Optimal Distal Fixation Construct for the Surgical Treatment of Thoracic Hyperkyphosis: A Biomechanical Analysis

• a, b - Stryker Spine

Background: Failure of the distal fixation construct is a significant problem for patients undergoing surgical treatment for thoracic hyperkyphosis. Previous studies have demonstrated the use of infralaminar hooks or sublaminar wires to augment pedicle screws to reduce the occurrence of pullout. However, a pullout test does not represent the cantilever bending mode of failure which occurs during forward bending, the assumed mode of failure.

Purpose: Compare the mechanical behavior of five constructs used to terminate dual-rod posterior spinal instruments in response to forward flexion moment.

Hypothesis: Infralaminar hooks or sublaminar wires significantly augment the mechanical performance of pedicle screws used at the terminal end of long spinal-constructs in response to forward bending.

Methods: Forty spinal units (T12-L2) obtained from calves were chosen due to their similarity to that of adolescent spines. Vertebral body and pedicle dimensions were measured with a caliper. Density was measured using DXA. Specimens were randomly assigned into five groups: (1) infralaminar hooks, (2) sublaminar wires, (3) pedicle screws, (4) pedicle screws+infralaminar hooks, and (5) Pedicle screws+sublaminar wires. Each specimen was instrumented with a dual rod construct with a transverse connector at T12. Both rods were seated in an open pedicle screw at T12 which acted as a fulcrum, with the fixation at L1 tested as the terminal construct. The L2 vertebra was mounted to a servo-hydraulic material test system using a custom device that applied cantilever bending to the construct. Each construct was pre-conditioned in compression for 25 cycles (0.5Hz). Construct stiffness and ultimate load were determined from load-displacement curves.

Results: Density and vertebral dimensions were equivalent among the groups (F=0.1-0.9, p>0.05). One wayanalysis of covariance showed all of the screw-construct groups to exhibit significantly higher stiffness and ultimate failure loads compared to either the hook or wire alone (P<0 .05). Post-hoc analysis demonstrated screw+hook construct (109±11Nm/mm) to be stiffer then either screws alone (88±17Nm/mm), or screw+wire (98±13Nm/mm), p<0.05. The had a higher failure load (1336±328N) the screw (1102±256N, p<0.05). mode of differed in various groups. wire and hook constructs failed by laminar fracture while pedicle simple pullout. Augmenting with hooks wires resulted vertebral body at most caudal vertebra.

Conclusion: Distal fixation constructs consisting of pedicle screws gives superior pullout strength and stiffness compared to wires or hooks alone. Augmenting the screws with infralaminar hooks or sublaminar wires provides additional enhancement in the mechanical performance of the construct.

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