Measurement accuracy in acetabular cup wear. Three retrospective methods compared with Roentgen stereophotogrammetry

Does Pelvic Orientation Influence Wear Measurement of the Acetabular Cup in Total Hip Arthroplasty—An Experimental Study

Roentgen stereophotogrammetric analysis (RSA) is the gold standard to detect in vivo material wear of the bearing couples in hip arthroplasty. Some surgical planning tools offer the opportunity to detect wear by using standard a.p. radiographs (2Dwear), whilst RSA (3Dwear) needs a special radiological setup. The aims of this study are to prove the interchangeable applicability of a 2Dwear approach next to RSA and to assess the influence of different pelvic positions on measurement outcomes. An implant-bone model was used to mimic three different wear scenarios in seven pelvic-femur alignment positions. RSA and a.p. radiographs of the reference and a follow-up (simulated wear) pose were acquired. Accuracy and precision were worse for the 2Dwear approach (0.206 mm; 0.159 mm) in comparison to the 3Dwear approach (0.043 mm; 0.017 mm). Changing the pelvic position significantly influenced the 2Dwear results (4 of 7, p < 0.05), whilst 3Dwear results showed almost no change. The 3Dwear is superior to the 2Dwear approach, as it is less susceptible to changes in pelvic position. However, the results suggest that a 2Dwear approach may be an alternative method if the wear present is in the range of 100–500 µm and a.p. radiographs are available with the pelvis projected in a neutral position.

The concept of a cementless isoelastic monoblock cup made of highly cross-linked polyethylene infused with vitamin E: radiological analyses of migration and wear using EBRA and clinical outcomes at mid-term follow-up

Background

The newest generation of cementless titanium-coated, isoelastic monoblock cup with vitamin E-blended highly cross-linked polyethylene (HXLPE) was introduced to the market in 2009. The aim of the present study was to obtain mid-term follow-up data including migration and wear analyses.

Methods

This prospective study investigated 101 primary total hip arthroplasty (THA) cases in 96 patients treated at a single institution. Patients were allowed full weight-bearing on the first day postoperatively. Harris hip score (HHS) and pain and satisfication on a visual analogue scale (VAS) were assessed at a mean follow-up of 79.0 months. Migration and wear were assessed using Einzel-Bild-Roentgen-Analyse (EBRA) software. Radiological acetabular bone alterations and complications were documented.

Results

At mid-term follow-up (mean 79.0 months, range: 51.8–101.7), 81 cases with complete clinical and radiological data were analyzed. Utilisable EBRA measurements were obtained for 42 hips. The mean HHS was 91.1 (range 38.0–100.0), VAS satisfaction was 9.6 (range 6.0–10.0), VAS rest pain was 0.2 (range 0.0–4.0), and VAS load pain was 0.6 (range 0.0–9.0). Mean migration was 0.86 mm (range: 0.0–2.56) at 24 months and 1.34 mm (range: 0.09–3.14) at 5 years, and the mean annual migration rate was 0.22 (range: − 0.24–1.34). The mean total wear was 0.4 mm (range: 0.03–1.0), corresponding to a mean annual wear rate of 0.06 mm per year (range: 0.0–0.17). Radiographic analysis did not reveal any cases of osteolysis, and no revision surgeries had to be performed.

Conclusions

After using vitamin-E blended HXLPE in cementless isoelastic monoblock cups, there were no obvious signs of osteolysis or aseptic loosening occurred. No patients required revision surgery after mid-term follow-up. Cup migration and wear values were well below the benchmarks considered predictive for potential future failure.

Trial registration

The trial registration number on ClinicalTrials.gov: NCT04322916 (retrospectively registered at 26.03.2020).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-03981-8.

Isolated Acetabular Liner Exchange for Polyethylene Wear and Osteolysis with Well-Fixed Metal Shell

Background

The isolated liner and head exchange procedure has been an established treatment method for polyethylene wear and osteolysis when the acetabular component remains well fixed. In this study, the mid-term results of this procedure were evaluated retrospectively.

Methods

Among the consecutive patients operated on from September 1995, two patients (three hips) were excluded because of inadequate follow-up, and the results of remaining 34 patients (34 hips) were evaluated. There were 20 men and 14 women with a mean age of 49 years. A conventional polyethylene liner was used in 26 cases and a highly cross-linked polyethylene liner was used in eight cases. In three cases, the liner was cemented in a metal shell because a compatible liner could not be used.

Results

After a follow-up of 5 to 20.2 years, re-revision surgery was necessary in 10 cases (29.4%): in eight for wear and osteolysis at 55 to 101 months after liner exchange and in two for acetabular loosening at 1 and 1.5 years after liner exchange. Re-revision surgery included all component revision (four cases), cup revision (four cases), and liner exchange (two cases). In all re-revision cases, a conventional polyethylene liner was used initially. There was no failure in the cases in which a highly cross-linked polyethylene liner was used.

Conclusions

The results of this study suggest that isolated acetabular liner exchange is a reasonable option for wear and osteolysis when the metal shell is well fixed. More promising long-term results are expected with the use of highly cross-linked polyethylene liners.

Acetabular revision arthroplasty using press-fitted jumbo cups: an average 10-year follow-up study

INTRODUCTION

Acetabular revision arthroplasty using jumbo cups for moderate-to-severe acetabular defects has varied outcomes. We evaluated the clinical and radiological outcomes of acetabular revision arthroplasty using a press-fitted jumbo cup and sought to identify factors that influence outcomes during intermediate follow-up.

MATERIALS AND METHODS

Eighty patients (47 men, 33 women; 80 hips) who underwent acetabular revision arthroplasty using press-fitted jumbo cups were included. The mean follow-up period was 10.4 years. Harris hip score (HHS), presence of groin pain, radiographic results, and Kaplan-Meier survival curves were evaluated. Implant design and surgery-related and patient-related factors were assessed to identify influential factors for cup loosening. Migration and wear analyses were performed using Einzel-Bild-Röntgen-Analyse software.

RESULTS

The mean preoperative HHS of 53 had improved to 77 at the final follow-up (p = 0.005). Nine patients experienced groin pain. Acetabular cup loosening was observed in seven cups (8.7%), and one jumbo cup was replaced with a reinforcement cage. The survival rate of the acetabular cup was 91% at 16 years according to the Kaplan-Meier analysis. Osteolysis was identified around the cup in six cases (7.5%). Acetabular cup loosening occurred more frequently in patients with conventional polyethylene liners than in those with highly cross-linked polyethylene liners (p = 0.045). The mean total migration was 1.52 mm, and the mean total wear was 0.98 mm. There was a positive correlation between total migration and total wear (p = 0.023; Spearman's rho = 0.388). The mean wear rate of the patients with the cup inclination angle < 50° was significantly lower than those with the cup inclination angle > 50° (p = 0.001). There were four cases of complications (three dislocations and one infection) that did not require revision surgery.

CONCLUSION

Press-fitted jumbo cups for acetabular revision arthroplasty exhibited encouraging results during follow-up for an average of 10 years. Use of highly cross-linked polyethylene liners and proper placement of the acetabular component with an inclination angle < 50° may contribute to better clinical outcomes after acetabular revision arthroplasty with jumbo cups.

Are Radiographic and Direct Measures of Acetabular Polyethylene Wear Comparable?

BACKGROUND

All polyethylene acetabular liners wear over time, and numerous methods for calculating linear wear rates exist. The objective of this study was to compare 2-dimensional wear rates between direct, micrometer measurements and the computerized, edge-detection method using Hip Analysis Suite (HAS) 8.0.4.3.

METHODS

Two groups of retrieved acetabular liners from Harris-Galante Prosthesis I and Harris-Galante Prosthesis II implants in situ for more than 10 years were evaluated. Group 1 (n = 18) contained liners with both early postoperative (<6 months) and prerevision radiographs taken within 1 month of explantation. Group 2 (n = 55) included liners with only prerevision X-rays (ie, 1 radiograph for wear assessment). Average and maximum direct linear wear was calculated from thicknesses measured at 6 consistent, well-separated locations (3 in the worn and 3 in the unworn regions) using a calibrated, digital micrometer. HAS 8.0.4.3 was used to calculate 2-dimensional wear from anteroposterior pelvic radiographs.

RESULTS

Aggregate wear rates calculated by HAS were higher than those calculated by the average of direct measurements for group 1 (P = .020) and group 2 (P < .001). However, comparing the maximum direct micrometer measurements to HAS showed no difference for either group 1 (P = .351) or group 2 (P = .451). Linear regression analysis showed a strong correlation between HAS and both average and maximum direct wear measures for both groups, though the coefficient for the direct maximum measurement comparisons were closer to one, indicating a better one-to-one correspondence between HAS and direct maximum wear.

CONCLUSION

To our knowledge, this is the first study to compare and validate 2-dimensional wear rates in polyethylene acetabular liners between direct measurements from retrieved components and a radiographic computer-assisted technique (as opposed to comparison against a phantom component). Wear rates determined by direct measurements from retrievals were consistent with computer-assisted 2-dimensional methods when comparing maximum wear measurements. In addition, a single prerevision radiograph appears to be sufficient to assess 2-dimensional in vivo wear.

Validation of a new 2-D technique for radiographic wear measurement of cemented, highly cross-linked polyethylene acetabular cups

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Mean wear penetration error of the system was −0.002 mm and the precision was 0.055 mm

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Above 0.25 mm penetration, wear direction error was within ±10° (median 1.15°)

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Good inter/intra observer agreement for clinical radiographs

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Penetration error equal to that of RSA for laboratory radiographs

A new 2-D radiographic wear measurement system has been developed which enables the low wear of highly cross-linked polyethylene acetabular cups to be accurately and precisely measured from standard, pelvis radiographs. The software was validated using radiographic images of a measurement jig which could vary the cup orientation and simulate the effect of pelvic tilt/rotation. Wear was simulated using accurately measured plastic shims to vary the position of the femoral head relative to the cup. The effects of varying “wear” penetration, “wear” direction, cup orientation and X-ray focus position were assessed. Further direct comparison tests were also carried out using radiostereometric analysis. Inter/intra-observer repeatability of the new system was assessed using clinical radiographs.

The mean (SD) “wear” penetration error was -0.002 mm (0.028 mm). The “wear” penetration precision was 0.055 mm. Changing the position of the X-ray focus point made no difference to the measurement error. Inter/intra-observer repeatability and limits of agreement had similar low values. Comparison tests with RSA showed the accuracy was similar.

Comparison of Polyethylene Wear before and after Hip Revision with Liner Exchange Fixed with the Original Locking Mechanism

Objective

To compare the wear of conventional ultra-high molecular weight polyethylene (CUHMWPE) and highly cross-linked polyethylene (HCLPE) in hip revision with liner exchange fixed with original locking mechanism using analysis of history medical data.

Methods

From Jan. 1, 2000, to Dec. 31, 2007, 26 patients (with 29 involved hips) underwent liner exchange revision fixed with the original locking mechanism due to wear of CUHMWPE and/or osteolysis. The mean age was 53 ± 9 years at the time of the primary total hip arthroplasty (THA) and 64 ± 9 years at the revision. The exchanged liners (Marathon, Depuy) were made of HCLPE. Annual X-rays were used to measure linear wear and osteolysis. The annual linear penetration was measured using PolyWare® software (Draftware Inc.). Annual Harris Hip Scores(HSS) were recorded.

Results

The mean follow-up time between the primary and revision THAs was 11 ± 2 years and 8 ± 2 years after revision. The mean Harris Hip Score(HHS) before primary THA, 1 year after primary THA, before revision and 1 year after revision was 43±5, 85±5, 71±6, 83±7 individually. The mean penetration of the CUHMWPE and HCLPE liners occurring in the first year were 0.44 ± 0.28 mm and 0.38 ± 0.14 mm, respectively (p = 0.211). The mean annual linear penetration of CUHMWPE and HCLPE from the second year onward were 0.29±0.09 mm and 0.08 ± 0.03 mm respectively (p <0.01). All THAs with CUHMWPE showed osteolysis on acetabular and/or femoral side before revision. No HCLPE liner showed osteolysis at the last follow-up. Conclusion: The CUHMWPE liner had a significantly higher wear rate than did the HCLPE liner. The HCLPE liner showed a satisfactory liner penetration rate after revision with isolated liner exchange fixed with the original locking mechanism.

2D measurements of cup orientation are less reliable than 3D measurements

BACKGROUND AND PURPOSE

2D analysis of metal-on-metal (MoM) hip arthroplasty (HA) has been conducted in several large series on conventional radiographs with the use of Ein Bild Roentegen Analyse (EBRA) software, but there have been no comparisons with 3D analysis in the literature. The main aim of this study was to quantify the agreement in measurements of cup version of large-diameter MoM hips obtained by EBRA and by 3D computed tomography (3D-CT). The secondary aim was to quantify the agreement for cup inclination. Lastly, we wanted to determine the inter- and intra-observer reliability of both methods.

PATIENTS AND METHODS

87 MoM hips in 81 patients were analyzed for cup inclination and version in 2D on conventional radiographs using EBRA software. The results were compared with 3D measurements using CT.

RESULTS

Cup version was underestimated by EBRA when compared to 3D-CT, by 6° on average with the pelvis supine and by 8° on average with the pelvis orientated to the anterior pelvic plane (APP). For inclination, the mean difference was no more than 1°. 53% of hips were within a 10° safe zone of 45° inclination and 20° version when measured by 3D-CT with the pelvis supine (and 54% with the pelvis in the APP). The proportion was only 24% when measured by EBRA. Inter- and intra-observer reliability of cup version is poorer using 2D analysis than when using 3D-CT.

INTERPRETATION

Errors in version in 2D were due to the difficulty in delineating the cup rim, which was obscured by a large-diameter metal head of the same radio-opacity. This can be overcome with 3D analysis. The present study demonstrates that measurements using EBRA have poor agreement and are less reliable than those with 3D-CT when measuring cup version and inclination in MoM hips.

Surveillance of Arc2f polyethylene after 15 years of implantation

BACKGROUND

To investigate the behavior of Arc2f polyethylene (PE) component after a minimum of 15 years post implantation and its possible reasons for failure.

METHODS

Between January 1988 and March 1995, 557 cementless total hip arthroplasties were performed in 578 patients. A total of 435 patients (450 hips) were followed up for 18.6 years (range 15-22).

RESULTS

At 10 years follow-up no cup needed revision surgery. After 15 years of implantation, we observed massive PE wear in 140 cups (31%) with large osteolytic lesions. A revision procedure was performed in all cases. Exchange only of the PE liner was sufficient in 78 cases, whilst in 59 the cup was also revised. PE insert thickness and the type of prosthetic head were found to be related to PE wear.

CONCLUSIONS

We postulate that the massive failure seen is attributed to the manufacturing of the PE insert. We advocate following all patients that have an implant lasting a decade after implantation for routine monitoring to identify those at risk for accelerated PE wear.

No superior performance of hydroxyapatite-coated acetabular cups over porous-coated cups

BACKGROUND AND PURPOSE

Hydroxyapatite (HA) coating is believed to improve bone-implant ingrowth and long-term survival of prostheses. Recent studies, however, have challenged this view. Furthermore, HA particles may produce third-body wear and initiate aseptic loosening of implants. We report the performance of HA- and porous-coated acetabular cups in a prospective randomized trial.

METHODS

This was an 8-year follow-up study of our previously published prospective randomized study to compare clinical outcomes, survival, periprosthetic bone mineral density, migration, and wear rates of HA- and porous-coated acetabular cups. Dual X-ray absorptiometry (DXA) and Ein Bild Roentgen Analyse (EBRA) measurements were used. 100 patients who underwent unilateral cementless total hip arthroplasty were randomized to either porous-coated cups or HA-coated cups. Patients were examined preoperatively and at 3, 6, and 9 months, and also 1, 3, and 8 years after surgery. 81 patients were available for 8-year follow-up, 40 with porous-coated cups and 41 with HA-coated cups.

RESULTS

Age, sex, bone mineral density, and clinical results (Harris hip score) were similar in the 2 groups. The survival, wear, and migration patterns of the cups were also similar in both groups. The results of periprosthetic bone mineral density scans in region of interest 2 was in favor of the porous-coated cups, but there were no differences between the 2 groups in all the remaining regions of interest.

INTERPRETATION

HA coating had no statistically significant effect on clinical results, survival, wear, or migration at the 8-year follow-up.

The efficacy of a "double-D-shaped" wire marker for radiographic measurement of acetabular cup orientation and wear

Historically, wire markers were attached to cemented all-plastic acetabular cups to demarcate the periphery and to measure socket wear. The wire shape was either a semi-circle passing over the pole of the cup, or a circle around the cup equator. More recently, "double-D" shaped markers were introduced with a part-circular aspect passing over the pole and a semi-circular aspect parallel to the equatorial plane. This configuration enabled cup retroversion to be distinguished from anteversion. In this study, the accuracy of radiographic measurement of cup orientation and wear was assessed for cups with "double-D" and circular markers. Each cup was attached to a measurement jig which could vary the anteversion/retroversion and internal/external rotation of the cup. A metal femoral head was fixed within the socket and radiographic images were created for all combinations of cup orientation settings. The images were measured using software with automatic edge detection, and cup orientation and zero-wear accuracies were determined for each setting. The median error for cup version measurements was similar for both types of wire marker (0.2° double-D marker, -0.24° circular marker), but measurements of the circular marker were more repeatable. The median inclination errors were 2.05° (double-D marker) and 0.23° (circular marker). The median overall "zero wear" errors were 0.19 mm (double-D marker) and 0.03 mm (circular marker). Measurements of the circular wire marker were much more repeatable.

Five-year results of the uncemented RM pressfit cup clinical evaluation and migration measurements by EBRA

This was a prospective consecutive, study of 50 patients (mean age 72 years, 56% males) treated unilaterally for primary osteoarthrosis using the uncemented, isoelastic, monoblock RM Pressfit Cup. Migration and wear were assessed using the EBRA (Ein-Bild-Röntgen-Analyse) software (201 radiographs from 36 patients). Mean migration was 0.82 mm after 2 years and 1.25 mm after 5 years. A decreasing migration rate over the period was observed. The mean annual wear rate was 0.09 mm/y. Five years after surgery, 84% of patients reached good or excellent Harris hip score values. No revisions related to the acetabular component were necessary. All cups appeared to have high primary stability.

Superior accuracy of model-based radiostereometric analysis for measurement of polyethylene wear: A phantom study

Objectives

The accuracy and precision of two new methods of model-based radiostereometric analysis (RSA) were hypothesised to be superior to a plain radiograph method in the assessment of polyethylene (PE) wear.

Methods

A phantom device was constructed to simulate three-dimensional (3D) PE wear. Images were obtained consecutively for each simulated wear position for each modality. Three commercially available packages were evaluated: model-based RSA using laser-scanned cup models (MB-RSA), model-based RSA using computer-generated elementary geometrical shape models (EGS-RSA), and PolyWare. Precision (95% repeatability limits) and accuracy (Root Mean Square Errors) for two-dimensional (2D) and 3D wear measurements were assessed.

Results

The precision for 2D wear measures was 0.078 mm, 0.102 mm, and 0.076 mm for EGS-RSA, MB-RSA, and PolyWare, respectively. For the 3D wear measures the precision was 0.185 mm, 0.189 mm, and 0.244 mm for EGS-RSA, MB-RSA, and PolyWare respectively. Repeatability was similar for all methods within the same dimension, when compared between 2D and 3D (all p > 0.28). For the 2D RSA methods, accuracy was below 0.055 mm and at least 0.335 mm for PolyWare. For 3D measurements, accuracy was 0.1 mm, 0.2 mm, and 0.3 mm for EGS-RSA, MB-RSA and PolyWare respectively. PolyWare was less accurate compared with RSA methods (p = 0.036). No difference was observed between the RSA methods (p = 0.10).

Conclusions

For all methods, precision and accuracy were better in 2D, with RSA methods being superior in accuracy. Although less accurate and precise, 3D RSA defines the clinically relevant wear pattern (multidirectional). PolyWare is a good and low-cost alternative to RSA, despite being less accurate and requiring a larger sample size.

A prospective randomised controlled trial comparing three alternative bearing surfaces in primary total hip replacement

The ideal bearing surface for young patients undergoing total hip replacement (THR) remains controversial. We report the five-year results of a randomised controlled trial comparing the clinical and radiological outcomes of 102 THRs in 91 patients who were <  65 years of age. These patients were randomised to receive a cobalt–chrome on ultra-high-molecular-weight polyethylene, cobalt–chrome on highly cross-linked polyethylene, or a ceramic-on-ceramic bearing. In all, 97 hip replacements in 87 patients were available for review at five years. Two hips had been revised, one for infection and one for peri-prosthetic fracture.

At the final follow-up there were no significant differences between the groups for the mean Western Ontario and McMaster Universities osteoarthritis index (pain, p = 0.543; function, p = 0.10; stiffness, p = 0.99), Short Form-12 (physical component, p = 0.878; mental component, p = 0.818) or Harris hip scores (p = 0.22). Radiological outcomes revealed no significant wear in the ceramic group.

Comparison of standard and highly cross-linked polyethylene, however, revealed an almost threefold difference in the mean annual linear wear rates (0.151 mm/year versus 0.059 mm/year, respectively) (p < 0.001).

Migration pattern of cementless press fit cups in the presence of stabilizing screws in total hip arthroplasty

The aim of this study was to evaluate the initial acetabular implant stability and late acetabular implant migration in press fit cups combined with screw fixation of the acetabular component in order to answer the question whether screws are necessary for the fixation of the acetabular component in cementless primary total hip arthroplasty. One hundred and seven hips were available for follow-up after primary THA using a cementless, porous-coated acetabular component. A total of 631 standardized radiographs were analyzed digitally by the "single-film-x-ray-analysis" method (EBRA). One hundred and one (94.4 %) acetabular components did not show significant migration of more than 1 mm. Six (5.6%) implants showed migration of more than 1 mm. Statistical analysis did not reveal preoperative patterns that would identify predictors for future migration. Our findings suggest that the use of screw fixation for cementless porous-coated acetabular components for primary THA does not prevent cup migration.

The final follow-up plain radiograph is sufficient for clinical evaluation of polyethylene wear in total hip arthroplasty. A study of validity and reliability

BACKGROUND AND PURPOSE

Radiostereometric analysis (RSA) is a highly accurate tool for assessment of polyethylene (PE) wear in total hip arthroplasty (THA); however, PE wear measurements in clinical studies are often limited to plain radiographs. We evaluated the agreement between PE wear measured with PolyWare software, which uses plain radiographs, and by model-based RSA, which uses stereo radiographs.

METHODS

Measurements of PE wear postoperatively and at final follow-up (after mean 6 years) on plain radiographs of 12 patients after cementless THA were evaluated with PolyWare software and the results were compared with those from RSA as the gold standard (Model-based RSA using elementary geometrical shape models; EGS-RSA). With PolyWare, we either used the final radiographic follow-up (PW1) only or both the postoperative follow-up and the final follow-up (PW2).

RESULTS

The 2D mean wear measured (in mm) was 0.80, 1.07, and 0.60 for the PW2, PW1, and RSA method. 2D intra-method repeatability was similar for PW1 and RSA with limits of agreement (LOAs, in mm) of ± 0.22, and ± 0.23, respectively. 2D inter-method concurrent validity was best between PW1 and EGS-RSA with LOAs of ± 0.55. For 2D linear wear measurements, the PW1 method had a clinical repeatability similar to that of RSA.

INTERPRETATION

PW1 is sufficient for retrospective determination of 2D wear from medium-term wear measurements above 0.5 mm, It alleviates the need for baseline plain radiographs, has a clinical precision similar to that of RSA, and is easy and inexpensive to use.

Dual mobility hip arthroplasty wear measurement: Experimental accuracy assessment using radiostereometric analysis (RSA)

INTRODUCTION

The use of dual mobility cups is an effective method to prevent dislocations. However, the specific design of these implants can raise the suspicion of increased wear and subsequent periprosthetic osteolysis.

HYPOTHESIS

Using radiostereometric analysis (RSA), migration of the femoral head inside the cup of a dual mobility implant can be defined to apprehend polyethylene wear rate.

STUDY OBJECTIVES

The study aimed to establish the precision of RSA measurement of femoral head migration in the cup of a dual mobility implant, and its intra- and interobserver variability.

MATERIAL AND METHODS

A total hip prosthesis phantom was implanted and placed under weight loading conditions in a simulator. Model-based RSA measurement of implant penetration involved specially machined polyethylene liners with increasing concentric wear (no wear, then 0.25, 0.5 and 0.75mm). Three examiners, blinded to the level of wear, analyzed (10 times) the radiostereometric films of the four liners. There was one experienced, one trained, and one inexperienced examiner. Statistical analysis measured the accuracy, precision, and intra- and interobserver variability by calculating Root Mean Square Error (RMSE), Concordance Correlation Coefficient (CCC), Intra Class correlation Coefficient (ICC), and Bland-Altman plots.

RESULTS

Our protocol, that used a simple geometric model rather than the manufacturer's CAD files, showed precision of 0.072mm and accuracy of 0.034mm, comparable with machining tolerances with low variability. Correlation between wear measurement and true value was excellent with a CCC of 0.9772. Intraobserver reproducibility was very good with an ICC of 0.9856, 0.9883 and 0.9842, respectively for examiners 1, 2 and 3. Interobserver reproducibility was excellent with a CCC of 0.9818 between examiners 2 and 1, and 0.9713 between examiners 3 and 1.

DISCUSSION

Quantification of wear is indispensable for the surveillance of dual mobility implants. This in vitro study validates our measurement method. Our results, and comparison with other studies using different measurement technologies (RSA, standard radiographs, Martell method) make model-based RSA the reference method for measuring the wear of total hip prostheses in vivo.

LEVEL OF EVIDENCE

Level 3. Prospective diagnostic study.

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.

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

Various formulas have been proposed to calculate the volume of prosthetic wear from the penetration depth of the head as assessed on plain radiographs, based on idealized, three-dimensional geometrical models of a prosthetic hip. However, for most published formulas no (or no simple) derivation is available and not all of them are correct. We describe a simple geometrical model that allows for transparent derivation of equations for various components of prosthetic wear volume and compare the calculated volumes with those obtained from published equations. These components are: (1) a right generalized cylinder resulting from a linear shift of a half spherical part of the prosthetic head into the hemispherical cup, (2) an additional wedge that is "cut" from the cup and (3) the wear from an optional additional cylindrical portion of the cup. We emphasize that calculation of a three-dimensional wear volume from linear penetration depth should be based on a geometrical concept that is transparent and simple enough for clinical research, such as the one presented. The incorrect formula of Kabo et al. should be completely abandoned.

Clinical and radiographic results and wear performance in different generations of a cementless porous-coated acetabular cup

We compared clinical results and wear performance in two different generations of a cementless porous-coated cup, analysing the long-term results of 83 uncemented Harris-Galante I cups (32-mm femoral head) and 93 uncemented Harris-Galante II cups (28-mm femoral head). All polyethylene liners were gamma irradiated in air. Polyethylene linear wear was estimated using a software package. The minimum follow-up was 10 years. Nine Harris-Galante I cups and two Harris-Galante II cups were revised due to aseptic loosening or polyethylene problems. The mean femoral head penetration at 6 weeks after surgery was 0.15 +/- 0.05 mm for the Harris-Galante I cups and 0.12 +/- 0.03 for the Harris-Galante II cups (p < 0.001);but mean wear was 0.13 +/- 0.23 mm per year for the Harris-Galante I cups and 0.11 +/- 0.10 for the Harris-Galante II cups (p = 0.740). Most of the metallic shells in both groups showed stable fixation. The so-called second-generation cups had lower initial polyethylene wear that resulted in less polyethylene wear at the latest the follow-up, but the overall wear rate was similar in both groups despite the different femoral head sizes and the improved locking mechanism.

Effectiveness of computer-navigated minimally invasive total hip surgery compared to conventional total hip arthroplasty: design of a randomized controlled trial

Background

Moderate to severe osteoarthrosis is the most common indication for Total Hip Arthroplasty (THA). Minimally Invasive Total Hip Surgery (MIS) and computer-navigated surgery were introduced several years ago. However, the literature lacks well-designed studies that provide evidence of superiority of computer-navigated MIS over a conventional THA technique. Hence, the purpose of this study is to compare (cost)effectiveness of computer-navigated MIS with a conventional technique for THA. It is our hypothesis that computer-navigated MIS will lead to a quicker recovery during the early postoperative period (3 months), and to an outcome at least as good 6 months postoperatively. We also hypothesize that computer-navigated MIS leads to fewer perioperative complications and better prosthesis positioning. Furthermore, cost advantages of computer-navigated MIS over conventional THA technique are expected.

Methods/design

A cluster randomized controlled trial will be executed. Patients between the ages of 18 and 75 admitted for primary cementless unilateral THA will be included. Patients will be stratified using the Charnley classification. They will be randomly allocated to have computer-navigated MIS or conventional THA technique. Measurements take place preoperatively, perioperatively, and 6 weeks and 3 and 6 months postoperatively. Degree of limping (gait analysis), self-reported functional status and health-related quality of life (questionnaires) will be assessed preoperatively as well as postoperatively. Perioperative complications will be registered. Radiographic evaluation of prosthesis positioning will take place 6 weeks postoperatively. An evaluation of costs within and outside the healthcare sector will focus on differences in costs between computer-navigated MIS and conventional THA technique.

Discussion

Based on studies performed so far, few objective data quantifying the risks and benefits of computer-navigated MIS are available. Therefore, this study has been designed to compare (cost) effectiveness of computer-navigated MIS with a conventional technique for THA. The results of this trial will be presented as soon as they become available.

Early clinical and radiographic analysis of the Fitmore cup

The Fitmore titanium mesh cementless acetabular component in 115 hip arthroplasties was reviewed at an average of 33 months of follow-up. None were revised nor had infection. One hip dislocated 4 years postoperatively. Two femoral components were revised. The average Harris Hip Score at the last follow-up was 90 points. In the 96 sets of radiographs available, there was no loosening or new radiolucency. One hip had nonprogressive osteolysis adjacent to a screw. This press-fit cup has its polar region flattened and is rim loading. Noncontact (gaps) at the acetabular floor is expected and is not critical for fixation. In all but 6 hips, these gaps filled. In 5 hips, a minimal gap (<or=0.5 mm) remained in part of the equatorial region. The Fitmore cup osseointegrates successfully.

Early polyethylene wear and excessive acetabular granuloma in an uncemented HA-coated total hip arthroplasty--midterm results of a prospective study

This is a prospective review of 135 HA-coated ABG I total hip arthroplasty (THA) systems with a mean clinical and radiographic follow-up of 8.5 years. The 5-year survival rate was 85%, but 22% of the patients were dissatisfied. Revision THA was already indicated in 28% of the patients, with 26% indicated for cup loosening. PE wear was detected by x-ray in 42%. Disproportionate substantial wear with an average linear loss of 2.6 mm at the inner rim of the insert was observed in 23% of the cases. The mean annual wear rate was calculated 0.1-0.25 mm/year. Laboratory examination of the retrieved PE revealed polishing, cracks, and subsurface delamination. Radiographic evidence of acetabular cysts were found to be excessive granuloma during surgery. Polarization microscopy revealed debris particles phagocytized by reticuloendothelial cells. Results confirm the general opinion that aseptic osteolysis is a cell-mediated process driven by the presence of particles generated from wear debris. The findings also suggest that the main reasons for the failure of the first-generation ABG hip system were an insufficient locking mechanism and poor PE congruency, and not solely poor PE quality.

Correction of error in two-dimensional wear measurements of cemented hip arthroplasties

The irregularity of individual wear patterns of total hip prostheses seen during patient followup may result partially from differences in radiographic projection of the components between radiographs. A method to adjust for this source of error would increase the value of individual wear curves. We developed and tested a method to correct for this source of error. The influence of patient position on validity of wear measurements was investigated with controlled manipulation of a cadaveric pelvis. Without correction, the error exceeded 0.2 mm if differences in cup projection were as small as 5 degrees. When using the described correction method, cup positioning differences could be greater than 20 degrees before introducing an error exceeding 0.2 mm. For followup of patients in clinical practice, we recommend using the correction method to enhance accuracy of the results.

EBRA-Digital release 2003 versus 1998: A comparison of instrument repeatability for measuring implant migration and wear

EBRA-Digital is an established method for measuring implant migration after total hip arthroplasty using digitized radiographs that has recently undergone a change in the software platform that may influence its precision. We assessed the precision of EBRA-Digital 2003 release and compared it to the previous 1998 release using consecutive, standardized, plain radiographic examinations made on the same day after repositioning in 29 patients. The precision of implant migration and wear measurements was similar between the two software releases, although analysis times were quicker using the 2003 release (p<0.01). Image file compression at a ratio of 30 resulted in poorer measurement precision for some variables. The EBRA 2003 software platform has similar precision to the previous release and allows faster measurement of implant migration and wear. The level of image file compression that is used affects the precision of these measurements. (Hip International 2005; 15: 226-9).

Validation of a digital image processing software package for the in vivo measurement of wear in cemented Charnley total hip arthroplasties

Computer-generated images were used to assess image processing software employed in the radiographic evaluation of penetration in total hip replacement. The images were corrupted using Laplacian noise and smoothed to simulate different modulation transfer functions in a range associated with hospital digital radiographic systems. With no corruption, the penetration depth measurements were both precise and accurate. However, as the noise increased so did the inaccuracy and imprecision to levels that may make changes in the penetration observed clinically difficult to discern between follow-up assessments. Simulated rotation of the wire marker produced significant bias in the measured penetration depth. The use of these simulated radiographs allows the evaluation of the software used to process the digital images alone rather than the whole measurement system.

Subsidence pattern of a cementless straight titanium alloy femoral stem - A radiographic study with EBRA-FCA

EBRA (Ein Bild Roentgen Analyse) is a computer-aided algorithm to assess migration of endoprosthetic components. Stem subsidence is measured in comparable radiographs, as defined by the software. Recent studies showed an improved accuracy of EBRA-FCA as compared to standard methods. For the BiCONTACT cementless femoral component, a prospective follow-up study of 250 cases has provided excellent results, with a survival rate of 97.1% after 11 years. For this migration study 93 hips with a complete radiographic follow-up (> 10 x-rays) over 120 months were included. Seventy-one stems were be measured; in 22 stems the measurement was impossible due to heterotopic ossification. Thirty-two hips (45.1%) showed initial subsidence within the first 24 months but no later, and 11 hips (15.5%) had a late onset of subsidence. In nine hips (12.7%) continuous sinking could be found. Nineteen hips (26.8%) had irregular patterns of migration. In 31.0% the overall migration was between 0.5-1mm after 120 months. Six stems (8.5%) had an absolute amount of subsidence exceeding 2mm after 120 months (one more than 3 mm). The mean subsidence was 0.2mm after three months and six months, 0.3mm after 12 months and reached 0.5mm after 10 years. EBRA-FCA is a practicable method to assess migration of the femoral component. There seems to be initial subsidence for about one half of the stems but further movements are rare, which might be a sign for osseo-integration. The radiographic findings correspond to the excellent clinical performance of this stem in long-term follow-up. (Hip International 2005; 15: 85-91).

Ten to twelve-year results with the Zweymüller cementless total hip prosthesis

Between January 1987 and December 1990, 221 Zweymüller cementless total hip arthroplasties were performed in 211 patients with idiopathic osteoarthritis. A total of 136 patients (142 prostheses) were evaluated at a mean follow-up of 134 months (SD 9.5). The study group consisted of 78 Hochgezogen and 64 Stepless stem prostheses, all with a threaded titanium cup and ceramic head. No clinical and radiological differences were found between the 2 stem prostheses. Seven cups had been revised because of aseptic loosening; 17 cups showed radiolucent lines, osteolysis, or migration. Mean linear polyethylene wear of 105 (74%) cups was 0.46 mm (SD 0.27), with an annual wear of 0.04 mm (SD 0.02). Wear did not correlate with pain, cup migration, radiolucent lines, or osteolysis. Cumulative survival was 96%. Zweymüller cementless total hip arthroplasty showed good midterm results.

Accuracy of radiographic and radiostereometric wear measurement of different hip prostheses: an experimental study

BACKGROUND

In vivo measurement of wear in the ball and socket articulation of total hip arthroplasties is of interest in the evaluation of both existing and new implants. Controversy reigns regarding the accuracy of different radiological measurement techniques and in particular how accuracy has been assessed.

MATERIAL AND METHODS

We assessed the accuracy of 2 radiostereometric (RSA) techniques for wear measurement and 3 standard radiographic techniques, namely Imagika (image analyzing software), Imagika corrected for head center displacement, and the Charnley Duo method. 5 custom-made adjustable phantoms with different prosthetic components were used.

RESULTS

In 20 measurements of all 5 phantoms at 3 levels of simulated wear (0.2 mm, 1.0 mm and 1.5 mm), the mean measurement error of the digital RSA examinations was 0.010 mm (accuracy 0.42). The corresponding error values for the three radiographic techniques were 0.19 (accuracy 1.3) for Charnley Duo, 0.13 (accuracy 1.3) for Imagika corrected, and 1.021 (accuracy 2.99) for Imagika. Measurement error decreased from 0.011 mm with ordinary RSA to 0.004 with RSA digital measurement. Head size, direction of wear in relation to the cup or type of prosthetic component did not influence the measurement error. The results of Charnley Duo and Imagika corrected were similar but the latter had an inexplicable systematic error in measuring one of the phantoms. Imagika had the worst results due to its inability to compensate for the out-of-head center effect. Alumina heads were difficult to analyze with all methods.

INTERPRETATION

By using the ISO standard for assessing accuracy, RSA can be expected to measure wear with an accuracy of about 0.4 mm irrespective of prosthetic component studied or direction of wear, whereas the best technique, in our study, based on standard radiographs can be accurate to about 1.3 mm.

Direct plain radiographic methods versus EBRA-Digital for measuring implant migration after total hip arthroplasty

We aimed to determine whether the precision and sensitivity of migration measurements after total hip arthroplasty (THA) using direct plain radiographic techniques could be made comparable to those of digital methods (EBRA-Digital; University of Innsbruck, Innsbruck, Austria) by careful control of radiographic technique and use of modern measuring tools. Precision was examined by analysis of consecutive radiographs taken after repositioning in 20 patients after hybrid THA. The precision (95% confidence interval) of measurements for cup migration using direct methods was +/-1.11 to 3.07 mm (x-axis) and +/-1.28 to 1.92 mm (y-axis). The precision of EBRA for cup measurements was +/-1.00 mm (x-axis) and +/-0.82 mm (y-axis). The precision of stem y-axis migration measurements was +/-1.12 to 6.91 mm using direct methods and +/-0.80 mm using EBRA. Migration of the stem (1.53 mm subsidence; P<.01) and the cup (0.53 mm cranial migration, P<.05) was detected using EBRA in 10 patients followed for 6 months after hybrid THA, but significant migration was not detectable using the most precise of the direct methods. Careful measures to standardize plain radiographs improve precision of direct radiographic measurements; however, their long-term sensitivity remains inferior to methods that employ quality control and measurement algorithms to measure migration from digitized radiographs.

Precision of EBRA-Digital software for monitoring implant migration after total hip arthroplasty

We assessed the precision of the EBRA-Digital software (EBRA, University of Innsbruck, Innsbruck, Austria) for measuring implant migration after total hip arthroplasty. Study subjects (n = 29) underwent consecutive, standardized, plain radiographic examinations of the hip on the same day after repositioning. The resulting radiograph pairs were digitized and analyzed using EBRA. The precision (95% confidence interval) of the method for measuring migration and wear was <+/-0.9 mm for both implant components. The 95% confidence intervals for measurement of cup inclination and anteversion and femoral stem/shaft angle were <+/-1.7 degrees. Measurement precision was not strongly related to patient gender, digitization method, or observer. The EBRA-Digital method has sufficient precision to detect clinically relevant migration to allow individual patient monitoring after total hip arthroplasty. The method requires careful patient positioning and radiographic technique to produce consistently images suitable for analysis.

Experimental assessment of precision and accuracy of radiostereometric analysis for the determination of polyethylene wear in a total hip replacement model

The purpose of this study was to develop and test a phantom model based on actual total hip replacement (THR) components to simulate the true penetration of the femoral head resulting from polyethylene wear. This model was used to study both the accuracy and the precision of radiostereometric analysis, RSA, in measuring wear. We also used this model to evaluate optimum tantalum bead configuration for this particular cup design when used in a clinical setting. A physical model of a total hip replacement (a phantom) was constructed which could simulate progressive, three-dimensional (3-D) penetration of the femoral head into the polyethylene component of a THR. Using a coordinate measuring machine (CMM) the positioning of the femoral head using the phantom was measured to be accurate to within 7 microm. The accuracy and precision of an RSA analysis system was determined from five repeat examinations of the phantom using various experimental set-ups of the phantom. The accuracy of the radiostereometric analysis, in this optimal experimental set-up studied was 33 microm for the medial direction, 22 microm for the superior direction, 86 microm for the posterior direction and 55 microm for the resultant 3-D vector length. The corresponding precision at the 95% confidence interval of the test results for repositioning the phantom five times, measured 8.4 microm for the medial direction, 5.5 microm for the superior direction, 16.0 microm for the posterior direction, and 13.5 microm for the resultant 3-D vector length. This in vitro model is proposed as a useful tool for developing a standard for the evaluation of radiostereometric and other radiographic methods used to measure in vivo wear.

Effect of radiographic quality on computer-assisted head penetration measurements

Even the most sophisticated computer-assisted radiographic techniques of measuring femoral head penetration into the polyethylene liner depend on the quality of the radiograph being evaluated, which varies greatly in clinical settings. The authors of this study sought to determine how the accuracy and reproducibility of three commercially available computer-assisted measurement systems differed when measuring optimal radiographs (with sharply defined component edges) and suboptimal radiographs (with less well defined edges). Using three computer-assisted measurement systems, the authors measured head penetration on simulated and clinical hip radiographs. All systems calculated head penetration as the movement of the head center relative to the cup center. To define the periphery of the prosthetic head and cup, one method (System One) used the human eye and a digitizing tablet, whereas the other two methods (System Two and System Three) used digital edge detection algorithms. For simulated hip radiographs, error was calculated as the absolute value of the difference between the known amount of head penetration, determined by a coordinate measuring machine, and the amount of penetration determined by the software. Three way analysis of variance showed a significant difference in absolute error among the three measurement techniques. System One had a significantly smaller absolute error (0.11 +/- 0.06 mm) than did System Two (0.25 +/- 0.25 mm) and System Three (0.19 +/- 0.13 mm). In addition, three-way analysis of variance showed that optimal radiographs were associated with a significantly lower absolute error (0.14 +/- 0.09 mm) than were suboptimal radiographs (0.23 +/- 0.22 mm). For optimal radiographs, there was no significant difference in error among the three measurement methods; all systems were accurate and reproducible. However, for suboptimal radiographs absolute error increased and varied widely, and a significant difference among the methods existed. These data show the susceptibility of head penetration measurements to radiographic technique and underscore the importance of good quality radiographs for all analyses of head penetration.

A single camera roentgen stereophotogrammetry method for static displacement analysis

A new method to quantify motion or deformation of bony structures has been developed, since quantification is often difficult due to overlaying tissue, and the currently used roentgen stereophotogrammetry method requires significant investment. In our method, a single stationary roentgen source is used, as opposed to the usual two, which, in combination with a fixed radiogram cassette holder, forms a camera with constant interior orientation. By rotating the experimental object, it is possible to achieve a sufficient angle between the various viewing directions, enabling photogrammetric calculations. The photogrammetric procedure was performed on digitised radiograms and involved template matching to increase accuracy. Co-ordinates of spherical markers in the head of a bird (Rhea americana), were calculated with an accuracy of 0.12mm. When these co-ordinates were used in a deformation analysis, relocations of about 0.5mm could be accurately determined.

Effect of low-friction ion-treated femoral heads on polyethylene wear rates

Polyethylene wear is a major contributor to osteolysis and subsequent aseptic loosening of prosthetic components in total hip arthroplasty. Use of ion implantation as a surface modification to the metallic bearing component of orthopaedic implants may be an effective means of reducing wear debris at the bearing interface. In July 1991, low friction ion treated femoral heads were introduced. This study evaluates the effect of the low friction ion treated femoral head on polyethylene wear. Fifty-five total hip arthroplasties (53 patients) with low friction ion treated femoral heads followed up a minimum of 3 years were matched with 55 total hip arthroplasties (47 patients) without low friction ion treated femoral heads for the same postoperative period. Socket wear was evaluated radiographically. Case matching and strict inclusion criteria were used to control for known factors influencing polyethylene wear. These criteria included: (1) cases matched for gender and age within 2 years; (2) diagnosis limited to osteoarthritis or avascular necrosis of the femoral head only; (3) femoral head diameter limited to 26 or 28 mm only; (4) hydroxyapatite coated femoral stem of the same design and a metal backed socket of the one of two designs with the same polyethylene insert; and (5) minimum followup of 3 years. The linear wear rate of polyethylene was 0.161 +/- 0.095 mm per year in the group without the low friction ion treated heads and 0.116 +/- 0.101 mm per year in the low friction ion treated group. The volumetric wear rates were 74.5 +/- 44.3 mm3 per year for the group without the low friction ion treated heads and 57.8 +/- 51.1 mm3 per year for the low friction ion treated group. Assuming the sensitivity of these measurements can detect these small differences in wear accurately, these results suggest low friction ion treated prosthetic heads are useful in reducing polyethylene wear at 3-year minimum followup.

Two-dimensional versus three-dimensional radiographic measurements of polyethylene wear

Because polyethylene wear adversely affects the outcome of total hip arthroplasty, reliable techniques for in vivo radiographic evaluation of femoral head penetration into the polyethylene liner are needed. With the recent development of software to measure three-dimensional femoral head penetration, the adequacy of two-dimensional head penetration measurements to estimate a three-dimensional process has been questioned. Thus, the purpose of the current study was to compare directly two-dimensional and three-dimensional radiographic measurements of femoral head penetration. Using two computer assisted radiographic techniques, the authors of this study measured the two-dimensional and the three-dimensional penetration of the femoral head into the polyethylene liner in 202 hips implanted with porous coated acetabular components. A comparison of two-dimensional and three-dimensional measurements showed that, on average, the mean amount of three-dimensional head penetration was larger than the mean amount of two-dimensional head penetration. However, linear regression analysis showed that the two measurements were highly correlated (r2 = 0.87, slope = 0.99, intercept = 0.08 mm). For most patients (95%), two-dimensional and three-dimensional measurements of femoral head penetration were nearly equal. However, there was a small subset of patients (5%) for whom three-dimensional radiographic penetration measurements were three times greater than corresponding two-dimensional radiographic penetration measurements. No common risk factor among this small subset of patients was identified. The current study provides practical information for the orthopaedic surgeon trying to assess polyethylene wear in hip replacement cases. It shows that for most patients head penetration can be measured sufficiently from anteroposterior radiographs alone. However, in a small percentage of patients (5% in the current study) the femoral head also moves perpendicular to the plane of the anteroposterior radiograph. For these cases, accurate analysis of head penetration requires three-dimensional analysis using anteroposterior and lateral radiographs.

The estimation of acetabular cup wear volume from two-dimensional measurements: a comprehensive analysis

This paper describes a mathematical investigation of the relationship between wear volume, wear depth and wear direction in acetabular components. The analysis takes into account the cylindrical and conical portions at the mouth of certain types of socket and also incorporates the effect of an initial radial discrepancy between the femoral head and socket. Published formulae for converting linear wear measurements to wear volumes are shown to be incorrect. Wear volume is shown to be highly dependent on the wear direction, increasing by more than 90 per cent over a 60 degrees range. Cylindrical and conical portions of a 22.225 mm cup may contribute up to about a third as much wear as the hemispherical socket. At low wear depths, the neglect of a radial discrepancy between the components can result in an overestimation of wear volume in excess of 100 per cent.

Discrepancy between penetration depths derived from radiographic and direct measurement of acetabular components

The most common technique for assessing penetration due to wear in acetabular components is with the aid of the most recent serial radiograph. This approach, which is often termed the uni-radiographic method, has been shown to underestimate the more reliable value of the penetration depth deduced from direct measurement of explanted sockets. In this article the causes of the discrepancies between the two data sets are explored. Ninety-six sockets were available from revision surgery for which both the penetration depth and angle could be measured using the shadowgraphic technique in both the coronal and wear planes. Further, the penetration depth for each of the sockets was also assessed from pre-revision X-rays. A significant discrepancy was observed between the penetration depths measured in the wear plane of the replica delta Pw and that measured from the radiograph, delta PX-ray. The discrepancy was greatest for loose sockets as opposed to those that were still fixed at revision surgery. Using the corresponding data from the shadowgraph measurements, it was possible to deduce that the errors have arisen from the radiographic measurement of wear in the coronal plane and the formula used in calculating delta PX-ray. If these errors (which cannot be calculated from the X-ray data alone) were taken into consideration, then the systematic bias between radiographic and shadowgraphic measurement was greatly reduced. The largest portion of the discrepancy was accounted for by wear occurring out of the plane of the radiograph, and this, in general, coincides with the coronal plane. Overall, these results indicate that the accurate measurement of wear from serial radiographs is not possible and that improved performance in terms of accuracy can only be achieved when a three-dimensional system is used. An alternative method for deducing the radiographic penetration depth is proposed which, theoretically, negates the error arising from the inaccuracy of the formula.

EBRA improves the accuracy of radiographic analysis of acetabular cup migration

EBRA (Ein Bild Röntgen Analyse) is a new computerized method measuring migration and wear of the acetabular cup, suggested to improve measurement accuracy. We evaluated possible errors of measurement and compared EBRA with standard methods. 1. We did repeated measurements on a single radiograph using the same reference lines. The reliability of the input procedure with standard measurements was significantly better than repeated digitization with EBRA. 2. In a more clinical test, a group of 10 patients was studied. 5 radiographs were taken of the same patient on the same day. EBRA improved the reliability of repeated radiographic examination significantly for migration measurements in the vertical direction. 3. To assess the inter- and intraobserver variations, repeated measurements were performed on the clinical series of pelvic radiographs of 10 patients. EBRA was significantly better than standard methods. With EBRA, errors of wear and migration measurements could be reduced, as compared to standard methods. The major improvement with EBRA was found for migration measurements in the vertical direction.