# Ottawa knee rule

Ottawa knee rule

Status: LIVE
Price: $0.005 USDC
`GET /api/calc/ottawa_knee`
HTML: https://magentlab.com/docs/ottawa-knee
Markdown: https://magentlab.com/docs/ottawa-knee.md

Not a medical device. Not a diagnosis, dose, or listing recommendation.

## What it computes

Apply the Stiell 1995 Ottawa knee rule and return whether a knee x-ray series is indicated from age and caller-assigned bone tenderness, flexion, and weight-bearing findings.

## When to use

When an agent has caller-assigned findings after blunt knee injury and needs the published knee-series radiography rule, not the Ottawa ankle or Pittsburgh knee rule.

## When not to use

- Not a diagnosis of fracture. magent does not examine the knee.
- This tool is the Ottawa knee rule only. It is not the Ottawa ankle or foot rule and not the Pittsburgh knee rule.
- Isolated patellar tenderness is caller-applied. magent does not infer isolation from other flags.
- Inability to bear weight means both immediately after injury and in the emergency department (four steps). One alone is not enough.
- Adult derivation as published (age 18-120). Age 55 or older meets the age criterion; 54 does not.

## Formula

```
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)
```

## Citation

[Derivation of a decision rule for the use of radiography in acute knee injuries](https://doi.org/10.1016/S0196-0644(95)70106-0)
Stiell IG, Greenberg GH, Wells GA, et al.
Ann Emerg Med. 1995;26(4):405-413
DOI: [10.1016/S0196-0644(95)70106-0](https://doi.org/10.1016/S0196-0644(95)70106-0)

## Worked example

### All findings absent

Given: `age=40, fibular_head_tenderness=false, flexion_less_than_90=false, isolated_patellar_tenderness=false, unable_to_bear_weight_immediately=false, unable_to_bear_weight_in_ed=false`

1. Age 40 (under 55), no fibular-head or isolated patellar tenderness, able to flex to 90 degrees, and able to bear weight
2. Knee x-ray is not indicated


### Age 55 with no other findings

Given: `age=55, fibular_head_tenderness=false, flexion_less_than_90=false, isolated_patellar_tenderness=false, unable_to_bear_weight_immediately=false, unable_to_bear_weight_in_ed=false`

1. Age 55 meets the age criterion (54 does not)
2. Knee x-ray is indicated


### Unable immediately but able in the ED

Given: `age=40, fibular_head_tenderness=false, flexion_less_than_90=false, isolated_patellar_tenderness=false, unable_to_bear_weight_immediately=true, unable_to_bear_weight_in_ed=false`

1. Unable immediately after injury, but able to take four steps in the ED
2. One weight-bearing failure alone is not enough; knee x-ray is not indicated


## Also searched as

- Ottawa knee rule
- Ottawa Knee Rules
- Stiell knee rule
- knee x-ray decision rule
- OKR knee
- Ottawa radiography knee
- Stiell 1995 knee
- fibular head x-ray rule

## Parameters

- `age` (integer, required): Age in years (18-120). Age ≥55 meets this criterion; 54 does not.
- `fibular_head_tenderness` (boolean, required): Bone tenderness at the head of the fibula. true or false.
- `isolated_patellar_tenderness` (boolean, required): Isolated patellar tenderness as assigned by the caller. true or false. magent does not infer isolation from other flags.
- `flexion_less_than_90` (boolean, required): Inability to flex the knee to 90 degrees. true or false.
- `unable_to_bear_weight_immediately` (boolean, required): Unable to bear weight immediately after injury (four steps). true or false. Counts only when also unable in the emergency department.
- `unable_to_bear_weight_in_ed` (boolean, required): Unable to bear weight in the emergency department (four steps). true or false. Counts only when also unable immediately after injury.

## Bulk (POST)

POST the same path with a JSON items array. Price is n times the GET unit. Invalid items return HTTP 400 before settlement.

Method: `POST /api/calc/ottawa_knee`
Max items: 25
Price: unit_amount * n (unit 5000 atomic)
Status: PREVIEW

- 1 to 25 items. Each item uses the same keys as the GET query parameters.
- The first invalid item fails the whole request (index in the 400 JSON). You are not charged.
- HTTP 200 is {count, path, items}. Each items[i] is the GET 200 body for that row.
- Catalog misses (empty matches, found=false) stay paid 200s inside that slot.
- x402 resource URL is this path plus n and body_sha256. Browsers that prefer text/html get the same paywall as GET; Pay retries POST with the same items.

Example request:

```json
{
  "items": [
    {
      "age": "40",
      "fibular_head_tenderness": "false",
      "flexion_less_than_90": "false",
      "isolated_patellar_tenderness": "false",
      "unable_to_bear_weight_immediately": "false",
      "unable_to_bear_weight_in_ed": "false"
    },
    {
      "age": "40",
      "fibular_head_tenderness": "false",
      "flexion_less_than_90": "false",
      "isolated_patellar_tenderness": "false",
      "unable_to_bear_weight_immediately": "false",
      "unable_to_bear_weight_in_ed": "false"
    }
  ]
}
```

Human paywall (two-item fixture): https://api.magentlab.com/paywall/bulk/ottawa-knee

Example 200:

```json
{
  "count": 2,
  "path": "/api/calc/ottawa_knee",
  "items": [
    {
      "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.",
      "citation": {
        "authors": "Stiell IG, Greenberg GH, Wells GA, et al.",
        "doi": "10.1016/S0196-0644(95)70106-0",
        "id": "stiell-1995",
        "source": "Ann Emerg Med. 1995;26(4):405-413",
        "title": "Derivation of a decision rule for the use of radiography in acute knee injuries"
      },
      "formula": "ottawa_knee",
      "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)",
      "age_years": 40,
      "criteria": [
        {
          "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",
          "met": false,
          "unable_to_bear_weight_immediately": false,
          "unable_to_bear_weight_in_ed": false
        }
      ],
      "xray_indicated": false
    },
    {
      "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.",
      "citation": {
        "authors": "Stiell IG, Greenberg GH, Wells GA, et al.",
        "doi": "10.1016/S0196-0644(95)70106-0",
        "id": "stiell-1995",
        "source": "Ann Emerg Med. 1995;26(4):405-413",
        "title": "Derivation of a decision rule for the use of radiography in acute knee injuries"
      },
      "formula": "ottawa_knee",
      "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)",
      "age_years": 40,
      "criteria": [
        {
          "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",
          "met": false,
          "unable_to_bear_weight_immediately": false,
          "unable_to_bear_weight_in_ed": false
        }
      ],
      "xray_indicated": false
    }
  ]
}
```

## Response fields

- `formula` (string): ottawa_knee
- `formula_expression` (string): Stiell 1995 knee-series indication rule
- `xray_indicated` (boolean): true when any published criterion is met. Not a diagnosis of fracture.
- `age_years` (integer): Age used for the ≥55 criterion
- `criteria` (array): Per-factor rows with factor and met; unable_to_bear_weight_both is both unable flags
- `citation` (object): Stiell 1995 Ann Emerg Med citation
- `disclaimer` (string): Not-a-device notice

## Example JSON

Frozen fixture. Not a live lookup.

```json
{
  "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.",
  "citation": {
    "authors": "Stiell IG, Greenberg GH, Wells GA, et al.",
    "doi": "10.1016/S0196-0644(95)70106-0",
    "id": "stiell-1995",
    "source": "Ann Emerg Med. 1995;26(4):405-413",
    "title": "Derivation of a decision rule for the use of radiography in acute knee injuries"
  },
  "formula": "ottawa_knee",
  "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)",
  "age_years": 40,
  "criteria": [
    {
      "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",
      "met": false,
      "unable_to_bear_weight_immediately": false,
      "unable_to_bear_weight_in_ed": false
    }
  ],
  "xray_indicated": false
}
```

## Errors

- HTTP 400 `invalid_age`: age must be an integer from 18 to 120 Returned before settlement; you are not charged.
- HTTP 400 `invalid_flag`: boolean clinical flags must be true or false Returned before settlement; you are not charged.
- HTTP 400 `invalid_items`: POST body must be a JSON object with only an items array Returned before settlement; you are not charged.
- HTTP 400 `invalid_item_count`: items must be an array of 1 to 25 objects Returned before settlement; you are not charged.
- HTTP 400 `invalid_item`: each items entry must be a JSON object Returned before settlement; you are not charged.

Shared HTTP 402 and 503: https://magentlab.com/docs/payments

## Related tools

- [Ottawa ankle rule](https://magentlab.com/docs/ottawa-ankle) — Apply the Stiell 1992 Ottawa ankle rule and return whether an ankle x-ray series is indicated from caller-assigned malleolar pain, malleolar bone tenderness, and inability to bear weight.

## Payment

Required query parameters must be present and valid. Invalid params return HTTP 400 JSON before settlement. magent does not charge. Fix the params, then request a new 402 quote for that complete URL. Catalog misses (empty matches, found=false) are valid paid 200s.

- GET the tool with Accept: application/json (do not send Accept: text/html unless you want the human paywall).
- HTTP 402 means you have not paid. Decode the base64 PAYMENT-REQUIRED header. HTTP 503 is not a new quote.
- Sign EIP-3009 (scheme exact, assetTransferMethod eip3009) with validBefore = now + accepts[0].maxTimeoutSeconds (600 seconds). That window starts when you sign and must cover wallet confirmation plus CDP verify and settle. Magent retries transient facilitator 429/5xx before answering. Do not send upto or batch-settlement payloads; magent does not offer those schemes.
- Retry the same URL with PAYMENT-SIGNATURE (base64 JSON echoing accepted, payload, and extensions when present).
- Success is HTTP 200 JSON plus PAYMENT-RESPONSE. Settlement happens before the tool runs. HTTP 503 settlement_uncertain or settle_failed: retry the same PAYMENT-SIGNATURE. Mint a new nonce only after a new HTTP 402.

- offered `exact` on eip155:8453: Fixed-price USDC on Base. The buyer authorizes the advertised amount (EIP-3009). magent settles before the tool runs (paymentFlow upfront). extra.assetTransferMethod=eip3009 extra.paymentFlow=upfront.

- not offered `upto`: Not offered. Usage-based: buyer authorizes a ceiling, seller settles the actual amount after work. Reserved for a future metered tool. Not for current calculators or code search.
- not offered `batch-settlement`: Not offered. Payment-channel vouchers claimed later in batches. Not the same as POST {items} bulk (that is still exact).

Same-path POST {items} is still scheme exact: one EIP-3009 authorization for n times the GET unit. That is not x402 batch-settlement (payment channels / vouchers).

Shared HTTP 402 and 503 table: https://magentlab.com/docs/payments

Stdio MCP (one process per host): https://magentlab.com/docs/mcp

## Guarantees

- Invalid query parameters return HTTP 400 before settlement. You are not charged.
- Settlement finishes before the tool executes (paymentFlow upfront). Uncertain or failed settlement fails closed (HTTP 503). Retry the same PAYMENT-SIGNATURE.
- HTTP 402 is unpaid only. Do not sign a second authorization because of a 503.
- Execution is timeout-bounded and concurrency-capped.
- Calculator inputs are stored encrypted for allowlisted operators (Magent Console). They are not in public logs, SSH lookups, catalog, or rollups. Request logs store path without query.