Relaxation Response of Lumbar Segments Undergoing Disc Space Distraction: Implications to the Stability of Stand-Alone Anterior Lumbar Interbody Implants

Purpose: To measure the distraction force and its relaxation over a period of up to 3 hours following disc space distraction.

Methods: A biomechanical study of 22 fresh human lumbar motion segments undergoing disc space distraction for insertion of anterior lumbar interbody implants as a function of the distraction magnitude and disc level. A discectomy was performed through a rectangular window in the anterior annulus. Disc-space distraction was performed using distraction plates attached to an MTS machine with the specimen in a saline bath. The distraction test battery consisted of two relaxation tests (2- and 4-mm distraction holds of 2-hours and 3 hours, respectively), a tension stiffness test before and after each relaxation test, and a recovery period before each distraction input. L1-2 and L2-3 segments (n=8) were distracted to simulate insertion of a rectangular implant. L3-4, L4-5, and L5-S1 segments (n=14) were distracted to simulate insertion of a lordotic implant. Disc heights, lordosis, and distraction force were measured for up to 3 hours following disc space distraction.

Results: Peak distraction force was larger for the 4-mm distraction (432±116 N) than for the 2-mm distraction (205±55 N) (p=0.00). The distraction force relaxed over time (p=0.00), approaching steady state values of 146±47 N at 2-mm and 290±93 N at 4-mm distraction. The maximum steady-state distraction force observed in the sample was less than 400 N. The distraction force reduced in magnitude by more than 20% of peak value in the first 15 minutes (21% for 2-mm and 23% for 4-mm distraction) Disc space distraction of 2 mm did not affect the behavior of the segment in tension. In contrast, 4-mm distraction caused 41±11% decrease in the specimens’ tension stiffness (p=0.00).

Conclusions: The “tightness of fit” that the surgeon notes immediately post interbody device insertion will degrade in the very early post-operative period. This suggests that the surgeon should be more than just “adequately” satisfied with how tight the interbody device feels in anticipation of load relaxation. Re-distraction of the disc space after 15 minutes may increase the preload on the implant; however, over-distraction should be avoided since it can cause soft tissue damage. Alternatively, supplemental stabilization may be useful in stabilizing the implant-bone interface during the early post-operative period.