Ballroom Dancing can be hard on the knees, but it doesn't have to be if you are aware of what causes this problem. The main culprits contributing to knee problems in Ballroom Dancing are: the Dance Floor, Dance Shoes and Dance Technique.
The dance floor should allow your feet to slide but it shouldn't be slippery. If the dance floor is too sticky, your feet won't have a natural swivel and this torque will be transferred to your knees. It would be better for your knees to have a slippery floor than a sticky floor. This is normally not a problem for Ballroom Dancers but occasionally you may attend special events, etc. that may have the dance floor on concrete, in a parking lot, etc. If you decide to dance on these types of surfaces, you need to change the way you move to limit the impact on your knees.
Ballroom Dance shoes should be just like the dance floor, allow your feet to slide but they shouldn't be slippery. This is normally not a problem either if the dancer is wearing real Ballroom Dance shoes. The Ballroom Dance shoe is designed to allow natural movement of the shoe on the floor, swiveling on both the ball and the heel of the foot. New and inexperienced dancers are most likely to have this problem because they have not yet invested in Ballroom Shoes. Any shoe with a rubber sole or even a shoe with a leather sole and a rubber heel can add to this problem. Many new dancers wear "DANCE" shoes but don't' realize that they aren't designed for Ballroom Dancing, they may be designed for "Jazz", "Hip Hop" etc. These shoes are designed to not slip or slip very little thus causing problems when dancing Ballroom. Dancing with non-ballroom shoes can cause your movement to feel sticky, awkward and unbalanced.
The Dance Technique of "BODY ROTATION" is probably the biggest threat to the knees, even if you are on a good floor and have the proper shoes. Generally speaking, rotation of the "body" is accomplished by "swiveling" and "Turning". In Latin/Rhythm dancing, the rotation is accomplished by "swiveling" on the feet. In Smooth/Standard dancing, the rotation is accomplished by "Turning" through the ankle, leg and hip. Keep in mind that there are always exceptions.
A "swivel" is the rotation of the body as a result of the action of the shoe on the floor. The rotation occurs only between the sole of the shoe and the floor (nowhere else in the body). To accomplish this, the ankle, leg and hip has to be toned enough to transfer all of the rotation to the shoe/floor surface. If the ankle, leg and hip are too relaxed, the majority of the torque of the rotation will be transferred to the knee. It is not a good idea for the knee to be exposed to this type of torque. In addition to the tone of the ankle, leg and hip, the knee of the "swiveling" leg should be slightly bent. This bend causes the "swivel" to occur on the ball of the foot instead of a flat foot (this reduces the friction of the "swivel"). The ball action is initiated by the bent knee, not by lifting with the ankle. The "swivel" has basically unlimited rotational possibilities (half turns, full turns, double turns, etc.).
A "Turn" is the rotation of the body as a result of the rotational flexibility of the ankle, leg and hip only (There may also be a very slight natural "swivel" action as well.) Try this: hold your arms out in front of you with your thumbs up, now "turn" your palms down, then "turn" your palms up. Notice that you "turned" your palms up and down through the rotation of your wrist, arm and shoulder. The ankle, leg and hip can rotate the foot in the same manner. This accommodates a smooth, even, controlled rotation of the body. The ankle, leg and hip need only be slightly toned to accomplish this rotation. The "turn" is limited in the amount of rotation (usually about 90 degrees or less). Trying to "turn" too far will create torque in the knees.
The dance technique of "LATIN/CUBAN MOTION" is another potential problem with the knees. Stand sideways in front of a full length mirror and step backwards and lock your knee (It is best to do this bare legged.). Most dancers have a slight hyper-extension backwards, but some have a scary amount of hyper-extension. "Latin/Cuban Motion" facilitates hyper-extending the knee, primarily on the backward step. The goal is to keep a straight leg on the backward step, and not allow the hyper-extension. This may be accomplished by employing a "Samba Tic". If someone would pretend to punch you in the stomach, your reaction would be to pull your abdominal muscles in to protect yourself. This is basically a "Samba Tic". In "Latin/Cuban Motion", you want to have this "Tic" all of the time when you dance, but it tends to slip out. The solution is to re-pull in the "Tic" on every step. Now back in front of the mirror. Pull the "Tic" in and step backwards. Your leg should be straight (no matter how much hyper-extension you have). Looking at the leg in the mirror, release the "Tic" and you will see your leg hyper-extend to some degree.
It is not uncommon for new dancers (untrained dancers) to try to execute all rotation as "turns". This is a real problem when dancing the Latin/Rhythm dances because most of the turns are more than 90 degrees. Generally speaking, Latin/Rhythm dances use "swivel" and Smooth/Standard dances use "turns". This is important to protecting your knees.