The role of proximal femoral support in stress development within hip prostheses

Bone remodeling commonly associated with implant loosening may require revision total hip replacement when there is substantial proximal femoral bone loss. Additionally, the surgical exposure required to remove primary implants may alter the proximal femur's structure. As a result, in many revision hip situations the proximal femur provides compromised support for the revision femoral component. Stress analyses of the proximal femur with extensively porous-coated prosthetic femoral components show that proximomedial femoral bone loss, ununited femoral osteotomy, and periprosthetic fracture can result in significant elevation of stress within revision prosthetic components. The first principal stress within prosthetic components can, in proximal bone loss conditions, be elevated significantly above a revision prostheses' fatigue strength. Loss of proximomedial bone is predicted to increase stress within a revision component by as much as 82%. An unhealed transverse femoral fracture or osteotomy is predicted to more than double the stress within a revision femoral component. In revision total hip replacement, efforts directed toward the restoration of proximal femoral bone and the use of larger prostheses may contribute to avoiding prostheses fatigue fracture. Similarly, protected weightbearing in patients with ununited femoral osteotomies and periprosthetic fractures may be important to preventing prosthetic fracture.     

Bone mineral density of the distal femur following uncemented total knee arthroplasty: a two-year follow-up of 11 knees using dual energy X-ray absorptiometry

Abstract In a prospective study, we evaluated the adaptive bone remodelling pattern of the distal femur using dual energy X-ray absorptiometry (DEXA) after total knee arthroplasty (TKA). Eleven patients underwent TKA with insertion of an Interax total knee system (Howmedica) because of primary osteoarthrosis of the knee. All patients received an uncemented femoral component with an uncoated cast-mesh ingrowth surface. The components had a 20-mm long central peg placed anteriorly just behind the anterior flange. DEXA scans were performed postoperatively at 2 weeks and at 3, 6, 12 and 24 months in the mediolateral plane of the distal femur and in the anteroposterior plane of the distal tibia and fibula. Compared to the immediate 2-week postoperative bone mineral density (BMD) value, we found no significant changes in BMD in 4 regions of interest (ROI) in the distal femur during the 2-year follow-up at 3 months. A temporary but significant decrease in BMD of 11.2% (95% CL, -17.3% to -5.0%) was observed in the most distal ROI just posterior to the fixation peg. Bone mineral content (BMC) of the distal tibia and fibula showed no significant changes in the operated and contralateral legs during the follow-up. The adaptive bone remodelling of the distal femur after TKA using an uncemented femoral component not previously examined by DEXA induced only a very limited and temporary loss of bone mineral.     

Periacetabular bone density after total hip arthroplasty a postmortem analysis

To clinically verify the bony response to a press-fit acetabular component, this study assessed 5 postmortem-retrieved pelves with unilateral total hip arthroplasties. Changes in periacetabular bone density between implanted and contralateral bone were assessed with dual energy x-ray absorptiometry and computed tomography. At a mean of 9.1 years postarthroplasty, bone density decreased an average of 1.5% to 7.1% proximal and 12.8% medial to the cup. This supports shorter-term in vivo investigations demonstrating periacetabular stress shielding proximal to press-fit cups as well as computer models predicting bone loss medially, but in much greater magnitudes. Unlike femoral remodeling, the average magnitudes of pelvic bone loss are not extensive; therefore, we question whether periacetabular remodeling should be a primary concern for orthopedic surgeons.     

Bone stock reconstruction for huge bone loss using allograft-bones,bone marrow,and teriparatide in an infected total knee arthroplasty

Bone stock reconstruction using allograft-bones, bone marrow (BM), and teriparatide (TPTD) is reported. Huge and extensive bone losses occurred in the medullary cavity of the femur and tibia of a 55-year-old female rheumatoid arthritis patient with severe osteoporosis after debridement of her infected total knee arthroplasty. Because of the risks of unstable prosthetic fixation and intra-operation fracture, we first reconstructed the bone stock. Chipped allograft bones mixed with BM were implanted in the bone defects, and TPTD was administrated for the osteoporosis therapy. Good bone formation was found by computed tomography after 4 months. Bone turnover markers and bone mineral density (BMD) were increased at 6 months. We confirmed good bone formation at the re-implantation surgery. The newly formed bone harvested during the re-implantation surgery showed active osteoblast-like lining cells. TPTD is known to enhance allograft bone union, mesenchymal stem cell differentiation into osteoblasts, and BMD. This tissue engineering-based technique might be improved by the various effects of TPTD. This method without any laboratory cell culture might be a good option for bone stock reconstruction surgery in ordinary hospitals.     

Effect of femoral stiffness on bone remodeling after uncemented arthroplasty

The current study examined the relationships among femoral stiffness, implant stiffness, and bone remodeling in 40 femurs retrieved at autopsy from 20 patients with unilateral uncemented hip replacements. The purpose of the study was to determine if the magnitude of periprosthetic bone loss after arthroplasty was correlated with, and could be predicted from, stem and femoral stiffness terms. For analysis, the contralateral normal femur was used as a control to represent the unremodeled condition of the in vivo implanted femur. Bone loss attributable to remodeling was quantified by video-densitometric analysis. Stiffness terms were calculated as the product of the elastic modulus and geometric properties digitized from cross-sectional slab radiographs. Femoral stiffness calculations accounted for variations in modulus attributable to patient differences in bone mineral density and geometric properties attributable to differences in the shape of individual femurs. Similarly, calculations of implant stiffness accounted for variations in implant shape. Results showed axial bone stiffness was the variable most strongly correlated with bone loss. Individual stem stiffness terms were not significantly correlated with bone loss. Multiple linear regression analysis, using stem-to-bone stiffness ratios as independent variables, accounted for 46% of the variance in bone loss data. In the regression analysis, the axial stem-to-bone stiffness ratio was the strongest correlate with bone loss. Although these results show the influence of mechanical stiffness factors on bone remodeling, other factors (hormonal status, drugs, disease, activity level) could represent the variance in bone loss data not accounted for in the study.     

Results of noncemented revision

The results of revision of total hip replacements with noncemented fixation are dependent on implant design, postoperative treatment, and the technique selected in relationship to the bone quality and quantity. When proximal femoral bone ingrowth fixation is desired an implant must have a double wedge design, which permits a high percentage contact between the host bone and the porous coating of the implant. Bone graft in either the femur or acetabulum should be used at a minimum. Every attempt to obtain host bone contact to porous coating should be done. When a bone graft is used to replace the proximal femur or when an acetabular bone graft covers more than 50% of the cup, the implant should be cemented into the bone graft. Bone structure affects the outcome of noncemented revisions more than the thickness of the bone. Even thin and weak femoral bone has dynamic remodeling capability. The structure influences result more according to the ability to contact the host bone to the porous coating. When this can be done, the bone will heal to the porous coating. Postoperatively, 40% of our patients are treated with a cast to protect bone grafts and weak bones that are important for stability of the implant. We believe that conservative postoperative rehabilitation for noncemented revision arthroplasty significantly improves the healing for the bone to the implant.     

Effects of material properties of femoral hip components on bone remodeling.

Bone loss around femoral hip stems is one of the problems threatening the long-term fixation of uncemented stems. Many believe that this phenomenon is caused by reduced stresses in the bone (stress shielding). In the present study the mechanical consequences of different femoral stem materials were investigated using adaptive bone remodeling theory in combination with the finite element method. Bone-remodeling in the femur around the implant and interface stresses between bone and implant were investigated for fully bonded femoral stems. Cemented stems (cobalt-chrome or titanium alloy) caused less bone resorption and lower interface stresses than uncemented stems made from the same materials. The range of the bone resorption predicted in the simulation models was from 23% in the proximal medial cortex surrounding the cemented titanium alloy stem to 76% in the proximal medial cortex around the uncemented cobalt-chrome stem. Very little bone resorption was predicted around a flexible, uncemented "iso-elastic" stem, but the proximal interface stresses increased drastically relative to the stiffer uncemented stems composed of cobalt-chrome or titanium alloy. However, the proximal interface stress peak was reduced and shifted during the adaptive remodeling process. The latter was found particularly in the stiffer uncemented cobalt-chrome-molybdenum implant and less for the flexible iso-elastic implant.     

Long stemmed total knee arthroplasty with interlocking screws: a computational bone adaptation study.

The ability of an interlocking screw fixation technique to minimize bone loss related to stress shielding in the tibia was investigated and compared to the abilities of cement and press-fit fixation. Full bony ingrowth has been associated with greater stress shielding than partial ingrowth; therefore, the effect of intimate bonding of the stem to bone on subsequent bone loss was also studied. A damage- and disuse-based remodeling theory was coupled with a two-dimensional finite element model of the tibia to predict changes in bone remodeling following long stemmed total knee arthroplasty (TKA) for four different fixation techniques (cement, press-fit, interlock with bony ingrowth, and interlock without bony ingrowth). Remodeling changes commenced with the model state variables--bone area fraction, mechanical stimulus, damage, and remodeling activity--at steady-state values predicted by the intact tibia simulation. After TKA and irrespective of fixation technique, the model predicted elevated remodeling due to disuse, in which more bone was removed than replenished. In regions below the tibial tray and along the cortices, the interlocking stem with full bony ingrowth and the cemented stem caused the least amount of bone loss. An interlocking stem with a smooth, matted finish did not reduce the bone loss associated with interlocking fixation.     

Surface replacement of the hip

Summary Surface replacement of the hip has been performed in 25 rabbits. Twenty animals were chosen for evaluation of the result of the operation after an average period of five months postoperatively.Tetracycline injections were given for fluorochrome labeling and radiographic controls have been made in vivo to check the position of the cup before the animals were sacrified.Histological examinations were made from slides, which were gained after the metal cup had been removed with the bone cement still in place.It was found that the bone underneath the cup survived the surface replacement and that there was a normal metabolism furtheron.The trabeculae even grew into the surface structures of the inner layer of the bone cement and there was no interposition of membranes of fibrous tissue.It could be demonstrated that fibrous tissue between bone and bone cement is found only in cases of loosening of the cup and the cement or in cases of insufficient contact between this interface from the moment of the operation on. This is possible when airbubbles arise in the bone cement and get in contact with the trabeculae of the femoral head.Fluorochrome labeling proved the formation of new bone underneath the cup and the survival of the osseous structures of the coxal end of the femur.We believe, that in clinical cases it is necessary to remove all predamaged cancellous bone from the head of the femur (i.e. in cases of avascular necrosis) before the cup is attached. Close contact between the bone cement and the healthy spongiosa is one of the conditions for success in surface replacement of the hip.     

Influence of porous coating level on proximal femoral remodeling. A postmortem analysis

This study used femurs retrieved at autopsy to compare the extent and location of bone remodeling between four patients implanted with proximally porous coated femoral prostheses and a matched group of four patients implanted with extensively porous coated femoral prostheses. The femoral components studied were large, cementless, straight, cobalt chrome stems and were identical except for the amount of porous coating. The contralateral normal femur of each patient also was retrieved, implanted with an identical prosthesis, and used as a control for bone mineral content. Dual energy x-ray absorptiometric analysis showed marked loss of bone mineral content in both groups of patients. The extensively coated group had less bone loss on average (18.4%) than did the proximally coated group (38.6%). There was no relationship between the extent of coating and the location of bone mineral loss; specifically, proximal coating did not protect against loss of bone mineral content proximally or distally in the femur. Videodensitometric analysis of cross sections of periprosthetic bone also showed that the extensively coated group tended to have less decrease in bone density than did the proximally coated group (14.3% versus 28.4%). Although one cannot presume that all proximally fixed stem designs would produce results similar to those presented here, these findings show that decreasing the extent of porous coating alone does not necessarily reduce proximal femoral bone loss.     

The femoral head expandable peg: improved periimplant bone properties following expansion in a cadaveric model

Purpose The femoral head expandable peg offers a new concept of bone-device purchase treating proximal femur fractures. We describe the expansion influence on both the periimplant bone properties and the intraosseous pressure.Methods A 10 × 8 mm cannulated peg consisting of a stainless steel oval-shaped transversely sectioned rod was tested on 13 femoral heads retrieved from patients with subcapital fracture treated by hemiarthroplasty. Normal saline solution was used to expand the diameter of its membrane from 7.8 to 10.5 mm, resulting in abutment of the distal peg to the compacted cancellous bone of the femoral head. The intraosseous pressure was monitored using a 1.4 mm drill-hole.Results Dual-energy X-ray absorptiometry and microradiography demonstrated increased periimplant bone density following peg expansion, without significant increase in the intraosseous pressure.Conclusion The new implant afforded improved periimplant bone density, less trabecular damage and preservation of bone stock, all contributing to reliable biomechanical support and potentially decreasing the high complication rate following screw fixation in osteoporotic femora. Further studies are needed in order to establish clinical safety and efficacy.     

Stemmed femoral knee prostheses: Effects of prosthetic design and fixation on bone loss

Although the revision rates for modern knee prostheses have decreased drastically, the total number of revisions a year is increasing because many more primary knee replacements are being done. At the time of revision, bone loss is common, which compromises prosthetic stability. To improve stability, intramedullary stems are often used. The aim of this study was to estimate the effects of a stem, its diameter and the interface bonding conditions on patterns of the bone remodeling in the distal femur.

We created finite element models of the distal half of a femur in which 4 types of knee prostheses were placed. The bone remodeling process was simulated using a strain-adaptive bone remodeling theory. The amount of such remodeling was determined by calculating the changes in bone mineral density in 9 regions of interest from simulated DEXA scans.

The computer simulation model showed that revision prostheses tend to cause more bone resorption than primary ones, especially in the most distal regions. Predicted long-term bone loss due to a revision prosthesis with a thin stem equalled that around a prosthesis with an intercondylar box. However, strong regional differences were found- the stemmed prostheses having more bone loss in the most distal areas and some bone gain in the more proximal ones. A prosthesis with a thick stem led to an increase in bone loss. When the prosthesis-cement interface was bonded, more bone loss was predicted than with an unbonded interface. These results suggest that a stem which increases stability initially may reduce stability in the long term. This is due to an increase in stress shielding and bone resorption.     

Stemmed femoral knee prostheses: effects of prosthetic design and fixation on bone loss

Although the revision rates for modern knee prostheses have decreased drastically, the total number of revisions a year is increasing because many more primary knee replacements are being done. At the time of revision, bone loss is common, which compromises prosthetic stability. To improve stability, intramedullary stems are often used. The aim of this study was to estimate the effects of a stem, its diameter and the interface bonding conditions on patterns of the bone remodeling in the distal femur. We created finite element models of the distal half of a femur in which 4 types of knee prostheses were placed. The bone remodeling process was simulated using a strain-adaptive bone remodeling theory. The amount of such remodeling was determined by calculating the changes in bone mineral density in 9 regions of interest from simulated DEXA scans. The computer simulation model showed that revision prostheses tend to cause more bone resorption than primary ones, especially in the most distal regions. Predicted long-term bone loss due to a revision prosthesis with a thin stem equalled that around a prosthesis with an intercondylar box. However, strong regional differences were found--the stemmed prostheses having more bone loss in the most distal areas and some bone gain in the more proximal ones. A prosthesis with a thick stem led to an increase in bone loss. When the prosthesis-cement interface was bonded, more bone loss was predicted than with an unbonded interface. These results suggest that a stem which increases stability initially may reduce stability in the long term. This is due to an increase in stress shielding and bone resorption.     

Factors affecting femoral bone remodeling after cementless total hip arthroplasty.

We performed a postmortem comparison of femurs from two patients who had bilateral cementless total hip arthroplasties with femoral prostheses of different stiffness implanted in their right and left hips. Radiographs of transverse sections of the four femurs demonstrated that all the prostheses were bone ingrown with the most ingrowth occurring distally where the porous coating contacted diaphyseal bone. In both patients, dual-energy x-ray absorptiometry analysis revealed that the femur implanted with the stiffer prosthesis had a 65% to 79% greater loss of proximal periprosthetic bone than the femur implanted with the more flexible prosthesis. One patient, however, had a dramatically greater total loss of bone from side to side than the other patient. In this patient, we believe that it was host factors more than the differences in stem stiffness that affected the bone-remodeling pattern. Although the two femurs with the stiffer prostheses had the greatest bone loss, the two femurs with the more flexible prostheses demonstrated radiographic signs of cantilever bending of the prosthetic stem and failure of proximal osseointegration. We are not aware of any other bilateral human postmortem analysis that so clearly illustrates the effect of stem stiffness on bone remodeling.     

The effect of alendronate on bone mineral density in the distal part of the femur and proximal part of the tibia after total knee arthroplasty

BACKGROUND: Bone mineral density around the knee is related to the mechanical properties of bone. Alendronate has been shown to be effective for the treatment of osteoporosis and for reducing the rate of osteoporotic fractures. The purpose of the present study was to investigate the effect of alendronate on bone mineral density in the distal part of the femur and proximal part of the tibia after total knee arthroplasty in women. METHODS: Ninety-six women with an average age of seventy years who were undergoing total knee arthroplasty were randomly divided into two groups. Patients in the study group received oral alendronate at a dose of 10 mg/day for six months, whereas patients in the control group did not. The bone mineral density in the distal part of the femur and proximal part of the tibia was determined preoperatively and at six and twelve months postoperatively. RESULTS: In the control group, the bone mineral density showed significant decreases of 13.8% (p < 0.001) and 7.8% (p = 0.003) in the distal part of the femur and of 6.5% (p = 0.002) and 3.6% (p = 0.141) in the proximal part of the tibia at six and twelve months, respectively. In the study group, however, the bone mineral density showed significant increases of 10.0% (p = 0.010) and 1.9% (p = 0.049) in the distal part of the femur and of 9.4% (p < 0.001) and 5.4% (p = 0.032) in the proximal part of the tibia at six and twelve months, respectively. The overall differences in bone mineral density between the study and control groups were significant (p = 0.011 for the proximal part of the tibia, and p = 0.033 for the distal part of the femur). CONCLUSIONS: We found significant postoperative decreases in bone mineral density in the distal part of the femur and proximal part of the tibia in women who had undergone total knee arthroplasty. Oral administration of alendronate for six months postoperatively significantly improved the bone mineral density. While the clinical benefits of alendronate after total knee arthroplasty remain unproven and the duration of follow-up in the present study was quite short, the improvement in bone mineral density may have a clinically important effect on prosthetic fixation and the rate of periprosthetic fractures after total knee arthroplasty.     

An Early-Life Femoral Shaft Fracture and Bone Mineral Density at Adulthood

High peak bone mass and density in early adulthood is an important protective factor against osteoporotic fractures in later life, but it is not known whether injuries to growing bones adversely affect the attainment of peak bone mass and density. The purpose of this study was therefore to examine with dual-energy X-ray absorptiometry the areal bone mineral density (BMD) of the injured and uninjured extremity (the femoral neck, trochanter area of the femur, distal femur, patella, proximal tibia and distal tibia), lumbar spine and distal radius of young adults with a history of an early-life femoral shaft fracture and to find out whether the fracture had affected the attainment of peak bone density of these patients. Thus, the BMD and clinical status of 41 patients (28 men, 13 women) who had sustained a femoral shaft fracture in childhood or adolescence (between 7 and 15 years of age, average 13 years before the study) were examined. The fracture had led to a statistically significant difference in BMD between the injured and uninjured side distal to the fracture site (men/women: distal femur, −3.7%/−3.9%; patella, −3.1%/−5.9%; proximal tibia, −2.0%/−4.6%; distal tibia, −3.4%/−5.2%), whereas the proximal femur did not show such differences. The male patients’ spinal BMD was significantly lower (−7.9%) than that in their age-, height- and weight-matched healthy controls. The female patients’ spinal BMD tended to be fairly comparable (−1.6%) to that of the controls (NS). In summary, this study indicates that early-life femoral shaft fracture results in a moderate (−2% to −6%) long-term side-to-side BMD difference distal to the fracture site. Patients’ spinal BMD values also tend to be lower than that of controls. Thus, a femoral shaft fracture sustained in childhood or adolescence seems to disturb somewhat the attainment of peak bone density, the important predictor of osteoporotic fractures in later life. Received: 23 December 1998 / Accepted: 1 April 1999     

The mechanical properties of immature osteopetrotic bone

Summary The elastic properties of the longer bones of the fore and hind limbs of an osteopetrotic Angus calf were measured using an ultrasonic technique. the same bones of a second Angus calf with unaffected bone tissue were also studied. The osteopetrotic bones were found to exhibit statistically significant lower values of density and mineral content than did the unaffected bones (P=.0054 andP=.0492, respectively.) This result is contrary to what has been reported in the literature. The variations of the elastic properties and the density around the circumference of both the immature osteopetrotic femur and the unaffected femur were found to be similar to those previously measured on normal adult bovine femora.     

Bony impingement affects range of motion after total hip arthroplasty: A subject‐specific approach

Hip range of motion after total hip arthroplasty has been shown to be dependent on prosthetic design and component placement. We hypothesized that bony anatomy would significantly affect range of motion. Computer models of a current generation hip arthroplasty design were virtually implanted in a model of pelvis and femur in various orientations ranging from 35° to 55° cup abduction, 0° to 30° cup anteversion, and 0° to 30° femoral anteversion. Four head sizes ranging from 22.2 to 32 mm and two neck sizes ranging from 10‐mm and 12‐mm diameter were tested. Range of motion was recorded as maximum flexion–extension, abduction–adduction, and axial rotation of the femur before any contact between prosthetic components or bone was detected. Bony impingement preceded component impingement in about 44% of all conditions tested, ranging from 66% in adduction to 22% in extension. Range of motion increased as head size increased. However, increasing head size also increased the propensity for bony impingement, which tended to reduce the beneficial effect of increased head size on range of motion. Reducing neck diameter had a greater effect on prosthetic impingement (mean, 3.5° increase in range of motion) compared to bone impingement (mean, 1.9°). This model allowed for a clinically relevant assessment of range of motion after total hip arthroplasty and may also be used with patient‐specific geometry [such as that obtained from preoperative computed tomography (CT) scans] for more accurate preoperative planning. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:443–452, 2008     

Dual energy X-ray absorptiometry analysis of peri-prosthetic stress shielding in the Birmingham resurfacing hip replacement

Introduction: Numerous reports in the literature refer to the femoral neck fracture rate in hip resurfacing. The aim of this study was to determine the bone mineral density and evidence of stress shielding around the femoral component of the Birmingham resurfacing prosthesis. Material and methods: Twenty-eight patients with primary unilateral osteoarthritis had a Birmingham resurfacing prosthesis. DEXA analysis of the proximal femur and femoral neck was performed and compared with the opposite unaffected side. Results: Total periprosthetic bone mineral density was 0.49% greater than the control, but this did not achieve statistical significance. Although the BMD of the femoral neck was slightly increased on the prosthetic side (1.002 g/cm²) as opposed to the control side, this difference did not reach statistical significance. Conclusion: The Birmingham resurfacing prosthesis does not appear to reduce femoral neck bone mineral density in comparison to the normal femoral neck bone density. We conclude that femoral neck fractures are unlikely to be due to stress shielding related to the prosthesis.     

What we have learned about long-term cementless fixation from autopsy retrievals

The current authors summarize insights gained from their experience studying uncemented porous-coated femoral and acetabular hip replacement components retrieved at autopsy. For femoral components, autopsy studies showed that osseointegration occurs over an average 35% of the porous surface with the most predictable ingrowth occurring near the termination of the porous coating. The bone ingrowth pattern causes a predictable bone remodeling pattern: an overall decrease in bone mineral content (mean, 23% loss) occurring on a gradient with the most loss proximally and the least distally. This pattern occurs regardless of the implant's level of coating, mirrors the gradient of strain reduction, and because it is recognized easily, can be used by orthopaedists to distinguish between components with and without bone ingrowth on standard radiographs. Studies also have shown that femoral bone loss is more related to the characteristics of the implanted femur than any other variable. On the acetabular side, bone ingrowth is more unpredictable and occurs randomly. Although the amount of acetabular bone ingrowth averaged 32%, it ranged from 3% to 84%. Unfortunately, a consistent pelvic remodeling pattern has not been associated with an implanted porous-coated uncemented cup. Consequently, orthopaedists cannot determine radiographically whether an acetabular component truly is bone ingrown. Overall, autopsy studies have contributed to a basic understanding of the histologic and radiographic appearance of osteointegrated porous-coated implants and have confirmed the durability of this three-dimensional fixation even in the presence of osteolysis.