title: Evaluation of mobility and stability in the Discover artificial disc: an in vivo motion study using high-accuracy 3D CT data.
abstract: OBJECTIVE
  Artificial disc replacement (ADR) devices are unlike implants used in cervical fusion in that they are continuously exposed to stress not only within the implant site but also at their site of attachment to the adjacent vertebra. An imaging technique with higher accuracy than plain radiography and with the possibility of 3D visualization would provide more detailed information about the motion quality and stability of the implant in relation to the vertebrae. Such high-accuracy studies have previously been conducted with radiostereometric analysis (RSA), which requires implantation of tantalum markers in the adjacent vertebrae. The aim of this study was to evaluate in vivo motion and stability of implanted artificial discs. A noninvasive analysis was performed with CT, with an accuracy higher than that of plain radiographs and almost as high as RSA in cervical spine.
  
  
  METHODS
  Twenty-eight patients with ADR were included from a larger cohort of a randomized controlled trial comparing treatment of cervical radiculopathy with ADR or anterior cervical decompression and fusion. Surgical levels included C4-7; 18 patients had 1-level surgery and 10 patients had 2-level surgery. Follow-up time ranged from 19 to 50 months, with an average of 40 months. Two CT volumes of the cervical spine, 1 in flexion and 1 in extension, were obtained in each patient and then spatially registered using a customized imaging tool, previously used and validated for the cervical spine. Motion between the components in the artificial disc, as well as motion between the components and adjacent vertebrae, were calculated in 3 planes. Intraclass correlation (ICC) between independent observers and repeatability of the method were also calculated.
  
  
  RESULTS
  Intrinsic motion, expressed as degrees in rotation and millimeters in translation, was detectable in a majority of the ADRs. In the sagittal plane, in which the flexion/extension was performed, sagittal rotation ranged between 0.2° and 15.8° and translation between 0.0 and 5.5 mm. Eight percent of the ADRs were classified as unstable, as motion between at least 1 of the components and the adjacent vertebra was detected. Five percent were classified as ankylotic, with no detectable motion, and another 8% showed very limited motion due to heterotopic ossification. Repeatability for the motion in the sagittal plane was calculated to be 1.30° for rotation and 1.29 mm for translation (95% confidence level), ICC 0.99 and 0.84, respectively. All 3 patients with unstable devices had undergone 1-level ADRs at C5-6. They all underwent revision surgery due to increased neck pain, and instability was established during the surgery.
  
  
  CONCLUSIONS
  The majority of the artificial discs in this study showed intrinsic mobility several years after implantation and were also shown to be properly attached. Implant instability was detected in 8% of patients and, as all of these patients underwent revision surgery due to increasing neck pain, this might be a more serious problem than heterotopic bone formation.
authors.affiliation: Stockholm Spine Center, Löwenströmska Sjukhuset, Upplands Väsby;|Department of Molecular Medicine and Surgery, Karolinska Institute, Stockholm;
authors.forename: Martin
authors.initials: M
authors.lastname: Skeppholm
authors.affiliation: Stockholm Spine Center, Löwenströmska Sjukhuset, Upplands Väsby;|Department of Molecular Medicine and Surgery, Karolinska Institute, Stockholm;
authors.forename: Per
authors.initials: P
authors.lastname: Svedmark
authors.affiliation: Department of Radiology, New York University School of Medicine, New York, New York.
authors.forename: Marilyn E
authors.initials: ME
authors.lastname: Noz
authors.affiliation: School of Information and Communication Technology, KTH, Royal Institute of Technology, Kista;
authors.forename: Gerald Q
authors.initials: GQ
authors.lastname: Maguire
authors.affiliation: Department of Molecular Medicine and Surgery, Karolinska Institute, Stockholm;
authors.forename: Henrik
authors.initials: H
authors.lastname: Olivecrona
authors.affiliation: Department of Orthopedic Surgery, Uppsala University Hospital, Uppsala, Sweden; and.
authors.forename: Claes
authors.initials: C
authors.lastname: Olerud
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000503:physiopathology / Q000601:surgery*; D055959:Intervertebral Disc Degeneration / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D019736:Prostheses and Implants; D016059:Range of Motion, Articular / Q000502:physiology*; D059145:Total Disc Replacement; D016896:Treatment Outcome
keywords: ADR = artificial disc replacement; HA = hydroxyapatite; HO = heterotopic ossification; ICC = intraclass correlation; NDI = neck disability index; RCT = randomized controlled trial; ROM = range of motion; RSA = radiostereometric analysis; artificial disc replacement; computed tomography; instability; mobility; revision surgery