Craniomaxillofacial

1. Biomechanical and Principles of
CMF Osteosynthesis
Dr. dr. I Nyoman Putu Riasa, SpBP-RE(K)
Plastic Reconstructive and Aesthetic Surgery, Faculty of Medicine Udayana University –
Prof Dr I.G.N.G. Ngoerah Hospital Denpasar

2. Biomechanical of human CMF
 Biomechanics is the study of the function of living materials
 Movement analysis in relation to anatomy, physiology, mechanical movement and
physical properties of osteosynthesis material
 Human movement biomechanics
 Basic musculoskeletal system
 Mechanical principles on musculoskeletal system
 Application of mechanics on human movement system

3. Biomechanics of Lower Jaw (Mandible)

4. Cantilever Beam Theory
 Intact mandible develops tension & compression zones
during normal function
 Tension zone at the upper margin and compression zone on
the lower margin
 The similar tension assumed working on mandible fracture
Loading
Tension
Compression
Chun –Li Lin, Yu-Tzu Wang, Chun-Ming Chang et al. Design Criteria for
Patient-specific Mandibular Continuity Defect Reconstructed Implant with
Lightweight Structure using Weighted Topology Optimization and Validated with
Biomechanical Fatigue Testing. Int J Bioprint. 2022; 8(1): 437.
Courtesy Dr Riasa

5. Evolution of Model
 A model of mandible behavior (1980s): consisting of a tension zone at the upper
margin and a compression zone at the lower margin. This model is inadequate for
describing fracture behavior, device behavior and variation in clinical results.
Traditional mandibular biomechanics describes plate placement for fracture repair by
defining a tension band (plate)along the upper margin and a compression plate along the
lower margin. This is an oversimplified, incorrect model.
 Suspended beam theory
 In search of ideal osteosynthesis, there is paradox of similar clinical outcome
obtained with use of small plate technique and reconstruction plate
 Circuit of force theory

6. Suspended Beam Theory
Randal H. Rudderman, Robert L. Mullen, John
H. Phillips. The Biophysics of Mandibular Fractures:
AnEvolution toward Understanding. Plast.Reconstr.Surg.
121:596,2008
⇢ The basis of application of
compression plate
and lag screw system

7. Circuit of Force Theory
 Masticatory muscles contraction and relaxation acting as a
deforming forces to the mandible based on its insertion
and origin.
 Any forces applied to lower jaw will causing different
tension and compression zones depending on the location
of the bite area as the forces transmitter
 These zones are dynamic and contingent on the bite target
location and muscle recruitment pattern
 Mandible behavior not only biomechanical of the bone but
also the effects of soft tissue (fascial and periosteal
attachment, effects of muscle contraction)
 The forces are transmitted not only through bone but
through soft tissues, creating circuits of force.
Digastric muscle
Temporalis
Masseter
Lateral pterygoid muscle
Tension
Compression
Randal H. Rudderman, Robert L. Mullen, John H. Phillips. The Biophysics of
Mandibular Fractures: AnEvolution toward Understanding. Plast.Reconstr.Surg.
121:596,2008
Cortesy: Dr Riasa

8. Randal H. Rudderman, Robert L. Mullen, John H. Phillips. The Biophysics of Mandibular
Fractures: AnEvolution toward Understanding. Plast.Reconstr.Surg.
121:596,2008
• 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.
• 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.
• 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 mar-gin and less separation, or
relative compression, of the lower margin.
• The result is tension at the upper border not on the bone but
on immediately adjacent soft tissue.

9. Randal H. Rudderman, Robert L. Mullen, John H. Phillips. The Biophysics of Mandibular
Fractures: AnEvolution toward Understanding. Plast.Reconstr.Surg.
121:596,2008

10. Tension & Compression Zones and Facial circuit of force
Compression
Tension
Loading
Concept 3: of 1980 Concept 4: 2008

11. Zygomatic Biomechanical Beam Theory
 The zygomatic arch experiences inferiorly
directed bending in the parasagittal plane, as
well as torsion in which the inferior margin of the
arch is rotated medially.
 The medially directed component of the
masseter muscle force further suggests that the
arch might experience mediolateral bending.
 An arch that is circular in cross section is equally
strong under bending in all directions, whereas a
blade-like arch (i.e., one that is much taller than it
is wide) would be much stronger
in parasagittal bending than in mediolateral bendi
ng
Amanda L Smith, Ian R Grossse. The Biomechanics of Zygomatic Arch Shape. The Anatomical Record. 299:1734–1752 (2016)

12. Evolution of Concepts
 Concept 1 (< mid 20th century): application of splinting technique to achieve
maxillomandibular fixation.1
 Concept 2 (20th century): The concept of rigid fixation using compression plate, lag
screw and reconstruction plate.2 Application of internal fixation providing the
option for early return to function, adequate healing, reducing the consequences of
immobilization of active joint. This principles advocated by AO/ASIF
 Concept 3: Understanding of biomechanical behavior of facial structure, supporting
development of semirigid fixation techniques.3
 Concept 4: multiple plate system repair (to increase stiffness and strength) versus
single small plate system repair (produced consistent favorable results.4
1. Dingman RO , Natvig P. Surgery of facial fractures, Philadelphia,1964, WB Saunders.
2. Bernd Spiessl (ed). New Concept in Maxillofacial Bone Surgery. Springer-Verlag Berlin Heidelberg, Germany, 1976.
3. Champy M, Lodde JP, Schmitt R, et al. J Maxillofac Surg 1978;6: 14.

13. Evolution of devices
 Wire loop: approximating segments to stabilize fractures
 Maxillomandibular wire splinting
 Arch bar: transfer of forces from one segment to the other through the bar during
loading. If the fracture segment are subject to displacement, the bar will serve to
prevent distraction and will loaded with tension (more efficient in tensile than
compression stress). Load sharing under compressive forces only happened if the
bone segment in contact. By applying MMF fixation/rubber, the biomechanical
behavior of function is eliminated due to restricted motion.

14.  Rigid fixation (ORIF): Plates and screws: modify the stress on fracture sites during
loading.
 Non locking system. As the screw tightens, compressive forces increase between the
plate and the bone surface and increase the friction forces between the plate and the
bone. More screws will increase frictional forces.
 Locking system: screw locks into additional treads in the plate. The stability rely on the
screw/bone interface and screw/plate interface
 External fixation: no reliance of the plate/bone interface for stability. Stability in
both system depends on the screw/bone contact, and local failure will result in
system failure
 Locking system and external fixation system behave in similar manner

15. Ideal System
 Materials and applications that stimulate the bone and geometry, do not interfere
with function and healing while providing adequate stiffness to resist excess
motion and create appropriate environment for healing to occur.
 The construct must able to sustain loading conditions without excessive stress
concentration that will damage the bone. Micromotion at the screw insertion site
will lead to future instability
 Because the forces flow along the area of greatest stiffness, rigid system (too stiff)
will causing concentration of stress leading to mobility on screw/bone interface
 During and following bone healing without screw loosening, the screw/plate/screw
path will continue to carry forces across fracture site

16. Load-sharing osteosynthesis
 Simple fracture pattern and
acceptable amount of bone stock
 Ideal osteosynthesis lines: Maxime
Champy lines
Torsional Forces

17. Semi Rigid Fixation (Miniplate): The Most Recent
Practice Osteosynthesis CMF
 The ideal line of osteosynthesis in the body region runs at the
vertical height of the tooth apices from the canine region to the
oblique line. This carries into the oblique ridge which turns into the
anterior outer rim of the ramus.
 All biomechanical models developed to date have shown that two
points of fixation (ie, two plates) provide much more stability than a
single one. The basal triangle decreases the bone buttressing and
interfragmentary support. This condition demands a degree of
stability beyond pure load sharing.
 The superior border plate is positioned on the ideal line of
osteosynthesis. The inferior border plate is located at the base of the
mandibular body in a longitudinal field below the course of the
mandibular canal.

18. Thank You