NIRS AUC data will be compared to Doppler ultrasound perfusion data using Spearman's correlation. Accounting for the repeated measurements within patients, mixed effects logistic regression analysis will be conducted and reciever operating characteristic curves will be determined for each NIRS AUC interval (i.e. 15 sec, 30 sec, 1,5 and 10 min), using the dichotomous Doppler ultrasound outcome as the gold standard. NIRs AUC data for perfused and non-perfused allograft poles will be compared using mixed effects regression analysis. Distributional assumptions will be investigated and where appropriate suitable transformations will be used.