Spondylolysis. Bending of the inferior articular processes of lumbar vertebrae during simulated spinal movements.
METHODS
This was a cadaveric lumbar motion segment experiment.


OBJECTIVE
To measure bending about the pars interarticularis during simulated flexion and extension movements, with and without high compressive forces. This was designed to indicate which activities are most likely to cause spondylolysis.


BACKGROUND
The mechanical etiology of spondylolysis is controversial.


METHODS
Motion segments were loaded to simulate: A, full flexion; B, full extension; C, high compression in flexed posture, and D, high compression in lordotic posture. Bending of the inferior articular processes about the pars was measured in the sagittal plane, using a two-dimensional "MacReflex" infrared motion analysis system.


RESULTS
In full flexion, tension in the intervertebral ligaments bent the processes forward and down (by 3.25 degrees +/- 3.07 degrees), whereas in the loading conditions B, C, and D, inter-facet compressive forces caused the processes to bend backward and upward, by 2.24 +/- 2.48 degrees, 0.40 +/- 0.23 degrees, and 0.74 +/- 0.56 degrees respectively.


CONCLUSIONS
Activities that involve alternating flexion and extension movements cause large stress reversals in the pars and pose the greatest threat of spondylolysis. Compressive loading has little effect.
Green
T P
TP
Department of Orthopaedic Surgery, University of Leicester, United Kingdom.
Allvey
J C
JC
Adams
M A
MA
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D013169:Spondylolysis / Q000209:etiology / Q000503:physiopathology*; D016474:Weight-Bearing / Q000502:physiology