title: Biomechanical evaluation of a new total posterior-element replacement system.
abstract: METHODS
  In vitro study to characterize the flexibility of a new total posterior-element system when instrumented to L4-L5 segments.
  
  
  OBJECTIVE
  The goal of this in vitro study was to investigate whether an optimized version of the TOPS implant (Impliant Ltd., Ramat Poleg, Israel) is capable to restore the physiologic motion characteristic of a spinal segment after facetectomy.
  
  
  BACKGROUND
  The TOPS implant is designed to replace the posterior elements of a functional spinal unit, to provide flexible restabilization and spinal alignment, while maintaining the intervertebral disc. The implant is composed of bilateral pedicle screws, connected with 2 crossbars in the transversal plane. The crossbars are joined together by an elastic element capable of transmitting tensile and compressive loads, as well as shear forces.
  
  
  METHODS
  Six human cadaver specimens (L3-S1) (median age 61 years: minimum 47 and maximum 74 years) were used for this in vitro experiment. The specimens were loaded with pure moments of +/-7.5 Nm in flexion/extension, lateral bending, and axial rotation. The following states were investigated: (1) intact; (2) after bilateral laminectomy, including facetectomy of the lower facet joints, of the upper vertebra L4; and (3) after device implantation. The range of motion (ROM), neutral zone, and intradiscal pressure were determined from a third cycle. In a second step, the ROM in axial rotation was determined as a function of different flexion/extension postures.
  
  
  RESULTS
  In the neutral position, the laminectomy and facetectomy increased the median values of the ROM in flexion plus extension, lateral bending right plus left, and significantly in axial rotation left plus right from: 8.2 degrees, 7.6 degrees, 3.6 degrees to 12.1 degrees, 8.5 degrees, and 8.5 degrees (Wilcoxon signed rank test; P < 0.05). After fixation of the implant, the ROM was again reduced to 6.8 degrees, 7.8 degrees, and 3.8 degrees. In a flexed posture, the ROM in axial rotation was slightly increased compared to the neutral position. With increasing extension, the axial rotation decreased linearly from 3.7 degrees in neutral position to 2.3 degrees in 4 degrees extension in the segment L4-L5. The characteristic of the intradiscal pressure versus load with the implant was similar to that of the intact specimen.
  
  
  CONCLUSIONS
  The TOPS implant almost ideally restored the ROM in lateral bending and axial rotation compared to that of the intact specimen. In the sagittal plane, 85% of the intact ROM could be obtained. The ROM in axial rotation as a function of flexion and extension angle also mimics the biomechanical behavior of the posterior complex of a lumbar spine. This relationship between ROM and posture emphasizes the importance of a proper implantation.
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, University of Ulm, Ulm, Germany. hans-joachim.wilke@uni-ulm.de
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Wilke
authors.forename: Hendrik
authors.initials: H
authors.lastname: Schmidt
authors.forename: Karin
authors.initials: K
authors.lastname: Werner
authors.forename: Werner
authors.initials: W
authors.lastname: Schmölz
authors.forename: Jörg
authors.initials: J
authors.lastname: Drumm
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena; D001860:Bone Plates; D001862:Bone Resorption; D001863:Bone Screws / Q000009:adverse effects; D004867:Equipment Design; D005260:Female; D006801:Humans; D020007:Implants, Experimental; D016268:Internal Fixators; D007796:Laminectomy; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D018583:Pliability; D011183:Postoperative Complications / Q000209:etiology; D011187:Posture; D011312:Pressure; D016059:Range of Motion, Articular; D012399:Rotation; D013123:Spinal Fusion; D016474:Weight-Bearing