The Feasibility of Optimal Surgical Result Prediction according to the Center of Rotation Shift after Multilevel Cervical Total Disc Replacement

Study Design: Retrospective design/spine clinic and tertiary referral hospital.Purpose: This study investigated the association between shifted location of center of rotation (COR) and subsequent surgical outcomes and identified radiological parameters that corresponded to COR change after multilevel cervical total disc replacement (MCTDR).Overview of Literature: Cervical total disc replacement (TDR) maintains normal cervical kinematics after surgery. However, there is a paucity of literature analyzing the relationship between radiological shifts of COR and subsequent clinical success, especially when it comes to the issue of MCTDR switch.Methods: This study included 24 consecutive patients treated with MCTDR following the diagnosis of multilevel cervical disc herniation or stenosis. Numeric rating scale, range of motion (ROM) at both C2–7 segment and TDR implanted levels, and location of COR at TDR level were evaluated pre- and post-MCTDR. These parameters were compared between patients who experienced successful and unsuccessful pain relief.Results: The inherent CORs relatively at ventrocranial coordinates demonstrated significant migrations to dorsocaudal location, more prominent shifts for the successful group, after MCTDR switch. The unsuccessful group showed markedly reduced C2–7 ROM and reduced angular improvement at C2–7 segment and MCTDR level compared with the successful group. Postoperative C2–7 ROM was related to postoperative COR along the x-axis.Conclusions: Aside from ROM preservation at both C2–7 and TDR levels, COR restoration from ventrocranial location close to normal coordinates by posterior and inferior shifts was marked as a clinical success after MCTDR. The COR position along the x-axis after MCTDR was an important factor to determine maintenance of C2–7 ROM.

Analysis of adjacent-segment cervical kinematics: the role of construct length and the dorsal ligamentous complex

OBJECTIVELateral mass fixation stabilizes the cervical spine while causing minimal morbidity and resulting in high fusion rates. Still, with 2 years of follow-up, approximately 6% of patients who have undergone posterior cervical fusion have worsening kyphosis or symptomatic adjacent-segment disease. Based on the length of the construct, the question of whether to extend the fixation system to undisrupted levels has not been answered for the cervical spine. The authors conducted a study to quantify the role of construct length and the terminal dorsal ligamentous complex in the adjacent-segment kinematics of the subaxial cervical spine.METHODSIn vitro flexibility testing was performed using 6 human cadaveric specimens (C2–T8), with the upper thoracic rib cage and osseous and ligamentous integrity intact. An industrial robot was used to apply pure moments and to measure segmental motion at each level. The authors tested the intact state, followed by 9 postsurgical permutations of laminectomy and lateral mass fixation spanning C2 to C7.RESULTSConstructs spanning a single level exerted no significant effects on immediate adjacent-segment motion. The addition of a second immobilized segment, however, created significant changes in flexion-extension range of motion at the supradjacent level (+164%). Regardless of construct length, resection of the terminal dorsal ligaments did not greatly affect adjacent-level motion except at C2–3 and C7–T1 (increasing by +794% and +607%, respectively).CONCLUSIONSDorsal ligamentous support was found to contribute significant stability to the C2–3 and C7–T1 segments only. Construct length was found to play a significant role when fixating two or more segments. The addition of a fused segment to support an undisrupted cervical level is not suggested by the present data, except potentially at C2–3 and C7–T1. The study findings emphasize the importance of the C2–3 segment and its dorsal support.