[Our Experience with RM Pressfit Vitamys Cup and Cementless Optimys Short Stem].

INTRODUCTION The purpose of our study was to evaluate the clinical outcomes in patients at 3-6 years follow-up after primary implantation of RM Pressfit Vitamys cementless elastic cup and cementless Optimys short stem, including bone tissue remodelling around both the components. MATERIAL AND METHODS The evaluation covered 49 joint replacements in patients who had undergone surgery at our department between 2012 and 2015. The age at the time of primary surgery ranged from 29 to 71 years, with the mean value of 59.2 years. Postoperatively, the position of endoprosthesis, changes in femoroacetabular offset, signs of osseointegration of the implant, bone tissue remodelling around both the components and potential signs of aseptic loosening were assessed. The functional status of the joint was evaluated according to the Harris scale. RESULTS The mean follow-up time of Vitamys cup and Optimys stem was 5.6 years and 4.6 years, respectively. We focused on the combination of RM Pressfit Vitamys (49x), Optimys (28x) and Bionit 2 (41x) implants. All the cups showed good osseointegration. Based on the comparisons with a postoperative X-ray, at least mild osteoporosis in the acetabular roof was confirmed in 6 cases. All Optimys femoral components were in direct contact with the Adams arch and with the endosteal side of lateral cortex of proximal femoral metadiaphysis. Femoroacetabular offset was slightly decreased in 5 patients only. The final evaluation in 2018 did not confirm any radiolucent lines or signs osteolysis around any of the components. In 2 stems only, distal migration less than 2 mm was obvious, with subsequent good osseointegration. Signs of stress shielding were present in 2 femoral components in the form of mild cortical atrophy in the region of the Adams arch. Distal femoral cortical hypertrophy was not observed, the greater trochanter did not show the loss of bone tissue in any of the patients. There were no signs of polyethylene wear. The mean value of HHS increased from 53 to 97 points. An excellent result was achieved in 44 total hip replacements, of which 100 points in 28 cases. In the remaining 5 patients the result was good. The survival rate of both the components was 100% according to the Kaplan-Meier analysis. DISCUSSION The successful functioning of cementless total hip arthroplasty is the correct placement of both components with good primary fixation. Excessive proximal and lateral shift of the centre of rotation results in increased load of endoprosthesis and risk of earlier aseptic loosening, its reduction leads to the weakening of pelvitrochanteric muscles. The shift of the centre of rotation from the original anatomical position should therefore not exceed 5 millimetres. Insufficient cup fixation always results in mechanical failure of an endoprosthesis. Distal migration of stems without contact with external femoral cortex with full weight-bearing of the operated lower extremity in the postoperative period does not constitute a sign of instability, but only its placement enables good osseointegration. Bone remodelling can be assessed by imaging techniques at 2 years after the primary implantation at the earliest. At places with lower load, the bone loss occurs and the loss of bone trabeculae can lead to the failure of fixation of the component. At places with load accumulation, the bone hypertrophy occurs that can be manifested by thigh pain. In case of cementless press-fit cup, the degree of bone remodelling depends on its elasticity, in case of stem on the used material, shape and fixation site. CONCLUSIONS The RM Pressfit Vitamys monobloc cup through its mechanical properties approximates the best the elasticity of bone tissue. The stress distribution around the implant is more symmetrical as against other conventional cementless cups. The Optimys stem enables the reconstruction of anatomical conditions corresponding a healthy hip joint. Respecting the rule of at least three-point fixation is a precondition for good and fast secondary stability of components. Minimising the wear of articulating surfaces and physiological remodelling of adjacent bone tissue are the main factors that help prolong the survivorship of both the components, while also securing more favourable conditions and better outcomes in case of necessity of reimplantation. Key words: cementless elastic cup, short cementless stem, femoroacetabular offset, stress shielding, osseointegration of the implant.