title: Radiographic and clinical evaluation of cage subsidence after stand-alone lateral interbody fusion.
abstract: OBJECTIVE
  Indirect decompression of the neural structures through interbody distraction and fusion in the lumbar spine is feasible, but cage subsidence may limit maintenance of the initial decompression. The influence of interbody cage size on subsidence and symptoms in minimally invasive lateral interbody fusion is heretofore unreported. The authors report the rate of cage subsidence after lateral interbody fusion, examine the clinical effects, and present a subsidence classification scale.
  
  
  METHODS
  The study was performed as an institutional review board-approved prospective, nonrandomized, comparative, single-center radiographic and clinical evaluation. Stand-alone short-segment (1- or 2-level) lateral lumbar interbody fusion was investigated with 12 months of postoperative follow-up. Two groups were compared. Forty-six patients underwent treatment at 61 lumbar levels with standard interbody cages (18 mm anterior/posterior dimension), and 28 patients underwent treatment at 37 lumbar levels with wide cages (22 mm). Standing lateral radiographs were used to measure segmental lumbar lordosis, disc height, and rate of subsidence. Subsidence was classified using the following scale: Grade 0, 0%-24% loss of postoperative disc height; Grade I, 25%-49%; Grade II, 50%-74%; and Grade III, 75%-100%. Fusion status was assessed on CT scanning, and pain and disability were assessed using the visual analog scale and Oswestry Disability Index. Complications and reoperations were recorded.
  
  
  RESULTS
  Pain and disability improved similarly in both groups. While significant gains in segmental lumbar lordosis and disc height were observed overall, the standard group experienced less improvement due to the higher rate of interbody graft subsidence. A difference in the rate of subsidence between the groups was evident at 6 weeks (p = 0.027), 3 months (p = 0.042), and 12 months (p = 0.047). At 12 months, 70% in the standard group and 89% in the wide group had Grade 0 or I subsidence, and 30% in the standard group and 11% in wide group had Grade II or III subsidence. Subsidence was detected early (6 weeks), at which point it was correlated with transient clinical worsening, although progression of subsidence was not observed after the 6-week time point. Moreover, subsidence occurred predominantly (68%) in the inferior endplate. Fusion rate was not affected by cage dimension (p > 0.999) or by incidence of subsidence (p = 0.383).
  
  
  CONCLUSIONS
  Wider cages avoid subsidence and better restore segmental lordosis in stand-alone lateral interbody fusion. Cage subsidence is identified early in follow-up and can be accessed using the proposed classification scale.
authors.affiliation: Department of Imaging Diagnosis, Universidade Federal de São Paulo, São Paulo, Brazil. luismarchi@gmail.com
authors.forename: Luis
authors.initials: L
authors.lastname: Marchi
authors.forename: Nitamar
authors.initials: N
authors.lastname: Abdala
authors.forename: Leonardo
authors.initials: L
authors.lastname: Oliveira
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Amaral
authors.forename: Etevaldo
authors.initials: E
authors.lastname: Coutinho
authors.forename: Luiz
authors.initials: L
authors.lastname: Pimenta
mesh_terms: D000328:Adult; D000368:Aged; D015331:Cohort Studies; D019299:Decompression, Surgical / Q000379:methods; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D016268:Internal Fixators / Q000592:standards*; D007403:Intervertebral Disc / Q000000981:diagnostic imaging / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011184:Postoperative Period; D011475:Prosthesis Failure / Q000009:adverse effects; D011859:Radiography; D012086:Reoperation / Q000379:methods; D012307:Risk Factors; D012720:Severity of Illness Index; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D016896:Treatment Outcome