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Ottawa Knee Rule Calculator (Stiell 1995)
x402
Ottawa Knee Rule Calculator (Stiell 1995) is a paid API for AI agents from api.magentlab.com, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-15).
Applies the Ottawa Knee Rule to determine whether a knee x-ray is indicated based on age and five clinical criteria
Price / call$0.005
Success—
StatusUnknown
How this service works
Ottawa knee rule (Stiell 1995). Knee x-ray is indicated when any of: age 55 or older, bone tenderness at the fibular head, isolated patellar tenderness (caller-applied), inability to flex to 90 degrees, or inability to bear weight both immediately after injury and in the emergency department (four steps). One weight-bearing failure alone is not enough. Not a diagnosis of fracture, not the Ottawa ankle or foot rule, and not the Pittsburgh knee rule. magent does not examine the knee.
Output
Returns whether a knee x-ray is indicated according to the Ottawa Knee Rule (Stiell 1995), based on the five clinical criteria: age ≥55, fibular head tenderness, isolated patellar tenderness, inability to flex to 90°, and inability to bear weight both immediately after injury and in the ED. The result reflects the rule's logic, not a diagnosis of fracture, and does not apply to the Ottawa ankle/foot rule or Pittsburgh knee rule.
Example use cases
ED triage after knee injury
“I have a 58-year-old patient who twisted their knee playing soccer. They have isolated patellar tenderness, no fibular head tenderness, can flex past 90 degrees, and were able to bear weight both immediately after and now in the ED. Does the Ottawa Knee Rule require an x-ray?”
Age-threshold clinical decision check
“Patient is 54 years old, has fibular head tenderness, and can't flex to 90 degrees — run the Ottawa Knee Rule for me: no isolated patellar tenderness, was able to bear weight right after injury, and still can here in the ED.”
Weight-bearing failure edge case
“My patient is 45, no fibular head tenderness, no patellar tenderness, can flex fully, but couldn't bear weight immediately after the injury — they can now though. Does the Ottawa Knee Rule still call for imaging?”
Technical details
Endpoint
GEThttps://api.magentlab.com/api/calc/ottawa_kneeWhen to prefer
Use this endpoint when you need a fast, deterministic application of the Ottawa Knee Rule (Stiell 1995) specifically — not the Pittsburgh knee rule or Ottawa ankle/foot rule. Ideal for ED decision-support workflows, clinical triage automation, or documentation assistance where the caller can supply all six required clinical parameters. Do not use for ankle/foot injuries or pediatric populations outside the age range.
Side effects
- Missing required query parameters (age, fibular_head_tenderness, isolated_patellar_tenderness, flexion_less_than_90, unable_to_bear_weight_immediately, unable_to_bear_weight_in_ed) returns a validation error
- Age outside the valid range (18-120) may return an error or undefined behavior
- Boolean fields supplied as strings instead of true/false may cause parsing errors
- Misapplying the rule (e.g. confusing with Ottawa ankle rule or Pittsburgh knee rule) yields a clinically incorrect invocation
- Payment failure (402) if USDC balance is insufficient
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| inputrequired | object | |
| output | object |
Output
Schema · Response
{
"type": "json",
"example": {
"path": "/api/calc/ottawa_knee",
"count": 2,
"items": [
{
"formula": "ottawa_knee",
"citation": {
"id": "stiell-1995",
"doi": "10.1016/S0196-0644(95)70106-0",
"title": "Derivation of a decision rule for the use of radiography in acute knee injuries",
"source": "Ann Emerg Med. 1995;26(4):405-413",
"authors": "Stiell IG, Greenberg GH, Wells GA, et al."
},
"criteria": [
{
"met": false,
"value": 40,
"factor": "age_ge_55"
},
{
"met": false,
"factor": "fibular_head_tenderness"
},
{
"met": false,
"factor": "isolated_patellar_tenderness"
},
{
"met": false,
"factor": "flexion_less_than_90"
},
{
"met": false,
"factor": "unable_to_bear_weight_both",
"unable_to_bear_weight_in_ed": false,
"unable_to_bear_weight_immediately": false
}
],
"age_years": 40,
"disclaimer": "Not a medical device. Deterministic published-formula or published-code output for autonomous agents. A licensed clinician remains responsible for patient care and billing submissions.",
"xray_indicated": false,
"formula_expression": "Knee x-ray indicated iff age >= 55 or fibular_head_tenderness or isolated_patellar_tenderness or flexion_less_than_90 or (unable_to_bear_weight_immediately and unable_to_bear_weight_in_ed)"
},
{
"formula": "ottawa_knee",
"citation": {
"id": "stiell-1995",
"doi": "10.1016/S0196-0644(95)70106-0",
"title": "Derivation of a decision rule for the use of radiography in acute knee injuries",
"source": "Ann Emerg Med. 1995;26(4):405-413",
"authors": "Stiell IG, Greenberg GH, Wells GA, et al."
},
"criteria": [
{
"met": false,
"value": 40,
"factor": "age_ge_55"
},
{
"met": false,
"factor": "fibular_head_tenderness"
},
{
"met": false,
"factor": "isolated_patellar_tenderness"
},
{
"met": false,
"factor": "flexion_less_than_90"
},
{
"met": false,
"factor": "unable_to_bear_weight_both",
"unable_to_bear_weight_in_ed": false,
"unable_to_bear_weight_immediately": false
}
],
"age_years": 40,
"disclaimer": "Not a medical device. Deterministic published-formula or published-code output for autonomous agents. A licensed clinician remains responsible for patient care and billing submissions.",
"xray_indicated": false,
"formula_expression": "Knee x-ray indicated iff age >= 55 or fibular_head_tenderness or isolated_patellar_tenderness or flexion_less_than_90 or (unable_to_bear_weight_immediately and unable_to_bear_weight_in_ed)"
}
]
}
}Tags
x402
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