In vitro biomechanical comparison of pedicle screws, sublaminar hooks, and sublaminar cables

Three types of posterior thoracolumbar implants are in use today: pedicle screws, sublaminar titaniumcables, and sublaminar hooks. The authors conducted a biomechanical comparison of these three implants in human cadaveric spines.

Spine specimens (T5-12) were harvested, radiographically assessed for fractures or metastases, and their bone mineral density (BMD) was measured. Individual vertebrae were disarticulated and fitted with either pedicle screws, sublaminar cables, or bilateral claw hooks. The longitudinal component of each construct consisted of bilateral 10-cm rods connected with two cross-connectors. The vertebral body was embedded in cement, and the rods were affixed to a ball-and-socket apparatus for the application of a distraction force. The authors analyzed 1) 20 vertebrae implanted with screws; 2) 20 with hooks, and 3) 20 with cables. The maximum pullout (MPO) forces prior to failure (mean +/- standard deviation) for the screw, hook, and cable implants were 972 +/- 330, 802 +/- 356, and 654 +/- 248 N, respectively (p = 0.0375). Cables allowed significantly greater displacement (6.80 +/- 3.95 mm) prior to reaching the MPO force than hooks (3.73 +/- 1.42 mm) and screws (4.42 +/- 2.15 mm [p = 0.0108]). Eleven screw-implanted vertebrae failed because of screw pullout. All hook-and-cable-implanted vertebrae failed because of pedicle, middle column, or laminar fracture.

These findings suggest that screws possess the greatest pullout strength of the three fixation systems. Sublaminar cables are the least rigid of the three. When screw failure occurred, the mechanism was generally screw back-out, without vertebral fractures.