The need for better analysis of observational studies in orthopedics. A retrospective study of elbow fractures in children.
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.
Randsborg
Per-Henrik
PH
Department of Orthopedic Surgery, Akershus University Hospital, Lørenskog, Norway. pran@ahus.no
Sivertsen
Einar A
EA
Skråmm
Inge
I
Saltyt Benth
J rat
Jr
Gulbrandsen
Pål
P
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
JNMA J Nepal Med Assoc. 2008 Apr-Jun;47(170):66-70
18709034
J Am Acad Orthop Surg. 1997 Jan;5(1):19-26
10797204
J Pediatr Orthop. 2004 May-Jun;24(3):245-8
15105716
J Bone Joint Surg Am. 2001 Mar;83(3):323-7
11263634
J Orthop Trauma. 1999 Jan;13(1):51-5
9892127
J Bone Joint Surg Am. 1974 Mar;56(2):263-72
4375679
J Pediatr Orthop. 2001 May-Jun;21(3):313-8
11371812
J Bone Joint Surg Br. 2008 Sep;90(9):1228-33
18757965
JAMA. 2009 Oct 14;302(14):1565-72
19826026
J Pediatr Orthop. 2001 Jan-Feb;21(1):27-30
11176349
J Orthop Surg (Hong Kong). 2004 Jun;12(1):76-82
15237126
Eur J Pediatr Surg. 2002 Dec;12(6):410-5
12548495
J Pediatr Orthop. 2002 Mar-Apr;22(2):203-7
11856931
J Bone Joint Surg Br. 2006 Apr;88(4):528-30
16567791
Surg Gynecol Obstet. 1959 Aug;109(2):145-54
13675986