The spinal lax zone and neutral zone: measurement techniques and parameter comparisons

Area of Science:

• Biomechanics
• Spinal Surgery
• Orthopedics

Background:

• The neutral zone (NZ) quantifies spinal motion but may not fully capture ligamentous laxity.
• Instability onset is theorized to increase the neutral zone before overall range of motion (ROM) increases.
• The upper boundary of the NZ is traditionally defined by the onset of ligamentous resistance.

Purpose of the Study:

• To introduce and define the lax zone (LZ) as a distinct parameter for spinal biomechanics.
• To hypothesize that the NZ is a subset of the LZ, influenced by joint friction.
• To experimentally differentiate and quantify the LZ and NZ in human cadaveric spinal specimens.

Main Methods:

• Experimental methods for determining the spinal LZ were developed.
• The LZ and NZ were quantified in six human C5-C6 cadaveric specimens under varying preload conditions (magnitude and direction).
• Parameter independence was verified by comparing LZ and NZ responses to load-deformation curve characteristics and specimen resting position.

Main Results:

• LZ variations correlated with the load-deformation curve's 'elbow,' while NZ variations reflected specimen resting position, confirming parameter independence.
• Both NZ and LZ demonstrated sensitivity to cervical discectomy in a separate dataset.
• The absolute change in LZ was greater, but the percentage change was smaller than NZ following discectomy.

Conclusions:

• The lax zone (LZ) is a distinct biomechanical parameter from the neutral zone (NZ), potentially offering a more accurate assessment of ligamentous laxity.
• The NZ may be more suitable for analyzing joint articulation changes under controlled loading conditions.
• The LZ may be more appropriate for evaluating ligament/disc alterations, especially in less controlled loading environments.