Influence of stem geometry on mechanics of cemented femoral hip components with a proximal bond

Cemented Stem Design in Total Hip Arthroplasty: Fixation Philosophies, Biomechanics, and an Updated Classification System

There is renewed interest in cemented femoral fixation in total hip arthroplasty in the United States, and to fully appreciate the evolution of cemented femoral stem designs, an understanding of their history and design rationale is essential. To adequately study the outcomes of modern-day designs, a comprehensive classification system is also necessary. The biomechanical principles, failure mechanisms, and clinical outcomes associated with various cemented femoral stems are described in this comprehensive review. In addition, an updated version of an existing classification system is described that incorporates the primary design characteristics which differentiate implants currently in use. In this classification, implants are categorized as taper-slip (Type I), which are subdivided by Anatomic (IA), Double-Taper (IB), and Triple-Taper (IC) with subclassification for Traditional and Line-to-Line implants. Composite beam (Type II) implants are similarly divided into Anatomic (IIA), Straight (IIB), and Wedge (IIC) with subclassification for Polished, Satin, or Roughened finish. This classification system provides a basis for comparing cemented femoral stems, thereby improving our understanding of the effect of design characteristics on survivorship to guide future advancements and improve clinical outcomes.

Analysis of hip joint loading during walking with different shoe types using instrumented total hip prostheses

Hip joint loads need careful consideration during postoperative physiotherapy after joint replacement. One factor influencing joint loads is the choice of footwear, but it remains unclear which footwear is favorable. The objective of the present study was to investigate the influence of footwear on hip joint loads in vivo. Instrumented hip endoprostheses were used for in vivo load measurements. The parameters resultant contact force (F_res), bending moment (M_bend) and torsional moment (M_tors) were evaluated during treadmill walking at 4 km/h with different shoe types. In general, footwear tended to increase hip joint loading, with the barefoot shoe having the least influence. F_res and M_bend were significantly increased during heel strike for all shoe types in comparison to barefoot walking, with everyday shoe (34.6%; p = 0.028 and 47%; p = 0.028, respectively) and men’s shoe (33.2%; p = 0.043 and 41.1%; p = 0.043, respectively) resulting in the highest changes. M_tors at AbsMax was increased by all shoes except for the barefoot shoe, with the highest changes for men’s shoe (+ 17.6%, p = 0.043) and the shoe with stiffened sole (+ 17.5%, p = 0.08). Shoes, especially those with stiff soles or elaborate cuishing and guiding elements, increase hip joint loads during walking. The influence on peak loads is higher for M_tors than for F_res and M_bend. For patients in which a reduction of hip joints loads is desired, e.g. during physiotherapy after recent surgery or to alleviate symptoms of osteoarthritis, low profile shoes with a flexible sole may be preferred over shoes with a stiff sole or elaborate cushioning elements.

Experimental and numerical static failure analyses of total hip replacement interfaces

In joint replacement surgery, the critical point in the success of cemented implants is the stabilisation between the implant and bone. The stronger the interlocking achieved on both the implant-cement interface and the cement-bone interface, the more durable is the surgical intervention. However, to date, it has not been possible to prevent aseptic loosening of hip implants, thus making a revision surgery necessary after a period of about 10 years. In this study, the tensile and shear strengths of the implant-cement and cement-bone interfaces and factors which can affect these strengths such as sandblasting parameters and implant material choice were investigated experimentally. The stresses on the total hip replacement interfaces were then determined via finite element analysis and the findings compared with the strength values obtained from the mechanical tests. The total hip replacement prosthesis was designed using SolidWorks software, and material properties and boundary conditions were modelled with the ANSYS Workbench software. Stresses due to the loads applied to the femur head had exceeded the highest tensile and shear strength values obtained by the classical test methods and damaged the contact surfaces in some regions. In light of these findings, the damage sites of the interfaces were determined.

Periprosthetic femoral fractures and trying to avoid them: what is the contribution of femoral component design to the increased risk of periprosthetic femoral fracture?

AIMS

Periprosthetic femoral fractures (PFF) following total hip arthroplasty (THA) are devastating complications that are associated with functional limitations and increased overall mortality. Although cementless implants have been associated with an increased risk of PFF, the precise contribution of implant geometry and design on the risk of both intra-operative and post-operative PFF remains poorly investigated. A systematic review was performed to aggregate all of the PFF literature with specific attention to the femoral implant used.

PATIENTS AND METHODS

A systematic search strategy of several journal databases and recent proceedings from the American Academy of Orthopaedic Surgeons was performed. Clinical articles were included for analysis if sufficient implant description was provided. All articles were reviewed by two reviewers. A review of fundamental investigations of implant load-to-failure was performed, with the intent of identifying similar conclusions from the clinical and fundamental literature.

RESULTS

In total 596 articles were initially identified, with 34 being eligible for analysis. Aggregate analysis of 1691 PFFs in 342 719 primary THAs revealed a significantly higher number of PFFs with cementless femoral implants (p < 0.001). Single-wedge and double-wedge (fit-and-fill) femoral implants were associated with a threefold increase in PFF rates (p < 0.001) compared with anatomical, fully coated and tapered/rounded stems. Within cemented stems, loaded-taper (Exeter) stems were associated with more PFFs than composite-beam (Charnley) stems (p = 0.004). Review of the fundamental literature revealed very few studies comparing cementless component designs.

CONCLUSION

Very few studies within the PFF literature provide detailed implant information. Cementless implants, specifically those of single-wedge and double-wedge, have the highest PFF rates in the literature, with most investigations recommending against their use in older patients with osteoporotic bone. This review illustrates the need for registries and future PFF studies to record implant name and information for future analysis. Furthermore, future biomechanical investigations comparing modern implants are needed to clarify the precise contribution of implant design to PFF risk. Cite this article: Bone Joint J 2017;99-B(1 Supple A):50-9.

Musculoskeletal-based finite element analysis of femur after total hip replacement

This article evaluates the effect of stress variation on adult femur following total hip replacement using musculoskeletal-based finite element analysis. The aim was to study the changes in stress distribution in the femur after total hip replacement by providing simulated in vivo loading and boundary conditions. The loading and boundary conditions were generated using a musculoskeletal modelling software ‘AnyBody’ and were applied on femur model, generated from the computed tomography (CT) scan data for standing posture of male patient. The results showed considerable variation in stress distribution pattern in the femur before and after total hip replacement, the metallic implant taking major loads of human body and transferring very less loads to the femur.

Influence of time in-situ and implant type on fixation strength of cemented tibial trays - a post mortem retrieval analysis

BACKGROUND

Loosening of the tibial tray is cited as the most common cause of failure in total knee arthroplasty but the mechanism remains unclear. Post mortem specimens provide a unique opportunity to investigate the clinical condition.

METHODS

Twenty two cemented components were serially retrieved in situ at autopsy from a university clinic. They were investigated for mechanical stability by pull-out, which was related to cement morphology and bone quality from CT scans, and to polyethylene wear by score analysis. Implants were grouped into three types: a particular fixed bearing design (n=8), a particular rotating platform design (n=5) and other mixed designs (n=9).

FINDINGS

Trends were observed for pull-out force to decrease with time in situ and increase with cement penetration but was unrelated to bone density or polyethylene wear. For the fixed bearing implants decreasing pull-out strength was related to an increasing proportion of failure at the bone-cement interface. For the mixed designs the opposite was observed. The rotating platform implants failed at the implant-cement interface.

INTERPRETATION

The analysis demonstrated that interface failure is dependent on the implant design, but that both the stem and the bone interfaces weaken with time in situ. Published findings for laboratory implantations have demonstrated that greater cement penetration improves fixation and this was reflected for clinical samples in this study.

Functional interface micromechanics of 11 en-bloc retrieved cemented femoral hip replacements

BACKGROUND AND PURPOSE

Despite the longstanding use of micromotion as a measure of implant stability, direct measurement of the micromechanics of implant/bone interfaces from en bloc human retrievals has not been performed. The purpose of this study was to determine the stem-cement and cement-bone micromechanics of functionally loaded, en-bloc retrieved, cemented femoral hip components.

METHODS

11 fresh frozen proximal femurs with cemented implants were retrieved at autopsy. Specimens were sectioned transversely into 10-mm slabs and fixed to a loading device where functional torsional loads were applied to the stem. A digital image correlation technique was used to document micromotions at stem-cement and cement-bone interfaces during loading.

RESULTS

There was a wide range of responses with stem-cement micromotions ranging from 0.0006 mm to 0.83 mm (mean 0.17 mm, SD 0.29) and cement-bone micromotions ranging from 0.0022 mm to 0.73 mm (mean 0.092 mm, SD 0.22). There was a strong (linear-log) inverse correlation between apposition fraction and micromotion at the stem-cement interface (r(2) = 0.71, p < 0.001). There was a strong inverse log-log correlation between apposition fraction at the cement-bone interface and micromotion (r(2) = 0.85, p < 0.001). Components that were radiographically well-fixed had a relatively narrow range of micromotions at the stem-cement (0.0006-0.057 mm) and cement-bone (0.0022-0.029 mm) interfaces.

INTERPRETATION

Minimizing gaps at the stem-cement interface and encouraging bony apposition at the cement-bone interface would be clinically desirable. The cement-bone interface does not act as a bonded interface in actual use, even in radiographically well-fixed components. Rather, the interface is quite compliant, with sliding and opening motions between the cement and bone surfaces.

Novel methods to study functional loading micromechanics at the stem-cement and cement-bone interface in cemented femoral hip replacements

We have developed a technique to directly observe the micromechanics of the stem-cement and cement-bone interfaces of cemented femoral stems under physiologically relevant loading conditions. Thick transverse sections of a stem-cement-femur construct were fixed to the base of a test frame. Ante- and retro-verting torques were applied to the femoral stem by screwing the stem (via a pair of through holes) to an axle, which was turned using a lever arm actuated by the test frame cross-head. The surface of each transverse section was serially digitally imaged during loading. The displacements of the stem, cement and bone were determined using digital image correlation. These data were then used to calculate the relative displacements across the interfaces. This method provides a path to more thorough understanding of load-transfer from femoral stem to femur.

The condition of the cement mantle in femoral hip prosthesis implantations--a post mortem retrieval study

Despite numerous studies demonstrating the characteristics of the optimal cement mantle in joint replacement, the clinical state of the cement mantle is rarely assessed. A random sample of 214 cemented implanted femoral hip components was retrieved post mortem from Hamburg, Germany, and sectioned to investigate the quality of the cement mantle. The most common observation made in at least one measured region per retrieval was debonding (82% of stems), followed by a thin cement mantle (74%), stem-bone contact (48%), soft tissue at the stem interface (44%), no cement-bone interdigitation (30%), a gap at the stem interface (28%), voids in the cement (22%) and cracks and blood in the cement mantle (<10%). 21% of stems demonstrated complete debonding of the interface. However, distributions of all other defects were local, with less than 10% of stems demonstrating any imperfection in more than 21% of the regions assessed. No progressive damage was observed with implantation duration. The results suggest that current implantation technique may be adequate for proper implant function over the service life in the older patient population. However, for younger and more active patients, perfection of the cementation technique is crucial, particularly in modern implant systems such as resurfacing. The frequency of almost all defects could be further reduced by careful implantation technique, providing the increased service life necessary for the ever younger, more physically demanding, patient population.

Effect of antibiotic loading on the shear strength at the stem-cement interface (Shear strength of antibiotic loaded cement)

The purpose of this study was to investigate the effects of addition of antibiotics into cement powder on the shear properties of the cement-metal interface. The approach involved adding 800 mg of teicoplanin to 40 g bone cement powder in the t-800 group, 1,600 mg teicoplanin in the t-1,600 group, and no antibiotic in the control group. Industrially prepared bone cement containing 500 mg of gentamicin was used as group g-500. Each group consisted of ten samples. Cement-metal interfaces were produced using metal discs with porous surfaces (1 microm) and templates at the third minute. Shear stability of specimens was measured in a material testing machine. The ANOVA test was used for comparison between the mean shear results of each group. Results showed that mean shear stress to failure values were 12.28+/-3.35 MPa for the control group, 11.72+/-3.09 MPa for the t-800 group, 13.25+/-2.36 MPa for the t-1,600 group and 13.09+/-2.58 MPa for the g-500 group. No statistically significant differences were found between results of the groups. Results of the study have proven that addition of 1,600 mg of teicoplanin or 500 mg gentamycin in 40 g of bone cement does not decrease the shear strength at the cement-metal interface significantly on the day of application.

The effect of cross-sectional stem shape on the torsional stability of cemented implant components

Stability of a cemented implant, once the stem-cement interface has debonded, is reliant upon stem geometry and surface finish. There are relatively few studies addressing the effect of cross-sectional stem shape on cemented implant fixation. The purpose of this investigation was to compare the torsional stability of five different stem cross-sectional shapes-circular, oval, triangular, rectangular with rounded edges, and rectangular with sharp edges-under monotonically increasing and cyclic loading conditions. Seven samples of each stem geometry were tested. Stems were potted in bone cement and loaded to 5 deg of rotation. For monotonic loading, torque was applied at a constant rate of 2.5 deg/min. For cyclic loading, a sine wave torque pattern was applied, with a maximum magnitude that began at 4.5 Nm for 1500 cycles and then increased by 2.25 Nm every 1500 cycles until 5 deg of rotation. The rectangular stem with the sharp edges always provided the greatest resistance to torque, followed by the rectangular with rounded edges, triangular, oval, and circular. These results, including the effects of sharp corners, may differ for modes of loading other than torsion. These experimental results support the findings of earlier finite element models, indicating stem shape has a significant effect on resistance to torsional loading.

Three-dimensional computer-aided design based design sensitivity analysis and shape optimization of the stem using adaptive p-method

The number of stem designs for total hip arthroplasty is increasing, and occasionally design changes have yielded unexpected clinical results. At present, we are not able to clearly identify which parameter of the stem is most important, and the optimum value of many parameters. The goals of this study were to identify which parameter is most important, to understand the effect of design change, and to find the optimum stem shape. For this purpose, we used adaptive p-method together with three-dimensional computer-aided design software program for the design sensitivity analysis (DSA) and shape optimization of the stem. The results suggested that increasing the lateral and medial width of the distal cross-section together with decreasing the medial-lateral width and the medial radius of the distal cross-section from the default value would lead to a decrease in the largest maximum principal stress of the distal cement. The medial width of middle cross-section, however, was not so simple. The result of DSA suggested that decreasing this parameter from the default value decreased the stress in the distal cement, but the optimum shape was obtained by increasing this parameter. The method used in this study will assist our engineers and surgeons in the process of modifying and optimizing the stem design.

Pre-coated femoral components in hybrid total hip arthroplasty

We present the medium-term results of hybrid total hip arthroplasties using pre-coated stems with a second-generation cementing technique. The 128 hips in 111 patients (18 men and 93 women) were followed up at a mean of 11 years after surgery. The mean age at the time of surgery was 61 years. Both components of one hip were removed at ten months after surgery for infection. None of the other 127 femoral components showed possible, probable, or definite loosening at the most recent follow-up. Five acetabular components were revised for aseptic loosening, recurrent dislocation, or displacement of the polyethylene liner from the metal shell. The mean Harris hip score at follow-up was 84 points. A pre-coated femoral component with a second-generation cementing technique provides good clinical function and survival in the medium term.

The effect of muscle loading on the simulation of bone remodelling in the proximal femur

A large number of finite element analyses of the proximal femur rely on a simplified set of muscle and joint contact loads to represent the boundary conditions of the model. In the context of bone remodelling analysis around hip implants, muscle loading affects directly the spatial distribution of the remodelling signal. In the present study we performed a sensitivity analysis on the effect of different muscle loading configurations on the outcome of the bone remodelling simulation. An anatomical model of the femur with the implanted stem in place was constructed using the CT data of the Visible Human Project dataset of the National Institute of Health. The model was loaded with three muscle force configurations with increasing level of complexity. A strain adaptive remodelling rule was employed to simulate the post-operative bone changes around the implant stem and the results of the simulation were assessed quantitatively in terms of the bone mineral content changes in 18 periprosthetic regions of interest. The results showed considerable differences in the amount of bone loss predicted between the three cases. The simplified models generally predicted more pronounced bone loss. Although the overall remodelling patterns observed were similar, the bone conserving effect of additional muscle forces in the vicinity of their areas of attachment was clear. The results of this study suggest that the loading configuration of the FE model does play an important role in the outcome of the remodelling simulation.

Computer simulation on fatigue behavior of cemented hip prostheses: a physiological model

This paper is concerned with the investigation on the fatigue failure of implant fixation by numerical approaches. A computer algorithm based on finite element analysis and continuum damage mechanics was proposed to quantify the fatigue damage rate of cement mantle under physiological conditions. In examining the interfacial debonding effect, the interface elements were introduced at cement-stem interfaces and calibrated with the increase of loading cycles. Current results reveal that the major sites for failure initiation are in the proximal anterior-medial regions and at the distal prosthesis tip, which clearly demonstrate the same failure scenario as observed in clinical studies. Such fatigue failures not only result in the corruption of cement-stem interfaces, but also greatly affect the cement stress distribution and the damage rate in subsequent loading cycles. Another significant result is that the predicted damage rate increases steadily with gait cycles. This trend in damage development is consistent with the findings obtained from fatigue tests available in literature. It is anticipated that presented methodology can serve as a pre-clinical validation of cemented hip prostheses.

Selection of fixation devices in proximal femur rotational osteotomy: clinical complications and finite element analysis

OBJECTIVE

To compare the postoperative stability between large cancellous screws fixation and dynamic hip screws fixation in transtrochanteric rotational osteotomy.

DESIGN

Finite element analysis was designed to validate the clinical outcomes of fixation failure in transtrochanteric rotational osteotomy.

BACKGROUND

From 1997 to 2000, transtrochanteric rotational osteotomy had been done in 20 osteonecrotic hips. Fixation with large cancellous screws as suggested by Sugioka had been used in the first five hips. Unfortunately, fixation failures were encountered in all of the five cases. The fixation mode was then shifted to a combination of plate and screws, no fixation failure had been observed postoperatively. A finite element analysis was designed to investigate the causes of fixation failure.

METHODS

Finite element models for four dynamic hip screw instrumented femora and four large screws instrumented femora simulating four different levels of bone cut were created. The von Mises stress distributions of each model were analyzed and compared for a loading condition simulating single-legged stance.

RESULTS

Analysis results confirmed our clinical observation that fixation with screws had higher stress concentration on the proximal femur. Improper bone cut, especially those that are too distal from the intertrochanteric line, had higher risks of fixation failure.

CONCLUSIONS

Transtrochanteric rotational osteotomy is a technically demanding procedure such that surgical principles should be abided carefully to avoid catastrophic complications.

RELEVANCE

Finite element analysis results demonstrated that dynamic hip screw fixation provides better stability and prevents fixation failure that corresponds with the clinical observation.

THA loading arising from increased femoral anteversion and offset may lead to critical cement stresses

Aseptic loosening of artificial hip joints is believed to be influenced by the design and orientation of the implant. It is hypothesised that variations in implant anteversion and offset lead to changes in the loading of the proximal femur, causing critical conditions to both the bone and cement. The goal of this study was therefore to analyse the role of these parameters on loading, bone strains and cement stresses in total hip arthroplasty (THA). A validated musculo-skeletal model was used for the analysis of muscle and joint contact forces during walking and stair climbing. Two different anteversion angles (4 degrees vs. 24 degrees ) and prostheses offsets (standard vs. long) were analysed. The loads for each case were applied to a cemented THA finite element model. Generally, stair climbing caused higher bone strains and cement stresses (max. +25%) than walking. Variations in anteversion and offset caused changes in the loading environment, bone strain distribution and cement stresses. Compared to the standard THA configuration, cement stresses were raised by increasing anteversion (max. +52%), offset (max. +5%) and their combination (max. +67%). Femoral anteversion, offset and their combination may therefore lead to an increased risk of implant loosening. Analyses of implant survival should consider this as a limiting factor in THA longevity. In clinical practice, implant orientation, especially in regard to pre- and post-operative anteversion, should be considered to be more critical.

Finite element analysis of the cervico-trochanteric stemless femoral prosthesis

OBJECTIVE

To investigate the biomechanical performance of a newly designed cervico-trochanteric stemless prosthesis by comparing the stress distribution with that of the traditional stem-type porous-coated anatomic prosthesis.

DESIGN

Three-dimensional finite element models were created for the intact femur, cervico-trochanteric implanted femur and porous-coated anatomic implanted femur. The stress distributions on the femur and the implant were compared. The effects of using two or three screws fixation for the cervico-trochanteric implanted femur were also investigated.

BACKGROUND

Local bone loss after implantation of traditional stem-type prostheses remains an unsolved problem during the long-term application of total hip replacement. The stress shielding effect and osteolysis were thought to be the two main factors that result in local bone loss after prosthesis implantation. In order to eliminate the mechanical and the biological causes of bone loss after total hip arthroplasty, a newly designed stemless femoral prosthesis was investigated.

METHODS

Three-dimensional finite element models were created for the intact, cervico-trochanteric (with two or three fixation screws), and porous-coated anatomic implanted femora with the geometry of a standardized composite femur. Analysis was performed for a loading condition simulating the single-legged stance. The von Mises stress distributions of each model were analyzed and compared.

RESULTS

The results can be summarized as follows: (1) Von Mises stress in the proximal, medial femur for the cervico-trochanteric implanted model was higher than that of the intact model and the porous-coated anatomic implanted model; (2) stress-shielding effect of the cervico-trochanteric models (with two or three fixation screws) were eliminated as compared with the porous-coated anatomic model; (3) no obvious difference in von Mises stress distribution for the cervico-trochanteric implanted model with two or three fixation screws.

CONCLUSIONS

The cervico-trochanteric femoral prosthesis may reduce the stress-shielding effect of the proximal femur and achieve a more physiological stress distribution on the proximal femur than that of the porous-coated anatomic prosthesis.

RELEVANCE

The new concept of cervico-trochanteric stemless prosthesis has proven to possess several advantages based on the current results, and may be an alternative for traditional stem-type prostheses in future clinical applications.

How much can a vibrational diagnostic tool reveal in total hip arthroplasty loosening?

OBJECTIVE

To investigate how much information a vibrational technique can reveal regarding the loosening of the femoral component of a total hip arthroplasty.

DESIGN

Numerical modal analysis using finite element method and computer simulation.

BACKGROUND

Existing work suggested that this technique is capable of revealing total hip arthroplasty loosening. However, the potentials of this technique are not fully studied yet. There is a need to establish the limitation of this technique.

METHODS

Numerical modal analysis was conducted to obtain the natural frequencies and mode shapes of femoral models under free vibration. An absent interface portion was hypothesized at specified locations the data from modal analysis were then used in a computer simulation to visualize the vibration diagnosis.

RESULTS

The effects of interface failure on the femoral component stiffness vary with the locations and sizes of failure. Nominal critical, reliable, and nondetectable levels of interface failure are found.

CONCLUSION

When the size of failure is more than one-third of the stem length, this tool is able to provide a reliable diagnosis. The critical detectable interface failure size is one-fifth of the stem length. It was found that the motion at higher harmonics is the most sensitive to the interface failure. This tool is unlikely to differentiate the slight differences in natural frequencies that occur when the interface failure length is less than one-sixth of the stem length. It is likely to fail in detecting failures located at the stem central portion regardless of their length.

RELEVANCE

A numerical study of the vibration tool is important prior to pursuing clinical trials of the tool and may guide the characterizations involved in the trials.

Precoated femoral component in primary hybrid total hip arthroplasty: results at a mean 10-year follow-up

This is a mid-term report at 10 years' mean follow-up of a study of a precoated femoral component used in primary hybrid total hip arthroplasty (THA). Of an original cohort of 98 hips undergoing THA performed by one surgeon, 75 hips in 65 patients (mean age, 67 years) were prospectively followed up for 7 to 12 years (mean, 10 years). All hips had the same porous coated acetabular component and a precoated femoral component (with an oval cross-section) implanted using Simplex bone cement (Howmedica, Rutherford, NJ). There was no femoral component loosening or revision. Two acetabular components in patients with rheumatoid arthritis and protrusio acetabulae had radiographic loosening; however, only 1 was symptomatic and was revised. Acetabular osteolysis was seen in 4 hips (5.3%), and minor femoral osteolysis was seen in 3 hips (4%). Used in this manner in this patient population, precoating is not detrimental to successful fixation at 10 years' mean follow-up of primary hybrid THA.

The John Charnley Award: Three-dimensional analysis of the cement mantle in total hip arthroplasty

Cemented fixation of the femoral stem is the gold standard for patients older than 60 years. The importance of reliably achieving an adequate cement mantle has been shown in many studies. Currently, inspection and grading of plain radiographs is the accepted method for study of the cement mantle. However, the reliability of plain radiographs for this purpose has been questioned. In addition, the interobserver agreement of current grading systems has been shown to be limited. A new in vitro method of cement mantle analysis is described. Plastic replicas of six contemporary stems were implanted into femurs from cadavers. The specimens were imaged with a computed tomography scanner. Detailed, computer-assisted analysis of mantle thickness was done. Comparisons were made between designs. A subset was compared with standard radiographs. Plain radiographs overestimated thickness and underestimated the deficiencies. There was significant variability in the mantle produced by the different designs. Commonly used designs had deficiencies in their mantles by standard criteria despite proper surgical technique. The importance of being fully acquainted with the particular implant one uses is emphasized by these results. This is a valuable technique for investigation of the effects on the cement mantle of implant design, surgical technique, and patient anatomy.

A mixed mode fracture toughness test of bone-biomaterial interfaces

Tissue-biomaterial interfacial bonding plays a significant role in the success of biomaterials used for load-bearing orthopedic and dental prostheses. The objective of this study was to develop a physically sound and practically effective technique for assessment of the strength of bone-biomaterial interfaces under mixed mode loading. A single-edge notched sandwich specimen was developed for this purpose, wherein a bilayer specimen comprising the interface between tissue and biomaterial was sandwiched between two holders and loaded under mixed modes. First, a closed form solution was derived for the sandwich specimen under the assumption of linear elasticity, based on a general solution for sandwich structures reported in the literature. Then, a correction factor was determined for the solution using finite element models to compensate for errors induced by finite interlayer thickness. Moreover, using the same FEA models, it was found that crack closure may occur when the shear component is dominant at the crack. However, its effects were estimated to be limited and negligible. Furthermore, as an example, the strength of a bone/dental cement interface under different loading modes was tested using this sandwich technique. It is expected that the mixed mode technique can provide an effective means for investigators to study the mechanical integrity of bone-biomaterial interfaces under complex loading conditions.

Hybrid total hip arthroplasty with a precoated offset stem. Four to nine-year results

BACKGROUND

Use of modern cementing techniques for fixation of femoral components in total hip arthroplasty has had excellent clinical and radiographic results in most patients. However, several authors have described early loosening of femoral components with roughened and precoated finishes. The purpose of this study was to examine the performance of the precoated Iowa stem, which has increased offset, and to compare the results with those of another cemented precoated femoral component with standard offset used at our institution.

METHODS

We carried out a prospective analysis of 102 primary hybrid total hip arthroplasties (a cementless acetabular component and a cemented femoral component) performed with use of the Iowa femoral component in ninety-five patients at our institution. The Iowa stem was used in hips that required greater offset than is available with standard stems as determined by preoperative templating. The average age of the patients at the time of the index procedure was sixty-nine years. Sixteen patients (seventeen hips) died before the forty-eight-month minimum follow-up period had elapsed. Two patients were lost to follow-up, and radiographic follow-up was incomplete for one. The mean duration of clinical and radiographic follow-up of the remaining eighty-two hips in the seventy-six surviving patients was sixty-five months (range, forty-eight to 104 months).

RESULTS

The average preoperative Harris hip score of 47 points (range, 16 to 69 points) improved to an average of 87 points (range, 24 to 100 points) at the time of the review. Two hips underwent femoral component revision. Four femoral stems were radiographically loose at an average of thirty-four months. Femoral osteolysis was seen in five hips (6 percent) at an average of fifty-four months postoperatively. No acetabular component was revised because of aseptic loosening. According to Kaplan-Meier analysis, the seven-year survival rate, with an end point of femoral revision, osteolysis, or stem debonding, was 90.6 percent (95 percent confidence interval, 0.87 to 0.94).

CONCLUSIONS

The prevalence of revision, osteolysis, and loosening after total hip arthroplasty with the Iowa femoral component at our institution was higher than that seen in our series of Harris Precoat stems, which had a survival rate of 98.4 percent (95 percent confidence interval, 0.97 to 1.00) at ten years with the same end points. The design of the Iowa stem may make it difficult to achieve a good cement mantle, and, in combination with the geometry and increased offset of the stem, may compromise the long-term survival of this cemented femoral component.

Loosening rates and bone lysis with rough finished and polished stems

In the senior author's 27-year experience in cementing femoral components, stems with smooth and rough surfaces were implanted. Prospective data have been kept on all patients allowing extensive long term followup. The polished Charnley stem was implanted in 168 hips from 1971 to 1975. To date, only four (2.4%) stems have required revision for aseptic loosening and four others for fracture. None showed significant bone lysis. The experience with the T-28 and TR-28 allows comparison of stems of similar geometry, but with different surface finishes. From 1972 to 1977, 209 polished T-28 stems were inserted and to date 18 (14 for fracture and four for loosening, 1.9%) have required revision. None had associated bone lysis. From 1977 to 1982, 227 second generation TR-28 matte surface finish stems (Ra 30) were implanted. Five required revision, three (2.2%) with major bone lysis. Radiographic review revealed minimal lysis in four (1.9%) polished T-28 stems and in 11 (4.9%) TR-28 stems, three of which showed major bone destruction. From 1980 to 1993, 1061 Iowa stems were implanted. In 1986 proximal precoat was added, which required additional roughening of the surface. In 1995 a 1.5% early loosening rate was reported with significant bone lysis, complicating revision. By 1998 34 (3.2%) stems have required revision, all associated with significant bone loss. Revision of one of 12 original Iowa (Ra 30) and 12 of 22 (Ra 80) grit blasted stems was needed before 5 years postoperatively. These early failures prompted the author to return to a polished stem with a geometry almost identical to the Charnley stem.