Introduction

Lumbar instability is a pathology of the spine in which there is abnormal mobility or an abnormal joint between two or more contiguous vertebrae. When a patient suffers from lumbar instability, there is excessive movement between the vertebrae and, progressively, a degeneration of the intervertebral joints and can affect the structures of the nervous system that pass through them.

  • Lumbar instability is an important cause of low back pain and can be associated with substantial disability.
  • Back pain is the largest cause of disability-adjusted living years based on the WHO survey of the global burden of disease.

 Spinal Stability

Spinal stability can be related to one, two or three “sub-systems” that greatly influence the spine. These are:

  • The spinal column and its ligaments
  • The nervous system (controls spinal movement)
  • Muscles, which move the spine

In a healthy state, the three systems interact and provide stability. When any one of these sub-systems becomes damaged e.g. age-related degeneration, fractures, neuromuscular disease, the other two sub-systems must compensate. When it comes right down to it, you really can’t tease apart one spinal subsystem from the other. Clinical instability is really a multi-system dysfunction.

The resulting Imbalance can lead to an unstable spine and pain – and can considerably worsen the quality of life of the patients, preventing them from carrying out their daily activities.

Within lumbar instability, we distinguish functional (clinical) instability and structural (radiographic) instability. 

  • Functional instability, which can cause pain despite the absence of any radiological anomaly, can be defined as the loss of neuromotor capability to control segmental movement during mid-range.
  • Structural or mechanical instability can be defined as the disruption of passive stabilisers, which limit the excessive segmental end range of motion (ROM).
  • There is also a possibility to have a combined instability.

Relevant Anatomy

Injury or damage to your spinal column is the most common cause of spinal instability. The spinal column is a complex structure, and there are numerous ways problems can develop in the subsystems.

Spinal Stabilization Systems:

  • Passive subsystem: intervertebral disc, ligaments, facet joints and capsules, vertebrae and passive muscle support.
    • Facet Joint Capsular ligaments (cover and support the facet joints) can become lax. When they do, they introduce excessive movement—and, therefore, instability—in the spine. Among the many possible causes of capsular ligament laxity are disc herniation, spondylosis, whiplash-related problems and more.
  • Active subsystem: spinal muscles and tendons, thoracolumbar fascia,
    • Spinal Muscles are the powerful engines that move the spine. They also provide stiffness and stability. Spinal instability can be the result of weakening or damage to the muscles that support the spine, e.g. multifidus, transverse abdominal. Ultrasound may show a wasting (which is a sign of weakness) of deep spinal muscles, while electromyography (EMG) is often used to detect muscle change.
  • Neural Subsystem
    • The nervous system is responsible for receiving messages about the position of the spinal bones and column and for producing impulses to move. These impulses are relayed to the muscles, signalling them to contract. Muscle contraction powers the spinal movements and provides stability. If these muscles are slow to contract or they do so in an abnormal pattern, you may have a disruption to this neural control sub-system. These two factors can be detected by an EMG test. These disruptions can cause changes in spinal movement patterns, which can be observed by a trained eye (or by a motion detector machine). Abnormalities in the neural control sub-system can also be detected by a nerve conduction study (NCS). An NCS is often performed along with an EMG to detect associated muscle irritation or damage. 

Aetiology

Causes of lumbar instability

The primary causes of lumbar instability can be classified as follows:

  • Degenerative: The most frequent cause of spinal instability in adults is degeneration or early wear of the intervertebral discs, the most important part of the spine, causing abnormal mobility of these structures. This can happen in people who have a greater genetic predisposition to it and those whose bodies are subject to overexertion. These degenerative problems are increased by poor posture, poor physical condition, muscular insufficiency, sedentary life, excess body weight, injury, etc.
  • Congenital:
    • The most frequent is spondylolisthesis, caused by spondylolysis ie bone defect at the junction of the facet joints.
    • Lumbosacral transition anomalies (lumbarization of the first sacral vertebra or sacralization of the fifth lumbar vertebra).
    • Anomalies in the alignment of the vertebral bodies, as in the case of scoliosis.
  • Acquired:
    •  
    • Pathologies that affect the lumbar spine, such as infections or tumours.

A constant morphological modification of the spine alters the biomechanical loading from back muscles, ligaments, and joints, and can result in back injuries.

  • Body mass, task asymmetry, and level of experience affect the scale and variability of spinal load during repeated lifting efforts.
  • In older people, bending and lifting activities produce loads on the spine that exceed the failure of vertebrae with low bone mineral density, which is linked with spinal degeneration.
  • The degenerative transformation has influence on the intervertebral discs, ligament and bone.

Clinical Presentation

Patients with lumbar instability are common patients with chronic recurrent low back pain, a constant nagging pain which gradually increases. This pain can also be a residue of acute complaints. There remains controversy about the exact meaning of the term lumbar instability. The following characteristics can indicate lumbar instability:

  • The feeling of instability, giving way
  • A visually observable or palpable hitch at a moving segment in the lumbar spine, mostly during change of position.
  • Segmental shifts or hinging associated with the painful movement.
  • Moving or jumping of the vertebra accompanied with pain in active trunk flexion or deflexion.
  • An increased mobility at the concerned movement segment, mostly in passive segmental lumbar flexion and extension.
  • Excessive intervertebral motion at the symptomatic level or an increased intersegmental motion at the level above the concerned movement segment.
  • Local pain.
  • Low back pain during long static load and deflexion.
  • Pain during change of position and while bending or lifting.
  • An abnormal motion sensation in postero-anterior movements of the vertebra.
  • Decreased repositioning accuracy.
  • Decreased postural control.
  • Decreased activation of stabilizing muscles.
  • Disruptions in the patterns of recruitment and co-contraction of the large trunk muscles (global muscle system) and small intrinsic muscles (local muscle system). This affects the timing of patterns of co-contraction, balance and reflexes.
  • Pain and the observation of movement dysfunction within the neutral zone.
  • A painful arc.
  • Gowers sign: the inability to return to erect standing from forward bending without the use of the hands to assist this motion.
  • Frequently crack or pop the back to reduce the symptoms, self-manipulation.

Diagnostic Procedure

Getting a diagnosis for spinal stability is based on the observable signs (factors that can be measured or objectively determined) and symptoms (your subjective experience, which may include pain, other sensations and things you notice about the way your back is functioning).

Conservative treatments

  • Patients should avoid over-exertions or strain, sudden movements and holding continued positions that cause sustained tension in the lumbar musculature, as well as any habit that worsens the prognosis of the disease.
  • Chiropractic treatment can help stabilize the section in the correct alignment to prevent reoccurrence.
  • Additionally, conservative treatments include the prescription of anti-inflammatories for short periods and an adequate physiotherapy program to exercise the muscles and thus stabilize the bone or ligament injury.
  • Since the degeneration of passive spinal stabilizing structures (ligaments, articular cartilage and discs) is very difficult to undo, the best prevention strategy is to keep the dynamic spinal stability mechanisms, that is to say, the spinal musculature, healthy and functional.

 Surgical management

For those patients whose pathology is very advanced and do not find relief in less aggressive treatments, there is the option of surgery. There are several types of surgery possible for intervertebral stabilization, but they will almost always require some type of implant that fulfils the stabilization function as a replacement for the failed anatomical structure. Whenever possible, a minimally invasive surgery technique (MIS) is used in order to respect the spinal musculature. Among the different techniques of this type, we can find the following:

  • Extreme lateral interbody fusion (XLIF): used for levels from L1 to L5.
  • Anterior approach arthrodesis: used for levels between L3 and S1, in which the disc is replaced with a mobile or fixed implant (ALIF), depending on the severity of the instability.

In those cases in which conventional stabilization cannot be performed e.g. due to age or health problems, an option is minimally-aggressive surgery This involves implanting an interlaminar or intrafacetary distraction device, located at the most posterior part of the vertebra.

When in addition to this, there is a need to decompress nerve structures such as the medulla or nerve roots, a more open type of surgery will be required to ensure proper release of the structures.