title: The effect of axial tibial torque on the function of the anterior cruciate ligament: a biomechanical study of a simulated pivot shift test.
abstract: OBJECTIVE
  Various techniques are used to produce the pivot shift phenomenon after anterior cruciate ligament (ACL) injury. In particular, the amount of applied axial tibial torque varies among examiners. Thus, the objective of this study was to determine the effect of the magnitude and direction of axial tibial torque in combination with valgus torque on the resulting knee kinematics during such a simulated pivot shift test.
  
  
  METHODS
  This was a biomechanical study that used cadaveric knees with the intact knee of the same specimen serving as a control.
  
  
  METHODS
  On 19 human cadaveric knees (age, 26 to 69 years), a constant 10-Nm valgus torque was applied at 15 degrees of knee flexion. Then, internal and external tibial torque was applied incrementally from 0 to 10 Nm and the resulting kinematics of the ACL-intact and ACL-deficient knee, as well as the in situ force in the ACL, were measured using a robotic/universal force-moment sensor testing system.
  
  
  RESULTS
  In response to isolated valgus torque, the coupled anterior tibial translation for the ACL-intact and ACL-deficient knee was 1.6 +/- 2.4 mm and 8.5 +/- 4.7 mm, respectively; therefore the difference between the ACL-intact and ACL-deficient knee was 6.9 +/- 3.4 mm. With an external tibial torque greater than 5 Nm, the tibia translated up to 4 mm posteriorly for both the ACL-intact and ACL-deficient knee. Whereas, internal tibial torque greater than 1.6 Nm caused a rapid increase in coupled anterior tibial translation up to 10.2 mm in the ACL-deficient knee, while causing only a gradual increase for the ACL-intact knee. With excessive internal torque of 10 Nm, the difference in coupled anterior tibial translation was only 4.4 +/- 2.2 mm, suggesting a decrease in the sensitivity of the test. Correspondingly, the in situ force in the ACL under 10 Nm valgus tibial torque was 43 +/- 17 N, and increased up to 87 +/- 32 N as a 10-Nm internal torque was added. By applying a 3.3-Nm external tibial torque in addition to the 10-Nm valgus torque, the in situ force decreased to 21 +/- 14 N.
  
  
  CONCLUSIONS
  This study showed that a minimal amount of internal torque in combination with valgus torque may be a suitable way to elicit a pivot shift from an ACL-deficient knee.
authors.affiliation: Musculoskeletal Research Center, Department of Orthopaedic Surgery, The University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Kanamori
authors.forename: Jennifer
authors.initials: J
authors.lastname: Zeminski
authors.forename: Theodore W
authors.initials: TW
authors.lastname: Rudy
authors.forename: Guoan
authors.initials: G
authors.lastname: Li
authors.forename: Freddie H
authors.initials: FH
authors.lastname: Fu
authors.forename: Savio L-Y
authors.initials: SL
authors.lastname: Woo
mesh_terms: D000328:Adult; D000368:Aged; D016118:Anterior Cruciate Ligament / Q000502:physiology* / Q000503:physiopathology; D000070598:Anterior Cruciate Ligament Injuries; D001696:Biomechanical Phenomena; D006801:Humans; D007593:Joint Instability / Q000175:diagnosis / Q000503:physiopathology; D007719:Knee Joint / Q000502:physiology* / Q000503:physiopathology; D008875:Middle Aged; D010808:Physical Examination / Q000295:instrumentation / Q000379:methods; D013977:Tibia / Q000502:physiology*; D019415:Torque; D016474:Weight-Bearing / Q000502:physiology