The floating shoulder: a biomechanical basis for classification and management

Area of Science:

• Orthopedic biomechanics
• Trauma surgery
• Anatomy

Background:

• The floating shoulder, characterized by ipsilateral clavicular shaft and scapular neck fractures, is often deemed unstable.
• Current treatment recommendations lack robust biomechanical data.
• This study investigates the structural contributions to stability in scapular neck fractures.

Purpose of the Study:

• To quantify the osseous and ligamentous contributions to the stability of experimentally induced scapular neck fractures.
• To evaluate the biomechanical impact of combined clavicular and scapular fractures on shoulder stability.

Main Methods:

• Twelve fresh-frozen human cadaveric shoulders underwent standardized scapular neck fracture creation.
• Specimens were tested in a custom apparatus using a materials testing device.
• Stability was assessed by measuring resistance to medial displacement after sequential disruption of associated bony and ligamentous structures.

Main Results:

• Scapular neck fractures alone resisted 183 N of medial force.
• Adding a clavicular fracture reduced stability by 30% (128 N).
• Complete instability (0 N resistance) occurred only after sequential disruption of the coracoacromial and acromioclavicular capsular ligaments, or coracoacromial and coracoclavicular ligaments.

Conclusions:

• Ipsilateral clavicular shaft and scapular neck fractures alone do not constitute a floating shoulder.
• Significant ligamentous disruption (coracoacromial and acromioclavicular capsular ligaments) is essential for the instability characteristic of a floating shoulder.
• These combined injuries likely involve substantial medial displacement of the glenoid fragment.