5-year clinical and radiographic follow-up of the uncemented Symax hip stem in an international study.

Background: The uncemented Symax hip stem is developed through optimization of the uncemented Omnifit hip stem. The Symax stem design combines an anatomical anteverted proximal geometry with a straight distal section. The proximal part is coated with a biomimetic hydroxyapatite (HA) coating for improved osseointegration to enhance load transfer and to minimize proximal bone loss. The distal part is treated with an anodization surface treatment in order to prevent distal bone apposition, which is expected to prevent distal loading and reduce proximal stress shielding. Aim of this study is to report mid-term clinical performance and evaluate whether the radiographic features are in line with the design principles of the Symax hip.

Methods: The biomimetic hydroxyapatite-coated uncemented Symax hip stem was evaluated in 80 patients during a 5-year prospective clinical international study. Harris Hip Score (HHS), Oxford Hip Score (OHS), and Western Ontario and McMaster Universities Arthritis Index (WOMAC) were performed preoperatively and postoperatively at 6 months and 1, 2, 3 and 5 years. Anteroposterior radiographs of the pelvis and axial radiographs of the operated hips were evaluated immediately postoperative and at follow-up 6 months and 1, 2, 3, and 5 years. Wilcoxon signed-rank test was used to analyse whether clinical outcome scores changed statistically significant over time. The overall percentage of agreement between two radiology assessment teams was used to evaluate observer agreement of radiology results. The Cohen's Kappa was evaluated as a measure of reliability to quantify the agreement between raters, corrected for chance agreement.

Results: Clinical outcome scores were excellent at 5 years with mean HHS of 98.1, mean OHS of 16.2 and mean WOMAC of 6.9. Only 2.7% of the patients had pain at rest or on weight-bearing, and mid-thigh pain was reported by 1.4% of the patients after 5 years. The percentage of agreement between radiology assessment teams was 94 to 100%, except for distal line formation (48%). Radiographic evaluation showed stable stems and signs of excellent progressive proximal fixation and favourable bone remodeling.

Conclusions: The excellent mid-term clinical and radiographic performances are in line with the design principles and coating properties of this new implant and earlier published results.

Trial Registration: http://ClinicalTrials.gov , NCT03469687 . Registered 19 March 2018 - Retrospectively registered.