Known as prepatellar bursitis, water on the knee is the inflammation of fluid-filled sacs called the bursae. There are several bursa sacs of the knee. On each knee, the prepatellar sac sits on top of the patella just under the skin. The injury is characterized by pronounced localized swelling directly over the kneecap and will occasionally extend to the quad area. Pain is exacerbated during the end range of flexion or when direct pressure is placed upon the bursa sac. Direct and repetitive trauma and repetitive motion are the primary causes of prepatellar bursitis, according to the textbook "Sports Injury Management." In rare instances bursitis can result from infection or spider bites. As stated in the "Principals of Athletic Training" textbook, bursal swelling can be immediate or develop over several days .
Direct Trauma
The most common cause of water on the knee is direct trauma from a single blow ,according to the textbook "Principles of Athletic Training." One example of a direct blow is falling on a flexed knee. This is common in the sport of basketball where an unprotected knee hits the floor after a player trips. Soccer is another sport where water on the knee can result from a direct blow. When two players are going after a loose ball an errand kick can hit another players knee cap causing bursitis.
Repetitive Trauma
Light repeated trauma to the anterior portion of the knee cap will cause water on the knee. For example in the sport of wrestling the athlete will often go in to a kneeling or crawling position. Getting in to this position on a daily basis during practices or matches can cause bursitis. In the working world prepatellar bursitis is often found in carpenters or tailors. These professions require the worker to repeatedly kneel to perform work tasks predisposing them to water on the knee.
Repetitive Motion
In rare cases the repeated flexion and extension of the knee can aggravate the prepatellar bursa. Friction occurs between subcutaneous tissue and the patellar tendon during knee flexion and extension, according to the textbook "Physical Rehabilitation of the Injured Athlete." Repeated friction will trigger bursal swelling. Activities require repetitive knee flexion and extension such as dancers or gymnasts are predisposed to prepatellar bursitis.
Secondary to Other Injury
In very rare cases bursa can swell secondary to another injury. Serious injuries such as a quadriceps tendon rupture or patellar fracture can bring about bursae inflammation. Another rare cause is from infection or insect bites such as a spider bite states the textbook " Physical Rehabilitation of the Injured Athlete." Spider bites can cause inflammatory response in nearby by tissue. The infection or insect bite can cause over stimulation of synovial fluid production causing a painful an inflamed bursae.
References
- "Sports Injury Management"; Marcia K. Anderson, Susan J. Hall; 1995
- "Principals of Athletic Training"; Daniel D. Arnheim , William E. Prentice; 2000
- "Physical Rehabilitation of the Injured Athlete"; James R. Andrews, Gary L. Harrelson, Kevin E. Wilk; 1998
