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Background. Most cervical spine procedures in patients with degenerative disc disease involve discectomy and remo­val of osteophytes in posterior vertebral body surfaces followed by interbody stabilisation with an interbody implant. Interbody implants are made of a variety of materials, differing in structural design, shape and surface topography. Considering that fusion between the implant and host bone is crucial for long-term positive outcomes, the choice of an appropriate implant is significantly important clinically and continues to be an important area of study. Material and methods. Relevant published studies indexed by Medline were identified via PubMed and reviewed. The findings were combined with the authors’ experiences. The database query was based on keywords related to implants in cervical spine surgery. This article presents the currently most popular types of implants by describing their properties and indicating their strengths and weaknesses as well as differences between different implant types. Results. Currently, the most popular interbody cages in cervical spine surgery are polyetheretherketone (PEEK) im­plants, titanium-coated PEEK implants and titanium implants. Besides the type of material used, the shape and surface structure of an implant appear to be of significant importance for a successful bony fusion. Conclusions. 1. 3D printing and the ability to produce 3-dimensional porous-surfaced implants opens up considerable pro­spects for this technique in the production of modern interbody implants. 2. Implants that facilitate the engagement (interlocking) of greater volumes of bone (e.g. porous implants) offer better implant fixation, with the type of material used being less important.

Background Implant subsidence is an undesirable effect after anterior cervical discectomy and fusion (ACDF). We investigated the relation between the rate of implant subsidence and the ratio of the implant surface area to the surface area of the adjacent bone. Methods We operated 170 disc spaces in a group of 104 patients. Two types of implants were used: 1) PEEK (polyetheretherketone) cages and 2) titanium-coated (TC) PEEK cages. Patients were randomised to receive a specific implant using a randomisation table. All implants had a surface area of 1.61 cm2. Based on computed tomography images, bone surface areas were calculated for vertebral bodies immediately adjacent to the interbody implants. The implant-to-bone surface ratio was then calculated for each disc space. Implant subsidence was assessed over 12 months of follow-up, and associations between implant subsidence, the type of implant, and the implant-to-bone surface ratio were investigated. Results Twelve months after the surgery, computed tomography was performed on 86 patients (144 disc spaces). Furthermore, in 166 disc spaces and 102 patients, conventional radiographs were obtained. Subsidence was observed in 21% of the examined intervertebral spaces, and it was more frequently associated with higher values of bone surface area and lower values of the implant-to-bone surface ratio. The type of implant (PEEK vs TC-PEEK cages) did not significantly influence the rate of implant subsidence. Conclusions Implant subsidence was significantly related to the value of a coefficient representing the ratio of the implant's surface area to the bone surface area of the adjacent vertebral bodies, with subsidence occurring significantly more rarely for coefficient values ≥ 0.37.

Most surgical procedures performed on account of degenerative disease of the cervical spine involve a discectomy and interbody fixation. Bone fusion at the implant placement site is evaluated post-operatively. It is agreed that computed tomography is the best modality for assessing bone union. We evaluated the results obtained with various methods based solely on conventional radiographs in the same group of patients and compared them with results obtained using a method that is a combination of CT and conventional radiography, which we considered the most precise and a reference method. We operated on a total of 170 disc spaces in a group of 104 patients. Fusion was evaluated at 12 months after surgery with five different and popular classifications based on conventional radiographs and then compared with the reference method. Statistical analyses of test accuracy produced the following classification of fusion assessment methods with regard to the degree of consistency with the reference method, in descending order: (1) bone bridging is visible on the anterior and/or posterior edge of the operated disc space on a lateral radiograph; (2) change in the value of Cobb’s angle for a motion segment on flexion vs. extension radiographs (threshold for fusion vs. pseudoarthrosis is 2°); (3) change in the interspinous distance between process tips on flexion vs. extension radiographs (threshold of 2 mm); (4) change in the value of Cobb’s angle of a motion segment (threshold of 4°); (5) change in the interspinous distance between process bases on flexion vs. extension radiographs (threshold of 2 mm). When bone union is evaluated on the basis on radiographs, without CT evidence, we suggest using the “bone bridging” criterion as the most reliable commonly used approach to assessing bone union.

Purpose This paper sets out to analyse mobility changes in segments adjacent to the operated segment. Additionally, it investigates the relationship between the degree of fusion in the operated disc space and mobility changes in the adjacent segments. Methods In total, 170 disc spaces were operated on in 104 consecutive patients qualified for one- or two-level surgery. The degree of mobility of segments directly above and below the implant insertion site was calculated. Measurements were performed the day before the surgery and 12 months post-surgery. Functional (flexion and extension) radiographs of the cervical spine and CT scans obtained 12 months post-surgery were used to evaluate the fusion status. The results were subjected to statistical analysis. Results Statistically significant increase in mobility was recorded for the segments situated immediately below the operative site, with a mean change in mobility of 1.7 mm. Complete fusion was demonstrated in 101 cases (71.1%), and partial fusion in 43 cases (29.9%). In the complete fusion subgroup, the ranges of both flexion and extension in the segments directly below the operative site were significantly greater than those in the partial fusion (pseudoarthrosis) subgroup. Conclusion The mobility of the adjacent segment below the implant insertion site was significantly increased at 12 months post-ACDF surgery. The range of this compensatory hypermobility was significantly greater in patients with complete fusion at the ACDF site than in cases of pseudoarthrosis. Implant subsidence was not associated with mobility changes in the segments directly above or directly below the site of ACDF surgery.