Restoration of Spinal Alignment and Disk Mechanics following Polyetheretherketone Wafer Kyphoplasty with StaXx FX

Specimens and Experimental Setup

Eight fresh-frozen human thoracic-lumbar specimens (age range, 67.8 ± 5.7 years; 4 male, 4 female), each consisting of 5 vertebrae (T10-L2, T12-L4, and L1-L5), were used. All specimens were radiographically screened to exclude pre-existing osteoporotic fractures within the tested levels, severe disk space collapse, bridging osteophytes, and vertebral neoplastic disease. Bone mineral attenuation of the index and adjacent levels was determined by using dual energy x-ray absorptiometry (Lunar Prodigy/DPX Series; GE Healthcare, Waukesha, Wisconsin). The specimens were thawed at room temperature (20°C) 24 hours before testing. The paravertebral muscles were dissected, while keeping the disks, ligaments, and posterior bony structures intact. The most cephalad and caudal vertebrae of each specimen were anchored in cups by using bone cement and pins.

The specimen was fixed to the testing apparatus at the caudal end and was free to move at the cephalad end. A moment was applied by controlling the flow of water into bags attached to loading arms fixed to the cephalad vertebra. A 6-axis load cell (model MC3A-6-1000; AMTI, Newton, Massachusetts) was placed under the specimen to measure the applied loads and moments. The apparatus allowed for continuous cycling of the specimen between specified maximal moment end points in flexion and extension. The load-displacement data were collected until 2 reproducible load-displacement loops were obtained. This required a maximum of 3 loading cycles. The collected data from the last cycle were used for analysis.

The motion of the cephalad vertebra relative to the caudal vertebra was measured by using an optoelectronic motion measurement system (model 3020, Optotrak; Northern Digital, Waterloo, Ontario, Canada). In addition, biaxial angle sensors (model 902-45; Applied Geomechanics, Santa Cruz, California) were mounted on the cephalad and caudal vertebrae to allow real-time feedback for the optimization of the preload path. The spines were instrumented with pressure transducers (model 060S-1000; Precision Measurement, Ann Arbor, Michigan) in the nucleus pulposus of the disks above and below the target vertebra. The pressure transducers were calibrated before the testing of each specimen by using a pressure chamber. Load and pressure data were collected throughout the loading cycle at a rate of 5 Hz, resulting in approximately 200 data points from 0 to 6 Nm.

Compressive preload was applied to the specimens according to the follower load concept¹⁵ where the compressive preload is applied along a path that follows the curve of the spine. By applying a compressive load along the follower load path, the segmental bending moments and shear forces due to the preload application are minimized, allowing a multisegment thoracolumbar spine to support physiologic compressive preloads without constraining the sagittal plane motion.¹⁶ The preload was applied by using bilateral loading cables attached to the cup holding the cephalad vertebra. The cables passed freely through guides anchored to the vertebrae adjacent to the target vertebra (Fig 1). To avoid creation of stress risers, the cable guide mounting technique did not violate the cortices of the vertebral bodies adjacent to the target vertebra. No cable guides were mounted to the target vertebra. The loading cables were connected to loading actuators under the specimen. The cable guide mounts allowed anteroposterior adjustments of the follower load path. The alignment of the preload path was optimized by adjusting the cable guide locations to minimize changes in the sagittal alignment of the specimen when a compressive load of up to 400 N was applied. The cables were coated with radiopaque barium solution to be visible on x-ray images. A radiopaque ball (19.05 mm in diameter) was used as a calibration marker for the x-ray images.

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Fig 1.

Testing setup shown in schematic (A) and photo (B).

Experimental Protocol

Each specimen was tested under flexion-extension moments (±6 Nm) with a 400-N compressive preload intact, after EPF, after VCF, and after kyphoplasty with PEEK wafers. Pressure was recorded in the disks above and below the middle (target) vertebra (Fig 2 A).

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Fig 2.

Testing sequence shown on specimen 6. Intact specimen with pressure transducers inserted in the disks above and below the middle vertebra (A). Dashed circle shows the location of the void created to selectively fracture the upper endplate of the middle vertebra (B). Arrow indicates location of pressure transducer in cephalad disk: Note the location of the transducer within the vertebral body space after EPF and VCF (C and D) and restoration of the intact transducer location after StaXx FX (E).

Experimental Creation of Endplate and VCF

A previously described technique was used to selectively fracture only the upper endplate of the middle vertebra.¹⁴ Through a small opening on the anterior wall, a void was created selectively under the upper endplate and was extended to one-third of the vertebral body trabecular content; thereby creating a “stress-riser” under the target endplate (Fig 2B). The specimen was flexed to 5 Nm and compressed by using the loading cables until a fracture of the upper endplate (loss of continuity or development of endplate concavity) without significant anterior height loss was evident on fluoroscopy (Fig 2C). After creation of the EPF, the specimen was loaded in flexion-extension (±6 Nm) with a compressive preload of 400 N. Following the moment loading, the specimen was again flexed to 5 Nm and compressed until an anterior height loss of 20%–30% was seen radiographically (Fig 2D). After creation of the vertebral compression fracture, the specimen was again loaded in flexion-extension (±6 Nm) with a compressive preload of 400 N.

Reduction of the Vertebral Kyphotic Deformity Using PEEK Wafer Kyphoplasty

After vertebral fracture, the specimen remained under a physiologic compressive preload of 150 N, representing the compressive preload on the lumbar spine in the prone position.¹⁷ PEEK wafer kyphoplasty was performed by using the StaXx FX device following the manufacturer's instructions. In this procedure, PEEK wafers with a thickness of 1 mm, width of 8 mm, and a length of either 20, 25, or 30 mm were inserted through a parapedicular approach (posterolateral vertebral body). The wafers were progressively inserted in a manner that each newly introduced wafer pushed the previously implanted wafers superiorly until the desired fracture reduction was achieved (Fig 2E). The wafers were inserted and positioned such that maximal reduction of the endplate deformation without overcorrection was observed radiographically by using sagittal, frontal, and 2 oblique views. Bone cement was then injected around the wafer stack to stabilize the PEEK wafers and fractured vertebra. The cement was allowed to harden for 1 hour with the specimen at room temperature and under a 150-N compressive load. The specimen was then retested in flexion-extension with a compressive preload of 400 N.

Data Analysis

CT scans (section increment, 0.6 mm; Sensation Cardiac 64, Siemens, Malvern, Pennsylvania) were taken of each specimen intact and following implant insertion to evaluate the placement of the PEEK wafers within the index vertebra. Endplate deformity, anterior vertebral height, vertebral kyphosis, and segmental kyphosis measurements were performed at the index level by using computer software (Image-Pro Plus, version 4.1.0; MediaCybernetics, Bethesda, Maryland) on digital fluoroscopy images taken with the specimen in neutral posture under 400-N compressive preload by 2 independent observers. The first observer (P.P.T.) is a neurosurgeon and fellowship-trained spine surgeon. The second observer (S.M.R.) is a PhD biomedical engineer with 11 years of experience in spine biomechanics. Pearson coefficients were calculated to determine interobserver reliability. The endplate deformity was measured as the vertebral height at the point of maximal endplate depression. To ensure that the height measurement was taken at the same location for each testing condition, the location of the maximal endplate deformation was first determined in the EPF condition as the point of the lowest EPF concavity. The distance from that location to the posterior rim along the inferior endplate was then measured and used to determine the location for the vertebral height measurements in all other testing conditions. Disk pressure was normalized so that values in neutral position under 400-N preload were taken to 0 to compensate for thermal drifting of sensors throughout testing. As a result, the change in pressure from neutral to full flexion for each testing condition was used for analysis. The data were analyzed by using repeated measures analysis of variance with Bonferroni correction for 4 comparisons with the level of significance set at P = 0.05. The SPSS statistical package, version 15.0 was used (SPSS, Chicago, Illinois).