• Koichi Sairyo, Shinsuke Katoh, Takahiro Sasa, Natsuo Yasui, Vijay K Goel, Sasidhar Vadapalli, Akiyoshi Masuda, Ashok Biyani, and Nabil Ebraheim.
• Department of Orthopedics, University of Tokushima, 3-18-15, Kuramoto, Tokushima 770-8503 Japan. sairyokun@hotmail.com
• Am J Sports Med. 2005 Apr 1; 33 (4): 583-90.

BackgroundUnilateral spondylolysis is common in youths; its clinical and biomechanical features, especially effects on the contralateral side, are not fully understood.HypothesisUnilateral spondylolysis predisposes the contralateral side to stress fracture, especially in athletes actively engaged in sporting activities involving torsion of the trunk.Study DesignCase series and descriptive laboratory study.MethodsThirteen athletes younger than age 20 with unilateral spondylolysis were included. The contralateral pedicle and pars of spondylolytic vertebrae were examined using computed tomography and magnetic resonance imaging. Using a finite element model of the intact ligamentous L3-S1 segment, stress distributions were analyzed in response to 400-N axial compression and 10.6-N.m moment in flexion, extension, lateral bending, and axial rotation. Unilateral spondylolysis was created in the model at L5. The stress results from the unilateral defect model were compared to the intact model predictions and correlated to the contralateral defects seen in patients.ResultsAmong 13 patients, there were 6 early-, 2 progressive-, and 5 terminal-stage defects. Three (23.1%) showed contralateral stress fracture. Among them, 2 belonged to the progressive-stage and 1 to the terminal-stage spondylolysis group. The remaining 4 patients in the terminal defect group showed stress reactions, such as sclerosis at the contralateral pedicle. In the finite element analysis model with an L5 left spondylolysis, the stresses at the contralateral and pars interarticularis were found to increase in all loading modes, with increases as high as 12.6-fold compared to the intact spine.ConclusionsUnilateral spondylolysis could lead to stress fracture or sclerosis at the contralateral side due to an increase in stresses in the region.Clinical RelevanceSurgeons should be aware of possibility of contralateral stress fractures in cases in which patients, especially athletes engaged in active sports, show unilateral spondylolysis and persistent low back pain complaints.

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