本研究目的為探討不同測量角度與國軍篩選扁平足測量法之相關性，並找出可以用來輔助診斷扁平足之角度。本研究共收集87名體檢役男之側位負重X光影像，依據參考文獻總共測量五種不同參數：跟骨第五蹠骨角(calcaneal-fifth metatarsal angle)、內側足弓角(medial arch angle, MAA)、跟骨角(calcaneal pitch angle, CP)、距骨(talus angle, TA)與距骨第一蹠骨角(talo-first metatarsal angle, TFM)，在本研究中，我們定義足弓角(arch angle)為跟骨第五蹠骨角，並以足弓角≧165度定為扁平足判斷標準。藉由皮爾森相關係數(Pearson correlation coefficient)評估測量參數與足弓角之相關性，並以receiver operating characteristic curve (ROC curve)比較不同測量參數之準確性，另外以kappa檢定觀察測量參數與足弓角之ㄧ致性(agreement)，研究結果顯示跟骨角與足弓角相關性最高且為負相關，在ROC曲線下涵蓋面積最多，且其角度測量也最方便，角度值較不易受人為因素與放射影像品質影響。以Youden Index 選擇適當之閥值，經kappa檢定後發現當跟骨角小於11°時可與足弓角達到很高的一致性(kappa score = 0.827)，由本研究可知，依照現行扁平足的診斷標準，當放射影像中足弓角不易定義時，可用跟骨角來輔助診斷。The purpose of this study was to evaluate the correlation between the arch angle and other radiographic parameters. Moreover, we tried to find a radiographic parameter which was able to enhance the flatfoot diagnosis. In this study, 87 lateral weight-bearing radiographies were obtained from the military recruits. Five radiographic measurements, including a calcaneal-fifth metatarsal angle (arch angle), a medial arch angle (MAA), a calcaneal pitch angle (CP), a talus angle (TA) and a talo-first metatarsal angle (TFM), were obtained to evaluate the osseous structure in flatfoot. The criterion for judging flatfoot was defined as the arch angle ≧ 165 degree. Two statistic methods, the Pearson correlation and the kappa statistic were used to find the correlation coefficient and the degree of association between the arch angle and other parameters, respectively. The receiver operating characteristic (ROC) curve was also obtained to assess the diagnostic accuracy of a test, and to compare the usefulness of different tests. According to our results, the CP was the most correlated to arch angle negatively (p<0.001, r=－0.881) and the greatest area under ROC curve (0.988). In addition, this angle was measuring easily and there were few effects on artificial disturbance and image quality. The optimal threshold value is selected by Youden index as maximized to 0.88 that the CP ≦ 11° as defined a flatfoot. The optimal sensitivity to 88%, specificity to 100%, positive predictive value to 100% and negative predictive value to 95.38% were achieved, respectively. The kappa test exhibited excellent agreement (kappa score = 0.827) related to the arch angle on the premise that the CP ≦ 11° was defined as a flatfoot. According to our results, the CP could improve the diagnosis of the flatfoot as an auxiliary parameter when the arch angle was not easily defined.