Maintenance of graft compression in the adult cervical spine

Factors Affecting the Nonlinear Force Versus Distraction Height Curves in an In Vitro C5-C6 Anterior Cervical Distraction Model

STUDY DESIGN

In vitro biomechanical study of cervical intervertebral distraction.

OBJECTIVE

To investigate the forces required for distraction to different heights in an in vitro C5-C6 anterior cervical distraction model, focusing on the influence of the intervertebral disk, posterior longitudinal ligament (PLL), and ligamentum flavum (LF).

SUMMARY OF BACKGROUND DATA

No previous studies have reported on the forces required for distraction to various heights or the factors resisting distraction in anterior cervical discectomy and fusion.

MATERIALS AND METHODS

Anterior cervical distraction at C5-C6 was performed in 6 cadaveric specimens using a biomechanical testing machine, under 4 conditions: A, before disk removal; B, after disk removal; C, after disk and PLL removal; and D, after disk and PLL removal and cutting of the LF. Distraction was performed from 0 to 10 mm at a constant velocity (5 mm/min). Force and distraction height were recorded automatically.

RESULTS

The force required increased with distraction height under all 4 conditions. There was a sudden increase in force required at 6-7 mm under conditions B and C, but not D. Under condition A, distraction to 5 mm required a force of 268.3±38.87 N. Under conditions B and C, distraction to 6 mm required <15 N, and further distraction required dramatically increased force, with distraction to 10 mm requiring 115.4±10.67 and 68.4±9.67 N, respectively. Under condition D, no marked increase in force was recorded.

CONCLUSIONS

Distraction of the intervertebral space was much easier after disk removal. An intact LF caused a sudden marked increase in the force required for distraction, possibly indicating the point at which the LF was fully stretched. This increase in resistance may help to determine the optimal distraction height to avoid stress to the endplate spacer.

Development of a Remodeled Caspar Retractor and Its Application in the Measurement of Distractive Resistance in an In Vitro Anterior Cervical Distraction Model

STUDY DESIGN

In vitro biomechanical study of the cervical intervertebral distraction using a remodeled Caspar retractor.

OBJECTIVE

To investigate the torques required for distraction to different heights in an in vitro C3-C4 anterior cervical distraction model using a remodeled Caspar retractor, focusing on the influence of the intervertebral disk, posterior longitudinal ligament (PLL), and ligamentum flavum (LF).

SUMMARY OF BACKGROUND DATA

No previous studies have reported on the torques required for distraction to various heights or the factors resisting distraction in anterior cervical discectomy and fusion.

METHODS

Anterior cervical distractions at C3-C4 was performed in 6 cadaveric specimens using a remodeled Caspar retractor, under 4 conditions: A, before disk removal; B, after disk removal; C, after disk and PLL removal; and D, after disk and PLL removal and cutting of the LF. Distraction was performed for 5 teeth, and distractive torque of each tooth was recorded.

RESULTS

The torque increased with distraction height under all conditions. There was a sudden increase in torque at the fourth tooth under conditions B and C, but not D. Under condition A, distraction to the third tooth required 84.8±13.3 cN m. Under conditions B and C, distraction to the third tooth required <13 cN m, and further distraction required dramatically increased torque. Under condition D, no marked increase in torque was recorded.

CONCLUSIONS

Distraction of the intervertebral space was much easier after disk removal. An intact LF caused a sudden marked increase in the force required for distraction, possibly indicating the point at which the LF was fully stretched. This increase in resistance may help to determine the optimal distraction height to avoid excessive stress to the endplate spacer. The remodeled Caspar retractor in the present study may provide a feasible and convenient method for intraoperative measurement of distractive resistance.

Compression and contact area of anterior strut grafts in spinal instrumentation: a biomechanical study

Background

Anterior bone grafts are used as struts to reconstruct the anterior column of the spine in kyphosis or following injury. An incomplete fusion can lead to later correction losses and compromise further healing. Despite the different stabilizing techniques that have evolved, from posterior or anterior fixating implants to combined anterior/posterior instrumentation, graft pseudarthrosis rates remain an important concern. Furthermore, the need for additional anterior implant fixation is still controversial. In this bench-top study, we focused on the graft-bone interface under various conditions, using two simulated spinal injury models and common surgical fixation techniques to investigate the effect of implant-mediated compression and contact on the anterior graft.

Methods

Calf spines were stabilised with posterior internal fixators. The wooden blocks as substitutes for strut grafts were impacted using a “pressfit” technique and pressure-sensitive films placed at the interface between the vertebral bone and the graft to record the compression force and the contact area with various stabilization techniques. Compression was achieved either with posterior internal fixator alone or with an additional anterior implant. The importance of concomitant ligament damage was also considered using two simulated injury models: pure compression Magerl/AO fracture type A or rotation/translation fracture type C models.

Results

In type A injury models, 1 mm-oversized grafts for impaction grafting provided good compression and fair contact areas that were both markedly increased by the use of additional compressing anterior rods or by shortening the posterior fixator construct. Anterior instrumentation by itself had similar effects. For type C injuries, dramatic differences were observed between the techniques, as there was a net decrease in compression and an inadequate contact on the graft occurred in this model. Under these circumstances, both compression and the contact area on graft could only be maintained at high levels with the use of additional anterior rods.

Conclusions

Under experimental conditions, we observed that ligamentous injury following type C fracture has a negative influence on the compression and contact area of anterior interbody bone grafts when only an internal fixator is used for stabilization. Because of the loss of tension banding effects in type C injuries, an additional anterior compressing implant can be beneficial to restore both compression to and contact on the strut graft.

Implications of the center of rotation concept for the reconstruction of anterior column lordosis and axial preloads in spinal deformity surgery

OBJECT

In thoracolumbar deformity surgery, anterior-only approaches are used for reconstruction of anterior column failures. It is generally advised that vertebral body replacements (VBRs) should be preloaded by compression. However, little is known regarding the impact of different techniques for generation of preloads and which surgical principle is best for restoration of lordosis. Therefore, the authors analyzed the effect of different surgical techniques to restore spinal alignment and lordosis as well as the ability to generate axial preloads on VBRs in anterior column reconstructions.

METHODS

The authors performed a laboratory study using 7 fresh-frozen specimens (from T-3 to S-1) to assess the ability for lordosis reconstruction of 5 techniques and their potential for increasing preloads on a modified distractable VBR in a 1-level thoracolumbar corpectomy. The testing protocol was as follows: 1) Radiographs of specimens were obtained. 2) A 1-level corpectomy was performed. 3) In alternating order, lordosis was applied using 1 of the 5 techniques. Then, preloads during insertion and after relaxation using the modified distractable VBR were assessed using a miniature load-cell incorporated in the modified distractable VBR. The modified distractable VBR was inserted into the corpectomy defect after lordosis was applied using 1) a lamina spreader; 2) the modified distractable VBR only; 3) the ArcoFix System (an angular stable plate system enabling in situ reduction); 4) a lordosizer (a customized instrument enabling reduction while replicating the intervertebral center of rotation [COR] according to the COR method); and 5) a lordosizer and top-loading screws ([LZ+TLS], distraction with the lordosizer applied on a 5.5-mm rod linked to 2 top-loading pedicle screws inserted laterally into the vertebra). Changes in the regional kyphosis angle were assessed radiographically using the Cobb method.

RESULTS

The bone mineral density of specimens was 0.72 ± 22.6 g/cm(2). The maximum regional kyphosis angle reconstructed among the 5 techniques averaged 9.7°-16.1°, and maximum axial preloads averaged 123.7-179.7 N. Concerning correction, in decreasing order the LZ+TLS, lordosizer, and ArcoFix System outperformed the lamina spreader and modified distractable VBR. The order of median values for insertion peak load, from highest to lowest, were lordosizer, LZ+TLS, and ArcoFix, which outperformed the lamina spreader and modified distractable VBR. In decreasing order, the axial preload was highest with the lordosizer and LZ+TLS, which both outperformed the lamina spreader and the modified distractable VBR. The technique enabling the greatest lordosis achieved the highest preloads. With the ArcoFix System and LZ+TLS, compression loads could be applied and were 247.8 and 190.6 N, respectively, which is significantly higher than the insertion peak load and axial preload (p < 0.05).

CONCLUSIONS

Including the ability for replication of the COR in instruments designed for anterior column reconstructions, the ability for lordosis restoration of the anterior column and axial preloads can increase, which in turn might foster fusion.