The compensation mechanism of cervical muscle dysfunction on spinal stability–an in vitro study using porcine model

Muscle injury/impairment results in decreased muscle forces. Decreased muscle forces can be compensated by synergic muscle forces. However, the effects of muscle dysfunction and compensation on spinal stability are not clear yet. Eight porcine cervical spine specimens (C2-T1) were tested by the spine flexibility testing apparatus. The apparatus was equipped with muscle force replication of three paired cervical muscles. The simulations of muscle recruitment included; no muscle recruitment, normal muscle recruitment, muscle dysfunction without compensation, and muscle dysfunction with two compensation strategies: the minimized muscle forces and the minimized axial forces. The spinal stability was examined by the neutral zone (NZ) and range of motion (ROM), which was the sagittal motion of specimen applied with external moment at 0.5 and 2 Nm respectively. Initial positions of specimens were also recorded. NZ and ROM were largest in the no muscle test, and smallest in the muscle dysfunction without compensation test. NZ and ROM of muscle dysfunction with minimal axial force compensation were larger than those with minimal muscle force compensation. This study concluded that: (1) the muscle dysfunction without compensation constrains spinal motion; (2) impaired muscle with compensations cannot stabilize cervical spine efficiently as normal muscles does; and (3) compensation strategy of minimal muscle forces provides better spinal stability than that of minimal axial forces.