Stiffness

An early definition simply maintained that instability was loss of stiffness.⁷ A later elaboration introduced a clinical dimension, to the effect that instability is a

Other engineers have disagreed, insisting that any definition of instability should include the sense of sudden, unpredictable behaviour; that a small load causes a large, perhaps catastrophic, displacement.⁶ They argue that loss of stiffness may simply describe loose or hypermobile segments that are not at risk of catastrophic collapse.

Indeed, any definition expressed simply in terms of stiffness is inadequate and inappropriate. It is inadequate because it raises the question ‘How much less stiff should a segment become before it is considered unstable?’. It is inappropriate because it does not convey the sense of impending failure. In that regard the definition that includes the terms ‘catastrophic displacement’ is more appropriate but there is still the question ‘What constitutes a “catastrophic displacement”?’.

There may well be conditions of the lumbar spine that involve loss of stiffness and the production of symptoms, but these do not necessarily constitute instability in the full sense of the word, and perhaps an alternative term should be applied, such as ‘segmental looseness’ or simply ‘hypermobility’.

Neutral zone

A refreshing new definition that has emerged is one that essentially defines instability as an increased neutral zone. Explicitly, the definition is

The neutral zone is that part of the range of physiological intervertebral motion, measured from the neutral position, within which the spinal motion is produced with a minimal internal resistance.⁹ In essence, although not exactly the same mathematically, it is similar to the length of the toe phase of the stress–strain curve that describes the behaviour of the segment (Fig. 16.1).

Figure 16.1 An archetypical stress–strain curve showing the location of the neutral zone (NZ).

This definition describes joints that are loose but early in range. Their ultimate strength may be normal but early in range they exhibit excessive displacement (Fig. 16.2). This definition captures the sense of excessive displacement; it captures the sense of excessive displacement under minor load but it defies the engineering sense of impending catastrophic failure. However, it does so deliberately and not totally without regard to catastrophe.

Figure 16.2 The stress–strain curve of a lumbar segment that exhibits instability in terms of an increased neutral zone (ΔNZ) compared to a normal curve.

The neutral zone concept directs attention away from the terminal behaviour of a joint to its earlier behaviour. This allows the definition to be applied to circumstances more common than those associated with impending failure of the spine; it is applicable to the conditions otherwise described as ‘looseness’. The sense of catastrophe, and hence instability, is nonetheless retained in a modified form.

As a joint moves through an extended neutral zone it is undergoing an inordinate displacement. If extrapolated, this behaviour predicts that the joint will eventually fall apart. Hence the sense of impending catastrophe applies. It transpires, however, that eventually the inordinate motion of the joint is arrested and catastrophe does not ensue. Nevertheless, during the neutral zone, the movement looks and feels inordinate and threatening.

Instability factor

The engineering definitions of instability describe what might be called terminal instability: the behaviour of a system at its endpoint. It is there that the sense of impending failure arises. Another interpretation addresses instability during movement rather than at its endpoint. It focuses on the quality of movement during range, not on terminal behaviour.

Flexion–extension of the lumbar spine is not a singular movement; it involves a combination of rotation and translation (see Ch. 8). Notwithstanding the range of motion, the quality of motion may be defined in terms of the ratio between the amplitude of translation and the amplitude of rotation. For each phase of movement there should be a certain amount of translation accompanied by an appropriate degree of rotation. If this ratio is disturbed, the motion becomes abnormal and the sense of instability may arise. In this regard, the instability would be defined as an inordinate amount of translation for the degree of rotation undergone, or vice versa.

Normal lumbar segments exhibit an essentially uniform ratio of translation to rotation during flexion–extension.¹⁰ The overall pattern of movement looks smooth; translation progresses regularly, as does rotation (Fig. 16.3). The ratio between translation and rotation at any phase of movement is the same as the ratio between total translation and total rotation.

Figure 16.3 A normal movement pattern of a lumbar segment in terms of the ratio between translation and rotation.

(Based on Weiler et al. 1990.¹⁰)

It may be defined that instability occurs when, at any time in the movement, there is an aberration to this ratio. The segment suddenly exhibits an inordinate translation for the degree of rotation undergone, or may translate without any rotation (Fig. 16.4).

Figure 16.4 A movement pattern of a lumbar segment showing an aberrant ratio between translation and rotation, and an abnormally high instability factor.

(Based on Weiler et al. 1990.¹⁰)

This definition conveys the sense of inordinate displacement but places it during the normal range of motion instead of at its endpoint. The segment may be terminally stable but expresses instability during range. The sense of catastrophe does not obtain in the conventional sense, in that the segment will not fall apart, but it is present qualitatively. For that brief moment when the unexpected inordinate movement suddenly occurs, the sensation will be the same as that of impending failure. The fact that the joint is ultimately stable is not sufficiently reassuring, for during the unstable phase the movement is alarming and qualitatively the same as if the spine were about to fall apart.

Special techniques are required to detect this form of instability. They involve taking serial radiographs of the motion, at least five exposures for the entire range of motion, and determining the ratios of translation to rotation for each phase. From these ratios, an instability factor (IF) can be computed, namely.

where (ΔT)_i is the range of translation for each phase of motion (i) and (Δθ)_i is the range of rotation for each phase.¹⁰ In normal spines, the instability factor has a mean value of 25 (mm radian⁻¹) and a standard deviation of 8.7. Values beyond the upper two SD range nominally qualify for instability.