title: Influence of implant rod curvature on sagittal correction of scoliosis deformity.
abstract: BACKGROUND
  Deformation of in vivo-implanted rods could alter the scoliosis sagittal correction. To our knowledge, no previous authors have investigated the influence of implanted-rod deformation on the sagittal deformity correction during scoliosis surgery.
  
  
  OBJECTIVE
  To analyze the changes of the implant rod's angle of curvature during surgery and establish its influence on sagittal correction of scoliosis deformity.
  
  
  METHODS
  A retrospective analysis of the preoperative and postoperative implant rod geometry and angle of curvature was conducted.
  
  
  METHODS
  Twenty adolescent idiopathic scoliosis patients underwent surgery. Average age at the time of operation was 14 years.
  
  
  METHODS
  The preoperative and postoperative implant rod angle of curvature expressed in degrees was obtained for each patient.
  
  
  METHODS
  Two implant rods were attached to the concave and convex side of the spinal deformity. The preoperative implant rod geometry was measured before surgical implantation. The postoperative implant rod geometry after surgery was measured by computed tomography. The implant rod angle of curvature at the sagittal plane was obtained from the implant rod geometry. The angle of curvature between the implant rod extreme ends was measured before implantation and after surgery. The sagittal curvature between the corresponding spinal levels of healthy adolescents obtained by previous studies was compared with the implant rod angle of curvature to evaluate the sagittal curve correction. The difference between the postoperative implant rod angle of curvature and normal spine sagittal curvature of the corresponding instrumented level was used to evaluate over or under correction of the sagittal deformity.
  
  
  RESULTS
  The implant rods at the concave side of deformity of all patients were significantly deformed after surgery. The average degree of rod deformation Δθ at the concave and convex sides was 15.8° and 1.6°, respectively. The average preoperative and postoperative implant rod angle of curvature at the concave side was 33.6° and 17.8°, respectively. The average preoperative and postoperative implant rod angle of curvature at the convex side was 25.5° and 23.9°, respectively. A significant relationship was found between the degree of rod deformation and preoperative implant rod angle of curvature (r=0.60, p<.005). The implant rods at the convex side of all patients did not have significant deformation. The results indicate that the postoperative sagittal outcome could be predicted from the initial rod shape.
  
  
  CONCLUSIONS
  Changes in implant rod angle of curvature may lead to over- or undercorrection of the sagittal curve. Rod deformation at the concave side suggests that corrective forces acting on that side are greater than the convex side.
authors.affiliation: Division of Human Mechanical Systems and Design, Laboratory of Biomechanical Design, Graduate School of Engineering, Hokkaido University, N. 13 W. 8 Kita-Ku, Sapporo 060-8628, Japan.
authors.forename: Remel Alingalan
authors.initials: RA
authors.lastname: Salmingo
authors.affiliation: Division of Human Mechanical Systems and Design, Faculty of Engineering, Hokkaido University, N. 13 W. 8 Kita-Ku, Sapporo 060-8628, Japan. Electronic address: tadano@eng.hokudai.ac.jp.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Tadano
authors.affiliation: Department of Orthopaedic Surgery, Eniwa Hospital, Kogane-Cho 2-1-1, Eniwa 061-1449, Japan.
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Abe
authors.affiliation: Advanced Medicine for Spine and Spinal Cord Disorders, Graduate School of Medicine, Hokkaido University, N. 15 W. 7 Kita-Ku, Sapporo 060-8638, Japan.
authors.forename: Manabu
authors.initials: M
authors.lastname: Ito
mesh_terms: D000293:Adolescent; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002648:Child; D005260:Female; D006801:Humans; D016268:Internal Fixators; D008297:Male; D019637:Orthopedic Procedures / Q000379:methods; D019736:Prostheses and Implants; D011475:Prosthesis Failure; D011859:Radiography; D012189:Retrospective Studies; D012600:Scoliosis / Q000000981:diagnostic imaging / Q000601:surgery*; D013123:Spinal Fusion / Q000379:methods; D055815:Young Adult
keywords: Adolescent idiopathic scoliosis; Biomechanics; Implant rod curvature; Rod deformation; Sagittal correction; Spine