title: Rod stiffness as a risk factor of proximal junctional kyphosis after adult spinal deformity surgery: comparative study between cobalt chrome multiple-rod constructs and titanium alloy two-rod constructs.
abstract: Little is known about the effect of rod stiffness as a risk factor of proximal junctional kyphosis (PJK) after adult spinal deformity (ASD) surgery.
  
  
  
  The aim of this study was to compare radiographic outcomes after the use of cobalt chrome multiple-rod constructs (CoCr MRCs) and titanium alloy two-rod constructs (Ti TRCs) for ASD surgery with a minimum 1-year follow-up.
  
  
  
  Retrospective case-control study in two institutes.
  
  
  
  We included 54 patients who underwent ASD surgery with fusion to the sacrum in two academic institutes between 2002 and 2015.
  
  
  
  Radiographic outcomes were measured on the standing lateral radiographs before surgery, 1 month postoperatively, and at ultimate follow-up. The outcome measures were composed of pre- and postoperative sagittal vertical axis (SVA), pre- and postoperative lumbar lordosis (LL), pre- and postoperative thoracic kyphosis (TK)+LL+pelvic incidence (PI), pre- and postoperative PI minus LL, level of uppermost instrumented vertebra (UIV), evaluation of fusion after surgery, the presence of PJK, and the occurrence of rod fracture.
  
  
  
  We reviewed the medical records of 54 patients who underwent ASD surgery. Of these, 20 patients had CoCr MRC and 34 patients had Ti TRC. Baseline data and radiographic measurements were compared between the two groups. The Mann-Whitney U test, the chi-square test, and the Fisher exact test were used to compare outcomes between the groups.
  
  
  
  The patients of the groups were similar in terms of age, gender, diagnosis, number of three-column osteotomy, levels fused, bone mineral density, preoperative TK, pre- and postoperative TK+LL+PI, SVA difference, LL change, pre- and postoperative PI minus LL, and location of UIV (upper or lower thoracic level). However, there were significant differences in the occurrence of PJK and rod breakage (PJK: CoCr MRC: 12 [60%] vs. Ti TRC: 9 [26.5%], p=.015; occurrence of rod breakage: CoCr MRC: 0 [0%] vs. Ti TRC: 11 [32.4%], p=.004). The time of PJK was less than 12 months after surgery in the CoCr MRC group. However, 55.5% (5/9) of PJK developed over 12 months after surgery in the Ti TRC group.
  
  
  
  Increasing the rod stiffness by the use of cobalt chrome rod and can prevent rod breakage but adversely affects the occurrence and the time of PJK.
authors.affiliation: Department of Neurosurgery, Spine Center, Seoul National University Bundang Hospital, Seoul National University College of Medicine, 82 Gumi-ro, 173 Beon-Gil, Bundang, Seongnam, Gyeonggi 463-707, Republic of Korea.
authors.forename: Sanghyun
authors.initials: S
authors.lastname: Han
authors.affiliation: Department of Neurosurgery, Spine Center, Seoul National University Bundang Hospital, Seoul National University College of Medicine, 82 Gumi-ro, 173 Beon-Gil, Bundang, Seongnam, Gyeonggi 463-707, Republic of Korea. Electronic address: hyunsj@snu.ac.kr.
authors.forename: Seung-Jae
authors.initials: SJ
authors.lastname: Hyun
authors.affiliation: Department of Neurosurgery, Spine Center, Seoul National University Bundang Hospital, Seoul National University College of Medicine, 82 Gumi-ro, 173 Beon-Gil, Bundang, Seongnam, Gyeonggi 463-707, Republic of Korea.
authors.forename: Ki-Jeong
authors.initials: KJ
authors.lastname: Kim
authors.affiliation: Department of Neurosurgery, Spine Center, Seoul National University Bundang Hospital, Seoul National University College of Medicine, 82 Gumi-ro, 173 Beon-Gil, Bundang, Seongnam, Gyeonggi 463-707, Republic of Korea.
authors.forename: Tae-Ahn
authors.initials: TA
authors.lastname: Jahng
authors.affiliation: Department of Neurosurgery, Asan Medical Center, University of Ulsan College of Medicine, 388-1 Pungnap-2dong Songpa-Gu, Seoul, 138-736, Republic of Korea.
authors.forename: Subum
authors.initials: S
authors.lastname: Lee
authors.affiliation: Department of Neurosurgery, Asan Medical Center, University of Ulsan College of Medicine, 388-1 Pungnap-2dong Songpa-Gu, Seoul, 138-736, Republic of Korea.
authors.forename: Seung-Chul
authors.initials: SC
authors.lastname: Rhim
mesh_terms: D000368:Aged; D002858:Chromium Alloys / Q000009:adverse effects*; D005260:Female; D006801:Humans; D016268:Internal Fixators / Q000009:adverse effects* / Q000592:standards; D007738:Kyphosis / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D010027:Osteotomy / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D011183:Postoperative Complications / Q000453:epidemiology / Q000209:etiology*; D011475:Prosthesis Failure / Q000209:etiology*; D011859:Radiography; D012447:Sacrum / Q000000981:diagnostic imaging / Q000601:surgery; D013123:Spinal Fusion / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D014025:Titanium / Q000009:adverse effects
chemical_list: D002858:Chromium Alloys; D014025:Titanium
keywords: Adult spinal deformity surgery; Cobalt chrome; Multiple-rod construct; Proximal junctional kyphosis; Titanium alloy; Two-rod construct