(L-473) A Novel Test Method For Pedicle Screw With UnifiMI Mechanical Integration Technology

Introduction:: Posterior pedicle screw fixation is common practice in many spinal surgeries; however, posterior pedicle screws are subject to loosening and failure, particularly in osteoporotic or low-density bone. Previous studies have found that the major cause of failure for these pedicle screws is transverse loosening as opposed to axial pullout. Forces acting on the lumbar spine in vivo are much more complicated. A new type of pedicle screw by OsteoCentric Technologies with patented UnifiMI geometry is created to provide primary stability and long-term osseointegration through specially designed threads. Unifi screw relies on an interlocking bone-implant interface that provides a distribution of forces from implants to bone and subsequently resists loads in multidirectional forces and bending moments; thereby limiting the toggling of implants and minimizing radial forces. This technology is promising in pedicle screw application for spine surgery. To evaluate the effect of the UnifiMI technology in pedicle screw application, we designed and implemented a biomechanical test that incorporates multiaxial cyclic testing and subsequent pullout testing. In literature, most studies which perform cyclic testing only focus on the uniaxial loading specifically in the cranio-caudal direction. Our multiaxial cyclic test was designed so that forces were applied to create moments in both the cranio-caudal direction as well as medial-lateral bending. Subsequent standard pullout testing was performed to evaluate fixating powers of Unifi pedicle screws versus standard pedicle screws. The hypothesis of the study is to demonstrate the superiority of the Unifi interlocking thread in pull-out resistance under multiaxial loads.

Materials and Methods:: Five freshly frozen human spines were received from the United Tissue Network (Phoenix, AZ) with an age range of 37-71 years old, BMI 27.3 to 41.8, and BMD 0.929 to 1.417. Each specimen was implanted in the lumbar region (L2-L4) with 6.5 mm Unifi pedicle screws and 6.5 mm standard pedicle screws (Acuity) on the contralateral side by a trained orthopedic spinal surgeon. Testing was performed using the MTS Model Bionix 370.02 Axial/ Torsional (MTS system, Eden Prairie, MN). A special apparatus holds the screw head in place and attachs to the top fixture which connects to the Bionix^® Spine Kinematics system. The Spine Kinematics system is a gimbal fixture that permits 6 degrees of freedom motions. It was equipped with a small load transducer Mini 58 (ATI Industrial Automation, Apex, NC) to facilitate accurate force feedback. Each prepared specimen went through a biomechanical test procedure first with a novel multiaxial cyclic test, then standard screw pullout. The multiaxial cyclic loads comprised of 4 subsequent motions (flexion, extension, medial bending, and lateral bending with bending moment control from -4 Nm to 4 Nm) and preload tension of 50N. A total of 12,000 cycles was performed at 2 Hz frequency. Specimen successfully passed the cyclic test if there was no screw loosening or axial displacement was not over 1 mm. All specimens then went through a standard pullout to failure test ASTM F543-A3 at a rate of 5mm/min. Stiffness, ultimate load and strain energy are parameters being measured for comparison.

Results, Conclusions, and Discussions:: All screws passed the cyclic test without any breakage or premature bone failure. Initially stiffness of both groups was less than 2.47% different (Unifi 1099.7 N/mm and Acuity 1127.3 N/mm). After multiaxial cyclic testing, the difference in stiffness became 9.76% while Unifi demonstrated an increase of 4.81% in stiffness (1153.9 N/mm) and Acuity had a decrease of 7.43% in stiffness (1046.6 N/mm). The total axial displacement or rotational change of both groups are insignificant. All specimens successfully passed the cyclic test. These parameters demonstrated the advantage of the interlocking thread of Unifi. Increased stiffness means better screw purchase into the bone material. The thread better engaged within the bone giving a stiffer bone screw interface. The multiaxial cyclic test was able to highlight the difference between the 2 thread technologies under multidirectional forces and bending moments. In the subsequent pullout test, stiffness, maximum load, and total strain energy were calculated and compared. Group Unifi’s values were all higher than those of Acuity but there was no statistically significant difference detected, possibly due to the small number of samples. The 2 groups were considered equivalent.
We successfully designed a multiaxial cyclic test that takes into account the multidirectional loads and moments within the physiological environment. Overall, the Unifi screw presents equivalent properties to standard pedicle screws but stands out when multiaxial loads are involved. UnifiMI technology can be implemented to avoid the risk of failure in spinal surgery application.

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