Polenov Neurosurgical Institute, the branch of North-Western Federal Medical Research Center, 

Saint Petersburg, Russia

 

Posterior dynamic stabilization is one of the relatively new techniques for treating degenerative diseases in the lumbar spine. This technique is based on the preservation of motions in the operated spinal motion segment (SMS). Interspinous dynamic stabilization was introduced into clinical practice in late 80s of the previous century. J. Senegas (1988) in his publication presented the titanium device, which was fixed between the spinous processes by means of dacron tape. The alternative technique for surgical treatment of degenerative diseases of the lumbar spine showed good long term results, and effective performance of the interspinous spacer of the first generation was found in 84.1 % of the cases within 10 years of follow-up [1]. Later the author improved the device by means of production with modern and more elastic material PEEK (Wallis Stabilization System, Abbott Spine Inc., Austin, TX).

At the present time all systems for interspinous stabilization are divided into dynamic (Coflex, DIAM, Viking), rigid (X-STOP, Wallis, BacJac, Aperius), percutaneous implanted (Helifix, Aperius) and telescopic ones (BridgePoint, PosteriorFusionSystem).

The literature review dedicated to biomechanics of interspinous spacers showed the main biomechanical effects relating to a fixed SMS:

-              Influence on the range of motion in an operated and adjacent SMS;

-              Increasing size of the spinal canal and intervertebral foramina;

-              Change in intradiscal pressure, load to IVD and facet joints;

-              Influence on local sagittal balance and the instant spinning axis in the operated SMS. 

F.M. Phillips et al. (2006) published the study of the cadaveric block of L1 sacrum. They estimated influence of the interspinous spacer DIAM on flexion, extension, lateral inclination and axial rotation after partial facetectomy and discectomy for operated and adjacent SMS in the lumbar spine [2]. The authors found that unilateral hemifacetectomy did not increase angle motion and the following discectomy increased it during flexion-extension, lateral flexion and axial rotation. Use of DIAM after discectomy restored angle movement up to the indexes, which were less than the indexes for an intact segment during flexion-extension. During lateral flexion DIAM decreases amount of movement induced by discectomy, but not to the levels of intact level. Use of the spacer did not influence on the amount of changed axial rotation after discectomy.

In 2010 S.P. Markin received the similar results: DIAM created the stabilizing effect by means of limitation of extensional linear mobility, decreasing amplitude of angle and linear motions in SMS to the normal values [3].

D.P. Lindsey et al. examined X-STOP and could observe significant decrease in flexion-extension at the surgery level, while the spacer did not influence on the other motions. The results of the follow-up also showed absence of significant changes in kinematics of the adjacent segment in all planes of motion [4].

P.D. Fuchs et al. investigated the range of motion after unilateral medial facetectomy, unilateral total facetectomy and bilateral total facetectomy with segmental fixation by means of X-STOP [5]. The authors concluded that the interspinous spacer:

-       significantly decreases flexion in the intact SMS and after unilateral medial and total facetectomy, as well as after bilateral total facetectomy;  

- significantly decreases extension in the intact SMS after unilateral medial and bilateral total facetectomy, but has no effect in unilateral total facetectomy;      

- does not have intense effect on the size of axial rotation;

- significantly decreases lateral flexion in all cases of facetectomy.

The in vitro study by K. Tsai found that Coflex limited motions in flexion-extension and in axial rotation both in partially and full destabilized segment [6].  

The similar results were found by S.P. Markin in his thesis work of Coflex and its influence on range of motion [3].

V. Lafage published the combined in vitro study with finite element analysis for estimation of the biomechanical effects of Wallis [7]. The intact, injured and fixed segments were compared during flexion, extension, lateral flexion and axial rotation. The experimental study showed decreasing range of motion during flexion and extension in the segment fixed with Wallis in comparison with the intact and injured SMS. The finite element analysis found decreasing pressure to the disk and increasing load to the spinous processes. J.C. Richards et al. conducted the study and showed that X-STOP significantly increased the square of the spinal canal (by 18 %) during extension, subarticular diameter – by 50 %, the diameter of the canal – 10 %, the square of the foramen – by 25 % and the height of the intervertebral foramen – by 41 %. It proved prevention of narrowing in the spinal canal and the intervertebral foramina during extension [8]. H. Celik estimated Coflex and its influence on the vertical size of the intervertebral foramen and confirmed absence of statistically reliable differences in sizes in pre- and postsurgical period, and the good results of surgical treatment were conditioned by microsurgical decompression [9].

A.Ya. Aleynik investigated DIAM and its influence on the operated SMS. They found increasing sizes of the intervertebral disk, the intervertebral foramina and the interosseous space in early intervals after surgery. However in 24 months he found the clear tendency to weakening the effect [10].              

In vitro studies involving small rodents showed that dynamic stabilization could result in reparation of the injured SMS as result of influence of compression forces.

K.E. Swanson (2003) published the measurements of intradiscal pressure and load to the fibrous ring after implantation of the interspinous spacer X-STOP in cadaveric materials in neutral position, flexion and extension. The study found that during extension the pressure to the posterior departments of the fibrous ring and the pulpal nucleus decreased by 63 % and 41 % correspondingly, by 38 % and 20 % in neutral position and in standing position. There were no significant changes in intradiscal pressure at the adjacent levels [11].

H. Wilke performed the study of the interspinous spacers Coflex, Wallis, DIA and X-STOP. He showed that intradiscal pressure significantly decreased during extension. There were no significant changes in other planes [12].

In 2010 S. Zheng et al. published the study dedicated to influence of different degrees of distraction of the interspinous processes on the intradiscal pressure [13]. The authors suggested that after implantation of the interspinous spacer the different degrees of distraction could redistribute load to the intervertebral disk. According to their opinion, the ideal implant should decrease pressure to the posterior departments of the intervertebral disk and to the pulpal nucleus, and to transfer most load from IVD to the interspinous processes during extension and in neutral position. The authors found positive correlation between height of the interspinous spacer and distribution of load: during increase in size of an implant beyond the distance between the spinous processes one can observe significant decrease and redistribution of load from the posterior departments of the fibrous ring to the implant and to the anterior departments of the fibrous ring that can accelerate degeneration of the intervertebral disk.

H. Wilke estimated the interspinous implants Coflex, Wallis, DIAM and X-STOP and their influence on the local sagittal balance in the fixed SMS. The cadaveric samples were used for creation of kyphosis within the range of 0.5⁰-0.7⁰, the basic value of angulation was 0⁰. The study showed different influence of each implant to the size of segmental lordosis: administration of DIAM caused increase in kyphosis, the interspinous spacers Wallis and DIAM did not make significant influence on kyphosis. Coflex did not cause statistically significant increase in kyphosis, however, the range of the received values was significantly higher [12].

S.P. Markin found that Coflex decreased the angle of lordosis in the hypermobile segment and the size of posterior translation. DIAM did not remove retrolisthesis, but it restored lordosis in kyphosis of the segment up to 5⁰ [3].

A. Anasetti et al. estimated influence of DIAM (10 mm and 14 mm) in two different positions: in standard and anterior displacement. The study found that the implant caused development of kyphosis in the operated segment. Both sizes of the implant and both positions resulted in posterior displacement of instant axis of rotation in flexion and extension. If implantation was without dacron tapes, instant axis of rotation displaced towards the center of IVD during flexion [14].

In 2011 we implemented the patented device made of nitinol with shape memory effect for dynamic stabilization – the interspinous distractor ILKODA. The mathematic modelling of behavior of the interspinous spacer ILKODA with the universal program for finite element analysis ANSYS allowed identifying influence of the distractor on SMS [15]:

-          0.2 cm displacement of the spinous process with uninjured IVD, and 0.4 cm for injured IVD;

-          efficient stabilization of SMS with two-fold weakening of IVD;

-          functioning regardless of priority vector of instability;

-          prevention of decreasing stability and increasing rigidity in segments near injured SMS.

 

Objective – to estimate the effects of the interspinous distractor ILKODA on the biomechanics of the operated motion segment of the lumbar spine.

 

MATERIALS AND METHODS

The present study is based on the analysis of surgical treatment of 35 patients with degenerative changes in the spinal motion segments of the lumbar spine (table 1). The exclusion criteria were:

-          segmental instability as result of injury and defect in the interarticular part of the arc;

-          degenerative antelisthesis > degree I;

-          intense segmental instability with need for implantation of rigid systems of fixation;

-          structural and non-structural deformations in the lumbar spine except for antalgic scoliosis;

-          local kyphosis at the level of implantation of the interspinous spacer; 

intense degeneration of the intervertebral disk (Pfirrmann Grade V);

-          degenerative stenosis of the spinal canal with need for total facetectomy and laminectomy;

-          concurrent somatic diseases which could influence on outcomes of surgical treatment;

-          body mass index > 30 kg/m²;

-          osteopenia and osteoporosis.

Table 1
Patient distribution according to nosological entities

All patients were distributed into two groups: the group A (n = 17) – the patients who received ILKODA interspinous distractor for treating segmental instability, and the group B (n = 18) – implantation for prevention of segmental instability.

Biomechanical influence of the distractor in the operated SMS was realized by means of measuring the height of the anterior and posterior departments of IVD, the height of intervertebral foramina (IVF), segmental Cobb angle, value of translation and angulation in unstable SMS according to X-ray examination in all patients before surgery and 12 months after surgical management.

The statistical analysis was realized with Statistica 12. Multi-way ANOVA was used for estimation of level of significance of the data. Quantitative variables were presented as M ± SD (M – mean arithmetic, SD – standard deviation). Qualitative elements were presented as absolute and relative (%) values. Depending on the type of distribution of quantitative variables, reliability of differences was estimated with Student’s test. Critical level of significance was p < 0.05.

 

RESULTS AND DISCUSSION

The analysis of the results showed that in case of monosegmental lesion the most common injured segment was L4-L5 (61.9 %), followed by L4-L5 (26.19 %) without significant differences in the comparison groups. In 20 % of the cases we observed multi-level lesions: L3-L4 segment, L4-L5 in 5 cases, L5-S1 in 2 cases. Fixation for L5-S1 did not include the interspinous distractor ILKODA because of the anatomic features of the spinous processes. The most common degenerative changes in L4-L5 are explained by the transitional characteristics from lumbar lordosis to sacral kyphosis, i.e. maximal loading (table 2).

Table 2
Patient distribution according to the level of surgical intervention

The measurement of the height in the anterior parts of IVD found that 12 months after surgical treatment one could observe reliable decrease in the height (p = 0.41). Dynamics of change in the height of the posterior departments of IVD allowed making the conclusion about the tendency in decreasing height in 12 months in comparison with presurgical values (p = 0.07). There were no reliable differences in dynamics of changes in vertical dimension of intervertebral foramen (p = 0.08). The comparison of the values between the groups A and B did not show any statistically significant differences. The analysis of the results showed that the interspinous distractor ILKODA did not influence on vertical dimension of intervertebral foramen and posterior departments of IVD. However statistically significant decrease in the anterior departments of IVD may be a result of progressing degenerative changes in the intervertebral disk, and preservation of the height in the posterior departments of IVD and vertical dimension of intervertebral foramen can be achieved by means of the interspinous spacer and redistribution of load to the implant and the anterior parts of the disk.

All included patients had disk degeneration which corresponded to only 3rd and 4th degrees according to Prirrman’s classification, because 1st and 2nd degrees do not have clinical significance, and 5th degree causes restabilization of SMS resulting in inexpediency of dynamic stabilization. The study allowed making the conclusion about the highest efficiency of ILKODA for 3rd degree of degeneration, when the most efficient increase in vertical size of intervertebral foramen is observed (p = 0.04) with increasing the sectional area of the foramen and tendency (p = 0.07) to increase the posterior parts of IVD. 4th degree of degeneration did not result in statistically significant changes in height of the anterior and posterior departments of IVD (p = 0.4) and intervertebral foramen (p = 1) after implantation of the interspinous spacer (table 3). However the positive clinical effect in view of regressing vertebral syndrome in the postsurgical period in this group can be explained by the volume of decompression of vascular neural formations in the spinal canal at the operated level.       

Table 3
Influence of interspinous spacer on the height of intervertebral disk and foramen

                              

The analysis of dynamic changes in Cobb segmental angle showed no reliable changes in SMS of the upper lumbar spine in both comparison groups, whereas SMS in L3-L4 and L4-L5 showed the positive trend to increasing angle after 12 months (p = 0.06 in the group A and 0.007 in the group B). Considering the fact that both groups demonstrated reliable decrease in the anterior parts of IVD without significant changes in the height of the posterior parts of IVD and IVF, we concluded that increasing Cobb angle (kyphosis in operated segment) was the logical process reflecting the events of further degeneration of the IVD and application of interspinous spacer’s distraction forces to the structures of the posterior supporting complex (table 4).      

Table 4
Influence of interspinous spacer on Cobb segmentary angle

Segmental instability (the group A) was found in 17 cases (48.6 %). The patients were included into this group on the basis of X-ray criteria by White and Panjabi. The translational instability was found in 15 cases, rotational instability – in 2 cases, translational instability with posterior vertebral displacement (retrolisthesis) was observed in 9 patients, 6 patients demonstrated priority vertebral displacement anteriad. The amount of presurgical sagittal translation was 4.8 ± 0.66 mm, after 12 months – 0.86  ± 0.8 mm that is statistically reliable (p = 0.003). The statistical preparation of the values of rotational instability was impossible because of very small amount of the patients. However during functional spinal radiography in postsurgical period we could observe decreasing angulation in operated SMS to the physiologic parameters (Fig. 1, 2).

Figure 1 Patient M., examination before surgery.À, B – functional X-ray images showrotational instability in the motion segment LIII-LIV; C, D –MRI scans in the axial and sagittal planes show degenerative spinal stenosis in motion segments LIII-LIVand LIV-LV

Figure 2 Patient M., examination in 12 months after surgery. A, B – there is no rotational instability in the motion segment LIII-LIV; C, D – CT-scans, general view of interspinous spacer ILKODA

       

The figures 3 and 4 present the results of surgical treatment of the patient with primary degenerative instability and priority vertebral displacement towards posterior direction (Fig. 3, 4).

Figure 3 Patient P., examination before surgery. A, B – lumbar functional X-ray showsretrolisthesisLIII in extension; C, D – MRI-scans show no evidence of spinal canal compression

Figure 4 Patient P., examination in 12 months after surgery.  There is no instability in LIII-LIV motion segment in extension

       

 

CONCLUSION

1.                 The interspinous distractor favorably changes biomechanics of operated spinal motion segment as result of redistribution of part of load from posterior departments of the intervertebral disk and facet joints to the implant and the anterior departments of the disk.

2.                 Expressive influence on biomechanics of operated SMS is made by the interspinous spacer ILKODA only in moderate degeneration of the intervertebral disk (degree III according to Pfirrmann).

3.                 In more intense degeneration of the intervertebral disk the clinical effect is better defined by the volume of performed decompression for the structures of the spinal canal.

4.                 The interspinous distractor ILKODA functions regardless of priority displacement of the vertebra.

5.                 Use of interspinous dynamic stabilization does not hinder the course of natural “degenerative cascade”.