Morrey and Chao used biplane radiographs of bone markers to break passive elbow motion into flexion, carrying angle and axial rotation.
Cadaver study of 2 elbows fitted with radiopaque wire markers and radiographed at increments of passive flexion and forearm rotation.
Key Findings
Carrying angle changed linearly with flexion, from 10° of valgus in extension to varus in full flexion (6° in one elbow, 10° in the other), whatever the forearm position.
The forearm also rotated about its own long axis during flexion: about 5° internally early in flexion and about 5° externally near the end.
Because the forearm was held fixed in pronation or supination, the authors attribute this rotation to the shape of the ulnohumeral surfaces and ligament constraints.
With the elbow extended, the ulna showed virtually no valgus deviation and no significant axial rotation during pronation. The radius deviated medially 6° to 10°.
Through flexion, the distance between the humeral and forearm coordinate centers varied by only 1.8 and 1.4 mm, inside Fischer's locus of under 3 mm. This supports a fixed flexion axis through the center of the trochlea.
The authors suggest hinged prostheses that block this axial rotation could partly explain early loosening. Only 2 specimens and passive motion were studied.
Board pearl
The flexion axis is fixed through the trochlear center, in the plane of the anterior humeral cortex on the lateral radiograph. The authors note this guides placement of a hinged prosthesis axis.
Authors' conclusion
As the elbow flexes, the carrying angle varies linearly as a function of flexion. In extension there is a valgus angulation, whereas when flexion is complete there is a varus angulation. The forearm rotates axially on the humerus during flexion.
Morrey and Chao used biplane radiographs of bone markers to break passive elbow motion into flexion, carrying angle and axial rotation.
Cadaver study of 2 elbows fitted with radiopaque wire markers and radiographed at increments of passive flexion and forearm rotation.
Key Findings
Carrying angle changed linearly with flexion, from 10° of valgus in extension to varus in full flexion (6° in one elbow, 10° in the other), whatever the forearm position.
The forearm also rotated about its own long axis during flexion: about 5° internally early in flexion and about 5° externally near the end.
Because the forearm was held fixed in pronation or supination, the authors attribute this rotation to the shape of the ulnohumeral surfaces and ligament constraints.
With the elbow extended, the ulna showed virtually no valgus deviation and no significant axial rotation during pronation. The radius deviated medially 6° to 10°.
Through flexion, the distance between the humeral and forearm coordinate centers varied by only 1.8 and 1.4 mm, inside Fischer's locus of under 3 mm. This supports a fixed flexion axis through the center of the trochlea.
The authors suggest hinged prostheses that block this axial rotation could partly explain early loosening. Only 2 specimens and passive motion were studied.
Board pearl
The flexion axis is fixed through the trochlear center, in the plane of the anterior humeral cortex on the lateral radiograph. The authors note this guides placement of a hinged prosthesis axis.
Authors' conclusion
As the elbow flexes, the carrying angle varies linearly as a function of flexion. In extension there is a valgus angulation, whereas when flexion is complete there is a varus angulation. The forearm rotates axially on the humerus during flexion.