#300 The Insertion Forces and Impact Involved in Trialing a Continuously Expandable TLIF Spacer and a Static Cage

General Session: What's New in Biologics and Biomechanics

Presented by: J. Torretti


J. Torretti (1)
B. Bucklen (2)
M. Moldavsky (2)
S. Khalil (2)

(1) Mount Nittany Medical Center, State College, PA, USA
(2) Globus Medical, Inc., Audubon, PA, USA


Introduction: Interbody spacers provide structural support and increase foramen height, but may require insertion forces larger than the bone-disc interface can tolerate. This study evaluated the insertion forces of a continuously expandable spacer for lumbar interbody fusion (CaliberĀ®, Globus Medical, Inc). This device is designed to overcome certain limitations of static implants; namely, difficulty of a TLIF approach at L5-S1 due to the L5 nerve root.

Methods: Ten human L5-S1 segments were used, five for a static spacer, and five for an expandable spacer. A custom pressure sensor was used to measure the force of impaction on the trials and spacers. A full TLIF was completed (resection of the facet joint plus posterolateral annulotomy). Pressure mapping software was used to measure the force of each mallet strike. Custom slap hammer knobs were machined onto the insertion instrumentation and the pressure film was placed on the slap hammer/knob interface. The number of strikes, average force per strike, and total force (strikes x force/strike) was recorded, following calibration. Static trials were inserted to tightness of fit, starting with the smallest size. The expandable spacer group used an expandable trial inserted at the smallest size.

Results: The number of strikes required to insert the expandable spacer trials was 2.6+/-0.9 strikes, and the expandable spacer 2.6+/-0.5 strikes (Fig.1) The number of strikes required to insert the static spacer trials was 8.4+/-3.8 strikes, and the static spacer 4.2+/-1.6 strikes. Per trial size, this results in 3.8+/-2.1 strikes for the static spacer, with an average of 2.2 trial sizes used. The average force/strike required to insert the spacers and trials was relatively constant. The total force (number of strikes x force/strike) required to insert the expandable spacer trials was 150+/-71.5 N, and the expandable spacer 180.4+/-58.9 N. The total force required to insert the static spacer trials was 378.3+/-123.3 N, and the static spacer 228.5+/-78.5 N.

Conclusions: The authors quantified the forces needed to trial and insert both an expandable interbody spacer and static spacer as used in a TLIF decompression and fusion. There was statistically less force required in the trialing phase of the expandable spacer when compared to the static spacer, due to the number of mallet strikes required for sequential trialing, while the average force/strike was relatively constant across trials and implants. While not significant, there was noticeably less force to insert the expandable implant.

Fig. 1. Work of Impacting Spacers and Trials