Increase or decrease in stability after nucleotomy? Conflicting in vitro and in vivo results in the sheep model

Abstract

Nucleotomy is a common surgical procedure to treat disc herniations. The potential occurrence of segmental instability after surgery, however, is suspected to necessitate re-operation and fusion. Although in vitro studies support the theory of destabilization after nucleotomy, a prior, in-house animal study contrarily revealed an increase in stability after surgery. To identify which structural compartment of the motion segment is decisive for increased stability after nucleotomy in vivo, the flexibilities of ovine motion segments were measured after different stepwise reductions at the anterior and posterior spinal column. Different test groups were used in which nucleotomy had been performed during surgery in vivo and under isolated in vitro conditions, respectively. In accordance with expectations, in vitro nucleotomy on ovine motion segments significantly increased flexibility. By contrast, nucleotomy significantly decreased flexibility 12 weeks after surgery. After removal of the posterior structures, however, the differences in flexibility diminished. The present results thus suggest that it might not exclusively be the trauma to the intervertebral disc during surgery which is decisive for post-operative stability, but rather adaptive mechanisms in the posterior structures. Therefore, care should be taken to minimize the damage to the posterior structures in the course of the surgical approach, which more likely compromises stability.

A comparison between porcine, ovine, and bovine intervertebral disc anatomy and single lamella annulus fibrosus tensile properties

“…Smaller sized animals such as the mouse and rat (recently Bailey et al, ; Holguin et al, ; Jin et al, ; Miyagi et al, ; Pelle et al, ) as well as larger animals such as the pig (recently Ivicsics et al, ; Hebelka et al, ; Yilgor et al, ), sheep (recently Reitmaier et al, ; Wade et al, ), and cow tail (recently Chan and Gantenbein‐Ritter, ; Eder et al, ; Farrell and Riches, ) have been extensively used in the literature. Substantial work has also been conducted on verifying the suitability of such animal models for the study of IVD disorders (Wilke et al, ; Beckstein et al, ; Neidlinger‐Wilke et al, ; Ohtori et al, ; comprehensive reviews by Alini et al, and Singh et al, ).…”

A pilot investigation of chemonucleolysis-induced intervertebral disc degeneration in the ovine lumbar spine

“…This has been observed by others and could be due to a reactive process involving the posterior elements in response to inflammation and overloading, and thus are attempting to stabilize the degenerate FSU’s. [49] This could also explain why overall FSU range of motion was ultimately unaltered in the degenerate study group. Similar phenomena have been observed in human degenerate lumbar IVDs in which spinal instability (i.e.…”

“…increase in neutral zone and overall range of motion) occurs in early- and mild degeneration,[50,51] however restoration or re-stabilization of axial spinal kinematics has been shown to occur as degeneration progresses. [49] However, further study of this in the sheep model is warranted.…”

Finite element comparison between the human and the ovine lumbar intervertebral disc

“…The mechanical properties of the nucleus pulposus and of the cartilaginous and bony endplates were taken from the literature 15 . A convergence analysis based on the flexibility has been performed 12 generating three different mesh sizes (mesh 1 had 4500 elements, mesh 2 56496 elements and mesh 3 had 90816 elements), and the model was then validated comparing the results with the flexibility data measured in vitro by Reitmaier et al 16 . Because the rotation difference between the meshes was lower than 5%, mesh 2 has been used for the study.…”

“…1). Because of the different anatomy and physiological loads of the two species, a moment of 3.75 Nm, correspondent to the physiological ROM of sheep, and a follower load of 130 N were applied 16 . The resulting load was equivalent to a complex load due to the combination of two main rotations ( Fig.…”