The thoracic facet joint biomechanics in some underlying mechanisms of so-called “non-specific” thoracic back pain.

Ukrainian Institute of muscle-skeletal medicine and Neurology
www.gongalsky.com.ua

Abstract:

Purpose is to detect the typical alterations in the thoracic spinal motion unit (SMU) classified as a local “non-specific” spinal pain.

Methods. 340 patients with “non-specific” spinal thoracic pain enrolled in the study.

The degenerative and biomechanical changes were observed in 3960 SMUs. The biomechanical alterations were determined in the affected (painful) SMUs and reproduced on the human cadaveric thoracic spine.

Results. The biomechanical alterations in all thoracic SMUs were detected more frequently than degenerative (59.06% and 40.94% accordingly).  The degenerative signs were present equally in painful and intact spinal units. The biomechanical X-ray alterations in the effected thoracic SMUs were: asymmetry of intervertebral gap, slight lateral dislocation with small lateral tilt of a vertebra and acantha deviation. The modeling on the human cadaveric spine showed that the most traumatic factor for SMU in the form of non-physiological movements of a vertebra was the fixed over-rotational dislocation of it resulted from the small lateral tilt of a vertebra body and minimal asymmetrical flexion in SMU. The vertebra rotation of an over-physiological range was accompanied with a facets subluxation in zygoapophyseal joints (ZAJ)

Conclusions. The biomechanical alterations in the thoracic SMUs are more prevalent and thus more significant in patients with “non-specific” back pain instead degenerative ones. Moreover, the degenerative X-ray features can be detected both in painful (affected) and in painless SMUs equally and therefore they are less significant clinically. The thoracic SMU with the fixed over-rotational dislocation of a vertebra has complicated biomechanics that involves traumatically the abundantly innervated ZAJ’s region: the dislocated facets, the intervertebral foramen deformation and involved deep short muscles contribute to the secondary nociceptive reactions. This kind of dislocation in thoracic SMU can be defined as a “facet subluxation pain syndrome”.

Key words: biomechanics of the thoracic spinal unit, rotational vertebra dislocation, thoracic zigoapophyseal joints, facet syndrome, non-specific spinal pain, facet subluxation pain syndrome

Introduction

The thoracic spinal pain is a common symptom experienced by the general population [1]. It is one of the most frequent reasons to visit people to a general practitioner [2]. Some of back pain events are known as a “non-specific pain”. However, the term of “non-specific” itself can be interpreted as a pathology that is indeterminate with an incomprehensible nature of it. There is not enough knowledge about mechanism and causes of “non-specific” spinal pain, and that “leads to conservatism in pain management and could be contra-productive for innovations in pain treatment” [3]. “Knowledge about the incidence and prognosis of mid-back (non-specific) pain in the general population is limited” too [4].

The chronic thoracic spinal pain is associated with zygoapophyseal joints (ZAJ) pathology in 42% of patients [5]. It can be managed by pharmacotherapy as well as local anesthesia with steroids [5] or radiofrequency ablation [6]. The positive effect of local anesthesia and ablation of spinal joint region confirmed to a certain extent that the non-specific spinal pain can be related to highly innervated ZAJs and describe as a facet syndrome. Nevertheless, the cause of facet nociceptive reaction still remains unclear [7].

It should be note that back pain is often experienced during awkward movements, heavy weight lifting, staying in an uncomfortable position or maintaining bad posture – all these factors can theoretically provoke some biomechanical alterations.  Such acute pain infrequently transform into chronic “non-specific”. At the same time, the course of back pain “cannot be specified in the majority of patients” [10]. This pain quite often disappears without any medical assistance or in pharmacotherapy [4], or any other sort of spinal procedure such as massage or any sort of manipulation on spine [8]. The question is why? What is the cause of thoracic spinal pain — degenerative or nevertheless biomechanical changes? The role of biomechanical changes as a possible source of the facet pain has not been thoroughly investigated yet in detailed: “…facet-specific biomechanical and physiologic mechanisms that need to be defined, highlighting possible trends for future biomechanical research” [9]. If the so-called “non-specific” spinal pain is really a result of biomechanical alterations in ZAJs, they must be proved experimentally or clinically. Unfortunately, the typical alterations in the thoracic SMU, as well as informative tests for the assessment of the facet joint pain (and in the thoracic level in particular) were not presented [8,10].

The Purpose of the study is to detect the typical alterations in the thoracic SMU classified as a local “non-specific” spinal pain.

Methods

340 patients (from 17 to 60 years old) who suffered from non-specific thoracic spinal pain and experienced it from several days to several years were investigated. The patients were selected for further analysis on the basis of their positive ZAJ signs: the pain should be increased during one of the active spinal movements – extension, frontal flexion, side flexions in two directions and in rotation [11]. The study did not include patients with specific diseases or their signs, such as spinal arthropathy, spondylitis, specific spondyloarthritis accompanied by positive laboratory tests or clinical inflammatory manifestations, myelopathy, patients with a reflected pain from the chest or from abdominal cavity, oncology, osteoporosis with vertebra fracture.

The X-ray thoracic spine carried out in standard frontal and side projections.  The 3960 thoracic SMUs of all investigated spines were examined to detect the degenerative and biomechanical signs. The degenerative features that were taken into account included sclerosis of the vertebral body plate, narrowing of the intervertebral gap space that due to disc degeneration, sharpening of the vertebral edges, Schmorl’s nodules, and spondylosis.  The biomechanical alterations incorporated the intervertebral gap asymmetry, vertebral body displacement, spinous process deviation, increased or decreased kyphosis. Only the “presence” or “absence” of the spinal degenerative and biomechanical signs detected by radiographic X-ray was taking into consideration and analyzed statistically, while their quantitative characteristics were not observed.

The affected aria was verified clinically with active motions of spine in different directions during which the patients showed the painful part of their spine. The defined (“painful”) SMU was verified by palpation of the apexes of thoracic spinous process and ZAJ projections to detect the most painful region. The asymmetrical one-side painful feeling during acantha swaying was the indicator of the effected SMU. The clinically detected SMUs were assessed on X-ray photos to determine the specific changes that could be a cause of pain.

The biomechanical alterations detected by an X-rays were modeled using the human cadaveric spine without soft tissues. The rubber tube was inserted into the spinal canal of four adjacent thoracic vertebras (Th2-Th5) to fix them in a natural position. The facet surfaces of ZAJs and apexes of spinous process were marked by colored black for better visualization. The upper and lower SMUs fixed with a wire by transverse processes to imitate the dislocations were observed on an X-rays.

Results and discussion

The assessment of X-ray signs in all investigated thoracic SMUs revealed that biomechanical signs were dominated compared to the degenerative ones (Fig.1). A lot of SMUs had several biomechanical and degenerative X-ray signs simultaneously.

The thoracic facet joint biomechanics in some underlying mechanisms of so-called “non-specific” thoracic back pain.

Fig.1. Percentage and quantity (in brackets) of the biomechanical and degenerative X-ray markers were detected in all 3,960 thoracic SMUs of the patients with “non-specific” back pain. Several biomechanical and degenerative X-ray signs were present simultaneously in each of many SMUs.

The biomechanical X-ray signs prevailed in patients with “non-specific” thoracic back pain and were detected in 7,037 SMUs (59,06%). The less frequent signs were degenerative – 4,878 (40,94%). Both some biomechanical and degenerative signs were present in each of many SMUs simultaneously. However, this stage of investigation showed the basic tendency of the biomechanical domination in spines with a “non-specific” pain.

The analysis of the X-ray signs appeared in the affected (“painful”) SMUs revealed the specific biomechanical alterations that were seen on all X-ray films: intervertebral space asymmetry, minimal lateral flexion, slight lateral displacement of the vertebral body accompanied by the acantha deviation. The deviation of the spinous process to the right with the lateral displacement of the lower right edge of the Th3 vertebral body seen on the frontal X-ray image is showed in Fig.2.

The thoracic facet joint biomechanics in some underlying mechanisms of so-called “non-specific” thoracic back pain.

Fig.2. The frontal plane X-ray film of the cervical-thoracic part of spine (one of the patients with “non-specific” thoracic back pain in Th3/4 SMU). The Th3 spinous process is deviated to the right (marked by straight arrow); the direction of the rotational displacement of the vertebra is shown by the rounded arrow. The slight lateral displacement of the lower right edge of Th3 vertebra can be observed. Wedging of the rotated Th3 vertebra as well as signs of vertebral torsion (signs of scoliosis) are absent.

The minor biomechanical alterations have seen on an X-ray are typically accompanied by pain during palpation of the displaced acantha and the projection of one of the relevant ZAJ (or a pair of them).

The displacements in SMUs detected by an X-ray and confirmed by clinical investigation were modeled using the human cadaveric spine to specify the biomechanical changes (Fig.3).

The thoracic facet joint biomechanics in some underlying mechanisms of so-called “non-specific” thoracic back pain.

Fig.3. The section the human thoracic spine, the frontal view. The model of Th3 vertebra rotation coupled with acantha displacement. The vertebrae are labeled with numbers. The dislocation is limited in a single SMU by fixing the upper and lower segments with a wire. The lateral deviation of the spinous process to the right accompanied by the lateral displacement of the lower right edge of the Th3 vertebra is visible. The ZAJs subluxations have different characteristics that can be seen as well (shown by arrows). The facets surfaces and apexes of the spinous processes are shadowed by colored black for better visualization.

The lateral deviation of acantha coupled with the minimal lateral displacement of the rotated vertebral body, plus the small lateral tilt of a vertebra with the local asymmetrical banding of a SMU are contributed to the intervertebral space asymmetry and lead to the complicated displacements in a pair of the zygoapophyseal joints. These visible movements of a vertebra remind the “asymmetric nod” of it.

At the side of acantha deviation the facet surfaces are displaced in the cranium-caudal direction (Fig. 4). The intervertebral foramen section is enlarged and joint’s capsule is overstretched.  The intervertebral space asymmetry is obviously seen in the picture as well.

The thoracic facet joint biomechanics in some underlying mechanisms of so-called “non-specific” thoracic back pain.

Fig.4. Model of the rotational displacement of the Th3 vertebra in Th3/4 SMU (the vertebrae are labeled by numbers). Lateral view from the direction of the acantha deviation side to. The facets are separated in cranium-caudal direction (marked by arrow).

ZAJ facets from the opposite side to acantha deviation (Fig.5) are separated in sagittal plane and thus narrowing the intervertebral foramen.

The thoracic facet joint biomechanics in some underlying mechanisms of so-called “non-specific” thoracic back pain.

Fig.5. Model of the rotational displacement of the Th3 vertebra in Th3/4 SMU (the vertebrae are labeled by numbers). Lateral view from the direction of opposite to the acantha deviation.

The facets are separated in the sagittal plane (follow the arrow), the intervertebral foramen is narrowed.

The measurements of the thoracic vertebra motional in the human thoracic spine without soft tissues showed that the most possible amplitude of the vertebral rotation in horizontal plain can reached 5° (for 3 mm of acantha deviation).  However, the real potential range of the thoracic vertebra rotation in a spine with saved soft tissues is smaller: the acantha side displacement varies between 1 to 2 mm (from 1.70 to 3.30 of acantha deviation). Such vertebra rotation makes the facets move apart from 0.7 to 1.3 mm correspondingly on the side opposite to acantha deviation [12].

The thoracic ZAJs injury with pain can be found in the most biomechanically loaded spinal zones located in the upper and middle thoracic SMUs. These zones are approximately near the Th3 and Th7 spinal vertebras.

The stability of a thoracic SMU is supported by soft tissues and taut ligaments, ribs arch and active muscle control to prevent the over-motion of it. Young people usually have stable spine due to proper elasticity of their spinal discs and ligaments that are taut. As a rule they have no complaints of pain. When a disk degenerates and loses its turgor, the resistance of intervertebral ligaments reduces which leads to the increased vertebra mobility. An extreme instability of SMU is the vertebral over-motion which exceeds the physiological range of vertebra motion coupled with the secondary facets over-dislocation. Awkward movements, lifting objects, being in uncomfortable posture or position are trigger for such subluxation in the spinal nodal zones. The spinal pain associated with the described biomechanical mechanisms usually starts about the age of 30 when the disk begins to degenerate. The spinal tissues degeneration and the secondary biomechanics alterations are two interrelated processes.

The dislocation in a pair of ZAJs in vertebra over-physiological rotation provokes the trauma of the capsules. The experimental investigations on animals with forming of the thoracic vertebra rotational subluxation showed that even a small fixed over-physiological facets dislocation led to the long-term swelling of the synovia that was deformed and squeezed between facets [13].

The rotational vertebra dislocation activates short spinal deep muscles (m.rotatores breves) which are located and innervated strictly segmentally. The mm.rotatores breves were studied by a needle electromyography in patients and on anesthetized animals during 6 months with the rotational thoracic vertebra dislocation [15]. The paravertebral short deep muscles can remain in tone (a persistent tonic activity) showing the “spontaneous” low-frequency electromyography activity with 8-12 hertz (the assimilation frequency of muscle) [15]. The pathophysiological significance of such muscle reflexo-segmental reaction is to prevent vertebra for further dislocation by fixing actively the affected SMU. The parts of deep paravertebral muscles get the signs of over-contracted myofibrils which were observed by electron microscopy [14,15]. The over-contracted areas were degenerated gradually forming the cicatrical tissue in which the vessels and their small non-myelinated or low-myelinated nerve fibers were involved [15]. Consequently, the effected deep paravertebral muscles can be additional sours of nociceptive afferentation in chronic back pain together with inflamed synovial fold in a dislocated ZAJ mentioned before.

The evidence of pain in thoracic vertebra sudluxation was specific segmental neural sell’s reaction in the lateral horn and in the substantia gelatinosa of spinal cord gray matter that participate in pain perceiving, control and “colored” of it [15,16].  The specific neural cells reaction in the spinal cord centers was observed during 6 months of experiment and confirmed the secondary chronic pain reactions on a vertebra dislocation.

The capsules of ZAJs are synovial and well innervated. In 1972 B.Wyke presented the concept of “Articular neurology” [17]. This concept can be applied to the ZAJs — the principle of joints synovial tissues organization and their innervation as well as pathophysiological reaction of abundantly innervated joints in damage are identical in all joints, including zygapophyseal. The results of the study demonstrate that the fixed rotational vertebra dislocation is a really significant clinical pathology which can disclose the underlying mechanisms in some cases in so called “non-specific” spinal pain. The accurate definition of the spinal pain associated with a fixed thoracic vertebra subluxation conforms the concept of a “facet pain syndrome” that can be specify by the term of “subluxation” and called as the “facet subluxation pain syndrome”.

It would not be superfluous to note the possibility of an innate asymptomatic lateral deformation of spinous process or achanta deviation in a scoliotic vertebra torsion. Both are typically painless during palpation.  Anyway the main tasks are to identify the effected (painful) SMU clinically, confirm and specify it not only with MRI, but also by using of the traditional X-ray investigation.

Conclusions

The biomechanical alterations in the thoracic SMUs are more prevalent and thus more significant in patients with “non-specific” back pain instead degenerative ones. Moreover, the degenerative X-ray features can be detected both in painful (affected) and in painless SMUs equally and therefore they are less significant clinically. The thoracic SMU with the fixed over-rotational dislocation of a vertebra has complicated biomechanics that involves traumatically the abundantly innervated ZAJ’s region: the dislocated facets, the intervertebral foramen deformation and involved deep short muscles contribute to the secondary nociceptive reactions. This kind of dislocation in thoracic SMU can be defined as a “facet subluxation pain syndrome”.

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