THE LIGAMENTO-MUSCULAR STABILIZING SYSTEM OF THE SPINE

Presented at NACOB 98:
North American Congress on Biomechanics
Canadian Society for Biomechanics - American Society of Biomechanics

University of Waterloo
Waterloo, Ontario, Canada
August 14-18, 1998

THE LIGAMENTO-MUSCULAR STABILIZING SYSTEM OF THE SPINE

M. Solomonow, M. Harris, B. Zhou, Y. Lu and R.V. Baratta
Bioengineering Laboratory, Dept. of Orthopedic Surgery
Louisiana State University Medical Center
2025 Gravier Street - Suite 400, New Orleans, Louisiana 70112

INTRODUCTION

The ligaments are well established as the major restraints of the extremity joints. Evolving work from the spine literature, however, clearly demonstrates that the spinal ligament have only minor role, if any, in maintaining the stability of the motion segments. (1,2) Conversely, the same literature clearly demonstrates that the spinal and abdominal muscles are responsible for the spine's stability. (3, 4)

What, therefore, is the role of the spinal ligaments? Several studies show that all spinal ligaments are equipped with mechanoreceptors of various types; fast adapting Pacinian as well as slow adapting Golgi and Ruffini organs.(5) Bare nerve endings of the nociceptive type were also shown to exist in the ligaments. Such receptors are known to signal spinal and central neural circuits of stress, strain, joint angle and limits, vibration and tissue damage.

The hypothesis of this research, therefore, is that mechanoreceptors in the spinal ligaments monitor strains, stresses and angles of the different motion segments, and reflexively initiate muscular activity which maintains the stability of the spine.

METHODS

Human Experiments: Three patients with structural lumbar spine deficits (disc herniation, stenosis, spondylolysthesis, etc.) scheduled for corrective spine surgery were tested. Once the skin incision was made over the spine and all fat and connective tissue were removed to expose the Dorso-lumbar Fascia, four pairs of EMG needle electrodes were inserted into the multifidus muscles, bilaterally, two levels above the level were structural/neurological damage was diagnosed. Bipolar stimulating electrodes was placed over the middle portion of the supraspinal ligament, and delivered rectangular pulses at 10 pps. Pulse intensity was adjusted to yield supramaximal EMG response in the recording electrodes. EMG was differentially recorded from each of the four channels.

Feline Experiments: The same protocol that was employed in humans was implemented on six feline preparations. In addition, a ligature was inserted around the supraspinal ligament of each of the motion segments from L-1/2 to L-6/7 (feline has 7 lumbar vertebrae). One at the time, each ligature was attached to the loading shaft of the Bionix MTS instrument, and a linearly increasing load was applied in the range of 0 - 50 Nt. Differential EMG recordings were made from the multifidus muscles of the L-1/2 to L-6/7 motion segments. The six EMG channels recorded unilaterally were stored on a computer for later analysis.

RESULTS

Human Experiments: Stimulation of the supraspinal ligament in two of the three patients resulted in recording of EMG from the multifidus muscles in the two levels above the level of structural damage. The EMG was present only when stimulating the mid-part of the supraspinal ligament, but not when directly applying the stimulus to the muscles. This confirmed that a ligamento-muscular reflex from mechanoreceptors to the multifidus muscles exists in humans.

Figure

Feline Experiments: Electrical stimulation of the feline supraspinal ligament resulted in bilateral EMG recording from the multifidus muscles. The strongest EMG was recorded at the level of ligament stimulation, with gradually weaker recording as far as 3 - 4 levels above and below.

Mechanical deformation of the supraspinal ligament resulted in EMG recordings from the multifidus muscles which were the strongest at the level of deformation or one above, and with weaker recordings as far as 3 - 4 levels above and below. The EMG intensity was nearly related to the linear increase in force. Defused, high frequency EMG appeared at high loads above 40 Nt, just below the rupture load of the ligament.

DISCUSSION

A ligamento-muscular reflex exists in the spine of humans and the feline model. The reflex is triggered by strains or stress in the supraspinal ligament that probably arises from motion of two vertebrae relative to each other. Activity (contraction) of the multifidus muscles seems to be directly related to the level of ligament deformation, and is intended to increase the stiffness of a given motion segment, as well as 2 - 3 motion segments above and below in order to maintain structural stability and prevent damage to the tissues (disc, ligaments, nerves, cord, etc. . .)

The finding are experimental confirmation to the stability theory proposed by Panjabi,(6) and are probably a strong component of the co-activation of spinal and abdominal muscles described by several groups. (3, 4 and others).

REFERENCES

1. McGill & Norman. Spine, 11:666, 1986.

2. Lucas & Bresler, UC-SF/B, Report #40, 1961.

3. Gramata & Marras. Spine, 20:913, 1995.

4. Zetterberg, et al. Spine, 12:1035, 1987.

5. Rhalmi, et al. Spine, 18:264, 1993.

6. Panjabi. J. Spinal Disorders, 5:383, 1992.