The previous post contained a summary of how a handful of talented individuals (Maitland, Kaltenborn, Mulligan etc.) embraced the teachings of previous practitioners and evolved the practice of manual therapy to a new level. Does it mean however, that the evolution stopped? Does it mean that manipulation reached a plateau? Certainly not!
The latter half of the 20th century saw an additional endeavour to delve deeper into the essential facets of manipulation. To actually learn what is it that we do, what is the reasoning behind the form of our applications, what we want to achieve and what is the physiological meaning of the feedback we get from our treatment.
Initially, pain and the mechanisms behind it came under the scope of clinical research. Given the period, the results were pioneering and a model was conceived. “The Gate Control Theory of Pain” was presented in Melzack and Wall’s work on Pain Mechanisms in 1965. That theory was adapted with enthusiasm in the beginning and influenced the practice and reasoning of many. Nevertheless, that theory was not the answer even though it was the predominant belief until 1987, when Gordon Waddell introduced the “Biopsychosocial Model” regarding the treatment of low back pain (Waddell, 1987).
But what are the benefits of studying pain for a manual therapy practitioner? Since manual 
therapy is often the intervention of choice on problems that cause chronic pain, it is fair to argue that by knowing the nature of pain a therapist can plan a more effective treatment. So, the study of pain, under the scope of manual therapy never stopped and in fact we now walk in an era where psychosocial factors are being added in the pain equation. For a manual therapist it is essential to track those factors that contribute to or maintaining the pain.
In the same manner, the audible crack that often accompanies high velocity-low amplitude techniques came in the centre of attention. Unsworth in 1971 was one of the first researchers that managed to accurately explain the audible crack and describe it in biomechanic terms.
The cracking of joints is due to the phenomenon of cavitation which is produced when the synovial fluid, under subatmospheric pressures, vaporizes and releases gas (Nitrogen). The eventual collapse of the previously formed cavities gives rise to the distinct sound often referred as “crack” (Unsworth et al., 1971).
As with pain, this finding was not the end of the research. Subsequently, manual therapists were facing the challenge of finding out what physiological and chemical changes are caused by the cavitation phenomenon to the nervous system. Why it alleviates the pain, as those who apply manipulation techniques claim, and why those techniques maintain that form all these years?
There was the claim that manipulation of the joints, capable of causing cavitation (audible crack), creates the conditions for the production and release into peripheral tissues of the neuropeptide “Substance P” (Brennan et al., 1992; Payan, 1992; Payan, 1989). Despite that fact, those physiological effects cannot be shown in imaging methods of any kind (Evans, 2002).
In what way exactly manipulation techniques can affect pain is not entirely understood up to this day but the previously reported evidence suggest a level of interaction between mechanical factors and neurophysiological responses to these factors (Bialosky et al., 2009; Pickar, 2002).
Additionally, there has been significant research on the clinical reasoning field in the last years. Procedures that were thought a simple routine, such as neck manipulation, now are suggested to be done with caution. There are many underlying pathologies that a manual therapist should keep in mind if not able to detect. One example is Cervical Arterial Dysfunction that may lurk as a secondary pathology in a patient’s cervical spine. Even though the evidence cannot clearly prove if an adverse effect can be produced by spinal manipulation, the red flags are there. Clinicians must only notice them and modify the chosen treatment (Kerry, 2008). Under that scope, it is suggested that manual therapists’ assessment should expand to procedures that were thought detrimental till now. By taking blood pressure measurements, the risk of causing unwanted effects on a patient is significantly reduced (Taylor and Kerry, 2013).
Of course there is need for continuous future research in order to determine the mechanisms of manipulation and how they truly affect the tissues. We are obligated to uncover and define those neurophysiological mechanisms that occur immediately after a manipulation technique. By discovering the ways that pain is reacting to the stimuli we apply then we will be more efficient. We will be able to modify our treatment and utilize our best skills towards the best result. Don’t you think that the ancients or the predecessors would want to know the answers if they had the means and the knowledge we now possess? I believe that everyone considers that question rhetorical.
As we can see, there is constant reviewing and revising of what we already know or thought we knew. Research doesn’t stop and this is exactly what a “healthy” and respected health profession must do. There is always the need of improving the reasoning which makes us able to make decisions. All the knowledge we gained through the course of history is not gone. It is there. It is always reviewed, revised and improved. The goal was always the same, to treat a patient and improve his quality of life by using the best of our knowledge and talents.
References:
Bialosky JE, Bishop MD, Price DD, Robinson ME, George SZ. The mechanisms of manual therapy in the treatment of musculoskeletal pain: A comprehensive model. Man Ther 2009; 14(5):531-8.
Brennan PC, Graham M, Hondras MA, McGregor M, Triano JJ. Quality control in a randomized clinical trial using cellular chemiluminescence. In: Evans WD. Mechanisms and effects of spinal high-velocity, low-amplitude thrust manipulation: Previous theories. J of Manip and Physiol Therap 2002; 25(4):251-262
Kerry R. Manual therapy and cervical arterial dysfunction, directions for the future: a clinical perspective. J Man Manip Ther 2008; 16(1):39-48.
Melzack, R., & Wall, P. Pain Mechanisms: A New Theory. Science, 1965; 150(3699):971-979.
Payan DG. Peripheral neuropeptides, inflammation and nociception. In: Evans WD. Mechanisms and effects of spinal high-velocity, low-amplitude thrust manipulation: Previous theories. J of Manip and Physiol Therap 2002; 25(4):251-262
Payan DG. Substance P: a neuroendocrine-immune modulator. Hosp Pract 1989; 24:67-80.
Pickar JG. Neurophysiological effects of spinal manipulation. Spine J 2002; 2(5):357–71.
Taylor JA, Kerry R. Vascular profiling: Should manual therapists take blood pressure? Manual Therapy 2013; 18(4):351-353.
Unsworth A, Dowson D, Wright V. Cracking joints. A bioengineering study of cavitation in the metacarpophalangeal joint. Ann Rheum Dis 1971; 30: 348-358.
Waddell G. A new clinical model for the treatment of low-back pain. Spine 1987; 12: 632-644.
Pictures:
Fig. 1: Here-Us-Now. In: www.here-us-now.com
Fig. 2: Pain Exit. In: www.themusculoskeletalelf.net
Fig. 3: Cavitation Mechanics. In: www.wikipedia.org
Fig. 4: Blood Pressure Measurement. In: www.3tend.com
Vasilis Georgopoulos 
A truly interesting article. Always a pleasure reading your research.