title: Robot-aided in vitro measurement of patellar stability with consideration to the influence of muscle loading.
abstract: BACKGROUND
  Anterior knee pain is often associated with patellar maltracking and instability. However, objective measurement of patellar stability under clinical and experimental conditions is difficult, and muscular activity influences the results. In the present study, a new experimental setting for in vitro measurement of patellar stability was developed and the mediolateral force-displacement behavior of the native knee analyzed with special emphasis on patellar tilt and muscle loading.
  
  
  METHODS
  In the new experimental setup, two established testing methods were combined: an upright knee simulator for positioning and loading of the knee specimens, and an industry robot for mediolateral patellar displacement. A minimally invasive coupling and force control mechanism enabled unconstrained motion of the patella as well as measurement of patellar motion in all six degrees of freedom via an external ultrasonic motion-tracking system. Lateral and medial patellar displacement were measured on seven fresh-frozen human knee specimens in six flexion angles with varying muscle force levels, muscle force distributions, and displacement forces.
  
  
  RESULTS
  Substantial repeatability was achieved for patellar shift (ICC(3,1) = 0.67) and tilt (ICC(3,1) = 0.75). Patellar lateral and medial shift decreased slightly with increasing flexion angle. Additional measurement of patellar tilt provided interesting insights into the different displacement mechanisms in lateral and medial directions. For lateral displacement, the patella tilted in the same (lateral) direction, and tilted in the opposite direction (again laterally) for medial displacement. With regard to asymmetric muscle loading, a significant influence (p < 0.03, up to 5 mm shift and 8° tilt) was found for lateral displacement and a reasonable relationship between muscle and patellar force, whereas no effect was visible in the medial direction.
  
  
  CONCLUSIONS
  The developed experimental setup delivered reproducible results and was found to be an excellent testing method for the in vitro analysis of patellar stability and future investigation of surgical techniques for patellar stabilization and total knee arthroplasty. We demonstrated a significant influence of asymmetric quadriceps loading on patellar stability. In particular, increased force application on the vastus lateralis muscle led to a clear increase of lateral patellar displacement.
authors.affiliation: Department of Orthopaedic Surgery, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72076, Tübingen, Germany. andrea.lorenz@med.uni-tuebingen.de.
authors.forename: Andrea
authors.initials: A
authors.lastname: Lorenz
authors.affiliation: Department of Orthopaedic Surgery, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72076, Tübingen, Germany. evgenij.bobrowitsch@med.uni-tuebingen.de.
authors.forename: Evgenij
authors.initials: E
authors.lastname: Bobrowitsch
authors.affiliation: Department of Orthopaedic Surgery, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72076, Tübingen, Germany. Markus.Wuenschel@uni-tuebingen.de.
authors.forename: Markus
authors.initials: M
authors.lastname: Wünschel
authors.affiliation: Department of Orthopaedic Surgery, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72076, Tübingen, Germany. christian.walter@med.uni-tuebingen.de.
authors.forename: Christian
authors.initials: C
authors.lastname: Walter
authors.affiliation: Department of Orthopaedic Surgery, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72076, Tübingen, Germany. wuelker@med.uni-tuebingen.de.
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Wülker
authors.affiliation: Department of Orthopaedic Surgery, University Hospital Tübingen, Hoppe-Seyler-Straße 3, 72076, Tübingen, Germany. ulf.leichtle@med.uni-tuebingen.de.
authors.forename: Ulf G
authors.initials: UG
authors.lastname: Leichtle
mesh_terms: D001696:Biomechanical Phenomena; D006801:Humans; D008422:Materials Testing / Q000295:instrumentation*; D009132:Muscles / Q000502:physiology*; D010329:Patella / Q000502:physiology*; D012371:Robotics; D016474:Weight-Bearing