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

Introduction:. The transverse acetabular ligament (TAL) antomy is not a well explored aspect of the hip joint with limited morphological description in the reviewed literature. It is often used as an anatomical landmark for orientation of the acetabular component in total hip arthroplasty (THA). There is debate as to whether it represents an appropriate guide to cup placement in THA. Present descriptions in orthopaedic literature conside it as a single plane structure to which the surgeon can align the cup. The aim of the current study was to investigate the morphology of the TAL and it was hypothesised that the current description of it being a plane would prove insufficient. Materials and methods:. Seven dry bone hemi-pelves were reconstructed using a microscribe and rhinoceros 4.0 3D software to visualise attachment sites. Three hips from two female donors were dissected to expose the acetabulum and the TAL. This structure was removed and a footprint taken of its perimeter and attachment sites for measurement of ligament length, breadth and area of attachment from digital photographs. Finally, 3D models of the dissected acetabuli with an outline of the TAL and attachment sites were created as before. Results:. The TAL extended beyond the acetabular notch, around the circumference of the acetabular rim. Two attachment sites were identified in each specimen, found at two sites in the superior half of the acetabular rim; one anterior and one posterior. In one specimen, an additional attachment site was identified on the posterior horn. TAL length in each specimen as measured from 2D digital photographs were 132 mm, 117 mm and 179 mm, with attachment areas of 215 mm. 2. , 150 mm. 2. and 350 mm. 2. , respectively. There was marked variation in ligament breadth both between and within individual specimens, ranging from 2.6 to 5.3mm in the smallest specimen and 3.2 to 6.3mm in the largest specimen. The whole structure as one does not conform to be a plane of orientation. Discussion:. Contrary to previous literature assumption, the TAL extended far beyond the acetabular notch. Likewise, its attachment sites were found further round the acetabular rim than previously described. The unexpected breadth may explain the disagreement between studies looking at the structure as an anatomical plane. Further biomechanical research may determine which part – if any – of the ligament, should the acetabular component be orientated to. The details will be presented in the paper