Wesley M. Johnson

Biomechanical evaluation of occipitocervical fixation devices.

Human cadaveric occipitocervical specimens were implanted with three types of instrumentation. The devices were tested biomechanically under three modes of loading to determine the stiffness of spinal constructs and the failure mechanisms of the constructs under extreme flexion. The devices tested were the AXIS Fixation System (with custom plate), the Y-Plate, and the Luque rectangle. No significant differences in stiffness among the devices were found under compression and flexion. The stiffnesses of the plate systems were statistically higher than the Luque rectangle in extension and torsion. In extreme flexion, the plate systems failed by fracture of the C2 pedicles. Modern plate systems, for occipitocervical fixation, provide more stiffness and stability than traditional wiring techniques. This study provides surgeons with information on the relative merits of modern plate and screw systems compared with traditional rod and wire constructs.

Load sharing in Premier and Zephir anterior cervical plates.

Study Design. An in vitro biomechanical study using a simulated anterior cervical discectomy and interbody fusion model to compare the load sharing properties of two semiconstrained cervical (Premier and Zephir) plates. Objectives. To determine the percent load transmission through these plates and grafts under simple axial compression. Summary of Background Data. No published data exist as to the load transmission through these semiconstrained plates. Methods. Cadaveric calf spines were subjected to axial compression loading while instrumented with an interbody graft and with the graft plus one of the plates. Load transmission was computed through an analysis of the load–displacement data. Results. A mean load transmission of 23% was shared by the Premier plate. The Zephir, a more constrained plate but still semiconstrained, shared a mean of 32% of the load. Conclusions. The semiconstrained plates tested allow more graft loading than some previously tested constrained plates. However, there are differences between the research methods used in these studies that provide a less than satisfactory comparison.

Craniocervical fixation with occipital condyle screws: Biomechanical analysis of a novel technique

Study Design. A human cadaveric biomechanical study comparing craniocervical fixation techniques. Objective. To quantitatively compare the biomechanical stability of a new technique for occipitocervical fixation using the occipital condyles with an established method for craniocervical spine fusion. Summary of Background Data. Stabilization of the occipitocervical junction remains a challenge. The occiput does not easily accommodate instrumentation because of access and spatial constraints. In fact, the area available for the implant fixation is limited and can be restricted further when a suboccipital craniectomy has been performed, posing a challenge to current fixation techniques. Occipital screws are also associated with the potential for intracranial complications. Methods. Six fresh frozen cadaveric specimens occiput-C4 were tested intact, after destabilization and after fixation as follows: (1) occipital plate with C1 lateral mass screws and C2 pars screws and (2) occipital condyle screws with C1 lateral mass screws and C2 pars screws. Specimens were loaded in a custom spine testing apparatus and subjected to the following tests, all performed under 50-N unconstrained axial preload: flexion, extension, lateral bending, and axial rotation at 1.5 Nm. The constructs were statistically compared with a one-way analysis of variance and compared with the intact condition. Results. Motions were reduced by ∼80% compared with the intact condition for both configurations under all motions. There were no statistically significant differences in the range of motion (ROM) between the 2 instrumentation conditions. The mean values indicated decreased ROM with the novel occipital condyle screw construct in comparison with the standard occipital plate and rod system. Conclusion. Craniocervical stabilization using occipital condyle screws as the sole cephalad fixation point is biomechanically equivalent with regard to the modes tested (ROM and stiffness) to the standard occipital plate construct.

Biomechanical comparison of anterior cervical spine instrumentation techniques with and without supplemental posterior fusion after different corpectomy and discectomy combinations: Laboratory investigation.

OBJECT The objective of this study was to compare the stiffness and range of motion (ROM) of 4 cervical spine constructs and the intact condition. The 4 constructs consisted of 3-level anterior cervical discectomy with anterior plating, 1-level discectomy and 1-level corpectomy with anterior plating, 2-level corpectomy with anterior plating, and 2-level corpectomy with anterior plating and posterior fixation. METHODS Eight human cadaveric fresh-frozen cervical spines from C2-T2 were used. Three-dimensional motion analysis with an optical tracking device was used to determine motion following various reconstruction methods. The specimens were tested in the following conditions: 1) intact; 2) segmental construct with discectomies at C4-5, C5-6, and C6-7, with polyetheretherketone (PEEK) interbody cage and anterior plate; 3) segmental construct with discectomy at C6-7 and corpectomy of C-5, with PEEK interbody graft at the discectomy level and a titanium cage at the corpectomy level; 4) corpectomy at C-5 and C-6, with titanium cage and an anterior cervical plate; and 5) corpectomy at C-5 and C-6, with titanium cage and an anterior cervical plate, and posterior lateral mass screw-rod system from C-4 to C-7. All specimens underwent a pure moment application of 2 Nm with regards to flexion-extension, lateral bending, and axial rotation. RESULTS In all tested motions the statistical comparison was significant between the intact condition and the 2-level corpectomy with anterior plating and posterior fixation construct. All other statistical comparisons between the instrumented constructs were not statistically significant except between the 3-level discectomy with anterior plating and the 2-level corpectomy with anterior plating in axial rotation. There were no statistically significant differences between the 1-level discectomy and 1-level corpectomy with anterior plating and the 2-level corpectomy with anterior plating in any tested motion. There was also no statistical significance between the 3-level discectomy with anterior plating and the 2-level corpectomy with anterior plating and posterior fixation. CONCLUSIONS This study demonstrates that segmental plate fixation (3-level discectomy) affords the same stiffness and ROM as circumferential fusion in 2-level cervical spine corpectomy in the immediate postoperative setting. This obviates the need for staged circumferential procedures for multilevel cervical spondylotic myelopathy. Given that the posterior segmental instrumentation confers significant stability to a multilevel cervical corpectomy, the surgeon should strongly consider the placement of segmental posterior instrumentation to significantly improve the overall stability of the fusion construct after a 2-level cervical corpectomy.

In vitro biomechanical comparison of an anterior and anterolateral lumbar plate with posterior fixation following single-level anterior lumbar interbody fusion.

OBJECT Anterior lumbar interbody fusion (ALIF) is often supplemented with instrumentation to increase stability in the spine. If anterior plate fixation provided the same stability as posterior pedicle screw fixation (PSF), then a second approach and its associated morbidity could be avoided. METHODS Seven human cadaveric L4-5 spinal segments were tested under three conditions: ALIF with an anterior plate, ALIF with an anterolateral plate, and ALIF supplemented by PSF. Range of motion (ROM) was calculated for flexion/extension, lateral bending, and axial torsion and compared among the three configurations. RESULTS There were no significant differences in ROM during flexion/extension, lateral bending, or axial torsion among any of the three instrumentation configurations. CONCLUSIONS The addition of an anterior plate or posterior PS/rod instrumentation following ALIF provides substantially equivalent biomechanical stability. Additionally, the position of the plate system, either anterior or anterolateral, does not significantly affect the stability gained.

Comparative in vitro biomechanical evaluation of two soft tissue defect products

A soft tissue defect is often an unavoidable consequence of various surgical procedures or a result of trauma. Recently, intraoperative use of xenograft as a patch to the soft tissue defect has become popular with various products available in the market. In this study, mechanical properties of the OrthADAPT™ Bioimplants (Pegasus Biologics, Irvine, CA), new xenograft products composed of collagen from equine pericardium, were evaluated individually and against an existing bioimplant product. The OrthADAPT™ Bioimplants have three subtypes which differ in the degree of crosslinking of collagen strands. The three products are named as FX, PX, and MX in the order of the degree of collagen crosslinking and likely durability in vivo, with FX most dense in crosslinking and hence most durable. The three subtypes underwent three destructive mechanical tests: tensile strength, suture pull-out strength, and burst strength test. In tensile strength and suture pull-out strength tests, the three products were compared with CuffPatch™, a similar collagen-based xenograft product from a competing manufacturer. In the burst strength test, the three products were compared with untreated equine pericardium tissue. In tensile strength and suture pull-out strength tests, the products FX and MX were shown to have mechanical properties that were comparable with CuffPatch, while the mechanical strength of PX was significantly inferior to FX and CuffPatch in tensile strength test. In burst strength test, there were no differences in mechanical properties among the three OrthADAPT Bioimplants. This study demonstrates the biomechanical equivalence of OrthADAPT with CuffPatch.

Interbody allograft in a skeletally immature spine model

The objective of this cadaveric biomechanical study was to establish further bovine spines as models for evaluating lumbar interbody allografts and to provide guidance for their use in pediatric humans. It is unknown whether interbody allografts can be used in the pediatric spine without failure of the host vertebral bone. Allografts were placed in cow and calf spines and loaded in compression. The cow spines were much stronger and stiffer than the calf, but moderate in vivo activities were estimated to result in loads on the allograft constructs that would result in host bone failure. Bovine spines were established as suitable models for the compressive behavior of interbody allografts in the human spine, when bone density is considered. Interbody allografts should continue to be used with adjunctive instrumentation so as to preclude host bone failure.