title: The role of the flexor carpi radialis muscle in scapholunate instability.
abstract: OBJECTIVE
  The flexor carpi radialis (FCR) muscle has been suggested to act as a dynamic scaphoid stabilizer. Because the FCR tendon uses the scaphoid tuberosity as a pulley to reach its distal insertion onto the second metacarpal, it has been hypothesized that FCR muscle contraction generates a dorsally directed vector that resists the scaphoid from rotating into flexion. The purpose of the present study was to validate that hypothesis and clarify the role of the FCR as a dynamic scaphoid stabilizer.
  
  
  METHODS
  Ten fresh cadaver wrist specimens were tested. A custom-designed testing apparatus was used to hold the forearm and wrist vertically, in neutral forearm rotation. A 6-degree-of-freedom, electromagnetic motion-tracking device, with sensors attached to the scaphoid, triquetrum, capitate, and radius, was used to monitor spatial changes in carpal alignment as a result of isometrically loading the FCR in 5 different wrist positions.
  
  
  RESULTS
  In all specimens and all wrist positions, the scaphoid consistently rotated into flexion when the FCR was loaded. It also exhibited variable degrees of pronation or supination, depending on whether the wrist was in flexion or extension. When the wrist was loaded in neutral position, the scaphoid consistently supinated and the triquetrum pronated, these differences being statistically significant (p < .05).
  
  
  CONCLUSIONS
  The scaphoid consistently rotated into flexion and supination when the FCR was loaded, while the triquetrum rotated in flexion and pronation. The positive effects of FCR muscle re-education in dynamic scapholunate instabilities can be explained not by this muscle's capability of extending the scaphoid, as has often been hypothesized, but by its ability to induce supination to the scaphoid and pronation to the triquetrum. Such opposite rotations are likely to result in a dorsal coaptation of the scapholunate joint with relaxation of the dorsal scapholunate ligament.
authors.affiliation: Department of Hand and Microsurgery, Ibacma Institut, and Son Llàtzer Hospital, Palma de Mallorca, Spain. gsalva@ibacma.com
authors.forename: G
authors.initials: G
authors.lastname: Salvà-Coll
authors.forename: M
authors.initials: M
authors.lastname: Garcia-Elias
authors.forename: M
authors.initials: M
authors.lastname: Llusá-Pérez
authors.forename: A
authors.initials: A
authors.lastname: Rodríguez-Baeza
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D008875:Middle Aged; D009068:Movement / Q000502:physiology*; D011403:Pronation; D013484:Supination / Q000502:physiology; D013710:Tendons / Q000502:physiology*; D014955:Wrist Joint / Q000503:physiopathology*