The Trident acetabular system is the second most common cementless cup implanted in the UK. Recent studies have shown that malseating of the liner can be as high as 16.4%. We felt this was very high and were prompted to review our series and early clinical outcomes. We reviewed 118 hips in 110 patients, implanted between from 2005-2007. We reviewed initial post operative X-rays using the technique described by Howcroft to identify malseating. The posterior approach was used in all cases. All cups were Trident PSL and all 85 Patients had OA, 10 RA, 8 AVN, 5 DDH, 3 OA post trauma, 2 Perthes, 2 Psoriatic Arthritis, 3 other. We only identified 3 malseated cups in 118 hips. 2 were in patients with OA secondary to trauma and 1 in primary OA. The rate of malseating for trainees operating was 5 % and only 1% when consultants were operating. There were no adverse events in these patients. No-one required revision. Oxford Hip Score (OHS) improved from 47 pre-op to 20 post op. This was compared to 47 and 22 in the correctly seated group (115 cases). Surprisingly the subgroup with the poorest OHS at 1 year had surgery for DDH, with a mean OHS of 31. The reasons for this are unclear. Contrary to other studies our malseating rate is very low. We do not feel that malseating is a problem with Trident if adequate exposure is obtained. In those patients with sclerotic bone, we suggest over reaming the rim of the acetabulum by 1mm to avoid excess deformation of the shell which may lead to difficulty with seating the liner. We suggest trainees are supervised closely when using Trident

Summary Statement. A FEA model built from CT-data of frozen cadaver has been validated and used for under-reaming experiments. 1 mm under-reaming can provide contact surface and micromotions that are acceptable and within the clinical relevance without high impact force. Introduction. Long-term cup fixation and stability in total hip arthroplasty (THA) is directly related to the bone ingrowths between the porous cup and the acetabulum. To achieve the initial cup setting, 1 mm of under reaming is becoming the gold standard for cementless cup and what is at stake is usually the actual contact between cup and acetabulum wall. During impact and cup placement, friction forces are generated from the “not permanent” deformations of the acetabular wall that are translated into a gap between the reamed bone and the cup. Clinically the surgeon objective is to have the gap extended to a limited portion of the cup in order to improve bone ingrowth. Hence, the need arises from examining this cup bone stability interface by examining the selected “under reaming” conditions, the surface of contact between the acetabular cup and the bone and its relation to the impact force resulting from the hammering of the cup. Patients & Methods. A validated finite element model built from CT data of fresh frozen hip cadavers has been used for under-reaming mechanically testing experiment. The model was constrained at the sacral and pubic joints to mimic the exact fixation and potting of the pelvis used for testing, and an “impactor” model was used to force the cup into the acetabular reamed socket for both 1 and 2 mm under reaming conditions of the selected cup sizes. Three impact conditions were simulated by imposing cup displacements equivalent to 80, 100 and 120% of the initial distance between the cup apex and the bone. The corresponding reactions forces were evaluated as ideal insertion forces. After the loading phase, a relaxing phase was defined by the removal of load to determine the equilibrium position between the friction forces and the elastic deformation of the actabulum bone. In our last phase, the cup is loaded with a 1500N along the femoral mechanical axis following the same loading conditions of our cup-bone interface experimental setup. Results. The value of under-reaming plays a significant role in the hammering force due to cup placement and has a high correlation with the surface in contact in all cases of implantation, as well as the final stability of the cup throughout loading. When comparing the 2 mm with 1 mm of bone under-reaming we found that the higher degree of under-reaming resulted in slightly greater surface area of contact between the cup and bone as well as reduced micromotion at loading up to 1500 N. However, the impact force requirements for 2mm under reaming was found to be much higher in all three cases investigated. Discussion/Conclusion. Our results indicate that 1 mm under reaming can provide contact surface and micromotions that are both acceptable and within the clinical relevance of cup bone stability without the need of high impact force needed to insert the cup to its desired depth. High insertion forces may lead or cause risk of fracture