# Ottawa Knee Rule Calculator (Stiell 1995) > 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 ## Facts - Endpoint: GET https://api.magentlab.com/api/calc/ottawa_knee - Price: $0.005/call - Payment: x402 - Status: unknown - Last checked: 2026-09-15 - Activations on Zero: 0 - Tags: x402 - Canonical page: https://www.zero.xyz/c/ottawa-knee-rule-calculator-stiell-1995-5991c402 - Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_G8phQc56JTrNayxhBhqje Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently. ## How to call it through Zero Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`): ```sh zero fetch --capability ottawa-knee-rule-calculator-stiell-1995-5991c402 ``` Example prompt: My patient is 62 years old, has bone tenderness at the fibular head, no isolated patellar tenderness, can flex to 90 degrees, couldn't bear weight right after the injury, and still can't bear weight here in the ED — does the Ottawa Knee Rule say we need an x-ray? ## When to prefer this 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. ## Known failure modes - 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 ## 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. ## Request schema (JSON Schema) ```json { "type": "object", "$schema": "https://json-schema.org/draft/2020-12/schema", "required": [ "input" ], "properties": { "input": { "type": "object", "required": [ "type", "method" ], "properties": { "type": { "type": "string", "const": "http" }, "method": { "enum": [ "GET", "HEAD", "DELETE" ], "type": "string" }, "headers": { "type": "object", "additionalProperties": { "type": "string" } }, "queryParams": { "type": "object", "required": [ "age", "fibular_head_tenderness", "isolated_patellar_tenderness", "flexion_less_than_90", "unable_to_bear_weight_immediately", "unable_to_bear_weight_in_ed" ], "properties": { "age": { "type": "integer", "description": "Age in years (18-120). Age ≥55 meets this criterion; 54 does not." }, "flexion_less_than_90": { "type": "boolean", "description": "Inability to flex the knee to 90 degrees. true or false." }, "fibular_head_tenderness": { "type": "boolean", "description": "Bone tenderness at the head of the fibula. true or false." }, "unable_to_bear_weight_in_ed": { "type": "boolean", "description": "Unable to bear weight in the emergency department (four steps). true or false. Counts only when also unable immediately after injury." }, "isolated_patellar_tenderness": { "type": "boolean", "description": "Isolated patellar tenderness as assigned by the caller. true or false. magent does not infer isolation from other flags." }, "unable_to_bear_weight_immediately": { "type": "boolean", "description": "Unable to bear weight immediately after injury (four steps). true or false. Counts only when also unable in the emergency department." } } } }, "additionalProperties": false }, "output": { "type": "object", "required": [ "type" ], "properties": { "type": { "type": "string" }, "example": { "type": "object" } } } } } ``` ## Response schema (JSON Schema) ```json { "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 submissi … (truncated) ``` ## More - Live health (JSON, refreshed every minute): https://www.zero.xyz/c/ottawa-knee-rule-calculator-stiell-1995-5991c402/health.json - [Zero catalog index](https://www.zero.xyz/llms.txt) - [Other services from api.magentlab.com](https://www.zero.xyz/host/api.magentlab.com/llms.txt)