Trochlea of the Humerus
Part of a Long Bone, Limb (Appendicular) Skeleton
Also known as: Spool of the Upper Arm Bone, Elbow Pivot
Classification
Part of a Long Bone, Limb (Appendicular) Skeleton
Dimensions
This feature is a spool-shaped ridge located at the bottom end of the upper arm bone. It is wider than it is tall and sits toward the inner side of the bone's base.
Key Features
It looks remarkably like a sewing thread spool or a simple pulley. It has a smooth, hourglass-shaped surface that sits right next to a rounded 'knob' on the bottom of the arm bone. You can identify it by the deep notch that runs around its center.
Similar Bones
It can be confused with the Capitulum, which is the rounded knob sitting right next to it. You can tell them apart because the Trochlea is shaped like a bowtie or spool, while the Capitulum is shaped like a round marble or a half-sphere.
Across Species
In many mammals, this structure acts like a pulley for the forearm. In specialized runners like horses, this joint surface is deeply grooved to keep the leg swinging in a perfect straight line for efficiency. In climbing animals like apes, it allows for a wide range of rotation and stability while hanging from branches.
Evolutionary History
This pulley-like surface evolved as land-dwelling vertebrates transitioned from a sprawling, lizard-like crawl to a more upright tucked-in limb position. This change allowed animals to support more weight and move with greater speed by locking the elbow into a stable forward-and-back motion.
Photography Tips
Lay the bone flat and take a picture looking directly at the bottom end. Good side-lighting will help show the deep groove of the 'spool' and the way it separates from the rounded knob next to it.
Notes
What is the blue highlighted called