Cervical disc arthroplasty is an alternative surgery to anterior cervical decompression and fusion for cervical disc degeneration. Cervical disc arthroplasty was developed to preserve range of motion (ROM) and to prevent the accelerated degeneration of adjacent segments. Goffin et al1 first reported the clinical use of the Bryan cervical disc prosthesis in 2002. Cervical disc arthroplasty with the Bryan disc has presented good clinical and radiographic results at long-term follow-up.2,3 Both the Bryan cervical disc prosthesis and ProDisc-C prosthesis are employed in our center. This study aimed to evaluate the 5-year radiographic outcome of cervical disc arthroplasty using the ProDisc-C prosthesis.
Patient inclusion criteria were single-level spondylotic myelopathy or cervical radiculopathy, which had not responded to non-surgical treatment. Exclusion criteria were previous cervical spine surgery, marked cervical instability, severe spondylosis at the level to be treated, ossification of the posterior longitudinal ligament, cervical spinal injury and active infection.
ProDisc-C cervical disc arthroplasty was first performed in our center in June 2006. Up to May 2013, 26 patients who had single-level ProDisc-C arthroplasty had completed around 5 years' follow-up. All the surgeries were performed by the same surgeon. The mean follow-up period was 63 months (56-76 months). The study group consisted of 16 men and 10 women, with age range 27-62 years (mean 44 years). Eight of the 26 patients presented with radiculopathy, 15 with myelopathy, and 3 with combined myelopathy and radiculopathy. The levels of surgery included C3/4 (3 levels), C4/5 (4 levels), C5/6 (16 levels) and C6/7 (3 levels).
Radiographic evaluation included static and dynamic flexion-extension lateral images. ROM at baseline and final follow-up were measured using flexion-extension lateral X-rays according to White's method.4
Heterotopic ossification (HO), according to McAfee's classification,5 was evaluated by lateral X-rays. HO was classified into grade I to grade IV.
Radiographic evidence of adjacent segment degeneration (ASD) included the presence of any of the following parameters:6,7 (1) new anterior or enlarged osteophyte formation; (2) narrowing of the disc space by ≥10%; or (3) calcification of the anterior longitudinal ligament (CALL).
ASD and HO were assessed by two independent orthopedic specialists. Disagreement was resolved by discussion or, if no consensus could be reached, by consultation with a third independent orthopedic specialist.
The results were expressed as mean ± standard deviation (SD) and the statistical analysis was conducted using paired t tests with SPSS version 13.0 software (SPSS Inc., USA). A P value <0.05 was considered statistically significant.
X-ray examination revealed the ROMs at the operated level to be 9.3°±3.7° at baseline and 7.3°±3.5° at final follow-up, with a significant difference (P <0.05).
At final follow-up, 17 of 26 levels (65.4%) had developed HO. According to McAfee's classification, three levels were classified as grade II, 13 levels as grade III and one as grade IV (ROM≤2°). The prevalence of spontaneous fusion was 4% (1/26). Eleven of 17 HO were detected on the anterior margin of the vertebral body, and 4 on the posterior margin. Both anterior and posterior HOs developed in 2 patients (Figure 1).
Forty nine adjacent segments were evaluated by lateral X-rays, and 18 (36.7%) were found to have developed ASD. Three adjacent segments were excluded because of a shoulder shadow. There were 9 cases of new anterior (n=8) or enlarged osteophyte formation (n=1), 4 cases of CALL (including 3 with progression of CALL and 1 case of new CALL), 2 cases of new anterior or enlarged osteophyte formation and CALL, 2 cases of narrowing of the disc space by ≥10%, and 1 case of new osteophyte formation and narrowing of the disc space (Figure 2).
HO after ProDisc-C arthroplasty
HO is common after cervical disc arthroplasty. McAfee et al5 classified HO into grades I to IV, and only grade IV HO segments lost motion. Mehren et al8 reported the HO rate was 65.2% at 1-year follow-up after ProDisc-C arthroplasty. Yi et al9 followed up 28 patients with ProDisc-C arthroplasty and 81 patients with Bryan disc arthroplasty. The HO rate of the ProDisc-C group was 71.4% at 20-month follow-up, which was higher than the Bryan disc group (21.0%). The mechanism for the formation of HO is not clear. Preventive factors for HO include using nonsteroidal anti-inflammatory drugs, sufficient washing after milling and so on.9–11
In our center, all the patients of ProDisc-C arthroplasty were prescribed nonsteroidal anti-inflammatory drugs for 2 weeks to prevent the formation of HO. In this study, the HO rate was 65.4% at 5-year follow-up. Our previous study reported the HO rate was about 40% after Bryan disc arthroplasty at long-term follow-up.12 Thus, the ProDisc-C prosthesis seems to have a higher rate of HO than the Bryan disc prosthesis.
Most of the HOs were detected on the anterior margin of the vertebral body. Keel cuts were made on the midline of the vertebral body to fit the ProDisc-C prosthesis, which may lead to the formation of HO on the anterior margin of the vertebral body. The motion patterns of the ProDisc-C prosthesis and Bryan disc prosthesis are completely different. The ProDisc-C prosthesis is a small radius ball and socket device with a fixed center of rotation. This device can only simulate part of the motion of the human cervical disc, which may contribute to the formation of HO.
Several types of cervical disc prosthesis are used in China. They are different in the design, biomechanical performance and surgical procedures.9,13,14 Further biomechanical analysis and clinical follow-up are needed to evaluate the performances of different types of prosthesis.
ROM after ProDisc-C arthroplasty
Cervical disc arthroplasty was designed to preserve ROM and to prevent increased degeneration of adjacent segments. Murrey et al15 and Zigler et al16 reported the follow-up of 103 patients with single-level ProDisc-C arthroplasty, and the average ROM was 8.5° at baseline, 9.4° at 2-year follow-up, and 8.1° at 5-year follow-up. In our study, the ROM was 9.3° at baseline and decreased to 7.3° at final follow-up.
We divided the patients into HO group (17 cases) and non-HO group (9 cases). The ROM of HO group was 6.4° and the ROM of non-HO group was 8.9°. The loss of motion may be related to the high rate of HO formation. According to the definition of HO, grade III and grade IV HO affect the ROM. One case with grade IV HO lost motion (ROM ≤2°), while 13 cases with grade III HO maintained ROM. However, long-term follow-up is still needed to evaluate the progression of grade III HO.
ASD after cervical disc arthroplasty
ASD was defined as the asymptomatic new radiographic evidence of degeneration at adjacent segments. Symptomatic degenerations at the adjacent segments were defined as adjacent segment disease. Robertson et al6 compared the incidence of ASD between 74 patients treated with Bryan disc arthroplasty and 158 treated with the Affinity Anterior Cervical Cage system at 2-year follow-up; the incidence of ASD detected by X-ray examination was significantly higher in the Cage series than in the artificial disc group. Burkus et al17 reported the long-term outcomes of cervical disc arthroplasty from a prospective randomized controlled clinical trial. Rates of surgery at adjacent segments tended to be lower in the cervical arthroplasty group compared with those in the cervical fusion group. Delamarter et al18 reported 5-year reoperation rates of ProDisc-C arthroplasty and cervical fusion from a prospective randomized clinical trial. The cervical fusion group had a statistically significantly higher rate of reoperation at the adjacent segments than the cervical disc arthroplasty group.
In this study, 49 adjacent segments were evaluated and 18 (36.7%) segments developed ASD. The most common types of ASD were new anterior or enlarged osteophyte formation, and included 9 cases. Sun et al12 reported the rate of ASD after single-level cervical fusion was about 60% at 5 years' follow-up. The rate of ASD after ProDisc-C arthroplasty in this study was lower at 36.7%. ProDisc-C arthroplasty may avoid the acceleration of ASD by preserving ROM.
ProDisc-C arthroplasty had acceptable radiographic results at 5-year follow-up, with preservation of ROM. However, more than 60% of patients developed HO. Thus, long-term follow-up studies with a large cohort are needed to evaluate the effects of HO after ProDisc-C arthroplasty.
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