The bounding box of the graphic is set on the base line. So you have to add some artificial depth to the graphic.
\starttext
\startreusableMPgraphic{tetrahedral} begingroup u := 10pt; draw (0,0)--(2u,0)--(u,sqrt(3)*u)--cycle; draw boundingbox currentpicture withpen pensquare scaled .1; endgroup; \stopreusableMPgraphic
\def\Tetrahedral{\lower5pt\hbox{\reuseMPgraphic{tetrahedral}}}% needs some fiddling :)
\startformula \Phi(\Tetrahedral)=F_{kln}^{ijm}\,v_iv_jv_kv_l\, \delta_{j^*nk^*}\,\delta_{l^*n^*i^*}\,\Phi(\emptyset) \stopformula
\stoptext
That's exactly what I was looking for … thanks very much! Out of curiosity just a few questions … why do you use grouping in the MetaPost code and why do you draw the bounding box explicitly? When I was experimenting with your suggestion a bit I tried leaving out those two commands at some point and surprisingly things still looked fine … And what's the difference between \startreusableMPgraphic and \startuseMPgraphic? Does caching play a role? Thanks again, Oliver