Background: Previous studies have indicated that the demands placed
on the medial ulnar collateral ligament of the elbow when it is subjected to
valgus torque during throwing exceed its failure strength, which suggests the
necessary dynamic contribution of muscle forces. We hypothesized that the
flexor-pronator mass assists the medial ulnar collateral ligament in
stabilizing the elbow against valgus torque.
Methods: Six cadaveric elbows were tested at 30° and 90° of
flexion with no other constraints to motion. A full medial ulnar collateral
ligament tear was simulated in each elbow. Muscle forces were simulated on the
basis of the centroids and physiological cross-sectional areas of individual
muscles. The biceps, brachialis, and triceps were simulated during flexor
carpi ulnaris, flexor digitorum superficialis, flexor digitorum superficialis
and flexor carpi ulnaris, and pronator teres-loading conditions. Kinematic
data were obtained at each flexion angle with use of a three-dimensional
Results: Release of the medial ulnar collateral ligament caused a
significant increase in valgus instability of 5.9° ± 2.4° at
30° of elbow flexion and of 4.8° ± 2.0° at 90° of elbow
flexion (p < 0.05). The differences in valgus angulation between each
muscle-simulation condition and the medial ulnar collateral ligament-intact
condition were significantly different from each other (p < 0.05), except
for the difference between the flexor carpi ulnaris contraction condition and
the flexor digitorum superficialis-flexor carpi ulnaris co-contraction
condition. This co-contraction provided the most correction of the valgus
angle in comparison with the intact condition at both 30° and 90° of
elbow flexion (1.1° ± 1.8° and 0.38° ± 2.3°,
respectively). Simulation of the flexor carpi ulnaris alone provided the
greatest reduction of the valgus angle among all individual flexor-pronator
mass muscles tested (p < 0.05), whereas simulation of the pronator teres
alone provided the least reduction of the valgus angle (p < 0.05).
Conclusions: The flexor-pronator mass dynamically stabilizes the
elbow against valgus torque. The flexor carpi ulnaris is the primary
stabilizer, and the flexor digitorum superficialis is a secondary stabilizer.
The pronator teres provides the least dynamic stability.
Clinical Relevance: The flexor-pronator mass is capable of
contributing valgus stability to the elbow. When considering injury
prevention, surgical techniques, and rehabilitation in throwing athletes, the
physician should give particular attention to optimizing the function of the
flexor carpi ulnaris and flexor digitorum superficialis.