Clinical anatomy of the spina musculi recti lateralis: A frequently overlooked variation of the greater wing of the sphenoid.

Background: The spina musculi recti lateralis (SMRL) is often visible along the lateral rim of the superior orbital fissure (SOF). Aim of this study is to characterize SMRL morphology and topography relative to known bony landmarks.

Methods: Orbits from 291 adult dry skulls and from 60 CT scans were analyzed to measure the distance between the SMRL and the SOF or the inferior orbital fissures (IOF) as well as its height, width and orientation. Processes other than SMRLs were also recorded. Fetal skulls were observed for comparison with adult samples.

Results: Forty-one per cent of orbits on dry skulls and 43.3% by CT showed an SMRL. Additional 32.9% of orbits on dry skulls had processes with a different shape. On average, SMRL were orientated almost along the transverse plane and showed implant bases as wide as 141.9° or as narrow as 36.8°. SMRLs were close to the infero-posterior angle of the orbital plate of the sphenoid, 1.21 ± 0.84 mm in front of the SOF, 5.8 ± 1.9 mm above the IOF and 12 ± 2.3 mm from the anterior end of the SOF. They were 1.58 ± 0.64 mm high and did not show any age or sex-related prevalence. By CT, the SMRL appeared as the insertion site for the lateral rectus, tendinous ring and, sometimes, inferior rectus.

Conclusions: The SMRL is a process of the sphenoidal orbital plate rather than of the SOF. It is also a reliable landmark for the insertion of the tendinous ring and lateral rectus. Orbital surgeons should be aware of this common variant of the orbital apex.