Ottawa Knee Rules Calculator

The Ottawa Knee Rule asks a single question: does this acutely injured knee need a radiograph at all. Imaging is indicated if any one of five findings is present - age 55 or older, isolated tenderness of the patella with no other bone tenderness, tenderness at the head of the fibula, inability to flex the knee to 90 degrees, or inability to bear weight for four steps both at the scene and in the department. Each item points at a mechanism that produces a fracture rather than a soft tissue injury. Age captures the rise in osteoporotic tibial plateau and patellar fractures; isolated patellar tenderness suggests a direct dashboard or fall impact; fibular head tenderness flags proximal fibular fractures, which can accompany peroneal nerve injury or a Maisonneuve pattern; loss of 90 degrees of flexion and weight-bearing failure reflect functional disruption rather than pain alone. Clinically significant fracture prevalence among adults with acute knee injury is only around 6 to 7 percent, so a rule with sensitivity near 98 to 100 percent and specificity of roughly 45 to 50 percent leaves a post-test probability well under 1 percent when all five criteria are negative. That supports safely omitting films in about a quarter to a third of presentations. The rule was validated in adults, applies only to blunt acute injury generally within 7 days, and is not intended for patients under 18, those with altered consciousness or paraplegia, isolated superficial skin wounds, or a return visit for reassessment of the same knee.

The rule came from the same Ottawa Civic Hospital research programme that produced the ankle rules. Ian Stiell, George Greenberg, Robert McKnight, and colleagues published the derivation in Annals of Emergency Medicine in 1995, then a prospective validation in JAMA in 1996 and a further multicentre implementation study in JAMA in 1997. The problem they targeted was the same one that motivated the ankle work: knee radiographs ordered after acute injury were negative in roughly 90 to 95 percent of cases, generating cost and delay for very little diagnostic yield. Candidate clinical findings were recorded prospectively before imaging and reduced by recursive partitioning to the smallest set that missed no clinically important fracture. External validation followed quickly - an early United States study by Stiell and others reported lower specificity than the Canadian cohorts, and a competing instrument, the Pittsburgh Decision Rule published by Seaberg and Jackson in 1994, offered higher specificity using mechanism of injury and age criteria. Pooled analyses, including a 2004 meta-analysis in Academic Emergency Medicine, ultimately supported the Ottawa rule as having near-perfect sensitivity across settings, which is the property that matters for a rule-out tool. Pediatric application was studied separately by Bulloch and colleagues in 2003, supporting use in children roughly 2 to 16 years old, though most guidelines still treat the adult evidence as the primary basis and recommend clinical judgment in growing skeletons.