It’s a critical commonplace to see Jeff’s leg in a cast as an emblem of his emasculation. This is valid to the extent that the broken leg prevents Jeff from fulfilling his role as a conventional hero. Those duties fall to Lisa, in large part, one of film’s first action heroines.
However, the symbolism isn’t Freudian. After the adventure, Jeff has both his legs in casts. If the first broken leg represents castration, does the second then represent a double castration? What the heck is a double castration? Such a nonsensical idea is, at any rate, at odds with the situation we find at the end of the film. There Jeff and Lisa have achieved rapprochement; presumably, Jeff will be able to rise to his conjugal duties (The portents are good, anyway: the temperature has dropped to a Spring-like 70° F; couples in the community around Jeff and Lisa are forming or re-forming; a love song is wafting through the air).
What then to make of the two broken legs?
What about this: The first broken leg doesn’t operate symbolically so much as it parallels another condition hidden from the viewer. From the very beginning of the film Jeff has had two broken legs, but only one was showing. By the end of the movie, “brokenness” has been transferred from his “third leg” to his second, signaling a resurgent virility.
[I’d like to thank W.E. Guilford for suggesting this reading to me]