title: Biomechanical Effects of Capsular Shift in the Treatment of Hip Microinstability: Creation and Testing of a Novel Hip Instability Model.
abstract: BACKGROUND
  A capsular shift procedure has been described for the treatment of hip instability; however, the biomechanical effects of such a shift are unknown.
  
  
  OBJECTIVE
  To create a cadaveric model of hip capsule laxity and evaluate the biomechanical effects of a capsular shift used to treat hip instability on this model.
  
  
  METHODS
  Controlled laboratory study.
  
  
  METHODS
  Eight cadaveric hips with an average age of 58.5 years were tested with a custom hip testing system in 6 conditions: intact, vented, instability, capsulotomy, side-to-side repair, and capsular shift. To create the hip model, the capsule was stretched in extension under 35 N·m of torque for 1 hour in neutral rotation. Measurements included internal and external rotation with 1.5 N·m of torque at 5 positions: 5° of extension and 0°, 15°, 30°, and 45° of flexion for each of the above conditions. The degree of maximum extension with 5 N·m of torque and the amount of femoral distraction with 40 N and 80 N of force were measured. Statistical analysis was performed by use of repeated-measures analysis of variance with Tukey post hoc analysis.
  
  
  RESULTS
  The instability state significantly increased internal rotation at all flexion angles and increased distraction compared with the intact state. The capsulotomy condition resulted in significantly increased external rotation and internal rotation at all positions, increased distraction, and maximum extension compared with the intact state. The side-to-side repair condition restored internal rotation back to the instability state but not to the intact state at 5° of extension and 0° of flexion. The capsular shift state significantly decreased internal rotation compared with the instability state at 5° of extension and 0° and 15° of flexion. The capsular shift and side-to-side repair conditions had similar effects on external rotation at all flexion-extension positions. The capsular shift state decreased distraction and maximum extension compared with the instability state, but the side-to-side repair state did not.
  
  
  CONCLUSIONS
  The hip capsular instability model was shown to have significantly greater total range of motion, external rotation, and extension compared with the intact condition. The greatest effects of capsular shift are seen with internal rotation, maximum extension, and distraction, with minimal effect on external rotation compared with the side-to side repair state.
  
  
  CONCLUSIONS
  The biomechanical effects of the capsular shift procedure indicate that it can be used to treat hip capsular laxity by decreasing extension and distraction with minimal effect on external rotation.
authors.affiliation: Congress Orthopaedic Associates, Pasadena, California, USA timothyjjackson@gmail.com.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Jackson
authors.affiliation: Orthopaedic Biomechanics Laboratory, Long Beach VA Healthcare System, Long Beach, California, USA.
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Peterson
authors.affiliation: Orthopaedic Biomechanics Laboratory, Long Beach VA Healthcare System, Long Beach, California, USA.
authors.forename: Masaki
authors.initials: M
authors.lastname: Akeda
authors.affiliation: Congress Orthopaedic Associates, Pasadena, California, USA.
authors.forename: Allyson
authors.initials: A
authors.lastname: Estess
authors.affiliation: Orthopaedic Biomechanics Laboratory, Long Beach VA Healthcare System, Long Beach, California, USA.
authors.forename: Michelle H
authors.initials: MH
authors.lastname: McGarry
authors.affiliation: Congress Orthopaedic Associates, Pasadena, California, USA.
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Adamson
authors.affiliation: Orthopaedic Biomechanics Laboratory, Long Beach VA Healthcare System, Long Beach, California, USA Department of Orthopaedic Surgery, University of California at Irvine, Irvine, California, USA.
authors.forename: Thay Q
authors.initials: TQ
authors.lastname: Lee
mesh_terms: D001696:Biomechanical Phenomena / Q000502:physiology; D002102:Cadaver; D006615:Hip / Q000503:physiopathology* / Q000601:surgery; D006621:Hip Joint / Q000601:surgery; D006801:Humans; D017746:Joint Capsule / Q000503:physiopathology* / Q000601:surgery; D007593:Joint Instability / Q000503:physiopathology* / Q000601:surgery; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology*; D012399:Rotation
keywords: biomechanics; capsule shift; hip; instability