Fig. 5

FDP Enthesis Regional Variation. Entire sample section (proximal – right; distal – left), showing FDP tendon (T) attachment; b, c and d indicate subsequent panel regions b) Proximal enthesis region. The 4 fibrocartilaginous enthesis zones are apparent. The approaching tendon fibres undergo a considerable angle change in reaching the tidemark (TM) and cortical bone (CB). The majority of the angle change occurs in the uncalcified fibrocartilage (UF) zone, demonstrated by the curved columns of fibrochondrocytes (white arrows), with straight tendon fibres in the calcified fibrocartilage (CF) zone. c) Middle enthesis region. A calcified fibrocartilage (CF) zone is present, although less thick than in the proximal region, with a variable layer of uncalcified fibrocartilage (UF) demonstrated by the limited but perceptible fibrochondrocytes (white arrow). Compared to the proximal enthesis region, the approaching tendon fibres are generally less acute to the horizontal, and there is less angle change between the approaching fibres and tidemark intersection fibres. d) Distal enthesis region. Areas of calcified fibrocartilage (CF) are sporadic and are interspersed between fibrous enthesis regions which lack any fibrocartilage. The absence of fibrochondrocytes proximal to the calcified fibrocartilage indicates no uncalcified fibrocartilage zone. Tendon fibres approach the tidemark (TM) more acutely than the middle enthesis region. Micrographs of a typical mid-sagittal section of an FDP enthesis, toluidine blue (a-d). e) Quantified angle change comparison between the 5 distance measurement points along the thesis. Mean ± standard error **p < 0.01, ***p < 0.001