title: Relaxation response of lumbar segments undergoing disc-space distraction: implications to the stability of anterior lumbar interbody implants.
abstract: METHODS
  A biomechanical study of human cadaveric lumbar spine segments undergoing disc-space distraction for insertion of anterior lumbar interbody implants.
  
  
  OBJECTIVE
  To measure the distraction force and its relaxation during a period of up to 3 hours after disc-space distraction as a function of the distraction magnitude and disc level.
  
  
  BACKGROUND
  Interbody implants depend on compressive preload produced by disc-space distraction (annular pretension) for initial stabilization of the implant-bone interface. However, the amount of preload produced by disc-space distraction due to insertion of the implant and its subsequent relaxation have not been quantified.
  
  
  METHODS
  Twenty-two fresh human lumbar motion segments (age: 51 ± 14.8 years) were used. An anterior lumbar discectomy was performed. The distraction test battery consisted of a tension stiffness test performed before and after each relaxation test, 2 distraction magnitudes of 2 and 4 mm, and a recovery period before each distraction input. The distraction forces and lordosis angles were measured. RESULTS.: Peak distraction force was significantly larger for the 4-mm distraction (431.8 ± 116.4 N) than for the 2-mm distraction (204.9 ± 55.5 N) (P < 0.01). The distraction force significantly decreased over time (P < 0.01), approximating steady-state values of 146.1 ± 47.3 N at 2-mm distraction and 289.8 ± 92.8 N at 4-mm distraction, respectively. The distraction force reduced in magnitude by more than 20% of peak value in the first 15 minutes and reduced by approximately 30% of the peak value at the end of the testing period. The spine segment relaxed by the same amount of force, regardless of the disc level (P > 0.05).
  
  
  CONCLUSIONS
  The "tightness of fit" that the surgeon notes immediately after interbody device insertion in the disc space degrades in the very early postoperative period, which could compromise the stability of the bone-implant interface.
authors.affiliation: Musculoskeletal Biomechanics Laboratory, Department of Veterans Affairs, Edward Hines Jr. VA Hospital, Hines, IL, USA.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Havey
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Voronov
authors.forename: Parmenion P
authors.initials: PP
authors.lastname: Tsitsopoulos
authors.forename: Gerard
authors.initials: G
authors.lastname: Carandang
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Ghanayem
authors.forename: Mark A
authors.initials: MA
authors.lastname: Lorenz
authors.forename: Michael R
authors.initials: MR
authors.lastname: Zindrick
authors.forename: Avinash G
authors.initials: AG
authors.lastname: Patwardhan
mesh_terms: D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D002102:Cadaver; D017586:Diskectomy; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000000981:diagnostic imaging / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011474:Prosthesis Design; D011475:Prosthesis Failure; D011859:Radiography; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000009:adverse effects / Q000295:instrumentation*; D013314:Stress, Mechanical; D013997:Time Factors