Estimation of the wear volume after total hip replacement. A simple access to geometrical concepts

Pyrocarbon hemiprostheses show little glenoid erosion and good clinical function at 5.5 years of follow-up

BACKGROUND

The success of traditional shoulder hemiarthroplasty (HA) with cobalt-chromium heads is limited by painful glenoid erosion with problematic bone loss. Hemiprostheses with pyrolytic carbon (PyC) heads have shown reduced glenoid erosion in experimental laboratory studies. Few in vivo data are available.

METHODS

We performed a single-center consecutive cohort study of 31 of 34 patients (91%) who underwent PyC HA between September 2013 and June 2018. In 11 of these patients, concentric glenoid reaming was additionally performed. The mean follow-up period was 5.5 years (range, 3.5-7 years). Standardized radiographs were taken, and clinical function (Constant score) and pain (visual analog scale score) were recorded. Anteroposterior radiographs were analyzed according to an established method by 2 independent observers: A line parallel to the superior and inferior glenoid rim was translated to the most medial point of the glenoid surface. A further parallel line was placed on the spinoglenoid notch. The distance between these 2 lines was measured. Measurements were scaled using the known diameter of the implanted humeral head component. To assess eccentric erosion, anteroposterior and axial images were classified according to Favard and Walch, respectively.

RESULTS

Mean medial glenoid erosion measured 1.4 mm at an average of 5.5 years of follow-up. In the first year, 0.8 mm of erosion was observed, significantly more than the average erosion per year of 0.3 mm (P < .001). Mean erosion per year was 0.4 mm in patients with glenoid reaming vs. 0.2 mm in those without reaming (P = .09). An evolution of glenoid morphology was observed in 6 patients, of whom 4 had a progression of the erosion grade. The prosthesis survival rate was 100%. The Constant score improved from 45.0 preoperatively to 78.0 at 2-3 years postoperatively and 78.8 at latest follow-up (5.5 years postoperatively) (P < .001). The pain score on a visual analog scale decreased from 6.7 (range, 3-9) preoperatively to 2.2 (range, 0-8) at latest follow-up (P < .001). There was a weak correlation (r = 0.37) between erosion and pain improvement (P = .039) and no correlation between erosion and change in Constant score (r = 0.06).

CONCLUSION

PyC HA caused little glenoid erosion and a sustained improvement in clinical function in our cohort at mid-term follow-up. PyC demonstrates a biphasic development of glenoid erosion, with a reduced rate after the first year. PyC HA should therefore be considered as an alternative to cobalt-chromium HA and to anatomical total shoulder arthroplasty for patients with a high risk of glenoid component complications.

Clinical and radiological results with a 36-mm cobalt-chrome prosthetic head, cross-linked Durasul liners associated with Allofit cups: a more than 10-year follow-up period

BACKGROUND

Highly cross-linked polyethylene is currently a common articulation surface used for total hip arthroplasty (THA).

AIM

The aim of the present study is in vivo assessment of highly cross-linked Durasul polyethylene linear and volumetric wear when associated with a 36-mm prosthetic femoral head.

METHODS

We retrospectively reviewed clinical and radiographic data of 78 patients (81 hips) having primary THAs using Durasul liner combined with a 36-mm CoCr prosthetic head. All of them were followed for more than 10 years. Patient outcome was assessed with the Harris Hip Score (HHS) preoperatively and at last follow-up. 2-D prosthetic head penetration into polyethylene, 3D wear rates and cup migration were evaluated.

RESULTS

The preoperative and last follow-up HHS were 50.43 +/- 10.42 and 97.44 +/- 5.51 respectively. The annual penetration of the prosthetic head into Durasul^® liner was 0.029 +/- 0.003 mm. The annual linear penetration and volumetric wear extrapolation rates using Charnley and Ilchmann formulas were 37.84% and 57.76% respectively of that seen with conventional polyethylene liner. At last follow-up, the total loss of material in Durasul represents only 0.15% of the initial polyethylene mass. We did not observe any significant cup migration in the study group.

CONCLUSIONS

Results are promising, and we believe that these data authorise the continued use of highly cross-linked polyethylene liner associated with a 36-mm prosthetic head for total hip arthroplasties in older patients. More long-term follow-up studies are mandatory before we feel comfortable with the project of using cross-linked polyethylene in young and active patients instead of ceramic-on-ceramic bearings.

Estimating the osteolysis-free life of a total hip prosthesis depending on the linear wear rate and head size

We present a model to estimate the osteolysis-free life of total hip arthroplasty, depending on linear wear rate and femoral head size. An estimate of the radiologic osteolysis threshold was calculated, which was based on volumetric wear. The osteolysis-free life of the cup was estimated from the quotient of the osteolysis threshold and volumetric wear rate, which was calculated from the linear wear rate. The impact of the direction of linear wear was determined by sensitivity analysis. From our review, we calculated a weighted mean polyethylene volume of approximately 670 mm³ as osteolysis threshold. Osteolysis-free life of less than 20 years was estimated for linear wear rates of 50 µm/year for head sizes of 32 mm or more, or for linear wear rates of 100 µm/year for any head size. For head sizes of 36 and 40 mm with a linear wear rate of 50 µm/year, the osteolysis-free period is estimated to be only 14.10 and 11.42 years, respectively. Sensitivity analysis showed reasonably robust results. With the aim of osteolysis-free life of more than 20 years, our study presents a viable model to determine maximum possible head size for articulations. Osteolysis-free period for 36 and 40 mm head sizes are far too low for conventional polyethylenes. As the threshold wear volume for highly crosslinked polyethylene is, as of yet, unknown, more research is warranted before our model can be generalized to XLPE.

Patient-specific hip geometry has greater effect on THA wear than femoral head size

In vivo linear penetration in total hip arthroplasty (THA) exhibits similar values for 28mm and 32mm femoral head diameter with considerable variations between and within the studies. It indicates factors other than femoral head diameter influence polyethylene wear. This study is intended to test the effect of patient׳s individual geometry of musculoskeletal system, acetabular cup orientation, and radius of femoral head on wear. Variation in patient׳s musculoskeletal geometry and acetabular cup placement is evaluated in two groups of patients implanted with 28mm and 32mm THA heads. Linear wear rate estimated by mathematical model is 0.165-0.185mm/year and 0.157-0.205mm/year for 28 and 32mm THA heads, respectively. Simulations show little influence femoral head size has on the estimated annual wear rate. Predicted annual linear wear depends mostly on the abduction angle of the acetabular cup and individual geometry of the musculoskeletal system of the hip, with the latter having the greatest affect on variation in linear wear rate.

The study of wear behaviors on abducted hip joint prostheses by an alternate finite element approach

An acetabular cup with larger abduction angles is able to affect the normal function of the cup seriously that may cause early failure of the total hip replacement (THR). Complexity of the finite element (FE) simulation in the wear analysis of the THR is usually concerned with the contact status, the computational effort, and the possible divergence of results, which become more difficult on THRs with larger cup abduction angles. In the study, we propose a FE approach with contact transformation that offers less computational effort. Related procedures, such as Lagrangian Multiplier, partitioned matrix inversion, detection of contact forces, continuity of contact surface, nodal area estimation, etc. are explained in this report. Through the transformed methodology, the computer round-off error is tremendously reduced and the embedded repetitive procedure can be processed precisely and quickly. Here, wear behaviors of THR with various abduction angles are investigated. The most commonly used combination, i.e., metal-on-polyethylene, is adopted in the current study where a cobalt-chromium femoral head is paired with an Ultra High Molecular Weight Polyethylene (UHMWPE) cup. In all illustrations, wear coefficients are estimated by self-averaging strategy with available experimental datum reported elsewhere. The results reveal that the THR with larger abduction angles may produce deeper depth of wear but the volume of wear presents an opposite tendency; these results are comparable with clinical and experimental reports. The current approach can be widely applied easily to fields such as the study of the wear behaviors on ante-version, impingement, and time-dependent behaviors of prostheses etc.

Effect of cup inclination on predicted contact stress-induced volumetric wear in total hip replacement

In order to increase the lifetime of the total hip endoprosthesis, it is necessary to understand mechanisms leading to its failure. In this work, we address volumetric wear of the artificial cup, in particular the effect of its inclination with respect to the vertical. Volumetric wear was calculated by using mathematical models for resultant hip force, contact stress and penetration of the prosthesis head into the cup. Relevance of the dependence of volumetric wear on inclination of the cup (its abduction angle ϑA) was assessed by the results of 95 hips with implanted endoprosthesis. Geometrical parameters obtained from standard antero-posterior radiographs were taken as input data. Volumetric wear decreases with increasing cup abduction angle ϑA. The correlation within the population of 95 hips was statistically significant (P = 0.006). Large cup abduction angle minimises predicted volumetric wear but may increase the risk for dislocation of the artificial head from the cup in the one-legged stance. Cup abduction angle and direction of the resultant hip force may compensate each other to achieve optimal position of the cup with respect to wear and dislocation in the one-legged stance for a particular patient.

Wear analysis of chamfered elongated acetabular cup liners

This study investigated the wear phenomena of chamfered acetabular cup liners. The liners have three parameters at the lengthened rim: the length of the elongation, the depth of the cup and the chamfer angle. Using published wear volume equations for cylindrically elongated liners, this study analysed the volume of the chamfer to obtain the exact theoretical wear volume of the chamfered liner. The criteria described in our previous paper were used to verify the accuracy of the proposed methodology. The results showed that a shallow cup depth (<2 mm) and a chamfer angle of about 30° could significantly reduce the wear volume arising from the elongation. When the chamfer angle was increased further, this effect became progressively less significant. The results suggest that a chamfer angle up to about 30° is appropriate, but the angle should not be larger than required to obtain the minimum femoral range of motion (ROM). The results of this study can also be integrated with the analysis of the kinetics and kinematics of liners. A compromise design incorporating a lower wear volume, sufficient ROM and a lower contact stress should be possible.

Polyethylene wear is related to patient-specific contact stress in THA

BACKGROUND

General numerical models of polyethylene wear and THA simulators suggest contact stresses influence wear. These models do not account for some patient-specific factors. Whether the relationship between patient-specific contact stress and wear apply in vivo is unclear.

QUESTIONS/PURPOSES

We therefore determined whether (1) contact stress distribution at the prosthesis-cup interface and (2) hip geometry and cup inclination are related to wear in vivo.

METHODS

We retrospectively reviewed the radiographs of 80 patients who had aseptic loosening of their THAs as determined by radiographic criteria. We determined linear penetration and volumetric wear using postoperative and last followup radiographs. Contact stress distribution was determined by the HIPSTRESS method. The biomechanical model was scaled to fit the patient's musculoskeletal geometry of the pelvis, trochanteric position, and cup inclination using the standard postoperative radiograph.

RESULTS

Linear penetration and volumetric wear correlated with peak contact stress. Polyethylene wear was greater in THAs with a medial position of the greater trochanter and smaller inclination of the acetabular cup.

CONCLUSIONS

Our observations suggest wear is specific to contact stresses in vivo.

CLINICAL RELEVANCE

Long-term wear in a THA can be estimated using contact stress analysis based on analysis of the postoperative AP radiograph.

In vitro evaluation of stiffness graded artificial hip joint femur head in terms of joint stresses distributions and dimensions: finite element study

The aim of the present work is to evaluate the artificial hip joint femur head that is made of Stiffness Graded (SG) material in terms of joint stresses distributions and dimensions. In this study, 3D finite element models of femur head that is made of SG material and traditional femur heads made of Stainless Steel (SS), Cobalt Chromium alloy (Co Cr Mo) and Titanium alloy (Ti) have been developed using the ANSYS Code. The effects on the total artificial hip joint system stresses due to using the proposed SG material femur head (with low stiffness at the outer surface and high stiffness at its core) have been investigated. Also, the effects on the polymeric cup contact stresses due to the use of different sizes of femur heads, presence of metal backing shell and presence of radial clearance (gap) between cup and femur head have been investigated. The finite element results showed that using SG femur head resulted in a significant reduction in the cup contact stresses even for small femur heads compared with Ti alloy, SS and Co Cr Mo femur heads. The presence of radial clearance resulted in significant increase in the cup stresses especially for small femur heads. Finally, the presence of SS metal backing shell resulted in slight increase in the hip joint stresses especially for small femur head joints. This work analyzes successfully the usage of proposed SG material as femur head in order to reduce the predicted stresses at the total hip joint replacement due to the redistribution of strain energy in the hip prostheses. Therefore, the present results suggest that minor changes in design and geometrical parameters of the hip joint have significant consequences on the long term use of the joint and should be taken into consideration during the design of the hip joint.

Wear patterns of, and wear volume formulae for, cylindrically elongated acetabular cup liners

This study analyzed the wear patterns of, and wear volume formulae for, cylindrically elongated acetabular cup liners. The geometric patterns of the wear surface were first classified, then wear volume formulae were derived by integral calculus. SolidWorks((R)) software or published formulae were used to verify the accuracy of the proposed formulae. The analytical results showed that the wear shape of the liner can be categorized into seven wear patterns, including the special case of wear at 90 degrees , and the seven corresponding wear formulae were derived. In addition, wear of the cylindrical elongation might add considerably to the volume loss of the liner, depending on the height and shape of the elongation and the depth and direction of the linear penetration, being maximally 21% in the investigated model. The proposed wear formulae and patterns will be useful for more accurate performance evaluation of existing hip components implanted in patients and for the designing of new hip components.

Evaluating the accuracy of wear formulae for acetabular cup liners

This study proposes two methods for exploring the wear volume of a worn liner. The first method is a numerical method, in which SolidWorks software is used to create models of the worn out regions of liners at various wear directions and depths. The second method is an experimental one, in which a machining center is used to mill polyoxymethylene to manufacture worn and unworn liner models, then the volumes of the models are measured. The results show that the SolidWorks software is a good tool for presenting the wear pattern and volume of a worn liner. The formula provided by Ilchmann is the most suitable for computing liner volume loss, but is not accurate enough. This study suggests that a more accurate wear formula is required. This is crucial for accurate evaluation of the performance of hip components implanted in patients, as well as for designing new hip components.

Effect of femoral head size on polyethylene wear and synovitis after total hip arthroplasty: a sonographic and radiographic study of 39 patients

BACKGROUND AND PURPOSE

The role of synovitis and high fluid pressure in the loosening process after total hip arthroplasty has gained increasing attention. We investigated the correlation between head size, polyethylene wear, and capsular distention.

PATIENTS AND METHODS

We analyzed 39 unrevised, radiographically stable hips that had been operated with 28 or 32 mm femoral heads 10 years earlier because of osteoarthritis. We evaluated radiographic signs of loosening, linear and volumetric polyethylene wear, body mass index, activity level, and age. Sonographic examination was performed to measure capsular distance i.e. the distance between the prosthetic femoral neck and the anterior capsule.

RESULTS

Linear wear was 0.09 mm/year and 0.18 mm/year in the 28 mm and 32 mm groups, respectively (p < 0.001). The volumetric wear was 51 mm(3)/year and 136 mm(3)/year (p < 0.001) and the capsular distance was 13 mm and 17 mm, respectively (p < 0.001). There was a correlation between linear wear (r = 0.54), volumetric wear (r = 0.62), and capsular distance (p < 0.001).

INTERPRETATION

Wear was greater for the larger femoral head and was correlated to capsular distension.