Biomechanical analysis of the effect of ‘intermediate screws’ in short segment posterior fixation of unstable burst fractures of thoracolumbar spine in calf spine model

INTRODUCTION

Unstable burst fractures are commonly seen in young individuals in the thoracolumbar region due to high energy trauma.^1,2 Approximately fifty percent of the affected population are estimated to have neurological deficits.^3-5 The affected individual is usually poly-traumatized, and the associated injuries warrant a quick surgical stabilization of the spine to aid early rehabilitation.

Short segment instrumentation was preferred in young individuals because of the preservation of motion levels. Posterior short segment spanning fixation (SSSF) depends on the integrity of the anterior column and is not reliable in an unstable burst situation.^6,7 Although circumferential fixation provides reliable and stable short segment constructs, it adds to the further morbidity of an already poly traumatized individual.⁸ Various augmentation techniques to SSSF have been described in the literature such as percutaneous balloon vertebroplasty, use of laminar hooks, and insertion of screws into the fractured level commonly termed as ‘intermediate screws’.^9-11 The short segment posterior fixation with intermediate screws (SSPI) was performed through the posterior approach familiar to most spine surgeons, and the associated morbidity was less. Our study aims to evaluate the biomechanical advantages of intermediate screws in experimentally induced unstable burst fractures in the calf spine model.

METHODS

This experimental study was conducted at the School Of Advanced Sciences, Vellore Institute Of Technology, Tamil Nadu, India between September 2014 and October 2015.

Institutional review board and ethics committee approved the study.

Specimen collection and preparation

Five calf spine specimens (all males, aged 4 to 6 months), including the last two thoracic and first three lumbar vertebrae, were freshly harvested from a local slaughterhouse. Plain radiographs were obtained to rule out any gross pathology. The specimens were double packed in polythene bags and stored at -70ºC. Before biomechanical testing, each specimen was thawed overnight, and all muscle tissue was cleared carefully, retaining the bony and discoligamentous anatomy. The end vertebrae were trimmed to fit the mounting cup and mounted using dental resin.

Biomechanical testing

Specimens were put to test for flexion extension, right-left lateral flexion, and axial rotation in clockwise and anticlockwise directions. The intact specimen was first examined. The same test protocol was repeated for the instrumented specimens with and without ‘intermediate screws’.

The mounted specimen was firmly fixed to the testing fixture on either end using four screws drilled through it. An electromagnetic three-dimensional motion tracking system (Polhemus, Inc., Colchester, VT) was used to record the orientation of the spine in space (Figure 1). The 6 degrees of freedom sensors were attached to the vertebrae above and below the index level. Neutral Point (NP) coordinates were measured for each test direction before loading. The non-destructive unidirectional bending moment was applied for each test direction with a servohydraulic universal testing machine (Tinius, Oslen, PA, USA) using a system of cables and pulleys.¹² The test direction was determined by the relative orientation of the specimen to the cables and pulleys. Flexion-extension was tested by orienting the cable sagittal to the specimen, while lateral flexion was tested by orienting the cable coronal to the specimen. Axial rotations were tested by the horizontal arrangement of the pulleys attached to the upper mounting fixture.

Three preconditioning loading cycles of 200 N were applied in the test direction at a displacement control mode of 5mm/s to correspond roughly to a bending moment of 7.5 Nm.¹³ The load-displacement curve (LDC) obtained from the fourth cycle was used to calculate the stiffness of the construct. The applied load, which was recorded by the load cell placed on the actuator arm of the testing device, was plotted against the displacement of the actuator arm to obtain the LDC.¹⁴ The stiffness of the construct was calculated from the slope of the elastic zone of LDC (Figure 2) and was expressed in N/mm.¹⁵ A continuous record of the relative motion of the vertebrae in space was obtained for the fourth loading cycle until the peak loading value of 200 N was reached. The coordinate values were converted into angles using custom-made software. The Range of Motion (ROM) was calculated as the angular difference between the NP and the end of the peak loading.^16,17 After completing the test for each direction, the apparatus was reconfigured for testing in another direction.

Creation of unstable burst fracture

After testing the intact spine biomechanically, an unstable burst fracture was created at the first lumbar vertebra using a previously described method.¹⁸ In short, the index vertebra L1 was weakened by making osteotomies in the upper third in an H-shaped fashion. A weight of 4.5 kg was dropped along a rail from a height of 1.25 meters onto the upper end of the mounted specimen, keeping in mild flexion. The specimen was then wrapped in saline-soaked gauze and immediately taken for computerized tomography (CT) scan. The fracture pattern was studied in detail with the help of CT with 3D reconstruction.

Instrumentation

Once the CT scan was done of the created fracture, the specimen was immediately instrumented with SSPI. Pedicle screws were inserted in the standard freehand manner [Figure 3]. Instrumentation was done for all specimens using titanium monoaxial pedicle screws of 5mm diameter and 34 to 38 mm length (Jayon, India). There were no visible pedicle violations during instrumentation, nor was there a need for screw repositioning. Plain radiographs of all specimens were obtained following instrumentation. Once the biomechanical testing was completed, the intermediate screw was removed and the resulting SSSF construct was again tested biomechanically following the same test protocol.

RESULTS

Instrumentation using SSPI has significantly decreased the ROM and increased the construct stiffness in all test directions. On testing the SSSF after removing the intermediate screws; ROM was found to increase significantly in all test directions-20.2% in flexion, 16.5% in extension, 14.5% in lateral flexion, and 23% in axial rotation (Figure 4). Moreover, the intermediate screws were found to significantly contribute to the construct stiffness-15.4%, 25.6%, 48.3%, and 160.2%, respectively, in flexion, extension, lateral bending, and axial rotation (Figure 5).

DISCUSSION

Traditionally, long segment fixation was used for stable fixation of an unstable burst fracture. Posterior long segment spanning fixation was used when instrumentation has relied only on the posterior fixation points. Long constructs work on the principle of three points bending when the load is applied.⁷ Later, with the advent of pedicle screws by Roy Camillie, transpedicular three-column fixation became popular.Although biomechanically superior to posterior anchoring devices, multilevel fixation using pedicle screws is associated with the high possibility of spontaneous fusion of joints spanned and not included in the arthrodesis.^19,20 Moreover, placing pedicle screws in the upper 155 thoracic level is challenging given the great variability in pedicle size, orientation, and surrounding vital anatomy.^21,22

Considering the biomechanical superiority of pedicle screw fixation over posterior-only fixation devices, Dick et al. popularised short segment spanning fixation (SSSF) of the fractured vertebra to decrease the instrumented motion segments.^7,23 SSSF works on the principle of cantilever bending or anterior column integrity.⁷ The pedicle screws spanning the fractured vertebra act as “fixed beams” through transpedicular three-column fixation imparting superior stability.⁷ However, later studies revealed the inadequacy of SSF with implant failure and progressive kyphosis.^7,23-24 McLain et al illustrated the mechanism of early failure of the SSSF.²⁷ This led to the implementation of other techniques of stable fixation preserving the motion segments.

Circumferential fixation reconstructs the anterior column with a stable device such as a cage to allow for a reliable stable short segment construct and aids indirect decompression of the neural canal. However, not many surgeons are familiar with this approach and the associated morbidity restricts its use in a poly-traumatized patient. Posterior augmentation of SSSF with intermediate screws gained popularity with its promising clinical results. SSSPI is a less morbid procedure that offers indirect spinal canal decompression and correction of kyphosis by ligamentotaxis.^22,23 SSSPI offers an additional anchoring point to fixation and decreases the progression of kyphosis.

Farrokhi et al in their prospective randomized study in 80 patients with burst fractures treated with SSSF and SSSPI concluded that inclusion of fracture level into the construct offered a better kyphosis correction, fewer instrument failures, lesser complications, and good functional outcome.¹¹ Butt et al in their review of 50 patients observed a reasonable correction of the deformity and neurologic recovery.²⁵ Previous biomechanical studies also have signified the role of intermediate screws. Lazzaro et al. estimated the average stability offered by the intermediate screws as 25% and established its superiority over cross-links in augmenting SSSF.²⁸ Wang et al compared the biomechanical strength of SSSF and SSSPI using monoaxial and polyaxial pedicle screws.²⁹ It concluded that there was no significant difference in instability with the SSSPI constructs using monoaxial and polyaxial screws. However, the construct stability was significantly high with SSSF in the sagittal plane using monaxial pedicle screws.²⁹ Both these biomechanical studies used a decrease in ROM recorded while loading using a 3D motion tracker as the stability parameter and did not calculate the stiffness of the construct from the LDC. Anekstein et al used the offset loading method using a universal testing device to apply the load and displacement was recorded using an extensometer mounted on the vertebrae above and below the index vertebra to record an LDC. They calculated the construct stiffness from the slope of LDC and ROM from the amplitude of the LDC.² Offset loading is not a method to apply pure unidirectional bending moments as it has an axial load vector which is also acting on the vertebrae. The ROM recorded in their study is only a displacement change rather than an angular change.

The primary limitation of this study was the use of calf spine models instead of human cadaveric spines, which was due to the restricted availability of human spines. Another limitation was the inability to apply a follower load, as the impact of such a load on a habitually quadrupedal animal has not been investigated. Nonetheless, the study offers valuable insight into the role of intermediate screws in short segment fixation.

CONCLUSION

Our study has the limitation of using calf spine instead of the human cadaveric spine. Easy availability, expandability, and matching motion kinetics as like the human cadaveric spine prompted us to use the calf spine model. The study results reflect the immediate post-fixation stability of the constructs. The durability of the fixation needs to be tested further using cyclical loading tests. We did not use a follower load to simulate the compressive load acting on the vertebral column due to the weight of the trunk because we were not sure about its effect on the vertebral column of a habitual quadruped animal. Intermediate screws reliably augment the construct stiffness of a short segment spanning fixation and contribute significantly to decrease the ROM.

ACKNOWLEDGEMENTS

Author would like to thanks to the fluid research grant IRB (EC)-ER-1-27-11-2013 for funding and making the work flow easy and success.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: The study was approved by the Institutional Ethics Committee

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