The Throwing Shoulder

James P. Sostak II

Carlos A. Guanche

INTRODUCTION

Throwing Motion

Throwing in baseball has been analyzed extensively, with the specific maneuvers being broken down into 6 phases (6). While the analysis of the baseball throw is the best understood and most studied, other throwing, racquet, and overhand sports have also been evaluated. The mechanics of a baseball throw are transferable, for the most part, to other sports with some modifications depending on the size of the ball or the maneuver being analyzed. The baseball throw, by virtue of its speed and frequency, however, is the most traumatic to the shoulder.
The phases of throwing are summarized as follows:
- Wind up
  - □ Readying phase
  - □ Minimal shoulder stress
  - □ Ground, legs, and trunk are force generators
- Early cocking
- Late cocking
  - □ Scapular retraction for stable throwing base
  - □ Maximal external rotation
  - □ Posterior translation of the humeral head as a result of abduction/external rotation (ABER)
  - □ Shear force across anterior shoulder of 400 newtons
  - □ Compressive force of 650 newtons generated by cuff
- Acceleration
  - □ Transition from eccentric to concentric forces anteriorly (vice versa posteriorly)
  - □ Rotation occurs at 7,000-9,000 degrees per second
  - □ Only one-third of the kinetic energy leaves with the ball (the remainder is dissipated through the extremity)
- Deceleration
  - □ Most violent phase (responsible for dissipation of energy not imparted to ball)
  - □ Distractive forces generated by the acceleration phase are countered by violent contraction of posterior rotator cuff and scapular stabilizers
  - □ Largest joint loads
    - Posterior shear force of 400 newtons
    - Inferior shear forces of > 300 newtons
    - Compressive forces of > 1000 newtons
    - Adduction torque > 80 newton-meter; horizontal abduction torque of 100 newton-meter
- Follow-through
  - □ Rebalancing phase
  - □ Compressive forces of 400 newtons
  - □ Inferior shear of 200 newtons
The entire motion takes less than 2 seconds with most of the time (1.5 seconds) taken up by the early phases (wind up and cocking).
- Three critical points in the motion:
  - □ Cocking: Point in process where full external rotation/abduction is achieved, shear force on labrum is maximum, biceps vector shifts posteriorly, and the energy generated by trunk/legs is transferred to shoulder; leads to potential injury situation for the shoulder at risk (2,3).
  - □ Acceleration: The body falls ahead of the shoulder while the internal rotators are maximally contracting and the angular velocity exceeds 7,000 degrees per second.
  - □ Deceleration: Scapular stabilizers and posterior rotator cuff contract violently to counter the distractive force of acceleration and lessen the load on the posterior inferior glenohumeral ligament (PIGHL).

Kinetic Chain of Throwing

The forces needed to propel the ball and generate the velocity of the throw require contributions from all body segments (4). These contributions help to minimize joint stress and allow for the forces to be passed to distal segments as the motion progresses.
The kinetic chain includes
- Force generators – ground, legs, trunk
- Force regulator – shoulder
- Force delivery – arm

Dead Arm

Any pathologic shoulder condition in which the thrower is unable to throw with preinjury velocity and control because of a combination of pain and subjective unease in the shoulder (4).

Glenohumeral Internal Rotation Deficit (GIRD)

Basic definition: Loss of glenohumeral internal rotation in the throwing shoulder compared to the nonthrowing shoulder.
What constitutes a clinically significant amount of GIRD is debated.
Acceptable level of GIRD, as defined by Burkhart et al. (2,3,4), is less than 20 degrees or less than 10% of the total rotation measured in the nonthrowing shoulder.
Clinical significance may occur when internal rotation loss exceeds external rotation gain in the throwing athlete.
GIRD can also be seen in asymptomatic throwers and, in this case, may be related to increased humeral retroversion (21).

PATHOPHYSIOLOGY

Current Theory

Centers around GIRD, caused by contracted PIGHL, as the essential problem in pathology of the thrower’s shoulder.
Previously, anterior capsular laxity due to repetitive microtrauma of the throwing motion had been suggested as the essential problem (9).
Recent basic science suggests that the thrower’s shoulder pathology may be the result of posterior capsular/ligament tightness rather than increased anterior laxity. There was a trend toward a less inferior position of the humeral head in the late cocking position when a posterior capsular contracture was simulated (7).

The Pathologic Cascade (2,3)

Primary event — posterior cuff and scapular stabilizer weakness; patient often asymptomatic

Muscle weakness leads to PIGHL overload as a static stabilizer, which leads to PIGHL fibrosis and contracture, clinically manifested as GIRD.

Posterior superior shift of humeral head on the glenoid occurs in the ABER position (late cocking phase) due to posterior ligament contracture. Can lead to excessive internal impingement beyond the physiologic level, excessive labral shear forces, and excessive posterior biceps vector, thus leading to painful shoulder
Posterior type II superior labrum anterior to posterior (SLAP) tear occurs next as labrum gives way due to increased shear stress combined with increased peel-back force from posterior biceps vector.
Undersurface posterior rotator cuff tears follow due to increased tensile, torsional, and compressive forces.
Tertiary event of the cascade is anterior capsular failure with resultant instability. This is often limited to veteran throwers, who despite proceeding through the previous points in the cascade over many years have been able to compensate, continuing to throw at an elite level, and only present once the instability tips them over the edge.

Labral Tears (SLAP)

Traction Mechanism

During deceleration, biceps muscle contraction is strong as both elbow extension and glenohumeral distractions occur. The biceps muscle has been shown to be essential to limiting torsional forces to the shoulder in the ABER position (17). By this mechanism, the effect on the superior labrum would be one of failure either by tension or direct compression.

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