Prospective two-year subsidence analysis of 100 cemented polished straight stems – a short

Between Jan 2009 and Oct 2010 a total of 285 primary total hip arthroplasty (THA) were performed at our institution, 100 (97 patients) of them were operated with the cemented twinSys straight stem and followed prospectively after 6, 12, 52 and 104 weeks. Median age at operation was 78 (68 to 93) years. 51 stems were implanted in female patients, 53 on the right side. Diagnoses were 79 osteoarthritis, 6 osteonecrosis and 15 femoral neck fractures. The other 185 primary THA during the study period were operated with the uncemented twinSys stem. Operations were performed or supervised by two senior consultants (TI, MC). Data analysis and EBRA-FCA was performed by an independent surgeon (WS) not beeing involved in the operations nor follow-ups. All patients agreed to participate in the study and approval of the local ethics committee (EKNZ (Ethikommission Nordwest Schweiz) 2015-125) was obtained. No patient was lost to follow-up.

The cemented twinSys stem has a polished surface (mean roughness Ra 0.4 ?m) and a triple taper with an angle in the lateral projection of 4° proximal und 1.5° distal and 5° in the ap projection. A standard and a lateralised version are available, whith the lateralisied version used in 45 of our cases as needed to reconstruct the offset and joint geometry. 97 stems were combined with a cementless RM pressfit cup (Mathys AG Bettlach, Switzerland), 3 stems with a Muller acetabular reinforcement ring (ARR) and a cemented PE cup. The RM pressfit® cup is made out of standard UHMW-PE, with a 28 or 32 mm articulation. The new RM vitamys® cup, made out of highly crosslinked PE, offers a 36 mm articulation, which we use nowadays if patients are at higher risk for dislocation. For all hips a ceramic head (Bionit 2®, Mathys AG Bettlach, Switzerland) was used, 35 heads had a 28 mm and 65 a 32 mm diameter.

All patients were operated in the routine setup of a university affiliated teaching hospital. Implant size, position and leg length were planned with a digital planning tool (AGFA® Orthopaedic Tools, Agfa HealthCare. N.V, Mortsel, Belgium) prior to surgery. 22 patients were operated in a supine position through a direct lateral transgluteal approach, and 78 with an anterior MIS approach as recently published [15]. Stems were cemented with a third-generation cementing technique with (Synplug®, Mathys AG Bettlach, Switzerland), cement syringe, vacuum-mixing, jet lavage but no proximal sealing using Palacos® G bone cement (Hereaus Medical, Dübendorf, Switzerland). Patients were mobilised either on the day of surgery or the day after with full weight bearing. Crutches were advised for comfort as needed for 6 weeks.

Radiological evaluation

Standardized digitalized radiographs of the pelvis (patient in supine position, centered on the symphysis, focus film distance 120 cm) were taken at 1 week, 12 weeks, 1 and 2 years postoperatively. The quality of the cement mantle was rated according to Barrack [17]. Varus/valgus alignment of the stem was measured on the postoperative ap radiograph, a deviation of more than 3° was defined as malalignment [18]. Debonding was defined as a radiolucent line at the prosthesis-cement-interface not visible on the first postoperative radiograph [18]. Osteolysis was defined as a progressive, newly developed endosteal bone loss with a diameter greater than 3 mm with an either scalloping or bead-shaped lucency at the cement-bone-interface [3]. Debonding and osteolysis were manually measured on the plain radiographs and reported accoording to their location in the Gruen zones [19]. Subsidence of the stem was measured using the software based EBRA-FCA method. For this method a minimum of 4 comparable digitalized x-rays are needed. An electronic coordinate system is placed on the x-rays to localize cup, stem and certain landmarks (Fig. 1). Compared to e.g. RSA no further markers have to be implanted and routine follow-up radiographs can be used. Therefore it’s an ideal tool to detect early subsidence or migration in huge cohorts [2, 3, 20].

Cup inclination was manually measured on the plain radiographs using the interteardrop line. Osteolysis around the cup was rated according to the zones described by DeLee and Charnley.