16. Evaluation of the cage subsidence performance of three porous lateral cage designs

Abstract

<h3>BACKGROUND CONTEXT</h3> Interbody cage subsidence remains a major complication after lumbar spine fusion surgery, particularly in lateral or other procedures that rely on indirect decompression. Porous cages are believed to reduce the risk of cage subsidence. Multiple porous cage designs exist, including 3D printed (3DP) titanium cages with body lattice and microporous endplates, 3DP titanium cages with a truss structure, and PEEK cages with surface porosity. Currently, there is a lack of carefully-controlled biomechanics studies to evaluate their relative performance. <h3>PURPOSE</h3> To evaluate the resistance to subsidence of three representative porous cage designs using ASTM standardized testing methods, as well as clinically relevant dynamic subsidence testing methods. <h3>STUDY DESIGN/SETTING</h3> Mechanical testing using foam blocks. <h3>PATIENT SAMPLE</h3> Three porous cage designs were included: a 3DP cage with stress-optimized body lattice and microporous endplates (stress-optimized titanium cage), a 3DP titanium cage with truss structure (truss titanium cage) and a PEEK cage with surface porosity (porous PEEK cage). All cage designs have the similar dimensions. N=5 testing constructs were used per cage design/loading mode. For titanium cages, one single cage was reused for all constructs. For PEEK cages, a new cage was used in each construct. <h3>OUTCOME MEASURES</h3> For ASTM F2267 static subsidence testing, the block stiffness at the implant-foam interface was reported. The greater the block stiffness, the more force was required to generate the same displacement. For the clinically relevant dynamic subsidence testing, the post-test subsidence displacement was reported, measured as the amount of displacement that each cage sank into the test block surface after a simulated 3-month period of physiologically relevant loading. <h3>METHODS</h3> Static subsidence testing per the ASTM F2267 standard, and novel dynamic subsidence testing, consisting of a physiologically relevant cyclic compressive load, were performed for each cage design to evaluate subsidence performance. Surface-conforming polyurethane foam test blocks that simulated the adjacent vertebral bodies were used in both the static and dynamic subsidence tests. All testing was performed on a calibrated servohydraulic load frame. Testing results were compared between three designs using one-way ANOVA with Bonferroni correction for post-hoc analysis. <h3>RESULTS</h3> In ASTM F2267 static compression testing, the stress-optimized titanium cage showed a significantly greater (27%) block stiffness of 2690.9 ± 92.7 N/mm than that of the truss titanium cage (2118.3 ± 19.9 N/mm, p<0.001), as well as 17.7% greater than that of the porous PEEK cage (2284.7 ± 64.5 N/mm, p=0.001). The porous PEEK cage also significantly outperformed the truss titanium cage (p=0.001) by 8%. In the clinically relevant dynamic subsidence testing, the truss Ti-cage showed the greatest amount of subsidence displacement of 0.141±0.007mm, significantly greater than that of the stress-optimized titanium cage by 153.3% (0.056±0.008mm, p<0.001) and that of the porous PEEK cage by 65.6% (0.085±0.004mm, p<0.001). The stress-optimized titanium cage showed a significantly lower subsidence displacement than that of the porous PEEK (p<0.001) by 34.6%. <h3>CONCLUSIONS</h3> Despite the common conception that porous cage design can reduce the risk of subsidence, the specific design rationale is worthy of rigorous investigation. Under the ASTM F2267 standard static subsidence testing and clinically relevant dynamic subsidence testing conditions, a 3DP titanium cage with a stress-optimized body lattice and microporous endplates showed the best subsidence performance, followed by the porous PEEK cage, and then the 3DP truss titanium cage design. <h3>FDA DEVICE/DRUG STATUS</h3> Modulus XLIF (Approved for this indication), Cohere XLIF (Approved for this indication), LATERAL SPINE TRUSS SYSTEM (Approved for this indication)