Background: The clinimetric properties of knee goniometry are essential to appreciate in light of its extensive use
in the orthopaedic and rehabilitative communities. Intra-observer reliability is thought to be satisfactory, but the
validity and inter-rater reliability of knee goniometry often demonstrate unacceptable levels of variation. This study
tests the validity and reliability of measuring knee range of motion using goniometry and photographic records.
Methods: Design: Methodology study assessing the validity and reliability of one method (’Marker Method’) which
uses a skin marker over the greater trochanter and another method (’Line of Femur Method’) which requires
estimation of the line of femur. Setting: Radiology and orthopaedic departments of two teaching hospitals.
Participants: 31 volunteers (13 arthritic and 18 healthy subjects). Knee range of motion was measured
radiographically and photographically using a goniometer. Three assessors were assessed for reliability and validity.
Main outcomes: Agreement between methods and within raters was assessed using concordance correlation
coefficient (CCCs). Agreement between raters was assessed using intra-class correlation coefficients (ICCs). 95%
limits of agreement for the mean difference for all paired comparisons were computed.
Results: Validity (referenced to radiographs): Each method for all 3 raters yielded very high CCCs for flexion (0.975
to 0.988), and moderate to substantial CCCs for extension angles (0.478 to 0.678). The mean differences and 95%
limits of agreement were narrower for flexion than they were for extension. Intra-rater reliability: For flexion and
extension, very high CCCs were attained for all 3 raters for both methods with slightly greater CCCs seen for
flexion (CCCs varied from 0.981 to 0.998). Inter-rater reliability: For both methods, very high ICCs (min to max: 0.891
to 0.995) were obtained for flexion and extension. Slightly higher coefficients were obtained for flexion compared
to extension, and with the Marker compared to the Line of Femur Method. For intra- and inter-rater reliability, the
mean differences (within 2 degrees) and 95% limits of agreement (within 5 degrees) were generally clinically
acceptable for both methods.
Conclusion: Photography potentially offers a superior method of measurement over standard goniometry as
visualising the centre of the knee is simplified in a two-dimensional plane and the permanent record provides
greater assessor transparency as well as opportunity to confer. The Marker and Line of Femur Methods have
moderate to substantial validity, but the inter- and intra-rater repeatability for trained observers are excellent with
both methods yielding small mean differences with narrow limits of agreement. The Line of Femur Method offers
the added advantage that it does not rely on inter-clinician consistency in identifying the greater trochanter