title: Medial or lateral dislocation (snapping) of a portion of the distal triceps: a biomechanical, anatomic explanation.
abstract: Medial and lateral snapping (dislocation) of the distal triceps over the epicondyle during elbow flexion has been reported but is frequently misdiagnosed and is not well understood. In this study a mathematical model was designed to simulate the effect that bony abnormalities at the distal humerus and soft tissue variations of the distal triceps have on the line of pull of the triceps. The predictions were then tested on prefabricated and fabricated plastic elbow models, as well as 8 cadaveric elbows. When the bony alignment was altered, varus angulation had the greatest effect: 30 degrees varus malalignment of the distal humerus displaced the centroid of the triceps vector medially by approximately 2.0 cm. Valgus malalignment had a lesser effect: 30 degrees valgus displaced it laterally by 1.5 cm. Negligible effects on the triceps line of pull were seen with internal or external malrotation and with flexion or extension malalignment. Of the soft tissue alterations, displacement of the triceps insertion had a greater effect than movement of the triceps origin. The triceps vector was displaced by approximately 70% of the amount of translation of the triceps insertion. The relationship between the triceps line of pull and the bony alignment is represented by the triceps (T) angle. Our use of the T angle to understand snapping triceps is analogous to the use of the quadriceps (Q) angle for patellar subluxation/dislocation. Treatment should aim to restore normal triceps biomechanics with soft tissue or bony procedures.
authors.affiliation: Department of Neurologic Surgery, Mayo Clinic Foundation, Rochester, Minn 55905, USA. spinner.robert@mayo.edu
authors.forename: R J
authors.initials: RJ
authors.lastname: Spinner
authors.forename: K N
authors.initials: KN
authors.lastname: An
authors.forename: K J
authors.initials: KJ
authors.lastname: Kim
authors.forename: R D
authors.initials: RD
authors.lastname: Goldner
authors.forename: S W
authors.initials: SW
authors.lastname: O'Driscoll
mesh_terms: D001132:Arm; D001696:Biomechanical Phenomena; D002102:Cadaver; D004551:Elbow Joint / Q000033:anatomy & histology* / Q000503:physiopathology*; D006801:Humans; D004204:Joint Dislocations / Q000503:physiopathology*; D008953:Models, Anatomic; D018482:Muscle, Skeletal / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology*; D012307:Risk Factors; D012680:Sensitivity and Specificity