Comparison of Pretic-I Versus Discover Disc Arthroplasty As Treatment for Single-level Cervical Disc Degenerative Disease With 5-year Follow-up

Background: The newly designed cervical disc prosthesis, Pretic-I, had been nished its limited clinical use for over 5 years. At a short-term follow-up of 2 years, we obtained satisfactory clinical results. The long-term clinical ecacy and safety of Pretic-I will now be analyzed. Methods: Peri-operative parameters included intra-operative blood loss, operation time, off-bed time. Clinical parameters included visual analogue scale (VAS) for arm and neck, neck disability index (NDI), and Japanese Orthopaedic Association (JOA) score. Radiological parameters included C2-7 Cobb angle, Shell angle, and the range of motion (ROM) of C2-7, functional segment unit (FSU), and adjacent FSU. The CDA-related complications included adjacent segment degeneration (ASDeg), adjacent segment disease (ASDis) , heterotopic ossication (HO), prosthesis subsidence, prosthesis displacement, and dysphagia. Results: A total 64 patients from two independent centers received a single-level CDA with Discover (n=32) and Pretic-I (n=32), and all of patients nished a 5-year follow-up. There’re no signicant differences between two groups in peri-operative parameters. The clinical parameters improved greatly in Pretic-I group (p(cid:0)0.0001), and there’s no statistical difference from Discover group. Furthermore, Pretic-I could slightly improve the cervical curvature (15.08±11.75 to 18.00±10.61, p=0.3079) and perfectly maintain the Shell angle (3.03±3.68 to 2.23±4.10, p=0.1988), cervical ROM (52.48±14.31 to 53.30±11.71, p=0.8062) and FSU ROM (12.20±4.52 to 10.73±4.45, p=0.2002). The incidence of high-grade HO (Grade III-IV) at the nal follow-up was signicantly lower in Pretic-I group than in Discover group (12.50% vs. 34.38%, p=0.0389, Statistical Power=95.36%). The incidences of other CDA-related complications in Pretic-I group were also well-accepted, comparable to the Discover group, without signicant differences. Conclusion: CDA with Pretic-I demonstrated a well-accepted and sustained clinical outcome, with a signicantly lower incidence of high-grade HO. This newly designed prosthesis is expected to become an alternative choice for cervical disc prosthesis in the future. its clinical safety and ecacy[10]. The purpose of this study was to verify the long-term effectiveness of Pretic-I. Through a comparative study of 5-year clinical follow-up with Discover prothesis, the clinical ecacy, radiological features and the incidence of CDA-related complications of Pretic-I were analyzed, so as to preliminarily evaluate whether it can meet the standards of large-scale clinical application.


Introduction
Cervical disc degenerative disease (CDDD) is a vital factor affecting the quality of life for the middle-aged and old people [1,2]. As an important alternative surgical method to treat CDDD, cervical disc arthroplasty (CDA) has been used in clinical practice for decades since 2002 [3,4], whose clinical effect is similar to that of anterior cervical discectomy and fusion (ACDF). Furthermore, it can reduce the incidence of adjacent segment degeneration (ASDeg) or adjacent segment disease (ASDis) by preserving the range of motion (ROM) of surgical segment to a certain extent [5]. It is now highly recommended in some appropriate circumstances.
With the clinical application of arti cial disc prosthesis becoming more and more widespread, several problems of CDA are gradually emerging, such as heterotopic ossi cation (HO), prosthesis displacement and even falling off, and some scholars also believe that the application of arti cial disc prosthesis cannot effectively reduce the incidences of ASDeg and ASDis. Therefore, the assessment of one arti cial disc prosthesis will always focus on the incidence of post-operative HO, prosthesis displacement, ASDeg and ASDis, etc [5][6][7][8]. With the deepening of the clinical application and research of arti cial disc prosthesis, it was found that many arti cial disc prosthesis footprints often do not match the cervical endplates, which may lead to subsidence, displacement, HO and some other complications mentioned above [9].
In order to better match the prosthesis with the endplate of patients, the new cervical disc prosthesis--Pretic-I was designed and developed as shown in Figure-1. The previous 2-year follow-up study has shown a positive result, demonstrating its clinical safety and e cacy [10]. The purpose of this study was to verify the long-term effectiveness of Pretic-I. Through a comparative study of 5-year clinical follow-up with Discover prothesis, the clinical e cacy, radiological features and the incidence of CDA-related complications of Pretic-I were analyzed, so as to preliminarily evaluate whether it can meet the standards of largescale clinical application.

Ethics Approval
This study has institutional review board (IRB) approval / research ethics committee approval, and the document has been attached to the submission.

Study Design
By collecting information on a total of 64 patients who underwent single level CDA surgery in two centers from June 2014 to January 2016, a retrospective, comparative, and double-center study was performed. During the follow-up period, the results at 5 time points were examined and evaluated: pre-operative stage, 1 week, 3 months, 1 year and the nal follow-up. The inclusion and exclusion criteria were clearly de ned (Table-1). Among the 64 patients, 32 of them who have received Discover prothesis over the same period were enrolled into the control group; while the other 32 of them who received Pretic-I prothesis were enrolled into the experimental group.

Surgical Technique
The patient was supine under general anesthesia, and the C-arm was used to determine the position of cervical vertebra and the operative segment. The surgery eld was fully exposed by the standard Smith-Robinson approach. After cutting and opening the anterior intervertebral disc annulus brosus, nucleus pulposus tissues were removed with nucleus pulposus forceps, and the cartilage endplate were scraped carefully. The dural membrane and bilateral nerve roots were con rmed to be free from compression. X-ray examination results in Figure-1 showed that the size of the prosthesis was determined with a trial mold, and then the prosthesis of the corresponding size was implanted.

Clinical Evaluation
In peri-operative parameters, intra-operative blood loss and operation time were mainly used to evaluate the operation di culty of the two prostheses and the damage to local tissues, while off-bed time (the time from operation to their off-bed activity), off-hospital time (the time from operation to their hospital discharge), and back-to-society time (the time from operation to their return to social life) were used to assess the recovery condition after surgery. The clinical effect evaluation mainly included: visual analog score (VAS) for neck and arm, neck disability index (NDI) and Japanese Orthopaedic Association Score (JOA) for cervical spine (only for patients with myelopathy or mixed-type CDDD). In addition, the clinical symptoms associated with ASDis were examined and evaluated to determine the presence of CDDD symptoms of adjacent segments. Dysphagia was also evaluated at every follow-up point. Furthermore, the recovery rate of each clinical parameters were calculated as follows: In addition to ASDeg, the complications such as HO, prosthesis subsidence and prosthesis displacement were evaluated. The extent of HOs was graded according to the modi ed McAfee grading system for disc prosthesis as previously described by Mehren et al (Table-2)[14]. Prosthesis subsidence was de ned as loss of more than 3 mm on the height of surgical FSU. Prosthesis displacement was de ned as horizontal movement with more than 3 mm.

Statistical Analysis
Analysis was conducted using Stata version 13.1 (Stata-Corp LP, College Station, TX). The level of signi cance was set at p 0.05. All of the clinical and radiological measurements were carried out by two independent and experienced observers. Chi-squared analysis and unpaired t test were used, respectively, for categorical and continuous data between groups, while paired t test was used to compare the data of the one group from different time points. Statistical power with a signi cant level (alpha) of 0.05 was calculated using G-Power software (version 3.1.9.4) when there was a statistical difference, and 80% is the minimum level for statistical signi cance [15]. All the data were expressed as mean ± standard deviations or percentages.

Patients'Characteristics
A total of 64 patients met the inclusion criteria who agreed to accept single level CDA. The speci c data in Table-3 showed no signi cant difference between the two groups in the basic characteristics. By the end of the nal follow-up, no patient needed the second operation for CDDD.

Peri-operative Conditions
In order to evaluate and compare the conditions of the two groups during and after surgery, a statistical analysis of peri-operative parameters was conducted.
As shown in Table-4, intra-operative blood loss and operative time in two groups were similar, without statistical differences. In addition, the post-operative recovery between both two groups was compared and analyzed, and as shown in Table-4, there was no signi cant difference in all of the parameters between the two groups.

Clinical Results
As shown in Table-5, the evaluation results of various clinical items such as VAS-neck, VAS-arm, NDI, and JOA in the two groups at the nal follow-up were signi cantly better than those before surgery in both groups. The comparison of clinical results between the two groups is shown in Table-6. It was noticed that there was no signi cant difference with valid statistical power in the results between two groups.

Radiological Results
As shown in Table-7, the Shell Angle of Pretic-I group decreased from 3.03°±3.68° to 2.23°±4.10° (p=0.1988), while that of Discover group decreased from 0.75°±3.42° to 0.09°±4.44° (p=0.1820), showing no signi cant difference. The C2-7 Cobb angle of the Discover group increased from 12.05°±12.17° preoperatively to 19.32°±9.32° at the nal follow-up (p=0.0104, Statistical Power=98.20%); while in the Pretic-I group, it increased from 15.08°±11.75° preoperatively to 18.00°±10.61° at the nal follow-up (p=0.3079). C2-7 ROM and FSU ROM showed no signi cant change at the nal follow-up in both groups (Table-7). With more detailed analysis, the ROM was divided into exion ROM and extension ROM, which also showed no signi cant changes. Figure-2 shows a typical case of patient with Pretic-I, demonstrating that after CDA with Pretic-I, the cervical curvature of the patient was greatly maintained and even improved; the cervical total ROM and FSU ROM were well preserved along the whole follow-up period. Looking at the whole follow-up period, as shown in Table-8, there was no signi cant difference with valid statistical power for each parameter of the two groups.

Complications
As shown in Table-9, the exion ROM, extension ROM and total ROM in the adjacent segment of the two groups were very close at each follow-up point.
Furthermore, as shown in Table-10, at the nal follow-up, ROM in the upper and lower FSU of both groups did not signi cantly change compared with that preoperative data. At each follow-up point, as shown in Table-11, there were no statistical differences between two groups in the listed items of ASD characteristics. In addition, there was similar incidence of CDDD-related symptoms in the adjacent segments between the two groups. Therefore, the incidence of cases that met at least one of the above descriptions was calculated, showing that at the nal follow-up, the total incidence of ASD (ASDeg and ASDis) in upper segment was 21.88% (7/32) in the Discover group and 18.75% (6/32) in the Pretic-I group, with no signi cant difference (p=0.7560). The total incidence of lower segment ASD (ASDeg and ASDis) was 15.63% (5/32) in the Discover group and 18.75% (6/32) in the Pretic-I group, also with no signi cant difference (p=0.7404). The incidence of HO between the two groups was further compared. As shown in Table-12, at the nal follow-up, the HO incidence was 65.63% (21/32) in the Discover group and 46.88% (15/32) in the Pretic-I group, respectively. There was no signi cant difference between the two groups (p=0.1306).
The incidence of Grade III-IV HO was 12.50% (4/32) in the Pretic-I group, which is signi cantly lower than the 34.38% (11/32) of Discover group (p=0.0389, Statistical Power=95.36%). The other related clinical complications of CDA between the two groups were further analyzed and compared. Although the incidence of subsidence of the nal follow-up in the Pretic-I group (12.50%, 4/32) appears to be lower than that in the Discover group (3.13%, 1/32), there is no signi cant difference (p=0.1623). As shown in Table-12, a small number of patients in both groups had suffered from dysphagia 1 week after surgery (12.50% in Discover group, 9.38% in Pretic-I group, p=0.6888). Fortunately, all symptoms of dysphagia gradually disappeared after 3 months without speci c treatment.

Discussion
CDA has become one of the surgical options for the treatment of CDDD, and is even superior to ACDF in some cases. However, it is possible for arti cial cervical discs to fail, mainly due to prosthesis displacement, subsidence, and heterotopic ossi cation. Martin et al. [9] proposed the concept of footprint mismatch in 2012, that is, if the end plate of the arti cial cervical disc prosthesis does not t well with the bone surface of the corresponding vertebral body, the related complication incidence will be signi cantly increased. Therefore, how to better match the two has become a point that needs to be paid attention to in the development of arti cial cervical disc prosthesis.
Our center has modi ed and redesigned the existing cervical disc prosthesis, known as Pretic-I in this article, and conducted a 5-year follow-up and comparative study among a limited number of patients. The evaluation focused on the following aspects: the di culty of implantation, the clinical e cacy, the ability to maintain cervical curvature and ROM, and the incidence of related complications.
In the rst two parts, there was no signi cant difference in peri-operative indicators between the Pretic-I and Discover groups, indicating that the di culty of operation was similar, and the recovery rate of patients receiving the two types of prostheses was also show no differences. At the same time, CDA with two prostheses were both associated with great clinical outcomes, and there was no signi cant difference in follow-up between the two groups at the same time point, assuring the clinical e cacy of Pretic-I prosthesis.
The radiological outcomes of the two groups were compared as an important part. By comparing the results of pre-operative and nal follow-up, it was found that both Discover and Pretic-I could improve cervical curvature. In addition, Shell angles of patients in both groups did not change signi cantly from 0.75°± 3.42° to 0.09°±4.44° in Discover group and from 3.03°±3.68° to 2.23°±4.10° in Pretic-I group from 1 week after surgery to the nal follow-up. Those results suggested that both prostheses can improve cervical curvature to a certain extent, and the prostheses themselves can maintain a perfect opening status. Similarly, by comparing the pre-operative and nal follow-up results, it was found that after 5 years of observation, the C2-7 ROM of the both groups changed slightly along the follow-up. It was observed that the C2-7 ROM and FSU ROM of the two groups maintained the same size as before surgery, and there was no signi cant difference between the two groups at each follow-up point, indicating that both prostheses can maintain the original ROM of cervical spine, which also best re ects the value of CDA surgery.
Finally, the CDA-related complications of the two groups were analyzed and compared. The changes in adjacent FSU ROM were rstly compared, revealing that all of the data had no signi cant difference with valid statistical power at each follow-up point. Accordingly, it was believed that the ROM of adjacent segments for the two groups did not change signi cantly during the whole follow-up, indicating that there was no obvious compensatory increase in adjacent segment ROM. Next, the degenerative radiological features and CDDD-related symptoms in adjacent segments of the two groups were analyzed to evaluate the incidences of ASDeg and ASDis, respectively, also showing no signi cant differences, which indicated that ASD incidences were similar between both groups. The incidence of high-grade HO in Pretic-I was signi cantly lower than in the Discover group (12.50% vs. 34.38%, p=0.0389, Statistical Power=95.36%), as was the incidence of prosthetic subsidence, although there was no statistically signi cant difference (3.13% vs. 12.50%, p=0.1306). This may re ect, to some extent, that the prosthetic endplate of Pretic-I is better matched with the bone surface of the vertebral body. No more differences in incidences of other complications were found.
In this study, it was the rst time to report the clinical e cacy of Pretic-I with a long-term follow-up. From the results obtained so far, it can be seen that Pretic-I has a good clinical e cacy and, after CDA with Pretic-I, the ROM of the cervical spine and FSU can be well maintained. At the same time, the incidence of postoperative complications in the Pretic-I group was also acceptable, and even showed some advantages in the high-grade HO. Generally, we believe that Pretic-I could be one safe and effective alternative to cervical disc prosthesis in the future. Tables Table-1

. The Inclusion and Exclusion Criteria
Inclusion Criteria Exclusion Criteria (1) The patient is over 18 years old; (2) The radiological results and clinical symptoms were consistent with the diagnosis of single-segment disc degeneration; (3) Soft disc herniation; (4) The degeneration located in C3-C7; (1) There're spinal protopathy, such as tumor, deformity, and infection; (2) Serious osteoporosis; (3) Rheumatoid arthritis; (4) Ankylosing Spondylitis; (5) Adjacent segment degeneration; (6) Severe narrowing of the degenerative space; (7) Ossi cation of posterior longitudinal ligaments; (8) The patient's mental state was unstable and he could not cooperate with the follow-up; The Images of Pretic-I Cervical Disc Prosthesis and Intra-operative Implantation Figure-  Radiological Results of Typical Case with Pretic-I Figure-2 showed the radiological results of one typical case with Pretic-I pre-operatively and 1 week, 3 months, 12 months, 60 months post-operatively. Figure-2a to -2c, cervical sagittal ROM condition at pre-operative stage. Figure-2d to -2f, cervical sagittal ROM condition at 1 week post-operatively. Figure-2g to -2i, cervical sagittal ROM condition at 3 months post-operatively. Figure-2j to -2l, cervical sagittal ROM condition at 12 months post-operatively. Figure-2m to -2o, cervical sagittal ROM condition at the nal follow-up.