Development of an in vivo method to investigate biomechanical and neurophysiological properties of spine facet joint capsules

Abstract

Facet joint capsules (FJC) may experience large mechanical deformation under spine motion. There has been no previous quantitative study of the relationship between capsular strain and sensory nerve activation in spine FJC in vivo. Space limitation in the cervical spine makes such a study difficult, as the facet joint must be loaded while simultaneously monitoring nerve discharge from nerve roots immediately adjacent to the loaded tissue. A new methodology was developed to investigate biomechanical and neurophysiological properties of spine facet joint capsules in vivo. The method incorporated a custom-fabricated testing frame for facet joint loading, a stereoimaging system, and a template-matching technique to obtain single afferent response. It was tested by loading goat C5-C6 FJC in vivo with simultaneous nerve root recordings and 3D strain tracking of the capsules. Preliminary data showed that 18 of 23 afferents (78.3%) were found to be mechanosensitive to tensile stretch, and five were not responsive, even under tensile load as high as 27.5 N. Mechanosensitive afferents in goat capsules had tensile strain thresholds of 0.119+/-0.080. Neural responses of all mechanosensitive units showed statistically significant correlations (all P<<0.05) with both capsular load (r(2)=0.744+/-0.109) and local strain (r(2)=0.868+/-0.088). This method enables the investigation of the correlation between tissue load, deformation and neural responses of mechanoreceptors in spine facet joint capsules, and can be adapted to investigate tissue loading and neural response of other soft tissues.

Fig. 1

Testing apparatus design. The fixture frame accommodates a computer-controlled actuator, a spine fixator and two synchronized stereoimaging CCD cameras. The inset displays detailed setup for nerve root recording and visual marker application on a C5–C6 facet joint capsule. Box A in the diagram and box B in the inset are further illustrated in Fig. 2a, b

Fig. 2

Illustration of the spine fixator (a) and the actuator adaptor setup (b). a The fixator was fixed on the spinous process by a screw (illustrated) between the beam bracket and the gusset bracket. b The actuator shaft was coupled to the load cell by adaptor 1, while adaptor 2 connected the anchoring hook with the load cell. Thick stainless steel wires (2.38 mm diameter) were used to make a 75° hook to assure that the hook did not bend under loading up to 89 N

Fig. 3

An example of identification of a single afferent unit. Five electrical stimuli of the same voltage, duration and frequency are shown superimposed. This train of electrical stimuli on joint capsules was reflected in root recordings as stimulus artifact. Due to their large amplitude, positive peaks of stimulus artifact are not shown. The inset shows that a single afferent was provoked by these stimuli because all five responses had the same waveshape and the same travel time from stimulated spot to recording site, indicating the same CV. Nerve activities are small in magnitude compared to the large stimulus artifact

Fig. 4

An example of template matching technique used in current study. Source action potentials were screened to match the specific pattern of a single afferent. If the match error of a source action potential fell within a margin of error of 30%, it was considered as one impulse of this afferent. A margin of error of 30% corresponded to a mean error percentage of 1–6% of the peak-to-valley amplitude of the waveform for each data point. Histograms could then be obtained for this afferent from multiunit nerve root recordings

Fig. 5a–d

Data summary for a representative 4-mm tensile test. a Multiunit impulses of action potentials. T on the voltage axis indicates a threshold level used to obtain multiunit histogram of discharge rates in b. b Multiunit discharge rate increased during stretch. c An identified C afferent (CV=2.48 m/s) with the waveshape shown in the inset exhibited increased discharge rate after a specific threshold. d Capsular load increased nonlinearly with time during stretch, and showed stress relaxation during the 10 s hold

Fig. 6a,b

Neural response of a representative mechanosensitive C afferent (CV=2.48 m/s) to tensile stretch. Nerve discharge increases above thresholds were linearly regressed against capsular load (a), and maximum principal Lagrangian strain (b). Both correlations were statistically significant