Nearly all previous studies in posterior tibial slope (PTS) and anterior cruciate ligament (ACL) injuries ignored age-related changes, and the published data are inconsistent. The objective of this study was to reveal age-related changes of PTS and its roles in ACL injury. Data for 2618 lower limbs were included initially based on the availability of lateral X-rays and a suitable femorotibial angle. The final 1431 subjects were analyzed according to age, sex, side, and injury status. Student t-tests, 1-way analysis of variance, and curve fitting were used to analyze data. The PTS in males was greater than that in females in the 0-9 and 30-39-year-old groups, but this pattern was reversed in the 40-49, 60-69, 70-79, and 80-89-year-old groups. The PTS was greater on the left side than on the right side in the 0-9, 10-19, 50-59, 60-69, and 80-89-year-old groups. The curve fitting for PTS demonstrated a trend of first decreasing and then increasing with aging. The PTS values differed significantly between knees with an ACL injury and those without in the 20-29, 30-39, and 40-49-year-old groups but not in the 50-59-year-old group. The PTS follows a trend of first decreasing and then increasing, and its role in ACL injury changes with advancing age. The higher PTS is only unrelated to the risk of ACL injury in age groups with a lower mean PTS value.Key words: Age-related -Posterior tibial slope -Anterior cruciate ligament -Injury -X-ray T he posterior inclination of the tibial plateau relative to the longitudinal axis of the bone, known as the posterior tibial slope (PTS), is important to know for the pathology of anterior cruciate ligament (ACL) injury.1 On the basis of large X-rays of the lower limbs of adults, Genin et al 1 reported that the PTS ranges from 08 to 188. Jiang et al 2 found that the PTS, as determined from lateral radiography of the knee, is 108 6 48 (range, 08 to 208). Chiu et al 3 studied 25 pairs of Chinese cadaveric tibias and concluded that the medial PTS is 14.88, the lateral PTS is 11.88, the PTS according to intramedullary radiographic measurement is 11.58, and the PTS based on extramedullary radiographic measurement is 14.78. Moreover, many studies have reported that an increased PTS is a risk factor for ACL injury. Brandon et al 4 found that an increased PTS is associated with noncontact ACL rupture in both males and females. Todd et al 5 and Hohmann et al 6 reported that an increased PTS is a possible risk factor for noncontact ACL injury only among women. Senis ik et al 7 found that there is a higher PTS in injured male soccer players compared with uninjured male players. Webb et al 8 suggested that an increased PTS is associated with a greater chance for further ACL injury after ACL reconstruction. Recently, Li et al 9 reported that a PTS of 58 or greater is a new risk factor for ACL reconstruction failure. However, some other studies refuted the conclusions above. Meister et al 10 found that the PTS is not an identifiable risk factor for noncontact ACL injury on the basis of 100 knee jo...