It was the first tool to aid in the early diagnosis of a slipped capital femoral epiphysis (SCFE), which if treated late or left untreated leads to crippling arthritis, leg length discrepancy and lost range of motion.

[3] Klein and three colleagues at Massachusetts General Hospital had seen the crippling effects in later life for a person with slipped capital femoral epiphysis; they were convinced that early diagnosis in childhood and "prophylactic procedures" were needed to treat the condition.

[3]: 233  The diagnosis was up until then made by comparing the X-ray of the suspected epiphysis with the "normal" hip on the opposite side, which in Klein's series had left 11 of 38 children undiagnosed.

In 2009, the classic definition of Klein's line was shown to miss 60% of SCFE cases on X-rays of 30 children between 8 and 16 years of age.

[1] The sensitivity and reliability is improved by measuring the epiphyseal width lateral to Klein's line, which if differing by 2 millimetres (0.079 in) or more between hips suggests the diagnosis of SCFE.