The appropriate hybrid surgical strategy in three-level cervical degenerative disc disease: a finite element analysis

Cervical degenerative disc disease (CDDD) has become a common condition causing pain and/or neurological deficit secondary to compression of the nerve roots or spinal cord [1, 2]. Anterior cervical discectomy and fusion (ACDF) is a standard surgical procedure for CDDD. This procedure aims to decompress the affected neural components, preserve intradiscal height, and provide mechanical stability [3]. However, an increase in motion, shear strain, and intradiscal pressure in adjacent vertebrae after ACDF has also been reported [4].

Cervical disc arthroplasty (CDA) is one of the most carefully studied and documented spinal procedures. CDA has been indicated to be a safe and effective alternative to ACDF. CDA can maintain physiologic motion, restore disc height and some viscoelastic properties, decrease the morbidity of fusion, and allow earlier return to function. In addition, CDA can mitigate the adverse biomechanical changes at adjacent vertebrae and consequently prevent the progression of adjacent segment degeneration (ASD) [5].

Some studies have examined the ability of prostheses to absorb vibrational and impact loads at adjacent segments, but cervical disc prostheses have little ability to provide viscoelastic properties anywhere near those of a normal hydrated disc. Therefore, the approved indication for CDA is restricted to single-level or two-level cervical disc disease. Although one prospective trial demonstrated efficacy and safety for three-level CDA, the study was off-label and should be considered experimental [6].

How to address three-level CDDD has become a problem. A novel hybrid surgery (HS) strategy consisting of ACDF and CDA was introduced to treat three-level cervical disease. Barbagallo et al. [7] was the pioneer in choosing the HS strategy for two-level and three-level CDDD. Despite the encouraging clinical outcomes of HS, few biomechanical tests have been used in the past to investigate HS strategies [8], and these strategies have lacked a detailed internal structural response to external loading. It is not yet clear which HS strategy represents the best choice or whether HS could be efficient and beneficial for three-level CDDD.

Mathematical models such as the finite element (FE) method can be applied to discover structural responses to external loading and, more importantly, to establish an internal structural response such as stress to external loading [9]. Although several models of the cervical spine have been demonstrated in recent studies, efforts to analyze the internal response, especially to evaluate three-level HS strategies, are lacking. Therefore, the purpose of this FE study is to target biomechanical analyses of the different HS strategies used in the treatment of three-level CDDD (one-level CDA and two-level ACDF).

Three-level CDDD is common in clinical practice [25], but surgeons are occasionally confronted with a patient requiring surgical treatment due to CDDD, involving multiple levels. Because of a paucity of data regarding three-level CDDD, the optimal management of this disorder remains controversial and case dependent. While ACDF has long been advocated for three-level CDDD, there is evidence supporting more substantial biomechanical effects in three-level fusion [26, 27]. The anterior approach may have a little more trauma than the posterior approach. There are still some annoying complications with the posterior approach such as axial pain and concomitant cervical kyphosis without fixation. The degree of degeneration in different levels is much variant among patients, some levels in some patients are still fit for arthroplasty surgery. HS strategy is a combination of replacement and fusion operation that can reduce the fusion segment and thus reach the goal of retaining as much ROM as possible, reducing the stress between the adjacent intervertebral disc and decreasing the unfusion rate.

To assess the ROM after three-level HS to find out how much ROM we can retain compares to the intact model, we chose to compare HS with intact cervical spine, because all degrees of freedom of C7 is constrained by the relative fixing inferior surface of the C7 vertebra. If the replacement segment is located at C6/7, it might affect the data. We should choose the segments of C3/4–C5/6 rather than C4/5–C6/7. In the present study, the ROM of segments C2/3 and C6/7 in the HS models confirmed an interesting phenomenon. The results indicated that there were no significant ROM changes in the segment adjacent to the operative segments compared to the intact group. Under flexion-extension, lateral bending, and axial torsion conditions, the total ROM of the HS models decreased to some extent in the present study. This result is consistent with the results from a study by Cho et al. [28]; the use of a cadaveric biomechanical test showed that two-level ACDF could decrease the entire ROM, and two-level CDA and hybrid ACDF/CDA did not show significant changes in the adjacent-level ROM. However, our study showed an encouraging result in that the M2 model had the closest total ROM to the intact group, especially under flexion-extension and axial torsion conditions. In this regard, the results of this study may indicate how to choose an optimal HS strategy in terms of preserving the maximum total ROM.

The increased ROM found at the CDA segments is probably caused by the excision of the ALL and the anterior parts of the annulus. Furthermore, the increased ROM may have resulted in increased stress of the facet joint force in the implanted segment [29]. An increase in the facet joint force after CDA was reported to accelerate degeneration in implanted segments [30], and subsequent alterations in cervical spine biomechanics result in hypertrophy of the ligamentum flavum and facet joint laxity. Some studies have indicated that the increased facet joint force in implanted segments might lead to the degeneration of new segments [31]. Lee et al. [20] analyzed the biomechanical changes in Mobi-C after CDA. The ROM in the CDA segment increased during flexion (33%), extension (56%), lateral bending (35%), and axial torsion (105%). The facet joint force increased by 210% in both fixed and mobile core models. In our study, the facet joint force at the CDA segments was different from the force at the corresponding segments in the intact cervical spine model. As the histogram clearly shows, the facet joint force of the CDA segments increased to some extent. However, the facet joint force adjacent to the treatment levels (C2/3, C6/7) was hardly different among the HS groups compared with the intact group. Based on the results described above, we believe that his procedure has no adverse effect on the facet joints of adjacent segments but on the facet joints before operation due to the increase in the facet joint force after CDA, which may be regarded as a risk factor that leads to new segment degeneration.

Over the last decade, some cervical devices have gained US Food and Drug Administration (FDA) approval [32]. We classified the cervical devices as constrained or mobile types according to the type of prosthesis implanted. Mobi-C belongs to the mobile type of devices; despite the fact that its insertion point slightly misses the center, it tends to have a biomechanical impact on the facet joint because the UHMWPE core of the prosthesis is translocated, while the pressure on the facet joint is high [33]. Our result is also in agreement with the results of the study mentioned above, showing increased pressure on the UHMWPE core in Mobi-C with a mobile core. However, our study showed that the maximal force value of the UHMWPE core was increased in the M2 strategy compared with the HS strategies. This finding indicates that the UHMWPE core may wear easily in the M2 strategy.

The present study, focusing on the changes in the ROM, facet joint force, and UHMWPE core, aimed to analyze the biomechanical performance after three-level HS. The conclusions are summarized as follows: (1) HS does not affect the ROM or facet joint force of the adjacent segment in the prevention of ASD, (2) the results of total ROM show that the M2 strategy is the best HS strategy in terms of preserving the maximum total ROM, (3) the ROM and the facet joint force in the CDA segments clearly increase, and (4) the maximal force value of the UHMWPE core is increased in the M2 strategy compared with the other strategies.

In our study, we try to find the best theoretical combination in these three models by analyzing the FE simulation. As the discussion above, through the comparison of ROM, the facet joint force after CDA, and the stress distribution of the prosthesis, we find that M2 model has a better theoretical outcome, especially in preserving the maximum total ROM. In clinical practice, we choose the hybrid surgery according to the strict surgical indications. If patients can be unconstrained to choose all the three combinations, M2 is recommended according to our theoretical results. If the replacement segment is the exclusive selection, we should choose the corresponding combination.

In the study, there are still several limitations. Though we have conducted some research about the cervical three-level HS simulation, there are much further improvements that shall be carried out. First, if the EF model can include the entire spine and surrounding muscles, the biomechanical simulation of the entire model will be greatly improved. The additional soft tissues such as muscles can imitate the dynamic effect of the active and passive contraction that might make our work much more integrated and authentic. Second, the EF model only reflects the biomechanical state of a particular time period and cannot reflect the continuous dynamic and biomechanical state of the whole time period. Finally, the results of the EF research can only theoretically support the researchers’ hypothesis, and the reliability of the results will ultimately need to be confirmed by cadaver specimens and clinical studies.