What is "Shen Ro'ah"?
Rashi: It is a crushed (Malbim - broken) tooth.
What is "Regel Mu'ades"?
Rashi: This is [slipping,] like "v'Lo Ma'adu Karsulai" (Tehilim 18:37).
How are these "Mivtach Boged"?
Rashi: He does not fulfill his promise on the day of affliction, like a crushed tooth or a dislocated [leg].
Malbim #1: One relies on the tooth to chew the food, and on the leg to take him to his destination, and it betrays (does not fulfill). So a reliance who betrays on the day of affliction - he trusted in him, and does not find his trust when needed.
Malbim #2: This hints to 'chew well (eat much), and it will strengthen your steps' (Shabbos 152a). When the tooth is broken, also the leg slips. Likewise, when one reliance betrays, others on whom he relies are drawn after him, and all betray him.
