Biomechanics of Dynamic Rod Segments for Achieving Transitional Stiffness With Lumbosacral Fusion

Abstract

The effect of the dynamic rod segment on the kinematics of the transition level was less pronounced than that of a fully rigid construct in vitro with this particular rod system. This experimental model detected no biomechanical alterations at adjacent intact levels with hybrid or rigid systems.

The Current Testing Protocols for Biomechanical Evaluation of Lumbar Spinal Implants in Laboratory Setting: A Review of the Literature

“…Most multisegmental studies involved one [ 32 , 33 , 35 , 36 , 38 , 40 , 42 , 44 – 49 , 51 – 56 , 59 , 65 , 66 , 68 – 70 , 72 , 74 , 75 ] (60.9%) or two [ 34 , 37 , 39 , 41 , 43 , 50 , 60 , 62 , 64 , 67 , 71 , 76 ] (26.1%) operative level(s) in their testing sample. Moreover, the L3-L4 and the L4-L5 were the most frequent operative levels used in multisegmental investigations (58.7%) that included one or more spinal segment as their operative level(s).…”

“…A follower compressive preload of 400 N during FE motion was used in thirteen (13) [ 44 – 50 , 56 , 61 , 65 , 69 , 70 , 76 ] of the nineteen (19) multisegmental publications reporting the use of some preload in at least one of their flexion-extension (FE) tests. There were four (4) publications [ 52 , 53 , 59 , 60 ] that mentioned the performance of an additional test where a 400 N follower compressive load was applied through a notched belt looped oriented midsagitally over the specimen using a compression-flexion apparatus, which were not included in Table 2 under the preload classification.…”

“…Moreover, the quasistatic protocol was only used in multisegmental studies (in 21.7% of the 46 articles), where the step size was reported in Nm, commonly as 1.5 Nm, except for one article that did not specify the actual step size ( Table 2 ). In terms of the length of the step, most investigations reported 45 sec [ 33 , 38 , 52 , 59 , 60 ] to allow creep deformation, while 30 sec [ 64 , 73 ] and 60 sec [ 58 ] were also seen. For single FSU studies, on the other hand, there was no evident trend for reporting the load rate as deg/sec or Nm/sec ( Table 2 ), and the following ranges were seen: 0.1–0.5 Nm/sec and 0.05–1.08 deg/sec, with the exception of the investigation performed by Clair et al [ 29 ] that reported the rate as 10 N/sec (off-centered loading method) ( Table 2 ).…”

“…There was 1 investigation that used the combination of 7.5 Nm with no axial preload [ 52 ] but did not state explicitly to have followed Wilke et al protocol. All other publications (14) that applied 7.5 Nm of torque in FE, LB and AR deviated from the protocol recommended by Wilke et al by (a) not mentioning the use of any preload [ 33 , 37 – 39 , 43 , 53 , 59 , 60 , 64 , 75 ] (from where 4 included additional FE testing using 400 N of follower preload [ 33 , 53 , 59 , 60 ]), (b) including 400 N of follower load in all motions [ 46 , 50 , 76 ], and (c) including 100 N of axial load for all motions [ 42 ]. As previously mentioned, 8 Nm was also used in 10 investigations, from where 4 did not mention the use of a compressive load [ 35 , 41 , 57 , 73 ], 4 included 400 N of follower load in all direction [ 45 , 56 , 69 , 70 ], and 2 used 0 N of compressive load [ 34 , 68 ].…”

Stability and Load Sharing Characteristics of a Posterior Dynamic Stabilization Device

“…On the contrary, in a study of a device with similar design characteristics to the PDS device used in the current study, Lazaro et al found significantly greater LB and AT ROM in the flexible rod compared to a titanium rod. 22 However, it is critical to note that this study investigated both implants as stand-alone devices with no interbody spacer, constituting a motion-preserving construct which is a commonly referenced configuration for this class of devices. 23 – 25 Since the current study was designed to elucidate differences in fixation and load sharing of the two devices used in an adjunct-to-fusion model, no evaluation of the devices as stand-alone fixation was conducted.…”