Ultrasonographic assessment of patellar tendon thickness at 16 clinically relevant measurement sites – A study of intra- and interrater reliability

Objectives
To determine intra- and interrater reliability of ultrasonographic imaging (USI) measurements of patellar tendon (PT) thickness using 16 measurement sites covering the entire tendon.

Design
Reliability study.

Setting
Physiotherapy outpatient clinic.

Participants
Twenty healthy and physically active volunteers (9 women). Mean age: 24 years (SD ± 2.73). Mean body mass: 75.8 kg (SD ± 11.8).

Main outcome measures
Intraclass correlation coefficient (ICC) and 95% limits of agreement (LOA) in cm and in percentage relative to the mean PT thickness.

Results
Intrarater reliability ranged from 0.59 to 0.87 and 0.59 to 0.93 for examiners I and II, respectively. Interrater reliability ranged from 0.37 to 0.89. Measurement precision for examiner I ranged from 0.05 to 0.09 cm (17.5%–26.7%) while ranging from 0.04 to 0.13 cm (13.3%–38.7%) for examiner II. Interrater measurement precision ranged from 0.07 to 0.15 cm (19.1%–42.5%).

Conclusion
In an attempt to replicate daily clinical USI practice, this was the first study extensively assessing reliability throughout the full range of the patellar tendon - revealing a considerable variation in intra- and interrater reliability as well as measurement precision throughout the 16 individual PT sites. In a clinical context, the low interrater reliability and precision found at the proximal tendon insertion site may have implications for USI of the symptomatic PT, as this is the site mainly associated with underlying pathologic changes. Further reliability studies are needed to clarify the region-specific reliability of the full length PT.