Shoulder Instability

What is it?

Shoulder instability is the abnormal relationship between the Humeral ball joint and the Glenoid socket, such that there is excessive movement between the two resulting in the loss of stability. This can be caused by several factors, both within the shoulder joint or capsule itself, or outside the joint involving the muscles or bones. Shoulder instability manifests either by “a feeling of looseness,” or loss of momentum and strength in the shoulder.

The classic example of this is the baseball pitcher who loses zing in their fastball. Other athletic motions that can signal instability include tennis serving, kayak paddle control, crew, wrestling, and lacrosse. In some cases, for sports such as swimming and gymnastics, athletes can actually benefit from the shoulders being a little “loose.”

The most obvious example of shoulder instability is a dislocated shoulder. A dislocated shoulder has gone full circle, from a little looseness, to stretching out the soft tissues, to the humeral ball joint actually jumping (usually forward) out of the glenoid socket. When shoulder dislocation occurs in a young individual (age 17-40), there is a very high probability that recurrent dislocations will occur in the future. We will focus primarily on adult shoulder instability (age 17 and up), although there is a section at the end on Pediatric Glenohumeral Instability.

Normal Anatomy

The shoulder is best thought of as a universal joint. It has a ball, which is actually a cartilage sphere, which makes up 2/3 of the top of the upper arm bone (also known as the humerus). It articulates with a relatively flat & oval glenoid bone, which is shaped similar to the racetrack at the Indy 500 – slight, high-riding curves at the outer edges. This socket is deepened by a lip of soft tissue around the entire glenoid bone, called the labrum. Much like how the chain-linked fence at the Indy 500 deepens the racetrack to keep the cars on the track, the labrum serves to keep the ball of the humerus within the joint.

This Glenoid-labral complex further functions like the suction cup you attach to a glass window. By maintaining a negative pressure within the shoulder joint, the labrum helps to keep their humeral ball anchored in the socket. Due to the flatness of the glenoid component, this makes the shoulder the most movable joint of the entire body. Free of a matching socket for the humeral ball like in the hip (ball & socket joint), or a mortise to cradle the humeral head like in the ankle (hinged joint), the shoulder can achieve remarkable ranges of motion unmatched anywhere else in the body. This system makes up the static stabilizers of the shoulder joint.

The shoulder joint is contained within a capsule. This capsule functions like a balloon, surrounding the Humeral ball and glenoid socket to keep lubricating fluid where it is needed. There are several regions of the capsule where it is thickened, to serve as addition restraints against the ball sliding out of the joint, depending on the position of the arm. These ligaments act as dynamic stabilizers of the shoulder joint. Dynamic stabilizers are called to function as the arm moves.

Several muscles surround the shoulder joint. Four muscles in particular (Supraspinatus, Infraspinatus, Teres Minor, and Subscapularis) originate in the chest & back to converge on the Humeral ball. These muscles come together to form what is commonly called the “rotator cuff”. To picture these muscles, imagine a 4-legged Tepee lying on its side. The rotator cuff muscles – the subscapularis, supraspinatus, infraspinatus, and teres minor – make up the muscle stabilizers of the shoulder joint. Muscle stabilizers control a wide variety of shoulder motion, including internal rotation (scratching your lower back), external rotation (opening a door), and forward flexion (reaching up). Several other important muscles make up the outer layer of shoulder stabilizers, including the deltoid, pectoralis major, Latissimus dorsi, and the biceps. The biceps muscle deserves special recognition, as parts of it are involved at all layers, and can function as a static, dynamic, or muscle stabilizer depending on position of the shoulder. The Biceps is anchored on the 12 o’clock position of the glenoid bone within the joint capsule. It then traverses over the top of the humeral ball, where it works to prevent the ball from traveling upward and banging into the acromion, or shoulder blade. As the biceps enters a small groove in the humeral head, it prevents forward migration of the ball external to the shoulder joint proper. The biceps is also a usual source of shoulder pain that radiates down the upper arm, and sometimes even affects the elbow.

Abnormal Anatomy

Shoulder instability occurs when one or more of the stabilizing systems of the shoulder fails. The static stabilizers can fail throughout a traumatic labral tear of either the anterior (Bankart lesion) or superior (SLAP lesion) portion of the labrum. This is usually associated with a dislocation where the arm is flung violently up and back (the windup phase of throwing). This can be seen when a basketball player going up for an overhead shot is stuffed by a blocker. Loss of the anterior or superior bumper allows the humeral ball to slide forward on the flat glenoid bone, resulting in dislocation.

Failure of the dynamic stabilizers, namely the anterior inferior glenohumeral ligament, is though to contribute to recurrent positional instability (“It bothers me only when I throw”). These ligaments are likely torn or stretched at the time of the initial injury. They no longer function as a check to prevent the humeral ball from sliding forward, hence the development of recurrent instability. Failure of muscle stabilizers tends to be more complex. Causes of muscle stabilizer failure are numerous, and can include inflammation (tendonitis), irritation (impingement), nerve damage due to trauma or ganglion, or rotator cuff tears.


A wide range of injury histories can be seen with instability. Usually, the common denominator is a history of traumatic shoulder injury that resulted in either a subluxation (dislocation) or separation. Subluxation is the partial sliding out of the humeral ball out of the socket, such that it can easily slide back into socket with moving the arm.

Shoulder instability has been historically classified as either traumatic or atraumatic. Traumatic instability is associated with a normal shoulder that incurs a traumatic injury that causes the shoulder to dislocate, or sublux, in one direction (usually anterior-inferiorly), and is almost always associated with failure of the static & dynamic stabilizers of the shoulder. There is a very high incidence of re-dislocation and recurrent instability in this group. While the initial event that causes dislocation is remarkable, subsequent events resulting in dislocation are less dramatic. For example, In one case a patient re-dislocated their shoulder by simply putting their arm behind their head on their pillow while watching a hockey game.

Atraumatic instability usually describes a systemic problem. Other joints in the body are usually loose (double jointed) as well. There may be a family history of this generalized ligamentous laxity. The patient usually has looseness in all planes of glenohumeral shoulder motion, which is known as multi-directional instability of the shoulder. Sometimes, these patients can make their shoulder joints pop out of place at will. There is usually no history of a traumatic event causing the issues. Atraumatic instability patients will usually experience looseness in both shoulders. This is a result of the atraumatic decompensation of the muscle stabilizing group caused by abnormally elastic collagen within the static labrum and dynamic capsular ligaments. Some folks summarize the differences between traumatic and atraumatic instability with the simple euphemisms “torn loose” (traumatic) or “born loose” (atraumatic).

Physician Exam

Examination of the shoulder is best accomplished by exposing the entire shoulder.  Wearing Tank tops assists the examiner in getting maximal benefit of the exam.  It is important to assess the degree of instability.  Either outright dislocation, subluxation, or apprehension to movement can characterize recurrent instability.

Apprehension refers to the fear that the shoulder may dislocate in certain positions.  This usually restricts maximal performance within a sport.  The range of motion of the shoulder joint will be compared with the opposite, non-involved shoulder.  Localized tenderness along the anterior glenoid rim will be sought if a labral tear is suspected.  The muscles of the rotator cuff will be tested against resistance.

The apprehension test will usually be positive in patients with recurrent instability.  Other special maneuvers performed by the examiner on the shoulder include the sulcus test, drawer test, push-pull test, and the fulcrum test.  Finally, a close assessment of the neurologic structures will be evaluated to insure that there is no nerve compromise.

Special Test

Many times, a confirmatory test will be ordered.  These include x-rays of the shoulder, which is important in determining a diagnosis for patients with a history of traumatic instability.  An MRI may also be ordered, which uses a special imaging machine to define the soft tissue and bony anatomy rather precisely.  In some cases, it may be necessary to add a special magnetic dye (gadolinium) to the shoulder joint to view an MRI-Arthrogram.  This aids in defining tears of the glenoid labrum.  These special tests can assist in the diagnosis, but do not take the place of a well performed physical exam & comprehensive patient history.

Differential Diagnosis

Other problems may mimic instability and are contained in the list of “other” diagnoses which may be considered.  Luckily for shoulder instability, this list is rather short and usually can be distinguished by physical exam or x-ray.  Soft tissue interposition, scapular winging due to nerve palsy, seizure disorder, electrical shock, violent muscle contraction with dislocation, tumors, and unrecognized fractures can all mimic the symptoms of shoulder instability.


Conservative Treatment

Conservative treatment usually the first option to restore shoulder function.  Physical therapy can assist in building up muscle strength and re-teaching proper motion to restore coordinated, strong muscle contractions that are important for maintaining glenohumeral stability.

The rotator cuff muscles play an important role in stabilizing the shoulder joint. Optimal control of neuromuscular forces is required to restore full shoulder function.  Your physical therapist will assist you in learning how to isolate individual muscles of the rotator cuff and strengthen them.  This strengthening is initially performed within the “stable range” of shoulder function.

In cases of atraumatic instability, studies have shown an 80% success rate with physical therapy.  Unfortunately, only 16% of patients with a history of traumatic instability improve.  Physical therapy is important, even in cases of traumatic instability, to improve the muscle and tissue tone prior to planned surgery.  It is important to avoid activities that stress the capsular and muscular structures.  Certain habits must be broken to avoid the “unstable” positions.  Any position, action, or sport that promotes shoulder subluxation or dislocation must be avoided.


Tremendous gains have been made in the past ten years for surgery on shoulder instability.  Most procedures can now be performed through the arthroscope as same-day outpatient procedures with the use of three small 1/2 inch incisions.  Bone anchors with attached sutures allow the reattachment of torn tissues.   Our preference is to use absorbable suture anchors in younger athletes with simple tear patterns.  This takes about 6 weeks for the tissues to heal to bone, so the anchors are around for plenty of time to allow for healing.  The body reabsorbs the absorbable suture anchor over 3 months, leaving no trace behind!  Lasers or even more advanced thermal controlled radiofrequency devices are used to shrink the redundant capsular tissues and stretched capsular ligaments.

In large studies performed at the U.S. Coast Guard Academy, Army West Point, and Naval Academy, success rates for arthroscopic procedures approached the success rates of open surgical techniques, at about 80% – 97%.  Morbidity as defined by hospital stay, patient pain levels, and return to regular activity was found to be far superior in the arthroscopic group.  Open techniques are still utilized for revisions, or in complicated cases.


After surgery, it is very important to get into a regularly scheduled physical therapy rehabilitation program.  Post-op, the arm will usually be in a sling with a special formfitting ice pack on the shoulder.  Ice can be discontinued after 2 or 3 days when comfortable.  Studies have shown a 50% reduction in narcotic pain medications post-operatively with the use of ice therapy.  Simple pendulum exercises can be performed at home three times a day for the first two weeks.  Then, depending on the repair, patients will be prescribed a full physical therapy program when stitches are removed.

The shoulder is kept “protected” for 6 weeks while soft tissue healing occurs.  Then, active range of motion and strengthening begin.  The goal is to have the patient return to activities of daily living within 12 weeks after surgery.  A more conservative timeline of 14-18 weeks is used to return athletes back to activity.

Frequently Asked Questions

Does Surgery hurt?
Pain is substantially less with the arthroscopic techniques used today than it was with older open techniques.  Patients usually report pain for 2-3 days that then relents to a dull tooth-ache like pain for 3-6 weeks.  As healing occurs, the pain is intermittent and is often associated with over-use or physical therapy sessions.  You’ll go home with strong and mild painkillers to assist you in dealing with the discomfort.  Cryotherapy or the use of ice sleeves has also been shown to substantially reduce the amount of perceived pain.

Will I be able to return to my sport?
The aim for these advanced arthroscopic techniques in sports medicine is to return athletes to their previous level of function as quickly as is safe for the individual.  There have been football players, baseball pitchers, wrestlers, crew team, lacrosse players, swimmers, and basketball players that have returned to their sport at NCAA division-I levels.  Over 90 Coast Guard Cadets have been commissioned into the U.S. Coast Guard after undergoing shoulder stabilization procedures.

I have one dislocation, and now my shoulder is just a little loose…should I be worried?
The answer to this question varies with each individual.  A comparison to the opposite, non-involved extremity will usually exhibit significant more shoulder laxity then was appreciated.  An early evaluation by an Orthopedic Surgeon can help advise you on the proper course of treatment to prevent a  re-dislocation from occurring.

I think I’ve got instability.  What do I do now?
In these days of Managed Care and Health plans, most insurance plans demand that you get referred to an Orthopedic Surgeon through your Primary Care Physician. Talk to your doctor about your symptoms, and request a referral to an orthopedic specialist.


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