Biomechanical studies performed on fresh frozen calf lumbar spines have shown that artificially induced bilateral interarticularis defects increase the intervertebral mobility not only at the involved level but also at the upper level adjacent to the lysis (20). The increased mobility of both the lytic segment and the adjacent level was significantly reduced by the Buck screw repair. However, if a pedicle screw system was applied across the involved segment, the mobility in the adjacent segment was increased. It would appear that fixation of the pars defect alone causes less adjacent-level mechanical stress than pedicle screw-rod fixation of the motion segment.

Deguchi et al. carried out a biomechanical comparison of various fixation techniques employed for the repair of spondylolytic defects (21). The procedures were carried out on calf cadaveric lumbar spines. The four techniques that were compared were Scott’s wiring technique (wire loops passed around each transverse process and tightened under the spinous process); modified Scott’s technique (wire loops around cortical screws placed in both pedicles and tightened under the spinous process); Buck’s technique; and screw-rod-hook fixation. All fixation techniques significantly increased stiffness in the spondylolytic segment. The screw-rod-hook fixation and Buck’s technique allowed the least motion across the spondylolytic defect when tested in flexion. The authors suggest that Buck’s technique resists forces and movements in all directions, is low profile, and is less expensive than the screw-rod-hook system, but it remains technically demanding. Screw breakage or nerve root irritation requiring screw removal has occurred following Buck’s technique (8,22).

Kip et al. carried out biomechanical testing of various pars defect repairs (23). He compared the Buck screw with the Morscher hook-screw and a modified Scott wiring technique. Overall, the Buck screw provided the stiffest and strongest repair (with a mean repair stiffness of 64% of the intact stiffness), followed closely by the Morscher hook-screw (with a mean repair stiffness of 52% of the intact stiffness). The figure 8 wiring repair (modified Scott repair) was the least stiff, with a mean repair stiffness of only 3% to 4% of the intact stiffness. The authors conclude that Scott wiring of pars defects is a tension band repair that
is loaded only in flexion. It is used to repair a defect whose probable cause is repetitive hyperextension of the spine and should be questioned on biomechanical grounds. Nevertheless, it is interesting to note that similar clinical results have been obtained with both Buck’s technique and Scott’s wiring technique in some series (21).

The advantages of direct repair include a high rate of defect healing, preservation of intervertebral motion by avoidance of fusion across the disc, and restoration of anatomy.

Buck (8) first reported the direct repair of a pars defect with a 4.5-mm stainless steel Arbeitsgemeinschaft für Osteosynthesefran or Association for the Study of Internal Fixation (AO) cortical lag screw in 1970 (Fig. 34.2). The defect was packed with cancellous bone graft and the patient was allowed to be mobile immediately following surgery. Kimura had described the insertion of bone-graft without internal fixation for spondylolysis and spondylolisthesis 2 years earlier (24). However, Kimura’s technique required the patients to remain on bedrest for 2 months and was not universally popular. Buck initially reported on 16 patients with only 1 failure (8). In 1979, he updated his results to a 75-patient series, reporting satisfactory results in 88% (25). Several series of patients were then reported in the literature with success rates varying from 72% (22) to 95% (25) (Table 34.1).

Van der Werf et al. presented a series of 12 patients in 1985, with 8 good, 2 fair, and 2 poor results (26). There were two documented pseudarthroses in this series, one with a fair result and the other with a poor result. Pedersen and Hagen described 18 patients who underwent a direct repair using Buck’s technique for a slip of Grade 1 or less (3). They reported 15 patients with a good or excellent result and 3 with a poor result (all with radiographic evidence of pseudarthrosis).

Roca et al. (27) reported on 15 patients with a slip of Grade 1 or less treated with the Buck method. There were 13 good results and 2 poor results (attributable to a
pseudarthrosis). In 1989, Bonnici et al. reported on 24 patients, 21 of whom obtained excellent results (28). Seventeen patients had adequate radiographic follow-up revealing 5 with screw loosening and 1 with a broken screw. Interestingly only 7 of 17 were noted to have healing of both defects radiographically, 5 patients had bridging of one defect, and 5 patients had no evidence of healing.