Pedicle screw anchorage of carbon fiber-reinforced PEEK screws under cyclic loading.

PURPOSE: Pedicle screw loosening is a common and significant complication after posterior spinal instrumentation, particularly in osteoporosis. Radiolucent carbon fiber-reinforced polyetheretherketone (CF/PEEK) pedicle screws have been developed recently to overcome drawbacks of conventional metallic screws, such as metal-induced imaging artifacts and interference with postoperative radiotherapy. Beyond radiolucency, CF/PEEK may also be advantageous over standard titanium in terms of pedicle screw loosening due to its unique material properties. However, screw anchorage and loosening of CF/PEEK pedicle screws have not been evaluated yet. The aim of this biomechanical study therefore was to evaluate whether the use of this alternative nonmetallic pedicle screw material affects screw loosening. The hypotheses tested were that (1) nonmetallic CF/PEEK pedicle screws resist an equal or higher number of load cycles until loosening than standard titanium screws and that (2) PMMA cement augmentation further increases the number of load cycles until loosening of CF/PEEK screws. METHODS: In the first part of the study, left and right pedicles of ten cadaveric lumbar vertebrae (BMD 70.8 mg/cm3 ± 14.5) were randomly instrumented with either CF/PEEK or standard titanium pedicle screws. In the second part, left and right pedicles of ten vertebrae (BMD 56.3 mg/cm3 ± 15.8) were randomly instrumented with either PMMA-augmented or nonaugmented CF/PEEK pedicle screws. Each pedicle screw was subjected to cyclic cranio-caudal loading (initial load ranging from - 50 N to + 50 N) with stepwise increasing compressive loads (5 N every 100 cycles) until loosening or a maximum of 10,000 cycles. Angular screw motion ("screw toggling") within the vertebra was measured with a 3D motion analysis system every 100 cycles and by stress fluoroscopy every 500 cycles. RESULTS: The nonmetallic CF/PEEK pedicle screws resisted a similar number of load cycles until loosening as the contralateral standard titanium screws (3701 ± 1228 vs. 3751 ± 1614 load cycles, p = 0.89). PMMA cement augmentation of CF/PEEK pedicle screws furthermore significantly increased the mean number of load cycles until loosening by 1.63-fold (5100 ± 1933 in augmented vs. 3130 ± 2132 in nonaugmented CF/PEEK screws, p = 0.015). In addition, angular screw motion assessed by stress fluoroscopy was significantly smaller in augmented than in nonaugmented CF/PEEK screws before as well as after failure. CONCLUSIONS: Using nonmetallic CF/PEEK instead of standard titanium as pedicle screw material did not affect screw loosening in the chosen test setup, whereas cement augmentation enhanced screw anchorage of CF/PEEK screws. While comparable to titanium screws in terms of screw loosening, radiolucent CF/PEEK pedicle screws offer the significant advantage of not interfering with postoperative imaging and radiotherapy. These slides can be retrieved under Electronic Supplementary Material.

Three-dimensional stabilization provided by the external spinal fixator compared to two internal fixation devices: a biomechanical in vitro flexibility study.

We performed an in vitro study to investigate the stabilization (i.e. motion reduction) provided by the external spinal fixator (ESF), and to compare the three configurations of the ESF with two internal fixation techniques. Six human cadaveric lumbar spine specimens (L3-S1) were subjected to multidirectional flexibility testing in six configurations: (1) intact, (2) ESF in neutral, (3) ESF in distraction, (4) ESF in compression, (5) translaminar facet screw fixation, and (6) internal transpedicular fixation. Both the ESF and the internal fixation systems stabilized the specimens from L4 to S1. In each testing configuration, pure bending moments of flexion-extension, bilateral axial rotation, and bilateral lateral bending were applied to the uppermost vertebra stepwise to a maximum of 10 Nm. The rigid body motion between the vertebrae was measured using an optoelectronic camera system, and custom software was used to calculate the intervertebral rotations. For each applied motion in all testing configurations, the total range of motion (ROM) of L4-S1 is reported. All three ESF configurations stabilized the spine significantly when compared to the intact specimen. The ESF in compression provided significantly more stabilization in flexion-extension than the two other ESF configurations, but no other significant differences were found between the three ESF modes. In flexion-extension the ESF stabilized the spine significantly when compared with the two internal fixation devices. Only in bilateral lateral bending was the ESF inferior to internal transpedicular fixation in providing stabilization. The results of the present study suggest that the ESF provides a high degree of stabilization for preoperative assessment of selected low back pain patients. Whether other non-mechanical factors affect the pain relief experienced by the patients remains unknown.

Is there a connection between the clinical response after an external fixation test or a subsequent lumbar fusion and the pre-test intervertebral kinematics?

STUDY DESIGN: Evaluation of a possible correlation of clinical symptoms of chronic low back pain (LBP) patients before and after application of external spinal fixation (ESF) with three-dimensional motion patterns. OBJECTIVE: To determine whether the intervertebral kinematics in chronic LBP patients correlate with pain relief after stabilization of the suspected painful segments. SUMMARY OF BACKGROUND DATA: Chronic LBP is a complex clinical entity with mechanical, biochemical, and psychosocial components. Although clinically controversial, ESF has been shown to reduce intervertebral motion, and thus, it provides a mechanism to investigate the mechanical aspect of LBP. METHODS: Thirty-four chronic LBP patients were tested with ESF of the suspected painful segments. The subjective pain relief experienced by the patients during the test and 2 years after spinal fusion surgery was determined, along with the Oswestry Disability Index. Before removal of ESF, the three-dimensional intervertebral kinematics of the painful segments (specifically range of motion, coupling patterns, and motion asymmetries) were analyzed with a precision optoelectronic camera system during active trunk motions. A series of linear correlations were performed between the clinical measures and the motion parameters. RESULTS: The subjective pain relief during the ESF test was nominally, but not significantly, associated with lateral bending asymmetry ( = 0.22) and "abnormal" axial rotation-lateral bending coupling ( = 0.13). The improvement in the Oswestry Disability Index both post-test and at 2 years after lumbar fusion surgery was significantly correlated with the extension range of motion ( = 0.049 and = 0.036, respectively) and the extension-to-flexion range of motion ratio ( = 0.035 and = 0.044, respectively). CONCLUSIONS: The "abnormal" motion patterns of chronic LBP patients did not correlate with subjective pain relief after ESF of the suspected symptomatic levels or with the midterm outcome of subsequent lumbar fusion surgery. The results suggest that preserved motion of the symptomatic segments before surgery is positively correlated with the clinical result of the subsequent spinal fusion in case of a positive ESF test result.

Three-dimensional motion patterns during active bending in patients with chronic low back pain.

STUDY DESIGN: The three-dimensional motion patterns of chronic low back pain (LBP) patients were studied during active motions. OBJECTIVE: To evaluate whether the motion patterns of coupling and asymmetry in chronic LBP patients differ from those in the normal population according to previous literature. SUMMARY OF BACKGROUND DATA: There exists little data that correlates vertebral motion with LBP symptoms despite extensive research. Several studies have suggested that patterns of motion are sensitive indicators of spinal pathology, including coupled motion patterns and asymmetry of motion. METHODS: In this study, 34 chronic LBP patients enrolled for an external fixation test performed active bending maneuvers, during which the three-dimensional intervertebral motions were measured continuously with an optoelectronic camera via markers attached to the percutaneous transpedicular screws. If a solid connection between each marker carrier and vertebrae was verified, the three-dimensional rotations of the vertebrae were analyzed with special reference to motion asymmetries and patterns of coupled motion. All data were compared to the normal values in the literature since the methodology is not possible in asymptomatic control subjects. RESULTS: Seven patients were removed from the analysis because of a loose screw-bone interface. In the remaining patients, three distinct patterns of motion were observed in each direction of movement. Significant differences in the extension-flexion ratio, the lateral bending asymmetry, and the coupled axial rotation-lateral bending ratio were found between the chronic LBP group and the normal literature values. CONCLUSIONS: Chronic LBP patients exhibited motion patterns altered from those of the normal population. Specific differences were observed in coupled axial rotation during lateral bending movement, in the symmetry between flexion and extension, and in the symmetry be tween right and left lateral bending.