What is the labrum?
The labrum is a type of cartilage found in the shoulder joint. The shoulder is a ball and socket joint where the arm meets the body (Figure 1). The arm bone (humerus) forms a ball at the shoulder which meets the socket which is part of the shoulder blade. These two bones are connected by ligaments which are tough tissues forming tethers that hold the bones in relationship to each other.
There are two kinds of cartilage in the joint. The first type is the white cartilage on the ends of the bones (called articular cartilage) which allows the bones to glide and move on each other. When this type of cartilage starts to wear out (a process called arthritis), the joint becomes painful and stiff (see Patient Guide to Shoulder Replacements). The labrum is a second kind of cartilage in the shoulder which is distinctly different from the articular cartilage. This cartilage is more fibrous or rigid than the cartilage on the ends of the ball and socket. Also, this cartilage is also found only around the socket where it is attached.
What is the function of the labrum?
The labrum has basically two functions. The first is too deepen the socket so that the ball stays in place. The best analogy is to picture the shoulder joint as a beach ball on a dinner plate. The ball of the humerus (the “beach ball”) is much larger than the flat socket ( the “dinner plate”). One thing that keeps the ball in the socket are the ligaments; these are the tethers that go from bone to bone which hold the bones together. The other way the ball is kept in the socket is the labrum (Figure 2).
The labrum is a thick tissue or type of cartilage that is attached to the rim of the socket and essentially forms a bumper which deepens the socket and helps keep the ball in place. In individuals where the labrum is too small or is torn due to an injury, the ball may slide part of the way out of the socket (called a “subluxation”) or all the way out of the socket (called a “dislocation”) ( See Patient Guide to Shoulder Instability. The labrum goes all the way around the socket and in most areas is firmly attached to the bone of the socket. In some areas it is not firmly attached and only recently have specialists determined which parts are normal and which parts reflect tearing of the labrum.
The second function of the labrum is as an attachment of other structures or tissues around the joint. For example, the ligaments that help hold the joint together attach to the labrum in certain key locations. If there is an injury to the shoulder that tears the ligaments, sometimes the labrum is pulled off of the rim of the bone as well (Figure III).
This injury usually involves a subluxation or dislocation of the shoulder and is usually due to trauma. The ball of the shoulder can dislocate toward the front of the shoulder (an anterior dislocation) or it can go out the back of the shoulder (called a posterior dislocation). In either case the labrum can be torn off of the bone. Usually when this happens the labrum does not heal back in the right location. Whether the joint continues to be unstable depends upon many factors.
The other structure that attaches to the labrum is the tendon of the biceps muscle (Figure 4a). The biceps muscle is the muscle on the front of the arm which gets firm with bending the elbow. While this muscle is quite large, it turns into a small tendon about the size of a pencil which attaches inside the shoulder joint. At the other end of the muscle is a large tendon which attaches beyond the elbow in the forearm. The portion which attaches in the shoulder actually goes through a small hole in the rotator cuff tendons designed specifically for that tendon. Once inside the joint the tendon attached in part to the bone near the socket and in part to the labrum at the top of the joint. This tendon can get torn where it attaches to the bone, where it attaches to the labrum or at both locations (Figure 4b).
What is a labrum tear?
A labrum tear can take several forms, and it is very easy to confuse these types. As a result, it is important that you discuss with your physician exactly what type of tear he/she is talking about. The first type of tear is one where the labrum is torn completely off of the bone (Figure 3). This is usually associated with an injury to the shoulder where the shoulder has subluxated or dislocated. Sometimes this type of tear occurs and the individual does not appreciate that the shoulder has slid out of the socket.
The second type of labrum tear is tearing within the substance of the labrum itself. The edge of the labrum over time may get frayed so that the edge is not smooth. This type of tearing is quite common and rarely causes symptoms. It is seen frequently in the shoulder as people get more mature (over forty years of age). Sometimes the labrum may have a large tear where a portion of the labrum gets into the joint and causes clicking and catching as the ball moves around in the socket. This type of tear is very rare, and most labrum tears do not cause these symptoms.
A third type of labrum tear is in the area where the biceps tendon attaches to the upper end of the socket. The socket can be divided into four regions: anterior (or front), posterior (or back), the upper end near your head (or superior), and the lower end (or inferior) which is toward the elbow (Figure 5).
The biceps tendon attaches at the superior end where it blends in with the labrum. The labrum runs from there around the joint, both in an anterior and in a posterior direction. Due to injury in this area where the biceps tendon attaches, the labrum also can get injured. The injury in this area can be mild or it can be severe. Because the injury typically involves the biceps tendon and the labrum, because it is at the superior end of the socket and because it can effect the
labrum attachments anterior and posterior to where the biceps attaches in this region, the acronym or abbreviation for this injury is a SLAP lesion. This stands for an injury which is Superior Labrum Anterior and Posterior. There have been several grading systems or classification systems of this injury. In a lesser injury the labrum is only partially detached in this area. In a more severe injury the whole labrum is pulled off of the bone along with the biceps tendon. The most common classification divides SLAP lesions into four types (Figure 6).
From John Hopkins Sports Medicine