The use of hard-on-hard hip prostheses has highlighted specific problems like the “stripe-wear” and the squeaking. Many authors have related these phenomena to a micro-separation between the cup and the head. The goal of the study was to model the hip kinematics under micro-separation regime in order to develop a computational simulator for total hip prosthesis including a joint laxity, and to use it to perform a sound analysis. A three-dimensional model of the Leeds II hip simulator was developed on ADAMS® software. A spring was used to introduce a controlled micro-separation (less than 500 microns) during the swing phase of the walking cycle. The increase of the load during the stance phase induced a relocation of the head in the cup. Values of the medial-lateral separation predicted from the model were compared to experimental data measured using a LVDT of less than 5 microns precision. Theoretical wear path predicted from the model was compared to the literature data. The frequencies of the vibratory phenomena were determined, using the Fourier transformation. There was an excellent correlation between the theoretical prediction and the experimental measurement of the medial-lateral separation during the walking cycle (0.92). Edge-loading contact occurred during 57% of the cycle according to the model and 47% according to the experimental data. Velocity and acceleration were increased during the relocation phase in a chaotic manner, leading to vibration. The contact force according to the model had also a chaotic variation during the micro-separation phase, suggesting a chattering movement. Fourier transformation showed many frequencies in the audible area. A three-dimensional computational model of the kinematics of the hip after total replacement was developed and validated with an excellent precision under micro separation. It highlighted possible explanations for the squeaking that may occur during either relocation phase or edge loading.
Recovery after femur fractures is slow, despite rapid bone union. Causes of disability require investigation. Forty patients with isolated, diaphyseal femur fractures treated with antegrade locked intramedullary nailing were prospectively studied. Functional outcome was measured using the Western Ontario – McMaster University Osteoarthritis Index (WOMAC) and Short Musculoskeletal Functional Assessment (SMFA). Pain scores from the groin, buttock, thigh, and knee six months following the injury were correlated with functional outcome. Severity of pain was highest at the knee. Both knee and thigh pain had strong correlations with functional outcome measures following diaphyseal femur fractures. Further investigation into post-traumatic pain is warranted. This study was performed to compare pain at the knee, thigh, buttock and groin with functional outcome scores 6 months following femur fractures. Pain at the knee and thigh correlated with functional outcome measures. Many patients with femoral fractures have prolonged disability. Knee pain is common, severe, and correlates with functional outcome. Forty skeletally-mature patients with diaphyseal femoral fractures treated with locked antegrade intramedullary nails were prospectively enrolled. Exclusion criteria included polytrauma, ipsilateral injuries, metaphyseal extension, and pathologic fractures. Functional outcomes were assessed using the Western Ontario-McMaster University Osteoarthritis Index (WOMAC) and the Short Musculoskeletal Function Assessment (SMFA). Patients were instructed to record pain in the groin, buttock, thigh or knee of the fractured extremity on a 10-point visual analog scale. Patients reported more pain at the knee (3.7 ± 3.1), compared to the thigh (2.5 ± 2.7), buttock (1.7 ± 2.7), and groin (1.0 ± 1.7) (p=0.003). Pain and functional outcomes were plotted on scatter graphs and correlations performed using the Spearman rank test. Strongest correlations were noted between knee pain and WOMAC pain (ρ=0.748, p<
0.001), function (ρ=0.701, p<
0.001), and SMFA (ρ=0.733, p<
0.001); and between thigh pain and WOMAC pain (ρ=0.705, p<
0.001), stiffness (ρ=0.707, p<
0.001), function (ρ=0.731, p<
0.001), and SMFA (ρ=0.723, p<
0.001). Weaker correlations were noted between groin and buttock pain and functional outcomes. Knee pain is common and severe after femur fractures. Knee and thigh pain correlate with functional outcomes. Further investigation should be directed to this common problem.
All polyethylene tibial components (APT) for total knee joint replacement have been recently reintroduced due to their past success and cost savings with respect to knee designs with a metal backed tibial tray (MBT). However, isolated cases of collapse of the medial bone in APT designs have been observed by the authors prompting this investigation. The objective of this study was to investigate the stress/strain distribution within the cancellous bone for the APT and MBT systems, particularly looking at the effects of coverage of the tray over the proximal tibia in each design. A three-dimensional finite element model of the proximal tibia implanted with a tibia tray was generated. An elliptical cylindrical tibia tray with a peg was modeled as being perfectly bonded to a PMMA layer on the superior surface of the cancellous and cortical bone. Gap size between the edge of the tray and outer of the cancellous bone, was introduced in the medial direction. Load was applied on the superior surface of the tibial insert in the medial side. Two lift-off loading cases were used, a low load of 800N (1 body-weight) and a high load of 3200N (4 x BW), both on the medial side. Permanent plastic deformation and collapse was allowed only in the cancellous bone, while all other materials were modeled elastically. Under low load conditions within the elastic limit, introducing a gap between the tray and the cortical bone produced a stress/strain intensity in the cancellous bone beneath the edge of the tray. The strain in the cancellous bone within the APT design was generally 3 times greater than the MBT design, however, peak strain values were similar at the edge of the tray. Whilst the strain increased with the introduction of a gap the resulting strain was not sensitive to the gap size for both designs. Under high load conditions, permanent plastic deformation and bone collapse were observed in the cancellous bone at the edge of the tibial tray in both designs where a gap was introduced. The maximum strain in the cancellous bone was found to be more sensitive to the gap size for the APT design than the MBT design. This can be contributed to the difference in the load transfer through the cancellous bone in the two designs. The MBT design with the more rigid tibial tray transfered higher load through the outer cortical bone than the APT design. The less rigid APT design resulted in progressive collapse of the cancellous bone beneath the tray. Particularly significant was the volume of highly stressed cancellous bone which was 4 times greater in the APT design compared to the MBT design. The results suggest that coverage may be a more important parameter for the APT design than the MBT design. The APT design may, therefore, be more suited to patients with better bone quality.
Low positive swing phase load (<
100N) Positive swing phase load (300N, as per standard ISO 14242–1) Negative swing phase load, leading to microseparation and joint laxity. All tests were carried out in 25% (v/v) new-born bovine serum, with gravimetric wear measurements completed every million cycles.
Following total hip replacement surgery, fluroscopy studies have shown that a mean separation of 2 mm can occur between CoCr femoral heads and UHMWPE acetabular cups during the swing phase of gait [ A physiological hip simulator was used, loads and motions were applied to approximate in vivo conditions. The alumina ceramic heads and polyethylene cups were 28 mm in diameter and were tested for 5 million cycles in 25% new born calf serum at 1 Hertz. Microseparation was achieved by displacing the femoral head inferiorly during swing phase, where the head contacted the inferior cup rim and was laterally displaced. On heel strike the head contacted the superior cup rim prior to relocation. The volumetric wear of the polyethylene inserts was approximately four times less under microseparation conditions (5.6 ± 5.3 mm3 per million cycles), in comparison to standard conditions (25.6 ± 5.3 mm3 per million cycles). Deformation of the cup rim was observed, but some of this was attributed to creep. It is postulated that this reduction in wear was due to the separation of the components in swing phase improving the entrapment of lubricant, hence reducing wear via a squeeze film lubrication mechanism. In conclusion, surgical procedures that produce a small and controlled amount of joint laxity and microseparation may lead to a reduction in wear of the polyethylene acetabular cups.