title: The need for better analysis of observational studies in orthopedics. A retrospective study of elbow fractures in children.
abstract: OBJECTIVE
  The conventional statistical methods employed in observational studies in orthopedics require the fundamental assumption that the outcomes are independent. However, fractures treated by the same surgeon cannot be regarded as being independent of each other and should be nested in the statistical analysis. If the effect on outcome of early rather than delayed surgery depends on the severity of the fracture, we have a case of interaction. This is rarely considered in orthopedic research, but could affect the conclusions drawn. The aim of this paper is to describe the concepts of multilevel modeling and interaction in orthopedics.
  
  
  METHODS
  In a cohort of 112 patients with single supracondylar humerus fractures, 78 patients were examined clinically on average 4 years after surgery. The range of motion was measured and the global satisfaction was assessed. The results were used to compare traditional least-squares regression analysis with a 2-level model with interactions.
  
  
  RESULTS
  We found that 25% of the variance in outcome could be attributed to between-surgeon variance. We identified an interaction between the surgeons' experience and the severity of the fractures that influenced the conclusions. The variable "number of pins" was not significant in the 2-level model (p = 0.07), while the ordinary least-squares analysis gave a result that was statistically significant (p = 0.01).
  
  
  CONCLUSIONS
  Researchers should consider the need for a 2-level model and the presence of interactions. Standard statistical methods might lead to wrong conclusions.
authors.affiliation: Department of Orthopedic Surgery, Akershus University Hospital, Lørenskog, Norway. pran@ahus.no
authors.forename: Per-Henrik
authors.initials: PH
authors.lastname: Randsborg
authors.forename: Einar A
authors.initials: EA
authors.lastname: Sivertsen
authors.forename: Inge
authors.initials: I
authors.lastname: Skråmm
authors.forename: J rat
authors.initials: Jr
authors.lastname: Saltyt Benth
authors.forename: Pål
authors.initials: P
authors.lastname: Gulbrandsen
mesh_terms: D001858:Bone Nails; D002648:Child; D002675:Child, Preschool; D002983:Clinical Competence; D004551:Elbow Joint / Q000601:surgery; D005260:Female; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods; D006801:Humans; D006810:Humeral Fractures / Q000175:diagnosis / Q000601:surgery*; D008297:Male; D008954:Models, Biological; D019370:Observation; D009985:Orthopedics; D017063:Outcome Assessment, Health Care / Q000379:methods; D011183:Postoperative Complications / Q000175:diagnosis; D012189:Retrospective Studies; D016896:Treatment Outcome; D000092464:Elbow Injuries
references.citation: JNMA J Nepal Med Assoc. 2008 Apr-Jun;47(170):66-70
references.pmid: 18709034
references.citation: J Am Acad Orthop Surg. 1997 Jan;5(1):19-26
references.pmid: 10797204
references.citation: J Pediatr Orthop. 2004 May-Jun;24(3):245-8
references.pmid: 15105716
references.citation: J Bone Joint Surg Am. 2001 Mar;83(3):323-7
references.pmid: 11263634
references.citation: J Orthop Trauma. 1999 Jan;13(1):51-5
references.pmid: 9892127
references.citation: J Bone Joint Surg Am. 1974 Mar;56(2):263-72
references.pmid: 4375679
references.citation: J Pediatr Orthop. 2001 May-Jun;21(3):313-8
references.pmid: 11371812
references.citation: J Bone Joint Surg Br. 2008 Sep;90(9):1228-33
references.pmid: 18757965
references.citation: JAMA. 2009 Oct 14;302(14):1565-72
references.pmid: 19826026
references.citation: J Pediatr Orthop. 2001 Jan-Feb;21(1):27-30
references.pmid: 11176349
references.citation: J Orthop Surg (Hong Kong). 2004 Jun;12(1):76-82
references.pmid: 15237126
references.citation: Eur J Pediatr Surg. 2002 Dec;12(6):410-5
references.pmid: 12548495
references.citation: J Pediatr Orthop. 2002 Mar-Apr;22(2):203-7
references.pmid: 11856931
references.citation: J Bone Joint Surg Br. 2006 Apr;88(4):528-30
references.pmid: 16567791
references.citation: Surg Gynecol Obstet. 1959 Aug;109(2):145-54
references.pmid: 13675986