A biomechanical analysis of the self-retaining pedicle hook device in posterior spinal fixation

Abstract

Regular hooks lack initial fixation to the spine during spinal deformity surgery. This runs the risk of posterior hook dislodgement during manipulation and correction of the spinal deformity, that may lead to loss of correction, hook migration, and postoperative junctional kyphosis. To prevent hook dislodgement during surgery, a self-retaining pedicle hook device (SPHD) is available that is made up of two counter-positioned hooks forming a monoblock posterior claw device. The initial segmental posterior fixation strength of a SPHD, however, is unknown. A biomechanical pull-out study of posterior segmental spinal fixation in a cadaver vertebral model was designed to investigate the axial pull-out strength for a SPHD, and compared to the pull-out strength of a pedicle screw. Ten porcine lumbar vertebral bodies were instrumented in pairs with two different instrumentation constructs after measuring the bone mineral density of each individual vertebra. The instrumentation constructs were extracted employing a material testing system using axial forces. The maximum pullout forces were recorded at the time of the construct failure. Failure of the SPHD appeared in rotation and lateral displacement, without fracturing of the posterior structures. The average pull-out strength of the SPHD was 236 N versus 1,047 N in the pedicle screws (P < 0.001). The pull-out strength of the pedicle screws showed greater correlation with the BMC compared to the SPHD (P < 0.005). The SPHD showed to provide a significant inferior segmental fixation to the posterior spine in comparison to pedicle screw fixation. Despite the beneficial characteristics of the monoblock claw construct in a SPHD, that decreases the risk of posterior hook dislodgement during surgery compared to regular hooks, the SPHD does not improve the pullout strength in such a way that it may provide a biomechanically solid alternative to pedicle screw fixation in the posterior spine.

Use of the Universal Clamp in adolescent idiopathic scoliosis for deformity correction and as an adjunct to fusion: 2-year follow-up

“…In the series of 15 patients with more severe, neurological scoliosis[100°, La Rosa et al achieved a 70% initial reduction (from 107 to 32°) and lost only 7°of correction after an average follow-up of 3 years [20]. The intermediate-term results reported here on the corrective efficacy of the UC in the coronal plane are not inferior to results observed with other spinal anchors that have known safety issues [2,[22][23][24][25][26][27]]. An initial average thoracic curve correction from 54°to 27°(50%) achieved in 75 AIS patients with hook hybrid constructs (before the development of the UC) was followed by a loss of 4°o bserved 2 years postoperatively [13].…”

Which factors prognosticate rotational instability following lumbar laminectomy?

“…Low BMC may thus be seen as an indication for using instrumentation after laminectomy. Note, however, also pull out strength of spinal implants proved to be dependent on bone mineral quality as measured by DXA [25] and this dependency needs to be taken into account when deciding on additional instrumentation.…”

Which factors prognosticate spinal instability following lumbar laminectomy?

“…However, also pull out strength of spinal implants, proved to be dependent on bone mineral quality as measured by dual X-ray absorptiometry (DXA) [25] and this dependency needs to be taken into account when deciding on instrumentation.…”