Anatomy Doesn’t Lie
One of my favorite books is Clinical and Radiological Anatomy of the Lumbar Spine by Nikolai Bogduk. It is currently in its 5th edition and is the most detailed and functional explanation of lumbar anatomy. The entire second chapter is dedicated to the interbody joints and the intervertebral disc, and although the book is great from cover to cover, this chapter is a must read. From the outset, Bogduk emphasizes the need for a disc to have strength to transfer the body weight through it (compression), transfer load through vertebrae (compression) and deformable and strong enough to handle vertebral rocking and twisting (torsion). The net result is one of the most magnificent structures in the human body. Here are a few anatomical facts of the lumbar disc:
- The lumbar discs are the largest avascular structures in the human body, and the central region of the largest discs in the adult can be almost 10mm from the nearest direct blood supply (Holm and urban, 1987)
- The outer 1/3 of the annulus is innervated with the inner and central parts being aneural.
- Degeneration of the lumbar disc consists of alterations of proteoglycan content. This is not uniform through the nucleus and annulus but is focal and varies by individual (Iatridis et. al. 2012). Proteoglcans are key moloecules for water binding and therefore alterations will alter structural integrity of the lumbar interbody joint.
- The discs of the human spine account for 20% of spinal height. This is why astronaut Scott Kelly was a full 2” taller in height after his 340 days on the International Space Station.
The Body Heals in a Predictable Manner
This section is going to discuss some anatomical facts about disc degeneration and healing. One of the theories of disc degeneration is that the endplate is susceptible to injury. This damaged area becomes vascularized (this region of the disc is avascular). This will decrease proteoglyan concentrations and also water content. As a result two things happen:
- The inner annulus delaminates
- Nuclear pressure falls.
This will alter the structural integrity of the disc. Meaning, that typically an endplate can resist loads of 37% to 50% of its failuture strength without failing 1000 – 2000 reps (Hansson, 1987 and Liu 1983). But, if damaged it can fail at as few as 100 repetitions (Hansson, Ibid).
Wait a minute, I thought this section was talking about healing … Well, we had to start with some explinations, but this alteraion in the disc can be a 2 year process, therefore, we need to factor that into our stratgies for our patients. If there is delamination and segmental compromise, a stabilization program and healthy confident movement strtaegies will have to be incorporated o
Pain is in the Brain
Let’s look a little closer at the topic of convergence. When a patient has referred pain into the buttock there are several potential sources of this pain. Anatomically the lumbrosacral region is innervated by dorsal rami and the deep tissues of the buttock are innervated by the ventral rami (superior and inferior gluteal nerves). Within the spinal cord the sensory neurons that receive input from different peripheral sites converge onto common neurons which relay to higher centers. Without additional sensory information, the brain is unable to determine whether activity in the common neuron was initiated by one or the other of its peripheral inputs. So, true somatic referred pain is diffuse and hard to localize and is usually described by patients as deep and achy in quality. Physiologically, the critical feature is that it is evoked by stimulation of the nerve endings in a structure that is the primary source of pain. Referred pain occurs because of a misperception of the origin of the signal which reaches the brain by a convergent sensory pathway. Letting patients know that leg pain is not “sciatica” and that they can have leg pain which is somatic referred is important. If they cannot make the distinction, then they will be fearful of their movements and think that all their pain is coming from the nerve. This will be discussed further in the next section.
We are Prone to Clinical Reasoning Errors During a Busy Clinic Day
Given the information above, if a patient complains of “sciatica”, it is important to not make the error that if they are feeling pain in the back of their thigh that it is from the sciatic nerve. Most likely this is somatic referred pain and not “neurogenic pain.”
|Somatic Referred Pain||Radicular Pain|
|Constant in position
Aching in Quality
Limb pain, narrow band, easily localized
2” wide (specific)
And, according to Bogduk There is no physiological evidence that constant, deep aching pain in the lower limb arises from nerve root irritation.
Treat the Person and You Win Every Time
Video of the Week
Short video looking at alternative treatments for posterior lateral disc protrusions.
Here is a chapter that Springer put online regarding the anatomy of the IVD, it is by Michael Adams, it is thorough and well referenced: The Intervertbral Disc
A little something extra …
Vox just published an article “A Comprehensive Guide to the New Science of Treating Lower Back Pain” worth the read and a lot of self reflection!
Want to discuss more manual therapy topics? Find a NAIOMT course in your area that’ll take your skills to the next level. Enjoy your week!
Terry Pratt, MS, PT, COMT, FAAOMPT
One thought on “Musculoskeletal Monday: The Lumbar Disc”
Thanks so much for this well written article! Great anatomy review.