Background:
In the lower back there is an angle formed by the vertical spine and the iliac crest apical which extends downwards over the sacrum and is termed the lumbosacral angle or sacro-iliac angle. This triangle contains the following structures outside of the spine, variously involved in the production of low back pain:
- The L4/5 & L5/S1 facet joints
- The L3, L4 & L5 nerves descending to form the sciatic nerve.
- The posterior braches of L3, L4, L5 and S1 nerves supplying the muscles and skin of the triangle.
- The sacro-iliac joint
- The medial (inner) division of the Cluneal Nerve.
Presentation and causation:
Symptoms arising in this area can cause significant and disabling pain which unfortunately can be mis-attributed:
The L3, L4 & L5 nerves descending to form the sciatic nerve may be irritated in the tunnel (foramen) as they exit the spinal canal (termed “exiting” foraminal nerves) by a variety of factors:
- Disc protrusions,
- Scarring and tethering around the nerve (Perineural Scarring),
- Bone spikes (Osteophytes) arising from the facet joint or vertebral margin,
- Overgrown (Hypertrophic) facet joints squeezing and displacing the nerve,
- Slippage (Spondylosis) of vertebrae one on each other due to loss of disc bulk, causing distortion and squeezing of the nerve,
- Facet joint cysts causing intermittent or chronic squeezing of the nerve(s).
These factors will produce the signs of sciatica or stenosis in the back, buttock, groin and leg.
The posterior braches of L3, L4, L5 and S1 nerves supplying the muscles and skin of the triangle can become irritated by the same factors leading to:
- Hypersensitivity (Allodynia) or pins and needles (Paraesthesiae) of the skin overlying the triangle and variously involving wider territories,
- Spasm of the long and deep muscles in the back causing offset of the spine to one side or in a twisted direction. This may cause secondary effects upon the facet joints or vertebral slippage.
The L4/5 & L5/S1 facet joints may be osteo-arthritic and may cause pain from irritation of the lining of the joint. However arthritis of the facet joints causing more than stiffness is thankfully quite rare.
- The facet joints appear to cause pain in the sacro-iliac triangle or sciatica by an indirect mechanism and self-aggravating vicious circle as follows:
- The facets of the joints override and irritate the nerve exiting the spine (“exiting” foraminal) because of malposture, excess body weight, loss of disc bulk or the other factors causal of sciatica
- The “exiting” foraminal nerve reacts with combinations of swelling, bruising and scarring which cause the nerve to become tethered and unable to escape the attentions of the facet joint.
- The “exiting” foraminal nerve becomes progressively trapped and irritated and causes increasing spasm in the long and deep muscles of the back which increases the compression upon the nerve and worsens the situation.
- Injections of steroid “into” the facet joint have been interpreted as proof that the joint is causal of the pain. In fact the steroids used may percolate round in to the foramen or leak through the capsule on to the inflamed “exiting” foraminal nerve. Thus soothed the spasm reduces and the vicious circle is interrupted for a time.
- The benefit of reduced spasm and preservation of foraminal volume can be extended by ablating the nerves to the facet joint.
- However the definitive treatment is Foraminoplasty which addresses the causal factors at their source.
The sacro-iliac joint can become arthritic in certain conditions such as Ankylosing Spondylitis or Rheumatoid Arthritis. Occasionally it may be involved in axial osteo-arthritis. Sadly it has become the focus of much over diagnosis as a cause of pain arising in the lumbo-sacral angle or sacro-iliac angle.
- The “myth” of sacro-iliac joint pain has arisen due to its proximity to the presentation of pain in the lumbo-sacral angle or sacro-iliac angle.
- Its role again has been fostered by the results of injections in to the joint or more commonly around the joint which have indirectly reduced the spasm in the muscles and that arising from the local nerves so ameliorating the impact of the factors impinging on the “exiting” foraminal nerves and so reducing the pain in the triangle.
- In addition the local spasm has led physiotherapists to detect reduced movements in the area. They in turn have attributed this to a reduction in the movements of the sacro-iliac joint rather than to spasm arising from the “exiting” foraminal nerves.
The medial (inner) division of the Cluneal Nerve produces pain in the lumbo-sacral angle or sacro-iliac angle which may radiate: These pains can radiate and be described as
- to the buttock, and needs to be distinguished from Piriformis spasm pain
- Deeply within the pelvis and occasionally to the coccyx
- Extending into the groin and inner thigh to the knee (mimicking hip joint pain)
- Passing down the posterior thigh to the upper calf (mimicking proximal sciatica)
- Radiating along the anterior and outer thigh (mimicking irritation of the lateral femoral cutaneous nerve of the thigh).
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