The influence of design parameters on cortical strain distribution of a cementless titanium femoral stem

Early Radiographic Fit and Fill Analysis of a New Metaphyseal-Filling Triple Taper Stem Designed Using a Large Computed Tomography Scan Database

Background

Numerous cementless stems are available to maximize implant stability, fit, and survivorship in total hip arthroplasty. Recently, a new metaphyseal-filling triple-taper collared stem was designed using femoral morphology data obtained from over 1300 computed tomography scans. The purpose of this study was to evaluate the radiographic fit and fill of this new stem in the coronal and sagittal dimensions.

Methods

In this retrospective review, postoperative radiographs of patients receiving this new stem were analyzed in accordance with previously published fit and fill analyses. All radiographs were taken 6 weeks postoperatively. Means and standard deviations were reported for all fit and fill parameters.

Results

Fifty-nine hips were analyzed from 55 patients undergoing total hip arthroplasty. The coronal proximal fill was 85.02 ± 8.06%, and coronal distal fill was 75.21 ± 9.71%. The sagittal proximal fill was 86.51 ± 8.77%, and sagittal distal fill was 59.17 ± 8.66%. Mean calcar collar coverage was 80.64 ± 19.6% and all patients had full seating of the collar. Six cases (10.2%) had a collar length greater than the calcar length, with a mean collar overhang of 0.7 ± 0.4 mm.

Conclusions

This new stem demonstrated significant proximal fill in both the coronal and sagittal planes and validates the design intent of this implant. This is the first study to evaluate sagittal fit and fill of a femoral stem. Long-term follow-up is required to understand the clinical impact these fit and fill characteristics may have on patients' long-term outcomes.

Comparison of two metaphyseal-fitting (short) femoral stems in primary total hip arthroplasty: study protocol for a prospective randomized clinical trial with additional biomechanical testing and finite element analysis

BACKGROUND

Total hip replacement has recently followed a progressive evolution towards principles of bone- and soft-tissue-sparing surgery. Regarding femoral implants, different stem designs have been developed as an alternative to conventional stems, and there is a renewed interest towards short versions of uncemented femoral implants. Based on both experimental testing and finite element modeling, the proposed study has been designed to compare the biomechanical properties and clinical performance of the newly introduced short-stem Minima S, for which clinical data are lacking with an older generation stem, the Trilock Bone Preservation Stem with an established performance record in short to midterm follow-up.

METHODS/DESIGN

In the experimental study, the transmission of forces as measured by cortical surface-strain distribution in the proximal femur will be evaluated using digital image correlation (DIC), first on the non-implanted femur and then on the implanted stems. Finite element parametric models of the bone, the stem and their interface will be also developed. Finite element predictions of surface strains in implanted composite femurs, after being validated against biomechanical testing measurements, will be used to assist the comparison of the stems by deriving important data on the developed stress and strain fields, which cannot be measured through biomechanical testing. Finally, a prospective randomized comparative clinical study between these two stems will be also conducted to determine (1) their clinical performance up to 2 years' follow-up using clinical scores and gait analysis (2) stem fixation and remodeling using a detailed radiographic analysis and (3) incidence and types of complications.

DISCUSSION

Our study would be the first that compares not only the clinical and radiological outcome but also the biomechanical properties of two differently designed femoral implants that are theoretically classified in the same main category of cervico-metaphyseal-diaphyseal short stems. We can hypothesize that even these subtle variations in geometric design between these two stems may create different loading characteristics and thus dissimilar biomechanical behaviors, which in turn could have an influence to their clinical performance.

TRIAL REGISTRATION

International Standard Randomized Controlled Trial Number, ID: ISRCTN10096716 . Retrospectively registered on May 8 2018.

Load transfer and periprosthetic fractures after total hip arthoplasty: Comparison of periprosthetic fractures of femora implanted with cementless distal-load or proximal-load femoral components and measurement of the femoral strain at the time of implantation

BACKGROUND

Little is known about the causes and mechanisms underlying periprosthetic fractures around femoral components particularly in relation to the stem design. In an in vitro study 20 pairs of fresh cadaveric femora were loaded to fracture axially and transversally.

FINDINGS

When proximal femoral strain was measured at the time of impaction of cementless stems the load transfer was determined by the underlying anatomy rather than by the shape of the stem, so that the so-called "load transfer" properties - proximal or distal - ascribed to stem designs are a myth. The axial-load and the transverse-load model were then exposed to loads to failure (fracture) and showed a biphasic pattern throughout independent of the impact direction. In the second phase, the fracture phase proper, the bone behaved like a brittle solid. Failure occurred very rapidly within less than 5 milliseconds. The forces to failure were between 2 and 11 kN. Most of the fractures (82.5%) occurred above the stem tip.

INTERPRETATION

Note that the study was confined to early preosteointegration fractures. Neither the stem design nor the impact direction, i.e. on the knee or on the side of the hip, was related to the fracture morphology.

Finite Element Analysis of porously punched prosthetic short stem virtually designed for simulative uncemented Hip Arthroplasty

Background

There is no universal hip implant suitably fills all femoral types, whether prostheses of porous short-stem suitable for Hip Arthroplasty is to be measured scientifically.

Methods

Ten specimens of femurs scanned by CT were input onto Mimics to rebuild 3D models; their *stl format dataset were imported into Geomagic-Studio for simulative osteotomy; the generated *.igs dataset were interacted by UG to fit solid models; the prosthesis were obtained by the same way from patients, and bored by punching bears designed by Pro-E virtually; cements between femora and prosthesis were extracted by deleting prosthesis; in HyperMesh, all compartments were assembled onto four artificial joint style as: (a) cemented long-stem prosthesis; (b) porous long-stem prosthesis; (c) cemented short-stem prosthesis; (d) porous short-stem prosthesis. Then, these numerical models of Finite Element Analysis were exported to AnSys for numerical solution.

Results

Observed whatever from femur or prosthesis or combinational femora-prostheses, “Kruskal-Wallis” value p > 0.05 demonstrates that displacement of (d) ≈ (a) ≈ (b) ≈ (c) shows nothing different significantly by comparison with 600 N load. If stresses are tested upon prosthesis, (d) ≈ (a) ≈ (b) ≈ (c) is also displayed; if upon femora, (d) ≈ (a) ≈ (b) < (c) is suggested; if upon integral joint, (d) ≈ (a) < (b) < (c) is presented.

Conclusions

Mechanically, these four sorts of artificial joint replacement are stabilized in quantity. Cemented short-stem prostheses present the biggest stress, while porous short-stem & cemented long-stem designs are equivalently better than porous long-stem prostheses and alternatives for femoral-head replacement. The preferred design of those two depends on clinical conditions. The cemented long-stem is favorable for inactive elders with osteoporosis, and porously punched cementless short-stem design is suitable for patients with osteoporosis, while the porously punched cementless short-stem is favorable for those with a cement allergy. Clinically, the strength of this study is to enable preoperative strategy to provide acute correction and decrease procedure time.

Characterization of Femoral Component Initial Stability and Cortical Strain in a Reduced Stem-Length Design

BACKGROUND

Short-stemmed femoral components facilitate reduced exposure surgical techniques while preserving native bone. A clinically successful stem should ideally reduce risk for stress shielding while maintaining adequate primary stability for biological fixation. We asked (1) how stem-length changes cortical strain distribution in the proximal femur in a fit-and-fill geometry and (2) if short-stemmed components exhibit primary stability on par with clinically successful designs.

METHODS

Cortical strain was assessed via digital image correlation in composite femurs implanted with long, medium, and short metaphyseal fit-and-fill stem designs in a single-leg stance loading model. Strain was compared to a loaded, unimplanted femur. Bone-implant micromotion was then compared with reduced lateral shoulder short stem and short tapered-wedge designs in cyclic axial and torsional testing.

RESULTS

Femurs implanted with short-stemmed components exhibited cortical strain response most closely matching that of the intact femur model, theoretically reducing the potential for proximal stress shielding. In micromotion testing, no difference in primary stability was observed as a function of reduced stem length within the same component design.

CONCLUSION

Our findings demonstrate that within this fit-and-fill stem design, reduction in stem length improved proximal cortical strain distribution and maintained axial and torsional stability on par with other stem designs in a composite femur model. Short-stemmed implants may accommodate less invasive surgical techniques while facilitating more physiological femoral loading without sacrificing primary implant stability.

Full-field in vitro measurements and in silico predictions of strain shielding in the implanted femur after total hip arthroplasty

Alterations in bone strain as a result of implantation may contribute towards periprosthetic bone density changes after total hip arthroplasty. Computational models provide full-field strain predictions in implant–bone constructs; however, these predictions should be verified using experimental models wherever it is possible. In this work, finite element predictions of surface strains in intact and implanted composite femurs were verified using digital image correlation. Relationships were sought between post-implantation strain states across seven defined Gruen zones and clinically observed longer-term bone density changes. Computational predictions of strain distributions in intact and implanted femurs were compared to digital image correlation measurements in two regions of interest. Regression analyses indicated a strong linear correlation between measurements and predictions (R = 0.927 intact, 0.926 implanted) with low standard error (standard error = 38 µε intact, 26 µε implanted). Pre- to post-operative changes in measured and predicted surface strains were found to relate qualitatively to clinically observed volumetric bone density changes across seven Gruen zones: marked proximal bone density loss corresponded with a 50%−64% drop in surface strain, and slight distal density changes corresponded with 4%−14% strain increase. These results support the use of digital image correlation as a pre-clinical tool for predicting post-implantation strain shielding, indicative of long-term bone adaptations.

Spontaneous fracture of diaphyseal stem of S-ROM femoral prosthesis

We present two cases of spontaneous fractures of the S-ROM femoral stem prosthesis implanted by different surgeons within 5 years of implantation. Both the stems fractured in the mid-distal stem at the junction of the main body and the slotted portion. Both fractures affected the posterior tine only. Our aim in publication is to ensure that this is an isolated problem and not an under-reported phenomenon. We are not aware of any previous reports of spontaneous fracture of the distal stem.

Radiographic fit and fill analysis of a new second-generation proximally coated cementless stem compared to its predicate design

The purpose of this study was to compare in vivo fit and fill analysis of a new second-generation proximally coated cementless stem compared to its predicate design. This prospective trial of 100 total hip arthroplasties compared specific radiographic "Fit and Fill" parameters between the two designs. Fit type was assessed by comparing the type of canal fill. Post-operative fill parameters such as mean stem-to-canal ratios and mean minimum and maximum gaps between the stems to the cortical bone in different sections and areas were compared. A significantly higher proportion of the second-generation stems had Type I fit (82% vs. 54%), had better stem to canal fill ratio in the middle (90.6% vs. 85.3%) and distal sections (88.1% vs. 78.6%) compared to the older design. The new second-generation stem design had a significantly better canal fit and distal canal fill in the medial and lateral portions.

In vitro assessment of Function Graded (FG) artificial Hip joint stem in terms of bone/cement stresses: 3D Finite Element (FE) study

BACKGROUND

Stress shielding in the cemented hip prosthesis occurs due to the mismatching in the mechanical properties of metallic stem and bone. This mismatching in properties is considered as one of the main reasons for implant loosening. Therefore, a new stem material in orthopedic surgery is still required. In the present study, 3D finite element modeling is used for evaluating the artificial hip joint stem that is made of Function Graded (FG) material in terms of joint stress distributions and stem length.

METHOD

3D finite element models of different stems made of two types of FG materials and traditional stems made of Cobalt Chromium alloy (CoCrMo) and Titanium alloy (Ti) were developed using the ANSYS Code. The effects on the total artificial hip joint stresses (Shear stress and Von Mises stresses at bone cement, Von Mises stresses at bone and stem) due to using the proposed FG materials stems were investigated. The effects on the total artificial hip joint system stresses due to using different stem lengths were investigated.

RESULTS

Using FG stem (with low stiffness at stem distal end and high stiffness at its proximal end) resulted in a significant reduction in shear stress at the bone cement/stem interface. Also, the Von Mises stresses at the bone cement and stem decrease significantly when using FG material instead of CoCrMo and Ti alloy. The stresses' distribution along the bone cement length when using FG material was found to be more uniform along the whole bone cement compared with other stem materials. These more uniform stresses will help in the reduction of the artificial hip joint loosening rate and improve its short and long term performance.

CONCLUSION

FE results showed that using FG stem increases the resultant stresses at the femur bone (reduces stress shielding) compared to metallic stem. The results showed that the stem length has significant effects on the resultant shear and Von Mises stresses at bone, stem and bone cement for all types of stem materials.

DIFFERENTIAL THERMOGRAPHY FOR EXPERIMENTAL, FULL-FIELD STRESS ANALYSIS OF HIP ARTHROPLASTY

A hip prosthesis implant produces a significant deviation in the stress pattern compared with the physiologic condition. In this work, the stress patterns are evaluated experimentally on synthetic femora, by means of thermoelastic stress analysis. Two factors have been considered: stem implantation and head offset. Stress maps were obtained using differential thermography and correlated to these factors.

Thermoelastic stress maps have demonstrated to be sensitive to the implant and the head offset. In detail, the standard deviation of stresses can reduce from –5% to –50% (with reference to the physiologic one), depending on stem design; peak stresses change their position or disappear for different implant position or press-fitting, the sensitivity of average stresses to the offset is at least equal to 0.07 MPa/mm.

On the whole, a methodology was developed, allowing the experimental evaluation and comparison of the stress distributions produced by different implants.

Bone stresses before and after insertion of two commercially available distal ulnar implants using finite element analysis

Distal ulnar arthroplasty is becoming a popular treatment option for disorders of the distal radioulnar joint; however, few studies have investigated how load transfer in the ulna is altered after insertion of an implant. The purpose of our study was to compare bone stresses before and after insertion of two commercially available cemented distal ulnar implants: an implant with a titanium stem and an implant with a cobalt chrome stem. Appropriately sized implants of both types were inserted into eight previously validated subject-specific finite element models, which were created by using information derived from computed tomography scans. The von Mises stresses were compared at eight different regions pre- and post-implantation. The bone stresses with the titanium stem were consistently closer to the pre-implantation stresses than with the cobalt chrome stem. For the loading situation and parameters investigated, results of these models show that insertion of the E-Centrix® ulnar Head may result in less stress shielding than the SBI uHead™ stem. Future studies are required to investigate other implant design parameters and loading conditions that may affect the predicted amount of stress shielding.

Surgical treatment options in patients with impaired bone quality

BACKGROUND

Bone quality should play an important role in decision-making for orthopaedic treatment options, implant selection, and affect ultimate surgical outcomes. The development of decision-making tools, currently typified by clinical guidelines, is highly dependent on the precise definition of the term(s) and the appropriate design of basic and clinical studies. This review was performed to determine the extent to which the issue of bone quality has been subjected to this type of process.

QUESTIONS/PURPOSES

We address the following issues: (1) current methods of clinically assessing bone quality; (2) emerging technologies; (3) how bone quality connects with surgical decision-making and the ultimate surgical outcome; and (4) gaps in knowledge that need to be closed to better characterize bone quality for more relevance to clinical decision-making.

METHODS

PubMed was used to identify selected papers relevant to our discussion. Additional sources were found using the references cited by identified papers.

RESULTS

Bone mineral density remains the most commonly validated clinical reference; however, it has had limited specificity for surgical decision-making. Other structural and geometric measures have not yet received enough study to provide definitive clinical applicability. A major gap remains between the basic research agenda for understanding bone quality and the transfer of these concepts to evidence-based practice.

CONCLUSIONS

Basic bone quality needs better definition through the systematic study of emerging technologies that offer a more precise clinical characterization of bone. Collaboration between basic scientists and clinicians needs to improve to facilitate the development of key questions for sound clinical studies.

Minimal stress shielding with a Mallory-Head titanium femoral stem with proximal porous coating in total hip arthroplasty

BACKGROUND

As longevity of cementless femoral components enters the third decade, concerns arise with long-term effects of fixation mode on femoral bone morphology. We examined the long-term consequences on femoral remodeling following total hip arthroplasty with a porous plasma-sprayed tapered titanium stem.

METHODS

Clinical data and radiographs were reviewed from a single center for 97 randomly selected cases implanted with the Mallory-Head Porous femoral component during primary total hip arthroplasty. Measurements were taken from preoperative and long-term follow-up radiographs averaging 14 years postoperative. Average changes in the proximal, middle and diaphyseal zones were determined.

RESULTS

On anteroposterior radiographs, the proximal cortical thickness was unchanged medially and the lateral zone increased 1.3%. Middle cortical thickness increased 4.3% medially and 1.2% laterally. Distal cortical thickness increased 9.6% medially and 1.9% laterally. Using the anteroposterior radiographs, canal fill at 100 mm did not correlate with bony changes at any level (Spearman's rank correlation coefficient of -0.18, 0.05, and 0.00; p value = 0.09, 0.67, 0.97). On lateral radiographs, the proximal cortical thickness increased 1.5% medially and 0.98% laterally. Middle cortical thickness increased 2.4% medially and 1.3% laterally. Distal cortical thickness increased 3.5% medially and 2.1% laterally. From lateral radiographs, canal fill at 100 mm correlated with bony hypertrophy at the proximal, mid-level, and distal femur (Spearman's rank correlation coefficient of 0.85, 0.33, and 0.28, respectively; p value = 0.001, 0.016, and 0.01, respectively).

CONCLUSION

Stress shielding is minimized with the Mallory-Head titanium tapered femoral stem with circumferential proximal plasma-sprayed coating in well-fixed and well-functioning total hip arthroplasty. Additionally, the majority of femora demonstrated increased cortical thickness in all zones around the stem prosthesis.

LEVEL OF EVIDENCE

Therapeutic Level III.

The effect of modular tapered fluted stems on proximal stress shielding in the human femur

The purpose of this study was to show a change in proximal femur surface strains following total hip arthroplasty and after the addition of BoneSource hydroxyapatite bone cement in the proximal region of an instrumented femur and to measure the surface strain on the proximal body. Seven third-generation composite femurs (Pacific Research Laboratories, Vashon, Wash) were instrumented with 12 uniaxial strain gages, 6 gages on the anterior face, and 6 gages on the posterior face of each femur. All femurs exhibited stress shielding since the strains in the proximal region were drastically reduced. There was a large decrease in strain in the mid-shaft region and small changes in strain in the distal region. The surface strains on the modular implant were relatively low.

Alterations in femoral strain following hip resurfacing and total hip replacement

Bone surface strains were measured in cadaver femora during loading prior to and after resurfacing of the hip and total hip replacement using an uncemented, tapered femoral component. In vitro loading simulated the single-leg stance phase during walking. Strains were measured on the medial and the lateral sides of the proximal aspect and the mid-diaphysis of the femur. Bone surface strains following femoral resurfacing were similar to those in the native femur, except for proximal shear strains, which were significantly less than those in the native femur. Proximomedial strains following total hip replacement were significantly less than those in the native and the resurfaced femur. These results are consistent with previous clinical evidence of bone loss after total hip replacement, and provide support for claims of bone preservation after resurfacing arthroplasty of the hip.

RAPID MANUFACTURING SYSTEM OF ORTHOPEDIC IMPLANTS

This study, aimed the development of a methodology for rapid manufacture of orthopedic implants simultaneously with the surgical intervention, considering two potential applications in the fields of orthopedics: the manufacture of anatomically adapted implants and implants for bone loss replacement. This work innovation consists on the capitation of the in situ geometry of the implant by direct capture of the shape using an elastomeric material (polyvinylsiloxane) which allows fine detail and great accuracy of the geometry. After scanning the elastomeric specimen, the implant is obtained by machining using a CNC milling machine programmed with a dedicated CAD/CAM system. After sterilization, the implant is able to be placed on the patient. The concept was developed using low cost technology and commercially available. The system has been tested in an in vivo hip arthroplasty performed on a sheep. The time increase of surgery was 80 minutes being 40 minutes the time of implant manufacturing. The system developed has been tested and the goals defined of the study achieved enabling the rapid manufacture of an implant in a time period compatible with the surgery time.

Segmentation of the canine population in different femoral morphological groups

OBJECTIVE

The incidence of femoral traumatology and hip dysplasia shows the need to design canine specific femoral implants in veterinary surgery. A good knowledge of femoral morphology, and particularly of intra-species variability, is required to develop a well-adapted canine femoral intramedullary implant. The aim of this study is to evaluate the morphological variability of the canine femur and to propose a segmentation of this population.

PROCEDURE

This study proposes different possibilities for the segmentation of a canine population of 103 dogs of various common breeds in relation to their femoral morphology. These segmentations were obtained with a statistical methodology, which takes into account 24 measured and calculated morphological parameters of 206 canine femurs.

RESULTS

The segmentation of this canine population into four or six homogeneous groups related to the femoral morphology were the two most relevant solutions. The total length of the femur and the femoral head diameter were the best discriminant parameters for this segmentation.

CONCLUSION

Knowledge of the variability of the femoral morphology in the canine species and the possibility of splitting the canine population into homogeneous morphological groups are useful for the design of specific canine femoral implants. The femoral morphological profiles of each group constitute an essential database for fitting the best orthopedic implant to the bone.

Cemented fixation with PMMA or Bis-GMA resin hydroxyapatite cement: effect of implant surface roughness

Implant surface roughness is an important parameter governing the overall mechanical properties at the implant-cement interface. This study investigated the influence of surface roughness using polymethylmethcrylate (PMMA) and a Bisphenol-a-glycidylmethacyrlate resin-hydroxyapatite cement (CAP). Mechanical fixation at the implant-cement interface was evaluated in vitro using static shear and fatigue loading with cobalt chrome alloy (CoCr) dowels with different surface roughness preparations. Increasing surface roughness improved the mechanical properties at the implant-cement interface for both types of cement. CAP cement fixation was superior to PMMA under static and dynamic loading.