The Impact of Forefoot Alignment and Arthritic Change on Conversion to Fusion after Synthetic Cartilage Implant Placement for Hallux Rigidus

Abstract

Category: Midfoot/Forefoot Introduction/Purpose: There are several treatments for hallux rigidus, including the use of a synthetic cartilage implant (SCI). Level I studies have shown success with these implants, but some patients do require conversion to arthrodesis for ongoing symptoms. The purpose of this radiographic review was to examine if baseline bone alignment, dimensions, and arthritis status are were different for patients who required conversion to arthrodesis compared to a control group with successful outcome. Methods: A radiographic review was conducted using data for from 15 hallux rigidus subjects implanted with an SCI enrolled in a during a previously concluded Level 1 clinical trial. Ten (10) Success subjects were identified as subjects those without conversion and with the lowest VAS pain levels indicating the most favorable outcome formed a success (control) group. Five (5) Conversion (study) subjects were identified as those with a conversion requiring revision to fusion and the with highest VAS scores indicating the least favorable outcome. Preoperative Weight-bearing AP and lateral radiographs from baseline visits were reviewed. Using previously validated Quantitative Motion Analysis software, two radiologists made the following measurements: first interphalangeal angle; first/second intermetatarsal angle; hallux valgus intraphalangeal angle; metatarsus adductus angle; metatarsal declination angle; first to second metatarsal length; functional ratio; and first metatarsal cortical thickness. Preoperative assessments of arthritis grade, metatarsalus primus elevatuselevation, sesamoid arthritis, and sesamoid station were also performed. Results: First metatarsal length was approximately 3% larger longer in the Conversion group compared to the Success group (85.2% vs. 82.5%). This was driven by the Conversion group having longer median normalized first metatarsal length (88.5% vs. 86.6%) and shorter median normalized second metatarsal lengths (104.0% vs. 106.3%). First metatarsal The second exception was cortical thickness was greater in the conversion group. This measurement was defined as the combined width of the medial and lateral cortices measured at the midline of the first metatarsal normalized to the full midline diameter of the first metatarsal. The median cortical thickness was almost 7% larger in the Conversion group compared to the Success group (40.1% vs. 33.2%). Nine Success subjects and 2 Conversion subjects had Grade 3 arthritis, while all other subjects had Grade 2. Three (60%) Conversion subjects and two (20%) Success subjects had evidence of metatarsus primus elevatus. Conclusion: Baseline joint alignment did not correlate with the need for conversion to arthrodesis. Conversion subjects had larger functional ratios longer first rays and greater cortical thickness. Published literature suggests smaller functional ratios result in increased second ray loading. It is possible the opposite is occurring in Conversion subjects, with longer first and shorter second metatarsal lengths resulting in increased loading of the SCI implant. The result indicates that shortening the first ray may be required to achieve a successful outcome with SCI. Success patients had a lower incidence of metatarsus primus elevatus.