title: Measurement of paradoxical and coupled motions following lumbar total disc replacement.
abstract: BACKGROUND
  Maintenance of segmental motion following lumbar total disc replacement (LTDR) is one of the theoretical advantages of spinal arthroplasty. This in vivo study examined paradoxical and coupled motions during sagittal plane movements following disc arthroplasty and compared these motions with those measured following lumbar discectomy.
  
  
  METHODS
  Ten patients following LTDR using ProDisc-L (Synthes, Inc., West Chester, Pennsylvania) and 8 patients following lumbar discectomy (LD) were enrolled. At 1-month, 1-year and 2-year postoperative time-points, patients performed flexion/extension starting from a neutral position, and the intervertebral rotations were determined with radiostereometric analysis. The amount of intended and coupled motion was compared in each group and at each postoperative time. The frequency of paradoxical motion was compared between the 2 groups, and the effects of intended motion, operative-level, number of levels, and postoperative time-point were examined.
  
  
  RESULTS
  The intended and coupled motions following LTDR and LD did not change over time and did not differ from each other for the flexion and total sagittal movements. The sagittal range of motion (ROM) of LTDR was significantly smaller than that of LD in extension (-0.6° ± 1.1° vs -2.2° ± 1.6°). LTDR exhibited a significantly higher rate of paradoxical motion when compared to LD (26.4% vs 6.7%). In LTDR, the rate of paradoxical motion at 1 month (40%) was significantly higher than at 1-year (21.1%) or at 2-year (25.0%). The presence of paradoxical motion was significantly less frequent at L4-5 (19.2%) when compared to L5-S1 (31.3%) or L2-3 (36.4%).
  
  
  CONCLUSIONS
  The overall sagittal ROM of LTDR was 3.5° ± 2.4° and not significantly different than LD. The current study did not demonstrate a difference in coupled motions between LTDR and LD. The rate of paradoxical motion was significantly higher in LTDR than in LD. In LTDR, there was a significantly lower rate of paradoxical motion seen at L4-5 and significantly higher rate seen in the earlier postoperative period.
  
  
  METHODS
  Prospective cohort study with good follow-up (level 1b).
authors: Asan Medical Center, Orthopedic Surgery, Seoul, Korea.
authors: Soo-An
authors: SA
authors: Park
authors: SUNY Upstate Medical University, Syracuse, New York.
authors: Nathaniel
authors: N
authors: Ordway
authors: VSAS Orthopaedics, Allentown, Pennsylvania.
authors: Amir
authors: A
authors: Fayyazi
authors: SUNY Upstate Medical University, Syracuse, New York.
authors: Bruce
authors: B
authors: Fredrickson
authors: SUNY Upstate Medical University, Syracuse, New York.
authors: Hansen A
authors: HA
authors: Yuan
keywords: Lumbar total disc replacement; coupled motion; disc arthroplasty; discectomy; paradoxical motion