Anatomy Study On Parameters For Cervical Artificial Disc Replacement

BackgroundCervical intervertebral disks, the main supporting and associating structures located between every two verteberae, bearing gravitation and redistributing the live load, perform an important function of motion retainer and shock absorber for the cervical spine. Degeneration of discs plays a key role in the pathogenesis of cervical syndrome. It influences people's health seriously. Concerns have been aroused extensively about the related theoretical and clinical research.In 1928, Stookey put forward the concept of cervical spondylosis. He believed that cervical spondylosis was one of the most frequent causes of cervical cord dysfunction. Then in 1956, Clarke and Robinson pointed out the pathological change of cervical spondylosis systematically. They confirmed that spondylosis was caused by the compressed cervical spinal cord with pathological changes and functional impairments due to tissue hyperplasia caused by the degenerated discs. Robinson and Smith introduced the operation of anterior discectomy and fusion with implanted bone graft in 1950s. From then on, anterior cervical decompression and fusion (ACDF) has been the preferred treatment for cervical spondylosis employed commonly in the domain of spine surgery, especially for cervical spondylotic myelopathy (CSM). Single or multiple fusion, however, always has effects on thecervical spine kinetics. Both the biomechanical and physiologic effects become increasingly more stressful on the adjacent functional spine unit (FSU). They may include: alteration in the axis of rotation, altered load sharing, loss of cushioning, hyper- or hypomobility of the FSU, declining performance of the nutritional supply system, and accelerated degeneration of the adjacent segments. Recently, longer-term outcome data suggests that there are significant radiographic and clinical consequences associated with fusion. Hilibrand et al. identified symptomatic adjacent-segment disease occurring for patients in their study group, with two-thirds of those patients requiring reoperation. Goffin et al. identified a 92% rate of adjacent-level radiological degeneration after fusion over a mean of 8.6 years.The concept of accelerated degeneration of adjacent disc levels as a consequence of increased stress caused by interbody fusion of the cervical spine has been widely accepted. Therefore, reconstruction of a failed intervertebral disc with a functional disc prosthesis should offer the same benefits as fusion while simultaneously providing motion and thereby protecting the adjacent level discs from the abnormal stresses associated with fusion. Functional disc prosthesis has been sought since the 1950s. Although there were a few early attempts, because of far more sophisticated motion mode and biomechanical environment, cervical disc replacement did not become a viable therapy until the late 1980s. Nowadays, several kinds of cervical disc prosthesis, such as Bryan cervical disc system, have been employed more frequently. But Cummins identified 2 failed cases of artificial cervical disc replacement due to mismatched prosthesis. In Chinese, Wang Yan et al. implanted Bryan disk prosthesis in Asian group firstly and then accumulated a few of successful cases, whereas, whether those cervical disc prosthesis match the cervical intervertebral disks of Asian or not is still in the midst because those prosthesis are designed originally for the Occidental.Some applied anatomy researches are employed in this study in order to provide related anatomical parameters for cervical artificial disk replacement in Chinese, or at least in southern groups of Chinese.Objectives(1) To achieve morphological parameters of cervical intervertebral discs, such as the height of disc space, disc space angle and disc-facet angle, by measuring adult volunteers' cervical lateral projection and to investigate whether the prosthesis designed according to the parameters of the Occidental fit the Asian's or not.(2) Anatomical measurement was performed on human cadaveric cervical spines to determine related anatomical parameters such as endplate thickness. Provide some available parameters of native human body for cervical disc replacement and then compare the data with other literature.(3) To investigate the change of parameters after implantation by embedding a Bryan cervical disc prosthesis and try to provide theoretical basis of cervical disc replacement.Methods and Materials(1) Sixty adult volunteers (range 19-45years)were random selected to take cervical X-ray photos (lateral view) in neutral, flexing and extending position. Photos were excluded from the study if: CD any cervical vertebrae were missing, (2) obvious morphological abnormal, (3) undergone surgical trauma or tumor, which would render measurement inaccurate. This left a sample of 43 volunteers, 24 males (average 31.04, range 22-43 years ), 19 females (average 30.16, range 23-39 years ), 215 discs. Then, given computer-assisted measurement, the anterior, middle, and posterior heights of intervertebral space in neutral, flexing and extending position were determined. In the same way, we achieved the disc-facet angle of C3-C7 and disk space angle of C2-3, C3.4, C4.5, C5-6 and C6.7. Statistics analyses were carry out atthe end.(2) Twelve fresh craniocervical specimens (8 males and 4 females) with a mean age of 35.6 years (range 24-51 years) were employed to take cervical X-ray photos (lateral view) in neutral. Photos were excluded from the study if: ? any cervical vertebrae were missing,? obvious morphological abnormal, (3) undergone surgical trauma or tumor, which would render measurement inaccurate. 1 specimen was excluded because of cervical trauma. Another one was preserved for the implanting. So there was 10 (6 males, 4 females) specimens enrolled. Given computer-assisted measurement, the anterior, middle, and posterior heights of intervertebral space were determined. Then the endplate thicknesses were measured with a sliding caliper.(3) A fresh craniocervical specimen (male, 38, atlantooccipital joint remained) was employed to investigate the change of disc space angle, disc-facet angle and the height of anterior column after the implanting of cervical disc prosthesis.Results(1) There were significant differences between the sexes, segments, locations and positions (P = 0.000). Anterior parts of discs play key roles in the intervertebral motion. When curve the spine, both anterior and middle parts of discs were depressed. Disc-facet angles of C3-C7 were 128.70 ±0.71° ,127.36 + 0.21° ,130.43 + 0.87 ° ,124.78+0.39° and 115.82+0.52° . There was a significant difference between the vertebral bodies. On the contrary, no significant difference was found between genders.Disc space angles of C2.3,C3-4,C4-5,C5-6,C6-7 were 9.44 + 2.23° ,8.48 + 2.03 ° ,7.15 + 2.27° ,6.42+1.90° ,3.86 + 1.71° . No significant difference was found between genders. On the other hand, there were significant differences between vertebrae.(2) Differences for the thickness of endplates were not significant between sexes(P = 0.567) and superior or inferior endplates (P = 0.097), but absolutely significant between central and surrounding zones (P = 0.000). Meanwhile, obvious interaction was found between segments and locations (P = 0.036).(3) According to the CT measured data, A piece of Bryan disc prosthesis was implanted into the male craniocervical specimen for three times, each time into a different disk, C3₄, C4.5 and C5.6. From preoperative to postoperative data, the space disc angle diminished, the average is 1.47° . On the contrary, the disc-facet angle and the height of anterior column increased averaging out at 5.74° and 2.07mm respectively.Conclusions(1) From flexion to extention, range of invertebral motion accounts a great proportion for the anterior part. It should be the underlying premise remaining the height and motion range of anterior part in the designing, manufacturing and clinical applying of artificial disc prosthesis. In the samples studied, some data are significant different from their counterparts in the literature according to the Occidental. So it is not assured that the cervical disc prosthesis designed originally for the western equally fit the Asian.(2) Endplate improves the crushing strength of cervical vertebrae. Whether reserve the endplate or not, prosthesis and its host should have a contact surface big enough so that the prosthesis can get itself instant and permanent fixed. Furthermore, it conduces to prevent the prosthesis subsidence or excursion in the long run.(3) It has been more than 50 years in the history of cervical artificial discs. But there still exist some restricting factors holding its applying back. It is out of question that motorization is one of the future trends of the internal fixation in the territory of spine surgery. However, further studies both in clinic and in laboratory should be required for the applying of cervical artificial disc replacement extensively in Asian.