION OF POSITIONING TEMPLATES
An intact hemi-mandible will be used for the fabrication of the three polymethylmethacrylate (PMMA) localization templates required for standardised and chronological preparation of the test samples and positioning of these samples in the mechanical testing device as follows:
• Rigid fixation template The purpose of this template is for rigid fixation of the proximal segment of the test module to the vertical fixation plate of the test jig.
Use of the prefabricated PMMA template will standardise and correlate the required receptacle holes through the coronoid/ramus region and the existing receptacle holes in the vertical fixation plate. Furthermore, the location of these receptacle holes in the mandible, will align the distal section of the test module to the free-rotating crib located on the horizontal rotational axis of the load application wheel to be employed for torque evaluation.
• Mini-plate positioning template The template for standardised mini-plate localisation on the ventral aspect of the external oblique ridge, is designed to accommodate two different plate positions in close proximity to one another. The upper position will be employed for tension/compression evaluation whereas torque will be derived from the inferiorly located structure. This approach is primarily a cost saving exercise which is unlikely to compromise biomechanical principles. The template will feature profile perforations imprinted to accommodate actual placement of the mini-plates for precise localization and screw access hole preparation. In addition, the screw access holes on the various mini-plates will have drill guides for accurate angular preparation and predetermined depth penetration for the fixation screws.
• Segmentation template The mini-plate position template will be modified to incorporate a guide for the introduction of standardized horizontally unfavourable osteotomies at the angle of the replica hemimandibles. The sectioning procedure will be undertaken simultaneously with the preparative procedures adopted for mini-plate localisation. The template will feature two corresponding and linearly aligned bi-cortical engaging guiding trenches, ± 5mm in length located on the upper and lower aspects of the template allowing orientation slots to be cut into the surface of the synthetic mandibles. Standardised sectioning will be obtained by linear connection of the prepared slots after removal of the template and employing a reciprocating saw with a blade width of 0,9mm for segmentation.
CHAPTER 1 INTRODUCTION
CHAPTER 2 REVIEW OF LITERATURE
2.1 Mono-Cortical Plating Strategies and its Biomechanical Problem
2.2 The Effect of Plating Techniques and Plate Orientation on Biomechanical Stability
2.3 Bite Force and its Clinical Relevance to Mono-Cortical Fixation
2.4 Angled (Slanted) Screw Application
2.5 Anatomical Considerations in Angled Screw Application
2.6 Surgical Approach and Clinical Relevance to Screw Angle Application
2.7 Screw Angle Comparative Biomechanical Stability Pilot Studies
CHAPTER 3 PROBLEM AND PURPOSE OF THE IN VITRO STUDY
3.1 Statement on the problem and purpose of the in vitro study
CHAPTER 4 EXPERIMENTAL PROCEDURES
4.1 The Biomechanical Testing Device
4.2 Tension/ Compression Evaluation
4.3 Torque Evaluation
4.4 Compilation of Mandible Samples
4.5 Fabrication of Positioning Templates
4.6 Mini-Plate Fixation Procedures
4.7 Load Displacement Evaluation
CHAPTER 5 RESULTS
5.1 Biomechanical Testing Device
5.2 Compression Load-Displacement Results
5.3 Statistical Analysis of Compression Evaluation
CHAPTER 6 INTERPRETATION OF RESULTS
6.1 Mathematical (Trigonometric) Formulation of Angled Screw Application Results
6.2 Defining and Measuring of STS (Screw Tip Shifting) (Figure 40)
6.3 Clinical/Experimental Significance of STS (Figure 40)
6.4 Defining and Measuring of Screw Tip Travel (STT)
6.5 Clinical Relevance of Trigonometric Calculations (Figure 40)
CHAPTER 7 DISCUSSION AND RECOMMENDATIONS
7.1 Discussion
7.2 Recommendations for future biomechanical stability investigation and geometrical design aspects of the ISI systems
7.3 The anatomical study related to the geometric ISI plate design
7.4 Surgical technique for ISI plate fixation
CHAPTER 8 CONCLUSION
ANNEXURE 1: REFERENCES
ADDENDA
