Bone creep and short and long term subsidence after cemented stem total hip arthroplasty (THA).

Stem-cement and cement-bone interfacial failures as well as cement fractures have been noted in cemented total hip arthroplasty (THA) as the cause of aseptic loosening. Attempts to reduce the risk of femoral component loosening include improving the stem-cement interface by various coatings, using a textured or porous coated stem surfaces or by using a tapered stem having a highly-polished surface. The latter approach, often referred to as "force-closed" femoral stem design, would theoretically result in stem stabilization subsequent to debonding and 'taper-lock'. Previous work using three-dimensional finite element analysis has shown a state of stress at the stem-cement interface indicative of 'taper-lock' for the debonded stem and indicated that stem-cement interface friction and bone cement creep played a significant role in the magnitudes of stresses and subsidence of the stem. However, the previous analysis did not include the viscoelastic properties of bone, which has been hypothesized to permit additional expansion of the bone canal and allow additional stem subsidence (Lu and McKellop, 1997). The goal of this study was to investigate the effect of bone viscoelastic behavior on stem subsidence using a 3D finite element analysis. It was hypothesized that the viscoelastic behavior of bone in the hoop direction would allow expansion of the bone reducing the constraint on bone over time and permit additional stem subsidence, which may account for the discrepancies between predicted and clinical subsidence measurements. Analyses were conducted using physiological loads, 'average peak loads' and 'high peak loads' for 'normal patient' and 'active patient' (Bergmann et al., 2010) from which short and long term subsidence was predicted. Results indicated that bone creep does contribute to higher stem subsidence initially and after 10 years of simulated loading. However, it was concluded that the "constraint" upon the cement mantle is not mitigated enough to result in stem subsidence equivalent to that observed clinically.

Cortical bone viscoelasticity and fixation strength of press-fit femoral stems: finite element model.

Many cementless implant designs rely upon a diaphyseal press-fit in conjunction with a porous coated implant surface to achieve primary or short term fixation, thereby constraining interface micromotion to such a level that bone ingrowth and consequent secondary or long-term fixation, i.e., osseointegration, can occur. Bone viscoelasticity, however, has been found to affect stem primary stability by reducing push-out load. In this investigation, an axisymmetric finite element model of a cylindrical stem and diaphyseal cortical bone section was created in order to parametrically evaluate the effect of bone viscoelasticity on stem push-out while controlling coefficient of friction (mu = 0.15, 0.40, and 1.00) and stem-bone diametral interference (delta = 0.01, 0.05, 0.10, and 0.50 mm). Based on results from a previous study, it was hypothesized that stem-bone interference (i.e., press-fit) would elicit a bone viscoelastic response which would reduce the initial fixation of the stem as measured by push-out load. Results indicate that for all examined combinations of mu and delta, bone viscoelastic behavior reduced the push-out load by a range of 2.6-82.6% due to stress relaxation of the bone. It was found that the push-out load increased with mu for each value of delta, but minimal increases in the push-out load (2.9-4.9%) were observed as delta was increased beyond 0.10 mm. Within the range of variables reported for this study, it was concluded that bone viscoelastic behavior, namely stress relaxation, has an asymptotic affect on stem contact pressure, which reduces stem push-out load. It was also found that higher levels of coefficient of friction are beneficial to primary fixation, and that an interference "threshold" exists beyond which no additional gains in push-out load are achieved.

Cortical bone viscoelasticity and fixation strength of press-fit femoral stems: an in-vitro model.

Cementless total hip femoral components rely on press-fit for initial stability and bone healing and remodeling for secondary fixation. However, the determinants of satisfactory press-fit are not well understood. In previous studies, human cortical bone loaded circumferentially to simulate press-fit exhibited viscoelastic, or time dependent, behavior. The effect of bone viscoelastic behavior on the initial stability of press-fit stems is not known. Therefore, in the current study, push-out loads of cylindrical stems press-fit into reamed cadaver diaphyseal femoral specimens were measured immediately after assembly and 24 h with stem-bone diametral interference and stem surface treatment as independent variables. It was hypothesized that stem-bone interference would result in a viscoelastic response of bone that would decrease push-out load thereby impairing initial press-fit stability. Results showed that push-out load significantly decreased over a 24 h period due to bone viscoelasticity. It was also found that high and low push-out loads occurred at relatively small amounts of stem-bone interference, but a relationship between stem-bone interference and push-out load could not be determined due to variability among specimens. On the basis of this model, it was concluded that press-fit fixation can occur at relatively low levels of diametral interference and that stem-bone interference elicits viscoelastic response that reduces stem stability over time. From a clinical perspective, these results suggest that there could be large variations in initial press-fit fixation among patients.

Primary total hip arthroplasty using a grit-blasted, press-fit femoral prosthesis.Long-term results with survivorship analysis.

BACKGROUND: This report presents the initial experience with the use of a grit-blasted, press-fit femoral prosthesis. METHODS: The first 300 consecutive primary total hip arthroplasty procedures using a collarless, three-dimensional tapered, straight, titanium alloy stem with a grit-blasted surface (performed in 299 patients) were evaluated with independent clinical and radiographic follow-up examinations up to sixteen years. Eight different types of acetabular components, including a cementless all-polyethylene socket in 80 percent of the cases, were used. Radiographic evaluation, performed by an independent observer using a zonal analysis method, included assessment of component migration, Enghs implant-bone femoral fixation score, implant-bone demarcations, and periprosthetic osteolysis. The average duration of long-term radiographic follow-up was 12.6 years (range; 10 to 16 years). RESULTS: At last examination only five hips were lost to follow-up and 69 patients were deceased. The overall mechanical failure rate (i.e. unstable with or without revision) of the femoral component was 2 percent. The femoral revision rate was 7 percent (two hips for aseptic loosening, five hips for septic loosening, and twelve hips with osseointegrated stems for severe progressive femoral osteolysis) and the acetabular revision rate was 27 percent (revised for either socket migration or progressive peri-acetabular osteolysis or both). Survivorship, based on any revision (femoral or acetabular) was 89 percent at ten years and 62 percent at fourteen years; survivorship of the femoral component was 95 percent at ten years and 90 percent at fourteen years. The incidence of femoral periprosthetic osteolysis, by radiographic examination at ten years or more was 47 percent, including 12 hips (5 percent) with distal endosteal osteolysis. Femoral implant-bone fixation was stable, bone-ongrowth in 97 percent; stable, fibrous-fixation in 1 percent, and unstable, fibrous-fixation in 2 percent of the cases. CONCLUSIONS: The grit-blasted, press-fit, collarless, tapered femoral component continued to per-form well clinically and radiographically up to sixteen years of follow-up despite the challenging environment of periprosthetic osteolysis associated with the acetabular component design. The high incidence of failure among the cementless all-polyethylene sockets was not unexpected and the use of this acetabular implant was discontinued in 1985. While considered a "first-generation" cementless stem, this implant is still in use virtually without design modifications. This study demonstrates the durability of the results of the grit-blasted femoral component and indicates that such an implant offers a viable alternative for fixation without bone cement. (Hip International 2002; 2: 55-72).

Dependency of cement mantle thickness on femoral stem design and centralizer.

Deficient cement mantles may be detrimental with regard to long-term outcome of cemented femoral stems. We performed a cadaver study on 48 left femora with 4 different stem designs (1 anatomic, 3 straight) to study the influence of stem design, centralizer, and femur type on cement mantle thickness. A radiographic and microradiograhic analysis was done. Overall, 88% of stems were aligned within 1 degrees of neutral in the frontal plane. In Gruen zones 1 through 7, we measured 24 thin cement mantles (<2 mm) in 19 specimens with no correlation to stem design or zone. In the sagittal plane, typical areas of thin cement mantles were identified in Gruen zones 8 and 9 (n = 39) and 12 (n = 21). The anatomic stem design carried the lowest risk (54%) of producing a thin cement mantle proximally in Gruen zones 8 and 9. The risk for straight stem designs was >90%. Straight stems without centralizer showed the highest risk of thin cement mantles in Gruen zone 12 (93%). Centralizers were efficient to prevent thin cement mantles in zone 12 but had no effect proximally. Lateral radiographs are essential to allow for adequate radiographic assessment of the cement mantle and stem alignment. There is a high risk of producing thin cement mantles in Gruen zones 8 and 9, in particular when straight stems are used. Posterior canal entry and low neck osteotomies are essential. Anatomic stems respect the anatomy, allow for more even cement mantles, minimize the risk of thin cement mantles without the use of centralizers, and may be considered in the femur with marked proximal bow.

Stem surface roughness alters creep induced subsidence and 'taper-lock' in a cemented femoral hip prosthesis.

The clinical success of polished tapered stems has been widely reported in numerous long term studies. The mechanical environment that exists for polished tapered stems, however, is not fully understood. In this investigation, a collarless, tapered femoral total hip stem with an unsupported distal tip was evaluated using a 'physiological' three-dimensional (3D) finite element analysis. It was hypothesized that stem-cement interface friction, which alters the magnitude and orientation of the cement mantle stress, would subsequently influence stem 'taper-lock' and viscoelastic relaxation of bone cement stresses. The hypothesis that creep-induced subsidence would result in increases to stem-cement normal (radial) interface stresses was also examined. Utilizing a viscoelastic material model for the bone cement in the analysis, three different stem-cement interface conditions were considered: debonded stem with zero friction coefficient (mu=0) (frictionless), debonded stem with stem-cement interface friction (mu=0.22) ('smooth' or polished) and a completely bonded stem ('rough'). Stem roughness had a profound influence on cement mantle stress, stem subsidence and cement mantle stress relaxation over the 24-h test period. The frictionless and smooth tapered stems generated compressive normal stress at the stem-cement interface creating a mechanical environment indicative of 'taper-lock'. The normal stress increased with decreasing stem-cement interface friction but decreased proximally with time and stem subsidence. Stem subsidence also increased with decreasing stem-cement interface friction. We conclude that polished stems have a greater potential to develop 'taper-lock' fixation than do rough stems. However, subsidence is not an important determinant of the maintenance of 'taper-lock'. Rather subsidence is a function of stem-cement interface friction and bone cement creep.

Primary total hip arthroplasty with use of the modular S-ROM prosthesis. Four to seven-year clinical and radiographic results.

BACKGROUND: A multicenter retrospective study was conducted to determine the clinical and radiographic results of primary total hip arthroplasty with insertion of the S-ROM modular femoral stem without cement in a series of patients who had been followed for four to seven years. Four centers participated in the study, with one contributing surgeon at each center. METHODS: Two hundred and eight consecutive patients who had a total hip arthroplasty with implantation of the S-ROM femoral prosthesis at one of the four centers during the study period were identified. Twenty-nine patients were lost to follow-up or had incomplete radiographic data, and twenty patients died from causes unrelated to the index arthroplasty. The remaining 159 patients formed the basis of this study. Sixteen of these patients had a bilateral procedure, resulting in 175 hips with complete clinical and radiographic data. The average age of the patients at the time of the index operation was fifty-nine years (range, twenty-two to ninety-three years). The duration of clinical follow-up averaged 5.3 years (range, four to 7.8 years), and the duration of radiographic follow-up averaged 4.9 years (range, four to 7.3 years). RESULTS: One patient (0.6 percent) had a failed femoral component, which was evidenced by progressive subsidence and lack of bone ingrowth. In addition, two patients (1 percent) had a revision of the acetabular component. The average Harris hip score increased from 35 points (range, 10 to 76 points) preoperatively to 91 points (range, 52 to 100 points) at the most recent follow-up examination. The radiographic evaluation revealed that 172 hips (98 percent) had stable bone ingrowth, two hips (1 percent) had stable fibrous ingrowth, and one hip (0.6 percent) had unstable fibrous ingrowth. Periprosthetic osteolytic lesions were noted in twelve hips (7 percent). The lesions were observed in the femur in eight hips, in the acetabulum in two hips, and in both the femur and the acetabulum in two hips. All femoral osteolytic lesions were localized within the greater trochanter or the proximal-medial portion of the femoral neck. No osteolytic lesions were evident distal to the stem-sleeve junction. CONCLUSIONS: Use of the modular S-ROM femoral prosthesis yielded excellent intermediate-term outcomes with respect to standard radiographic and clinical criteria. The issue regarding the theoretical increase in the rate of osteolysis due to metal debris generated at the modular femoral stem-sleeve junction was specifically addressed. We found that the rate of osteolysis in this series was not notably higher than that in other series reported in the orthopaedic literature. Although many possible factors may influence the rate of osteolysis in total hip arthroplasty, this finding suggests that the potential increase in osteolysis theoretically associated with this modular femoral implant was not observed at intermediate-term follow-up. Although longer follow-up is warranted so that the potential for osteolysis can be evaluated fully, no osteolytic lesions were evident distal to the stem-sleeve interface at the time of intermediate-term follow-up. This finding suggests that there is a circumferential seal at the modular junction of the stem that prevents the distal egress of wear debris.

Influence of delayed injection time on the creep behavior of acrylic bone cement.

It has been proposed that creep of acrylic bone cement may contribute to loosening of cemented total joint replacements. If true, it is important that factors affecting creep of bone cement are identified. The objective of this study was to evaluate the effect of an operator-controlled variable, injection time, on the creep behavior of acrylic bone cement. Results from this investigation showed that injection time significantly (p < 0.0001) influenced creep behavior of bone cement. "Delayed" injection time of acrylic bone cement increased creep by approximately 5 times in 24 h compared to specimens prepared according to standard injection procedures.

Axisymmetric finite element analysis of a debonded total hip stem with an unsupported distal tip.

A tapered femoral total hip stem with a debonded stem-cement interface and an unsupported distal tip subjected to constant axial load was evaluated using two-dimensional (2D) axisymmetric finite element analysis. The analysis was performed to test if the mechanical condition suggest that a "taper-lock" with a debonded viscoelastic bone cement might be an alternative approach to cement fixation of stem type cemented hip prosthesis. Effect of stem-cement interface conditions (bonded, debonded with and without friction) and viscoelastic response (creep and relaxation) of acrylic bone cement on cement mantle stresses and axial displacement of the stem was also investigated. Stem debonding with friction increased maximum cement von Mises stress by approximately 50 percent when compared to the bonded stem. Of the stress components in the cement mantle, radial stresses were compressive and hoop stresses were tensile and were indicative of mechanical taper-lock. Cement mantle stress, creep and stress relaxation and stem displacement increased with increasing load level and with decreasing stem-cement interface friction. Stress relaxation occur predominately in tensile hoop stress and decreased from 1 to 46 percent over the conditions considered. Stem displacement due to cement mantle creep ranged from 614 microns to 1.3 microns in 24 hours depending upon interface conditions and load level.

Hydroxyapatite-coated proximal ingrowth femoral stems. A matched pair control study.

A matched pair study of 2 groups of 42 uncemented total hip replacements were compared retrospectively after a minimum 3-year followup. Forty-two hips were implanted with a hydroxyapatite coating on the proximal femoral patched porous surfaces; 42 hips had patched porous-coated stems without hydroxyapatite. There were no clinical differences between the matched groups by any criteria of measurement. There was no statistically significant difference between the matched groups for femoral stem fixation at all followup intervals. At the 3-year followup, 90% of the femoral stems in the hydroxyapatite porous group, and 83% in the porous control group achieved stable bony fixation. Hydroxyapatite-coated femoral stems demonstrated accelerated bone remodeling characterized by proximal cancellous hypertrophy. The percentage of femoral stems exhibiting cancellous hypertrophy was significantly greater at all followup intervals. This study did not demonstrate any clinical advantage of hydroxyapatite being added to a porous-coated surface. The radiographic changes of bone remodeling seen with hydroxyapatite are not known to improve durability of the hip arthroplasty. This study again condemns the use of patched porous-coating and titanium-bearing surfaces.

Creep characteristics of hand- and vacuum-mixed acrylic bone cement at elevated stress levels.

Compressive creep testing of cylindrical specimens machined from two commercial self-polymerizing acrylic bone cements demonstrated measurable creep strains with higher creep strains for the hand-mixed cement specimens compared to vacuum-mixed cement ones. The average creep strains of hand-mixed cement, after 6 h of constant load, ranged from 0.11% at 10.5 MPa to 14.0% at 50 MPa of applied stress. Vacuum mixing reduced the average creep strain to 6.7% after 6 h of applied stress at 50 MPa. There were no significant differences in the creep response between the two types of acrylic cements. The difference in creep resistance of the two cements was reduced after vacuum mixing (P = .013), which also significantly reduced the cement's internal porosity.

Radiation therapy to prevent heterotopic ossification after cementless total hip arthroplasty.

Clinical and roentgenographic observations on a postoperative low-dose radiation of 1000 cGy for prevention of heterotopic ossification in high-risk patients after 48 noncemented total hip arthroplasties demonstrated no overall statistical difference in the Harris hip scores between the treated group (92.2) and the control group (91.0). There was no difference in the incidence of radiolucent lines about the components, and there were no revisions for aseptic loosening in the treated group. The incidence of clinically significant (Brooker Grades III and IV) heterotopic bone formation was lower in the treated group (7%) than in the control group (32%). This difference in the incidence of heterotopic bone formation was statistically significant. Radiation therapy was effective in preventing postoperative heterotopic bone formation in biologically fixed total hip implants.

Factors related to failure of structural bone grafts in acetabular reconstruction of total hip arthroplasty.

Structural bone grafts in acetabular reconstruction of total hip arthroplasty have come into more common usage with the increasing number of revision arthroplasties. The authors present the outcome of 40 bone grafts with follow-up periods of 2-8 years. Primary and revision arthroplasties with both cemented and noncemented fixation are included. Factors that had a statistically significant relationship to failure of the bone graft included fit and fixation of bone graft to host, fit and fixation of prosthesis to host, union of bone graft to host, and the presence or absence of the confluence of the anterior and posterior columns of the acetabulum. Other findings of interest were a higher failure rate of multiple grafts as compared to a single graft and freeze-dried femoral heads. Migration of the socket of more than 3 mm is statistically related to failure of the socket.

Anatomic Porous Replacement hip arthroplasty: first 100 consecutive cases.

One hundred consecutive Anatomic Porous Replacement (Intermedics Orthopaedics, Austin, TX) hip replacements were followed for 4 years. Eighty percent of patients had bone ingrowth fixation, 14% stable fibrous, and 6% unstable fibrous (loose) with 4% revised. Only two hips changed fixation grade after 2 years. Bone remodeling showed cancellous hypertrophy of the cortex, usually along the lateral cortex, and 7% had stress shielding (atrophy) of the proximal cortices. Adaptive bone remodeling almost always occurred in type B and C bone. Bone remodeling was statistically related to bone type, prosthetic fill, stem-bone ratio, and collar loading. We concluded that proximal bone ingrowth fixation with proximal load transfer provides good clinical results and excellent bone remodeling. Also, collar loading improves bone response. To expand this fixation in a predictable fashion to all type B and some type C bone requires geometry changes, which have been accomplished in the Anatomic Porous Replacement II.

A radiographic and histological analysis of 17 retrieved ultrahigh-molecular-weight polyethylene porous-coated femoral stems.

Radiographic and histological and evaluation for failure of 17 retrieved implants was performed. The implants consisted of a titanium stem with a 3-mm ultrahigh-molecular-weight polyethylene coating and an outer 1.5-mm ultrahigh-molecular-weight polyethylene fiber layer. Particular attention was paid to osteolysis, subsidence, debris, and foreign body reaction. Radiographic analysis showed subsidence (14 of 17), osteolytic lesions (9 of 14), and unique posterior medial exostoses (8 of 14). Histological analysis showed no significant evidence of acrylic bone cement either as debris or reactive tissue. There was an intense foreign body reaction directly related to the amount and size of polyethylene debris.

Revision of failed total hip arthroplasties with uncemented porous-coated anatomic components.

This series represents a relatively short follow-up study of patients who were treated with cementless revisions for failed previous arthroplasties. Many of these cases required extensive bone grafting to the acetabulum and often to the femur. Despite extensive bone grafting, there were no infections. There has been but a single graft resorption after a hemiarthroplasty conversion for recurrent dislocations. To date, all other grafts have remained intact and have shown signs of union. Even though the acetabular components were not anchored in place by adjuvant fixation devices such as screws, migration of the acetabular component has not been a problem. All other components have remained stable, and the supporting grafts appear to have united successfully. Femoral revision has been more technically demanding because the largest stem possible should be placed within the femur to prevent subsidence and provide good stabilization in the proximal metaphyseal area. These short-term results compared favorably with similar series of cemented revisions. Patient selection is important and there are definite candidates for cemented femoral components, particularly with first-time revisions in elderly patients. If there is massive osteolysis in the femur, cemented revision is probably not indicated. Long stems should not be used unless necessary. Cortical defects at the tip of the standard stem obviously would require bypassing the stress riser with a longer stem. If, however, the cortex is intact in this region and stability can be achieved, revision should be carried out with a relatively short stem. Techniques for cementless revision are demanding, but with meticulous attention to detail and technical perfection, the method has a most encouraging prognosis. Longer follow-up evaluations will be necessary to make an accurate evaluation of graft incorporation, but short-term results are encouraging to both surgeons and patients.

Radiographic performance of two different total hip cemented arthroplasties. A survivorship analysis.

Detailed serial radiographic analysis of the femoral and acetabular components of 367 Charnley (cobalt-chrome alloy) and 383 STH (titanium alloy) primary cemented total hip prostheses was conducted. The follow-up of the Charnley prosthesis ranged from 1 to 16 years (mean follow-up, 9 years), and for the STH from 1 to 11 years (mean follow-up, 80 months). Both single period and survivorship analyses were done to evaluate the radiographic performance. The analysis revealed that the curved STH prostheses' overall radiographic performance was less satisfactory than the Charnley prostheses. However, the STH with a straight stem had significantly better radiographic performance compared with the Charnley prostheses in all parameters except in the incidence of bone-cement radiolucent lines.

Two-year follow-up of the PCA noncemented total hip replacement.

The early results with the PCA total hip replacement have been most gratifying, especially the absence of complications related to the acetabular component. The radiographic evaluation was done critically, and the finding of no progressive acetabular radiolucencies was unexpected. Longer-term evaluation of these interfaces is necessary, but the short-term results have been encouraging. No components have migrated despite the absence of adjunct fixation mechanisms such as screws and flanges. The pressfit achieved with the roughened hemispherical surface has been adequate, and the fixation with the two outrigger pegs appears to have been sufficiently stable to preserve the prosthetic stability and has resulted in successful anchorage of all the components. The results with femoral components are obviously related to technique. In a few early cases when undersized prostheses were used, loosening occurred, and four of these components advanced to detectable loosening. One of these components was revised since this analysis. Attention to detail with maximal filling of the proximal femur apparently led to improved results, with successful anchorage in all subsequent implantations. The application of the dimensional analysis before surgery may indicate those cases in which a tight fitting metaphysis cannot be achieved. In elderly patients who have osteoporotic bone, a substantial mismatch exists between the size of the metaphysis and the diaphysis, and it may be advisable to continue with cement in these cases. By applying the dimensional analysis to preoperative templating, the surgeon may be sufficiently informed to know that a tight fit can be achieved at surgery. No catastrophic failures have occurred. If loosening does occur because of undersizing of the prosthesis, the process appears to be gradual and, although associated with pain, does not result in sudden failure. Despite the prosthesis not being anchored by cement or collar, no sudden subsidence of the components has resulted. Patients' clinical performance has been somewhat slower when compared to cemented series. Performance seems to accelerate once weight bearing occurs, however, and after 2 years no difference exists between this series and a corresponding cemented series. Noncemented total hip arthroplasty appears to offer as good if not better results than cemented total hip arthroplasty, if performed correctly and in the appropriate patient. Successful outcome depends greatly on technique, but when technically adequate implantation has been performed, the results have been gratifying.(ABSTRACT TRUNCATED AT 400 WORDS)