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Orbital Blowout Fractures
Experimental Evidence for the Pure Hydraulic Theory
John S. Rhee, MD;
John Kilde, MD;
Narayan Yoganadan, PhD;
Frank Pintar, PhD
Arch Facial Plast Surg. 2002;4:98-101.
Background The mechanism of injury and the underlying biomechanics of orbital blowout
fractures remain controversial. The "hydraulic" theory proposes that a generalized
increased orbital content pressure results in direct compression and fracturing
of the thin orbital bone.
Objective To examine the pure hydraulic mechanism of injury by eliminating the
factor of globe-to-wall contact and its possible contribution to fracture
thresholds and patterns.
Materials and Methods Five fresh human cadaver specimens were used for the study. In each
cadaver head, 1 orbit was prepared to mimic the normal physiologic condition
by increasing the hypotony of the cadaver globe to normal intraocular pressure
(15-20 mm Hg) with intravitreous injection of isotonic sodium chloride solution
(saline). The second orbit served as a "hydraulic control," whereby the globe
and orbital contents were exenterated and replaced by a saline-filled balloon
at physiologic intraocular pressure. A 1-kg pendulum measuring 2.5 cm in diameter
was used to strike the cadaver heads. Drop heights ranged from 0.2 m to 1.1
m (1960 mJ to 10 780 mJ energy). Each head was struck twice, once to
each orbit. Direct visualization, high-speed videography, and computed tomographic
scans were used to determine injury patterns at various heights between the
2 orbits.
Results A fracture threshold was found at a drop height of 0.3 m (2940 mJ).
Fracture severity and displacement increased with incremental increases in
drop height (energy). Fracture displacement, with herniation of orbital contents,
was obtained at heights above 0.5 m (4900 mJ). Isolated orbital floor fractures
were obtained at lower heights, with medial wall fractures occurring in conjunction
with floor fractures at higher energies ( 6860 mJ). The globe intact side
and balloon (hydraulic control) side showed nearly identical fracture patterns
and levels of displacement at each drop height.
Conclusions This study provides support for the "hydraulic" theory and evidence
against the role of direct globe-to-wall contact in the pathogenesis of orbital
blowout fractures. In addition, the orbital floor was found to have a lower
threshold for fracture than the medial wall. Preliminary threshold values
for fracture occurrence and soft tissue displacement were obtained.
From the Department of Otolaryngology and Communication Sciences, Medical
College of Wisconsin (Drs Rhee and Kilde); and the Department of Neurosurgery,
Zablocki Veterans Affairs Medical Center (Drs Yoganadan and Pintar), Milwaukee,
Wis.
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