Measurement of polyethylene thickness in total hip arthroplasty. A technique analysis

Retrieval Analysis of Large-Head Modular Metal-on-Metal Hip Replacements of a Single Design

BACKGROUND

There are limited publications examining modular metal-on-metal (MoM) total hip implants in which a comprehensive analysis of retrieved components is performed. This study examines 24 retrieved modular MoM implants from a single manufacturer and compares retrieval analytics; bearing surface damage, wear, and modular taper corrosion against patient, surgical and implant characteristics to elucidate significant associations.

METHODS

Clinical, patient, and surgical data were collected including age, body mass index, blood metal ion levels, and cup inclination. Damage assessment was performed visually in addition to surface profilometry. Acetabular liners and femoral heads were measured for volumetric wear. Femoral head taper bores were similarly measured for material removal due to corrosion and fretting.

RESULTS

Patients with MoM-related reasons for revision showed significantly higher levels of blood metal ion levels. Bearing wear was strongly associated with blood metal ion levels and was significantly increased in cups placed more vertically. Younger patients tended to have higher body mass indices as well as poorer cup placement.

CONCLUSION

This work details a broad range of analyses on a series of modular MoM total hip implants from a single manufacturer of which there are few published studies. Acetabular cup inclination angle was deemed a primary cause of revision surgery through increased MoM wear, high metal ion levels in the blood, and subsequent adverse local tissue reactions. Heavy patients can increase the surgical difficulty which was shown to be related to poor cup placement in this cohort.

Cementless total hip arthroplasty using the Spongiosa-I fully coated cancellous metal surface: a minimum twenty-year follow-up

BACKGROUND

We present the results of cementless total hip arthroplasty performed with use of an anatomically adapted femoral stem and hemispherical cup with a fully coated Spongiosa-I metal surface, which was designed to achieve a surface similar to human cancellous bone. The purpose of the present retrospective case series was to determine the long-term outcomes of this hip arthroplasty system after a minimum of twenty years of follow-up.

METHODS

Between 1983 and 1985, 209 consecutive total hip arthroplasty procedures (199 patients) were performed with use of the first-generation Spongiosa metal-surface chromium-cobalt total hip implant with an articulating surface consisting of a ceramic head and an ultra-high-molecular-weight polyethylene liner. We report the clinical and radiographic outcomes, the rates of and reasons for revision, and the influence of sex and age on outcome and complications.

RESULTS

At the time of the latest follow-up, twenty-seven patients had died and thirteen patients had been lost to follow-up; none of these forty patients had had revision surgery. The outcomes for 159 patients (169 prostheses) were reviewed. The mean duration of follow-up was 262 months (range, 242 to 275 months). There were nineteen revisions, including fourteen revisions of the femoral stem, two revisions of the acetabular cup, and three revisions of both components. The mean Harris hip score for patients who did not undergo revision surgery was 82 points. The probability of survival of both components at twenty years, with revision for any reason as the end point, was 97%. The probability of survival of the acetabular component was 98%, and the probability of survival of the femoral component only was 86%. The probability of component survival was significantly increased among older patients.

CONCLUSIONS

The results of cementless hip arthroplasty with use of the first-generation Spongiosa implant were excellent at a minimum of twenty years of follow-up. The probability of survival of the acetabular component exceeded that of the femoral stem.

Validation of two and three-dimensional radiographic techniques for measuring polyethylene wear after total hip arthroplasty

BACKGROUND

Three-dimensional radiographic techniques have been developed to estimate in vivo polyethylene wear of total hip replacements. We are not aware of any published study examining the accuracy of these in vivo methods. Our objective was to validate two radiographic techniques by comparing their results with those obtained directly from retrieved specimens.

METHODS

A coordinate measuring machine was used to probe the interior bearing surface of seventeen acetabular liners that had been retrieved at revision surgery. Solid models were created to determine volumetric polyethylene loss and linear depth and direction of wear. Two in vivo radiographic techniques (PolyWare and the Martell Hip Analysis Suite) were used to calculate two-dimensional linear, three-dimensional linear, and volumetric wear. The radiographic analysis was done independently, and the results were compared with the known values of polyethylene wear derived with use of the coordinate measuring machine.

RESULTS

Correlation coefficients comparing the values for two-dimensional and three-dimensional linear wear and volumetric wear derived with the PolyWare radiographic technique with the values derived with the coordinate measuring machine were r(2) = 0.78, r(2) = 0.75, and r(2) = 0.91, respectively (all p < 0.001), and the correlation coefficients comparing the values derived with the Martell Hip Analysis Suite radiographic technique with those derived with the coordinate measuring machine were r(2) = 0.80, r(2) = 0.84, and r(2) = 0.91, respectively (p < 0.001). The average absolute difference between the radiographic estimates and the results derived with the coordinate measuring machine was approximately 19% (range, 13% to 24%).

CONCLUSIONS

There was good agreement between the wear estimates made with both in vivo techniques and the measurements of the retrieved polyethylene liners made with the coordinate measuring machine. Two-dimensional wear analysis (based on anteroposterior radiographs) accounted for most of the polyethylene wear, while one technique of three-dimensional wear analysis (PolyWare) demonstrated some additional wear in the lateral plane.

The influence of shape and sliding distance of femoral head movement loci on the wear of acetabular cups in total hip arthroplasty

Wear of the polyethylene acetabular component is the most serious threat to the long-term success of total hip replacements (THRs). Greatly reduced wear rates have been reported for unidirectional, compared to multidirectional, articulation in vitro. This study considers the multidirectional motions experienced at the hip joint as described by movement loci of points on the femoral head for individual THR patients. A three-dimensional computer program determined the movement loci of selected points on the femoral head for THR patients and normal subjects using kinematic data obtained from gait analysis. The sizes and shapes of these loci were quantified by their sliding distances and aspect ratios with substantial differences exhibited between individual THR patients. The average sliding distances ranged from 10.0 to 18.1 mm and the average aspect ratios of the loci ranged from 2.5 to 9.2 for the THR patients. Positive correlations were found between wear rate and average sliding distance, the inverse of the average aspect ratio of the loci and the product of the average sliding distance and the inverse of the average aspect ratio of the loci. Patients with a normal hip joint range of motion produce multidirectional motion loci and tend to experience more wear than patients with more unidirectional motion loci. Differing patterns of multidirectional motion at the hip joint for individual THR patients may explain widely differing wear rates in vivo.

Multidirectional deformation in fully congruent acetabular components

Most clinical studies have used femoral head migration as an index of acetabular wear, but a previous study showed multiple wear vectors in 30% of retrieved acetabular components with noncongruent liners. The origin of multiple wear vectors is unclear, and it has been suggested that polyethylene creep in a noncongruent shell might influence deformation on the articular surface. We used shadowgraph and volumetric methods to evaluate the extent and direction of surface deformation of 37 retrieved polyethylene liners that were fully congruent to a single design of metal backing. The results show that multiple deformation vectors are relatively common in retrieved acetabular cups (27% in this study) and are independent of congruency between liner and metal backing, rim impingement, and backside creep. Polyethylene liners with multiple wear vectors were significantly thinner than those of cups with a single vector. The origin of multiple vectors is still unclear, but clinical and laboratory studies measuring linear wear alone without recognizing multiple vectors underestimate total in vivo volumetric wear.

Assessment of polyethylene wear in total hip replacement

The three-dimensional technique is a method for the measurement of polyethylene wear in patients with total hip joint replacement. Application of image processing technology allows automation of point selection from digital images of radiographs scanned into the computer. Validation of image processing modifications reveals a three-fold increase in accuracy and a 40-fold increase in reproducibility compared with manual input of points from a digitizer during bench testing. A review of three-dimensional technique application to clinical patients gives information on the factors that influence polyethylene wear. Increasing age, activity level, femoral head size, decreasing polyethylene thickness, and insertion of total hip prostheses without cement all increase polyethylene wear. Restoration of femoral offset during total hip replacement seems to decreases polyethylene wear. No apparent difference in polyethylene wear rate could be found between two groups of patients, one group had a stainless steel-polyethylene articulation and the other had a ceramic-polyethylene articulation. Measurement of the serial polyethylene wear of individual patients reveals a high rate of femoral head penetration during the first 2 years after total hip replacement using metal-backed acetabular components inserted without cement. Interpretation of this femoral head penetration as true polyethylene wear may be erroneous, however, because creep of the polyethylene and acetabular liner movement within its metal shell cannot be measured.

Socket wear in ceramic-on-polyethylene total hip arthroplasties: fixed versus rotating heads

We measured radiographically the polyethylene socket wear in 34 hip prostheses with a 32 mm Al2O3-ceramic head with a rotating bearing and in 37 prostheses with a "fixed" ceramic head. The mean follow-up was 12 years in both groups. The mean annual linear wear of the polyethylene was 0.034 and 0.069 mm, respectively, (Mann-Whitney U-test p < 0.0001) in the "rotation" and the "fixed" group. A rotating bearing between the head and neck in a modular total hip system seems to reduce socket wear as compared to fixed taper junctions.

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.

Dimensional characteristics of uncomplicated autopsy-retrieved acetabular polyethylene liners by ultrasound

A new, in vitro ultrasound-based method to measure the thickness of acetabular polyethylene components was developed and applied to 26 uncomplicated autopsy retrieved components and 40 unused components. The average age at total hip arthroplasty was 62 years and the average time in service of the retrieved components was 49 months. The clinical notes indicated that each of the arthroplasties was functioning well at the time of the patients death. Thickness distributions in the retrieved components had two distinct patterns. Eighteen of the retrieved components (69%) had their thinnest areas self-contained and located near the polar point. In the other 8 retrieved components (31%) the areas of minimum thickness were adjacent to the rim. Thickness distribution in the unused cups was predominately concentric with the thinnest area located near the polar point (85% of the cups). Detection limits for dimensional change were established based on the variability found in the unused liners. Fifteen of the 26 retrieved components (58%) had areas of reduced thickness which exceeded the detection limits; the average thickness reduction rate for these components was 0.076 mm per year. The other 11 retrieved components (42%) lacked such areas. The 15 cups with areas of reduced thickness had a longer time in service (63 +/- 18 months) than the 11 cups without areas of reduced thickness (32 +/- 25 months) (p = 0.003), but no other clinical factor (age, gender, Harris hip score, size and inclination of the cup, type of femoral fixation) was associated with these 15 cups. Cylindrical models to estimate volumetric change tended to underestimate the actual changes, suggesting that the actual particulate burden may be greater than previously appreciated. Finding that the pattern of thickness reduction can vary suggests that distinctive hip loading modes may be present postoperatively in patients with total hip arthroplasty. The wear rates of these components are consistent with wear rates calculated from radiographic data for well-functioning implants and are considerably lower than wear rates calculated for surgically-retrieved implants, indicating that autopsy-derived retrievals may be more representative of the majority of components currently in service than surgically-derived retrievals.

Catastrophic failure of the acetabular component in a ceramic-polyethylene bearing total hip arthroplasty

Recent research in total hip arthroplasty has focused on attempts to decrease wear at the femoral head-acetabulum articulation, to limit the production of debris that is believed to lead to osteolysis and prosthetic loosening. The use of ceramic-on-polyethylene bearing surfaces has been reported to produce lower wear rates and therefore may increase the life expectancy of the joint arthroplasty. Problems with this bearing have been reported to be due to ceramic femoral head fracture. Reported here are 2 cases of catastrophic failure of total hip arthroplasties, involving a ceramic femoral head, caused by failure of the polyethylene acetabular liner, with subsequent penetration of the femoral head through the acetabular shell.

Polyethylene wear in Scanhip arthroplasty with a 22 or 32 mm head: 62 matched patients followed for 7-9 years

We measured radiographic polyethylene wear in patients with Scanhip arthroplasty and no clinical or radiographic signs of loosening. The patients were divided into 2 groups according to head sizes. 32 patients (33 hips) had an implant with a 22 mm and 30 patients (34 hips) with a 32 mm head. They were followed for 7-9 years. The groups were matched for diagnosis, sex, weight, age, and time of follow-up. The mean linear wear with a 22 mm head was 1.1 mm and with a 32 mm head 1.5 mm (p 0.004), which corresponds to a yearly wear rate of 0.15 mm and 0.18 mm, respectively. The mean difference in volumetric wear was greater, 420 mm3, as compared to 1239 mm3.