I am an OutPatient Ortho PT. I've been doing this for over 10 years and seen many TKR. I am not going to tell you that he will definitely be able to get up onto the ladder because there are some people that have complications (which I hope this is not your husband's case) but I will agree with the type of ladder. If it's a one step ladder, you might want to look into getting an extension. I say this mainly because it will make it that much easier to continue to do what you two love to do. When you are in Out Patient, let your PT know what the ultimate goals are because kneeling is one of the last things that he will get back. The PT should be able to show you different ways of making kneeling easier and getting you back into the boat with no issues.
Good Luck, and if all goes well, you will be boating and swimming with ease by late spring (I would say earlier, but the lake won't be ready for you!!
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