1
Dr. Mohammed Alruby
Mechanics
with
orthopedic
treatment
Prepared by
Dr Mohammed Alruby
‫االمل‬ ‫اطال‬ ‫من‬
----
‫العمل‬ ‫اساء‬

2
Dr. Mohammed Alruby
Mechanics consideration in orthopedic force
The mechanical consideration in orthopedics are related to orthopedic force magnitude, duration
and direction,
1- Force produced by functional appliances:
a- Direct measurement of force.
b- Indirect measurement of force; electromyographic studies.
Measuring bite force during functional appliance
wear
2- Posterior force on maxilla ((headgear))
3- Anterior force on maxilla ((face mask) (reverse headgear)
4- Transverse force on maxilla (RME)
(1) Force produced by functional appliances:
During Class II correction with functional appliances, the jaws muscles stretched as a result of
mandibular advancement and exert a posteriorly directed force on the mandible, either activity or
by their elastic properties, this force is transmitted through the functional appliance to the maxilla.
A- Direct measurement of force produced by functional appliances:
= it has been reported that, inter-maxillary forces exerted by functional appliances during
treatment vary between 25 to 500 gm.
= the mean inter-maxillary forces by functional appliances for Class II correction 100gm
at upright position and 123gm at retroclined position.
B- Indirect measurement of force produced by functional appliances:
Is delivered by:
1-Electromyograpgic studies of jaws muscles:
= functional appliances act via an antro-posterior and vertical displacement of the
mandible, causing stretching of oral and facial soft tissue and muscles and myotonic reflex
which responsible for tension exerted on teeth and bony structures during treatment.
= EMG studies revealed that, EMG activities of jaw closing muscles decrease as an
immediate neuromuscular advancement, while those of jaws muscles opening increase.
Lateral pterygoid hypothesis:
= hypothesis stated that, hyperactivity of lateral pterygoid muscle is considered to be
responsible for condylar growth.
= EMG studies demonstrate that, the lateral pterygoid activity increases immediately after
insertion of forward positioning appliance but activity decrease markedly after 4-6 months
of treatment.
= resection of lateral pterygoid in experimental animals led to diminished condylar growth,
so some authors considered the lateral pterygoid muscle to be critical importance in
functional therapy, but on the other hand, there are several studies revealed, there is no
role of lateral pterygoid muscle on condylar growth.
Muscle adaptation to mandibular advancement:
= EMG activities remain altered only during the 1st
months of treatment about 6 months of
treatment then slowly return to initial level.
= studies of pig master muscles and medial pterygoid adapted to mandibular advancement
by transformation of type II into type I fibers.

3
Dr. Mohammed Alruby
= another study found atrophic changes in jaw muscles in response to mandibular
advancement ((mild atrophy on masticatory muscle)) because decreased functional activity
during treatment.
2- measuring bite force by functional appliances wear:
The magnitude of force delivered by functional appliances is variable and may be affected
by the following factors:
a- Individual variation in muscle tone and maximum bite force: human studies on
functional appliances suggest that, the skeletal changes has inter-individual variation.
= studies evaluate the biting force on molar during functional appliance treatment, they
concluded that maximum molar bite force increase in growing child but decrease during
activator treatment.
b- The thickness of construction bite:
= increased inter-occlusal jaw opening may lead to some changes in the bite force and
electro-myographic activities of jaw muscles.
= an increase in the vertical thickness of construction bite to 10mm may provide an
immediate effect of reducing masseter and anterior temporalis muscle activity.
c- The amount of mandibular advancement:
= for 3mm continuous enlargement or large single advancement, there is no differences
in orthopedic and orthodontic force.
= for 1mm continuous advancement, the response diminished but still significant.
(2) Posterior force on maxilla: ((headgear):
= the principle of head gear treatment is to apply a retroactive force of maxillary 1st
molars
in order to restrict the forward growth of maxilla.
= it has been shown that retroactive force in excess of 450gm needed are needed to obtain
skeletal effect, but when use light force (150-200gmm) only dento-alveolar changes was observed.
= there is a common agreement among clinician that: ideal amount of force for orthopedic
maxillary retraction is 350-450gmm / side.
NB: the latest evidence suggests that optimal condition for achieving orthopedic change in the
maxilla by headgear are; 1- the force is sufficient magnitude (1000gm/side)
2- most of teeth incorporated into the appliance.
** direction of headgear:
==Direction of headgear forces changed according to type of treatment needed as: vertical,
cervical, and occipital pull and sometimes there is some combination can occur.
== the center of resistance of maxilla is located on a line perpendicular to the functional occlusal
plane at the distal contact of the maxillary first molars, at one half the distance from the functional
occlusal plane to inferior border of the orbit.
== there are two center of resistance of the maxilla, on the right and left side.
== cervical traction causes the maxilla to move downward and backward, it oppose forward
growth and enhance downward growth.
== occipital traction is the method of choice for patients with open bite tendencies.
== some clinicians used combination of cervical and occipital approaches in high angle cases, to
combine the favorable maxillary growth restriction by cervical traction and better control of the
vertical dimension by the occipital traction.
(3) Anterior force on maxilla: facemask- reverse headgear:
Most studies from 1983 till 2008; the optimum force 447gm, 27,5 degrees to the occlusal plane for
15,2hr/day

4
Dr. Mohammed Alruby
Studies was made in two groups: group I received 50gm protraction force from canine region
directed downward 30 degree to the occlusal plane. Group II 500gm is applied extra-orally 20mm
above the maxillary occlusal plane.
For both groups, there is protraction on maxilla but in group I their counter clockwise rotation,
while in group II the maxilla translated forward without rotation, so: the force application from
near the center of resistance of maxilla was an effective method to prevent the unwanted side effect,
such as counter clockwise rotation of maxilla.
(4) Transverse force on maxilla RME:
= the force of palatal expansion often can reach 2 – 10 pounds to achieve the desired orthopedic
effect. The force level depends on appliance: single activation of Hass or Hyrax appliance produce
2-10 pounds of force with rapid initial decay followed by slower decay curve.
= younger patients dissipate loads faster than older patients, the separation of the central incisors
occurred between activations, and was not accompanied by any drop recorded load.
Relation to the center of resistance during expansion:
= from the clinical view: the placement of jack screw should be as close to center of resistance as
possible to effect a more translatory movement of maxillary halves as well as the teeth.
=== in primary dentition: this should be in the middle of palate between primary 1st
and 2nd
molars
=== in mixed and permanent dentition: this location moves posteriorly between 2nd
premolars or
2nd
primary molars and 1st
permanent molars.
= when jack screw is activated, the force that is applied create equivalent moment at the center of
resistance of each maxilla. It is possible to direct the force from the jack screw through the center
of resistance to produce pure bodily movement.
Center of rotation:
1st
: located at the fronto-nasal suture----seen from frontal view.
2nd
: located at posterior of mid palatal suture in the third molar area—when viewed from occlusal.
Duration of force:
= duration of force application in removable functional appliances as recommended by many
clinician ranges between 12 -16 hours.
= continuous force produces longer duration maintenance.
= continuous force produces 1.3mm than intermittent force (0.8mm) sutural separation.