• Jeffrey M Toth, Felix Ankomah, Noriaki Kawakami, and Koki Uno.
• Department of Orthopaedic Surgery, The Medical College of Wisconsin Inc., 8701 W. Watertown Plank Road, Box 26509, Milwaukee, WI, 53226-0509, USA. jtoth@mcw.edu.
• Eur Spine J. 2022 Sep 1; 31 (9): 2301-2310.

PurposeThe SHILLA™ Growth Guidance system is a stainless-steel rod and screw system used for Early Onset Scoliosis which incorporates a unique flanged set screw designed to capture the rod, while allowing it to slide as the patient grows. Concomitant with this design is the potential for generation of wear debris and for an inflammatory host response. We hypothesized that the magnitude of the host response adjacent to the unlocked screws and rods would be greater than the host response to the locked rod/screws.MethodsSeven tissue samples adjacent to locked (3) and unlocked screws (4) from three SHILLA patients (mean implantation time of 19 post-operative months) with infantile idiopathic scoliosis were obtained as part of an explant analysis protocol during a PMDA-approved clinical trial in Japan. Gross appearance, high-resolution radiographs, and histology were assessed. ISO Standard 10993 Part 6 was used to assess the host response.ResultsAll three locked screw had no metallosis. In contrast, metallosis for unlocked screw tissue samples were rated as "ubiquitous" (2/4), "focal" (1/4), or "absent" (1/4). Microscopic metallic debris was found intracellularly and within interstices of fibrous connective tissues more frequently adjacent to unlocked screws compared to locked screws. Cell type and population scoring consistently showed a modestly larger inflammatory response (macrophages) in the unlocked tissue samples.ConclusionsThe peri-prosthetic tissue response to the unlocked rods/screws had a higher reactivity grade (slight reaction, Δ = 4.0) per ISO 10993 Part 6 compared to the locked screws in three patients with the SHILLA™ Growth Guidance scoliosis system.© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

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