title: Head-shaft angle changes during internal and external shoulder rotations: 2-D angulation in 3-D space.
abstract: Restoration of native head-shaft angle (HSA) is critical for treatment of proximal humerus fracture. However, HSA has not been properly investigated according to the humeral rotation. This study was designed to analyze the relationship between the humeral rotation and the HSA at 1° increments, and clarify its serial changing pattern according to the humeral rotation.
  
  
  
  The angulation of HSA would be undervalued when the humerus is being rotated externally and it would be overvalued when it is being rotated internally.
  
  
  
  Eight dried cadaveric normal humeri were CT scanned. They were analyzed using computer-aided design with a standardized neutral position. HSA was the angle between the humeral shaft axis (SA) and the humeral head axis (HA). SA and HA were the best-fit lines through center of all the best-fitting circles in every cross section along the humeral shaft and within the humeral head, respectively. Each 3D model was rotated 30° internally and 45° externally relatives to the SA at 1° increments with the camera was fixed at antero-posterior view of neutral position. Angulation of HSA in every rotational degree was documented as ratio relatives to the angulation of HSA in neutral position.
  
  
  
  The average HSA at neutral position was 133±1.93°. HSA was underestimated by 8±1.9% and it was overestimated by 20±5.1% at the maximum external rotation (ER) and internal rotation (IR), respectively. HSA was underestimated by 1% in every 5.8° of ER and overestimated by 1% in every 1.5° of IR. Rotational misalignments within 10° of IR and 18° of ER could be tolerated (P>.05).
  
  
  
  HSA was underestimated at ER and was overestimated at IR. This information could be useful for surgeons in restoring the native HSA for treatment of proximal humerus fracture.
  
  
  
  Basic research study.
authors.affiliation: Department of Orthopaedics Surgery, Asan Medical Center, College of Medicine, University of Ulsan, Seoul, Republic of Korea.
authors.forename: A
authors.initials: A
authors.lastname: Adikrishna
authors.affiliation: Department of Orthopaedics Surgery, Asan Medical Center, College of Medicine, University of Ulsan, Seoul, Republic of Korea.
authors.forename: H
authors.initials: H
authors.lastname: Hong
authors.affiliation: Department of HCI and Robotics, University of Science and Technology, Daejon, Republic of Korea; Center of Robotics, Korea Institute of Science and Technology, Seoul, Republic of Korea.
authors.forename: M F
authors.initials: MF
authors.lastname: Deslivia
authors.affiliation: Department of Hand Surgery, Ningbo No. 6 Hospital, Ningbo, Zhejiang, China.
authors.forename: B
authors.initials: B
authors.lastname: Zhu
authors.affiliation: Department of Hand Surgery, Affiliated Hospital of Nantong University, Nantong, Jiangsu, China.
authors.forename: J
authors.initials: J
authors.lastname: Tan
authors.affiliation: Department of Orthopaedics Surgery, Asan Medical Center, College of Medicine, University of Ulsan, Seoul, Republic of Korea. Electronic address: jeonchoi@gmail.com.
authors.forename: I-H
authors.initials: IH
authors.lastname: Jeon
mesh_terms: D001696:Biomechanical Phenomena; D002102:Cadaver; D018483:Diaphyses / Q000000981:diagnostic imaging / Q000502:physiology; D006801:Humans; D058430:Humeral Head / Q000000981:diagnostic imaging / Q000502:physiology*; D012399:Rotation; D012785:Shoulder Joint / Q000000981:diagnostic imaging / Q000502:physiology*; D014057:Tomography, X-Ray Computed
keywords: 3D analysis; CAD; Head-shaft angle; Humeral external rotation; Humeral internal rotation; Proximal humerus; Proximal humerus fracture