Cervical fusion for treatment of degenerative conditions: development of appropriate use criteria.

BACKGROUND CONTEXT: High quality evidence is difficult to generate, leaving substantial knowledge gaps in the treatment of spinal conditions. Appropriate use criteria (AUC) are a means of determining appropriate recommendations when high quality evidence is lacking. PURPOSE: Define appropriate use criteria (AUC) of cervical fusion for treatment of degenerative conditions of the cervical spine. STUDY DESIGN/SETTING: Appropriate use criteria for cervical fusion were developed using the RAND/UCLA appropriateness methodology. Following development of clinical guidelines and scenario writing, a one-day workshop was held with a multidisciplinary group of 14 raters, all considered thought leaders in their respective fields, to determine final ratings for cervical fusion appropriateness for various clinical situations. OUTCOME MEASURES: Final rating for cervical fusion recommendation as either "Appropriate," "Uncertain" or "Rarely Appropriate" based on the median final rating among the raters. METHODS: Inclusion criteria for scenarios included patients aged 18 to 80 with degenerative conditions of the cervical spine. Key modifiers were defined and combined to develop a matrix of clinical scenarios. The median score among the raters was used to determine the final rating for each scenario. The final rating was compared between modifier levels. Spearman's rank correlation between each modifier and the final rating was determined. A multivariable ordinal regression model was fit to determine the adjusted odds of an "Appropriate" final rating while adjusting for radiographic diagnosis, number of levels and symptom type. Three decision trees were developed using decision tree classification models and variable importance for each tree was computed. RESULTS: Of the 263 scenarios, 47 (17.9 %) were rated as rarely appropriate, 66 (25%) as uncertain and 150 (57%) were rated as appropriate. Symptom type was the modifier most strongly correlated with the final rating (adjusted ρ2 = 0.58, p<.01). A multivariable ordinal regression adjusting for symptom type, diagnosis, and number of levels and showed high discriminative ability (C statistic = 0.90) and the adjusted odds ratio (aOR) of receiving a final rating of "Appropriate" was highest for myelopathy (aOR, 7.1) and radiculopathy (aOR, 4.8). Three decision tree models showed that symptom type and radiographic diagnosis had the highest variable importance. CONCLUSIONS: Appropriate use criteria for cervical fusion in the setting of cervical degenerative disorders were developed. Symptom type was most strongly correlated with final rating. Myelopathy or radiculopathy were most strongly associated with an "Appropriate" rating, while axial pain without stenosis was most associated with "Rarely Appropriate."

Effects of motion segment level, Pfirrmann intervertebral disc degeneration grade and gender on lumbar spine kinematics.

MRI allows non-invasive assessment of intervertebral disc degeneration with the added clinical benefit of using non-ionizing radiation. What has remained unclear is the relationship between assessed disc degeneration and lumbar spine kinematics. Kinematic outcomes of 54 multi-segment (L1-Sacrum) lumbar spine specimens were calculated to discover if such an underlying relationship exists with degeneration assessed using the Pfirrmann grading system. Further analyses were also conducted to determine if kinematic outcomes were affected by motion segment level, gender or applied compressive preload. Range of motion, hysteresis, high flexibility zone size and rotational stiffness in flexion-extension, lateral bending and axial rotation were the kinematic outcomes. Caudal intervertebral discs in our study sample were more degenerative than cranial discs. L5-S1 discs had the largest flexion-extension range of motion (p < 0.005) and L1-L2 discs the lowest flexion high flexibility zone size (p < 0.013). No other strict cranial-caudal differences in kinematic outcomes were found. Low flexibility zone rotational stiffness increased with disc degeneration grade in extension, lateral bending and axial rotation (p < 0.001). Trends towards higher hysteresis and lower range of motion with increased degeneration were observed in flexion-extension and lateral bending. Applied compressive preload increased flexion-extension hysteresis and augmented the effect of degeneration on hysteresis (p < 0.0005). Female specimens had about one degree larger range of motion in all rotational modes, and higher flexion extension hysteresis (p = 0.016). These results suggest that gender differences exist in lumbar spine kinematics. Additionally high disc loads, applied compressive preload or applied moment, are needed to kinematically distinguish discs with different levels of degeneration. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1389-1398, 2016.

Relaxation response of lumbar segments undergoing disc-space distraction: implications to the stability of anterior lumbar interbody implants.

STUDY DESIGN: A biomechanical study of human cadaveric lumbar spine segments undergoing disc-space distraction for insertion of anterior lumbar interbody implants. OBJECTIVE: To measure the distraction force and its relaxation during a period of up to 3 hours after disc-space distraction as a function of the distraction magnitude and disc level. SUMMARY OF BACKGROUND DATA: Interbody implants depend on compressive preload produced by disc-space distraction (annular pretension) for initial stabilization of the implant-bone interface. However, the amount of preload produced by disc-space distraction due to insertion of the implant and its subsequent relaxation have not been quantified. METHODS: Twenty-two fresh human lumbar motion segments (age: 51 ± 14.8 years) were used. An anterior lumbar discectomy was performed. The distraction test battery consisted of a tension stiffness test performed before and after each relaxation test, 2 distraction magnitudes of 2 and 4 mm, and a recovery period before each distraction input. The distraction forces and lordosis angles were measured. RESULTS.: Peak distraction force was significantly larger for the 4-mm distraction (431.8 ± 116.4 N) than for the 2-mm distraction (204.9 ± 55.5 N) (P < 0.01). The distraction force significantly decreased over time (P < 0.01), approximating steady-state values of 146.1 ± 47.3 N at 2-mm distraction and 289.8 ± 92.8 N at 4-mm distraction, respectively. The distraction force reduced in magnitude by more than 20% of peak value in the first 15 minutes and reduced by approximately 30% of the peak value at the end of the testing period. The spine segment relaxed by the same amount of force, regardless of the disc level (P > 0.05). CONCLUSION: The "tightness of fit" that the surgeon notes immediately after interbody device insertion in the disc space degrades in the very early postoperative period, which could compromise the stability of the bone-implant interface.

Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae.

OBJECT: There is limited data on the pullout strength of spinal fixation devices in the thoracic spine among individuals with different bone quality. An in vitro biomechanical study on the thoracic spine was performed to compare the pullout strength and the mechanism of failure of 4 posterior fixation thoracic constructs in relation to bone mineral density (BMD). METHODS: A total of 80 vertebrae from 11 fresh-frozen thoracic spines (T2-12) were used. Based on the results from peripheral quantitative CT, specimens were divided into 2 groups (normal and osteopenic) according to their BMD. They were then randomly assigned to 1 of 4 different instrumentation systems (sublaminar wires, pedicle screws, lamina claw hooks, or pedicle screws with wires). The construct was completed with 2 titanium rods and 2 transverse connectors, creating a stable frame. The pullout force to failure perpendicular to the rods as well as the pattern of fixation failure was recorded. RESULTS: Mean pullout force in the osteopenic Group A (36 vertebrae) was 473.2 ± 179.2 N and in the normal BMD Group B (44 vertebrae) was 1414.5 ± 554.8 N. In Group A, no significant difference in pullout strength was encountered among the different implants (p = 0.96). In Group B, the hook system failed because of dislocation with significantly less force than the other 3 constructs (931.9 ± 345.1 N vs an average of 1538.6 ± 532.7 N; p = 0.02). In the osteopenic group, larger screws demonstrated greater resistance to pullout (p = 0.011). The most common failure mechanism in both groups was through pedicle base fracture. CONCLUSIONS: Bone quality is an important factor that influences stability of posterior thoracic implants. Fixation strength in the osteopenic group was one-fourth of the value measured in vertebrae with good bone quality, irrespective of the instrumentation used. However, in normal bone quality vertebrae, the lamina hook claw system dislocated with significantly less force when compared with other spinal implants. Further studies are needed to investigate the impact of different transpedicular screw designs on the pullout strength in normal and osteopenic thoracic spines.

Response of Charité total disc replacement under physiologic loads: prosthesis component motion patterns.

BACKGROUND CONTEXT: Total disc replacement (TDR) has been recommended to reduce pain of presumed discogenic origin while preserving spinal motion. The floating core of Charité TDR is professed to allow the replication of the kinematics of a healthy disc under physiologic loads. While segmental motion after Charité TDR has been measured, little is known about the effects of a physiologic compressive preload on vertebral motion and the motion of prosthesis components after TDR. PURPOSE: (1) Does Charité TDR allow restoration of normal load-displacement behavior of a lumbar motion segment under physiologic loads? (2) How do the prosthesis components move relative to each other under physiologic loads when implanted in a lumbar motion segment? STUDY DESIGN: A biomechanical study using human lumbar spines (L1-sacrum). METHODS: Five lumbar spines (age: 52+/-9.3) were used. Specimens were tested under flexion (8 Nm) and extension (6 Nm) moments with compressive follower preloads of 0 N and 400 N in the following sequence: (i) intact, (ii) Charité TDR at L5-S1, (iii) simulated healed fusion at L5-S1 with Charité TDR at L4-L5. Segmental motion was measured optoelectronically. Motions between prosthesis end plates and core were visually assessed using sequential digital video-fluoroscopy over the full range of motion. Here we report on kinematics of 10 Charité TDRs: 5 at L5-S1 and 5 at L4-L5. RESULTS: Charité TDR increased the flexion-extension range of motion of lumbar segments (p<.05). At 400 N preload, the range of motion increased from intact values of 6.8+/-4.4 to 10.0+/-2.4 degrees at L5-S1 and from 7.0+/-2.6 to 10.8+/-2.9 degrees at L4-L5. Charité TDR increased segmental lordosis by 8.1+/-6.9 degrees at L5-S1 (p<.05) and 5.4+/-3.5 degrees at L4-L5 (p<.05). Four patterns of prosthesis component motion were noted: (1) angular motion only between the upper end plate and core, with little or no visual evidence of core translation (9 of 10 TDRs at 0 N preload and 5 of 10 TDRs at 400 N preload); (2) lift-off of upper prosthesis end plate from core or of core from lower end plate (observed in extension in 9 of 10 TDRs under 0 N preload only); (3) core entrapment, resulting in a locked core over a portion of the range of motion (observed in extension in 8 of 10 TDRs under 400 N preload); (4) angular motion between both the upper and lower end plates and core, with visual evidence of core translation (1 of 10 TDRs at 0 N preload, 5 of 10 TDRs at 400 N preload). The pattern of load-displacement curves was substantially changed under a physiologic preload in 8 of 10 TDRs; instead of a relatively gradual change in angle with changing moment application as seen for an intact segment, the TDR displayed regions of both relatively small and relatively large angular changes with gradual moment application. CONCLUSIONS: Charité TDR restored near normal quantity of flexion-extension range of motion under a constant physiologic preload; however, the quality of segmental motion differed from the intact case over the flexion-extension range. Whereas some TDRs showed visual evidence of core translation, the predominant angular motion within the prosthesis occurred between the upper end plate and the polyethylene core. Likely factors affecting the function of the Charité TDR include implant placement and orientation, intraoperative change in lordosis, and magnitude of physiologic compressive preload. Further work is needed to assess the effects of the prosthesis motion patterns identified in the study on the load sharing at the implanted level and polyethylene core wear.