By Abby Sims
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CC Sabathia, now on the DL due to an accumulation of fluid in his right knee, is reportedly likely to undergo another MRI after having the knee drained on Monday.

The diagnosis — at least the one reported in the media — was inflammation, which isn’t much of a diagnosis at all. It’s what caused the inflammation that is of the utmost concern. Though it could have simply been overuse, it is important to investigate further in order to treat properly and work to prevent future episodes.

Sabathia’s initial MRI, which was negative for meniscal involvement, may not have been as definitive as desired due to his condition at the time. The lefty, who had surgery in 2010 to repair a meniscus in the knee, commented that this time around he first noted the swelling when pitching on May 4th.


Every knee contains some fluid — called synovial fluid — which helps to lubricate the joint to minimize friction and facilitate movement. This fluid is contained within the joint capsule, which surrounds all synovial joints. The capsule, which is lined with a membrane known as the synovial membrane, also serves to supplement the ligaments (which connect one bone to another) to provide joint stability.


A buildup of fluid within the joint capsule — like Sabathia’s — is called an effusion. This is distinguished from edema, which is when fluid accumulates in the soft tissues outside the joint capsule.

A knee with an effusion feels like it contains a water balloon. In contrast, edema feels like it stretches the skin and is often accompanied by redness. Both may cause the area to feel warm to the touch.

Possible causes of edema include trauma from a contusion (bruise), soft tissue injury such as muscle strains or inflammation of a bursa.


Any number of pathologies can result in fluid buildup. Some of these may be from sudden trauma, while others may be due to more chronic conditions. Trauma to the knee that causes fracture or tearing of ligaments — such as the ACL or PCL — can cause that fluid to be bloody. Damage to the highly vascularized synovial membrane can also result in hemorrhage into the joint.

If the synovium becomes inflamed, causing pain and swelling, it too can cause joint effusion. This is known as synovitis and can result from overuse.

There are two types of cartilage in the knee (view image). The medial (innermost) meniscus and lateral (outermost) meniscus are composed of fibrocartilage and serve to create a better fit for the femur and tibia at the knee and act as shock absorbers. The other — articular cartilage — provides a smooth glassy surfacing at the ends of many bones. It facilitates movement while also protecting the integrity of the bones. Both meniscal and chondral (articular cartilage) injuries can result in a localized collection of fluid. However, because both types of cartilage lack a significant blood supply, isolated injuries to these structures are not likely to result in bloody fluid.

Though meniscal and chondral injures can be caused by trauma — particularly when it involves twisting motions — both are also often caused by degenerative wear and tear. A gradual onset of symptoms, including a proliferation of fluid, might result from the latter.

Osteoarthritis, which, among other things, entails a wearing of the chondral surface, is a common cause of inflammation and results in various degrees of deformation of the knee.

Rheumatoid arthritis, another condition that can affect the knee, also results in pain and swelling.

At the knee, injuries often occur to several structures simultaneously. Additionally, the presence of one or more pathologies may also predispose to the onset of others (more on this as it relates to ligaments here).

Lastly, other non-traumatic causes of fluid within the knee might be localized infection or gout.

Follow Abby on Twitter @abcsims.

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