Fukuda and colleagues measured how much the acromioclavicular, conoid and trapezoid ligaments each restrain the AC joint, and how that share changes with the direction and size of clavicle displacement.
Cadaver study of 12 shoulders from 7 fresh cadavers (aged 50 to 70) at the Mayo Clinic biomechanics laboratory.
Methods
Model
12 shoulders from 7 fresh‑frozen cadavers, aged 50 to 70
4 had rotator cuff ruptures
Ligament length measured in 5 by biplane radiography
Constructs
Intact joint
Sequential sectioning of the AC, conoid and trapezoid ligaments
Loading
12 displacement and rotation modes, each tested in 3 or 6 specimens
Loaded until force rose steeply or reached 100 N or 5 N·m
Ligaments cut one at a time, medial to lateral
Outcome measures
Share of total constraint carried by each ligament
Distance between ligament insertions at extreme clavicle positions (biplane radiographs)
Key Findings
In many displacement directions, the AC ligament carried more of the restraint at small displacements, while the coracoclavicular ligaments, chiefly the conoid, took over at larger displacements.
The AC ligament supplied about 90% of ligamentous restraint to posterior clavicle displacement at both small and large displacements.
Against anterior displacement, the AC ligament gave up to 50% of the resisting force early, and the conoid's share rose to 70% as displacement increased.
The authors suggested that small displacements, at about 10 N of constraint force, might reflect daily‑living loads and large displacements, at about 90 N, injury loads.
With further superior rotation, the conoid overtook the AC ligament and provided an average of 82% of the restraining torque.
The trapezoid gave less restraint in both the horizontal and vertical planes, apart from axial compression of the clavicle toward the acromion.
Board pearl
Superior displacement was resisted mainly by the AC ligament at small loads, about two‑thirds of the force, and mainly by the conoid at larger loads.
Authors' conclusion
If maximum strength of healing after an injury to the acromioclavicular joint is the goal, all ligaments should be allowed to participate in the healing process. Some repair methods, notably those involving excision of the distal part of the clavicle, may not make this possible.
Fukuda and colleagues measured how much the acromioclavicular, conoid and trapezoid ligaments each restrain the AC joint, and how that share changes with the direction and size of clavicle displacement.
Cadaver study of 12 shoulders from 7 fresh cadavers (aged 50 to 70) at the Mayo Clinic biomechanics laboratory.
Methods
Model
12 shoulders from 7 fresh‑frozen cadavers, aged 50 to 70
4 had rotator cuff ruptures
Ligament length measured in 5 by biplane radiography
Constructs
Intact joint
Sequential sectioning of the AC, conoid and trapezoid ligaments
Loading
12 displacement and rotation modes, each tested in 3 or 6 specimens
Loaded until force rose steeply or reached 100 N or 5 N·m
Ligaments cut one at a time, medial to lateral
Outcome measures
Share of total constraint carried by each ligament
Distance between ligament insertions at extreme clavicle positions (biplane radiographs)
Key Findings
In many displacement directions, the AC ligament carried more of the restraint at small displacements, while the coracoclavicular ligaments, chiefly the conoid, took over at larger displacements.
The AC ligament supplied about 90% of ligamentous restraint to posterior clavicle displacement at both small and large displacements.
Against anterior displacement, the AC ligament gave up to 50% of the resisting force early, and the conoid's share rose to 70% as displacement increased.
The authors suggested that small displacements, at about 10 N of constraint force, might reflect daily‑living loads and large displacements, at about 90 N, injury loads.
With further superior rotation, the conoid overtook the AC ligament and provided an average of 82% of the restraining torque.
The trapezoid gave less restraint in both the horizontal and vertical planes, apart from axial compression of the clavicle toward the acromion.
Board pearl
Superior displacement was resisted mainly by the AC ligament at small loads, about two‑thirds of the force, and mainly by the conoid at larger loads.
Authors' conclusion
If maximum strength of healing after an injury to the acromioclavicular joint is the goal, all ligaments should be allowed to participate in the healing process. Some repair methods, notably those involving excision of the distal part of the clavicle, may not make this possible.