December 5, 2011
Rounding Up the Usual Suspects: Impingement
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?
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
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.
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.
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.
Corey Dawkins is an author of Baseball Prospectus. Follow @CoreyDawkinsBP