Causes and Effects of Serratus Anterior Weakness
Serratus anterior weakness primarily results from long thoracic nerve injury, which can occur through trauma, neurological conditions, or idiopathic causes, leading to scapular winging and significant functional impairment of the shoulder. 1
Primary Causes of Serratus Anterior Weakness
Neurological Causes
- Long thoracic nerve injury - The most common cause 1
- Trauma (direct or indirect)
- Compression along the chest wall
- Parsonage-Turner syndrome (neuralgic amyotrophy)
- Viral illness
- C7 radiculopathy - Often overlooked but can present with serratus anterior weakness 2
Traumatic Causes
- Sports injuries, particularly in throwing athletes 3
- Repetitive stress from overhead activities
- Direct trauma to the shoulder region
- Iatrogenic injury during surgical procedures 4
Other Causes
- Idiopathic - No identifiable cause in some cases 4
- Inflammatory conditions - Rare cases reported in rheumatoid arthritis 5
- Overuse injuries in athletes, particularly throwers 3
Pathomechanics in Athletic Populations
In throwers, serratus anterior weakness often develops as part of a complex pathomechanical process:
- Repetitive stress leading to microinstability
- Extreme forces during throwing (internal rotation torques up to 67 N-m)
- Distraction forces equal to body weight during throwing motion
- Compressive forces up to 1090 N directed posteriorly during deceleration 3
Effects and Clinical Presentation
Functional Impairments
- Scapular winging - The hallmark sign 1
- Medial translation of the scapula
- Rotation of the inferior angle toward the midline
- Prominence of the vertebral border
- Inability to maintain proper scapular position during arm elevation
- Decreased shoulder strength and endurance
- Limited range of motion during abduction with external or internal rotation 3
Pain and Discomfort
- Pain during arm elevation and forward reaching activities
- Discomfort in anterior/anterolateral shoulder during movement
- Pain during throwing, especially during release and deceleration phases 3
Scapular Dyskinesis
- Poor coordination of scapular upward rotation
- Inadequate posterior tilting during arm elevation
- Contributes significantly to secondary rotator cuff injury 3
Secondary Complications
- Secondary shoulder impingement syndrome
- Rotator cuff dysfunction
- Decreased throwing velocity and precision in athletes
- Muscle atrophy in rotator cuff and deltoid regions 3
Assessment Findings
- Visible winging of the scapula, especially when pushing against resistance
- Weakness in forward flexion and abduction of the arm
- Scapular dyskinesis during arm elevation
- In C7 radiculopathy cases, winging may only be apparent when pushing with hands at waist level 2
Clinical Course and Prognosis
Most cases of serratus anterior weakness due to long thoracic nerve injury resolve with conservative treatment, but recovery can take up to 2 years. In cases where conservative management fails after 12 months, surgical intervention may be considered, with pectoralis major tendon transfer being the preferred surgical approach. 1, 4
Common Pitfalls in Management
- Failing to recognize early warning signs in athletes
- Returning to throwing activities before adequate rest and rehabilitation
- Ignoring scapular dyskinesis when treating related rotator cuff injuries
- Not addressing the entire kinetic chain in rehabilitation 3
- Overlooking C7 radiculopathy as a potential cause of serratus anterior weakness 2
Understanding the causes and effects of serratus anterior weakness is essential for proper diagnosis and management, particularly in athletic populations where early intervention can prevent long-term disability.