Rounding of the inferior rectus muscle as an indication of orbital floor fracture with periorbital disruption.

Objective: To determine if rounding of the inferior rectus muscle on coronal computed tomographic (CT) scans predicts disruption of the periorbita in orbital floor fractures and therefore predicts delayed enophthalmos.

Design: Cadaveric study with CT scan analysis.

Setting: Cadaveric laboratory and CT scanner at a tertiary care hospital in London, Ontario.

Methods: Each orbit of each cadaveric head was randomly assigned to have either intact or disrupted periorbita. Progressively larger orbital floor fractures were made and CT scans were taken before fractures and after each fracture to assess the shape of the inferior rectus muscle. Measurements were made of the length of the long and short axis of the inferior rectus muscle using CT analysis software. The short to long axis ratio was then compared.

Main outcome measures: Change in the short to long axis ratio of the inferior rectus muscle and correlation of the ratio with intact and disrupted periorbita.

Results: Orbital floor fractures measuring 1 x 1 cm show an increase in the short to long axis ratio (rounding) of the inferior rectus muscle only if the periorbita is disrupted. Orbital floor fractures measuring 2 x 2 cm show rounding of the inferior rectus muscle regardless of whether the periorbita is intact or disrupted; however, the degree of rounding is greater if the periorbita is disrupted.

Conclusions: For small orbital floor fractures, rounding of the inferior rectus muscle predicts periorbital disruption. This may therefore represent an indication for early surgical repair to prevent delayed enophthalmos.