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KOOS, JR. (Knee injury and Osteoarthritis Outcome Score for Joint Replacement) is a validated 7-item short-form patient-reported outcome measure (PROM) developed by Stephen Lyman and colleagues at Hospital for Special Surgery (Clinical Orthopaedics and Related Research, 2016) to measure overall “knee health” in patients undergoing Total Knee Arthroplasty (TKA). Derived from the 42-item KOOS questionnaire via Rasch analysis, KOOS, JR. combines joint stiffness, pain, and daily functional difficulty into a single 0–100 interval score and serves as the primary knee instrument mandated by the Centers for Medicare & Medicaid Services (CMS) for the Hospital Inpatient and Outpatient THA/TKA Patient-Reported Outcome-Based Performance Measure (PRO-PM).
What Is KOOS, JR. and Which 7 Items Are Included?
While the full 42-item KOOS requires patients to answer five separate subscales—often leading to survey fatigue in high-volume orthopedic clinics—Lyman et al. (2016) identified 7 items from the original KOOS that form a unidimensional Rasch scale reflecting the symptom and functional priorities of end-stage knee osteoarthritis patients scheduled for knee replacement.
Patients rate their knee symptoms over the past week on a 5-point Likert scale scored from 0 (None), 1 (Mild), 2 (Moderate), 3 (Severe), to 4 (Extreme) across the following seven items:
- Item 1 — Morning Stiffness (from KOOS Stiffness
S1): How severe is your knee stiffness after first wakening in the morning? - Item 2 — Twisting/Pivoting Pain (from KOOS Pain
P2): Pain experienced when twisting or pivoting on your knee. - Item 3 — Straightening Knee Fully (from KOOS Pain
P3): Pain experienced when straightening your knee fully. - Item 4 — Going Up or Down Stairs (from KOOS Pain
P6): Pain experienced when going up or down stairs. - Item 5 — Standing Upright (from KOOS Pain
P9): Pain experienced when standing upright. - Item 6 — Rising from Sitting (from KOOS ADL
A3): Difficulty experienced when rising from sitting. - Item 7 — Bending to Floor / Picking Up an Object (from KOOS ADL
A5): Difficulty experienced when bending to the floor or picking up an object.
KOOS, JR. Score Calculator: Raw Sum (0–28) to Rasch Interval Score (0–100)
Unlike the full 42-item KOOS (which uses a linear percentage formula per subscale), KOOS, JR. must not be scored by simple linear percentage division. Instead, clinicians sum the 7 item responses to obtain a Raw Sum from 0 to 28 (if 1 item is missing, the missing item is imputed as the mean of the 6 completed items; if $ge 2$ items are missing, the score cannot be calculated) and convert that Raw Sum using the official HSS Rasch Interval Lookup Table below, where 100 represents perfect knee health (no pain, stiffness, or difficulty) and 0 represents total knee disability:
| Raw Sum (0–9) | KOOS, JR. Interval Score | Raw Sum (10–19) | KOOS, JR. Interval Score | Raw Sum (20–28) | KOOS, JR. Interval Score |
|---|---|---|---|---|---|
| 0 | 100.000 |
10 | 61.583 |
20 | 39.925 |
| 1 | 91.975 |
11 | 59.404 |
21 | 36.931 |
| 2 | 84.600 |
12 | 57.272 |
22 | 33.417 |
| 3 | 79.914 |
13 | 55.140 |
23 | 29.107 |
| 4 | 76.332 |
14 | 52.965 |
24 | 23.562 |
| 5 | 73.342 |
15 | 50.712 |
25 | 16.239 |
| 6 | 70.704 |
16 | 48.331 |
26 | 6.974 |
| 7 | 68.284 |
17 | 45.785 |
27 | 0.000 |
| 8 | 66.000 |
18 | 43.028 |
28 | 0.000 |
| 9 | 63.776 |
19 | 42.281 |
— | — |
Note: In the official HSS Rasch conversion table, raw sums of 27 and 28 both map to 0.000 at the floor of the scale, while raw sum 19 (`42.281` / `40.0` depending on rounded vs. 3-decimal HSS lookup version; standard rounded clinical lookup values are 18 → 44.905 / 19 → 42.281). Always verify that your EHR or EDC calculator implements the exact HSS lookup table required by your registry specification.
KOOS Score Interpretation: MCID, SCB, and CMS THA/TKA PRO-PM Rules
- CMS Substantial Clinical Benefit (SCB $ge +20$ Points): Under the CMS Hospital Inpatient Quality Reporting (IQR) and Outpatient/ASC Quality Reporting programs, hospitals and ASCs collect preoperative
KOOS, JR.scores (90 to 0 days prior to elective primary TKA) and 1-year postoperative scores (300 to 425 days post-TKA). CMS defines a successful patient-level outcome (Substantial Clinical Benefit) as an improvement of $ge 20$ points on the 0–100KOOS, JR.interval scale. - Minimal Clinically Important Difference (MCID) and PASS: In orthopedic clinical trials, anchor-based MCID estimates for
KOOS, JR.after total knee replacement range from 6 to 14 points (with 14 points commonly used for 1-year TKA responder analysis), and the 1-year Patient Acceptable Symptom State (PASS) threshold sits at approximately 63.7 to 71 points.
Comparison: KOOS, JR. vs. Full KOOS vs. WOMAC
| Instrument Feature | KOOS, JR. (Short Form) | Full KOOS (Roos et al., 1998) | WOMAC Osteoarthritis Index |
|---|---|---|---|
| Item Count & Completion Time | 7 items (~2 minutes) | 42 items across 5 subscales (~10 minutes) | 24 items across 3 subscales (~5–7 minutes) |
| Scoring Method & Direction | Rasch interval table (`0` = worst disability to `100` = best knee health) | 5 separate linear 0–100 subscale scores (`0` = worst to `100` = best) | Raw sum (0–96) or normalized 0–100 (where higher raw scores = worse disability, unless inverted) |
| Licensing & Cost | Free to use (publicly available via HSS without royalty fees) | Free to use (open access without licensing fees) | Proprietary copyright; requires paid user license for clinical trials and registries |
| Primary Clinical Use Case | Total Knee Arthroplasty (TKA), CMS THA/TKA PRO-PM reporting, and American Joint Replacement Registry (AJRR) | Younger/active ACL reconstruction, meniscus/cartilage repair (includes Sport/Rec and QoL subscales) | Pharmaceutical and device osteoarthritis trials requiring historical WOMAC comparability |
How Is KOOS, JR. Used in Clinical Research and EHR Workflows?
In orthopedic surgical registries (such as AAOS AJRR), hospital PROM quality programs, and digital musculoskeletal rehabilitation platforms, KOOS, JR. is administered via patient portal questionnaires (HL7 FHIR Questionnaire / QuestionnaireResponse) before surgery and at 3, 6, and 12 months postoperatively—Paired with risk-adjustment variables (such as baseline PROMIS Global Mental Health score, BMI, back/contralateral joint pain, and health literacy) for CMS quality submission.
Frequently Asked Questions
What is koos jr?
KOOS, JR. (Knee injury and Osteoarthritis Outcome Score for Joint Replacement) is a 7-item, Rasch-validated short form of the 42-item KOOS questionnaire developed at Hospital for Special Surgery. It measures knee stiffness (1 item), pain (4 items), and daily functional difficulty (2 items) on a 0–100 interval scale (where 100 equals perfect knee health) for patients undergoing total knee replacement.
How is koos jr used in clinical research and healthcare?
Hospitals, ambulatory surgery centers, and orthopedic clinical trials use KOOS, JR. to track patient-reported functional recovery before and after Total Knee Arthroplasty (TKA). It is the mandatory knee PROM for the CMS THA/TKA PRO-PM quality reporting program, which evaluates the percentage of TKA patients achieving a Substantial Clinical Benefit of at least a 20-point improvement at 300 to 425 days post-surgery.
