title: The effect of relative needle diameter in puncture and sham injection animal models of degeneration.
abstract: METHODS
  Biomechanical study and literature review.
  
  
  OBJECTIVE
  To quantify the acute effect of needle diameter on the in vitro mechanical properties of cadaver lumbar discs in the rat and sheep. To review published in vivo animal studies and evaluate disc changes with respect to the relative needle size.
  
  
  BACKGROUND
  There are many cases where a disc needle puncture or injection is applied to animal models: puncture injuries to induce degeneration, chemonucleolysis to induce degeneration, and delivery of disc therapies. It is not clear what role the size of the needle may have in the outcome.
  
  
  METHODS
  Mechanics were measured after sham phosphate buffered saline injection with a 27 G or 33 G needle in the rat and with a 27 G needle in the sheep. A literature review was performed to evaluate studies in which animal discs were treated with a needle puncture or a sham injection. For each study, the ratio of the needle diameter to disc height (needle:height) was calculated.
  
  
  RESULTS
  When the rat was injected with a 27 G needle (52% of disc height), the compression, tension, and neutral zone stiffnesses were 20% to 60% below preinjected values and the neutral zone length was 130% higher; when injected with a 33 G needle (26% of disc height), the only affected property was the neutral zone length, which was only 20% greater. When the sheep was injected with a 27 G needle (10% of disc height), none of the axial properties were different from intact, the torsion stiffness was not different, and the torque range was 15% smaller. Twenty-three in vivo studies in the rat, rabbit, dog, or sheep were reviewed. The disc changes depended on the ratio of needle diameter to disc height as follows: significant changes were not observed for needle:height less than 40%, although between 25% and 40% results were variable and some minor nonsignificant effects were observed, disc changes were universal for needle:height over 40%.
  
  
  CONCLUSIONS
  A needle puncture may directly alter mechanical properties via nucleus pulposus depressurization and/or anulus fibrosus damage, depending on the relative needle size. As more basic science research is aimed at treating disc degeneration via injection of therapeutic factors, these findings provide guidance in design of animal studies. Such studies should consider the relative needle size and include sham control groups to account for the potential effects of the needle injection.
authors.affiliation: Department of Orthopaedic Surgery, McKay Orthopaedic Research Laboratory, University of Pennsylvania School of Medicine, Philadelphia, PA, USA. delliott@mail.med.upenn.edu
authors.forename: Dawn M
authors.initials: DM
authors.lastname: Elliott
authors.forename: Chandra S
authors.initials: CS
authors.lastname: Yerramalli
authors.forename: Jesse C
authors.initials: JC
authors.lastname: Beckstein
authors.forename: John I
authors.initials: JI
authors.lastname: Boxberger
authors.forename: Wade
authors.initials: W
authors.lastname: Johannessen
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Vresilovic
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000379:methods; D004195:Disease Models, Animal; D006801:Humans; D007278:Injections, Spinal / Q000009:adverse effects* / Q000295:instrumentation*; D009339:Needles / Q000009:adverse effects*; D019636:Neurodegenerative Diseases / Q000209:etiology / Q000503:physiopathology*; D011677:Punctures