Medial patellar luxation occurs when the patella (knee cap) moves medially (to the middle of the body), followed by the patellar ligament returning to its normal position. When the patellar ligament luxates, a "popping" can be felt.
A subluxating patella is a knee cap that is unstable but does not slip out of joint. A patella that comes out of joint on its own is a luxating patella. Although the problem can be caused by trauma, it is more often genetic.
A traumatic injury may occur if your Boston's leg gets caught and he struggles to pull free, forcing the knee out of normal alignment. Rough play where the dog's leg is pulled can also result in injury. An exam by your veterinarian can determine if the knee is stable. This is best done by your veterinarian since the ligaments are easily stretched if too much pressure is applied.
In cases of genetic patellar luxation, there is an abnormal development of the leg. A small groove in the joint allows the patella to move freely while remaining in alignment. If this groove is too shallow or if the leg is bowed, the ligaments holding the patella may become damaged. The ligaments themselves may also be weak. Any or all of these conditions will lead to problems.
If your BT has subluxating patellas, keep him from becoming obese.Excess weight only puts more pressure on the joints.Surgery is usually required when the patella becomes loose and pops in and out on its own. Surgery involves deepening the groove in the bone that the knee cap sits in. Recovery time is approximately 6 weeks and may include physical therapy.
Photo "Poser" courtesy of d70focus