PURPOSE: The primary objective was to examine the reliability of the Copenhagen Achilles length measure (CALM) in patients with an Achilles tendon rupture and secondary to examine the reliability of Achilles tendon resting angle (ATRA) and Achilles tendon length measure (ATLM).

METHOD: The study was executed as a cross-sectional study on two different groups: one focused on CALM and the other on ATRA/ATLM. CALM was performed on 56 patients at four timepoints during the first year after injury, whereas ATRA/ATLM were carried out on 28 patients. Intra- and inter-rater reliabilities were determined using the intra-class correlation coefficient (ICC), the standard error of the measurement (SEM), and the minimal detectable change (MDC).

RESULTS: For CALM, all measurements, both for injured and non-injured sides as well as for elongation, indicated excellent relative reliability (ICC ≥ 0.75). During the four timepoints, the inter-rater absolute reliability had an SEM that ranged between 0.3 and 0.8 cm (1-4 SEM%) for injured and non-injured sides and 0.3-0.6 cm (18-29 SEM%) for elongation. On an individual level, the inter-rater absolute reliability had an MDC ranging between 0.8 and 2.2 cm (4-11 MDC%) for injured and non-injured sides and 0.8-1.7 cm (47-81 MDC%) for elongation. In the case of ATRA, relative reliability was excellent (ICC ≥ 0.75), and for ATLM, it was fair to excellent (ICC 0.58-0.79). ATRA presented a lower measurement error than ATLM.

CONCLUSION: Copenhagen Achilles length measure showed excellent relative reliability, but had a significant measurement error at four timepoints the first year following an Achilles tendon rupture.

LEVEL OF EVIDENCE: II.