Some thoughts about HVT

This paper is an overview of the technical issues involved in the use of HVT (high velocity, low amplitude thrusts). It might be useful for those needing a deeper background while learning this art.

[I wrote this back in 2002, and have kept it as it was. Re-reading it so many years later and with so much more clinical experience, I find that I largely agree with the thoughts I had back then.]

First, it is worth clarifying what type of problem in a spinal facet joint requires manipulation. Some definitions;

  • A locked segment won’t move appropriately in any direction, even after soft-tissue and articulation of the segment. This is a potential candidate for HVT. It is important to distinguish normal joint locking from the lack of movement typical of osteo-arthrosis, but this is generally only a consideration in the older patient.
  • A tense segment is restricted typically in just one plane of movement, and responds quite well to soft-tissue and other indirect approaches. Tense segments should not be manipulated.

Some Ideas About Levers

We use these to achieve two main objectives

  • The first is to harness force (levers alter the forces input by the operator).
  • The second is to direct force.

We describe them in terms of movements (e.g. rotation). Classical physics and engineering descriptions of levers assume rigid structures. This is not the case with the human body where, rather than one rigid lever, there is a multitude of connective tissues that have different orientations, elasticities, rigidities and weights. Furthermore, human muscle tissue can exert contractile force to resist levers and thrusts.

Class 1 Lever
A Class 1 Lever – a see-saw

Levers harness force. We can see this in the classic seesaw. This is a class 1 lever. There are two other types reflecting other ways in which force, fulcrum and resistance can be spatially arranged. Some muscles (all muscles are class 3 levers) rely on leverage to generate more force.

Class 2 and 3 Levers
Class 2 and 3 Levers

A muscle that has an insertion a long way away from the joint (e.g. brachioradialis) will, for a given amount of muscle mass, produce more force than a muscle whose insertion is closer (e.g. triceps brachii). Positioning the patient using bones and soft-tissues, therefore generates a number of complicated levers which all serve to multiply the force exerted by the operator. For example, it should be evident that a lumbar roll is more forceful than a cervical spine thrust. It should also be clear that this is necessary to overcome the greater resistance of the lumbar spine.

Brachioradialis
Brachioradialis illustrating class 3 levers

Levers direct force. This is important in tissues that do not operate in the ‘rigid’ manner of machines. They do this in three ways.

  1. General Levers. These orientate tissues such that further application of forces localise at the desired area. Example: the way we use thoracic and pelvic rotation with lower limb placement to balance tension around a lumbar segment. Force behaves like water in that it takes the path of least resistance. If you correctly balance your target segment between the levers it has two benefits. Firstly, you need to produce less force to achieve the desired result. Secondly, you increase specificity.
    Some authors (e.g. Gibbons 2001) explain the use of what I have termed general levers by reference to the ‘locking’ of adjacent spinal segments. This locking is produced by side-bending and rotating to opposite sides – supposedly consistent with Fryette’s laws of vertebral motion (Fryette 1954) – while leaving the target segment ‘free’ to thrust. There has been great debate in the profession about these laws, but whether or not Fryette is right about the behaviour of vertebral segments, I am unsure that actual locking of segments above and below the target segment is what in practice happens. Instead, I prefer to think of tensioning the area around the target segment and that this tensioning occurs through multiple components, producing a strain through all the elastic tissues, rather than the articular barriers created by the opposing surfaces of facet joints. Would relying on the locking of adjacent facet joints to achieve force localisation be desirable?
  2. Local Levers. These alter the tension of the target segment such that a force (the thrust) will safely cavitate that joint. We can adjust this local tension by small movements of the general levers, or by adding additional directions of tissue movement such as compression, traction, translation etc. It is not clear exactly what happens when we make these ‘micro’ adjustments as there are some quite complicated issues to consider when thinking about what actually happens in a cavitation. For example, what does the application of levers (tension) have on intra-capsular pressure before the thrust itself? Some authors argue that these small local forces stretch the capsule and its adjacent tissues in such a way that an increase in intra-capsular volume safely produces the cavitation without taking the joint to an anatomical end of range. Others argue that nothing much happens at the level of the segment itself and that the joint itself is ‘in neutral’ before the thrust (in other words, it is the general levers and the speed and direction of the final thrust that are important). But we do make alterations to the local segment tension in some thrusts, for example, the use of traction in sitting cervical HVT’s to offset the effect of gravity on the segment.
  3. Thrust Levers. This delivers the final thrust force. An example is how we further rotate the pelvis to deliver the thrust in a lumbar roll. There is a great deal of debate about the ‘direction’ of thrust and the need to be aware of facet orientation before and after the application of leverage. In my opinion, this issue is essentially a practical matter in that it is important to direct forces in such a way that facet apposition (blocking) does not occur. This is most important in thrusts that use quite short levers and where structural integrity is critical, a good example being the cervical spine. Note that successfully avoiding facet apposition does not alone guarantee safety! Theoretically, the fastest way to increase intra-capsular volume is through a thrust direction that is perpendicular to the plane of the joint. But even this assumes that ‘slack’ in the capsule is evenly distributed. In reality, issues to do with practicality and safety of tissues supersede theoretical matters. Hence, providing facet apposition is avoided, the direction of thrust matters only in so far as we can generate the easiest (i.e. least force) and safest technique.

More Definitions

Cavitation. The audible pop sound made when manipulating a synovial joint. These pops may spontaneously happen in people from time to time. For example, as they twist their lumbar spine, or move their neck. Popping of joints is also possible in peripheral synovial joints.

Long Levers. When using levers that are quite a distance from the target segment. Example: using the lower extremities and pelvis in a lumbar roll. Long levers increase force.

Short Levers. When using levers that are close to the target segment. Example: a cervical thrust.

Nil Levers. Actually, using little leverage (no significant amplification of operator force). Example: prone thoracic thrust.

HVT (high-velocity thrust). This term refers to the final impulse of the thrust, which in this case is fast.

Low Amplitude. Here the final thrust movement (after the application of levers) has very low amplitude. This is a core part of the way we teach HVT, so more appropriately the kind of spinal thrust taught at the UCO could be called a ‘high velocity, low amplitude thrust’ (or HVLA). We conventionally abbreviate it to HVT. There is a significant inverse relationship between amplitude and speed of thrust.

CLT. Combined leverage thrust – an older terminology no longer much used but meaning the same as HVT and perhaps more applicable to HVT’s with only two levers (e.g. rotation and side-bending in the neck).

MLT. Minimal leverage thrust – not meaning (despite the term) that little leverage is applied, but rather that by the use of many (i.e. more than two or three) general levers, focussed tension at the target segment can be created with a minimum of tissue strain. In other words, no one lever has a dominant proportion of the generated tension. This approach is introduced in the 4th Year and requires a greater expertise in sensing barrier tension and in executing extremely rapid low amplitude thrusts. Perhaps MLT should mean multiple leverage thrust?

Components. Meaning levers, and a term (to the best of my knowledge) introduced by Hartman.

Manipulate. The way we use this term at the UCO is to mean HVT. Bear in mind that other schools and professions may use it in the general sense to mean any kind of technique (e.g. soft-tissue manipulation).

Mobilisation. This is a term previously associated with physiotherapy meaning articulation (of a joint), but bear in mind that some osteopaths use it to mean HVT.

Barrier Tension. This is the feeling of tissue tension generated by a combination of general and local levers at which the operator can perform the final thrust to produce cavitation of the joint.


Review of Physical Principles

HVT is a forceful technique compared to, for example, functional technique. It is useful to review the basic principles of forces, energy etc. The primary constraint we have with physical tissues is the amount of movement (or strain) we can safely put through the target tissues (in this case, a facet joint). A secondary constraint is that we want to avoid unnecessary force ‘flooding’ into surrounding tissues. For the purpose of the following discussion, we will discuss the final impulse that aims to cavitate the joint, although the concepts equally well apply to all the prior stages of the HVT and, indeed, most other techniques.

Force is a product of Mass * Acceleration. The mass of an object is its quantity of matter. Patients vary in mass terms! You apply force to a patient’s tissues when you move them. Assuming no leverage, then how quickly you accelerate tissues and how much mass you are working on (e.g. a large versus a small patient) determines the force you are using. What we do with an impulse is to generate a sudden force with a definite endpoint. We stop the force generation at a certain point (aiming just short of the amplitude point). Practically, this means that we can consider the acceleration of the mass of the patient to be the same as the speed achieved. Remembering the earlier discussion about how levers amplify forces, then it is evident that the larger the patient and for any given amplitude, to achieve the required force requires either more acceleration (which we can think of as speed) or more leverage. All the following terms (work and power) are just ways of measuring energy consumption.Work

 

Work is a term used to describe over what distance you have generated a given amount of force. Work = Force * Distance. Work is simply energy that has been used. Hence to minimise the amount of energy you need to ‘put into’ a patient – something that is an important aim of safe technique – you need to limit the amount of force used and what amplitude you apply it over.

Power is a term used to describe how much work you are doing per unit time. In other words, how fast have you done your work or used your energy? Power  = (Force * Distance) / Elapsed Time. Since Distance / Elapsed Time is simply Speed, another way of thinking about power is that it is Force * Speed. Put another way in terms of an impulse, the power you have used in a technique (for any given amount of leverage) is how much mass did you have to move, what was the speed at the end of the impulse and how fast did you get the tissues to that speed?

Velocity is a vector quantity meaning the speed and direction of mass. Combined with the term amplitude we can now see the derivation of the term high velocity, low amplitude thrust. It neatly emphasises the basics of HVT viz. a high speed, a definite safe direction of thrust, and a low (safe) amplitude.

To re-emphasise – given that distance (what we call amplitude) is our primary limiting factor for safe technique, and that we can estimate, through experience, the amount of force required to generate tissue change in any given patient, the variable we need to control is that of speed. It is intuitively and logically apparent that there is an inverse relationship between amplitude and speed. Thus speed without control of direction and without a definite stop or arrest point is potentially dangerous (see Hartman).


What Causes the Pop?

It is generally agreed that the pop sound itself is either the sound of dissolved gases (carbon dioxide, nitrogen etc.) bubbling out of the (synovial) solution within the joint membrane, or the sound of the joint capsule ‘snapping’ in or involuting during the thrust. The first of these two hypotheses is the most favoured. The motive force in both cases is hypothesized to be that the thrust produces an expansion of the intra-capsular volume. As the capsule, though not providing a perfect seal from the adjacent tissues, is a good seal for the brief period of time through which a thrust is delivered, this expansion in intra-capsular volume must be accompanied by a drop in joint pressure (Boyle’s law). Hence the first hypothesis (dissolved gases) is rather like opening a coke bottle quickly in that the faster the pressure drop, the more gases bubble out of solution per unit time. Based on general clinical experience, sometimes the cavitation seems to be needed to achieve therapeutic success, and sometimes not!


Explanation of Effect

The above paragraph explains (perhaps) the origin of the pop.

I feel we can accept as real that locked facet joints do occur and that they can persist for some time, disrupting the smooth integration of the surrounding spinal areas. Also grounded in solid evidence, both clinical and research, is the belief that HVT does something to improve this state of affairs.

But a number of questions remain;

  1. What actually happens to a locked joint that requires an HVT to resolve?
  2. How does the HVT help?
  3. Is the cavitation part of the solution, or just an accessory result (or artefact)?

One approach seeks answers in the internal (intra-capsular) state of the joint. For example, some kind of abnormal joint pressure (too much synovial fluid, too little, the wrong kind of gases, and so on).

Another is to look outside the joint at the surrounding small musculature (in the case of spinal facet joints, rotatores and the other erector spinae that cross a maximum of two segments).  Here the suggestion is that some kind of disruption to the normal, extremely local, muscular control of the segment has occurred that ‘holds the facet joint in place’ in an abnormal way.

Other suggestions combine the above, postulating that small changes in intra-capsular states trigger disruption of smooth muscular control (through an ‘arthro-kinetic’ feedback loop), and vice-versa.

All of the above, of course, capable of happening in the first place through some kind of trauma (an uncontrolled movement in one or more planes).

Through the HVT, some kind of successful reset of the above problem occurs.

I’m sure most practitioners would agree that spinal facet joints that have been locked for quite some time both feel different during palpation than merely tense joints and actually sound different during an HVT release. Though unverified, these observations seem to point to physical changes happening somewhere in the joint’s capsule – local musculature loop.


Conclusion

Spinal facet joints do not seem to need a high rate of acceleration to enable a cavitation. In fact, quite slow movements of segments can result in a pop or pops. We have all seen this when certain patients make slow active movements on examination. It may be that the explanation lies in the distinction made at the start of this article between tense and locked facet joints.  When patients ‘click’ their own joints, the majority of the time they are self-manipulating tense, not locked joints. The forces they put into their own spine are not focussed, and just follow the path of least resistance towards the tense segment.

All of the above discussion emphasises the need for controlled speed. Why is this?

The reason is that the need for speed during efficient spinal manipulation is not to produce cavitation per se (i.e. by rapidly stretching the joint itself). Instead, it is because of the need to deliver the appropriate force to a target segment that is ‘waiting’ to cavitate. Tissues are not rigid levers but instead constantly change. Holding tissue tension locally and in the surrounding areas is like trying to focus on a moving target. We have already seen that speed reduces the need either for increased leverage or for increased amplitude. But we also need speed to ensure that force is not dissipated into surrounding tissues during the thrust, something that seems to happen during slow impulses probably (but not only) because of the difficulty in maintaining barrier tension. Instead, speed allows the impulse force to flow straight and economically into the target segment.

You will know that ‘bad’ ways to counteract lack of controlled speed include;

  • increasing leverage to prevent this force dissipation.
  • using more force.

And patient self-manipulation – either deliberate, or accidental via an innocuous movement – is highly likely to be of tense, not locked joints. The art of manipulation is to distinguish facet joints that are capable of movement but are tense, and which should not be manipulated, from facet joints that are locked and can thus benefit from manipulation.


 

References

Fryette H, 1954, Principles of Osteopathic Technique, AAO, Newark

Gibbons P and Tehan P, 2001, Journal of Bodywork and Movement Therapies, April 2001, 110-119

Haas M, Journal of Manipulative and Physiological Therapies, May 1990, 204-246

Hartman L, Handbook of Osteopathic Technique, 1997, Chapman and Hall, London

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