Medial elbow anatomy: a paradigm shift for UCL injury prevention and management

Key points

1. 01Two tendinous septa exist between PT-FDS and FDS-FCU muscles, forming an integrated tendinous complex with the brachialis tendon and deep aponeuroses
2. 02The joint capsule merges with the tendinous complex distally at the sublime tubercle with approximately 7 mm attachment width, wider than previously reported
3. 03The PT, FDS, FCU, and brachialis muscles likely work as a coordinated unit rather than independently to stabilize the medial elbow
4. 04The MRI T-sign can be classified into three types based on the newly defined attachment widths: type 1 under 1 mm (normal variant), type 2 of 1-7 mm (capsular detachment), type 3 over 7 mm (tendinous complex detachment)
5. 05Rehabilitation programs such as the Thrower's Ten may need revision to include finger flexor exercises given the FDS role in the tendinous complex

How it was conducted

Design

Descriptive cadaveric anatomical study

Specimens

23 embalmed cadaveric elbows from 16 Japanese donors (8 male, 8 female; mean age at death 83 years, range 49-99)

Analysis methods

Macroscopic dissection and measurement (17 elbows), Masson trichrome histology (6 elbows), micro-CT with radiopaque markers (7 elbows)

Primary aim

Characterize medial elbow tendinous structures and joint capsule in relation to the UCL

Reliability

Interclass correlation coefficient 0.89-0.96 for measurements

What they found

• Capsular attachment width at anterior border of tendinous complex (Ca): 6.4 +/- 0.5 mm
• Capsular attachment width at posterior border of tendinous complex (Cp): 6.2 +/- 0.4 mm
• Maximum capsular attachment width at sublime tubercle (Cm): 7.3 +/- 0.6 mm
• Distance from anterior border of tendinous complex to point of maximum capsular attachment width (Da): 3.6 +/- 0.5 mm
• Distance from posterior border of tendinous complex to point of maximum capsular attachment width (Dp): 2.8 +/- 0.4 mm
• Length of ulnar attachment at anterior border of tendinous complex (La): 26.4 +/- 1.3 mm
• Cartilage surface width without capsular attachment was too small to measure (approximately 0-1 mm)
• Histology confirmed the two TS, brachialis intramuscular tendon, and deep FDS and FCU aponeuroses could not be separated from each other
• Micro-CT confirmed anterior borders of the two TS corresponded to the anterodistal and posterodistal slopes of the medial epicondyle and the anterior and posterior bases of the sublime tubercle

Limitations

• Purely anatomical cadaveric study with no biomechanical or clinical outcome data
• All cadavers were elderly (mean 83 years), not matched to the peak age of overhead throwing athletes
• Six of the 17 elbows used for macroscopic measurement came from the same three individuals, reducing specimen independence
• Findings require validation in future biomechanical and clinical studies before direct clinical application

Why it matters

For patients

Overhead athletes, particularly baseball pitchers, may benefit from rehabilitation programs targeting finger flexor strength, which this anatomy suggests plays a greater stabilizing role than previously recognized.

For clinicians

The new three-type T-sign classification and the broader-than-expected capsular attachment width at the sublime tubercle may improve MRI diagnostic precision and help select patients for repair versus reconstruction.

For readers

This study challenges the conventional ligament-centric model of medial elbow stability by showing that the UCL is part of a larger, inseparable tendinous complex, which has implications for how all periarticular injuries at the medial elbow are interpreted.