General Session: Biomechanics
Presented by: S. Khayatzadeh - View Audio/Video Presentation (Members Only)
S. Khayatzadeh(1), L. Voronov(1,2), J. Goodsitt(1), Z. Smith(3), A. Germanwala(1,4), G. Carandang(1), R. Havey(1,2), C. Lauryssen(5), C. Schätz(6), A. Ghanayem(2), A. Patwardhan(1,2)
(1) Edward Hines Jr. VA Hospital, Hines, IL, United States
(2) Loyola University Chicago, Orthopaedic Surgery and Rehabilitation, Maywood, IL, United States
(3) Northwestern University, Neurological Surgery, Chicago, IL, United States
(4) Loyola University Chicago, Neurological Surgery, Maywood, IL, United States
(5) Lauryssen Neurosurgical Spine Institute, Austin, TX, United States
(6) Orthopaedic Clinic, Markgroeningen, Markgröningen, Germany
Introduction: Forward position of the head relative to the shoulders, assessed radiographically using the horizontal offset distance between the head and the base of the cervical spine (C0-T1 SVA), is a measure of cervical sagittal imbalance. Clinical studies have suggested cervical sagittal imbalance as a risk factor for adjacent segment pathology. In a laboratory study presented at ISASS15, we showed the presence of non-physiologic SVA increases the mechanical burden on discs adjacent to cervical multilevel fusions. In the present study, we investigated the extent to which the use of total disc arthroplasty (TDA) would influence the motions at adjacent segments as compared to fusion in the presence of cervical sagittal malalignment.
Methods: Nine human cadaveric cervical spines (Occiput-T1; 34.6±12.1years) were tested: (1) intact; (2) after lateral mass screws and rods across C4-C5-C6 to simulate 2-level fusion; (3) after removing posterior rods and performing 2-level TDA at C4-C5 and C5-C6 using compressible disc prostheses (M6-C, Spinal Kinetics). Two types of tests were run on each specimen, starting with the intact state: (i) Range of Motion (ROM) test- measured segmental motions across C4-C5-C6 under ±2.0Nm flexion-extension moments; and (ii) Increasing SVA test- quantified segmental angular compensations necessitated by progressively increasing anterior head-offset (C0-T1 SVA) using a previously reported experimental setup. These tests were repeated after 2-level fusion and after 2-level TDA at C4-C5-C6. SVA test data were analyzed to calculate motions across the implanted (C4-C5-C6) and adjacent (C3-C4, C6-C7) segments for the highest common value of C0-T1 SVA across all specimens. Contributions of adjacent segments towards C3-C7 angular motion were compared between intact, after C4-C6 fusion and after C4-C6 TDA. Two independent surgeon-experts did blind assessment of TDR implantations.
Results: ROM Test: Posterior fixation across C4-C5-C6 yielded excellent immobilization with < 2.0º motion in flexion-extension at C4-C5 and C5-C6. TDA resulted in significantly greater segmental motion than fusion (p< 0.05), with post-TDA C5-C6 motion significantly more than at C4-C5 (11.1° vs. 7.3°, p< 0.05) (Fig. 1A). SVA Test: With increasing C0-T1 SVA, while maintaing horizontal gaze, the C3-C7 segments flexed while C0-C2 segments extended. The contributions of adjacent C3-C4 and C6-C7 segments towards C3-C7 angular motion increased significantly after C4-C5-C6 fusion compared to intact (C3-C4: 56% vs. 9.2%, p< 0.05; C6-C7: 37.5% vs. 19%, p< 0.05) (Fig. 1B). TDA across C4-C5-C6 significantly reduced compensatory motion of adjacent segments compared to fusion (p< 0.05); restoring the post-TDA C6-C7 motion to the intact level (p=0.7). The post-TDA C3-C4 motion, while significantly less than the post-fusion motion (p< 0.05), remained higher than the intact level (p< 0.05).
Conclusions: The results demonstrated that segments above and below 2-level fusion are subjected to increased motion compensation in the presence of sagittal malalignment (excessive SVA). Disc prostheses have the potential to mitigate these effects. Therefore, restoring adequate sagittal alignment may be particularly important when planning multilevel cervical fusions.