Functional adaptation and ingrowth of bone vary as a function of hip implant stiffness

The purpose of the present study was to test the hypothesis that cortical bone loss, trabecular bone density and the amount of bone ingrowth vary as a function of stem stiffness in a canine cementless hip replacement model. The study was motivated by the problem of cortical bone atrophy in the proximal femur following cementless total hip replacement. Two stem stiffnesses were used and both designs were identical in external geometry and porous coating placement. The high stiffness stem caused approximately 26% cortical bone stress-shielding and the low stiffness stem caused approximately 7.5% stress-shielding, as assessed by beam theory. Each group included nine adult, male canines who received unilateral arthroplasties for a period of six months. The animals with the low stiffness stems tended to lose less proximal cortical bone than the animals with high stiffness stems (4% +/- 9 as opposed to 11% +/- 14), but the difference was not statistically significant (p = 0.251). However, the patterns of bone ingrowth into the implant and change in medullary bone density adjacent to the implant were fundamentally different as a function of stem stiffness (p < 0.01). Most importantly, while the high stiffness group had peaks in these variables at the distal end of the stem, the low stiffness group had peak values proximally. These different patterns of functional adaptation are consistent with the idea that reduced stem stiffness enhances proximal load transfer.     

Maintenance of proximal cortical bone with use of a less stiff femoral component in hemiarthroplasty of the hip without cement. An investigation in a canine model at six months and two years

A canine model of hemiarthroplasty of the hip was used to determine if the use of a less stiff femoral stem can reduce the amount of bone loss induced by stress-shielding. Two types of stem were used: the stiffer stems were made of a titanium alloy, and the less stiff stems were composed of a cobalt-chromium-alloy core with an outer polymer layer. The stems were identical in shape, and both types were circumferentially coated along their entire length (except for the distal five millimeters) with commercially pure titanium fiber metal. Ten dogs with each type of stem were followed for six months, and twelve dogs with each type of stem were followed for two years. Loss of cortical bone from the proximal part of the femur was associated with both types of stem, but typically 50 per cent less bone was lost with the less stiff implants. Most of the cortical loss occurred at the subperiosteal surface. The amount of medullary bone adjacent to the proximal and distal aspects of both types of stem increased; the less stiff stems were associated with a greater increase in the proximal region, and the stiffer stems were associated with a greater increase in the distal region. Similarly, there were peaks in the amount of bone growth into the proximal and distal portions of both types of stem, with a greater peak in proximal bone growth into the less stiff stems and a greater peak in distal bone growth into the stiffer stems.     

Stability of proximal femoral grafts in canine hip arthroplasty

In a canine model, the fixation stability of a prosthesis and proximal bone graft composite were measured relative to the distal femur. One group had the prosthesis graft composite cemented into the distal femur. The second group had the prosthesis graft composite press fit into the distal femur for biologic ingrowth. Displacements of the proximal femoral grafts relative to the host bone in each group were measured after ex vivo (acute with graft) implantation and 4 months after implantation. A third group with no osteotomy (acute intact) simulated perfect graft to host bone union. Relative displacements representing 6 degrees freedom (translation and rotation) were calculated from the displacement values measured by 9 eddy current transducers. Measurements of displacement were used to test the hypothesis that distal press fit fixation equals distal cement fixation at 4 months after implantation. In all cases the measured translations and rotations of the graft to implant construct were small and of a magnitude that should encourage bone ingrowth (< 0.05 mm and < 0.1 degree, respectively). The stability of the press fit group at 4 months was not significantly different from the cemented group in axial and transverse displacement during axial and transverse loading, respectively. There was no difference in stabilities at 4 months between distal press fit and cemented fixation in hip replacements requiring a proximal femoral graft.     

Subsidence of an uncemented canine femoral stem

Factors contributing to subsidence were analyzed by radiographic evaluation and mechanical testing of 36 canine cadaver femora during and after insertion of an uncemented porous-coated femoral stem and by radiographic evaluation of 35 canine total hip arthroplasties. Mean percentage of canal fill in immediate postoperative radiographs, and percentage of canal fill at midimplant and distal implant locations, were accurate predictors of subsidence. Force required to implant the femoral stem was strongly correlated with force required for implant subsidence. Femoral morphology and percentage of canal fill at the middle and distal sites were accurate predictors of subsidence. Implants in femora with a stovepipe morphology (canal flare index less than or equal to 1.8) were six times more likely to subside than implants in femora that had a normal appearance (canal flare index 1.8 to 2.5), and 72 times more likely to subside than implants in champagne-fluted femora (canal flare index greater than or equal to 2.5). Femora with more than 85% mean, middle, or distal canal fill were less likely to subside.     

The effect of hip stem material modulus on surface strain in human femora

Human femora were used to compare the changes in bone surface strain resulting from decreasing the material modulus of a collarless hip stem to determine whether a highly elastic stem increased bone loading. Three substrate materials were tested: titanium (modulus of elasticity 110 GPa), carbon fiber composite (modulus of elasticity 52 GPa), and polymethylmethacrylate (PMMA, modulus of elasticity of 1.9 GPa). Two separate analyses were performed in which femora were implanted randomly with one of the three stem types. Results showed that assembly strains did not differ significantly among different materials. There was a large strain reduction in the proximal region of the femora for all stem substrates relative to the intact femur. Although there was statistically greater surface shear strain as the material modulus decreased, the PMMA stem did not substantially increase bone loading.     

Cementless implant composition and femoral stress. A finite element analysis

Proximal atrophy and thigh pain are recognized problems with some cementless femoral stems in total hip arthroplasty. It is thought that reduced femoral stress from alterations in load transfer caused by an intramedullary stem contributes to proximal femoral atrophy. An increase in flexural rigidity and bone stress near the stem tip is thought to contribute to thigh pain. A three-dimensional finite element analysis study was performed to calculate stresses in the proximal femur and bone near the stem tip before and after implantation of a collared, proximally coated, cementless femoral prosthesis. The influence of prosthetic material was examined by changing implant composition from cobalt chrome to titanium alloy and leaving all other parameters constant. Femoral stress was increased twofold immediately below the collar with the titanium implant compared with the cobalt chrome. However, the proximal femoral stress in the titanium implanted model was still 1/10 that in the corresponding region of the unimplanted femur model. At the stem tip, as much as a 30% reduction in femoral stress was seen with the titanium stem compared with the cobalt chrome. These findings suggest biomechanical evidence of an advantage for titanium as an implant material compared with cobalt chrome for cementless femoral stems.     

Cement-mantle thickness affects cement strains in total hip replacement

Bone cement is commonly used to affix femoral implants to the bone during total hip reconstruction. Previous studies suggest that the expected life of a cemented femoral implant may depend on the thickness of the cement mantle surrounding the implant and the implant geometry. The purpose of this study was to determine whether different cement-mantle thicknesses and femoral stem sizes affected strain patterns in the bone cement around cemented femoral stems. Two different sizes of cobalt-chromium stems were cemented into composite femora with varying cement-mantle thickness. Strain gages were embedded in the cement mantle and the implanted stems were loaded axially and under conditions simulating walking and standing. An increase in stem size with the same cement-mantle thickness (approximately 2.2 mm) caused a 65% decrease in proximal medial cement strains. Increasing cement mantle thickness from 2.4 to 3.7 mm caused substantial strain reductions in the distal cement (40-49%). We conclude that increased cement-mantle thickness around femoral stems may increase the fatigue life of a bone-implant system by reducing peak strains within the cement.     

The Otto Aufranc Award. Skeletal response to well fixed femoral components inserted with and without cement

Previous studies evaluating femoral remodeling after total hip arthroplasty have used clinical radiographs and dual energy xray absorptiometry. Limitation of these techniques make it impossible to quantify the magnitude of bone loss in terms of cortical thinning and cortical bone area and bone mineral density changes. Femoral cortical bone remodeling after cemented and cementless replacement was quantified and possible determinants of bone remodeling in terms of clinical and radiographic variables were evaluated. Forty-eight anatomic specimen femora from 24 patients with unilateral cemented and cementless hip replacements were analyzed. Cortical thickness, cortical bone area, and bone mineral density was assessed in 4 quadrants at 5 discrete levels. The maximum cortical bone loss by level was at the middle section for the cemented femurs and at the midproximal and middle sections for the cementless femurs. However, if one examines individual quadrants, the proximal medial cortex still represents the specific region of maximal bone loss for both types of implant fixation. The posterior cortex had substantially more bone loss, even in the diaphyseal levels, than had been previously appreciated. A strong correlation was noted between the bone mineral density of the control femur and the percentage decrease of bone mineral density in the remodeled femur. Based on this data, it seems that the less dense the bone is before hip replacement surgery, the greater the extent of bone loss after total hip arthroplasty regardless of the fixation type.     

The reaction to nailing or cementing of the femur in rats. A microangiographic and fluorescence study

Bone reaction to cement and to a cementless stem was studied in the rat femur with histological fluorescence and microangiographic techniques. Periosteal and endosteal apposition, and consequent remodelling, appeared as a reaction to reaming rather than caused by cement or a cementless stem. Every change in bone began with proliferation, progression and orientation of the vessels. Endosteal apposition was absent in cemented femurs because the entire medulla was occupied by the acrylic cement, but remodelling of the subendosteal cortex followed medullary revascularisation which was far advanced after 90 days. In cementless stems, endosteal apposition of primary woven bone and remodelling was the basis for bony ingrowth and anchorage through bony bridges. Our results suggest that the pattern of blood supply is relevant to the structural organisation of mature lamellar bone around the implant. Cemented stems have maximum anchorage and stability as soon as they are inserted, but this decreases with time as revascularisation occurs. Cementless stems can reach maximum integration later after insertion, and revascularisation is less critical because they usually do not fill the canal completely.     

From the RSNA refresher courses. Total hip arthroplasty: radiographic evaluation

Expected appearances of total hip arthroplasty vary according to type of implant, its method of fixation (cemented, porous coating for bone ingrowth, press fit), and whether it is a revision. Cemented arthroplastic components normally may show 1-2-mm-wide radiolucent zones at cement interfaces. Definite loosening is diagnosed when progressive widening of the radiolucent zone, migration of a cemented component, or change in alignment is seen. In cementless arthroplasty, normal findings include calcar resorption, radiolucent zones up to 2 mm in width, cortical thickening, periosteal reaction, endosteal sclerosis, and even subsidence of the femoral component that stabilizes at less than 1 cm. The most reliable radiographic signs of loosening in cementless arthroplasty are progressive subsidence, migration, or tilt of the component. Because subsidence or change in alignment may be very subtle, serial radiography and measurement are often required for diagnosis. Other signs that indicate loosening include bead shedding (in porous-coated prostheses), extensive cortical hypertrophy, endosteal bone bridging at the stem tip, endosteal scalloping, and a radiolucent zone wider than 2 mm. In revision arthroplasty, wide radiolucent zones and subsidence are common. The diagnosis of revision failure is based on progressive widening of the radiolucent zones and change in component position after 12 months.     

A histomorphometric and histologic analysis of the implant interface in five successful, autopsy-retrieved, noncemented porous-coated knee arthroplasties

Five clinically successful, primary uncemented porous-coated anatomic knee implants were retrieved postmortem, 13-56 months after implantation, and were sectioned and evaluated histologically and histomorphometrically for bone ingrowth. The prosthesis-bone interface was divided into the following four zones: (1) the tissue prosthetic surface interface; (2) the beaded area; (3) the immediate beadless area; and (4) the marrow space. Although fibroosseous ingrowth was present in all cases, it varied quantitatively with each case and component. Average component bone ingrowth for the prosthesis interface (Zones 1 and 2) of patellae was 29%; tibias, 6%; and femora, 8%. In Zone 3, the percentage of bone apposed to the prosthesis for the patellae was 53%; tibias 36%; and femora, 32%. Zone 4, the marrow space, was not quantitated. The fibrous tissue filling nonbone-ingrown porous space in Zone 2 appeared "ligamentoid," connecting bone to beads within Zone 2 and between Zones 2 and 3. Zone 3 exhibited a bony plate formation parallel to the prostheses. No significant inflammation was noted. Overall there was more bone ingrowth into Zone 3 than Zones 1 and 2 with greater bone ingrowth found in the patellar components. The implant interface in clinically successful noncemented porous-coated prostheses of this design is characterized histologically by a noninflammatory fibroosseous ingrowth of varying degrees, and the fibrous component of this composite structure exhibits a highly organized pattern.     

Hydroxyapatite-coated total hip femoral components in patients less than fifty years old. Clinical and radiographic results after five to eight years of follow-up

One hundred and thirty-three patients (152 hips) who were an average of thirty-nine years old (range, sixteen to forty-nine years old) received a proximally hydroxyapatite-coated femoral prosthesis as part of a total hip arthroplasty and were followed for a minimum of five years (average, 6.4 years; range, five to 8.3 years) or until revision. The average Harris hip score was 47 points (range, 22 to 77 points) preoperatively and 93 points (range, 49 to 100 points) at the time of the latest clinical evaluation. Two patients who had a well fixed femoral implant had activity-limiting pain in the thigh at the time of the most recent examination. Radiographic changes consistent with bone-remodeling (cortical hypertrophy and bone condensation) typically were seen around the mid-part of the shaft of the prosthesis. Forty-eight (32 per cent) of the 148 hips that were included in the radiographic analysis demonstrated a small amount of erosive scalloping in either zone 1 or zone 7 of Gruen et al., and intramedullary osteolysis was suspected in only one hip. All stems were radiographically osseointegrated according to a modification of the criteria described by Engh et al. Four stems were revised, but none of the revisions were performed because of mechanical failure (two stems were revised in conjunction with a revision of the cup because of pain; one, because of an infection; and one, after a traumatic femoral fracture that occurred six years postoperatively). Thus, the rates of aseptic and mechanical failure were both 0 per cent. The combined rate of failure, which included the two stems that were revised because of pain and the two stems that were associated with pain that limited activity, was 2.6 per cent (four of 152 stems). The over-all clinical results associated with hydroxyapatite-coated femoral components were excellent in this group of young patients after intermediate-term follow-up. A review of serial radiographs showed mechanically stable implants with osseous ingrowth, evidence of stress transmission at the middle part of the stem, and minimum endosteal osteolysis.     

Aseptic loosening of the femoral implant after cemented total hip arthroplasty in dogs: 11 cases in 10 dogs (1991-1995)

Aseptic loosening of the femoral implant (ALFI) was diagnosed in 10 dogs that had undergone cemented total hip arthroplasty (THA). One dog had bilateral ALFI. Loosening developed at the stem-cement interface a mean of 30 months after THA. The most common clinical sign was intermittent subtle or non-weight-bearing lameness. On radiographs obtained after THA, the distal stem tip was in contact with the cortical endosteum in all dogs. Radiographic changes at the time of diagnosis of ALFI included asymmetric periosteal reaction along the femoral diaphysis (n = 11), radiolucent zone at the stem-cement interface (6), altered implant position (4), and femur fracture (1). Surgical revision resulted in a good or excellent outcome in 9 dogs. In 1 dog, the implant became infected. In another, aseptic loosening recurred. Aseptic loosening of the femoral implant was significantly more common when initial positioning of the implant resulted in contact between the distal tip of the implant and cortical endosteum than when there was no contact.     

Fibrous dysplasia of the proximal part of the femur. Long-term results of curettage and bone-grafting and mechanical realignment

We reviewed the long-term outcomes of treatment of fibrous dysplasia of the proximal part of the femur in twenty-two patients (twenty-seven femora). There were fifteen male patients and seven female patients. Patients who had monostotic disease had no involvement of the calcar femorale, fewer microfractures, less deformity, and stronger bone that could support internal fixation. Patients who had polyostotic disease had frequent involvement of the calcar femorale; more microfractures; severe deformity, including shepherd's crook deformity; and, in many instances, bone that could not support internal fixation. Twenty-two of the twenty-seven femora had a microfracture at the time of the initial presentation. At least one osteotomy was performed in four femora that had monostotic disease and in nine femora that had polyostotic disease. Curettage and cancellous or cortical bone-grafting did not appear to have any advantage compared with osteotomy alone in the treatment of symptomatic lesions, as all grafts resorbed with persistence of the lesion. At the time of the latest follow-up evaluation, no lesion had been eradicated or had decreased in size. A satisfactory clinical result was achieved in twenty patients (twenty-four femora): nine who had monostotic disease and eleven who had polyostotic disease. Two patients who had polyostotic disease and an endocrinopathy (one of whom had bilateral involvement) had an unsatisfactory result. All three femora in these two patients had a neck-shaft angle of less than 90 degrees at the time of the most recent follow-up evaluation. Varus deformity of the proximal part of the femur is best treated with valgus osteotomy and internal fixation early in the course of the disease. If the calcar of the femoral neck is involved or if the quality of the bone is such that internal fixation is not possible, a medial displacement valgus osteotomy can provide a more mechanically favorable position for healing of the microfracture.     

Histological and biomechanical analysis of bone and interface reactions around hydroxyapatite-coated intramedullary implants of different stiffness: a pilot study on the goat

We hypothesized that reduced stem stiffness of orthopaedic implants contributes to a high risk of loosening, since interface stresses and relative motions may exceed a tolerable range. To study this hypothesis, three types of load-bearing implant with different stiffnesses were inserted into the tibia of the goat. Histological analysis was performed of bone repair after insertion of the implant, bone ingrowth, interface disruption and loosening. A finite element model of the configuration provided the quantitative range of interface stresses and relative motions for the present experiment. The implants were made out of stainless steel, hollow titanium and a thin titanium core covered with a polyacetal coating. The stiffness ratios of these implants were approximately 10:4:1, respectively. All implants were coated with a layer of hydroxyapatite (HA) in order to minimize the possible biological effects of the different implant materials. Irrespective of the type of implant, there was a repair phase that lasted 6-12 weeks. The stiff implants functioned well. Large areas of bone bonding to the HA layer were found after the repair phase at 12 weeks postoperatively. After 24 weeks, some signs of loosening were observed. More loosening occurred with the hollow titanium and polyacetal implants, mainly during the repair phase. Three hollow titanium and three polyacetal coated implants survived this period, and were killed after 24 weeks. The integrity of the HA layer at the bone-implant interface of the titanium implants was good. In the polyacetal implants, the repair reaction of the cortical bone was incomplete. Bone ingrowth into HA was largely lacking. In conclusion, we found significant differences in the repair and interface reactions around implants of different stiffness. Stiff implants showed favourable initial interface conditions for bone ingrowth. Intermediate and flexible implants provoked unfavourable interface conditions for initial bone ingrowth. The finite element study showed that the flexible stems produce larger micromotions and higher interface stresses at the bone-prosthesis interface than the stiff stems, indicating an explanation for the histological findings.     

One-stage lengthening of femoral nonunion augmented with human bone morphogenetic protein

Fifteen patients with posttraumatic shortened atrophic femoral nonunions were treated with one-stage lengthening. The alloimplant was composed of allogeneic antigen extracted autolyzed human bone perfused with partially purified human cortical bone morphogenetic protein associated with noncollagenous protein and used as graft. The composite was lyophilized and sterilized with ethylene oxide. All 15 nonunions were atrophic diaphyseal and were lengthened through intercalary segmental defects bridged with the human bone morphogenetic protein composite alloimplants stabilized to the medial femoral cortex through plate osteosynthesis and lag screw fixation. One lengthened proximal femur had fatigue failure of the plate and was treated successfully by exchange plating. The average increase in length was 2.8 cm (range, 1.5-5 cm) and an average percentage increase in length of 8% (range, 4%-132%) of the residual shortened femur. The human bone morphogenetic protein composite produced an immediate reactive bone formation in the host bone and progressive remodeling of the donor recipient interfaces. There were no infections, allergic reactions, clinical rejection of the human bone morphogenetic protein composite alloimplants, or evidence of malignant disease. One-stage femoral lengthening augmented with human bone morphogenetic protein composite graft bridged the intercalary defect, remodeled the atrophic host bone and restored bone continuity within 1 to 2 years. Human bone morphogenetic protein composite alloimplants are a substitute of autogeneic bone graft and offer an alternative to iliac crest bone without the associated morbidity.     

Preclinical testing of total hip stems. The effects of coating placement

The long-term fixation endurance of noncemented hip stems in total hip arthroplasty is subject to incompatible design goals. To reduce stress shielding and periprosthetic bone loss, proximal fixation and load transfer are indicated. However, to prevent interface motion and promote interface-bonding security, fixation preferably should be maximized over the entire stem surface. In this study, the authors questioned whether hydroxyapatite coatings could be applied in patterns that reduce bone resorption, while maintaining safe interface stress levels. For that purpose, strain-adaptive bone-remodeling theory was applied in 3-dimensional finite element models, to simulate the long-term postoperative bone resorption process. During the process, the adaptation of interface stresses was monitored, and its effects on interface failure probability evaluated. This analysis was done for a fully coated stem, a 1/3 proximally coated stem, a smooth uncoated, press-fitted stem, and a stem with 5 proximal patches of circumferential stripes. The uncoated stem reduced bone loss dramatically, but promoted interface motions and distal pedestal formation. In all cases, the gradual bone-remodeling process increased the interface security of the coated stems. Bone loss and interface failure probability were not very different for the fully and 1/3-coated stems. Stripe coating reduced bone resorption considerably, while increasing long-term interface failure probability only slightly. The investigators concluded that the initial stability and the ingrowth potential of such a stem design are likely to be inadequate.     

Epidermal growth factor receptor ectodomain in the urine of patients with squamous cell carcinoma

Female Sprague-Dawley rats were ovariectomized (OVX) or sham-operated at 3 months of age and maintained untreated for 1 year after surgery. Baseline control and OVX rats were killed at the beginning of treatment when the rats were 15 months of age and 1 year postovariectomy. The remaining rats were treated with hPTH 1-34 (80 micrograms/kg BW, 5 days/week) or vehicle for 10 weeks. Quantitative bone histomorphometry was performed on undecalcified longitudinal sections of the proximal femur from each rat. Baseline OVX rats exhibited cancellous and cortical osteopenia at the femoral neck as their mean cancellous bone volume and cortical width were significantly decreased compared to the means for baseline control rats. In addition, baseline OVX rats had increased osteoblast and osteoclast surfaces and a greater cancellous bone formation rate than baseline control rats. OVX rats remained osteopenic with no further bone loss from the femoral neck after 10 weeks of vehicle treatment. In contrast, cancellous bone volume and cortical width in OVX rats treated with PTH were increased to the level of vehicle-treated control rats. The hormone restored lost bone in the femoral neck of OVX rats by markedly stimulating both cancellous and cortical bone formation. These histomorphometric findings in concert with recent biomechanical studies of bone strength indicate that the femoral neck of aged OVX rats is a promising sample site for studies of the prevention and treatment of bone loss induced by estrogen depletion.     

Bilateral symmetry of biomechanical properties in rat femora

In many studies, bone healing and remodeling have been examined in various animal models using one femur as a control for the contralateral femur based on the assumption that they are bilaterally symmetrical. Symmetry studies have been limited mainly to geometrical properties. The purpose of this study was to determine whether or not there is symmetry in the mechanical properties of rat femora. Two strain gauges were attached to the anterior surface parallel to the long axis of explanted femora of retired female breeder and 120-day-old male Sprague Dawley rats. Femora were mechanically tested in cantilever bending and the strain values were recorded. Moments of inertia, cortical areas, and moduli of elasticity were determined from strains and cross-sectional properties. Female femora showed a bilateral strain difference of less than 2.2% and an elastic modulus difference of less than 8.7%. Males had less than 2.0% and 7.9% differences for strain and elastic moduli, respectively. Statistical analysis showed no significant difference between left and right femoral strain values for the females, but modulus differences were significant different at the p = 0.05 level. There was no significant difference in strain and modulus values for the males, indicating mechanical and geometrical symmetry of their femora.     

Is postmenopausal estrogen therapy associated with neuromuscular function or falling in elderly women? Study of Osteoporotic Fractures Research Group

Thirty-one cases of endoprosthetic proximal femoral reconstruction after resection for bone tumors are reported. The minimum follow-up period was 2 years (average, 63 months). There were two local recurrences, two deaths from pulmonary metastases, two postoperative infections (1 superficial and 1 deep), both responding to therapy, one postoperative dislocation of a bipolar endoprosthesis, and two cases of loosening of acetabular cups on the same patient. Clinical results (Enneking grade) showed 27% E, 56% G, 14% F, and 3% P. Diaphyseal remodeling results (Rizzoli grade) were 29% A, 6% B, 49% C, 10% D, and 6% E. Anchorage (International Society of Limb Salvage grade) was assessed as 97% E and 3% F, whereas interface (International Society of Limb Salvage grade) was 100% E. Hip (bipolar) articulation was graded as 30% E, 56% G, 11% F, and 3% P. Initial rigid stabilization of the stem with cross-fixation screws allows for excellent bone ingrowth, but presents the problem of proximal cortical atrophy. Bipolar hip components are easy to insert and offer greater inherent stability and so are to be recommended for use in tumor surgery. The results suggest good medium- to long-term results with respect to wear. A new radiographic grading system is presented for bipolar arthroplasty. Survivorship of the femoral component in this series is 100% at a maximum follow-up period of 8 years.