Measurement error of lumbar total disc replacement range of motion.
METHODS
A retrospective review of lumbar total disc replacement (TDR) radiographs.


OBJECTIVE
To determine the error and variability in measuring TDR radiographic range of motion (ROM).


BACKGROUND
Motion preservation is the driving force behind lumbar TDR technology. In the recent literature, sagittal radiographic TDR ROM as low as 2 degrees has been reported. In these studies, ROM was determined by using the Cobb method to measure TDR sagittal alignment angles in flexion-extension lateral radiographs. However, previous studies in the spinal deformity literature have shown that the Cobb method is very susceptible to measurement error.


METHODS
There were 5 observers, including 2 attending orthopedic spine surgeons, 1 spine fellow, 1 fifth-year resident, and 1 fourth-year resident, who measured the ROM of 50 ProDisc II (Synthes Spine Solutions, New York, NY) TDRs on standard flexion-extension lumbar spine radiograph sets. Repeated measurements were made on 2 occasions using the Cobb method. Measurement variability was calculated using 3 statistical methods.


RESULTS
The 3 statistical methods resulted in extremely similar values for TDR ROM observer variability. Overall, the intraobserver variability of TDR ROM measurement was +/-4.6 degrees, and interobserver variability was +/-5.2 degrees .


CONCLUSIONS
To be 95% certain that an implanted TDR prosthesis has any sagittal motion, a ROM of at least 4.6 degrees must be observed, which is the upper limit of intraobserver measurement variability for a TDR with a true ROM of 0 degrees. To be 95% certain that a change in TDR ROM has occurred between 2 measurements by the same observer, a change in ROM of at least 9.6 degrees must be observed (the entire range of +/-4.6 degrees intraobserver variability). ROM measurement variability should be considered when evaluating the success or failure of motion preservation in lumbar TDR.
Lim
Moe R
MR
Department of Orthopaedic Surgery, University of North Carolina-Chapel Hill, Chapel Hill, NC 27599-705, USA. Moe_Lim@med.unc.edu
Loder
Randall T
RT
Huang
Russel C
RC
Lyman
Stephen
S
Zhang
Kai
K
Sama
Andrew
A
Papadopoulos
Elias C
EC
Warner
Kristin
K
Girardi
Federico P
FP
Cammisa
Frank P
FP
D000328:Adult; D019643:Arthroplasty, Replacement / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D011859:Radiography; D016059:Range of Motion, Articular; D015203:Reproducibility of Results; D012189:Retrospective Studies; D013679:Technology, Radiologic / Q000379:methods / Q000706:statistics & numerical data*