Cervical kinematics after fusion and bryan disc arthroplasty.
BACKGROUND
Disc arthroplasty has been shown to provide short-term clinical results that are comparable with those attained with traditional anterior cervical discectomy and fusion. One proposed benefit of arthroplasty is the ability to prevent or delay adjacent level operations by retaining motion at the target level and eliminating abnormal adjacent activity. This paper compares motion parameters for single-level anterior cervical discectomy and fusion and disc replacement patients at the index level and adjacent segments.


METHODS
Radiographic data from patients enrolled in a prospective, randomized clinical trial were selected for kinematic assessment of cervical motion. All patients received either a single-level fusion with allograft and anterior cervical plate (Atlantis anterior cervical plate, n=13) or a single-level artificial cervical disc (Bryan Cervical Disc prosthesis, n=9) at either C5/C6 or C6/C7. Flexion, extension, and neutral lateral radiographs were obtained preoperatively, immediately postoperatively, and at regular intervals up to 24-month time points. Cervical vertebral bodies were tracked on the digital radiographs using quantitative motion analysis software (QMA, Medical Metrics) to calculate the functional spinal unit motion parameters including range of motion (ROM), translation, and center of rotation. If visible, the functional spinal unit parameters were obtained at the operative level, and also the level above and the level below.


RESULTS
As expected, significantly (P<0.006 at 3, 6, 12, and 24 mo) more flexion/extension motion was retained in the disc replacement group than the plated group at the index level. The disc replacement group retained an average of 6.7 degrees at 24 months. In contrast, the average ROM in the fusion group was 2.0 degrees at the 3-month follow-up and gradually decreased to 0.6 degrees at 24 months. The flexion/extension ROM both above and below the operative level was not statistically different for the disc-replaced and fusion patients, however, mobility increased for both groups over time. The anterior/posterior translation that occurs with flexion/extension motion remained unchanged for the disc replacement group at the level above the target disc preoperatively and postoperatively. In contrast, the translation increased for the level above the fusion. At the 6-month follow-up, the increase in translation was significantly greater for patients that were fused (P<0.02) than for patients that received a disc replacement. This change was not significant at 12 months.


CONCLUSIONS
Previous studies have shown the Bryan disc to maintain mobility at the level of the prosthesis. The long-term clinical benefit of maintenance of motion is postulated to be the ability to delay or avoid adjacent level operations. This study reveals that there is no difference in flexion/extension ROM at the level above and below either a fusion or Bryan arthroplasty. There is, however, an increase in anterior/posterior translation at the cephalad adjacent level in patients with arthrodesis while the Bryan arthroplasty retains normal translation for the same amount of flexion/extension at the adjacent level.


CONCLUSIONS
The Bryan disc may delay adjacent level degeneration by preserving preoperative kinematics at adjacent levels.
Sasso
Rick C
RC
Indiana Spine Group and Indiana University School of Medicine, Indianapolis, IN 46260, USA. rsasso@indianaspinegroup.com
Best
Natalie M
NM
D000328:Adult; D019643:Arthroplasty, Replacement / Q000295:instrumentation* / Q000379:methods / Q000706:statistics & numerical data; D001696:Biomechanical Phenomena; D001860:Bone Plates; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology / Q000601:surgery*; D017586:Diskectomy / Q000295:instrumentation* / Q000379:methods / Q000706:statistics & numerical data; D005260:Female; D019416:Head Movements / Q000502:physiology; D006801:Humans; D007091:Image Processing, Computer-Assisted; D016268:Internal Fixators; D007403:Intervertebral Disc / Q000293:injuries / Q000503:physiopathology / Q000601:surgery; D007405:Intervertebral Disc Displacement / Q000601:surgery*; D007595:Joint Prosthesis / Q000706:statistics & numerical data*; D008297:Male; D008875:Middle Aged; D011183:Postoperative Complications / Q000175:diagnosis / Q000503:physiopathology; D019919:Prosthesis Implantation / Q000295:instrumentation / Q000379:methods / Q000706:statistics & numerical data; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology; D020127:Recovery of Function / Q000502:physiology; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods / Q000706:statistics & numerical data; D016896:Treatment Outcome