《骨空論》膝痛不可屈伸，治其背內。Gu Kong Lun (SW60): "When the knees ache and cannot bend or stretch, treat the back."
Here this is literally the back (i.e., not the posterior/back of the knee). Gao Shishi says that since the Bladder Taiyang channel governs the sinews (c.f., Ling Shu 10), then we needle the back to reestablish normal bending and stretching of the vessels. Wang Bing says this means needling Da Zhu BL-11. In Tung's acupuncture we agree... For knee pain (especially chronic) we bleed Jin Dou (金斗 = Po Hu BL-42), Jin Ji (金吉 = Gao Huang Shu BL-43) and Jin Ling (金陵 = Shen Tang BL-44). Much of Tung's acupuncture agrees with other systems of classical acupuncture based on the Huang Di corpus.