Jeremy CT Fairbank MD FRCS, Nuffield Orthopaedic Centre, Oxford
Patients with chronic low back are frequently referred to orthopaedic surgeons. The majority of these patients does not require surgical treatment, and should be managed by non-operative methods. There are a small proportion who may benefit from surgical stabilisation of the spine. This article gives a brief resume of the rationale for this approach.
The surgeon and patient have to take a decision after weighing up the risks and benefits of the procedure (often hard to define) against the current and expected disability. This approach should only be considered when other non-operative methods of management have been fully explored.
Spinal fusion (or stabilisation) may be regarded as either:
(i) an attempt to speed up the natural; progress of degenerative changes towards a functional
(ii) alkalosis through spondylosis
iii) or a prevention of progressive deformity or instability.
The evidence on which to base these decisions is sparse. The positive or "Pro-view" is given by Sonntag and Marciano Spine 1995; 24 S: 138S - 142S, and the "Anti- view by Turner et al. [1]. Large trials of spinal fusion are in progress, but are not yet reported.
Rationale
This assumes that back pain may be generated from the low back by "mechanical" means or through a source of "inflammatory" agents (usually assumed to be the intervertebral disc) irritating neural tissues.
(i) "Mechanical" back pain is generated by movement, and may be controlled by rest or immobilization by external splintage (corsets, braces, plaster), or internal splintage (fusion, with or without internal fixation, or limitation of movement by special implants). In some cases there may be instability of the spine, actual or perceived, which can be controlled by spinal fusion.
(ii) "Inflammatory" pain may be managed surgically by excision of a whole disc, or at least a substantial part of it, and replacement of it by allograft or autograft bone, cages made of various materials containing bone, or even artificial disc prostheses.
(iii) Both "mechanical" and "inflammatory" pain may be involved in patients with previous root decompression surgery
Unfortunately reality has not always followed expectation, and the results of treatment have varied considerably. Only recently have these methods started to be analyzed by randomised controlled trials.
Spinal instability
Spinal stability is a much abused term. There are at least three ways in which this can be conceived:
1) in mechanical terms, summarized in White and Punjabi's [2] definition of stability: "A condition of the spine under normal physiological loading where there is neither abnormal strain nor excessive or abnormal motion in the functional spinal unit (FSU)." Mechanical instability may occur through:
- fractures
- congenital anomalies (failures of formation or segmentation)
- spondylolisthesis
- iatrogenic damage to the spine
- degenerative changes in the spine
2) In temporal terms, where symptoms wax and wane more or less predictably with time. Some patients report increasing frequency and duration of attacks of pain.
3) in perceptual terms, where the spine feels unstable, although no abnormal motion or position can be detected by conventional radiography. Perceived or functional instability is sometimes called "Instability syndrome". The patient complains of giving way; getting stuck; a ratchety flexion in the spine and, occasionally, a sensation of disconnection of the top of the body from the bottom.
This is all very well. Sometimes surgery is spectacularly successful, but in others it is not. Failures of the surgical approach can be put down to two main areas: the patient and the surgical methodology.
The patient
A technically correct operation achieving a solid fusion may not relieve pain, indeed it can make it worse. Here there may be a variety of reasons for this:
1) A mismatch between the patient's expectation and the reality of the outcomes of major surgery.
2) The wrong levels may be selected for surgery. This depends on clinical assessment, imaging (usually MRI), provocative discography, facet blocks, and (rarely) external fixation. None of these methods is totally reliable.
3) Smoking cigarettes has been associated in many studies with a high pseudarthrosis rate.
4) Involvement in litigation or workman's compensation has long been recognized as being associated with poor clinical results.
5) There is a technical failure in the surgery.
Surgical methodology
The object of most procedures is to obtain a solid bony fusion, avoiding damage to the surrounding soft tissues. A wide variety of methods are available, and comparisons between them are difficult. Postero-lateral fusions tend to be easier to perform, but are probably less reliable in terms of both fusion rates and in immobilizing a segment (or functional spinal unit). Interbody fusions, either from the back (PLIF), or from the front (ALIF) are more likely to fuse with instrumentation, but are technically more difficult to perform, and carry a higher risk of complication. Spinal instrumentation has evolved rapidly, increasing in complexity and expense. It has been difficult to demonstrate that its use has any advantage. Some, but not all studies, suggest that the use of instrumentation increases the fusion rate. Unfortunately this is not necessarily accompanied by an improvement in clinical results. A meta-analysis by Boos et al gives details of published reports of spinal fusion and the role of instrumentation. [4]
Investigation
It is likely that a significant proportion of patients have discogenic or segmental pain. MRI will diagnose degenerative discs in almost everyone. Unfortunately it cannot tell you which one hurts. Proponents of this view use provocative discography to identify painful segments. Discography can identify segments where pain is reproduced by injection of saline into the disc and adjacent normal discs. It identifies patients with inappropriate responses (over-reaction to local anaesthetic or skin penetration by needle) and inappropriate or multilevel response to disc injection. The "normal" disc is usually pain free when injected.
Many surgeons will not operate on patients showing inappropriate reactions to discograms. However the technique is observer dependent and by no means foolproof. Discography is still condemned by a vociferous group, but is widely used by surgeons using spinal fusion to treat back pain. The evidence can be seen in a large review by the North American Spine Society [5]. The arguments for and against are given by Bogduk and Modic respectively.[6]
Conclusion
We still have along way to go before we can reliably predict who will benefit from a surgical approach. At the Nuffield Orthopaedic Centre we are anxious to establish a proper place for surgery. We are currently Grantholders for the MRC Spine Stabilisation Trial, which aims to recruit 1000 patients in this and other UK centres into a study comparing a surgical with a non-operative approach in patients that we think may benefit from surgery, but are uncertain of the outcome.
Method
References
1. Turner J, Ersek M, Herron L, et al. Patient outcomes after lumbar spinal fusions. JAMA 1992;268:907-911.
2. White A, Panjabi M. Clinical biomechanics of the spine. 2nd ed. Philadelphia: Lippincott, 1990.
3. Frymoyer J, Selby D. Segmental instability. Rationale for treatment. Spine 1985;10:280-286.
4. Boos N, Webb J. Pedicle screw fixation in spinal disorders: a European view. Eur Spine J 1997;6:2-18.
5. Guyer R, Ohnweiss D. Contemporary concept in spine care: Discography: Position statement of the North American Spine Society Diagnostic and Therapeutic Committee. Spine 1995;20:2048-2059.
6. Bogduk N, Modic M. Controversy: Lumbar discography. Spine 1996;21:402-404.
7. Garfin S, Mardjetko S, Connolly P, et al. Historical Cohort Study of Pedicle Screw Fixation in Thoracic, Lumbar, and Sacral Spine Fusions. Spine 1994;19(20-S):2254-S - 2305-S.
8. Zdeblick TA. A prospective, randomized study of lumbar fusion. Preliminary results. Spine 1993;18(8):983-91.
9. Thomsen K, Christensen F, Eiskjaer S, Hansen E, Fruensgaard S, Bunger C. The Effect of Pedicle Screw Instrumentation on Functional Outcome and Fusion Rates in Posterolateral Lumbar Spinal Fusion: A Prospective, Randomized Clinical Study. Spine 1997;22:2813-2822.
10. Mardjenko S, Connolly P, Schott S. Degenerative lumbar spondylolisthesis. A meta-analysis of the literature 1970-1993. Spine 1994;19:S 2256-S 2265