OBJECTIVETo evaluate the biomechanical effect and clinical results of hook screw fixation after direct repair of lumbar spondylous defects in the pars interarticularis.

METHODSL(2)-L(6) spines of 8 fresh-frozen and thawed calf cadavers were used for mechanical testing. Bilateral spondylous defects were created in the L(4)vertebra. The intervertebral rotation ranges between L(4) and L(5) were scanned and computerized in various states of motion, such as flexion/extension, lateral bending and torsional loadings applied on the intact spine and the spondylous spine when the spondylous spine was fixed with modified Scott's fixation, hook screw fixation and Buck's fixation sequentially and respectively. Between July 2002 and February 2004, 14 young male patients (aged 15-31 years) suffering from symptomatic lumbar spondylolysis were treated with TSRH hook screw fixation after direct repair of the defects. MacNab criteria were used to assess their pre-and post-operative status.

RESULTSEach fixation technique could significantly increase the intervertebral rotational stiffness and made the stiffness return to nearly the intact level. Hook screw technique provided more rotational stability than the others. Hook screw and Buck's techniques provided more flexion/extension stability than modified Scott's technique. Neither complication nor instrumental failure was observed in this study. The mean follow-up period was 21 months. All the patients except one acquired union during the follow-up period. Thirteen patients had a "good" or "excellent" result according to MacNab criteria.

CONCLUSIONSHook screw fixation shows biomechanical advantages and is safe and effective for young patients with lumbar spondylolysis.