Biomechanical evaluation of immediate stability with rectangular versus cylindrical interbody cages in stabilization of the lumbar spine

The newly designed rectangular cage (Fig 1) (made by Corin Spinal, Gloucestershire, UK) had a tapered rectangular design to restore the lumbar lordosis. The large central cavity allowed adequate space for packing large amount of cancellous bone graft inside the cage. The teeth on the superior and inferior surfaces were designed to engage into the endplate to provide additional stability. The cages were designed to be used in pair at each level, and could be used for either ALIF or PLIF procedures. The rectangular cage was inserted by its smaller diameter into the disc space, and then rotated by right angle to engage its teeth into the endplate, and to align the longer diameter of the cages cranio-caudally.

Ten functional spinal units (FSU; five L2-L3, five L4-L5) of human cadaver lumbar spine from 5 subjects (3 male, 2 female) were tested. The donors had a mean age of 76.4 years (range, 68 – 82 years). X-ray and bone densitometry were done to rule out any metastatic or metabolic bone disease. Varying degrees of degenerative changes were found in all of them. From the same cadaver spine one FSU was tested with the rectangular cages and the other with the cylindrical cages. The specimens were stored at -20°C until the 48 hours before testing.

On the day before testing the thawed specimens were stripped carefully all the soft tissues leaving the ligaments and joint capsules intact. Specimens were then potted with plaster of Paris in aluminium pots of the loading jig. To improve anchorage, screws were introduced obliquely into the vertebral bodies close to the endplates away from the disc space.

Each of the ten specimens was tested for flexibility of the intact spine, after stabilization with a pair of the either rectangular (Corin) or cylindrical (BAK) cages and after additional posterior stabilisation with a pair of translaminar facet screws, as described by Montesano and Magerl et al [5]. Five specimens were tested for each type of implant.

The cages were introduced through the anterior aspect, following the manufacturer's guidelines for surgical technique and estimation of desired disc height. Following stabilization with the cage A-P and lateral radiographs were obtained to ensure correct placement of the implants with adequate restoration of the disc height (Fig 2 and 3).

Flexibility was tested in a servo-hydraulic material-testing machine (Dartec Ltd., Stourbridge, UK) (Fig 4). Special loading jig of similar design as described by Chiba et al [6] was used for mounting the potted specimens eccentrically in the loading frame, to test the flexion-extension movement (Fig 5). The length of the lever arm from the axis of motion was 10 cm. A 50 N of preload was applied with a lever arm of 40 cm in the opposed direction, at the bottom end of the specimen (Fig 4). The specimens were rotated by 90° in the loading jig for testing the lateral bending movement (Fig 6A and 6B). A bending moment was used to produce flexion-extension and lateral bending by applying a vertical load to the eccentrically mounted specimen through the 10 cm lever arm. A preload of 50 N was applied to a lever arm of 40 cm from the fulcrum, in the opposite direction, at the bottom end of the specimen, as described by Chiba et al [6]. Two digital goniometers, placed at the junction of the loading frame and the lever arm, measured the angular displacement at the FSU with application of the bending moment.

Axial rotation was tested by mounting the specimens in the centre of the material-testing machine, so that the axis of torsion lies midway between the centre and the posterior edge of the vertebral body in the sagittal plane. Torsion load was applied directly by a rotary actuator on the machine and the axis of torsion of the specimen was aligned to the centre of the actuator. A 200 N compressive load was applied throughout the tests with the linear actuator.

The specimens were loaded with a continuous cyclical bending moment of 5 Nm in either direction, at a constant rate of 0.5 Nm per second, for four cycles at a time. The load and angular deformation data were captured from the third cycle. The software (Datamanager 96, Dartec Ltd, Stourbridge, UK) produced the load-deformation curves directly from the captured data (Fig 7).

Because of the small number of specimens tested in each group, non-parametric methods were used for statistical analysis using SPSS for Windows version 10.0 (SPSS Inc. Chicago, Illinois) statistical software. The range of movement (ROM) of each specimen after cage fixation and after additional posterior stabilisation was normalised (ratio of ROM of stabilized to intact specimen) with respect to that of the intact FSU. The ROM for the intact specimens between the two cage-groups was compared for any difference using the Mann-Whitney Test. The difference in ROM in the individual cage group between intact specimen, after cage insertion and after additional posterior stabilisation were analysed with the related sample Wilcoxon test. The difference in ROM between the cage groups was compared using the Mann-Whitney test. The critical level of significance was 0.05.

The range of flexion-extension after insertion of both rectangular and cylindrical cages were significantly reduced as compared to that of the intact specimens (p < 0.05 related sample Wilcoxon test). The normalised median flexion-extension after stabilization with the rectangular and the cylindrical cages were 0.438 (range 0.187 – 0.695), and 0.423 (range 0.208 – 0.631) respectively. There was no significant difference (p = 0.602 Mann-Whitney test) between the two cage groups (Fig 9).

The range of lateral bending was significantly reduced after both types of cage insertion (p < 0.05). The normalised median lateral bending after stabilisation with the rectangular and the cylindrical cages were 0.365 (range 0.198 – 0.825) and 0.615 (range 0.301 – 1.032) respectively. There was a trend of better stability with the rectangular cages compared to the cylindrical cage group, but the difference was not significant (p = 0.117, Mann-Whitney test), (Fig 9).

The range of axial rotation was significantly reduced after both types of cage insertion (p < 0.05). The normalised median axial rotation after stabilisation with the rectangular and the cylindrical cages were 0.479 (range 0.293 – 0.801) and 0.421 (range 0.209 – 0.758) respectively. The difference was not significant (p = 0.917, Mann-Whitney test), (Fig 9).

The ROM after additional posterior fixation with translaminar screws was significantly reduced as compared to cage insertion alone for most of the movements for both cage groups (p < 0.05 related sample Wilcoxon test), except in two situations. The range of lateral bending in the rectangular group, and the range of flexion-extension in the cylindrical cage group were only marginally different following additional posterior stabilization (p = 0.08 for both).

There was no significant difference between the two cage groups, in ROM in any direction, following additional posterior stabilisation (p = 0.465 for all the movements, Mann-Whitney test), (Fig 10).

In the present study, both cage designs significantly reduced movement in all directions when compared to that of intact specimens. In flexion-extension the stability was almost identical for both types of cages investigated. This is consistent with reports of earlier investigators [4, 11, 13, 16].

Lund et al [4] noted inability of two types of rectangular cages (Brantigan carbon fiber, and Contact^®) to resist axial rotation. In fact ROM in axial rotation significantly increased. In contrast, the cylindrical cages (Ray TFC) provided a marginally superior stability against axial rotation compared to the control. Tsantrizos et al [11] observed superior stability with a ScrewCage compared to the other cage designs. The superior stability to rotation with screw-in cages may be related to the screw threads engaging the endplate. In a biomechanical study on cadaver spine, using BAK cages and translaminar screw, Rathonyi et al [17] observed very poor rotational stability in specimens with poor endplate contact. They concluded that the quality of endplate contact may be the most important factor for axial rotational stability. This may explain the poor rotational stability with rectangular cages as observed by Lund et al [4] The Contact^® cages have smooth surfaces to fit the endplate contour. The Brantigan carbon fiber cages have serrations on their cranial and caudal surface, which prevent cage migration but they are not designed to cut into the endplate.

In our study, insertion of the rectangular cages (Corin) increased the rotational stability compared to the control, and the stability was comparable to that with the cylindrical (BAK) cages. This may be the effect of the teeth in these rectangular cages engaging into the endplates.

Both Lund et al [4] and Tsantrizos et al [16] observed no difference in lateral bending stability between the cylindrical and rectangular cage constructs. In our study rectangular cages produced a marginally superior stability in lateral bending motion (p = 0.117) compared to the cylindrical cage constructs. Although the difference was not statistically significant, this may indicate a small advantage of a rectangular over a cylindrical shape. Theoretically, there is a possibility of side to side rocking movement of the vertebra over the cylindrical cages inserted in sagittal plane.

Posterior stabilization with translaminar screws was described by Montesano and Magerl et al [5]. Although the stability achieved by stand-alone translaminar or transfacetal screw fixation is less rigid compared to pedicle screw-rod instrumentations [20], most investigators suggested that translaminar screws provide sufficient stability in all directions, when combined with anterior column support [17, 21].

Most studies suggest that supplemental posterior fixation using pedicle screw-rod construct improves stability in all direction, and also levels off any difference in stability between stand alone interbody implant constructs [4, 16]. In a cadaver spine study Rathonyi et al [17] reported that stand alone BAK cages failed to provide stability in extension and axial rotation. However, supplemental translaminar screw fixation significantly increased stability in both axial rotation and extension. In a similar study Oxland et al [13] reported significant increase in stability with stand-alone cages (BAK and Syncage) in all directions except in extension; addition of translaminar screw fixation significantly increased the stability in extension.

In the present study additional posterior stabilization with translaminar screw fixation significantly increased the stability in all directions except in two situations. With rectangular cages there was no significant further increase in stability to lateral bending after posterior fixation (p = 0.08). A similar effect was observed with cylindrical cages where the difference in stability in flexion-extension between the stand-alone cages and additional posterior stabilization was not significant (p = 0.08).

Most investigators agree that interbody cages provide good stability in flexion and lateral bending but little or no stability in extension and axial rotation [4, 11, 13, 16, 17, 19]. The loss of stability in extension and axial rotation may be related to the obligatory damage to the specific anatomical structures needed for cage insertion. Stability in extension depends most on the distraction of the anterior annulus and stability in rotation depends primarily on the integrity of the facet joints. It may be anticipated that with anterior cage insertion (ALIF) damage to the anterior longitudinal ligament and annulus will lead to loss of stability in extension. In contrast, with posterior cage insertion (PLIF) damage to the facet joints will lead to greater loss of stability in rotation. The cage diameter for cylindrical cages and cage height in rectangular cages (where cage is inserted and rotated 90°) dictates the extent of medial facetectomy needed for cage insertion.

With posterior cage insertion (PLIF) Tsantrizos et al [16] reported marginal changes in extension, but stability in axial rotation decreased significantly, more with Ray TFC than with Contact^® cages. Lund et al [4] found increased range of axial rotation with posterior insertion of both Contact^® and Brantigan cages, and no significant change in extension with any cage design compared to the control.

With anterior cage insertion (ALIF) Oxland et al [13] found no stabilization in extension but significant stabilization in axial rotation using BAK cages in cadaver spine. Rathonyi et al [17] found a similar decrease in stability in extension but no change in axial rotation.

Tencer et al [14] evaluated a cylindrical cage (Ray TFC) in different orientations within the interbody space in calf and human cadaver spine. There was no significant difference with the direction of cage insertion except when posterior placement damaged facets or lamina. In this case torsional stiffness was reduced. Stiffness achieved with anterior implantation decreased when the anterior annulus was damaged.

The rectangular cage used in the present study is designed for insertion from both an anterior and posterior direction, but was tested for anterior insertion only. We observed increased stability in axial rotation, and in flexion-extension in both the cage groups.

The principle limitation in this study was the use of a constrained system for applying load. Rotations were allowed in one axis only, and coupled motions were prevented. It has been described in the literature that a closer estimate of a physiological load-deformation in any specimen may only be obtained by applying pure moment, with six degrees of freedom, allowing coupled motion [22] However, it is expected that a constrained system would affect the load-deformation and the ROM patterns almost equally, for both the stabilization system. Therefore, a constrained system may not give a true estimate of a physiological ROM but will be good enough to compare two different stabilization systems. The constrained system used here is certainly much simpler, faster, and allows cyclical loading of the specimen, as opposed to the stepwise quasi-static loading often applied during pure moment testing. Additionally, it produced a precise measurement of load-deformation curve and ROM on repeat testing on the same specimen, which helps to identify relatively small difference between the two systems.

The second limitation was that, like many other biomechanical studies [3, 8, 11] the flexion-extension ROM was recorded in this study as one composite movement. In practice however, the cage insertion should have a different effect on flexion as compared to extension. Consequently, our study may have missed the lack of stability in extension with the cages. This fact was stressed by Lund et al [4]. The reason behind our method is that we used an eccentric loading jig to apply a continuous cyclical load, to produce flexion-extension movement. The hysteresis curve was directly produced by the software (Datamanager 96, Dartec Ltd, Stourbridge, UK) in our experimental setup from a continuous flexion-extension movement. This offered a more sensitive and precise record of the flexibility of the spine, which improved the comparison between the two different cage systems.

The other limitation is that our study does not provide the effect on stability with posterior insertion of the rectangular cage. The cage is designed to be rotated 90° after insertion requiring medial facetectomy equal to the cranio-caudal height of the cages. Therefore the axial rotation stability observed with the stand-alone cages after anterior insertion may not hold true for their posterior insertion.