Load-carrying capacity of the human cervical spine in compression is increased under a follower load.
ABSTRACT An experimental approach was used to test human cadaveric cervical spine specimens.
To assess the response of the cervical spine to a compressive follower load applied along a path that approximates the tangent to the curve of the cervical spine.
The compressive load on the human cervical spine is estimated to range from 120 to 1200 N during activities of daily living. Ex vivo experiments show it buckles at approximately 10 N. Differences between the estimated in vivo loads and the ex vivo load-carrying capacity have not been satisfactorily explained.
A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 250 N. The experimental technique applied loads that minimized the internal shear forces and bending moments, loading the specimen in nearly pure compression.
A compressive vertical load applied in the neutral and forward-flexed postures caused large changes in cervical lordosis at small load magnitudes. The specimen collapsed in extension or flexion at a load of less than 40 N. In sharp contrast, the cervical spine supported a load of up to 250 N without damage or instability in both the sagittal and frontal planes when the load path was tangential to the spinal curve. The cervical spine was significantly less flexible under a compressive follower load compared with the hypermobility demonstrated under a compressive vertical load (P < 0.05).
The load-carrying capacity of the ligamentous cervical spine sharply increased under a compressive follower load. This experiment explains how a whole cervical spine can be lordotic and yet withstand the large compressive loads estimated in vivo without damage or instability.
Article: Alteration of load sharing of anterior cervical implants with change in cervical sagittal alignment.[show abstract] [hide abstract]
ABSTRACT: Anterior cervical discectomy and fusion (ACDF) is often supplemented with the application of an anterior plate to improve the stability of the fusion segment. While plate design has been shown to influence stress shielding of the graft, little is known about how the kyphotic alignment of a fused cervical segment affects the load sharing between the anterior plate and the osteoligamentous structures of the spine. The aim of this study was to characterize load sharing between an anterior plate and the osteoligamentous structures of the cervical motion segments in kyphotic versus normal lordotic alignment following single-level ACDF using fresh ovine cervical spines (C3-C6). The loading protocol involved preloading the spine with a 20 N compressive force and applying quasi-static moments (up to 2.1 Nm) in the sagittal plane to simulate flexion and extension. Stiffness of the fusion segment was measured from the moment-rotation plot. Normal lordotic alignment was replicated by insertion of a fibular allograft 2 mm taller than the interbody space. Kyphotic alignment was simulated by removing the graft and reapplying a shorter anterior cervical plate. The average segmental sagittal angulation at C4-C5 was 5.2+/-1.6 degrees of lordosis for the normal lordotic group and 6.8+/-2.3 degrees of kyphosis for the kyphotic group. With flexion, the plate shared 52.8% of the applied load in the normal lordotic group, and 70.1% in the kyphotic group (p<0.03). In extension, the amount of load-share by the plate in the normal lordotic group was comparable to that of the kyphotic group (52.7% vs. 40.7%, p=0.16). This study shows that kyphotic alignment of the cervical fusion segment increases the load sharing of the anterior plate under flexion loading.Medical Engineering & Physics 11/2007; 30(6):768-73. · 1.62 Impact Factor
Article: Cervical disc prosthesis versus arthrodesis using one-level, hybrid and two-level constructs: an in vitro investigation.[show abstract] [hide abstract]
ABSTRACT: The purpose of this experimental study was to analyse cervical spine kinematics after 1-level and 2-level total disc replacement (TDR) and compare them with those after anterior cervical arthrodesis (ACA) and hybrid construct. Kinematics and intradiscal pressures were also investigated at adjacent levels. Twelve human cadaveric spines were evaluated in different testing conditions: intact, 1 and 2-level TDR (Discocerv™, Scient'x/Alphatec), 1 and 2-level ACA, and hybrid construct. All tests were performed under load control protocol by applying pure moments loading of 2 N m in flexion/extension (FE), axial rotation (AR) and lateral bending (LB). Reduction of ROM after 1-level TDR was only significant in LB. Implantation of additional TDR resulted in significant decrease of ROM in AR at index level. A second TDR did not affect kinematics of the previously implanted TDR in FE, AR and LB. One and 2-level arthrodesis caused significant decrease of ROM in FE, AR and LB at the index levels. No significant changes in ROM were observed at adjacent levels except for 1-level arthrodesis in FE and hybrid construct in AR. When analysis was done under the displacement-control concept, we found that 1 and 2-constructs increased adjacent levels contribution to global ROMC3-C7 during FE and that IDP at superior adjacent level increased by a factor of 6.7 and 2.3 for 2-level arthrodesis and hybrid constructs, respectively. Although 1- and 2-level TDR restored only partially native kinematics of the cervical spine, these constructs generated better biomechanical conditions than arthrodesis at adjacent levels limiting contribution of these segments to global ROM and reducing the amount of their internal stresses.European Spine Journal 08/2011; 21(3):432-42. · 1.97 Impact Factor
Article: Examination of cervical spine kinematics in complex, multiplanar motions after anterior cervical discectomy and fusion and total disc replacement[show abstract] [hide abstract]
ABSTRACT: Background: The biomechanical behavior of total disc replacement (TDR) and anterior cervical discectomy and fusion (ACDF) in complex multiplanar motion is incompletely understood. The purpose of this study was to determine whether ACDF or TDR significantly affects in vitro kinematics through a range of complex, multiplanar motions. Methods: Seven human cervical spines from C4-7 were used for this study. Intact cervical motion segments with and without implanted TDR and ACDF were tested by use of unconstrained pure bending moment testing fixtures in 7 mechanical modes: axial rotation (AR); flexion/extension (FE); lateral bending (LB); combined FE and LB; combined FE and AR; combined LB and AR; and combined FE, LB, and AR. Statistical testing was performed to determine whether differences existed in range of motion (ROM) and stiffness among spinal segments and treatment groups for each mechanical test mode. Results: ACDF specimens showed increased stiffness compared with the intact and TDR specimens (P .001); stiffness was not found to be different between TDR and intact specimens. ACDF specimens showed decreased ROM in all directions compared with TDR and intact specimens at the treated level. For the coupled motion test, including AR, LB, and FE, the cranial adjacent level (C4/C5) for the intact specimens (2.7°) showed significantly less motion compared with both the TDR (6.1°, P .009) and ACDF (6.8°, P .002) treatment groups about the LB axis. Testing of the C4/C5 and C6/C7 levels in all other test modes yielded no significant differences in ROM comparisons, although a trend toward increasing ROM in adjacent levels in ACDF specimens compared with intact and TDR specimens was observed. Conclusions: This study compared multiplanar motion under load-displacement testing of subaxial cervical motion segments with and without implanted TDR and ACDF. We found a trend toward increased motion in adjacent levels in ACDF specimens compared with TDR specimens. Biomechanical multiplanar motion testing will be useful in the ongoing development and evaluation of spinal motion–preserving implants.International Journal of Spine Surgery. 12/2012; 1(6):190-194.