Inclinometers and apps are better than goniometers for measuring knee extension range of motion

Key points

1. 01Inclinometer had the best interrater reliability (ICC 0.80) and the smallest minimal detectable change (3.7 degrees between testers)
2. 02Smartphone app performed similarly to the inclinometer for interrater reliability (ICC 0.79, MDC 4.0 degrees)
3. 03Universal goniometer had poor interrater reliability (ICC 0.36) and a 10.4-degree interrater MDC, making it unsuitable for detecting small changes between different clinicians
4. 04All three devices showed excellent intrarater reliability (ICC 0.92-0.94), meaning any device is adequate when the same clinician measures repeatedly
5. 05Knee extension measurements can be taken at any point in an assessment session with minimal effect on results

How it was conducted

Design

Single-visit observational cohort reliability study

Participants

92 ACL-injured or -reconstructed patients (84 male, 8 female; mean age 29.5 years) at Aspetar Orthopaedic and Sports Medicine Hospital, Doha, Qatar

Groups

72 post-ACL reconstruction (mean 6.0 months post-op), 20 conservative/preoperative patients (mean 4.4 months post-injury)

Devices compared

Universal goniometer, inclinometer (Empire Magnetic Polycast Protractor), and smartphone app (iHandy Level Free)

Primary outcomes

Intrarater, interrater, and test-retest intraclass correlation coefficients (ICC2,1) and minimal detectable change at 95% CI (MDC95)

Statistical analysis

Two-way mixed ICC with absolute agreement; Bland-Altman plots and joint plots for systematic error

What they found

• Intrarater ICC was excellent for all three devices: goniometer 0.93 (95% CI: 0.90-0.95), inclinometer 0.94 (95% CI: 0.91-0.96), smartphone app 0.92 (95% CI: 0.89-0.95)
• Interrater ICC was poor for the goniometer at 0.36 (95% CI: 0.17-0.53), good for the inclinometer at 0.80 (95% CI: 0.71-0.86), and good for the smartphone app at 0.79 (95% CI: 0.70-0.86)
• Test-retest ICC (n=75): inclinometer 0.89 (95% CI: 0.84-0.93), smartphone app 0.86 (95% CI: 0.79-0.91), goniometer 0.83 (95% CI: 0.74-0.89)
• Intrarater MDC95: inclinometer 2.0 degrees, smartphone app 2.2 degrees, goniometer 3.5 degrees
• Interrater MDC95: inclinometer 3.7 degrees, smartphone app 4.0 degrees, goniometer 10.4 degrees
• Test-retest MDC95: inclinometer 2.6 degrees, smartphone app 2.9 degrees, goniometer 5.4 degrees
• A statistically significant interrater mean difference of 1.145 degrees (p<0.0001) was found for the smartphone app only; no significant systematic bias was found for the other devices or conditions

Limitations

• The sample was predominantly adult males, limiting generalizability to females and adolescents
• This was a single-visit study with short intervals between measurements; reliability at longer intervals (days or weeks apart) is unknown
• Two different smartphone devices with different operating systems (Android and iOS) were used, which may have influenced app reliability results
• Testers could not be blinded to the measurement method, which may have influenced results

Why it matters

For patients

ACL patients should be aware that results from knee extension measurements may differ depending on which device and which clinician is used, and that goniometer readings in particular should be interpreted cautiously when compared across visits with different therapists.

For clinicians

Clinicians measuring knee extension ROM in ACL patients between multiple raters should use an inclinometer or smartphone app rather than a universal goniometer, as only these devices have acceptable interrater reliability and a clinically meaningful MDC below 5 degrees.

For readers

This is the first reliability study conducted specifically in ACL patients for all three common measurement devices, providing device-specific MDC values that can guide clinical decision-making when monitoring rehabilitation progress.