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Direct lateral interbody fusion (DLIF) is a powerful surgical tool in a spine surgeon's armamentarium to address lumbar spine pathology minimally invasively. It has been widely adopted amongst spine surgeons to gain access to the lumbar disc through a small incision on a patient's flank. Through a retroperitoneal approach, the surgeon can perform discectomy, prep end plates, and insert an interbody cage with a much longer (lateral-medial) and wider (anterior-posterior) footprint than posterior lumbar interbody fusion (PLIF). Using either transpsoas or anterior-to-psoas (ATP) technique, the surgeons are now able to address lumbar spine pathologies affecting L2 to S1 minimally invasively. Both approaches are acceptable and widely adopted amongst spine surgeons. There are many commercially available FDA-approved DLIF cages in market for spine surgeons to use. The cages made of polyetheretherketone (PEEK) and titanium are most commonly used. Many surgeons who use PEEK cages in DLIF supplement the construct with recombinant human bone morphogenic protein-2 (rhBMP-2) to achieve fusion. PEEK has an inert and hydrophobic property which does not induce osteogenesis. In-vitro studies demonstrate absence of bone formation through PEEK. By adding rhBMP-2, it may induce robust osteoinduction around PEEK, achieving fusion across disk space over time. On the other hand, porous titanium has all three properties: osteoconductivity, osteoinductivity, and osseointegration. This may create a more favorable environment for fusion, which obviates the need for rhBMP-2 supplementation. Both types of cages are approved for the study's indications and are routinely used in DLIF at Penn. The surgical staff is qualified to implant both devices.

Aside from the type of cage used, subjects' DLIF procedures will not be altered for the purposes of the study. There are no known differences in the risks and benefits of the two cages being studied. Cage choice in routine care is based on each individual surgeon's standard-of-care preference. As such, there is a possibility of the patient getting either of these cages placed, should they choose to not participate in the study.