Introduction: Hook displacement or pullout is a common complication compromising the stability of spinal instrumentation. The two most common causes are the loss of the optimal adjustment between hook and lamina during the connection of the implant to the rod and the displacement of the hook during correction maneuvers.

Therefore, a partially constrained rod-implant link was conceived allowing for free rotation in the sagittal plane while maintaining the possibility for transverse loading during correction maneuvers. One of the possible benefits of this system is the preservation of the adjustment between hook and lamina.

Purpose of Study: To compare the adjustment obtained between hook and lamina by using a partially constrained pivot link (PL), connecting the hook to the rod while allowing for rotation in the sagittal plane, versus the common fully constrained link (FCL) connecting the hook rigidly to the rod.

Methods: A plastic model of the lumbar spine was instrumented on one side with a L1 supralaminar hook and a L3 infralaminar hook. Seven lordotic configurations (range: −45° to −30°) were randomly assigned to the model. A prebent rod with a −41° lordosis between the fixation points was used for all tests. Compression was applied to the claw construct until the best fit between hooks and laminae was achieved. The PL hooks were secured to the rod by a top-loading clip system allowing for rotation in the sagittal plane until final fixation with an incorporated setscrew. The FCL hooks were secured to the rod with a top-loading plug screw. The length of the hook blade in contact with the lamina, the initial and final lordosis of the construct were measured.

Results: The mean length of the hook blade in contact with the lamina was 6.9 mm for the PL infralaminar hooks versus 4.2 mm for the FCL infralaminar hooks (p< 0.0005). There was no statistically significant correlation between the degree of initial lordosis and the amountof contact achieved by both the FCL and PL infralaminar hooks (FCL: r = −0.052; PL: r = −0.585).

Discussion: Using a partially constrained pivot link achieves a larger contact between hook and lamina than the common fully constrained link. This was statistically highly significant at the level of the strategically most important infralaminar hooks in a lordotic construct. Early clinical experience using a spinal instrumentation based on the pivot link principle in seventy patients seems to confirm the enhanced strength of fixation. Especially, the management of spinal deformities in patients with severe osteoporosis or dystrophic lesions of the spine is significantly improved. Significant implant volume reduction allows for the use of this system even in young children.