Stress Shielding and Aseptic Loosening in Fully Porous-Coated vs Smooth Stemmed Tibial Implants in Total Ankle Arthroplasty

CATEGORY: Ankle Arthritis; Ankle INTRODUCTION/PURPOSE: Porous coating on the osseous interface of implants has gained increased popularity in total ankle replacement surgery. With the integration of porous coating into the newer generation of implants, this theoretically increases the chances of osseous ingrowth and survivorship in total ankle replacements. Alternatively, literature in hip replacement surgery has shown increased rates of stress shielding, aseptic loosening, thigh pain, and cystic formation around stemmed femoral implants extensively coated with porous surfaces. While some ankle prostheses have integrated porous coating technology with stemmed tibial implants, there is little to no research investigating the potential negative effects this may have on longevity or aseptic loosening secondary to a stress shielding effect. METHODS: Our study identified 9 patients who underwent total ankle arthroplasty via a smooth-stemmed tibial prosthesis and matched these patients based on implant age with patients who received fully porous-coated tibial implants. We reviewed radiographs and compared postoperative rates of backside loosening and cystic formation secondary to stress shielding between the two groups. We also compared rates of heterotopic ossification between the two groups. A homoscedastic t-test was used to identify any statistical significance, which was defined as P < 0.05. RESULTS: While the smooth-stem group showed no rates of backside cystic formation or stress shielding, the follow-up matched porous-coated group showed a rate of 55% of cystic formation and potential loosening secondary to stress shielding observed on final radiographic follow up (P < 0.01). There was no statistical difference between the two groups in their rates of heterotopic bone formation (P = 0.37). CONCLUSION: Despite having a relatively small sample size, this data shows that there is some consistency with the hip arthroplasty literature suggesting an increased risk for stress shielding in fully porous-coated stemmed tibial implants compared to smooth- stemmed tibial implants in total ankle arthroplasty.