Shear stress and von Mises stress distributions in the periphery of an embedded acetabular cup implant during impingement.

As literature implies, daily activities of total hip arthroplasty (THA) patients may include movements prone to implant-implant impingement. Thus, high shear stresses may be induced at the acetabular implant-bone interface, increasing the risk of implant loosening. The aim of the current study is to determine whether or not impingement events may pose an actual risk to acetabular periprosthetic bone. An existing experimental workflow was augmented to cover complete three-dimensional strain gage measurement. von Mises and shear stresses were calculated from 1620 measured strain values, collected around a hemispherical cup implant at 2.5 mm interface distance during worst-case impingement loading. A shear stress criterion for acetabular periprosthetic bone was derived from the literature. At the impingement site, magnitudes of von Mises stress amount to 0.57 MPa and tilting shear stress amount to -0.3 MPa at 2.5 mm interface distance. Conclusion can be drawn that worst-case impingement events are unlikely to pose a risk of bone material failure in the periphery around fully integrated cementless acetabular hip implants in otherwise healthy THA patients. As numerical predictions in the literature suggested, it can now be confirmed that impingement moments are unlikely to cause acetabular implant-bone interface fixation failures.

Effects of various anchoring components and loading conditions on primary stability of acetabular revision implant.

PURPOSE: In revision total hip arthroplasty, until today, orthopaedic surgeons are missing evidence-based guidelines on cementless acetabular cup fixation. METHODS: 5 finite element models were generated featuring the following anchorage strategies: 1 short peg, 1 long peg, 2 long screws, 3 short screws and zero anchoring components for reference. The micromotions at the implant-bone interface were analyzed for 3 different loadcases, "Seated leg-crossing" (joint force 940 N, impingement force 750 N), "Normal gait" (joint force 1820 N), and "Stumbling" (joint force 4520 N). RESULTS: Within the same loadcase, percentages of interface area below 28 µm are nearly identical in all anchorage strategies. The average percentage of interface area below 28 µm is 31% for "Seated leg-crossing", 17% for "Normal gait", and 11% for "Stumbling". Maximal von Mises stresses in "Normal gait", for example, reach 12 MPa in the short peg, 48 MPa in the long peg, 15 MPa in 1 of the 2 long screws, and 85 MPa in 1 of the 3 short screws. CONCLUSIONS: Common orthopaedic practice, to use peg or screw fixation alternatively according to bone availability or other clinical aspects, can be confirmed. The short peg may be a good alternative to the long peg with regard to the preservation of bone stock. However, the current study implies that the extent of potential osseointegration depends less on the chosen anchorage strategy but strongly on postoperative loading conditions. Total hip patients should be instructed on adequate postoperative activities.

Validation of density-elasticity relationships for finite element modeling of human pelvic bone by modal analysis.

In total hip arthroplasty and particularly in revision surgery, computer assisted pre-operative prediction of the best possible anchorage strategy for implant fixation would be a great help to the surgeon. Computer simulation relies on validated numerical models. In the current study, three density-elasticity relationships (No. 1-3) from the literature for inhomogeneous material parameter assignment from CT data in automated finite element (FE) modeling of long bones were evaluated for their suitability for FE modeling of human pelvic bone. Numerical modal analysis was conducted on 10 FE models of hemipelvic bone specimens and compared to the gold standard provided by experimental modal analysis results from a previous in-vitro study on the same specimens. Overall, calculated resonance frequencies came out lower than measured values. Magnitude of mean relative deviation of numerical resonance frequencies with regard to measured values is lowest for the density-elasticity relationship No. 3 (-15.9%) and considerably higher for both density-elasticity relationships No. 1 (-41.1%) and No. 2 (-45.0%). Mean MAC values over all specimens amount to 77.8% (No. 1), 78.5% (No. 2), and 83.0% (No. 3). MAC results show, that mode shapes are only slightly influenced by material distribution. Calculated resonance frequencies are generally lower than measured values, which indicates, that numerical models lack stiffness. Even when using the best suited (No. 3) out of three investigated density-elasticity relationships, in FE modeling of pelvic bone a considerable underestimation of model stiffness has to be taken into account.

Finite element analysis of shear stresses at the implant-bone interface of an acetabular press-fit cup during impingement.

After total hip replacement (THR) impingement of the implant components causes shear stresses at the acetabular implant-bone interface. In the current study the finite element method (FEM) was applied to analyse the shear stresses at a fully bonded implant-bone interface assuming total ingrowth of the cup. The FE model of a press-fit acetabular component and the proximal part of the femoral component incorporates non-linear material and large sliding contact. The model was loaded with a superior-medial joint load of 435 N simulating a two-legged stance. Starting at initial impingement, the femoral component was medially rotated by 20 degrees . The peak tilting shear stress of -2.6 MPa at the impingement site takes effect towards the pole of the cup. The torsional shear stress at the impingement site is zero. On each side of the impingement site, there are extrema of torsional shear stress reaching -1.8 and 1.8 MPa, respectively. The global peak shear stress during impingement may indicate a possible starting point for cup loosening. The pattern of the torsional shear stresses suggests that besides the symmetric lever-out, an additional asymmetrical tilting of the cup occurs that can be explained by the orientation of the applied joint load.