Geometry of inferior endplates of the cervical spine

doi:10.1016/j.clineuro.2016.01.027

Clinical Neurology and Neurosurgery

Volume 142, March 2016, Pages 132-136

https://doi.org/10.1016/j.clineuro.2016.01.027 Get rights and content

Highlights

•
The inferior endplates on the middle-sagittal plane were classified into 3 types according to the location of the concave apex.
•
The inferior endplates on the specific coronal plane were classified into 3 types: concave, flat and irregular.
•
On the middle-sagittal plane, the inferior endplate concave depth from C2 to C7 were between 1.7 and 2.8 mm, showing no significant difference.
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The average sagittal diameters of inferior endplate of females for each level were significantly smaller than those of males.
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The average transverse diameters of inferior endplate had an increasing tendency from C2 to C7, especially C6 and C7 increasing the most.

Abstract

Objectives

Device subsidence is a well-known complication following cervical disc arthroplasty. Its occurrence has been closely tied with the endplate-implant contact interface. But current literature on the geometry of cervical endplate is very scarce. The aim of this anatomical investigation was to analyze geometry of inferior endplates of the cervical vertebrae, thereby identifying the common endplate shape patterns and providing morphological reference values consummating the design of the implant.

Patients and methods

Reformatted CT scans of 85 individuals were analyzed and endplate concave depth, endplate concave apex location, sagittal diameter of endplate, coronal concave angle, as well as transverse diameter of endplate were measured in mid-sagittal plane and specified coronal plane. According to the endplate concave apex location, the inferior endplates in mid-sagittal plane were classified into 3 types: type I with posteriorly positioned apex, type II with middle situated concave apex and type III with anteriorly positioned apex. Moreover, the inferior endplates in specified coronal plane were also classified into three types: concave, flat and irregular.

Results

Based on visual assessment, for the mid-sagittal plane, type I endplate accounted for 26.9% of all the 510 endplates of 85 individuals, while the proportion of type II and type III endplates were 53.9 and 19.2% respectively. For the specified coronal plane, 68.6% of all the 510 endplates were evaluated as concave, 26.9% as flat and the remaining 4.5% as irregular. Among all measured segments, C3 had the largest endplate concave depth values in mid-sagittal plane, while C7 the least; C5 and C6 had the largest sagittal endplate diameter values, while C2 the least. For each level, the sagittal endplate concave depth and endplate diameter of females were significantly smaller than those of males (P < 0.05). Among all measured segments, C7 had the least coronal concave angle. Gender did not influence coronal concave angle significantly (P > 0.05). Increasing from C2 to C7, the endplate transverse diameters of females were significantly smaller than those of males (P < 0.05).

Conclusion

The exact shape and geometry of cervical endplate are crucial for the design and improvement of cervical disc prosthesis. Gender difference of sagittal and transverse diameters of cervical endplate should be given more attention when implanting a disc prosthesis. These endplate geometrical parameters should be taken into consideration when calculating most suitable geometric parameters of new disc prosthesis.

Introduction

Cervical disc replacement has been widely used in the treatment of degenerative cervical disc diseases. However, artificial cervical disc replacement may fail because of device-related complications, such as subsidence, heterotopic ossification, device wear and migration [1]. Subsidence is the most common complication following cervical disc arthroplasty with an incidence of 3–10% [1]. It is suggested that one desired artificial cervical disc should mirror the shape of both endplates of the same cervical segment in all three dimensions to gain a maximal contact area of the endplate-prosthesis interface [2]. Nevertheless, the endplate designs of those common cervical disc prostheses are oversimplified in contrast to the morphological complexity of vertebral endplates. That is to say, the profile of most currently available artificial cervical disc prostheses is limited to flat endplate or at the best minor convexity. This oversimplification of the design of cervical disc prosthesis would be bound to bring about insufficient contact area between endplate and disc prosthesis, finally resulting in subsidence of disc prosthesis.

The morphology of cervical endplate could have practical significance for cervical disc replacement. It has been demonstrated that endplate geometrics, such as the endplate concave angle and the sagittal or transverse diameter of endplate are crucial for artificial cervical disc design [3], [4]. Besides, endplate shape has been related with clinical outcomes of cervical disc replacement [5]. Thus, the exact shape and geometry of cervical vertebral endplate could be of important clinical significance in the design and improvement of cervical disc prosthesis.

The purpose of this study was to analyze the sagittal and coronal geometry of cervical inferior endplates from C2 to C7 by employing processed data digitized CT scans. These messages were used to describe the common cervical endplate shape and consummate the design of cervical disc prosthesis.

Section snippets

Patients

Eighty-five patients (average age 42 years, range 26–61 years) that underwent a cervical CT scans (Light Speed VCT, GE Healthcare, London, UK) were retrospectively selected from the Picture Archiving and Communication System (PACS). All the patients were scanned for head and neck symptoms and complained of no spinal problems. Scans with evidence of trauma, tumor, deformity or infection were excluded.

Images

All the CT scans images were imported into the Mimics 10.01 software to be multi-planar

Sagittal endplate shape

Based on visual assessment, for the MSP, type I endplate accounted for 26.9% of all the 510 endplates of 85 individuals, while the proportion of type II and type III endplates were 53.9% and 19.2% respectively. The proportion of different endplate shapes from C2 to C7 is present in Table 1.

Sagittal endplate concave depth (ECD)

Among all cervical segments, the inferior ECD values from C2 to C7 were between 1.7 and 2.8 mm, demonstrating no significant difference (P > 0.05, see Table 2). In addition, C3 had the largest mean value of ECD

Discussion

Device subsidence is a commonly reported complication following artificial disc arthroplasty, and it may also be the main cause resulting in the failure of artificial disc replacement [6], [7]. A small degree of subsidence may provide some benefits, including immediate stability of the instrumentation and promotion of biological fusion. However, when aggravated, it can result in loss of anterior column height, progressive deformity, and failure of the instrumentation [8], [9], [10].

Conclusions

The exact shape and geometry of cervical vertebral endplate are crucial for the design and improvement of cervical disc prosthesis. Gender difference of sagittal and transverse diameters of endplates should be given more attention when implanting a disc prosthesis. These endplate geometrical parameters should be taken into consideration when calculating most suitable geometric parameters of new implant.

Conflicts of interest

None.

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A novel biomimetic artificial intervertebral disc (bioAID) replacement implant has been developed containing a swelling hydrogel representing the nucleus pulposus, a tensile strong fiber jacket as annulus fibrosus and titanium endplates with pins to primarily secure the device between the vertebral bodies. In this study, the design safety of this novel implant was evaluated based on several biomechanical parameters, namely compressive strength, shear-compressive strength, risk of subsidence and device expulsion as well as identifying the diurnal creep-recovery characteristics of the device.
The bioAID remained intact up to 1 kN under static axial compression and only 0.4 mm of translation was observed under a compressive shear load of 20 N. No subsidence was observed after 0.5 million cycles of sinusoidal compressive loading between 50 and 225 N. After applying 400 N in antero-posterior direction under 100 N axial compressive preload, approximately 2 mm displacement was found, being within the range of displacements reported for other commercially available cervical disc replacement devices. The diurnal creep recovery behavior of the bioAID closely resembled what has been reported for natural intervertebral discs in literature.
Overall, these results indicate that the current design can withstand (shear-compression loads and is able to remain fixed in a mechanical design resembling the vertebral bodies. Moreover, it is one of the first implants that can closely mimic the poroelastic and viscoelastic behavior of natural disc under a diurnal loading pattern.
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Current intervertebral disc finite element models are hard to validate since they describe multi-physical phenomena and contain a huge number of material properties. This work aims to simplify numerical validation/identification studies by prioritizing the sensitivity of intervertebral disc behavior to mechanical properties. A 3D fiber-reinforced hyperelastic model of a C6-C7 intervertebral disc is used to carry out the parametric study. 10 parameters describing the extracellular matrix and the collagen network behaviors are included in the parametric study. The influence of varying these parameters on the disc response is estimated during physiological movements of the head, including compression, lateral bending, flexion, and axial rotation. The obtained results highlight the high sensitivity of the disc behavior to the stiffness of the annulus fibrosus extracellular matrix for all the studied loads with a relative increase in the disc apparent stiffness by 67% for compression and by 57% for axial rotation when the annulus stiffness increases from 0.4 to 2 MPa. It is also shown that varying collagen network orientation, stiffness, and stiffening in the studied configuration range have a noticeable effect on rotational motions with a relative apparent stiffness difference reaching 6.8%, 10%, and 22%, respectively, in lateral bending. However, the collagen orientation does not affect disc response to axial load.
Analysis of Functional Outcomes and Risk Factors for Facet Joint Distraction During Anterior Cervical Discectomy and Fusion for Cervical Spondylotic Myelopathy
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According to the ROC analysis, we determined that the optimal cutoff threshold for CPD was 12.45 mm. It is noteworthy that in studies of the morphometry of the cervical endplate, the inferior endplate of the adjacent vertebral bodies is arc shaped, whereas the superior endplate of the adjacent vertebral bodies is flat.26,27 As shown by our study in Table 4, the ΔAIH is positively correlated with the CPD.
This study aimed to clarify functional outcomes of facet joint distraction (FJD) and identify specific risk factors for excessive FJD during single-level anterior cervical discectomy and fusion (ACDF) for cervical spondylotic myelopathy (CSM).
This study retrospectively analyzed 100 patients who underwent single-level ACDF for CSM from January 2016 to May 2020. Anteroposterior and lateral radiographs were obtained before surgery and 12 months after surgery. Radiographic parameters including anterior intervertebral height (AIH), posterior intervertebral height, facet joint gap, cage posterior depth (CPD), upper vertebral length, cervical segmental Cobb angle (CSCA), C2–C7 Cobb angle, and C2–C7 sagittal vertical axis were analyzed. Functional outcomes were evaluated using the modified Japanese Orthopedic Association Score, visual analog scale (VAS), and Neck Disability Index (NDI).
Comparison between the appropriate FJD and excessive FJD groups showed statistically significant differences in the NDI, VAS, CPD, and ΔAIH (P < 0.05). Multivariate logistic regression analysis showed that independent factors associated with excessive FJD were as follows: a ΔAIH > 2.28 mm (odds ratio [OR] = 6.792, 95% confidence interval [CI] = 1.885–24.470, P = 0.003), CPD > 12.45 mm (OR = 5.876, 95% CI = 1.828–18.895, P = 0.003), and post-CSCA < 0° (OR = 6.251, 95% CI = 1.275–30.633, P = 0.024). Furthermore, receiver operating characteristic curve analysis for the multilevel logistic regression model produced an area under the curve of 0.783 (P < 0.001).
Patients with an FJD of >0.905 mm had worse NDI and VAS pain scores, but not a poorer modified Japanese Orthopedic Association Score recovery rate. Our findings suggested that a ΔAIH > 2.28 mm, CPD > 12.45 mm, and post-CSCA < 0° were independent risk factors for excessive FJD after single-level ACDF for CSM.
Three-dimensional morphometric analysis of cervical vertebral endplate anatomy: A systematic literature review
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Specifically, for superior cervical endplates, Soo et al. [9] reported a significant relationship between gender and APD and TD (P < 0.01, R2 = 0.27–0.41) and that on average, superior cervical endplates in males were wider (in terms of TD) by 0.6 mm (P = 0.05) and deeper (in terms of APD) by 1.6 mm (P = 0.0001) than those in females. For inferior cervical endplates, males had significantly larger APDs for than females for C2 to C7 and larger TDs for C6 and C7, whilst no significant difference was detected between genders for TD from C2 to C5 [10]. Whilst Kim et al. [13] also identified a significant correlation between gender and cervical endplate diameters (P < 0.05), no significant correlation was found between endplate diameters and age.
Cervical fusion and arthroplasty are common surgical procedures where devices are inserted into cervical interbody spaces. These spaces are defined superiorly and inferiorly by adjacent vertebral body endplates. Having sufficient data on cervical endplate shape and size contributes to superior device designs associated with better functional outcomes and less postoperative complications. This study presents a systematic review of current literature on cervical endplate morphology and variation with age, gender and spinal level.
A systematic review was conducted on 4 electronic databases using combinations of search terms (“endplate”, “cervical”, “cervical spine”, “morphology” and “anatomy”). Relevant articles (n = 10) were selected with additional studies identified through their citations (n = 2).
Current literature on cervical endplate morphology is limited. Most studies reported that cervical endplate diameters and surface area (SA) increased with descending vertebral level. Only two studies investigated the relationship between age and cervical endplate diameters, with conflicting results. Females generally had smaller cervical endplate diameters and inferior vertebral endplates were more concave than superior counterparts. Relationships between gender, age and cervical endplate SA, concavity and concavity apex location were not well reported.
Literature on three-dimensional (3D) cervical endplate and interbody space morphology is limited. Existing studies are based on linear and area parameters measured from two-dimensional (2D) imaging modalities (computed tomography (CT) and X-rays) in specific planes or 3D reconstructions of scanned spines (living or cadaveric). These 2D parameters do not capture the 3D shape of cervical endplates and studies differ on how they define these parameters, making data difficult to compare. Further studies utilising methods that better capture the 3D shape of cervical endplates would benefit our understanding of vertebral endplates and, thereby, cervical interbody space shape. In turn, greater morphological knowledge of cervical interbody spaces could aid in the design of new generation devices.
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2020, Clinical Neurology and Neurosurgery
Despite encouraging clinical results, the postoperative segmental kyphosis was observed in many cervical disc arthroplasty (CDA) studies. Most studies discussed technical factors related to kyphosis. However, the cervical endplate, which contacted directly with prostheses and distributed the compressive load across the vertebral body, was seldom investigated. The purpose of the study was to investigate the effects of the inferior endplate morphology on clinical and radiological outcomes after C5/C6-level CDA with Prestige-LP Disc.
One hundred and thirty-two patients with C5/C6-level CDA were retrospectively reviewed. Their preoperative inferior endplates on the mid-sagittal plane were visually classified into 3 types: type I with the endplate concavity apex located posteriorly, type II with the apex located in the middle and type III with the apex located anteriorly. Sagittal diameter of endplate, endplate concavity depth and endplate concavity apex location were measured. Japanese Orthopedic Association (JOA), visual analogue scale (VAS) and Neck Disability Index (NDI) scores were used to evaluate clinical outcomes. Range of motion (ROM) and sagittal alignment including C2-7 and C5-6 angle were assessed.
The type I, type II and type III endplates accounted for 25.00 %, 51.52 % and 23.48 % of 132 individuals. Most patients achieved significant reduction in VAS scores and NDI but the significant increase in JOA scores. No substantial differences in clinical outcomes were found among the 3 endplate types. Compared with preoperative values, the C5-6 ROM, C2-7 ROM and C2-7 angle were preserved, while the C5-6 angle increased significantly from kyphosis to lordosis. At the last follow-up, type I endplate had the least C5-6 angle (2.03°, 3.94° and 4.46° for type I, II and III endplates) and the highest incidence of segmental kyphosis at C5-C6 level (33.30 %, 14.70 % and 9.70 % in the same order, P = 0.028).
Patients achieved satisfactory clinical outcomes after C5/C6-level CDA without significant differences among three endplate types. Type I endplate had a high incidence of segmental kyphosis at C5-C6 level.
Effects of New Cage Profiles on the Improvement in Biomechanical Performance of Multilevel Anterior Cervical Corpectomy and Fusion: A Finite Element Analysis
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Previous studies have noted that the fewer points for anterior screw fixation and the drawbacks of the conventional cage profile are the main implant-related factors that induce cage subsidence and subsequent instability.14,40,41 For a conventional cage, the small footprints on the end and the geometric mismatching between the domed end plate and flat end substantially reduce the contact area with the end plate, which results in a large stress concentration and facilitates cage subsidence.14,40,42 To address these issues, some cages with new profiles have been reported to be used for vertebral body reconstruction in single-level ACCF.16-18
For multilevel cervical fusion, anterior corpectomy and fusion (ACCF) induces more implant-related complications than anterior diskectomy and fusion (ACDF), which implies that the biomechanical stability of ACCF may be insufficient. The aim of this study was to assess whether the optimization of the cage profiles could improve the biomechanical performance of multilevel ACCF.
Three finite element models were constructed and compared, including 3-level ACDF, 2-level ACCF using a conventional cage for reconstruction, and 2-level ACCF using a new cage for reconstruction. The ends of the new cage possessed additional end rings and emulated the end plate geometries. The ranges of motion (ROMs) of the surgical segments and the stress peaks in the end plate, fixation system, and screw-bone interface were compared.
Compared with preoperative status, ACDF and ACCF reduced the segmental ROMs by 96.1%–98.2%. The end plate stress peaks were the highest in ACCF using the conventional cage (10.1–18.6 MPa), followed by ACCF using the new cage (7.7–14.3 MPa) and ACDF (5.3–9.1 MPa). ACDF induced the highest stress peaks in the fixation system and screw-bone interface (32.5–39.3 MPa and 12.1–12.7 MPa, respectively), followed by ACCF using the conventional cage (20.4–31.7 MPa and 10.3–13.6 MPa, respectively) and ACCF using the new cage (18.6–25.7 MPa and 9.7–12.6 MPa, respectively).
The application of the new cage decreased the risks of cage subsidence and instrument-related complications in multilevel ACCF. Under the condition where cage subsidence was prevented, ACCF was superior to ACDF in terms of construct stability and avoiding instrument-related complications.

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Geometry of inferior endplates of the cervical spine

Highlights

Abstract

Objectives

Patients and methods

Results

Conclusion

Introduction

Section snippets

Patients

Images

Sagittal endplate shape

Sagittal endplate concave depth (ECD)

Discussion

Conclusions

Conflicts of interest

Spine J.

Spine J.

Spine J.

Bone

Spine J.

Stress analysis of the interface between cervical vertebrae endplates and the Bryan, Prestige LP, and ProDisc-C cervical disc prostheses: an in vivo image-based finite element study

Spine (Phila Pa 1976)

Is there a difference in shape of the endplates in sagittal plane? A morphometric study of the lumbosacral spine (Abstract)

Spine J.

Geometry of the intervertebral volume and vertebral endplates of the human spine

Ann. Biomed. Eng.

Complications of artificial disc replacement: a report of 27 patients with the SB Charite disc

J. Spinal Disord. Tech.

Complications and reoperations of the SB Charite lumbar disc prosthesis: experience in 75 patients

Eur. Spine J.