Clinical and Biomechanical Evidence Validating Pain Centralization and Directional Preference

Ronald Donelson, MD, MS

SelfCare First, LLC

Definitions of Centralization and Directional Preference

For patients presenting with lumbar or cervical pain, the centralization phenomenon (CP) and directional preference (DP) are often identifiable in patients’ histories as intermittent pain that comes with movements and positions with the spine bent in one direction and goes away with the opposite direction.  These two clinical findings are then elicited and observed during a specialized clinical exam using repeated end-range test movements. This examination was first described by McKenzie and is part of a paradigm of care that has come to be known as Mechanical Diagnosis and Therapy (MDT).  CP is characterized as a prompt retreat of any radiating or referred pain back to the lumbar or cervical center or midline, which is routinely followed by elimination of that central pain as the result of further performance of a single direction of repeated end-range lumbar testing.1  This specific change in pain location can occur with flexion, extension, or laterally-directed testing.  There is typically only a single direction of testing that elicits this unique pain response and that direction is referred to as the patient’s “directional preference”.

CP and DP Prevalence

When this specialized examination is conducted by clinicians well-trained in MDT methods of examination, the prevalence of CP and DP in the LBP population is surprisingly high, reported as 70-89% in those with acute LBP and 32-52% in those with chronic LBP.2-13

CP and DP Reliability and Outcome Validity

There are now many studies focused on CP and DP that report positive findings in three important domains of research:  1. high reliability in identifying DP and DP and that large subgroup of LBP patients in whom two clinical findings are elicited;13-21 2. CP and non-CP findings are strong predictors of excellent and poor outcomes respectively;5-7, 9, 11-13, 17, 22, 23 and 3. DP-specific treatments, consisting of directional exercises and posture strategies, consistently produce good-to-excellent outcomes, demonstrated in many prospective cohort studies,5-7, 9, 11-13, 17, 22, 23and now eight randomized clinical trials.10, 24-30

A Pathoanatomic Mechanism for CP and DP?

An additional form of validation of the CP/DP subgroup has emerged linking these two clinical findings to symptomatic disc pathology.  This includes the two commonly acknowledged forms of disc pain: sciatica due to a compressive herniated nucleus pulposus (HNP) and internal disc disorders.

A substantial percentage of patients with sciatica have been shown to have a DP, meaning their leg pain “centralizes”, or retreats out of their leg to the center of their back during their baseline testing assessment.7, 10, 11  They then routinely report excellent outcomes when treated with directional exercises and posture modifications that match their DP.

The second form of disc pain is tied to painful internal disc pathology and is identifiable only with discography, a controversial imaging study.  Two studies both report a strong correlation between discography findings and whether or not those patients could centralize their pain during a pre-discographic MDT assessment.2, 8  One study reported CP had a 100% specificity in making a pathoanatomic diagnosis of internal disc disruption (i.e. discogenic pain) based on discographic findings in subjects not distressed or severely disabled, and an 80-89% specificity for distressed or severely disabled individuals.8

Such high specificity enables the identification of a large subgroup of chronic LBP patients with internal disc disruption who, according to a large number of cohort and RCTs, have an excellent chance of rapid recovery with non-surgical treatment consisting of uni-directional exercises and posture strategies.  This further begs the question of how many chronic axial pain patients undergo an unnecessary spinal fusion or disc arthroplasty who are never examined properly to identify their DP?  How many would jump at the opportunity to be so evaluated that might well identify the means to a rapid and full non-surgical recovery?

Biomechanical Studies

Meanwhile, a large number of biomechanical studies of internal disc dynamics have established that normal disc nuclei move posteriorly within the disc with lumbar flexion loading and then move back anteriorly with subsequent extension loading.31-37  Such a directional reversal of nuclear displacement precisely matches the reversible symptomatic response that accompanies intentional directional loading tests to elicit CP and DP during this MDT assessment.  This directional nuclear displacement model explaining CP and DP would relate to both the internal disc pain as well as external nerve compression forms of disc pain.

One study provides additional insight into the validity of directionally reversible disc pathology.34  In an effort to investigate mechanisms by which CP and DP clinical findings could be linked to a painful intervertebral disc, the investigators asked:  could posterior displacement of nuclear material produced by repeated flexion loading be reversed and returned to its central location within the disc simply by applying repeated extension loading to the disc?

Eighteen porcine C3-4 cervical spines were repeatedly loaded in either pure flexion or combined flexion and side flexion.  Eleven of those specimens prolapsed and were then loaded repeatedly in pure extension. The prolapsed nucleus was successfully reduced in 5 of the 11 specimens as a result of this extension reversal testing.  The remaining six did not change.  Those that reduced had greater disc space height than those that did not, while neither the morphology of the herniation (circumferential or radial) nor the angle of lordosis of the specimens predicted their response to extension loading.

The investigators concluded that disc prolapse from repeated flexion could be reversed and directed back toward the center of the disc, in these cases by a direction of bending and loading that was directly opposite to the direction that produced the prolapse.

A Mechanism for Reported Pre-Surgical Turn-Arounds?

Kopp et al in 1986 reported on 67 Navy personnel with sciatica, neural deficits, and loss of their extension range-of-motion who were admitted to consider disc surgery due to lack of response to treatment.7  Hearing that McKenzie hypothesized that extension might restore, or at least reduce, posterior nuclear displacement, they tested all 67 patients with some extension movements and positioning upon admission to the hospital.  For 34, there was either a pain reduction or they were at least no worse with this testing.  Those 34 were prescribed extension exercises to be performed several times a day while remaining in the hospital.  All 34 (100%) regained their full lumbar extension range and eliminated all symptoms within 2-5 days. None required surgery.

The other 32 underwent laminectomy and discectomy based on their failure to improve with conservative care.

Pre-operatively, there was no difference between these two groups with regard to age, gender, pain below the knee, or any of the neurological signs or SLR.  Conventionally monitored clinical findings, i.e. radicular symptoms, straight-leg-raise test, neurological signs, or symptoms, did not distinguish between these two management groups.

These patients all most likely had nuclear displacement as the source of their pain, resulting in neural deficits and loss of extension range of motion.  Based on the Scannell study, it is quite likely that McKenzie’s theory is accurate: that progressively applied extension can often reduce or correct pain-producing posterior nuclear displacement and protrusions.

So twenty years ago, these Kopp, et al concluded:  “Some of these patients responded so dramatically to extension therapy that the use of extension exercises as a therapeutic modality is recommended".

Conclusion

This biomechanical evidence that reversing the directional loading on a displaced disc nucleus can restore a more central nuclear position is a match for the centralizing pattern of pain response seen so often when symptomatic patients perform repeated lumbar end-range extension testing.  This disc displacement/reduction model also nicely fits both types of commonly acknowledged disc pain production which perhaps helps explain the very high prevalence of DP and CP reported in so many studies. 

It is now quite defendable that CP and DP discovery may identify patients whose pain is caused by some degree of excessive nuclear displacement and in whom a mechanism for reducing that displacement can be found, whether the pain is caused by the displacement irritating posterior anular nociceptors or is being generated by nerve root compression from an HNP.

As further research unfolds, if this reversible nuclear model proves to be the mechanism by which so many patients develop and can then centralize their pain with a DP (70-89% of acute LBP), it would suggest that disc pathology is the dominant underlying LBP generator, regardless of whether the pain is limited to the low back or radiates to full sciatica.  Of great significance is that the prevalence of non-specific LBP would be reduced to a low percentage.  The cost benefit of using this assessment in the surgical selection process alone would be immense.

References

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