Muscle biomechanics

Biomechanics and mechanisms of mandibular fractures

The masticatory musculature plays a key role in the pathogenesis of fractures and the subsequent behavior of bone fragments. The mandible is controlled by two pairs of muscles with symmetrical attachment points: elevators and depressors.

Fractures often occur in "weak" spots (the thinnest and most curved areas where the bone breaks due to bending): the condylar process, the angle of the jaw, the mental foramen, and the canine region. Depending on the impact area, a fracture can be direct (at the site of force application in a narrow area) or indirect (away from the impact site when acting on a wide area).

Three biomechanical mechanisms of fractures are distinguished:

• Shear mechanism: a longitudinal fracture of the ramus occurs. Condition — an impact from bottom to top at the base of the jaw (anterior to the angle, in a narrow area). An unsupported section of the bone shifts relative to the fixed fragment.
• Compression mechanism: leads to a transverse fracture of the ramus. The impact is directed from bottom to top along the base in the angle region over a wide area. The ramus, fixed in the articular fossa, undergoes severe compression by opposing forces.
• Avulsion mechanism: an impact from top to bottom on the chin with tightly clenched teeth causes a powerful reflex contraction of all masticatory muscles. As a result, the strong temporalis muscle avulses the thin coronoid process to which it is attached.

The displacement of the resulting fragments is dictated by three factors: contraction of the attached muscles (this is the determining factor), the continued action of the traumatic force, and the fragment's own weight.

Biomechanics of the muscular system in labor (expulsion phase)

Expulsive efforts (pushing) represent an involuntary reflex muscular act. It is based on the Ferguson reflex: when the fetal presenting part exerts pressure on the pelvic nerve plexuses, cervical endings, and perineal muscles, the parturient woman experiences an irresistible urge to push.

The biomechanics of fetal expulsion includes the following stages:

1. The parturient woman holds her breath.
2. A sharp contraction of the diaphragm and abdominal wall musculature occurs.
3. Intra-abdominal and intrauterine pressure increases significantly.

Since the uterus is rigidly fixed by the ligamentous apparatus (broad, round, and uterosacral ligaments), all the energy of the generated pressure is directed exclusively toward expelling the fetus. The fetus performs complex movements, advancing along the path of least resistance — the pelvic axis. The presenting part descends onto the pelvic floor, stretches the vulvar cleft, after which the baby is born completely, and the hindamniotic fluid is discharged.

The role of the myometrium in the third stage of labor

After the birth of the child, the third (placental) stage of labor begins, normally lasting from 5 to 30 minutes. Its essence lies in the detachment of the placenta (placenta, fetal membranes, and umbilical cord) from the underlying uterine wall.

The main biomechanical driver of this process is placental contractions (colicky contractions of the myometrium). Detachment is also facilitated by a significant decrease in the uterine cavity volume, the location of the placenta in the functional layer of the mucosa (which facilitates separation from the basal layer), and the lack of contractile ability in the placenta itself.

Depending on the localization of the onset of detachment, two biomechanical variants are distinguished:

• Schultze mechanism (most frequent): detachment starts from the center. Due to the contraction of the muscular wall, the placenta is elevated in the form of a mound, and uteroplacental vessels rupture. Blood accumulates, forming a retroplacental hematoma, which pushes the placenta into the uterine cavity. Uterine contraction, hematoma growth, and gravity lead to complete expulsion. The placenta is born turned inside out (covered by the amniotic membrane, fetal surface facing outward).
• Duncan mechanism: detachment starts from the edge. A retroplacental hematoma does not form, as blood freely flows down, stripping the fetal membranes. With each new placental contraction, new areas are separated. The process is aided by the placenta's own mass — the detached edge hangs into the cavity and pulls it down.

Class and exam questions

What else to study on this topic

• Topography of mandibular "weak" spots (condylar process, angle, mental foramen).
• Differences between direct and indirect bone fractures.
• Anatomical structure of the conceptus (placenta, fetal membranes, umbilical cord).
• The role of the functional and basal layers of the uterine mucosa.
• The pelvic axis and the biomechanics of fetal advancement.

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.