Effect of an Intervertebral Disk Spacer on Stiffness After Monocortical Screw/Polymethylmethacrylate Fixation in Simulated and Cadaveric Canine Cervical Vertebral Columns

Abstract

Objective: To determine the biomechanical effect of an intervertebral spacer on construct stiffness in a PVC model and cadaveric canine cervical vertebral columns stabilized with monocortical screws/polymethylmethacrylate (PMMA).Study Design: Biomechanical study.Sample population: PVC pipe; cadaveric canine vertebral columns Methods: PVC model -PVC pipe was used to create a gap model mimicking vertebral endplate orientation and disk space width of large-breed canine cervical vertebrae; 6 models had a 4-mm gap with no spacer (PVC group 1); 6 had a PVC pipe ring spacer filling the gap (PCV group 2). Animals -large breed cadaveric canine cervical vertebral columns (C2-C7) from skeletally mature dogs without (cadaveric group 1, n=6, historical data) and with an intervertebral disk spacer (cadaveric group 2, n=6) were used. All PVC models and cadaver specimens were instrumented with monocortical titanium screws/PMMA. Stiffness of the 2 PVC groups was compared in extension, flexion, and lateral bending using non-destructive 4-point bend testing. Stiffness testing in all 3 directions was performed of the unaltered C4-C5 vertebral motion unit in cadaveric spines and repeated after placement of an intervertebral cortical allograft ring and instrumentation. Data were compared using a linear mixed model approach that also incorporated data from previously tested spines with the same screw/PMMA construct but without disk spacer (cadaveric group 1).Results: Addition of a spacer increased construct stiffness in both the PVC model (P< .001) and cadaveric vertebral columns (P<.001) compared to fixation without a spacer. Conclusions:Addition of an intervertebral spacer significantly increased construct stiffness of monocortical screw/PMMA fixation.

Kinematic behavior of a novel pedicle screw‐rod fixation system for the canine lumbosacral joint

“…This device has previously been used in conjunction with String‐of‐Pearl plates to achieve cervical distraction‐stabilization in dogs . The rationale for using it in combination with the screw‐rod system was that in addition to facilitating fusion, it will provide effective load sharing and decrease the risk of fatigue and subsequent implant failure . To introduce the distraction bolt into the L7‐S1 intervertebral space in vivo, the cauda equina is retracted using a long, narrow instrument…”

“…39 The rationale for using it in combination with the screw-rod system was that in addition to facilitating fusion, it will provide effective load sharing and decrease the risk of fatigue and subsequent implant failure. 26 To introduce the distraction bolt into the L7-S1 intervertebral space in vivo, the cauda equina is retracted using a long, narrow instrument. 40 Limitations: As with any cadaveric experiment, this study has a number of limitations that should be considered when interpreting the data.…”

“…Mechanical testing was performed using a custom 4‐point bending fixture . The specimen was subjected to nondestructive compressive axial loads through a servo‐hydraulic material testing machine (Model 858, MTS Systems Corporation, Eden Prairie, Minnesota) operating under load‐control (Figure ).…”

“…Mechanical testing was performed using a custom 4-point bending fixture. 26 The specimen was subjected to nondestructive compressive axial loads through a servo-hydraulic material testing machine (Model 858, MTS Systems Corporation, Eden Prairie, Minnesota) operating under load-control ( Figure 6). Loads were applied from 0 to 150N at the L6-L7 and L7-S1 junctions in the dorso-ventral (DV) direction to F IGUR E 4 Photographs of the cadaveric specimen (A) in dorsal view after dorsal laminectomy and annulectomy, (B) in dorsolateral view with the intervertebral spacer connected to the applicator instrument while the spacer is screwed into the intervertebral space (note the Ellis pin for the motion tracker cranial to the applicator and the spacer), (C) in dorsolaterocaudal view showing the polyaxial screws, clamps on both sides and a connecting rod applied on the right side ZINDL ET AL.…”

Biomechanical Comparison of 2 Veterinary Locking Plates to Monocortical Screw/Polymethylmethacrylate Fixation in Canine Cadaveric Cervical Vertebral Column

“…Various distraction‐stabilization methods for treatment of canine cervical spondylomyelopathy have been described clinically, with a few having been assessed biomechanically . It has been sufficiently demonstrated that bicortical implants are not safe to use in the canine cervical spine and investigation has shifted to monocortical fixations such as monocortical screws/polymethylmethacrylate (PMMA) and locking plates .…”