  BACKGROUND: Proptosis is characterized by a protrusion of the eyeball due to
an increase of the orbital tissue volume. To recover a normal eyeball
positioning, the most frequent surgical technique consists in the osteotomy of
orbital walls combined with the manual loading on the eyeball. Only a rough
clinical rule is currently available for the surgeons but it is useless for
this technique. The first biomechanical model dealing with proptosis reduction,
validated in one patient, has been previously proposed by the authors. METHODS:
This paper proposes a rule improving the pre-operative planning of the
osteotomy size in proptosis reduction. Patient-related poroelastic FE models
combined with sensitivity studies were used to propose two clinical rules to
improve the pre-operative planning of proptosis reduction. This poroelastic
model was run on 12 patients. Sensitivity studies permitted to establish
relationships between the osteotoemy size, the patient-related orbital volume,
the decompressed tissue volume and the eyeball backward displacement. FINDINGS:
The eyeball displacement and the osteotomy size were non-linearly related: an
exponential rule has been proposed. The patient-related orbital volume showed a
significant influence: a bi-quadratic analytical equation liking the osteotomy
size, the orbital volume and the targeted eyeball protrusion has been
established. INTERPRETATION: Two process rules derived from patient-related
biomechanical FE models have been proposed for the proptosis reduction
planning. The implementation of the process rules into a clinical setting is
easy since only a sagittal radiography is required. The osteotomy size can be
monitored using optical guided instruments.
