title: Measurement of total disc replacement radiographic range of motion: a comparison of two techniques.
abstract: OBJECTIVE
  Current methods used to measure total disc replacement (TDR) radiographic range of motion (ROM) have not been previously evaluated. Sagittal ROM is measured by determining the change in the Cobb angle of the prosthesis from the flexion to the extension radiographs. Either the metallic endplates or the keels of the TDR prosthesis can be used as radiographic landmarks in measuring ROM. We hypothesized that use of the prosthesis keels as radiographic landmarks, when compared with the use of prosthesis endplates, might lead to more precise measurements of TDR sagittal ROM.
  
  
  METHODS
  Two observers (a fifth-year orthopedics resident and an attending orthopedic spine surgeon) measured the ROM of 51 Prodisc II TDRs on standard flexion and extension lumbar spine radiograph sets. Repeated measurements were made on two occasions using either the keels or the endplates as landmarks. Precision was defined as the mean of the absolute differences between measurements.
  
  
  RESULTS
  For observer A, the mean absolute difference between two measurements was 1.4 degrees with the keel method compared with 3.0 degrees with the endplate method (P < 0.001). For observer B, the mean absolute difference between two measurements was 1.8 degrees with the keel method and 3.3 degrees with the endplate method (P < 0.001). When the interobserver differences were examined, the mean absolute difference was 1.8 degrees with the keel method and 3.3 degrees with the endplate method (P < 0.001).
  
  
  CONCLUSIONS
  Our results show that the use of TDR prosthesis keels as radiographic landmarks, when compared with the use of prosthesis endplates, yields greater precision in ROM measurement. For TDR prostheses with a keel, we recommend using the keel to measure ROM.
authors.affiliation: Hospital for Special Surgery, New York, NY, USA. moelim@post.harvard.edu
authors.forename: Moe R
authors.initials: MR
authors.lastname: Lim
authors.forename: Federico P
authors.initials: FP
authors.lastname: Girardi
authors.forename: Kai
authors.initials: K
authors.lastname: Zhang
authors.forename: Russel C
authors.initials: RC
authors.lastname: Huang
authors.forename: Margaret G
authors.initials: MG
authors.lastname: Peterson
authors.forename: Frank P
authors.initials: FP
authors.lastname: Cammisa
mesh_terms: D000328:Adult; D019643:Arthroplasty, Replacement; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000601:surgery*; D008875:Middle Aged; D015588:Observer Variation; D011859:Radiography; D016059:Range of Motion, Articular; D013679:Technology, Radiologic / Q000379:methods