Stability of the lumbar spine after intradiscal electrothermal therapy☆☆☆★
Accepted: April 4, 2000;
Abstract
Lee J, Lutz GE, Campbell D, Rodeo SA, Wright T. Stability of the lumbar spine after intradiscal electrothermal therapy. Arch Phys Med Rehabil 2001;82:120-2. Objective: To assess the stability of the human lumbar cadaveric spinal motion segment before and after treatment with intradiscal electrothermal therapy (IDET). Design: An in vitro biomechanic analysis of 5 human cadaveric spinal motion segments by using nondestructive biomechanic testing in flexion/extension, lateral bending, and axial rotation with loads of 0N, 600N, and 1200N. Setting: University-based hospital research center. Cadavers: Spinal unit specimens (upper and middle lumbar) from 5 human cadavers (age range, 39-79yr). Interventions: A spinal catheter consisting of a thermal-resistive heating coil was placed circumferentially into the outer annulus by using the standard extrapedicular discographic technique through a 17-gauge introducer needle. The disc was then heated in a saline bath (37°C) from 65°C up to 90°C for a total of 17 minutes. Main Outcome Measure: The stability of the spinal segments was measured before and shortly after IDET. Stability of the spine was measured as the compliance of the spine (the angular deformation afforded by the spine under applied bending moments). Results: With increasing preloads, there is a decrease in motion of the spinal segment in all planes of testing. However, there was no significant difference (p >.05) in the stability of the lumbar spine before and after treatment with IDET. Conclusions: IDET does not destabilize the spinal motion segment in vitro. © 2001 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation
CHRONIC LUMBAR PAIN is a common source of disability. Although most patients with lumbar pain improve in 3 months, approximately 5% will have chronic disabling lumbar pain and up to 60% will have recurrent episodes of lumbar pain.1Historically, it has been difficult to isolate a specific anatomic lesion that causes the chronic lumbar pain. However, with recent advances in techniques for precision lumbar spinal injection, the anatomic source of pain can be found in most patients with chronic lumbar pain.2 In many patients the source has been a painful degenerative disc.3
For a structure to cause pain, it must have a nerve supply. The posterior longitudinal ligament and the lamina of the outer annulus contain nociceptive free nerve endings, which are the terminal nerve endings of the sinuvertebral nerve,4, 5 and the outer one third of the annulus contains nerve endings with nociceptive neurotransmitters (substance P, calcitonin, vasoactive intestinal peptide).6, 7 In patients with chronic discogenic pain, nociceptive fibers have even been shown to grow into the diseased disc.5, 8
Traditional medical treatment for patients with chronic discogenic pain has included trials of oral medication, epidural steroid injections, intradiscal steroid injections, exercise therapies, manual therapies, back school, and lifestyle modification. Despite the best nonsurgical treatment, there remains a subset of patients who suffer symptoms that negatively affect their quality of life and their ability to maintain or resume gainful employment. Traditionally, these patients are offered the option of living with the chronic disabling pain or aggressive surgical intervention in the form of a lumbar fusion, which is a salvage procedure with the potential for significant patient morbidity and failure rates as high as 40%.9
Intradiscal heating methods have recently emerged as a minimally invasive treatment option for patients with chronic discogenic lumbar pain who have decided against or are not yet ready for lumbar fusion. Percutaneous intradiscal laser nucleotomy is an ablative procedure in which the nucleus pulposus is vaporized,10, 11 causing a reduction in the volume of the disc. This procedure is typically used to treat patients with radicular pain greater than axial pain, typically from a contained disc herniation. Nonablative intradiscal heating can be accomplished by using a radiofrequency probe,12, 13 or more recently, a thermal-resistive spinal catheter.14 The proposed mechanisms of action of these techniques are collagen modulation, cauterization of granulation tissue, and annular denervation.14 Houpt et al,15 in an in vitro experiment, found that the outer annular temperatures created by using a radiofrequency probe were insufficient (<42°C) to be cytotoxic to nerve fibers.16 They concluded that current intradiscal radiofrequency therapeutic protocols are insufficient to cause annular denervation and questioned the role of intradiscal heating by using this technique.
Intradiscal electrothermal therapy (IDET)a is a newer method that can produce annular temperatures sufficient to cause nerve fiber death.14 IDET uses a 30-cm semirigid catheter with a 6-cm active tip that contains a thermal-resistive heating coil. The catheter is placed by using the standard extrapedicular discographic technique, through a 17-gauge introducer needle, into the outer annulus in the region of the discographically proven symptomatic annular tear. After the catheter has been placed, controlled electrothermal heat is generated starting at 65°C and increased to 90°C over a 17-minute period. Clinical studies17, 18 of patients with chronic discogenic pain who have been treated with IDET have found high patient satisfaction rates (73%-76%) with this new method of intradiscal therapy.
Although IDET clinical studies have shown promise, concerns have been raised regarding the safety of this procedure and, more specifically, whether there are any deleterious biomechanic effects to the disc after intradiscal heating. This in vitro study assessed the stability of the human lumbar cadaveric spinal motion segment before and after treatment with IDET. This was accomplished by measuring the compliance of the segment in all 3 planes of motion (flexion/extension, lateral bending, axial rotation) under axial compressive preloads chosen to span those occurring in vivo across the spine during activities of daily living (ADLs).
Methods
Frozen human lumbar spine specimens were obtained from donor organizations and then examined fluoroscopically to eliminate those segments with over 50% disc space collapse, fractures, tumors, spondylolisthesis, or other serious osseous pathology. Five spinal unit specimens from 4 spines were tested: the L1-L2 and L3-L4 levels from a 58-year-old woman; the L3-L4 levels from a 79-year-old man; the L2-L3 level from a 39-year-old man; and the L2-L3 level from a 42-year-old woman. The ends of the 2 vertebral bodies were then fixed into molds by using an epoxy; the molds were mounted into a 6 degree-of-freedom apparatus attached to an MTS hydraulic testing system. Three-dimensional motions of the spinal segment were recorded by using a 3-space electromagnetic tracker system.b
The compliance of the spinal units was determined from the slope of the angular deformation versus the applied moment curve obtained through nondestructive physiologic loading in flexion/extension, lateral flexion, and axial rotation. The specimen was cycled with a torque of 7 to 8 Nm until a steady state was achieved and the displacement data was recorded. To simulate in vivo conditions, testing was performed with compressive axial preloads of 1200N, 600N, and 0N applied across the unit. Preliminary testing with additional spinal units showed that the test was repeatable and that the results were independent of the testing sequence.
After the baseline pretreatment biomechanic testing was completed, the specimen was removed from the MTS machine and taken to the fluoroscopy suite. By using the standard discographic extrapedicular approach, a 17-gauge introducer needle was placed into the middle of the disc. A semirigid spinal catheter was then advanced through the introducer needle into the outer annulus (fig 1).
Fig. 1
Axial fluoroscopic image showing the introducer needle in the center of the disc and the IDET catheter in the outer annulus.
The stiffness (degrees of motion per Nm) of the spinal segment was determined from the torque versus displacement data. Stability was measured as the compliance of the spine (the angular deformation afforded by the spine under applied bending moments). Each spinal segment was used as its own control. Statistical analysis of the data pre- and posttreatment was performed by using the paired Student t test, and a significant difference was defined as p <.05.
Results
Compliance decreased with increasing preloads in all planes of motion. The greatest degree of overall motion occurred in lateral flexion, followed up by flexion/extension, and then axial rotation. Because of technical difficulties, the sample size (n = 4) was limited in lateral flexion and axial rotation with a preload of 1200N; in all other cases, the sample size was maintained (n = 5). Statistical analysis by using Student's t test revealed no significant differences (p >.05) between pre- and posttreatment stability of the spinal motion segments for any of the 3 planes tested at any of the preloads (table 1).
| Preload (N) | Pretreatment (deg/Nm) | Postreatment (deg/Nm) | n |
|---|---|---|---|
| Flexion/Extension | |||
| 0 | .81±.24 | .86±.27 | 5 |
| 600 | .58±.22 | .60±.23 | 5 |
| 1200 | .41±.27 | .47±.28 | 5 |
| Lateral Flexion | |||
| 0 | .98±.31 | .99±.32 | 5 |
| 600 | .68±.26 | .68±.27 | 5 |
| 1200 | .54±.28 | .56±.27 | 4 |
| Axial Rotation | |||
| 0 | .27±.09 | .28±.10 | 5 |
| 600 | .13±.06 | .14±.06 | 5 |
| 1200 | .09±.04 | .10±.04 | 4 |
Data presented as mean ± standard deviation and expressed in degrees of motion per Nm at varying preloads.
Discussion
If IDET is going to be universally accepted as a minimally invasive treatment option for patients with chronic discogenic lumbar pain, it needs to be not only efficacious, but also safe. This study sought to determine if IDET adversely affected the segmental compliance of the lumbar spine. To the best of our knowledge, this is the first study to analyze the biomechanic effects of intradiscal heating on the compliance of the human spinal motion segment. Our results showed no significant in vitro effects from treatment with IDET. Though it has been postulated that IDET “stiffens” the spinal segment,14 based on the data presented here, this does not appear to be the case in vitro.
The preloads used in the experiment were based on previously reported data that estimated the loads on the lumbar spine during normal ADLs. The compressive force that acts on the lumbar spine of an average young man is approximately 500N when he is standing, 700N when sitting, and 3 to 6kN when lifting moderate to heavy weights from the floor.19 We chose loads that simulate those seen in vivo though not exceeding loads (1200N) that may potentially damage the spinal segment in vitro.20
A limitation of the current study is the small sample size (n = 5), thus, the statistical power of the study is low. A calculation of the sample size required to obtain a statistical power of 0.7 was performed, assuming a normal distribution. Given the small differences in stiffness before and after treatment, the sample size necessary would be n = 70. Time and financial requirements of such a large study were not feasible.
Another limitation was the use primarily of upper and middle lumbar segments. In clinical cases, most disc pathologies are found in the lower lumbar segments. Upper and middle lumbar segments resulted from the age of the specimens combined with the technical limitations of performing IDET on segments with disc space narrowing of more than 50%. In the majority of specimens, severe degenerative changes were noted at the lower lumbar levels.
A final limitation of the study was that it was performed in vitro a short time after treatment and may not necessarily reflect what would happen to the treated lumbar disc and spinal motion segment in vivo over time. Animal models will be required to investigate biologic (cell-mediated) processes, such as the tissue's healing response to intradiscal heating. However, mechanical testing of cadaveric spines is appropriate for investigating biomechanic changes of the motion segment before and after treatement with IDET.21 The mechanical properties of cadaveric spines tested in vitro reflect the in vivo properties, provided that preliminary creep tests are used to reduce the hydration of the disc.22
Establishing the biomechanics of a joint by measuring its compliance (or alternatively the inverse property, joint stiffness) is a well-accepted technique for the spine, as well as for other joints, such as the knee and shoulder, that rely on soft tissue structures for joint stability. To simulate a range of in vivo conditions, a range of compressive joint loads was applied across the spinal unit during testing.23 As expected, the spine became less compliant (more stiff) as compressive load increased. The added loads did not alter, however, the lack of significant difference between the pre- and posttreatment compliance measurements.23 Similar in vivo measurements of patients undergoing IDET will be necessary to confirm the clinical relevance of these findings.24 Such measurements are more difficult to perform, however, because a number of additional variables are present that cannot be adequately controlled during the test.
Conclusion
IDET does not adversely effect the stability of the human lumbar cadaveric spinal motion segment as reflected in the compliance of the segment in flexion/extension, lateral bending, and axial rotation. Further studies are needed to analyze the potential histologic changes induced by IDET in vitro, as well as the healing response in vivo over time.
Suppliers
a. SpineCATH; Oratec Interventions, Inc, 3700 Haven Ct, Menlo Park, CA 94025.
b. Polhemus Inc, 1 Hercules Dr, PO Box 560, Colchester, VT 05446.
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