Usage: 3 forms in 3 verses
Form Translated As     In Verse(s)
μεταφρένων his upper back LXX.Deut.32.11
μεταφρένοισ his upper back LXX.Ps.91.4
μετάφρενα her upper back LXX.Ps.68.13

Liddell-Scott-Jones

μετάφρενον (metaphrenon) the upper back G:N-N

μετάφρενον, τό,

properly part behind the midriff (μετὰ τὰς φρένας), broad of the back: hence, generally, back, μεταφρένῳ ἐν δόρυ πῆξεν ὤμων μεσσηγύς [Refs 8th c.BC+]; μ. ἠδὲ καὶ ὤμους (of a woman) [Refs 8th c.BC+]; in plural, of a single person, [Refs 8th c.BC+]; ὤμους καὶ μετάφρενα, of a woman, [Refs 7th c.BC+]— Epic dialect word, used by [Refs 5th c.BC+]

__II = {τὸ μεταξὺ τοῦ νώτου καὶ ὀσφύος κατὰ τὴν τῶν φρενῶν πρόσφυσιν} [Refs 2nd c.AD+]