Safety and preliminary efficacy of allogeneic bone marrow-derived mesenchymal stromal cells (BMSC) for painful lumbar facet arthropathy: study protocol for a randomised, double-blinded, placebo-controlled, crossover phase II clinical trial. - Yan D; Durand NC; Wang Z; Osborne MD; Stone JA; Gupta S; Miller DA; Witter DM; Eldrige JS; Quiñones-Hinojosa A; Zubair AC; Qu W (BMJ open, 2026)