Bottom line: The first documented endoscopic look inside a knee joint was presented in 1912 by the Danish surgeon Severin Nordentoft. The first clinical series of knee arthroscopies in patients was reported by Eugen Bircher in 1921–1926. The modern era began in the 1950s when Masaki Watanabe introduced the mass-produced Watanabe No. 21arthroscope, enabling routine diagnostic and therapeutic knee arthroscopy and worldwide adoption in the 1960s. Arthroscopy Journal+2Arthroscopy Journal+2PubMed

A brief, evidence-based timeline

• 1912 — First recorded knee endoscopy (arthroscopy concept).
  Severin Nordentoft presented “Endoscopy of Closed Cavities” at the German Surgical Society (Berlin), describing an endoscope for use in the knee to detect meniscal lesions. This is the earliest published record linking endoscopy to the knee joint (patient vs cadaver use is debated), and many historians credit him as the first arthroscopist. Arthroscopy JournalPubMedScienceDirectrise.aana.org

• 1919 — First knee arthroscopic examination in Japan (traditional credit).
  Kenji Takagi performed an arthroscopic examination of the knee using a cystoscope; this work seeded the Japanese school of arthroscopy that later advanced the optics and instruments. abjs.mums.ac.irPMC

• 1921–1926 — First sizeable clinical series in patients.
  Swiss surgeon Eugen Bircher published multiple papers on internal derangements of the knee and performed ~60 endoscopic procedures, typically preceding meniscectomy—considered the first large-scale clinical use of knee arthroscopy. PubMedScienceDirect

• 1958/1959 — Modern arthroscope and routine practice.
  Masaki Watanabe, Takagi’s student, developed the Watanabe No. 21 arthroscope, produced at scale in 1959. Its improved optics/illumination made arthroscopy practical and reproducible, marking the start of modern knee arthroscopy. Arthroscopy JournalScienceDirectryortho.com

• 1965–1967 — North American adoption.
  Robert W. Jackson brought Watanabe’s scope to Toronto in 1965 and reported early North American experience in 1967, catalyzing wider dissemination and eventually professional societies dedicated to arthroscopy. PMC

So, when did knee arthroscopy “start”?

It depends on what you mean by “start”:

• First described/recorded: 1912 (Nordentoft). Arthroscopy Journal

• First clinical series in patients: 1921–1926 (Bircher). PubMed

• Start of the modern, widely adopted era: 1958–1959 with Watanabe No. 21; global spread in the 1960s. Arthroscopy JournalPMC

This framing aligns with current historical analyses in the Arthroscopy journal and related orthopedic literature. Arthroscopy Journal+2Arthroscopy Journal+2

References (selected academic sources)

• Kieser CW. Severin Nordentoft: The First Arthroscopist. Arthroscopy. 2001. (PDF of original article). Arthroscopy Journal

• Kieser CW. Eugen Bircher (1882–1956): the first knee surgeon to use diagnostic arthroscopy. Arthroscopy. 2003. PubMedArthroscopy Journal

• Jackson RW. A History of Arthroscopy. Arthroscopy. 2010. Arthroscopy Journal

• Solheim E. Milestones in the early history of arthroscopy. Arthroscopy Techniques, History, and Education. 2022. ScienceDirect

• DeMaio M. Giants of Orthopaedic Surgery: Masaki Watanabe, MD. Clin Orthop Relat Res. 2013. (PMC). PMC

• Razi M. ISKAST: the Way We Have Made and Upfront. Arch Bone Jt Surg. 2018. (notes Takagi 1919).