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Thermographic imaging for amputation level viability assessment

We have previously described the use of skin oxygen saturation (SO₂) measurements to predict healing viability in lower limb amputations for critical limb ischaemia.[1] The technique involves determining the degree of tissue hypoxia (DTH)1 which is defined as the percentage of SO₂ values along the leg with a value of less than 10%. It was found that if the DTH was less than 15% or the mean SO₂ at two specified sites along the line of a proposed below knee amputation (BKA) was 30% or greater, healing at the BKA level was successful. The technique, using the above criteria and in combination with thermographic imaging[2], has been applied to routine clinical practice in the University Hospital of North Durham since 1999. The results of an initial audit of these techniques was published in 2002[3] and demonstrated a 94% healing rate of BKAs was being achieved at a BKA to above knee amputation (AKA) ratio of 9:2. The quantitative assessment of DTH is the primary criterion that is used to recommend amputation level, and to date temperature gradients in the thermographic image have only been used as a qualitative guide.

During a recent survey of 33 amputations carried out in the 3 year period 2004-2006, not only was the predictive power of DTH re-assessed, but also temperature gradients were measured retrospectively along the limb from the thermographic images. 9 patients were omitted from the audit: 3 died before the operation, 1 had a flexion contracture, and 5 that were recommended a BKA received an AKA on clinical grounds.

Of the 24 patients that lay within the inclusion criteria 2 were predicted for, and underwent, a AKAs, 2 were predicted for and underwent BKAs but needed to be revised to an AKA because of apparent stump ischaemia. The remaining 20 patients were recommended and underwent BKAs that successfully healed.

Analysis of the thermographic images revealed that the difference in the temperature gradients between the AKA and BKA groups over the first 5cm distally was significant (T test, p=0.03) and also that there was a significant difference (p=0.005) in absolute temperatures 5cm distal to the tibial tuberosity between the two groups. However, despite these significant differences, the overlap between the BKA and AKA groups was too great to be able to formulate a predictive tool from the results.

Further scrutiny of the records of the patients whose BKAs were revised showed that in one case the amputation was carried out 2 months after the assessment. However, anecdotally in the second case (amputation 1 week after assessment) the temperature along the limb was the lowest of the entire cohort.

In conclusion, as in the 2002 audit, skin SO₂ remains a robust predictor of healing viability in the critically ischaemic limb. However, it is intended to carry out a larger retrospective analysis of thermographic images to investigate whether there is a critical limb temperature below which a BKA is not viable. This could play a role in improving even further the accuracy of the assessment.

References:

1 Harrison DK, McCollum PT, Newton DJ, Hickman P and Jain AS; Amputation level assessment using lightguide spectrophotometry; Prosthetics and Orthotics International 1995; 19:139-147
2 Spence VA, Walker WF, Troup IM, Murdoch G; Amputation of the ischaemic limb: Selection of the optimum site by thermography 1981; Angiology; 32:155-169
3 Hanson JM, Harrison DK, Hawthorn IE; Tissue spectrophotometry and thermographic imaging applied to routine clinical prediction of amputation level viability; In "Functional Monitoring and Drug Tissue Interaction"; SPIE Proc. Series 2002; 4623:187-194

Publication

Harrison DK, Gaylard, LG, Singh DB. Thermographic imaging for amputation level viability assessment: Just a pretty picture or a quantitative tool? Thermology Int. 17: 79-80.

Contact

Dr. David K. Harrison, Durham Unit, Durham. Tel: +44 (0)191-333-2215.

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