title: Are More Screws Better? A Systematic Review of Anchor Density and Curve Correction in Adolescent Idiopathic Scoliosis.
abstract: METHODS
  Systematic review of clinical studies.
  
  
  OBJECTIVE
  To critically evaluate existing literature to determine whether increased anchor or implant density (screws, wires, and hooks per level fused) results in improved curve correction for adolescent idiopathic scoliosis (AIS) surgery.
  
  
  BACKGROUND
  Wide variability exists in the number of screws used for AIS surgery. High numbers of pedicle screws are increasingly used, but there is limited evidence to support this as best practice.
  
  
  METHODS
  Online English-language databases were searched to identify articles addressing anchor density. Articles were reviewed for anchor type/number, radiographic measures, and patient-reported outcomes.
  
  
  RESULTS
  Of 196 references identified, 10 studies totaling 929 patients met the inclusion criteria. Reported mean anchor density varied from 1.06 to 2.0 implants per level fused. Mean percent coronal curve correction varied from 64% to 70%. Two studies (463 patients) analyzed hook, hybrid (combined hooks and screws), and screw constructs as a single cohort. Both found increased correction with high-density constructs (p = .01, p < .001), perhaps as a result of the hooks and hybrid constructs. Eight retrospective studies and 1 prospective randomized, controlled trial had predominantly screw constructs (466 patients). Increased anchor density was not associated with improved curve correction. The studies evaluating screw density are significantly underpowered to detect a difference in curve correction.
  
  
  CONCLUSIONS
  Wide heterogeneity in anchor density exists in the surgical treatment of AIS. Reports evaluating the effects of increased anchor density are mostly retrospective and significantly underpowered to detect a difference in curve correction and patient outcomes. At this time, there is insufficient evidence to show that anchor density affects clinical outcomes in AIS.
authors.affiliation: Department of Orthopaedic Surgery, Mayo Clinic, 200 1st Street SW, Rochester, MN 55905, USA. Electronic address: larson.noelle@gmail.com.
authors.forename: A Noelle
authors.initials: AN
authors.lastname: Larson
authors.affiliation: Department of Mechanical Engineering, Polytechnique Montreal, P.O. Box 6079, Downtown Station, Montreal, Quebec H3T1C5, Canada.
authors.forename: Carl-Eric
authors.initials: CE
authors.lastname: Aubin
authors.affiliation: Department of Orthopaedic Surgery, University of Minnesota, 2450 Riverside Ave South, Suite R200, Minneapolis, MN 55454, USA.
authors.forename: David W
authors.initials: DW
authors.lastname: Polly
authors.affiliation: Department of Orthopaedic Surgery, University of Minnesota, 2450 Riverside Ave South, Suite R200, Minneapolis, MN 55454, USA.
authors.forename: Charles G T
authors.initials: CG
authors.lastname: Ledonio
authors.affiliation: Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, 820 2nd Avenue, Suite 7A, New York, NY 10017, USA.
authors.forename: Baron S
authors.initials: BS
authors.lastname: Lonner
authors.affiliation: Department of Orthopedics, Nemours/Alfred I. DuPont Hospital for Children, 1600 Rockland Road, P.O. Box 269, Wilmington, DE 19899, USA.
authors.forename: Suken A
authors.initials: SA
authors.lastname: Shah
authors.affiliation: Department of Orthopedics, Texas Scottish Rite Hospital for Children, 2222 Welborn Street, Dallas, TX 75219, USA.
authors.forename: B Stephens
authors.initials: BS
authors.lastname: Richards
authors.affiliation: Department of Orthopedic Surgery, The Children's Hospital, 13123 E. 16th Avenue, B060, Aurora, CO 80045, USA.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Erickson
authors.affiliation: Department of Orthopaedic Surgery, Hunnewell 2, Boston Children's Hospital, Boston, MA 02115, USA.
authors.forename: John B
authors.initials: JB
authors.lastname: Emans
authors.affiliation: Department of Orthopaedic Surgery, University of Iowa Hospitals and Clinics, 01026 JPP, Iowa City, IA 55242, USA.
authors.forename: Stuart L
authors.initials: SL
authors.lastname: Weinstein
keywords: Anchor point; Curve correction; Implant; Scoliosis; Screw