O'Brien CS, Martinez SA. Comparative Orthopedic Research Laboratory, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, WA 99164, USA. Vet Surg 2009 Oct 38 7 868-73 OBJECTIVE: To evaluate potential iatrogenic medial meniscal (MM) damage during tibial plateau leveling osteotomy (TPLO) and to establish a safe zone (SZ) for hypodermic needle (HN) identification of the medial aspect of the stifle joint. STUDY DESIGN: Prospective cohort. ANIMALS: Cadaveric canine stifles (n=40). METHODS: HN (20 or 25 G) were inserted through the medial collateral ligament (MCL) of the femorotibial joint and through the SZ insertion points. The medial meniscus was inspected for iatrogenic damage. Statistical comparison of MM damage caused by different needle sizes and insertion sites was performed using Fisher's exact test with significance at P< .05. RESULTS: Twenty-gauge group: 65% of stifles had minor MM damage with MCL insertion compared with 35% of stifles with SZ insertion (P=.0049). Severe MM damage occurred in 25% of stifles with MCL insertion compared with 0% of stifles with SZ insertion (P=.0014). Twenty-five-gauge group: 85% of stifles had minor MM damage with MCL insertion compared with 30% after SZ insertion (P=.0011); however, no severe MM injury was noted. CONCLUSIONS: HN insertion though the MCL can produce iatrogenic damage to the MM. Use of a 25 G HN and SZ site for insertion reduced the frequency and severity of MM damage. CLINICAL RELEVANCE: HN insertion into the medial aspect of the femorotibial joint during TPLO can cause gross iatrogenic MM damage, which may contribute to the incidence and misdiagnosis of latent MM injuries after TPLO.