Risk factors for unsuccessful acetabular press-fit fixation at primary total hip arthroplasty.

BACKGROUND: Surgeon at primary total hip arthroplasty sometimes cannot achieve sufficient cementless acetabular press-fit fixation and must resort to other fixation methods. Despite a predominant use of cementless cups, this issue is not fully clarified, therefore we performed a large retrospective study to: (1) identify risk factors related to patient or implant or surgeon for unsuccessful intraoperative press-fit; (2) check for correlation between surgeons' volume of operated cases and the press-fit success rate. HYPOTHESIS: Unsuccessful intra-operative press-fit more often occurs in older female patients, particular implants, due to learning curve and low-volume surgeons. MATERIALS AND METHODS: Retrospective observational cohort of prospectively collected intraoperative data (2009-2016) included all primary total hip arthroplasty patients with implant brands that offered acetabular press-fit fixation only. Press-fit was considered successful if acetabulum was of the same implant brand as the femoral component without additional screws or cement. Logistic regression models for unsuccessful acetabular press-fit included patients' gender/age/operated side, implant, surgeon, approach (posterior n=1206, direct-lateral n=871) and surgery date (i.e. learning curve). RESULTS: In 2077 patients (mean 65.5 years, 1093 females, 1163 right hips), three different implant brands (973 ABG-II™-Stryker, 646 EcoFit™ Implantcast, 458 Procotyl™ L-Wright) were implanted by eight surgeons. Their unsuccessful press-fit fixation rates ranged from 3.5% to 23.7%. Older age (odds ratio 1.01 [95% CI: 0.99-1.02]), female gender (2.87 [95% CI: 2.11-3.91]), right side (1.44 [95% CI: 1.08-1.92]), surgery date (0.90 [95% CI: 1.08-1.92]) and particular implants were significant risk factors only in three surgeons with less successful surgical technique (higher rates of unsuccessful press-fit with Procotyl™-L and EcoFit™ [P=0.01]). Direct-lateral hip approach had a lower rate of unsuccessful press-fit than posterior hip approach (P<0.01), but there was no correlation between surgeons' volume and rate of successful press-fit (Spearman's rho=0.10, P=0.82). Subcohort of 961 patients with 5-7-years follow-up indicated higher early/late cup revision rates with unsuccessful press-fit. DISCUSSION: Success of press-fit fixation depends entirely on the surgeon and surgical approach. With proper operative technique, the unsuccessful press-fit fixation rate should be below 5% and the impact of patients' characteristics or implants on press-fit fixation is then insignificant. Findings of huge variability in operative technique between surgeons of the presented study emphasize the need for surgeon-specific data stratification in arthroplasty studies and indicate the possibility of false attribution of clinically observed phenomena to patient-related factors in pooled data of large centers or hip arthroplasty registers. LEVEL OF EVIDENCE: Level III, retrospective observational case control study.

High contact hip stress is related to the development of hip pathology with increasing age.

BACKGROUND: High contact hip stress is believed to be one of the key biomechanical factors involved in the hip cartilage degeneration and osteoarthritis. Accordingly, with increasing age high contact hip stress is expected to cause elimination of subjects from the population of healthy hips, but its predictive value has not been evaluated so far. The objective of the paper is to investigate whether the exposure of healthy hips to estimated high contact hip stress is related to the development of hip pathology with increasing age. METHODS: A cross-sectional age- and gender-matched analysis of the peak contact hip stress calculated from pelvic geometry was made in 103 adult subjects with healthy hips. The peak contact hip stress was calculated from anterior-posterior pelvic radiographs of healthy hips by using a mathematical model of the human hip in the static one-legged stance. FINDINGS: In both female and male population, the average values of the peak contact hip stress normalized to the body weight are significantly higher and the values are also more dispersed in younger subjects when compared to older subjects. INTERPRETATION: The hip joints which remain healthy in the old age have lower average estimated peak contact hip stress. These results are consistent with the explanation that subjects with high estimated peak contact hip stress are more likely to develop hip disease in the course of life.

Shape transformation of giant phospholipid vesicles at high concentrations of C12E8.

Giant unilamellar phospholipid vesicles were prepared by the method of electroformation from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC). We studied the influence of different concentrations of the surfactant octaethyleneglycol dodecylether (C(12)E(8)) on the spontaneous shape transformations of POPC vesicles at room temperature. In accordance with previous results, we observed that low concentration of C(12)E(8) increased the speed of the characteristic vesicle shape transformation, starting from the initial shape with thin tubular protrusion, through beaded protrusion where the number of beads gradually decreased, to final spherical shapes with invagination, whereby the average mean curvature of the vesicle membrane monotonously decreased. In contrast, higher concentration of C(12)E(8) initially induced the shape transformation in the "opposite direction": in the protrusion, the number of beads gradually increased and eventually a tube was formed whereby the average mean curvature of the vesicle membrane gradually increased. However, at a certain point, an abrupt shape change took place to yield the vesicle with invagination. In this transition, the average mean curvature of the vesicle membrane discontinuously decreased. After this transition, the vesicle began to shrink and finally disappeared. We discuss possible mechanisms involved in the observed transformations.

Mathematical estimation of stress distribution in normal and dysplastic human hips.

By using a mathematical model of the adult human hip in the static one-legged stance position of the body, the forces acting on the hip, peak stress in the hip joint and other relevant radiographic and biomechanical parameters were assessed. The aims were to examine if the peak stress in dysplastic hips is higher than in normal hips and to find out which biomechanical parameters contribute significantly to higher peak stress. The average normalized peak stress in dysplastic hips (7.1 kPa/N) was markedly higher (to approximately 100%) than the average normalized peak stress in normal hips (3.5 kPa/N). The characteristic parameters that contributed to higher peak stress in dysplastic hips included the smaller lateral coverage of the femoral head, the larger interhip distance, the wider pelvis, and the medial position of the greater trochanter. These results are consistent with the hypothesis that stress distribution over weight-bearing surface of the hip joint is the relevant parameter for assessment of the risk for developing coxarthrosis.

Peak contact stress in human hip during gait.

The contact stress in a human hip is not uniform and it changes with different body positions. The changing location of the peak contact stress during gait may indicate the predilection sites for further development of osteoarthritis in the hip. On the basis of laboratory measurements and by using mathematical models of forces and stresses in human hip we determined the points of the peak contact stress in successive phases of gait. Results show that the peak stress points are mostly located in the posterior-medial portion of the weight bearing area, which corresponds well to the clinical observations. It is also shown that in the pathological conditions of hip dysplasia the peak contact stress trajectory is located more laterally and anteriorly.

Evaluation of biomechanical status of dysplastic human hips.

The evaluation of the biomechanical status of human hip joint was performed. Required parameters were assessed from standard antero-posterior rentgenographs and the value of contact stress on the weight bearing area is calculated. We analysed 45 dysplastic human hips and compared measured values to 81 healthy hips. The peak contact stress on the weight bearing area is considerably and statistically significantly higher in dysplastic hips compared to healthy hips. The Wiberg centre-edge angle is statistically significantly smaller in group of dysplastic hips compared to healthy hips.

Weight bearing area during gait in normal and dysplastic hips.

The size and the shape of the weight bearing area in adult human hips depend on the forces acting in the hip and therefore change during the body motion. In this work the size and the shape of the weight bearing area in several phases of gait are estimated. The forces acting in the hip were determined through laboratory measurements and analyzed by mathematical models. The dysplastic hips are distinguished from the normal ones by a smaller center-edge angle of Wiberg while the time course of the forces acting in the hip is assumed to be the same in both cases. It is shown how radial articular stress is distributed over the weight bearing area in both cases. In normal human hips the weight bearing area occupies a rather large portion of the acetabulum-femoral head contact area while in dysplastic hips stress distribution is unfavourably concentrated in a smaller weight bearing area.

Mathematical modelling of stress in the hip during gait.

A mathematical model is developed for calculating the contact stress distribution in the hip for a known resultant hip force and characteristic geometrical parameters. Using a relatively simple single nonlinear algebraic equation, the model can be readily applied in clinical practice to estimate the stress distribution in the most frequent body positions of everyday activities. This is demonstrated by analyzing the data on the resultant hip force obtained from laboratory observations where a stance period of gait is considered.