Dynamic Characteristics of the Intact, Fused, and Prosthetic-Replaced Cervical Disk

Defining and measuring objective and subjective spinal stiffness: a scoping review

PURPOSE

Examine and identify the breadth of definitions and measures of objective and subjective spinal stiffness in the literature, with a focus on clinical implications.

METHODS

A scoping review was conducted to determine what is known about definitions and measures of the specific term of spinal stiffness. Following the framework by Arksey and O'Malley, eligible peer-reviewed studies identified using PubMed, Ebsco health, and Scopus were included if they reported definitions or measures of spinal stiffness. Using a data abstraction form, the studies were classified into four themes: biomechanical, surgical, pathophysiological, and segmental spinal assessment. To identify similarities and differences between studies, sixteen categories were generated.

RESULTS

In total, 2426 records were identified, and 410 met the eligibility criteria. There were 350 measures (132 subjective; 218 objective measures) and 93 indicators of spinal stiffness. The majority of studies (n = 69%) did not define stiffness.

CONCLUSION

This review highlights the breadth of objective and subjective measures that are both clinically and methodologically diverse. There is no consensus regarding a standardised definition of stiffness in the reviewed literature.

Biomechanical analysis of lumbar spine with interbody fusion surgery and U-shaped lumbar interspinous spacers

Previous research indicates whole-body vibration may lead to low back pain. The aim of this study is assessing the dynamic characteristics of a lumbar spine with Coflex and Coflex-F (commercial implants used as lumbar interspinous spacers) and effect of lumbar interbody fusion surgery. A transient dynamic analysis is performed on three numerical lumbar spine models under the loading condition of a vertical sinusoidal force of ±40 N with a compressive follower preload of 400 N. Also, Coflex-F model with and without interbody fusion surgery is analyzed under the same loading condition. The results show that the maximum value and vibration amplitude of von Mises stress in annulus ground substance (AGS) and intradiscal pressure (IDP) at implanted segment decrease from healthy model to Coflex model, and Coflex-F model. By contrast, for adjacent segments the maximum value of implanted models are larger than that of healthy model. The maximum value of endplates with and without cage are 2.44 MPa and 1.73 MPa (L4 inferior endplate), 1.94 MPa and 1.42 MPa (L5 superior endplate), respectively. The vibration amplitude of Coflex-F model with fusion surgery is smaller than that without fusion surgery. Coflex and Coflex-F not only protect implanted segment but also have a negative effect on adjacent segments. Inserting cage for Coflex-F model can absorb vibration energy at adjacent segments.

Kinematics of the cervical adjacent segments after disc arthroplasty compared with anterior discectomy and fusion: a systematic review and meta-analysis

STUDY DESIGN

Systematic review and meta-analysis.

OBJECTIVE

To determine the kinematics of the adjacent segments and global cervical spine after cervical arthroplasty compared with anterior cervical discectomy and fusion (ACDF).

SUMMARY OF BACKGROUND DATA

Adjacent segment pathology after ACDF is a significant concern. Arthroplasty may decrease the risk of adjacent segment degeneration by maintaining normal spinal kinematics compared with fusion. However, the differences in the in vivo kinematics of the adjacent segments after cervical fusion versus arthroplasty have not been clearly established.

METHODS

A systematic literature review of studies comparing adjacent segment kinematic changes between fusion and arthroplasty was performed. We included randomized controlled trials and cohort studies that compared cervical arthroplasty with ACDF in adults with degenerative disease and reported on at least 1 outcome of interest. Meta-analysis was performed using a random-effects model where appropriate. The standardized mean difference of changes from baseline to follow-up between treatment groups was determined. Recommendations were made using Grades of Recommendation Assessment, Development, and Evaluation criteria.

RESULTS

We identified 12 studies, including 7 randomized controlled trials, 4 cohort studies, and 1 case-control study that evaluated kinematic measurements at the adjacent segments or the global cervical spine (C2-C7) after cervical arthroplasty compared with ACDF. We found no statistically significant differences between treatment groups in the change in range of motion (ROM) at the cranial or caudal adjacent segments from baseline to 2 years of follow-up. However, there was low evidence that the global cervical spine (C2-C7) had significantly greater change in ROM after arthroplasty compared with ACDF: patients had a greater angular ROM as measured up to 2 years after arthroplasty. We found no statistically significant differences between treatment groups in the change in the horizontal or vertical centers of rotation at the adjacent segments as measured up to 2 years after surgery. Regarding sagittal alignment, the cranial and caudal adjacent segments both became significantly more lordotic after arthroplasty compared with fusion at 1 to 2 years after surgery. However, there was no statistically significant difference between treatment groups in the change in global cervical sagittal alignment from baseline to 2 years. CONCLUSION.: There is no statistically or clinically significant difference in the adjacent segment ROM or centers of rotation after cervical arthroplasty compared with ACDF. However, the change in sagittal alignment at the cranial and caudal adjacent segments was significantly more lordotic after arthroplasty compared with fusion. In addition, although we found that there was no statistically significant difference between treatment groups in the change in global cervical (C2-C7) sagittal alignment, there was a significantly greater change in the angular ROM of the cervical spine at up to 2 years after arthroplasty than occurred after fusion.

CONSENSUS STATEMENT

Patients can be advised that single-level arthroplasty and ACDF result in clinically similar kinematic changes at short-term follow-up. Strength of Statement: Strong.

A comparison of the shock-absorbing properties of cervical disc prosthesis bearing materials

Background Data

Cervical arthroplasty offers theoretical advantages over traditional spinal fusion, including elimination of adjacent segment disease and elimination of the risk of pseudoarthrosis formation. Initial studies of cervical arthroplasty have shown promising results, however, the ideal design characteristics for disc replacement constructs have not been determined. The current study seeks to quantify the differences in the shock absorption characteristics of three commonly used materials in cervical disc arthroplasty.

Methods

Three different nucleus materials, polyurethane (PU), polyethylene (PE) and a titanium-alloy (Ti) were tested in a humidity- and temperature-controlled chamber. Ten of each nucleus type underwent three separate mechanical testing protocols to measure 1) dynamic stiffness, 2) quasi-static stiffness, 3) energy absorption, and 4) energy dissipation. The results were compared using analysis of variance.

Results

PU had the lowest mean dynamic stiffness (435 ± 13 N/mm, P < .0001) and highest energy absorption (19.4 ± 0.1 N/mm, P < .0001) of all three nucleus materials tested. PU was found to have significantly higher energy dissipation (viscous damping ratio 0.017 ± 0,001, P < .0001) than the PE or TI nuclei. PU had the lowest quasi-static stiffness (598 ± 23 N/mm, P < .0001) of the nucleus materials tested. A biphasic response curve was observed for all of the PU nuclei tests.

Conclusions

Polyurethane absorbs and dissipates more energy and is less stiff than either polyethylene or titanium.

Level of Evidence

Basic Science/Biomechanical Study.

Clinical Relevance

This study characterizes important differences in biomechanical properties of materials that are currently being used for different cervical disc prostheses.

Investigation of the intervertebral disc and fused joint dynamics through experimental modal analysis and the receptance coupling method

Identification of intervertebral disc (IVD) dynamics is important in understanding the spine mechanism and behavior. This paper experimentally identifies the dynamics of the bovine caudal IVD using experimental modal analysis and the inverse receptance coupling method. Experimental modal analysis was performed on free-free mounted bovine caudal vertebrae joined by an IVD and a fused IVD joint. Shear, rotational, and axial dynamics of the joints are identified by curve fitting of the frequency response functions, and identifying the damping ratio, stiffness, and modal frequency in each axis. The identified dynamics are compared with the IVD joints with and without fusion. Results provide important insight into IVD dynamics and fused IVD dynamics. This method can be extended to identify human IVD joint dynamics.

Síndrome do disco adjacente à fusão (Síndrome de Transição) na coluna cervical: resultados segundo critérios clínicos e radiológicos

OBJETIVO: o objetivo deste trabalho foi identificar a ocorrência da síndrome do disco adjacente secundária à fusão intervertebral, no segmento cervical, e os fatores prognósticos associados. MÉTODOS: foram analisados os prontuários e exames de imagem de 209 indivíduos submetidos ao tratamento cirúrgico da doença degenerativa cervical, sendo 169 casos por via anterior e em 40 por via posterior, no HC-FMUSP, no período de março de 1993 a janeiro de 2007. A avaliação clínica foi quantificada pela escala da Japanese Orthopaedic Association (JOA), com média de segmento de 80 meses. A análise radiológica foi baseada nos critérios de instabilidade de White e Panjabi em todos os casos. A degeneração discal foi mensurada segundo a escala de Kellgren antes e após a cirurgia. RESULTADOS: no total, diagnosticou-se a síndrome do disco adjacente em 30 casos (17,75%), com média de intervalo de tempo de incidência de 28,5 meses após a fusão ou 2,9 casos/ano. Houve um aumento das alterações degenerativas nos exames radiológicos em níveis adjacentes à fusão por via anterior em 41,42% e em 52,5% por via posterior sem fusão, avaliados pela escala de Kellgren, com taxa de degeneração de 6,7% ao ano, sem correlação com alterações clínicas. CONCLUSÕES: alterações no alinhamento fisiológico sagital da coluna e a sua gravidade predispõem ao desenvolvimento da síndrome. Não foi observada incidência da síndrome em casos de pseudoartrose ou utilização de metacrilato, portanto onde não ocorreu a fusão. A utilização de descompressão e fusão, por via anterior, está associada à possibilidade de desenvolvimento de um novo quadro de mielorradiculopatia no período pós-operatório. Fusão em um nível que envolva a quinta ou sexta vértebras cervicais, fixação com placas e pré-existência da evidência radiográfica de degeneração de disco adjacente mostraram-se como os grandes fatores de risco para o seu surgimento.

Centering of cervical disc replacements: usefulness of intraoperative anteroposterior fluoroscopic guidance to center cervical disc replacements: study on 20 discocerv (scient'x prosthesis)

STUDY DESIGN

This is a prospective randomized computed tomographic scan study on the centering of cervical disc prosthesis (Discocerv; Scient'X) with and without anteroposterior (AP) fluoroscopic guidance.

OBJECTIVE

Analyze interest of AP fluoroscopic guidance for coronal positioning in cervical disc replacements.

SUMMARY OF BACKGROUND DATA

This series consisted of 20 patients. One group of 10 patients was operated using only lateral fluoroscopic guidance (L guidance) and the other group of 10 patients was operated using both lateral and AP fluoroscopic guidance (AP + L guidance). Total disc replacements positioning is analyzed in the 2 groups.

METHODS

All patients had a computed tomographic scan 24 hours after surgery. Specific reconstructions were obtained from the native slices. Three planes P1, P2, and P3 are defined to quantify centering of the prosthesis in axial sagittal and coronal planes. RESULTS.: In the coronal plane P1, there is no difference in lateralization between the L guidance (absolute value of average M = 0.93 mm; SD = 0.59 mm) and AP + L guidance groups (M = 1.28 mm; SD = 0.75 mm). In the axial plane, there is no difference in lateralization between the L guidance and AP + L guidance groups. In the L guidance group, average was 1.96 degrees (SD = 1.43 degrees ) and 3.18 degrees (SD = 2.94 degrees ) in AP + L guidance. There is no significative difference between 2 groups in coronal (P = 0.26) and axial plane (P = 0.19).

CONCLUSION

Unci are reliable landmarks for coronal centering of total disc replacements. AP fluoroscopic guidance does not improve this positioning.

Rest cannot always recover the dynamic properties of fatigue-loaded intervertebral disc

STUDY DESIGN

An in vitro biomechanical study using adolescent porcine discs.

OBJECTIVE

To find the effect of fatigue loading and rest on the dynamic properties of healthy intervertebral disc.

SUMMARY OF BACKGROUND DATA

The fatigue loading is a risk factor for low back pain. The disc dynamic properties describe the shock attenuation capability of disc. Knowledge of effect of fatigue loading and rest on the disc dynamic properties can be beneficial for the study of fatigue loading induced spinal disorder.

METHODS

Specimens were divided into short-term (0.5 hours at 5 Hz, n = 9) and long-term (2 hours at 5 Hz, n = 9) fatigue loading groups. The specimen was applied with fatigue loading, followed by a 12-hours rest, and then applied with the second fatigue loading again. The impulse loading was applied at 0, 10, 20, and 30 minutes during short-term group and at 0, 0.5, 1, and 2 hours during long-term group using a drop-tower apparatus. The stiffness, damping coefficient, and ratio were calculated using impulse loading information. Dynamic properties between first and second fatigue loading were compared.

RESULTS

The stiffness increased, but the damping coefficient and ratio decreased with fatigue loadings. A 12-hours rest can fully restore the dynamic properties of fatigue loaded disc to original status. However, the degradation of dynamic properties during the subsequent fatigue loading was faster than the previous fatigue loading if the disc experienced a long-term fatigue loading.

CONCLUSION

A 12-hours rest can largely restore disc dynamic properties during fatigue loading; hence fully recover disc dynamic properties. A long-term fatigue loading squeezes the disc fluid and injures the disc integrity. The 12-hours rest can recover the fluid loss but not the injury of disc. Hence, the disc degradation during the second fatigue loading was faster than the first fatigue loading.