The reason why working on the knees is more dangerous for adolescents is due to the anatomy of the knee, and especially the specifics of it's anatomy during periods of growth. Most long bones in the body have a growth plate at each end (proximal and distal) to allow for development. These growth plates are more active at various times in our lives, however the rate of growth, and the exact ages that it fully closes are different in every individual. Typically the growth plates around the knees do not settle until about 12-19 years of age (1, 2).
The adolescent leg grows approximately 23 mm/year, with most of that coming from the knee (15 mm/year):
- distal tibia - 5 mm / yr (3/16 in)
- proximal tibia - 6 mm / yr (1/4 in)
- distal femur - 9 mm / yr (3/8 in)
- proximal femur - 3 mm / yr (1/8 in)
The tendon from the knee cap, the patella tendon, attaches onto the tibia at the tibial tuberosity, which also happens to be where one of the major growth plates in your lower leg is (the proximal tibial epiphyseal plate). Many young students begin to develop painful and unsightly lumps on the front of their knee from constant pulling in this area during periods of growth, termed Osgood Schlatters disease.
Osgood Schlatters disease is not actually a disease, but a syndrome that was just named after the doctor who first described it, hence its rather scary name! It is very common in adolescent athletes, due to an unfortunate anatomical situation. In the knee, there is a tendon (the patella tendon), that connects the kneecap (patella) onto the shin bone (tibia). The knee cap is very important in acting as a pulley, to transfer force through the knee without the muscles and tendons rubbing over the joint. Unfortunately, where the tendon attaches onto the tibia is exactly where the growth plate is situated in young adults. During periods of growth, there is a lot of cell turnover happening in this area, and if the tendon is repeatedly pulled (as in running and jumping) an area of inflammation may occur around the growth plate. In severe cases, a small piece of bone may actually pull up, away from the shin bone, and a visible lump will be seen at the top of the shin.
Aggressive work on the knees can actually cause the bone be cleaved off, due to too much load in this area during a period of sudden growth or intense activity. The photos below are from an 11 year old dancer who was doing a significant amount of dancing during a sudden period of growth. You can see the damage to the bone where the patella tendon attaches to the lower leg.
While excessive jumping will aggravate the condition, if you suffer from osgood schlatters you should not have to stop ballet or sport completely. You should abstain from jumping when the knees are tender, and must continually focus on correct placement of the feet and hips. Please also consult a sports physical therapist who is accustomed to dealing with younger people as there are also many things that you can do to decrease the symptoms to allow you to continue dancing, including;
- Gentle stretching of the Quadriceps.
- Strengthening the buttock muscles and other muscles around the hips to reduce the load on the knees.
- Taping techniques to redistribute the load going through the tendon.
- Icing the tendon after exercise, on a gentle stretch, to reduce the amount of inflammation.