Shear stress in epiphyseal growth plate is a risk factor for slipped capital femoral epiphysis.
BACKGROUND
Slipping of the capital femoral epiphysis is an important orthopaedic problem of early adolescence. Many hypotheses about its etiology have been examined, yet the underlying mechanisms have not yet been fully elucidated. We examined elevated shear stress in the epiphyseal growth plate and elevated contact hip stress exerted on the femoral head as risk factors for slipping of the capital femoral epiphysis.


METHODS
Two groups of hips were compared: a group of 100 hips contralateral to the slipped ones and a group of 70 age- and gender-matched healthy hips. The characteristics of individual hips were incorporated by means of geometrical parameters determined from standard anteroposterior radiographs. Shear stress was calculated by using a mathematical model where the femoral neck was considered to function as an elastic rod. Contact hip stress was calculated by the HIPSTRESS method.


RESULTS
Hips contralateral to the slipped ones had higher average shear stress (0.81 vs 0.51 MPa; P < 0.001) and more vertically inclined physeal angle (55.4 vs 63.2 degrees.; P < 0.001) in comparison to healthy hips. Shear stress in the contralateral hips to the slipped ones remained significantly higher even when normalized to the body weight (1400 vs 1060 Pa/N; P < 0.001). There was no significant difference in the average contact hip stress (1.86 vs 1.74 MPa; P = 0.145).


CONCLUSIONS
Elevated shear stress, but not elevated contact stress, is a risk factor for slipping of the capital femoral epiphysis.


METHODS
III (prognostic study, case-control study).
Zupanc
Oskar
O
Department of Orthopaedic Surgery, University Medical Center Ljubljana, Ljubljana, Slovenia.
Krizancic
Miran
M
Daniel
Matej
M
Mavcic
Blaz
B
Antolic
Vane
V
Iglic
Ales
A
Kralj-Iglic
Veronika
V
D000293:Adolescent; D001696:Biomechanical Phenomena; D002648:Child; D004839:Epiphyses, Slipped / Q000000981:diagnostic imaging / Q000209:etiology* / Q000503:physiopathology; D005260:Female; D005500:Follow-Up Studies; D006132:Growth Plate / Q000000981:diagnostic imaging / Q000503:physiopathology*; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000503:physiopathology*; D006801:Humans; D008297:Male; D011859:Radiography; D012189:Retrospective Studies; D012307:Risk Factors; D013314:Stress, Mechanical