Jonas Thanner

Radiostereometry of Hip Prostheses: Review of Methodology and Clinical Results

Radiostereometric analysis is a science that enables reliable measurements to be made from radiographs. The method involves several steps including insertion of spheric tantalum markers, radiographic examinations, measurements of radiographic films, and calculations of three-dimensional movements. The precision of the method corresponding to the 99% significance interval varies between 0.15 and 0.6 mm and 0.3 degree and 2 degrees when applied to total hip replacement depending on the technique used. Measurements of implant micromovement during 1 to 2 years after surgery have proved to be of value to predict later clinical failure because of aseptic loosening and revision. Subsidence of the femoral stem or proximal migration of the acetabular cup between 1 and 2 mm has indicated increased risk of early or intermediate term revisions in those prosthetic designs studied so far. Minimum early migration has been recorded for clinically well documented nonpolished stems and polyethylene cups, which probably is one explanation for their long term success. This small amount of early micromotion also has been found in porous coated and screw fixated press fit cups and all hydroxyapatite coated designs hitherto studied. As a first step in a clinical evaluation of new implants or surgical techniques, the predictive value of radiostereometric analysis measurements can be used to reduce the number of patients exposed to the potential risk of clinical failure.

The Otto Aufranc Award. Highly cross-linked polyethylene in total hip arthroplasty: randomized evaluation of penetration rate in cemented and uncemented sockets using radiostereometric analysis

The annual wear rate in polyethylene articulations should be 0.1 mm or less to avoid future osteolysis. Highly cross-linked polyethylene shows an 80 to 90% wear reduction in hip simulator testing, but the clinical documentation of this new polyethylene is still inadequate. We evaluated the highly cross-linked polyethylene in two prospective randomized clinical studies. Thirty-two patients (12 men, 20 women; 64 hips) with a median age of 48 years (range, 29-70 years) with bilateral primary or secondary arthrosis of the hip had hybrid total hip arthroplasty with liners made of highly cross-linked polyethylene on one side and conventional polyethylene on the other. Twenty-seven patients in this study have passed 2 years followup. A further group, comprised of 60 patients (61 hips) with a median age of 55 years (range, 35-70 years), was randomized to receive either highly cross-linked polyethylene or conventional cemented all-polyethylene of the same design. Forty-nine of these 60 patients have been observed for 3 years. In both studies all patients received Spectron stems with 28-mm Co-Cr heads. Radiostereometric examinations with the patient supine or standing were done at regular intervals. Wear was measured in the supine position from the first postoperative week, whereas standing examinations were initiated 3 months after the operation. The penetration rate almost was identical in the study and control groups at 6 months after the operation. Thereafter, the penetration rate leveled out in the two groups with highly cross-linked polyethylene. At 2 years the highly cross-linked polyethylene liner showed 62% lower proximal penetration and 31% lower total (three-dimensional) penetration when the patients were examined in supine position. The highly crosslinked all-polyethylene cemented cups showed lower proximal penetration in both positions. The better wear performance of highly cross-linked polyethylene could increase the implant longevity. Longer followup is needed to evaluate if this new material is associated with less occurrence of osteolysis.

5-year experience of highly cross-linked polyethylene in cemented and uncemented sockets: two randomized studies using radiostereometric analysis.

Background Laboratory tests and early clinical studies have shown that highly cross-linked polyethylene (PE) markedly improves wear resistance compared to conventional PE. We evaluated this type of PE in two randomized clinical studies using radiostereometric analysis (RSA). The 2- and 3-year follow-up of these studies have already been reported. We found a lower penetration rate for the highly cross-linked PE than for conventional PE. We now report the outcome after 5 years. Patients and methods 60 patients (61 hips) with a median age of 55 years were randomized to receive either highly cross-linked PE (Durasul) or conventional cemented all-polyethylene sockets of the same design. 55 patients (56 hips) were followed for 5 years. In the second study, 32 patients (64 hips) with a median age of 48 years and with bilateral primary or secondary osteoarthritis of the hip had hybrid total hip arthroplasty with liners made of highly cross-linked PE (Longevity) on one side and conventional PE on the other. 23 patients in this study have passed the 5-year follow-up. Results Bedding-in and creep for the Durasul all-PE was reached by 2 years, and by 1 year for the Longevity liners. In both control groups with conventional PE, this was reached by 6 months. The steady-state wear rate was close to zero in the two study groups and 0.06 mm/ year for the conventional PE in the two control groups. Interpretation We found that use of highly crosslinked PE instead of conventional PE reduced the wear by more than 95%, which supports continued use of this type of PE in young, active patients.

Highly cross-linked polyethylene in cemented THA: randomized study of 61 hips.

Highly cross-linked polyethylene (PE) has been introduced as an alternative bearing material in total hip arthroplasty (THA) because of high wear resistance in laboratory tests but the clinical experience of this material is limited. We evaluated a highly cross-linked PE (warm irradiated adiabatic melting, absorbed dose, 95 kGy) in a randomized study of cemented THAs. Cups of the same design but made of conventionally gamma irradiated PE (absorbed dose, 25–40 kGy) constituted the control group. Sixty-one hips (30 women, 30 men) with a median age of 55 years (range, 35–70 years) were included. All patients received a Spectron stem with 28-mm CoCr head. Radiostereometric examinations with the patient supine or standing were done at regular intervals. Wear was measured with the patient in the supine position from the first postoperative week, whereas examinations done with the patient standing were initiated 3 months after the operation. Dual xray absorptiometry and conventional radiography were used to evaluate the bone mineral density and the radiolucencies around the acetabular component. Fifty-two patients (53 hips; 25 highly cross-linked, 28 control) have been followed up for 2 years. At the 2-year followup, the highly cross-linked cups showed 50% reduction of proximal wear compared with the control group, when the patients were studied standing. When evaluated supine, the difference in proximal wear did not reach significance. The migration of the socket, the relative changes of periprosthetic bone mineral density, and the progression of radiolucencies between the immediately postoperative followup and 2-year followup did not differ. Highly cross-linked PE showed increase resistance to wear. Different mechanical properties of the two types of PE studied did not alter the performance of the cup in terms of fixation, periprosthetic bone loss, and radiographic appearance. However, the followup is short and these results are preliminary.

Evaluation of Boneloc®: Chemical and mechanical properties, and a randomized clinical study of 30 total hip arthroplasties

We evaluated the mechanical, chemical and clinical properties of Boneloc cement using radiostereometry and a series of laboratory tests. Compared to a standard cement (Palacos) the new cement displayed reduced tensile strength, elastic modulus, curing and glass transition temperatures. The amount of MMA extracted during 3 weeks in methanol was smaller for the Boneloc, but the total amount of released monomers was larger. The adhesion to stainless steel and bone did not differ. Radiostereometric analysis during the first postoperative year in 30 patients randomized to fixation of hip prostheses using either of the 2 cements displayed increased proximal migration of the cup and increased stem subsidence when Boneloc had been used. Part of the stem subsidence occurred inside the cement mantle. On the basis of these findings, we conclude that the inferior fixation in the Boneloc group is mainly caused by its mechanical properties. Other mechanisms, such as increased release of monomers, may also be important.

Evaluation of a femoral stem with reduced stiffness. A randomized study with use of radiostereometry and bone densitometry.

Background: Femoral stems with reduced stiffness were introduced in total hip arthroplasty to facilitate proximal load transfer and thereby reduce periprosthetic bone loss. Poor fixation and unacceptably high revision rates turned out to be a major problem with these prostheses. The purpose of the present study was to evaluate the early performance of a low-modulus stem (one that is less stiff) with the potential to address the problem of fixation by the use of a surface coating. The coating consisted of a titanium porous mesh proximally covered with a mixture of hydroxyapatite and tricalcium phosphate.Methods: Thirty-nine men and twenty-six women (sixty-eight hips) with noninflammatory arthritis were studied. The patients were randomized to receive either a porous-coated composite stem with reduced stiffness (Epoch) or a stiff stem with a porous coating (Anatomic). Both stems were inserted without cement, had a ceramic coating on the proximal two-thirds, and were supplied with tantalum beads. At the time of the operation, tantalum markers were also placed into the proximal part of the femur. The patients were followed for up to two years after the operation and were evaluated with use of repeated radiostereometric measurements, measurements of bone-mineral density, conventional radiography, and Harris hip scores.Results: Both stems showed optimum fixation with median subsidence and stem rotations that were close to zero. Repeated measurements of bone-mineral density revealed early loss of bone mineral in all Gruen regions in both treatment groups. The loss of bone mineral around the Epoch stems was significantly reduced at two years in Gruen regions 1, 2, 6, and 7 (p < 0.0005 to 0.04). Measurements on postoperative radiographs showed no difference in stem positioning or fill, but the Epoch stems had significantly more endocortical contact on both the anteroposterior (p < 0.0005) and the lateral radiograph (p = 0.02).At two years postoperatively, the Epoch stems had fewer sclerotic lines surrounding the stem (p £ 0.002) and less sclerosis at the tip of the prosthesis (p = 0.001) compared with the Anatomic stems. The clinical results in terms of the Harris hip score, which was determined in all hips, and pain or discomfort, which was evaluated in thirty-seven hips treated at the same hospital, were not found to be different, with the numbers available.Conclusions: Contrary to previous studies of other designs with reduced stiffness, the Epoch stem achieved excellent primary fixation. Despite this rigid fixation, the proximal loss of bone-mineral density was less than that associated with the stem with a stiffer design. These results should encourage additional long-term studies with a larger patient population.

Poor outcome of the PCA and Harris-Galante hip prostheses : Randomized study of 171 arthroplasties with 9-year follow-up

155 patients (171 hips) with a mean age of 50 years (24-64) were randomized to uncemented PCA (84 hips) or Harris-Galante type I (87 hips) total hip arthroplasty. Clinical and radiographic evaluations were done regularly. The improvements in the Harris hip and pain scores did not differ. Osteolysis developed in 5 PCA and 17 Harris-Galante hips. 13 hips in the PCA and 16 in the Harris-Galante (HG) group were revised because of mechanical failures and 1 hip (HG) because of infection after a mean follow-up of 9 years. Decreased 10-year survival rate, based on revision as end-point, was noted for the PCA (85%), compared with the Harris-Galante cup (99%). The corresponding survival rate of the PCA stem (96%) was higher than that observed for the Harris-Galante design (86%). When radiographic failures were included, the survival rates of the 4 different components dropped to between 73% and 94%. These findings indicate that further revisions will be necessary and continuous radiographic follow-up is indicated to enable revision before severe bone destruction has occurred. Although the PCA and the Harris-Galante designs differed as regards the survival of the individual components, the overall clinical and radiographic survival rates of these cementless total hip arthroplasties were poor.

Increase in early polyethylene wear after sterilization with ethylene oxide: radiostereometric analyses of 201 total hips.

We evaluated polyethylene wear by measuring femoral head penetration in 201 THA (median age 62 (31-81) years, 117 women) extracted from 5 randomized studies aimed to assess various fixation principles. There were 30 cemented all-polyethylene Lubinus cups sterilized by gamma irradiation in a reduced oxygen environment, 65 porous-coated Trilogy cups with liners gamma-sterilized in inert gas. Moreover, 37 cemented cups were sterilized with ethylene oxide (Reflection all-poly) and 69 porous-coated cups had liners sterilized in ethylene oxide (Reflection). 28 mm femoral heads were used in all cups. The patients were followed with repeated radiostereometric measurements (RSA) up to 2 years. The activity level of the patients was evaluated by a questionnaire. After 2 years, cups with polyethylene sterilized in EtO had almost twice the proximal and 3D penetration rates, as compared with gamma-sterilized polyethylene. The penetration did not differ between the gamma-irradiated designs. Using stepwise linear regression analysis, we found that the type of sterilization, age and weight were the most important predictors and that they determined the direction of the proximal penetration rate. Activity score, male gender and proximal migration of the cup had little effect. The accelerated wear observed with the EtO-sterilized polyethylene causes concerns about long-term problems and especially in younger patients.

POROUS CUPS WITH AND WITHOUT HYDROXYLAPATITE-TRICALCIUM PHOSPHATE COATING : 23 MATCHED PAIRS EVALUATED WITH RADIOSTEREOMETRY

Migration, wear, and presence of radiolucencies were studied in 23 matched pairs of patients operated with porous-coated acetabular cups with additional screw fixation. All implants had the same type of titanium fiber mesh. In each pair, one of the cups was plasma-sprayed with a coating consisting of 70% hydroxylapatite (HA) and 30% tricalcium phosphate (TCP). Radiostereometric analysis up to 2 years after the operation revealed smaller rotations around the horizontal axis in cups with HA/TCP coating. The migration of the cup center was not significantly influenced. Evaluation of femoral head penetration in 12 of the matched pairs did not reveal any significant difference. Immediately after operation, implants with HA/TCP coating had more central radiolucencies, which, despite minimal migration, disappeared during the follow-up. The clinical results did not differ between the 2 groups. The findings of less tilting and diminishing radiolucencies in the cups with HA/TCP coating suggest a more complete ingrowth of bone and a better sealing of the interface.

Different loss of BMD using uncemented press-fit and whole polyethylene cups fixed with cement: Repeated DXA studies in 96 hips randomized to 3 types of fixation

Background In cemented THA, aseptic loosening of the cup is more common than loosening of the stem, while periprosthetic osteolysis of the socket resulting in difficult reconstruction problems has emerged as the most significant problem with cementless cup fixation. Patients and methods 90 patients (96 hips) scheduled for THA were stratified in three groups according to the method of fixation of the acetabular component: acrylic bone cement with fluoride (Cemex-F), porous-coated press-fit cup with ceramic coating (Trilogy, uncemented) and acrylic cement with gentamicin (Palacos). All patients received the Spectron EF stem. Acetabular bone mineral density was measured with dual-energy X-ray absorptiometry (DXA) 1 week postoperatively, and after 12 and 24 months. The periprosthetic BMD was evaluated in 5 ROIs positioned around the acetabular component. Results In the uncemented sockets, the BMD had decreased proximally and medially to the cup after 2 years. The difference was significant in the proximal region as compared to the control group (Palacos). No difference was noted between the 2 groups with cemented components after 2 years. Stepwise linear regression analysis showed that loss of periprosthetic BMD in the proximal high-pressure region after 2 years increased with higher postoperative BMD and when the uncemented design had been used. Interpretation Contrary to previous studies of cemented stems, the use of fluoride cement did not influence the periprosthetic BMD 2 years after the examination. Increased loss of BMD with use of uncemented press-fit cups in the region in which osteolytic lesions are commonly found suggests that stress shielding may initiate the development of this complication. Longer follow-up will, however, be necessary to substantiate this hypothesis.