John Gill Commentary Deuteronomy 28:35

"Jehovah will smite thee in the knees, and in the legs, with a sore boil, whereof thou canst not be healed, from the sole of thy foot unto the crown of thy head." — Deuteronomy 28:35 (ASV)

The Lord shall smite thee in the knees, and in the legs, with
a sore botch, that cannot be healed
Which in those parts as it is very painful, so is not easily cured; and this which is threatened was incurable by the art of man, as others in (Deuteronomy 28:27) ; and which should not stop there in the lower parts of the body, but proceed and spread:

from the sole of thy foot unto the top of thy head ;
and so be filled with them, as Job was with his boils and ulcers.