BIOMECHANICAL COMPARISON OF POSTERIOR THORACOLUMBAR INSTRUMENTATION SYSTEMS
Paxinos O, Zindrick M, Voulgaris P, Lorentz M, Sartori M, Patwardhan A.
Loyola University Medical Center, Maywood, IL USA
Posterior instrumentation of the thoracolumbar spine is achieved by pedicle screws, screws supplemented with wires, sub laminar wires, and pedicle or laminar hooks. Screw insertion techniques as well as wire and hook slippage are particular clinical concerns that they have not been addressed by previous studies of isolated unilateral implant strength. This study compared the strength of complete segmental posterior instrumentation systems.
A total of 80 thoracic and lumbar vertebrae from 10 donors were used in this study. Bone mineral density (BMD) of each individual vertebra was measured using pQCT. Each spine was divided in groups of four vertebrae and one of the four different instrumentation systems was randomly assigned. Instrumentation was inserted bilaterally using standard techniques. The implants were attached to two rods of 10 cm length. The rods were then connected with two custom made transverse connectors forming a rectangular frame that closely reproduced the conditions experienced by the implant-bone interface in vivo. Pullout vertical to the system was done using cables that were attached to the rods in both ends.
Median value for BMD was 155.25 mg/ml in the group of vertebrae instrumented with screws, 137.6 mg/ml in the hook system, 182.65 mg/ml in the screw and sulaminar wire, and 186.05 mg/ml in the wire group. Median value for pullout strength was 1186.51 N for the screws and wires, 587.42 N for the hooks, 1161 N for the wires and 730.8 N for the screws. Using ANOVA the differences between the four systems were not found statistically significant. Most failures occurred in the pedicle-body interface.
The decisive factor in the strength provided by different posterior instrumentation systems is the bone quality and not the system characteristics.
BIOMECHANICAL COMPARISON OF ALIF AND PLIF CAGE CONSTRUCTS
Paxinos O, Voulgaris P, Zindrick M., Serhan H.
251 Hellenic Air Force General Hospital, Athens, Greece
Loyola University Medical Center, Chicago, IL, USA
Fusion is considered the standard of care offered to patients with discogenic pain that have failed all conservative methods of treatment. Interbody fusion can be achieved by an anterior (ALIF), or posterior (PLIF) approach. The three most commonly used lumbar interbody cage constructs are: (i) ALIF + translaminar facet screws (TLFS), (ii) ALIF + transpedicular instrumentation (TPI), (iii) PLIF + TPI. This study tested the hypothesis that there are no significant differences in the stability provided by these three constructs.
Eleven segments (7 PLIF and 4 ALIF) from 7 human lumbar spine specimens were tested. Three dimensional load-displacement behavior of each motion segment was measured before and after instrumentation in flexion (8Nm), extension (6Nm), lateral bending (6Nm) and rotation (5Nm).
PLIF cages with TPI increased the segment stiffness in flexion-extension by 85%, in lateral bending 83%, and in axial rotation 67%. ALIF cages with TLFS increased the segment stiffness in flexion-extension by 93%, in lateral bending 94%, and in axial rotation 90%. ALIF cages with TPI increased the segment stiffness in flexion-extension by 91%, in lateral bending 95%, and in axial rotation 90%. No statistical differences were found in the stiffness provided in flexion-extension and lateral bending when both ALIF constructs were compared to PLIF. Rotational stiffness was significantly greater in both ALIF constructs compared to PLIF(p<0.01).
This study provides evidence that both ALIF and PLIF interbody cages provide equal stability when they are supplemented with posterior instrumentation with the ALIF constructs being stiffer in rotation. This is the first study that has directly compared ALIF and PLIF cage constructs supplemented with posterior instrumentation.
ACDF WITH A LOCKED PLATE EFFECTIVELY STABILIZES THE COMPLETE DISLOCATED LOWER CERVICAL SPINE
Paxinos O, Zindrick M, Voulgaris P, Ghanayem A, Havey B
251 Hellenic Air Force Hospital, Athens Greece
Loyola University Medical Center Maywood IL, USA
Flexion distraction injuries of the cervical spine are most commonly treated with awake traction. ACDF is recommended when the MRI shows disk prolapse. Some authors advice a combined anterior and posterior instrumentation and others suggest that ACDF alone is enough.
A total of 8 human cervical spine specimens were used from C3 to C7. A flexibility test was conducted in flexion and extension using a moment of 0.8 Nm. Each specimen was tested in the intact state and after C5-C6 discectomy and ACDF using a constrained (locked) plate. During the diskectomy the PLL was also divided simulating a usual surgical step that ensures decompression of the cord. The spine was then tested again after disruption of the intraspinatus initially and then again after laceration of the ligamentum flavum and the capsule of the facet joints.
Average total ROM of the intact segment was 7.53 degrees and after ACDF was 1.28 degrees. This was statistically significant (p<0.05). Average ROM after division of the intraspinatus was 1.93 degrees and after complete facet capsule and ligamentum flavum disruption 1.98 degrees. There was not any statistical significant difference (p<0.05) in the total ROM between any step of the posterior element disruption and the ACDF.
In conclusion, a locked plate provides enough initial stability to treat complete posterior element deficient, flexion-distraction injuries. Age and bone quality may be a concern and additional external immobilization may be needed until fusion matures.
Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae.
Odysseas Paxinos, Voulgaris Panagiotis, Michael R Zindrick.
Department of Orthopaedic Surgery,
Loyola University, Chicago, Maywood, Illinois, USA
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).
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.
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.
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.