1. Biomechanics of Mandibular fracture GUIDED BY :- DR. NITIN BHOLA SIR. DR. ANENDD SIR. PRESENTED BY :- TANUJ PATIL (2 ND MDS)

2. CONTENTS  Introduction  Surgical Anatomy of Mandible.  “CHAMPY’S PRINCIPLE”  Fractures at the angle of the mandible:- -Vertically favourable & unfavorable fractures:- -Horizontally favourable & unfavorable fractures:-  Fractures at the symphysis and parasymphysis:-  Fractures of the condylar process:-  Fracture of the coronoid process:-  Different studies on biomechanics :-  Posterior Body/Angle Fractures :-  Molar Loading (Posterior Body) :-  Fracture at TMJ

3. Introduction  Biomechanics is the study of the function of living materials.  The mandible is the second most commonly fractured part of the maxillofacial skeleton because of its prominence and its position.  The mandibular body is a composed of external and internal cortical layers surrounding a central core of cancellous bone.  In the chin region the cortical bone is thickest at the lower border, where as more Posteriorly - Relatively thin.

4.  The maxillary and mandibular teeth, in occlusion form a very sensitivity balanced system; Any displacement caused by fragments leads to diminution of masticatory function and comfort.  The main aim in fracture treatment is therefore the restoration of normal occlusion.

5.  “Gysi in 1921” was one of the first to suggest that the function of the mandible could be likened to a “lever system”.  He argued that the mandible functioned as a class III lever, where the muscle force vector was between the fulcrum (TMJ) and the bite point, necessitating loads be transmitted through the TMJ.  Ellis and Throckmorton. Mandibular Condylar Process Fractures. J Oral Maxillofac Surg 2005.

6. Illustration of the 3 classic lever systems. The arrow represents the muscle force; the triangle represents the axis or fulcrum, the rectangle is the load or resistance force. Class I levers are used in seesaws, catapults, and use of a crowbar. A class II lever is used in the function of a wheelbarrow. Ellis and Throckmorton. Mandibular Condylar Process Fractures. J Oral Maxillofac Surg 2005.

7. ChAMPY’S PRINCIPLE  “Champy et al” ultimately developed the technique into a practicable clinical method.  Based on a methametical model of the mandible and taking into account the active biting forces applied to the mandible and performing different experimental evaluation, they were able to define the strains created within the bone by muscular activity.  Moments of flexion were found at the upper border of the mandible, increasing progressively from the front of the teeth to a maximum of approximately 600 N in the angles.

8.  Then are also tortion movements between the canines, which increases in sleight towards the middle to 1000 N.  They recommended that, behind the mental foramen only one plate should be applied, immediately below the dental roots and above the inferior alveolar canal.  In front of the mental foramen, in order to neutralize higher torsion forces between the canines another plate near the lower border of the mandible is necessary in addition to the sub apical plate.

10.  It was observed by “Champy” that the healing of the mandibular fractures is adequate even in absence of compression and with monocortical screws and non-compression plates also stabilize the fragments sufficiently.  The need for bulky plates is also not felt and thinner plates as recommended by him offer reasonable stability.  The kind of fixation offered by the Champhy’s monocortical plates is semirigid.  “Champy” observed that the osteosynthetic activity is optimum along certain areas during the healing of the fracture and he recommend the application of the plates along these osteosynthetic lines to facilitate good and early healing.  In this technique only one cortex comes in full contact while the other opens up to a minor extent.

11. Champy’s lines

12.  In the “mandibular anterior area” as the torsional forces are optimum two plates are used, one along each Champhy’s lines.  In the canine and premolar area while placing the plates intraorally, care should be taken to prevent damage to the branches of the mental nerve.  The “mandibular angle fractures” are treated by placing the plate along the external oblique ridge through an intraoral incision.  The subcondylar fractures are fixed through preauricular or Risdon’s submandibular incision with retromandibular extension or by Hind’s post ramal approach.  The low subcondylar fractures are easy to plate, but the high subcondylar fractures are very difficult to manage as the proximal fragment is usually displaced severely and difficult to control.

13.  When the miniplates are used, they should be properly adapted and contoured to the buccal surface of the bone.  The gap between the plate and the bone should be minimal.  The screws should be secured firmly and tightened until no more turning of the screw is possible with moderate force.

14. Muscle attachments and displacement of fractures  The periosteum is a most important structure in determining the stability or otherwise of a mandibular fracture.  The periosteum of the mandible is stout and unyielding and gross displacement of fragments cannot occur if it remains attached to the bone.  Perisoteum may be stripped from the bone ends by the extremity of the force applied, but frequently it yields to the accumulation of blood seeping from the ruptured cancellous bone.

15. Fractures at the angle of the mandible  Fractures at the angle of the mandible are influenced by the medial pterygoid- masseter ‘sling’ of which the medial pterygoid is the stronger component.  Fractures in this region have been classified as:-  Vertically favourable or unfavourable. and  horizontally favourable or unfavourable.  It must be remembered that vertically and horizontally unfavourable fractures may be undisplaced if the periosteum is undisturbed.  The concept is only important when the periosteum has been ruptured or stripped from the bone.

16. A favourable facture line will, however, make the reduced fragments easier to stabilize.  The presence of an erupted tooth on the posterior fragment will sometimes prevent gross displacement of this fragment in an upward direction if its crown impacts on the opposing upper tooth.

17. Vertically favourable & unfavorable fractures:-  If the vertical direction of the fracture line favours the unopposed action of the medial pterygoid muscle, the posterior fragment will be pulled lingually.  When the fracture line is viewed from the superior aspect of the mandible is passing from buccal cortical plate to the lingual cortical plate with the buccal end lying mesially and the lingual end of the line lying distally.  In such a situation, the distal fragment will be drawn closer to the proximal fragment due to the pull of the medial pterygoid muscle and the fracture segments will come closer rather than getting separated in bucco-lingual plane and thus the fracture is called vertically favorable

18. In vertically unfavorable case:-  The fracture line passes buccolingual with the buccal end lying mesially, when viewed from superior border.  The distal fragment in this case will easily get drifted lingually due to the pull of the medial pterygoid muscle, thus there will be separation of the fragments in the buccolingual plane due to the unfavorable muscle pull, and the fracture is said to be vertically unfavorable.

20. Horizontally favourable & unfavourable fractures:-  If the horizontal direction of the fracture line favours the unopposed action of the masseter and medial pterygoid muscle in an upward direction, the posterior fragment will be displaced upwards.  Horizontally Unfavorable :- the fracture line runs superoinferiorly with its superior and lying anteriorly and the inferior end lying posteriorly thus, the muscle pull becomes unfavorable and drifts the proximal and distal fracture segments apart.

22. Horizontally Favorable :-  When viewed from the buccal side (horizontally), the fracture line runs superoinferiorly with its superior end lying posteriorly than its inferior end.  In this situation the muscle pull from the temporalis and masseter muscle will pull the distal segment superiorly and the suprahyoid muscles will put the proximal segment inferiorly, thus drawing both the segments closer to each other.

23. Fractures at the symphysis and parasymphysis  In the symphysis region muscle attachments are also important.  The mylohyoid muscle constitutes a diaphragm between the hyoid bone and the mylohyoid ridge on the inner aspect of the mandible.  in transverse midline fractures of symphysis and the mylohyoid & geniohyoid muscles act as a stabilizing force  An oblique fracture in this region will tend to overlap under the influence of the geniohyoid / mylohyoid diaphragm

25.  When a bilateral parasymphyseal fracture occurs it usually results from considerable force which disrupts the periosteum over a wide area.  Such a fracture is readily displaced posteriorly under the influence of the genioglossus muscle and to a lesser extent the geniohyoid.  It is often state that such a fracture removes the attachment of the tongue to the mandible and allows the tongue to fall back and obstruct the oropharynx.  As the tongue is still firmly attached to the hyoid bone which in turn remains connected to the mandible by the posterior parts of the mylohyoid muscle.

26. Bilateral fracture of parasymphysis. The anterior fragment displaced backwards by the genial tubercle

27.  the intrinsic muscles of the tongue continue to exert control and the tongue remains forward in the oral cavity.  Voluntary control loss

28. Direct and Indirect Fracture  When the force is applied to the bone, the cortex (buccal cortex) at the site of application of the force undergoes compression and the other cortex (lingual cortex) of the bone undergoes tension.  If the force is greater than the compressive and the tensile strength of the bone a fracture ensues.  When the force is applied to the mandible, the point of application of the force is compressed and the resultant vector travels along the bone and applies tensile force on the point intersected by this vector.

29.  Direct fracture :- The fracture occurs at the site of direct application of the force and the fracture occurs at this site due to compression of the bone.  Indirect fracture:- A fracture also occurs at the site intersection of the resultant vector by tensile forces.  The force applied to the symphysis menti results in a direct fracture at the symphysis.  The vector travels to the condylar necks bilaterally and induce indirect fractures of subcondylar areas bilaterally, as the condylar necks are weak.

30.  In case of bilateral subcondylar fracture, when the proximal segment gets displaced medially due to the pull of lateral pterygoid and the distal segment is pulled superiorly, shortening of the ramus takes place.  This phenomenon is called ‘telescoping of the fracture’ which results in the anterior open bite with gagging of the last molars.

31. Fractures of the condylar process  When a fracture of the condylar neck occurs the condylar head is frequently displaced and sometimes dislocates from the articular fossa.  The most frequent direction of displacement is medially and forward under the influence of the lateral pterygoid muscle.  The importance of this muscle as a displacing force is more dramatically illustrated in those cases where anteromedial dislocation occurs some days after injury in a previously undisplaced fracture.

32. Fracture of the coronoid process  This is a rare fracture which is said to be brought about reflex muscular contraction of the strong temporalis muscle which then displaces the fragment upwards towards the infratemporal fossa.  Fractures of the coronoid process are rarely isolated and are usually simple and linear with little displacement  cause trismus and swelling in the region of the zygomatic arch.  Most condylar fractures can and should be treated via-clsoed techniques if the occlusion is compromised. Early jaw mobilization and physical therapy are indicated to prevent ankylosis or limited jaw movements.

33.  There may be swelling in the retromolar area and a lateral crossbite.  Treatment is usually instituted only if the occlusion is compromised or if the fractured coronoid process impinge on the zygomatic arch, inhibiting mandibular movement.

34. Different studies on biomechanics  The facial skeleton which is connected with the cranial vault and cranial base can be divided for convenience into three parts,  The upper third of facial skeleton - cranial vault and comprising of frontal bone  The middle third (central midfacial bone)- the maxilla, the nasoethmoid,  and lateral midfacial bone—  and the lower third comprising of rigid bone—mandible, with its condylar articulation to base of skull.  This constitutes with the oral cavity and other associated soft tissues, ‘‘viscerocranium’’.

35. Nahum AM (1975) The biomechanics of maxillofacial trauma. Clin Plast Surg 2:59  “Nahum in 1975 “he individually studied tolerance levels of each bone in midface and mandible.  In which mandible is least resistance to impact of 100 gm  The mandible is much more sensitive to lateral than to frontal impacts.  The anatomic configuration of the mandible approximates a rigid semicircular link with pinned joints at its free ends.  When attempts were made to apply a force anteriorly to the midline symphysis of the mandible, instability was encountered unless the line of force passed through the condylar processes.  For this reason, a force direction was necessary which combined anterior and submental vertical orientation.

36.  Mandible has complex geometry with differing modes of fracture failure and fracture tolerance levels for each area, the tolerance level increases in proportion to the relative size and area of the mandible involved.  Nahum in the discussion of the biomechanics of maxillofacial trauma suggested that the injuries are entirely predictable according to certain variables:  (1) Force and direction of collision;  (2) Impact interface geometry (shape and texture of opposing surfaces)  (3) energy absorbing characteristics of the opposing objects; and  (4) use of restraints such as seat belts

38.  The force of impact can be derived from the Equation F = ma (Force = mass x acceleration)  in which if the head mass is 15 pounds and the acceleration is 80 g (easily obtainable in a 30 mph collision),  the force on the face would be 1200 pounds, which exceeds the fracture limit of most of facial bones.  According to geometry of face, protruding areas are most likely to sustain injury; thus the nasal bones are most commonly injured, followed by malar bones, orbital rims, and symphysis of the mandible.

39. “Banks P (1992) Killey’s fractures of the middle third of the facial skeleton”. 5th edn (Indian edn) Varghese Publishing House. First Indian Reprint, India.  “Banks in Killey’s fractures of the middle third of the facial skeleton” states that, because of the relative fragility of the midfacial skeleton, it acts as a cushion for trauma directed towards the cranium from an anterior or anterolateral direction.  It is analogous to a ‘‘match box’’ sitting below and in front of a hard shell containing the brain, and differs quite markedly from the rigid projection of the mandible below.  They concluded when this force impacts middle third of facial skeleton is cushioned sufficiently so that it may not even lead to loss of consciousness, though causing considerable damage to the bones and soft tissue of the face.

40.  If, however, the mandible alone withstands the impact, the cushioning effect is reduced and blows to the mandible are transmitted directly to the base of the skull through temporomandibular articulation.  This in turn means that relatively minor mandibular fractures may be associated with a surprising degree of head injury; hence the effectiveness of the boxers knockout punch, according to them.

41. Biophysics principle applied to mandible  The behavior of an intact system (normal mandible) differs from the behavior seen when a fracture is present.  An intact mandible develops tension and compression zones during normal function, and these zones are dynamic and contingent on the bite target location and muscle recruitment pattern.  The fixation device acts to transfer forces across the fracture zone. Both tensile and compressive stresses can be generated at a fracture site when devices are applied The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

42.  All forces must remain in equilibrium during function regardless of the number or type of plates applied.  A complete force circuit will be established, with stress distributions seen along regions that provide greatest resistance to deflection and are therefore stiffest.  Because no single position along the bone surface in the human mandible is subject to only one type of stress, any plate applied will have to maintain stability in a variety of stress conditions (compression and tension) while resisting rotation and shear forces during loading at various bite locations. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

43.  If a fracture site must experience some load for maturation, one of two conditions must exist during healing:  (1) The external dynamic forces present after injury and plate application are initially significantly lower than normal, then increase toward normal during healing to contribute to greater stress at the fracture site; and  (2) The plate screw systems gradually lose some stability at the screw/bone interface, reducing the force flow through the plate as the bone/bone interaction at the fracture bears more load. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

44. Ruddermans’ FRACTURE biomechanics  The most basic fracture conditions are reviewed:- 1.Posterior body/angle fracture with bite load anterior, posterior, and contralateral to the fracture; 2.Symphyseal fracture with midline and posterior bite load. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

45. Posterior Body/Angle Fractures :- (Incisor Loading (Midline Load)  This involves a fracture position at the posterior body/angle region with a central (incisor) bite target.  The bite target is the point of force transition between the upper (maxilla) and lower (mandible) dental segments.  The bite target completes a force circuit between the mandible and midface,  where the load is transferred through this substance secondary to force generated by muscular actions. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

46.  As muscular contraction occurs, the masseteric sling (masseter and medial pterygoid musculature) generates an upward movement of the posterior mandible.  Most obvious movement occurs at the fracture site with the mouth open.  The midline load position (target) acts as a constraint around which the mandible rotates.

47.  When the fracture is anterior to the attachment of the masseter, regardless of the orientation of the fracture  (oblique, oriented anterosuperior or anteroposterior),  The segment posterior to the fracture will rotate, resulting in separation along the upper margin and less separation, or relative compression, of the lower margin. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

48.  The inferior mandible margin will experience some degree of compression, only if the segments are in contact, during movement.  Posterior body/angle fracture with incisor loading will result in intact soft tissues surrounding the fracture on the upper margin experiencing tensile forces and lower margin tissues and bone experiencing compressive forces.

49.  Molar Loading (Posterior Body) :-  Conditions change dynamically as the bite target moves posteriorly approaching the fracture location.  Posterior body/angle fracture with more posterior (molar) loading.  When the fracture is anterior to the bite target, a shear component may be seen in combination with the rotatory movement, effecting further displacement of the fracture segments.

51.  When the bite target is contralateral to the fracture and the fracture is within the attachment region of the muscle, the ipsilateral soft-tissue/muscle components may assist in stabilizing the fracture from additional movement caused by muscle contraction, depending on fracture conditions. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

52. Symphysis Fracture Incisor Loading (Midline Load)  This scenario involves a central incisor bite target with the fracture position at the symphysis region.  Behavior pattern significantly different from that predicted by accepted tension/compression cantilever theory.  A curved structure, suspended by soft tissue, with the active component of the force generation laterally positioned (human mandible),presents with behavior more consistent with a suspended Beam.

53. Cantilever theory has generally been depicted as a hemimandible loaded at the midline with the implied region of tension along the upper margin.

54.  Finite element analysis studies indicate tensile stress at the midline in an intact system, with greater tensile stress along the lingual surface than along the buccal surface.  When a midline fracture is present, the incisor load position (target) acts as a constraint around which the mandible rotates.  Activation of the masseteric sling will produce a rotation around an anteroposterior axis of a hemimandible (fracture at the midline) because of the point of attachment of the muscle and the curved structure of the mandible. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

55.  The effect of this rotation and movement will be seen at the midline as separation of the lower border of the mandible greater than separation of the upper border.  A compressive force along the upper Mandible border will occur if the segments are in contact.  The inferior margin of bone will not experience tension unless substantial soft tissue remains in contact with the fracture segments. The Biophysics of Mandibular Fractures: An Evolution toward Understanding. Randal H. Rudderman, M.D. Robert L. Mullen, Ph.D. John H. Phillips, M.D.

56.  When a bite load is generated at the fracture location, three effects contribute to displacement of the segments at the fracture site:  (1) rotatory,  (2) axial (translation), and  (3) shear.  If the bite target is adjacent to the fracture, one segment may move vertically relative to the other, resulting in sliding (shear) at the fracture location.

57. Symphysis Fracture Molar Load (Posterior Load)  A midline fracture will experience similar relative displacement patterns with an incisor bite target or molar bite target.  With no bony contact at the central segment fracture site (symphysis), the lateral mandibular segments will rotate with midline distraction opposed by soft-tissue attachments spanning the fracture.  The end effect will be displacement at the inferior border (tension of the soft tissue) and compression of the upper border.  Tensile stresses are predictably generated at the upper mandible margin when a body/angle fracture is exposed to incisor bite load conditions.

58. STABILIZATION TECHNIQUE  Force distribution and loading patterns on the plate/screw construct and local bone are substantially different when one versus two plates are used for treatment of mandibular fractures.  In a two plate system, one plate is typically placed at the upper border and one is placed at the lower border.  Single-Plate Systems:- Upper Border Plate  When an incisor (midline) bite target is present, the upper border plate at the fracture site will experience primarily tensile loads, with a minimal bending component.

59.  With appropriate screw insertion, the screws adjacent to the fracture will experience the majority of the load up to 90 percent of the stress on the central two screws.  As a load is applied, tensile stresses begin to develop in the plate between the screws across the fracture site.  The stress is further directed into the bone by means of the bone/ screw interface.  As healing occurs (bone growth and maturation at the fracture site), the healing tissues gradually begin to contribute to the transfer of forces generated during loading.

60.  These studies indicate the probability of gradually increasing bite forces with time after injury. Even when the fracture is completely healed, some of the load continues to be carried by the plate.  Therefore, the system does not return to the preinjury stress state while plates are present and remain firmly attached.

61.  Lower Border Plate:-  A single plate placed along the lower border of a mandible body fracture, with an incisor bite load, will need to resist distraction at the upper border.  The load condition becomes more bending, not pure tension or compression. In a four screw/ plate scenario, when the plate is subject to equal loading. all of the screws now are subjected to equal loading.  If a single screw becomes loose and fails to carry a load before adequate healing in this condition, the entire construct is subject to failure.

62.  Two Plate System:-  As a load is applied, compressive stress begins to develop within the bone/plate system.  The central two screws in a four-screw system carry the majority of the compressive load.  The plate will therefore experience varying amounts of compressive loads depending on the amount of load transferred at the fracture site.  If no bone-to-bone contact is present, all compressive loads will be transferred through the plate.

63.  If any one of the inner two screws becomes mobile, the load then shifts to the outer screw.  This effect will occur in either plate (upper or lower) under tensile or compressive load conditions.  The possibility of loss of three-dimensional stability significantly increases with screw mobility.

64. ANGLE FRACTURES AND SOFT-TISSUE STABILIZATION  The angle geometrically is a thinner construct than the anterior mandible.  The molar region functions with an efficiency greater than other regions.  The masseter–medial pterygoid sling in the posterior body–angle region is oriented to provide for mostly vertical force during function.  Compressive effects are exerted as muscle contraction occurs with a bite target in this location.  The muscle, once activated, becomes stiffer, and the muscle fascia component acts to stabilize the area.

65.  A single-plate application has been used successfully for the treatment of angle fractures.  The location of plate application is often along the oblique line (upper border) of the mandible. This plate when loaded experiences stress conditions dissimilar to the single plate placed along the inferior border.  The upper plate becomes loaded primarily in tension with an incisor bite condition. The central two screws are maximally loaded during function.  A molar bite load would tend to distract the lower border. However, if minimal displacement of the fracture occurs during injury, the soft tissue spanning the fracture zone may help stabilize the fracture region.

66.  Activation of masseter muscle causes stiffening which may reduce mobility of fracture site.  Because application of this plate requires minimal dissection, a small plate device placed along the superior margin, with minimal dissection along the fracture, may provide conditions where the nondisplaced soft tissue can contribute to stability during function.

67. Tmj guidence in mandibular movement  Transmitting forces between the mandible and skull, the TMJ also plays a role in controlling jaw movements.  But disease and injury do limit mandibular protrusion and lateral excursions because these movements are more dependent on condylar translation.  In addition, limited condylar translation may introduce excessive lateral deviation during opening.

68. relative loading of the TMJ biting at the incisors or the molar.  When biting at the incisor, the joints are loaded equally (A).  When biting at the molar, the opposite, balancing side joint is loaded more heavily than the working side joint.(B)  The relative joint loading can be determined by where a line connecting the bite point with the mean force vector (FV) of all the elevator muscles intersects with the intercondylar axis.  If the elevator muscles function equally, the mean force vector (FV) is located in the midline.(c) Ellis and Throckmorton. Mandibular Condylar Process Fractures. J Oral Maxillofac Surg 2005.

69.  When biting at a molar (D), the opposite, balancing side joint is loaded more if the mean force vector is still located in the midline.  In most individuals, the mean force vector shifts somewhat toward the working side so that the balancing side is not loaded as heavily as it otherwise would. However, it is still more heavily loaded than the working side joint.

71. HOW DOES ONE FUNCTION WHEN ONE TMJ IS DAMAGED?  Under normal circumstances, when one bites on the left molar, the right (contralateral or balancing side) TMJ bears more load than the left (ipsilateral or working side) TMJ.  “Greaves states” If the right TMJ were injured, one way to reduce loading it when biting on the left molar would be to selectively increase left masseter activity and selectively decrease right masseter activity. Doing so produces greater loading of the left, uninjured TMJ.  When biting on the right (ipsilateral or working side) molar, alteration of the muscle activity ratio is not necessary because the normal left-side joint carries the greater load.

73.  When biting on the side opposite the fracture, the fractured joint would be expected to be loaded more than the nonfractured side joint (A).  However, it has been demonstrated that in patients with such injuries, the mean force vector moves toward the uninjured side so that the relative loading of the damaged joint is reduced (B).

74.  When biting on the nonfracture-side molar, the working side masseter muscle was 1.5 times more active than the masseter on the balancing side.  Another functional component of the masticatory system that could be hindered by a condylar process fracture is mandibular mobility and asymmetrical motion.  With fracture, the normal translation and rotation of the mandibular component of the TMJ can be upset.  Several studies have shown dramatic differences in the amount of lateral deviation on opening and protrusive excursion after patients were treated closed for condylar process fractures

75.  “Lindahl’s” studies show persistent symptoms of dysfunction of the masticatory system (eg, clicking, pain, deviation on opening) to occur frequently in adults but rarely in children with condylar fractures.  How can one ever achieve normal occlusion with bilateral displaced condylar process fracture?  If the mandible functions as a “class III lever system”, as demonstrated for at least the power stroke of mastication, then loss of the fulcrum (ie, TMJ) should cause posterior collapse and premature contact of the terminal molars, creating an anterior open bite.

77.  While this happens in many and perhaps most patients, it does not occur in all. Some patients have the remarkable ability to bring their teeth into the proper occlusal relationship soon after injury, even without any treatment.  Justify..!!! Here neuromuscular adaptations are play role position in bilateral condylar fractures.  At this time, there is no articulation with the temporal bone to provide vertical skeletal support for the posterior ramus.  Thus, the only mechanism whereby the mandible can be positioned into a normal occlusal relationship early after injury is by complex neuromuscular adaptations in the muscles of mastication

78.  It has been shown that individuals with bilateral condylar process fractures selectively in- crease the EMG output in the posterior temporalis fibers.  The effect of this increased activity is to provide a posteriorly directed vector onto the coronoid process, which, all other things being equal, can rotate the anterior portion of the mandible superiorly, bringing the incisors into contact  Free-body (A) and lateral skull (B) diagrams showing that with selective contraction of the posterior temporalis muscles and minimal activity within the elevators and suprahyoids, the mandible can rotate “closed.  An axis of rotation is created somewhere within the ramus to allow this to occur (C).

80.  “Hjorth et al” similarly showed that most of the muscles in patients treated for condylar process fractures normalized with time, although some asymmetry of the masseter muscles occurred even after up to 1 year.  “Ellis and Throckmorton” showed asymmetry of EMG values when biting on the side opposite the fracture that continued for at least year with greater asymmetry in the patients treated open with greater asymmetry in the patients treated open.  “Talwar et al” showed that patients with bilateral fractures showed abnormal (but not asymmetric) activity initially, but were significantly different from controls only at the 6-week and 6-month intervals.

82.  Complete biomechanics theory includes not just bone/device interaction but also nutrition and metabolism, bone healing, and application techniques, including operator skill.  In fracture repair, the reduction process should not contribute to additional system damage.  The fixation system should provide adequate stiffness and strength to allow for early return to function and the system should not continue to significantly modify the stress distributions after healing has occurred.

83.  Thank you