title: In vitro-analysis of kinematics and intradiscal pressures in cervical arthroplasty versus fusion--A biomechanical study in a sheep model with two semi-constrained prosthesis.
abstract: BACKGROUND
  As an alternative technique to arthrodesis of the cervical spine, total disc replacement (TDR) has increasingly been used with the aim of restoration of the physiological function of the treated and adjacent motions segments. The purpose of this experimental study was to analyze the kinematics of the target level as well as of the adjacent segments, and to measure the pressures in the proximal and distal disc after arthrodesis as well as after arthroplasty with two different semi-constrained types of prosthesis.
  
  
  METHODS
  Twelve cadaveric ovine cervical spines underwent polysegmental (C2-5) multidirectional flexibility testing with a sensor-guided industrial serial robot. Additionally, pressures were recorded in the proximal and distal disc. The following three conditions were tested: (1) intact specimen, (2) single-level arthrodesis C3/4, (3) single-level TDR C3/4 using the Discover® in the first six specimens and the activ® C in the other six cadavers. Statistical analysis was performed for the total range of motion (ROM), the intervertebral ROM (iROM) and the intradiscal pressures (IDP) to compare both the three different conditions as well as the two disc prosthesis among each other.
  
  
  RESULTS
  The relative iROM in the target level was always lowered after fusion in the three directions of motion. In almost all cases, the relative iROM of the adjacent segments was almost always higher compared to the physiologic condition. After arthroplasty, we found increased relative iROM in the treated level in comparison to intact state in almost all cases, with relative iROM in the adjacent segments observed to be lower in almost all situations. The IDP in both adjacent discs always increased in flexion and extension after arthrodesis. In all but five cases, the IDP in each of the adjacent level was decreased below the values of the intact specimens after TDR. Overall, in none of the analyzed parameters were statistically significantly differences between both types of prostheses investigated.
  
  
  CONCLUSIONS
  The results of this biomechanical study indicate that single-level implantation of semi-constrained TDR lead to a certain hypermobility in the treated segments with lowering the ROM in the adjacent levels in almost all situations.
authors.affiliation: Orthopedic Department, Hannover Medical School, Diakoniekrankenhaus Annastift gGmbH, Anna-von-Borries-Str. 1-7, 30625, Hannover, Germany. dorothea.daentzer@ddh-gruppe.de.
authors.forename: Dorothea
authors.initials: D
authors.lastname: Daentzer
authors.affiliation: Laboratory for Biomechanics and Biomaterials, Department of Orthopedics, Hannover Medical School, Anna-von-Borries-Str. 1-7, 30625, Hannover, Germany. welke.bastian@mh-hannover.de.
authors.forename: Bastian
authors.initials: B
authors.lastname: Welke
authors.affiliation: Laboratory for Biomechanics and Biomaterials, Department of Orthopedics, Hannover Medical School, Anna-von-Borries-Str. 1-7, 30625, Hannover, Germany. hurschler.christof@mh-hannover.de.
authors.forename: Christof
authors.initials: C
authors.lastname: Hurschler
authors.affiliation: Department of Radiology, Asklepios Clinic Nord, Standort Heidberg, Tangstedter Landstr. 400, 22417, Hamburg, Germany. nathalie.husmann@gmx.de.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Husmann
authors.affiliation: Department of Spine Surgery, Klinikum Bad Bramstedt GmbH, Oskar-Alexander-Str. 26, 24576, Bad Bramstedt, Germany. christina_jansen@gmx.de.
authors.forename: Christina
authors.initials: C
authors.lastname: Jansen
authors.affiliation: Clinic for Orthopedics and Traumatology, Asklepios Clinic Hamburg, Eißendorfer Pferdeweg 52, 21075, Hamburg, Germany. c.flamme@asklepios.com.
authors.forename: Christian Heinrich
authors.initials: CH
authors.lastname: Flamme
authors.affiliation: Laboratory for Biomechanics and Biomaterials, Department of Orthopedics, Hannover Medical School, Anna-von-Borries-Str. 1-7, 30625, Hannover, Germany. berna.richter@aesculap.de.|Aesculap AG, Am Aesculap-Platz, 78532, Tuttlingen, Germany. berna.richter@aesculap.de.
authors.forename: Berna Ida
authors.initials: BI
authors.lastname: Richter
mesh_terms: D000818:Animals; D001178:Arthroplasty; D001696:Biomechanical Phenomena; D002574:Cervical Vertebrae / Q000502:physiology / Q000601:surgery*; D007403:Intervertebral Disc / Q000502:physiology / Q000601:surgery*; D011312:Pressure; D019736:Prostheses and Implants; D016059:Range of Motion, Articular; D012756:Sheep; D013123:Spinal Fusion