We present a retrospective study comparing patients after ACDF with either stand-alone TTN- or PEEK-cages. No differences in clinical outcome could be found between the two groups. Radiological comparison revealed no differences regarding the rate of solid arthrodesis or cage subsidence. A significant segmental lordotic correction could be achieved by both cage-types.
PEEK versus Titanium
The overall rate of cage subsidence was 17.4% in both of the groups. The loss of segmental correction in all of our patients at the last follow-up could be regarded as a consequence of cage subsidence as well, but it is of note that we defined a substantial subsidence as a minimum of 2 mm in lateral radiographs .
The incidence of cage subsidence appeared not to be affected by the differences in modulus of material elasticity as has been assumed by some authors [16, 18]. Niu et al. used cages of different sizes in the compared TTN- and PEEK-groups, although the pre-operative disc height of both groups was comparable. The PEEK-group received cages not bigger than 6 mm, while the TTN-group received cages of no less than 7 mm and 8 mm in 86% of the operated segments and even received 9 mm cages in 5 operated levels . The application of cages of a substantial size may result in an increased stress of the endplates and thus increased risk of subsidence [22–24]. In the cited study solely the measurement of disc height revealed the loss of correction that led to the conclusion that TTN-cages tend to a higher loss of correction. It is not conclusively discussed, why the segmental angulation did not change accordingly to the reduction of the disc height. In our study we observed that the validity of disc height measurement is inferior to the measurement of the Cobb angle of the cervical spine. Furthermore, in the mentioned study the PEEK-cages were filled with cancellous allograft bone, while the TTN-cages were filled with local bone and calcium phosphate bone extender, which may result in different radiological outcomes due to different osteogenic properties. The study of Chou and colleagues favoured PEEK over TTN, but examined only 9 patients in the PEEK-group . Therefore, the statement that PEEK-cages are superior to TTN-cages in maintaining interspace height and achieving fusion is not entirely conclusive.
Both cage-types offer certain advantages in spine surgery. TTN-implants likely provide a good osseointegration . Furthermore, their surface structure appears to be comparably resistent to microbial adhesion, although of course many factors affect the incidence of infection [25, 26].
The PEEK’s modulus of elasticity is close to that of cortical bone and its radiolucency allows for a more accurate assessment of osseous fusion on plain radiographs. It does not compromise MRI-examinations, which is of particular interest in follow-up examinations of patients suffering from myelopathy and neoplastic diseases [27, 28].
When comparing PEEK- with TTN-cages the occurrence of TTN-cage subsidence is believed to be related to the higher modulus of elasticity [16, 18], but comparison of cages of the same material reveals a multifactorial genesis of cage subsidence [20, 29, 30].
The patients in our PEEK-group were significantly older than in the TTN-group. This could be a random effect, but could also reflect the intuitive reaction of the surgeon towards potentially weaker bone substance in elder patients and consecutive choice of a cage-material with a more favourable modulus of elasticity. Also older patients can be expected to have weaker bone and therefore a potentially higher rate of subsidence. This phenomenon is a potential bias that possibly could have favoured the TTN group. A subsidence of TTN-cages is observed in 13 to 45% of cases in larger series [10, 20]. The reported rate of PEEK-cage subsidence varies from 11 to 18% [17, 24, 31]. This shows that even the PEEK’s favourable modulus of elasticity does not prevent a cage subsidence. The different rate of cage subsidence in various studies dealing with synthetic cages might be also due to different criteria (1 or 2 mm) and measurement methods [10, 20, 24]. High resolution digital radiographs and digital measurement tools enable a more precise analysis of various conditions in the spine.
The distance of the implant from the anterior rim, a big cage size, small contact area of endplates and cage or overdistraction with subsequently increased stress of the endplates are possible explanations for a cage subsidence [10, 20, 22–24, 32]. Our cages had an identical surface area and were placed close to the anterior rim. The ratio of cage size and disc height was no influencing factor on cage subsidence.
Bone quality, graft placement, force, shape of the implant and preparation of the endplates are further major factors that influence cage subsidence [20, 29, 30].
Revision surgery in case of cage subsidence without clinical symptoms was deemed unnecessary in our series. This goes conform with other studies [10, 24, 31].
Cervical profile and disc height
The mean cervical lordosis of our patients measured around 13° and is comparable with patients suffering from DDD [12, 33, 34]. It reflects an already substantial loss of lordosis that normally is about 34° . We managed to improve the cervical lordosis, but still did not achieve nearly normal alignment, which is hardly possible by a monosegmental approach. Furthermore, it is reported that patients may develop a kyphotic angulation at the levels above surgery  and subsequent loss of a transient lordotic overall correction. The loss of correction in our PEEK-group does not reach the level of significance, but is notable and could be explained by a disease progression and loss of muscle strength of the older patient collective.
Many authors determine the disc height to measure the achievement and loss of correction [16, 36], but the shape of cervical vertebral bodies is, in contrast to the thoracal spine, not consistently rectangular. The concavity of the cervical disc space and the common presence of osteophytes in DDD can compromise the validity of the measurement. Our interobserver error determining disc height was higher than for the measurement of angles, thus we used the measurement of the Cobb angle to determine the achievement and loss of correction [11, 12, 33, 34].
The higher rate of bone formation in our TTN-group could be explained by three factors: 1.The patients were younger and could possibly muster larger osteogenic abilities. 2. The Plasmapore® coating of the TTN-cages enlarges the surface and might increase osteoconductive properties compared to the PEEK-cage. 3. Cage subsidence and subsequent exposure of cancellous bone inside the cage might promote fusion in certain cases .
The fusion rate of empty TTN-cages is reported to reach even 100%  and exceeds the fusion rate of empty PEEK-cages that is reported to be 72% . One possible theory claims that fusion of empty cages may occur as the result of endplate failure and subsequent filling of a cage by fracture fragments. In this case the elastic mode of PEEK might prove to be disadvantageous for some patients according to the authors . Nonetheless, it must be considered that fusion is not mandatory for a clinical success, and a loss of disc height and a potential segmental kyphosis might result from a cage subsidence. The comparatively little number of bone formation in our study may relate to the choice of diagnostic means and criteria to asses bony fusion. Assessing bone formation within the cages  was no criterion in our PEEK-group. This would have compromised a proper comparison with the TTN-group on lateral radiographs. Also even the PEEK-cages are not entirely radiolucent to allow a certain assessment of bone within the cages. For these reasons only the presence of anterior or posterior bone formation was rated.
The absence of segmental movement was our main criterion to rate a segment as a solid arthrodesis or pseudarthrosis. While in many studies bony union of the operated segment is regarded as the main criterion for a stable fusion [37, 38], the study of functional flexion-extension radiographs and the position of the spinous processes can reveal motion nonetheless  or show a stable segmental status despite the lack of osseous trabeculation . The absence of bony fusion can occur with absence of motion even for a long-term period, which is therefore accepted as a successful criterion for fusion in lateral radiographs [1, 2, 5, 13, 21, 33]. Nonetheless, we preferred the term solid arthrodesis instead of fusion to describe the absence of motion. The comparison of our patients with and without bone formation revealed no clinical difference as well . A CT-scan would have allowed a more precise statement of bony ingrowth, but was not performed routinely in our series to avoid an unnecessary radiation exposure.
Surgery and study design
The surgeries were performed by three neurosurgeons in identical surgical technique in the same neurosurgical department. Nonetheless, this represents a potential bias of the study. The retrospective study design represents the major limitation of the study. The range of follow-up examinations provides a larger degree of interpretation compared to prospective studies with fixed follow-up appointments for all included patients. Preclinical data are limitedly available, and the only clinical outcome measurement is Odom’s criteria. The clinical comparison of patients suffering from myelopathy and radiculopathy is difficult, but we have focused on the radiological results of our study, particularly the rate of cage subsidence.
Postoperative external immobilization is often required when stand-alone devices are used. However, we did not observe any case of cage extrusion in our study population. This may be due to the surface structure of the cages, which either feature teeth (PEEK) or are coated with Plasmapore® and use a fixation ring (TTN). Some surgeons advocate rigid internal fixation and cervical immobilization postoperatively to prevent graft migration and nonunion and to enhance fusion [40, 41]. The outcomes of rigid plate fixation have been equivocal to surgery without internal fixation in some studies [42–44]. In degenerative spondylosis the application of an internal fixation or a collar does not seem to influence the clinical or radiological outcome in a negative manner .