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December 5, 2011

Collateral Damage

Rounding Up the Usual Suspects: Impingement

by Corey Dawkins and Ben Lindbergh

This article on impingement kicks off an in-depth series on some of the most common injuries we see in baseball. While impingement is an official diagnosis, it describes a dynamic process or a collection of conditions rather than a single end result, such as a torn ACL. Complicating matters for the fans is the fact that the end result is what is usually reported. Pedro Martinez and his case of rotator cuff inflammation, which developed into a partial rotator cuff tear? Impingement played a role. Martinez is an extreme example of how shoulder impingement can derail a career, but how many times have we heard of a pitcher developing rotator cuff inflammation and spending more time on the disabled list than expected?

Brief Anatomy
There are actually four different joints that make up the shoulder complex: glenohumeral, scapulothoracic, acromioclavicular, and sternoclavicular. Each of these must be functioning properly in order for the shoulder to function normally while performing baseball activities. The sternoclavicular (SC) joint is the only joint connecting the shoulder to the axial skeleton. The medial end of the clavicle attaches to the sternum with a cushioning disc between them, much like a meniscus, which provides a very strong connection that takes a large amount of force to injure. The joint is further strengthened by very strong ligaments. The other three joints are the ones we are most concerned with when discussing impingement.

The glenohumeral (GH) joint is the joint we usually picture when we imagine the shoulder joint. It is a multiaxial ball-and-socket joint, a stricture which permits movement in any plane but makes it inherently unstable. The head of the humerus is much larger than the surface it articulates with on the scapula, called the glenoid fossa, and it resembles a golf ball on a tee. The labrum is a structure that adds stability to the joint by forming a rim of fibrocartilage that attaches on the glenoid fossa. Rotator cuff muscles pass from their origins on the scapula, through the subacromial space, and insert on the greater and lesser tuberosities on the humerus.

The scapulothoracic joint is not what most consider a true joint, and it represents the articulation between the front of the shoulder blade and the back of the upper ribs. It is primarily musculotendinous in nature, relying upon major muscles such as the trapezius, serratus anterior, and rhomboids for its control. The most important function of the scapulothoracic joint is to properly align the glenoid fossa during the throwing motion to improve stability and function of the glenohumeral joint.

The acromioclavicular (AC) joint is the joint situated at the point where the lateral edge of the clavicle articulates with the acromion, a process coming off the scapula. The acromion can be categorized into three types: flat, curved, and hooked. The hooked acromion is associated with a higher incidence of impingement and rotator cuff tears. The coracoacromial ligament attaches at the tip of the acromion, which is where bone spurs commonly form. Portions of the long head of the biceps and the rotator cuff pass through the subacromial space.

Types of impingement and their causes
Impingement primarily comes in two varieties: subacromial and internal. Both intrinsic and extrinsic factors play a role in subacromial impingement’s development, including mechanical wear of the rotator cuff, vascular supply to tendons, degenerative tendinopathy, inflammation, poor posture, muscle weakness, altered kinematics, or soft tissue contractures, to name a few. Subacromial impingement typically occurs between 70 and 120 degrees of abduction, although large anatomic variants or other pathologies may alter those numbers. Anatomical variants such as an abnormally hook-shaped acromion or a large amount of spurring off the acromion increases the possibility of rotator cuff issues as the subacromial space is further decreased. The subacromial bursa is also often involved, especially once it becomes thickened and less pliable in the cases of chronic inflammation. The long head of the biceps also can become involved, and in extreme cases, more prone to rupturing.

Throwing is the main cause of subacromial impingement in baseball players. During the pitching motion, a tremendous amount of force is produced and must be safely dissipated. During arm deceleration, if the body is unable to dissipate the force produced, the humeral head can translate superiorly. This leads to the structures between the humeral head, coracoacromial arch, and the undersurface of the acromion being pinched, and inflammation or tearing can result. Structures at risk include the rotator cuff muscles, subacromial bursa, and long head of the biceps tendon.

Internal impingement is a little more difficult to assess but is common in overhead athletes. With internal impingement, contact between the undersurface of the rotator cuff muscles and the posterosuperior lip of the labrum occurs during the late cocking and early acceleration phases of throwing. This is the result of the arm dragging behind the body as it first starts to accelerate to the highest angular velocities on record. With proper training and muscular balance, the incidence and effects of internal impingement can be minimized. This repetitive contact between the structures can lead to undersurface partial rotator cuff tears, fraying or tears of the labrum, or cystic changes in the humeral head.

Internal derangement does not develop for one reason in particular. Over time, most pitchers at the professional levels develop some level of anterior laxity as their shoulders undergo significant stress during the late cocking and early acceleration phases of throwing. It has been suggested that excessive horizontal abduction has also led to anterior laxity. With balanced muscle strength throughout the throwing motion, the laxity can usually be contained. Once the muscles begin to fatigue, the risk of increasing the laxity rises. As a result, internal impingement likely is the result of a multitude of factors, including muscle weakness, anterior laxity, improper mechanics, and microinstability, to name a few.

Proper scapular positioning is vital to prevention of both types of impingement. The trapezius, rhomboids, serratus anterior, and pectoralis minor all help to control the position of the scapula. The scapula normally should be able to elevate the acromion to decrease impingement in the subacromial space and coracoacromial arch. Importantly, it also can rotate around an axis to properly position the glenoid fossa during throwing. When there are muscle spasms, tightness, or weakness, the scapular motions can become compromised and increase the likelihood of impingement.

Presentation
In cases of subacromial impingement, the player will complain of pain with overhead activities, even routine ones that take place off the field. Pain associated with reaching for something high on a shelf or sleeping on the affected side are common complaints. The pain is generally located over the anterior or lateral shoulder, with posterior shoulder pain being uncommon for subacromial impingement. Obviously, pitchers complain of pain and decreased throwing velocity with cases of subacromial impingement. Strength imbalances between the anterior and posterior musculature can lead to an improperly positioned scapula and can play a role in subacromial impingement. Poor posture and rounded shoulders—quick, everyone sit up straight—are also common. X-rays are taken to rule out any bony pathologies, such as a hooked acromion, but MRIs provide a better picture of the soft tissue injuries.

Players with internal impingement generally experience pain while throwing, and pitchers often describe stiffness and difficulty getting loose. There is not as much pain in daily life as there is during pitching. Pitchers often suffer decreased velocity and command. Pain occurs during the late cocking and early acceleration phases of pitching as the humeral head glides and pinches the posterosuperior labrum and rotator cuff. The location of the pain varies depending on any concurrent injuries but generally is posterosuperior in nature. Tenderness in the area is also often present. Internal rotation and horizontal adduction are usually affected and different from baseline values. Weakness of the external rotators—supraspinatus, infraspinatus, and teres minor—along with other muscles acting on the scapula is also common. Internal impingement can be identified through imaging by noting common findings associated with it, such as undersurface rotator cuff injury and posterosuperior labral changes. Non-contrast MRIs can be helpful in advanced cases, but contrast MRIs provide the best definition of the labrum.

Treatment
Treatment for subacromial impingement always involves nonsurgical management at first. As is the case with every injury, rest from the provocative activities is required. Anti-inflammatory medication and modalities are usually initiated, as are limited physical therapy exercises. The use of corticosteroid injections can be beneficial, but it is purely a temporary fix. Repetitive injections are rarely used because of the detrimental effects they have on tendon integrity.

Long-term physical therapy is the most effective option in avoiding a recurrence of injury, especially in those with Type-I or flat acromions. If conservative therapy fails, then surgical intervention is the obvious next step. The surgery can be open or arthroscopic in nature and involves debridement of any scar tissue and bone spurs and limited resection of the coracoacromial ligament. The anterior portion of the acromion can also be shaved away in order to effectively increase the amount of subacromial space while maintaining most of the origin of the deltoid muscle.

Internal impingement also does not automatically lead to surgery. Conservative management through activity modification and physical therapy is successful in most cases. Players avoid throwing for at least two weeks and get their inflammation under control. Modalities or medications are used to further aid in decreasing inflammation, and physical therapy exercises include strengthening of the external rotators, scapular retractors, and scapular protractors.

Rehabilitation is progressed into a thrower’s ten program with increasing exercise intensity before returning to an interval throwing program. Generally, an interval throwing program involves throwing every other day while gradually increasing to up to 180 feet. Only after successfully progressing through the flat ground throwing program can the pitcher return to a mound program. If the internal impingement is advanced, it may require surgical intervention.

While the patient is under anesthesia during surgery, the shoulder is again examined. This eliminates any muscle or positional guarding present while the athlete is awake and ensures a more accurate diagnosis. Although there can be an open repair depending on the surgical plan, the surgeon will generally examine all of the structures under arthroscopy prior to any repair. There could be many structures injured, including the labrum, rotator cuff, and/or humeral head. In addition to repairs to those structures, the anterior capsule will be thoroughly evaluated, and the surgeon will make a determination as to whether it needs to be tightened to limit anterior laxity.

After everything inside the joint capsule is addressed, the surgeon turns his focus to the subacromial space. Any scar tissue is removed, as are any spurs off the acromion. Rehabilitation following surgery will also depend on the exact procedure and repair performed.

Summary
Impingement can come in a few different flavors, but one commonality is that in all cases, it’s not easy to pinpoint a single cause. Because of the pure repetitive nature of pitching, pitchers are more prone to impingement. Both types of inpingement demand conservative management as the initial course of action and only require surgery in the most advanced cases or if conservative therapy fails. Impingement occurs much more often than is reported, and a large majority of the reported cases of rotator cuff tendinitis, frayed labrums, or even partial rotator cuff tears are the result of impingement over the years. The best way to avoid impingement is to have good mechanics early on without muscle imbalances or strength deficits—Pedro Martinez and Brandon Webb probably wish they had.

Corey Dawkins is an author of Baseball Prospectus. 
Click here to see Corey's other articles. You can contact Corey by clicking here
Ben Lindbergh is an author of Baseball Prospectus. 
Click here to see Ben's other articles. You can contact Ben by clicking here

Related Content:  Shoulder,  Rotator Cuff,  The Process

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