Bone Remodeling after Ulna Head Replacement in Distal Radioulnar Joint Arthroplasty: A Radiographic Comparison between a Partial and a Total Ulna Head Concept.

Ulna head arthroplasty has become an eligible solution for injury or disease in the distal radioulnar joint. Bone resorption beneath the prosthetic head is often reported, but mechanism poorly understood.  The aims were to evaluate bone remodeling and radiological instability in two conceptually different distal radioulnar joint arthroplasties: the total and the partial ulna head replacement.  We conducted a retrospective radiographic assessment of 51 ulna head arthroplasties; 26 Herbert ulna (total ulna head replacement) and 25 First Choice (partial ulna head replacement), to analyze periprosthetic bone resorption and radiologic instability. Intraoperative/immediate postoperative and 1-year radiographs were reviewed by two independent assessors. The radiographic follow-up averaged 13 (10-17) months. The size of the stem in relation to the diameter of the ulna (filling ratio) was measured on the intraoperative/immediate postoperative radiographs. Bone resorption beneath the collar of the prothesis was measured on the 1-year radiographs and expressed as a bone resorption index (BRI) between the length of the resorption and the length of the implant stem. Radiological stability was measured on both the preoperative and the 1-year lateral radiographs.  The total ulna head prothesis presented with more extensive bone resorption beneath the prosthetic head than the partial ulna head prothesis at 1-year post surgery ( <0.001). The filling ratio did not influence the 1-year bone resorption and there was no difference regarding radiological instability between the two prosthetic designs.  The pattern of bone adaptions after an ulna head prothesis may differ due to design and concept of the prosthesis.