Every ortho whom I've spoken with (and it has to be over ten in-person) has said 2nd premolar is the one to pull. Only justification for the first is something like a failing tooth.
There's no way first premolars is standard, nor should it be given how much bone and root loss comes with it.
Well, the salient point I was wanting to make was which pre-molar he should EXPECT to be the ones to be removed as to retrocline his proclined lower teeth in order to create a sufficient overjet for advancement via the BSSO
My web research of 'Which teeth are the most often/frequently extracted for braces?' kicked up plenty of sites that said the first pre-molars are the most frequently removed for orthodontic reasons. Of course, that doesn't preclude the second one from being selected for the right reasons, nor does it preclude what ever circumstances orthos you consulted with conveyed it 'should' be the 2nd PM to extract. Not to mention that for this particular poster who intially confused a pre-molar with an incisor to be extracted, I just wanted to pass along the MOST LIKELY PM he would be having extracted. Here's an example of sites kicked up that mention which PMs have high frequency of being extracted.
"....CONCLUSION
The number of orthodontic cases involving extractions has decreased over time. The frequency of tooth extraction observed in this study, over a period of 32 years, decreased by approximately 20%.
The teeth most often extracted were four first premolars, followed by the option of extracting only maxillary first premolars...."
Ref=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4816586/
Thing is the OP's ceph for the lower jaw where his teeth are to be extracted for the BSSO show an obtuse angle formed by the MPA and the inclination of lower incisor. So, I do think the pre-molar that will be extracted would be the 1rst PM which will give immediate relief as to closing in on that angle. Basically, I'm thinking in terms of which PMs do they most often extract for immediate relief for over proclined front lower teeth as to which ones he can expect to be removed. Since extraction refers to the lower jaw for advancement BSSO and his ceph shows an obtuse angle the inclination of the lower incisor makes with the MPA and that's the one they like to 'close in' on via plucking out the 1rst PM, I think it's fair to say IN PRACTICE, it's the 1rst PM they pluck and he can expect that one to be targeted for the extraction whether or not it has 2 roots.