Six fresh-frozen human cervical spines (C2–C7) with a mean age of 80 years (range 71–90 years) were kept at − 21 °C in triple-sealed bags. The spines were thawed overnight at 6 °C prior to the biomechanical testing. Computed tomography (CT) scans performed in all specimens showed age-related degeneration, however, no fractures, no ossifications of the posterior longitudinal ligament (OPLL), no bridging spondylophytes or syndesmophytes and no osseous ankylosis were revealed. Soft tissue was removed, leaving the ligaments, capsules, thecal sac and supporting structures intact. To fix the specimens firmly in place on the simulator, the cranial and caudal vertebrae (C2 and C7) were embedded in a casting resin (Ureol FC 53, Vantico GmbH, Wehr, Germany) in the test fixtures. Segmental motion was not restricted in any way, and the C4–C5 disk was oriented in the horizontal plane. The six specimens were consecutively tested under the following segment conditions: native, after unilateral hemilaminectomy with bilateral decompression without/with fixation (BDZ/BDF), unilateral hemilaminectomy with bilateral decompression and unilateral foraminotomy without/with fixation (UFZ/UFF), unilateral hemilaminectomy with bilateral decompression and bilateral foraminotomy without/with fixation (BFZ/BFF), and laminectomy without/with fixation (LAZ/LAF) (Fig. 1).
For lateral mass fixation, 3.5-mm screws (titanium) with a length of 16 or 18 mm were used (Ennovate Cervical System, Aesculap AG, Tuttlingen, Germany). The instrumentation was performed from C3–C6. To ensure a standardized instrumentation irrespective of the amount of bone removal, an embedding fixture was used, which kept the initially embedded native segment condition constant. The operations were performed by a board-certified neurosurgeon with ample experience in spine surgery (IF).
The surgical procedure to perform bilateral decompression via hemilaminectomies was as follows (Fig. 1). First, the medial borders of the facet joints were identified, allowing a hemilaminectomy of C4 and C5 using a spherical diamond drill and a Kerrison rongeur. Care was taken to preserve the integrity of the facet joints. The next step was the removal of the base of the spinous processes with a 5-mm spherical diamond drill, beginning at the medial edge of the hemilaminectomies and ending near the contralateral medial part of the facet joints, thereby thinning the inner contralateral hemilaminae. Then, bilateral undercutting of the adjacent intact laminae was performed. The yellow ligament was thinned out dorsolaterally in the vicinity of the nerve roots. With a sharp hook, the lateral ligament edge on the side of the approach was elevated. Lastly, the ligament was removed with a Kerrison rongeur until the very first segment of the contralateral dorsal nerve roots was exposed. For the following segment conditions UFZ and BFZ, a foraminotomy was added uni- or bilaterally. Finally, for the segment condition LAZ, laminectomy of C4 and C5 was performed using a Kerrison Rongeur and a diamond drill to remove the contralateral hemilaminae and spinous processes of the involved segments.
For stabilization, the facet joints and lateral masses were identified, and screws were placed bilaterally with the technique described by Magerl et al. [12]. Rods of appropriate size were selected and bent to match the contour of the lateral masses and secured by set screws (segment conditions BDF, UFF, BFF, LAF). Starting with the native condition, measurements were performed after each subsequent step of bony removal, without and with stabilization. The sequence of measurements is depicted in Fig. 2.
The test method complies with the testing criteria for spinal implants [13]. The specimens were loaded with ± 2.5 Nm for flexion/extension, lateral bending and axial rotation at room temperature on a spinal simulator based on the principles of Crawford et al. [14]. The load cycle consists of a displacement-controlled loading phase with velocity of 3°/s due to the laxity of the specimen followed by a load-controlled phase starting at 2% below the maximum load (2.5 Nm) which was kept constant for 1 s once the maximum load (2.5 Nm) was reached. The kinematics, i.e., the six components of motion according to Panjabi [15], were measured with a non-contacting three-dimensional ultrasonic motion analysis system (Zebris, Isny, Germany) across the entire non-embedded, free length of the specimen. The characteristic parameters range of motion (ROM) and neutral zone (NZ) were analyzed from the hysteresis curves of the third loading cycle, according to Wilke et al. [13].
For statistical analyses on ROM and NZ to determine differences between the specific segment conditions, repeated-measures analysis of variance followed by a post hoc test (Least Significance Difference Test) was performed. Prior to analysis, the normal distribution of the data (p-p plots) and the homogeneity of variance (Levene test) were verified. A significance level of p = 0.05 was defined, and all statistical analyses were performed using Statistica 13 (TIBCO Software, Inc.).