An investigation of the relationship between thermal imaging and digital thermometer testing at the knee

Abstract

BACKGROUND: A number of research papers and theoretical clinical models summarising how temperature of the skin over
the knee may be altered according to different pathological processes have been published. Thermal imaging (TI) is generally
regarded as the ‘Gold’ or ‘reference’ standard for measuring skin temperature, however this technology is not widely accessible
to most musculoskeletal physiotherapists working in clinical environments. This is largely due to the time required for analysis
of the thermal images and the high cost of the equipment. A digital thermometer (DT) is portable with a convenient display of
results which could offer an inexpensive substitute.

PURPOSE: The aim of this study was to determine the interchangeability between thermal imaging and a digital thermometer, using Bland-Altman limits of agreement, to determine skin temperature differences between right and left knees.

METHODS: Seventy-one healthy participants in the age group of 8 to 40 participated in the study. Data were collected in two
phases. The first phase was as part of a public engagement event at the Lancashire Science Festival where school children were
invited to learn about science. The second phase of data collection took place as part of a PhD study where staff and students at the university were recruited via electronic advert and posters displayed around the campus. All subjects were free from lower back or lower limb problems and had not had any previous lower limb surgery.

RESULTS: Matched paired t tests showed no significant difference between temperature difference between right and left using
DT and TI (t = 1.41, df = 69, P = 0.08). The DT and TI were interchangeable to measure knee skin temperature difference with a limit of agreement of –0.64 and 0.75; this limit of agreement is acceptable based on previous literature where skin temperature
differences between affected and non-affected knees are equal to or greater than 1◦C.

CONCLUSION: This study concludes that an inexpensive handheld digital thermometer shows acceptable agreement with a
thermal imaging camera. Clinically a handheld digital thermometer has the potential to play an important role in the localized
assessment of skin temperature in physiotherapy and can offer an inexpensive substitute to thermal imaging; due to the massive
difference in cost it is worth considering the adoption of digital thermometry in routine musculoskeletal physiotherapy practice.