Risk of Arthroplasty in Patients with Subchondral Insufficiency Fractures of the Knee: A Matched Study of the Implantable Shock Absorber (ISA) using a Validated Predictive Mo

Introduction: Subchondral insufficiency fractures of the knee (SIFK) can result in high rates of osteoarthritis and arthroplasty. The Implantable Shock Absorber (ISA) implant is a titanium and polycarbonate urethane device which reduces load on the medial compartment of the knee by acting as an extra-articular load absorber while preserving the joint. The purpose of this study was to evaluate whether partially unloading the knee with the ISA altered the likelihood of progression to arthroplasty utilizing validated predictive risk model (SIFK score). Methods: A retrospective case-control (2:1) study was performed on patients with SIFK without any previous surgery and those implanted with the ISA with outcome being progression to arthroplasty compared to non-operative treatment at 2 years. Baseline and final radiographs, as well as MRIs, were reviewed for evaluation of meniscus or ligament injuries, insufficiency fractures and subchondral edema. Patients from a prospective study were matched using the exact SIFK Score, a validated predictive score for progression to arthroplasty in patients with SIFK, to those with the ISA implant. Kaplan-Meier analysis was conducted to assess survival. Results: Total of 57 patients (38 controls:19 ISA) with mean age of 60.6 years were included. The SIFK score was matched exactly between cases and controls. The 2-year survival rate of 100% for the ISA group was significantly higher than corresponding rate of 61% for the control group (p<0.01). In ISA, 0% of the patients converted to arthroplasty at 2 years, and 5% (1 patient) had hardware removal at 1 year. When stratified by risk, the ISA group did not have a significantly higher survival compared to low-risk (p=0.3) or medium-risk (p=0.2) controls, though it had a significantly higher survival for high-risk groups at 2-years (100% vs 15%, p<0.01). Conclusion: SIFK of the medial knee can lead to significant functional limitation and high rates of conversion to arthroplasty. Implants such as the Implantable Shock Absorber have the potential to alter progression to arthroplasty in these patients, especially those at high-risk.