Femoral neck fracture following hip resurfacing

Biomechanical properties of artificial bones made by Sawbones: A review

Biomedical engineers and physicists frequently use human or animal bone for orthopaedic biomechanics research because they are excellent approximations of living bone. But, there are drawbacks to biological bone, like degradation over time, ethical concerns, high financial costs, inter-specimen variability, storage requirements, supplier sourcing, transportation rules, etc. Consequently, since the late 1980s, the Sawbones® company has been one of the world's largest suppliers of artificial bones for biomechanical testing that counteract many disadvantages of biological bone. There have been many published reports using these bone analogs for research on joint replacement, bone fracture fixation, spine surgery, etc. But, there exists no prior review paper on these artificial bones that gives a comprehensive and in-depth look at the numerical data of interest to biomedical engineers and physicists. Thus, this paper critically reviews 25 years of English-language studies on the biomechanical properties of these artificial bones that (a) characterized unknown or unreported values, (b) validated them against biological bone, and/or (c) optimized different design parameters. This survey of data, advantages, disadvantages, and knowledge gaps will hopefully be useful to biomedical engineers and physicists in developing mechanical testing protocols and computational finite element models.

Accuracy of guide wire placement for femoral neck stabilization using 3D printed drill guides

BACKGROUND

The goal of stabilization of the femoral neck is to limit morbidity and mortality from fracture. Of three potential methods of fixation, (three percutaneous screws, the Synthes Femoral Neck System, and a dynamic hip screw), each requires guide wire positioning of the implant(s) in the femoral neck and head. Consistent and accurate positioning of these systems is paramount to reduce surgical times, stabilize fractures effectively, and reduce complications. To help expedite surgery and achieve ideal implant positioning in the geriatric population, we have developed and validated a surgical planning methodology using 3D modelling and printing technology.

METHODS

Using image processing software, 3D surgical models were generated placing guide wires in a virtual model of an osteoporotic proximal femur sawbone. Three unique drill guides were created to achieve the optimal position for implant placement for each of the three different implant systems, and the guides were 3D printed. Subsequently, a trauma fellowship trained orthopedic surgeon used the 3D printed guides to position 2.8 mm diameter drill bit tipped guide wires into five osteoporotic sawbones for each of the three systems (fifteen sawbones total). Computed Tomography (CT) scans were then taken of each of the sawbones with the implants in place. 3D model renderings of the CT scans were created using image processing techniques and the displacement and angular deviations at guide wire entry to the optimal sawbone model were measured.

RESULTS

Across all three percutaneous screw guide wires, the average displacement was 3.19 ± 0.12 mm and the average angular deviation was 4.10 ± 0.17^o. The Femoral Neck System guide wires had an average displacement of 1.59 ± 0.18 mm and average angular deviation of 2.81 ± 0.64^o. The Dynamic Hip Screw had an average displacement of 1.03 ± 0.19 mm and average angular deviation of 2.59 ± 0.39^o.

CONCLUSION

The use of custom 3D printed drill guides to assist with the positioning of guide wires proved to be accurate for each of the three types of surgical strategies. Guides which are used to place more than 1 guide wire may have lower positional accuracy, as the guide may shift during multiple wire insertions. We believe that personalized point of care drill guides provide an accurate intraoperative method for positioning implants into the femoral neck.

Peri-prosthetic bone remodeling of hydroxyapatite-coated compaction short stem was not affected by stem alignment

Background

To improve implant survival through accelerated early bone remodeling during total hip arthroplasty (THA), hydroxyapatite (HA) is widely used as a bioactive coating, which is believed to enhance initial fixation by osseointegration. We aimed to investigate the relationship between stem insertion alignment and postoperative bone mineral density (BMD) changes in patients with full hydroxyapatite-coated (HA) compaction short stem and short tapered-wedge stem.

Methods

This retrospective cohort study enrolled 115 consecutive patients (115 joints) undergoing THA using the full HA compaction short (n = 59) and short tapered-wedge (n = 56) stems. Stem alignment, including anteversion, valgus, and anterior tilt were measured by a three-dimensional template using computed tomography data. Post-operative peri-prosthetic BMD was measured by dual-energy X-ray absorptiometry. The relationship between stem alignment and BMD changes in the stems was analyzed.

Results

Patterns of peri-prosthetic BMD changes were similar in both groups. Stem insertion alignments of anteversion, valgus, and anterior tilt were different between the two stem types. Stem alignment of valgus and anterior tilt did not affect peri-prosthetic BMD in either of the stem type. An absolute anteversion difference between stem anteversion and original canal anteversion caused significant peri-prosthetic BMD loss in Gruen zones one and seven in the tapered-wedge stem. However, stem alignment of absolute anteversion difference did not affect BMD changes in the HA compaction stem.

Conclusions

Peri-prosthetic bone remodeling remained unaffected by stem alignment after THA with the new short full HA compaction stem.

Birmingham mid-head resection periprosthetic fractures: Case report

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Birmingham Mid-Head Resection (BMHR) prostheses are implants used to treat hip osteoarthritis in young patients with the goal of preserving as much bone stock as possible.

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Compared to total hip arthroplasty, BMHR prostheses offer advantages such as lower rate of dislocation and lower rate of infection and sepsis.

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The revision rate of BMHR devices is higher than that of conventional total hip arthroplasty devices. Furthermore, these revisions occur earlier with the former.

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The main complication of BMHR prostheses are femoral neck fratures, which are also the most common reason for revision.

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Patterns of BMHR periprosthetic fractures usually described in the literature are subcapital and transcervical ones. Nevertheless, more patterns can be found and different therapeutic attitudes must be adopted.

Introduction

Total hip arthroplasty in young patients can cause problems when it comes to choosing a suitable implant. The Birmingham Mid-Head Resection prosthesis (BMHR) offers the option of preserving bone stock despite its poor quality in the femoral neck. Femoral neck fractures are a known complication of hip resurfacing prostheses and the main reason for revision surgery. Retaining the femoral implant may cause difficulties in osteosynthesis carried out with implants habitually used for intertrochanteric or femoral neck fractures (e.g., a screw-plate device or a cephalomedullary nail) [2].

Presentation of case

We present the case of an 81-year-old patient who underwent surgery for right hip osteoarthritis and received a Birmingham Mid-Head Resection prosthesis. She had no history of previous fractures due to frailty. While on holiday, she sustained a periprosthetic fracture as a result of a fall. While the most common fracture in resurfacing prosthesis is produced in the femoral neck, in this case the patient sustained a subtrochanteric fracture. This fracture was surgically treated by means of open reduction and internal fixation with trochanteric plate and three cerclages.

Discussion

Olsen et al. [8] described two fracture patterns: transcervical vertical shear type and subcapital type. In our patient's case, the fracture pattern was different to those described, as the fracture started in the cervical area and reached the subtrochanteric area. This change in the standard periprosthetic fracture pattern leads to a change in the therapeutic attitude that must be adopted.

Conclusion

BMHR prostheses are metal-on-metal implants that resulted from the development of the standard resurfacing prostheses used to treat hip osteoarthritis in young patients with the goal of preserving as much bone stock as possible.

In this paper we will describe a rare complication in this type of prosthesis and how it was surgically treated after reviewing the available literature.

Computer-assisted navigation in Birmingham hip resurfacing: A case report

Component malpositioning during Birmingham hip resurfacing increases the risk for component wear, metallosis, component loosening, and the likelihood of dislocation and revision surgery. Computer-assisted navigation can increase the accuracy to which components are placed, and the utilization of this technology in Birmingham hip resurfacing is increasing. The present report summarizes the accuracy of acetabular component positioning in a Birmingham hip resurfacing case utilizing navigation. Intraoperative C-arm fluoroscopy following the use of the navigation tool confirmed excellent seating, positioning, and stability of the acetabular component. In addition, post-operative antero-posterior radiographs confirmed device accuracy and revealed a stable joint with no evidence of acetabular loosening or femoral fracture. Computer-assisted navigation may therefore be an effective tool to improve the accuracy of component positioning during Birmingham hip resurfacing.

Biomechanical Analysis Using FEA and Experiments of Metal Plate and Bone Strut Repair of a Femur Midshaft Segmental Defect

This investigation assessed the biomechanical performance of the metal plate and bone strut technique for fixing recalcitrant nonunions of femur midshaft segmental defects, which has not been systematically done before. A finite element (FE) model was developed and then validated by experiments with the femur in 15 deg of adduction at a subclinical hip force of 1 kN. Then, FE analysis was done with the femur in 15 deg of adduction at a hip force of 3 kN representing about 4 x body weight for a 75 kg person to examine clinically relevant cases, such as an intact femur plus 8 different combinations of a lateral metal plate of fixed length, a medial bone strut of varying length, and varying numbers and locations of screws to secure the plate and strut around a midshaft defect. Using the traditional “high stiffness” femur-implant construct criterion, the repair technique using both a lateral plate and a medial strut fixed with the maximum possible number of screws would be the most desirable since it had the highest stiffness (1948 N/mm); moreover, this produced a peak femur cortical Von Mises stress (92 MPa) which was below the ultimate tensile strength of cortical bone. Conversely, using the more modern “low stiffness” femur-implant construct criterion, the repair technique using only a lateral plate but no medial strut provided the lowest stiffness (606 N/mm), which could potentially permit more in-line interfragmentary motion (i.e., perpendicular to the fracture gap, but in the direction of the femur shaft long axis) to enhance callus formation for secondary-type fracture healing; however, this also generated a peak femur cortical Von Mises stress (171 MPa) which was above the ultimate tensile strength of cortical bone.

Stem anteversion mismatch to the anatomical anteversion causes loss of periprosthetic bone density after THA

The short tapered-wedge stem is popular worldwide because it potentially preserves more bone stock during total hip arthroplasty (THA). However, stem version mismatch may affect physiological stress distribution. In this study, we analyzed the correlations between periprosthetic bone mineral density (BMD) changes and anteversion mismatch in patients who underwent THA using a short tapered-wedge stem. The study included 44 patients (44 joints) who underwent THA with a Tri-Lock stem. At baseline and at 6 and 24 months postoperatively, the BMDs in the seven Gruen zones were evaluated using dual-energy X-ray absorptiometry. BMD changes and stem alignment, that is, anteversion and stem anteversion mismatch to the anatomical canal anteversion, were analyzed. Significant negative correlations were found between BMD changes and absolute anteversion error in Gruen zones 1 and 7 at 6 and 24 months postoperatively (zone 1, 6M; RR= -0.48, p < 0.001) (zone 7, 6M; RR= -0.46, p = 0.002) (zone 1, 24M; RR= -0.47, p = 0.001) (zone 7, 24M; RR= -0.40, p = 0.007). We further demonstrated that excessive stem anteversion mismatch to the anatomical canal anteversion causes stem point contact with the cortical bone in the distal portion and affected proximal periprosthetic BMD loss after THA. We recommend that the native anatomical anteversion angle should be used as a reference for inserting the tapered-wedge stems.

CUSTOMIZED GUIDE FOR FEMORAL COMPONENT POSITIONING IN HIP RESURFACING ARTHROPLASTY

Objective:

To prove the accuracy of a customized guide developed according to our method.

Methods:

This customized guide was developed from a three-dimensional model of proximal femur reconstructed using computed tomography data. Based on the new technique, the position of the guide pin insertion was selected and adjusted using the reference of the anatomical femoral neck axis. The customized guide consists of a hemispheric covering designed to fit the posterior part of the femoral neck. The performance of the customized guide was tested in eight patients scheduled for total hip arthroplasty. The stability of the customized guide was assessed by orthopedic surgeons. An intraoperative image intensifier was used to assess the accuracy.

Results:

The customized guide was stabilized with full contact and was fixed in place in all patients. The mean angular deviations in relation to the what was planned in anteroposterior and lateral hip radiographs were 0.5º ± 1.8º in valgus and 1.0º ± 2.4º in retroversion, respectively.

Conclusion:

From this pilot test, the authors suggest that the proposed technique could be applied as a customized guide to the positioning device for hip resurfacing arthroplasty with acceptable accuracy and user-friendly interface. Level of Evidence IV, Cases Series.

Biomechanical optimization of subject-specific implant positioning for femoral head resurfacing to reduce fracture risk

Peri-prosthetic femoral neck fracture after femoral head resurfacing can be either patient-related or surgical technique-related. The study aimed to develop a patient-specific finite element modelling technique that can reliably predict an optimal implant position and give minimal strain in the peri-prosthetic bone tissue, thereby reducing the risk of peri-prosthetic femoral neck fracture. The subject-specific finite element modelling was integrated with optimization techniques including design of experiments to best possibly position the implant for achieving minimal strain for femoral head resurfacing. Sample space was defined by varying the floating point to find the extremes at which the cylindrical reaming operation actually cuts into the femoral neck causing a notch during hip resurfacing surgery. The study showed that the location of the maximum strain, for all non-notching positions, was on the superior femoral neck, in the peri-prosthetic bone tissue. It demonstrated that varus positioning resulted in a higher strain, while valgus positioning reduced the strain, and further that neutral version had a lower strain.

Influence of PEEK Coating on Hip Implant Stress Shielding: A Finite Element Analysis

Stress shielding is a well-known failure factor in hip implants. This work proposes a design concept for hip implants, using a combination of metallic stem with a polymer coating (polyether ether ketone (PEEK)). The proposed design concept is simulated using titanium alloy stems and PEEK coatings with thicknesses varying from 100 to 400 μm. The Finite Element analysis of the cancellous bone surrounding the implant shows promising results. The effective von Mises stress increases between 81 and 92% for the complete volume of cancellous bone. When focusing on the proximal zone of the implant, the increased stress transmission to the cancellous bone reaches between 47 and 60%. This increment in load transferred to the bone can influence mineral bone loss due to stress shielding, minimizing such effect, and thus prolonging implant lifespan.

Resurfacing head size and femoral fracture: Are registry conclusions on head size justified?

BACKGROUND

Joint registries report that peri-prosthetic fractures are the most common reason for early revision of a hip resurfacing arthroplasty (HRA) and are twice as likely with small implant sizes. However, a national survey found peri-prosthetic fracture to be strongly associated with surgical accuracy. We therefore asked whether the force required to induce a peri-prosthetic fracture: (1) was significantly lower when using smaller implants and (2) correlated to the size of implant used, when surgery was performed accurately.

METHODS

To ensure an adequate power, we calculated our sample size from pilot data. Forty-four femurs were tested in two experiments. The first experiment tested femurs with either a small (48 mm) or a large (54 mm) HRA implant. The second involved testing femurs with a range of implant sizes. A rapid prototyped femur-specific guide ensured accurate implantation. Specimens were then vertically loaded in a servo-hydraulic testing machine till fracture. Displacement (mm) and force (N) required for fracture were recorded.

RESULTS

A median force of 1081 N was required to fracture specimens implanted with small 48-mm heads, while 1134 N was required when a 54-mm head was used (U = 77, z = -0.054, p = 0.957). Implant head size and force required to fracture were not related, r = 0.12, p = 0.63.

CONCLUSIONS

The force required to induce a resurfacing peri-prosthetic fracture was not related to the size of the implant. The increased failure rate seen in all registries is unlikely to be directly the result of this single variable. Correctly performed resurfacing arthroplasty is highly resistant to fracture.

Birmingham Mid-Head Resection Periprosthetic Fracture

Total hip arthroplasty in the young leads to difficult choices in implant selection. Until recently bone conserving options were not available for younger patients with deficient femoral head bone stock. The novel Birmingham Mid-Head Resection (BMHR) device offers the option of bone conserving arthroplasty in spite of deficient femoral head bone stock. Femoral neck fracture is a known complication of standard resurfacing arthroplasty and is the most common reason for revision. It is unknown whether this remains to be the case for the BMHR neck preserving implants. We report a case of a 57-year-old male, who sustained a periprosthetic fracture following surgery with a BMHR arthroplasty. This paper illustrates the first reported case of a BMHR periprosthetic fracture. The fracture pattern is spiral in nature and reaches to the subtrochanteric area. This fracture pattern is different from published cadaveric studies, and clinicians using this implant should be aware of this as revision is likely to require a distally fitting, rather than a metaphyseal fitting stem. We have illustrated the surgical technique to manage this rare complication.

Birmingham Hip Resurfacing: A Single Surgeon Series Reported at a Minimum of 10 Years Follow-Up

We report outcomes on 120 Birmingham Hip Resurfacings (BHRs) (mean age 50 years) at a minimum of ten-years follow-up. Cases were performed by one surgeon and included his learning curve. Six hips were revised, with no revisions for infection, dislocation, or adverse reaction to metal debris. Ten-year survival was 94.2% (95% confidence interval (CI) 88.8%-98.7%) for all revisions and 96.1% (95% CI 91.5%-99.8%) for revisions for aseptic loosening. Gender (P = 0.463) and head size (P = 0.114) did not affect revision risk. Mean post-operative Harris hip score was 84.0. Contrary to previous independent reports, good outcomes into the second decade were achieved with the BHR in both men and women. Longer term follow-up will confirm whether these promising outcomes in women continue.

The impact of proximal femoral morphology on failure strength with a mid-head resection short-stem hip arthroplasty

Mid-head resection short-stem hip arthroplasty is a conservative alternative to conventional total hip replacement and addresses proximal fixation challenges in patients not suitable for hip resurfacing. It is unclear whether proximal femoral morphology impacts the ultimate failure load of mid-head resection implanted femurs, thus the aim of this study was to investigate the effect of native neck-shaft angle (NSA) and coronal implant alignment on proximal femoral strength. In total, 36 synthetic femurs with two different proximal femoral morphologies were utilized in this study. Of them, 18 femurs with a varus NSA of 120° and 18 femurs with a valgus NSA of 135° were each implanted with a mid-head resection prosthesis. Femurs within the two different femoral morphology groups were divided into three equal coronal implant alignment groups: 10° valgus, 10° varus or neutral alignment. Prepared femurs were tested for stiffness and to failure in axial compression. There was no significant difference in stiffness nor failure load between femurs implanted with valgus-, varus- or neutrally aligned implants in femurs with a NSA of 120° (p = 0.396, p = 0.111, respectively). Femurs implanted in valgus orientation were significantly stiffer and failed at significantly higher loads than those implanted in varus alignment in femurs with a NSA of 135° (p = 0.001, p = 0.007, respectively). A mid-head resection short-stem hip arthroplasty seems less sensitive to clinically relevant variations of coronal implant alignment and may be more forgiving upon implantation in some femoral morphologies, however, a relative valgus component alignment is recommended.

Evaluation of a patient specific femoral alignment guide for hip resurfacing

A novel alternative to conventional instrumentation for femoral component insertion in hip resurfacing is a patient specific, computed tomography based femoral alignment guide. A benchside study using cadaveric femora was performed comparing a custom alignment guide to conventional instrumentation and computer navigation. A clinical series of twenty-five hip resurfacings utilizing a custom alignment guide was conducted by three surgeons experienced in hip resurfacing. Using cadaveric femora, the custom guide was comparable to conventional instrumentation with computer navigation proving superior to both. Clinical femoral component alignment accuracy was 3.7° and measured within ± 5° of plan in 20 of 24 cases. Patient specific femoral alignment guides provide a satisfactory level of accuracy and may be a better alternative to conventional instrumentation for initial femoral guidewire placement in hip resurfacing.

Biomechanical analysis using infrared thermography of a traditional metal plate versus a carbon fibre/epoxy plate for Vancouver B1 femur fractures

Traditional high-stiffness metal plates for Vancouver B1 femur shaft fractures below the tip of a hip implant can cause stress shielding, bone resorption, and implant loosening. This is the first study to compare the biomechanics of a traditional metal plate versus a low-stiffness carbon fibre/epoxy composite plate for this injury. A total hip replacement was implanted in two previously validated intact artificial femurs. Femurs were fitted with either a metal or composite plate and had a 5 mm fracture gap created to simulate a Vancouver B1 shaft fracture. Femurs were cyclically loaded using 5 Hz at 7° of adduction with an average axial load of 800 N (range = 400–1200 N). Overall mechanical stiffnesses and femur and plate thermographic stresses were obtained. Femur/metal plate stiffness (698 N/mm) was only 12% higher than femur/composite plate stiffness (625 N/mm). The femur with the composite plate had 22.7% higher combined average stress compared to the femur with the metal plate, having specific differences of 29.5% (anterior view), 33.9% (posterior view), 1.0% (medial view), and 26.4% (lateral view). The composite plate itself had an average 21.1% reduction in stress compared to the metal plate. The composite plate reduced stress shielding, yet provided adequate stiffness.

The biomechanical effect of notch size, notch location, and femur orientation on hip resurfacing failure

For hip resurfacing, this is the first biomechanical study to assess anterior and posterior femoral neck notching and femur flexion and extension. Forty-seven artificial femurs were implanted with the Birmingham hip resurfacing (BHR) using a range of notch sizes (0, 2, and 5 mm), notch locations (superior, anterior, and posterior), and femur orientations (neutral stance, flexion, and extension). Implant preparation was done using imageless computer navigation, and mechanical tests measured stiffness and strength. For notch size and location, in neutral stance the unnotched group had 1.9 times greater strength than the 5-mm superior notch group (4539 N versus 2423 N, p=0.047), and the 5-mm anterior notch group had 1.6 times greater strength than the 5-mm superior notch group, yielding a borderline statistical difference (3988 N versus 2423 N, p = 0.056). For femur orientation, in the presence of a 5-mm anterior notch, femurs in neutral stance had 2.2 times greater stiffness than femurs in 25° flexion (1542 N/mm versus 696 N/mm, p = 0.000). Similarly, in the presence of a 5-mm posterior notch, femurs in neutral stance had 2.8 times greater stiffness than femurs in 25° extension (1637 N/mm versus 575 N/mm, p = 0.000). No other statistical differences were noted. All femurs failed through the neck. The results have implications for BHR surgical techniques and recommended patient activities.

Strict component positioning is necessary in hip resurfacing

BACKGROUND

Hip resurfacing arthroplasty has some advantages, including improved metal-on-metal articulation, a lower dislocation rate and preserved femoral bone. This procedure is a surgical option for younger and more active patients with osteoarthritis and osteonecrosis of the femoral head. Although there have been some reports about the efficacy of this technique, others report serious complications caused by metal debris. Additionally, femoral neck preservation adversely decreases the head-neck ratio and results in postoperative impingement.

METHODS

We evaluated the range of motion after hip resurfacing with various component orientations and optimal component orientations to avoid postoperative impingement using computer simulations in 10 male patients with osteonecrosis.

RESULTS

The mean ranges of motion in flexion, extension, abduction, adduction and internal rotation at 90° of flexion were 92.4° ± 13.8°, 25.7° ± 13.8°, 38.0° ± 11.1°, 29.1° ± 10.0° and 20.9° ± 11.5°, respectively. The oscillation angle in flexion and extension motion was 118.1° ± 10.3°. More than 100° of flexion was acquired in 79 of 240 simulations (32.9 %), and more than 20° extension was acquired in 142 simulations (59.2 %). Combined anteversion was significantly correlated with maximal flexion and extension angles. The component safe zone to fulfill the range of motion criteria varied among patients, and 4 of 10 patients had no safe zone.

CONCLUSIONS

Postoperative impingement occurs relatively frequently in hip resurfacing because of preservation of the femoral neck and component malpositioning. The safe zone of the acetabular component to avoid postoperative impingement is very narrow. Greater care should be taken regarding patient selection, rigorous preoperative planning and accurate component positioning.

The First SICOT Oral Presentation Award 2011: imageless computer-assisted femoral component positioning in hip resurfacing: a prospective randomised trial

PURPOSE

The aim of the study was to evaluate the effects of imageless computer-assisted surgery (CAS) on the accuracy of positioning of the femoral component and on the short-term clinical outcome in hip resurfacing (HR) using a randomised prospective design.

METHODS

A total of 75 consecutive patients undergoing HR were randomly allocated to CAS and conventional implantation, respectively. Preoperatively and six months post-operatively standardised pelvic anteroposterior X-ray images, the total Western Ontario and McMaster Universities Osteoarthritis Index, the Harris Hip Score and the EQ-5D utility index were evaluated in a blinded manner. The primary end point of the study was a post-operative femoral component malpositioning in five degrees or more either varus or valgus absolute deviation from the planned stem shaft angle.

RESULTS

Patient demographics and algofunctional scores did not differ between the CAS and conventional implantation samples. Using CAS fewer femoral components were positioned in five or more degrees absolute deviation (4/37 vs 12/38, Fisher's exact p = 0.047; 95 % confidence interval for the primary end point's incidence difference: +3 %; +39 %); the respective incidences of five or more degrees of varus deviation were 0/37 vs 5/38. One conversion to a stemmed prosthesis (CAS group) was performed for periprosthetic femoral neck fracture. Radiological signs of superolateral femoral neck/implant impingement were observed in two cases (one CAS-based and one conventional implantation).

CONCLUSIONS

The accuracy of femoral HR component positioning was significantly improved using CAS. However, one major complication necessitated early revision in the CAS group at six months of observation. Apart from that adverse event no inter-group differences were observed for the short-term clinical outcome. Future studies need to address the clinical long-term relevance of CAS in HR.

Computer navigation experience in hip resurfacing improves femoral component alignment using a conventional jig

BACKGROUND

The use of computer navigation has been shown to improve the accuracy of femoral component placement compared to conventional instrumentation in hip resurfacing. Whether exposure to computer navigation improves accuracy when the procedure is subsequently performed with conventional instrumentation without navigation has not been explored. We examined whether femoral component alignment utilizing a conventional jig improves following experience with the use of imageless computer navigation for hip resurfacing.

MATERIALS AND METHODS

Between December 2004 and December 2008, 213 consecutive hip resurfacings were performed by a single surgeon. The first 17 (Cohort 1) and the last 9 (Cohort 2) hip resurfacings were performed using a conventional guidewire alignment jig. In 187 cases, the femoral component was implanted using the imageless computer navigation. Cohorts 1 and 2 were compared for femoral component alignment accuracy.

RESULTS

All components in Cohort 2 achieved the position determined by the preoperative plan. The mean deviation of the stem-shaft angle (SSA) from the preoperatively planned target position was 2.2° in Cohort 2 and 5.6° in Cohort 1 (P = 0.01). Four implants in Cohort 1 were positioned at least 10° varus compared to the target SSA position and another four were retroverted.

CONCLUSIONS

Femoral component placement utilizing conventional instrumentation may be more accurate following experience using imageless computer navigation.

Predictors of femoral neck fracture following hip resurfacing: a cadaveric study

We aimed to establish if radiological parameters, dual energy x-ray absorptiometry (DEXA) and quantitative CT (qCT) could predict the risk of sustaining a femoral neck fracture following hip resurfacing. Twenty-one unilateral fresh frozen femurs were used. Each femur had a plain digital anteroposterior radiograph, DEXA scan and qCT scan. Femurs were then prepared for a Birmingham Hip Resurfacing femoral component and loaded to failure. Results demonstrated that gender and qCT measurements showed strong correlation with failure load. QCT could be used as an individual measure to predict risk of post-operative femoral neck fracture. However, when qCT is unavailable; gender, pre-operative DEXA scan and Neck Width measurements can be used together to assess risk of post-operative femoral neck fracture in patients due to undergo hip resurfacing.

The biomechanics of three different fracture fixation implants for distal femur repair in the presence of a tumor-like defect

The femur is the most common long bone involved in metastatic disease. There is consensus about treating diaphyseal and epiphyseal metastatic lesions. However, the choice of device for optimal fixation for distal femur metaphyseal metastatic lesion remains unclear. This study compared the mechanical stiffness and strength of three different fixation methods. In 15 synthetic femurs, a spherical tumor-like defect was created in the lateral metaphyseal region, occupying 50% of the circumference of the bone. The defect was filled with bone cement and fixed with one of three methods: Group 1 (retrograde nail), Group 2 (lateral locking plate), and Group 3 (lateral nonlocking periarticular plate). Constructs were tested for mechanical stiffness and strength. There were no differences between groups for axial stiffness (Group 1, 1280 ± 112 N/mm; Group 2, 1422 ± 117 N/mm; and Group 3, 1403 ± 122 N/mm; p = 0.157) and offset torsional strength (Group 1, 1696 ± 628 N; Group 2, 1771 ± 290 N; and Group 3, 1599 ± 253 N; p = 0.816). In the coronal plane, Group 2 (296 ± 17 N/mm) had a higher stiffness than Group 1 (263 ± 17 N/mm; p = 0.018). In the sagittal plane, Group 1 (315 ± 9 N/mm) had a higher stiffness than Group 3 (285 ± 19 N/mm; p = 0.028). For offset torsional stiffness, Group 1 (256 ± 23 N/mm) had a higher value than Group 3 (218 ± 16 N/mm; p = 0.038). Group 1 had equivalent performance to both plating groups in two test modes, and it was superior to Group 3 in two other test modes. Since a retrograde nail (i.e. Group 1) would require less soft-tissue stripping in a clinical context, it may be the optimal choice for tumor-like defects in the distal femur.

Biomechanical stress maps of an artificial femur obtained using a new infrared thermography technique validated by strain gages

Femurs are the heaviest, longest, and strongest long bones in the human body and are routinely subjected to cyclic forces. Strain gages are commonly employed to experimentally validate finite element models of the femur in order to generate 3D stresses, yet there is little information on a relatively new infrared (IR) thermography technique now available for biomechanics applications. In this study, IR thermography validated with strain gages was used to measure the principal stresses in the artificial femur model from Sawbones (Vashon, WA, USA) increasingly being used for biomechanical research. The femur was instrumented with rosette strain gages and mechanically tested using average axial cyclic forces of 1500 N, 1800 N, and 2100 N, representing 3 times body weight for a 50 kg, 60 kg, and 70 kg person. The femur was oriented at 7° of adduction to simulate the single-legged stance phase of walking. Stress maps were also obtained using an IR thermography camera. Results showed good agreement of IR thermography vs. strain gage data with a correlation of R(2)=0.99 and a slope=1.08 for the straight line of best fit. IR thermography detected the highest principal stresses on the superior-posterior side of the neck, which yielded compressive values of -91.2 MPa (at 1500 N), -96.0 MPa (at 1800 N), and -103.5 MPa (at 2100 N). There was excellent correlation between IR thermography principal stress vs. axial cyclic force at 6 locations on the femur on the lateral (R(2)=0.89-0.99), anterior (R(2)=0.87-0.99), and posterior (R(2)=0.81-0.99) sides. This study shows IR thermography's potential for future biomechanical applications.

Effect of cementing technique and cement type on thermal necrosis in hip resurfacing arthroplasty--a numerical study

Femoral fractures within resurfacing implants have been associated with bone necrosis, possibly resulting from heat generated by cement polymerization. The amount of heat generated depends on cement mantle volume and type of cement. Using finite element analysis, the effect of cement type and volume on thermal necrosis was analyzed. Based on CT-data of earlier implantations, two different models were created: a thick mantle model, representing a low-viscosity "cement filling" technique, and a thin mantle model, representing a high viscosity "cement packing" technique. Six cement types were analyzed. The polymerization heat generation and its effect on bone necrosis were predicted. In the thin cement mantle models, no thermal necrosis was predicted. Thick cement mantle models produced thermal necrosis at the cement-bone interface depending on cement type. In the worst case, 6% of the bone at the cement-bone interface became necrotic, covering almost the entire cross-sectional area. The current findings suggest a potential thermal drawback of thick cement mantles, although it is unclear whether thermal bone necrosis significantly affects implant fixation or increases the fracture risk. Furthermore, our study showed distinct differences between the heat generated and resulting thermal damage caused by the various cement types.

Biomechanical measurements of axial crush injury to the distal condyles of human and synthetic femurs

Few studies have evaluated the ‘bulk’ mechanical properties of human longbones and even fewer have compared human tissue to the synthetic longbones increasingly being used by researchers. Distal femur fractures, for example, comprise about 6% of all femur fractures, but the mechanical properties of the distal condyles of intact human and synthetic femurs have not been well quantified in the literature. To this end, the distal portions of a series of 16 human fresh-frozen femurs and six synthetic femurs were prepared identically for mechanical testing. Using a flat metal plate, an axial ‘crush’ force was applied in-line with the long axis of the femurs. The two femur groups were statistically compared and values correlated to age, size, and bone quality. Results yielded the following: crush stiffness (human, 1545 ± 728 N/mm; synthetic, 3063 ± 1243 N/mm; p = 0.002); crush strength (human, 10.3 ± 3.1 kN; synthetic, 12.9 ± 1.7 kN; p = 0.074); crush displacement (human, 6.1 ± 1.8 mm; synthetic, 2.8 ± 0.3 mm; p = 0.000); and crush energy (human, 34.8 ± 15.9 J; synthetic, 18.1 ± 5.7 J; p = 0.023). For the human femurs, there were poor correlations between mechanical properties versus age, size, and bone quality (R2 ≤ 0.18), with the exception of crush strength versus bone mineral density (R2 = 0.33) and T-score (R2 = 0.25). Human femurs failed mostly by condyle ‘roll back’ buckling (15 of 16 cases) and/or unicondylar or bicondylar fracture (7 of 16 cases), while synthetic femurs all failed by wedging apart of the condyles resulting in either fully or partially displaced condylar fractures (6 of 6 cases). These findings have practical implications on the use of a flat plate load applicator to reproduce real-life clinical failure modes of human femurs and the appropriate use of synthetic femurs. To the authors’ knowledge, this is the first study to have done such an assessment on human and synthetic femurs.

A preliminary biomechanical assessment of a polymer composite hip implant using an infrared thermography technique validated by strain gage measurements

With the resurgence of composite materials in orthopaedic applications, a rigorous assessment of stress is needed to predict any failure of bone-implant systems. For current biomechanics research, strain gage measurements are employed to experimentally validate finite element models, which then characterize stress in the bone and implant. Our preliminary study experimentally validates a relatively new nondestructive testing technique for orthopaedic implants. Lock-in infrared (IR) thermography validated with strain gage measurements was used to investigate the stress and strain patterns in a novel composite hip implant made of carbon fiber reinforced polyamide 12 (CF/PA12). The hip implant was instrumented with strain gages and mechanically tested using average axial cyclic forces of 840 N, 1500 N, and 2100 N with the implant at an adduction angle of 15 deg to simulate the single-legged stance phase of walking gait. Three-dimensional surface stress maps were also obtained using an IR thermography camera. Results showed almost perfect agreement of IR thermography versus strain gage data with a Pearson correlation of R(2) = 0.96 and a slope = 1.01 for the line of best fit. IR thermography detected hip implant peak stresses on the inferior-medial side just distal to the neck region of 31.14 MPa (at 840 N), 72.16 MPa (at 1500 N), and 119.86 MPa (at 2100 N). There was strong correlation between IR thermography-measured stresses and force application level at key locations on the implant along the medial (R(2) = 0.99) and lateral (R(2) = 0.83 to 0.99) surface, as well as at the peak stress point (R(2) = 0.81 to 0.97). This is the first study to experimentally validate and demonstrate the use of lock-in IR thermography to obtain three-dimensional stress fields of an orthopaedic device manufactured from a composite material.

Hip resurfacing revision rates: radiological audit of risk factors

INTRODUCTION

We performed a retrospective radiological audit of the hip resurfacings carried out in our trust over a five-year period. Abnormal cup inclination angle (CIA) and stem shaft angle (SSA) are recognised risk factors for revision in hip resurfacing. Our aims were to identify the CIA and SSA for hip resurfacings in our trust, to determine the revision rate in a CIA of ≥60° and an SSA of >0° varus, thereby identifying a high risk group for close, long-term follow up.

METHODS

A total of 247 patients underwent hip resurfacing in our trust between April 2003 and March 2008. The CIA and SSA were recorded. Of the 247 patients, 26 were excluded as there were no appropriate radiographs and so results were analysed for 221 patients.

RESULTS

The mean CIA was 47.6°. Over a third of the patients (34%) had a CIA of >50° and 13% had >60°. The mean SSA was 1.4° varus. Over two-thirds of the patients (67%) had a varus SSA. There were six revisions but one was excluded as it was secondary to infection. The revision rate was 10% in patients with a CIA of ≥60° and 1% in those with a CIA of <60° (p=0.017), and 1% in a varus and 4% in a valgus SSA ((p)>0.05) respectively.

CONCLUSIONS

The measurement of the CIA and SSA in hip resurfacings has identified a high risk group for close long-term follow up. There is already a 10% revision rate in those patients with a CIA of >60°. Hip resurfacing may generate a large revision burden in the 'average' surgeon's hands and all hospitals/surgeons should review their radiological outcomes critically and identify those at risk of revision.

The biomechanical effect of changes in cancellous bone density on synthetic femur behaviour

Biomechanical researchers increasingly use commercially available and experimentally validated synthetic femurs to mimic human femurs. However, the choice of cancellous bone density for these artificial femurs appears to be done arbitrarily. The aim of the work reported in this paper was to examine the effect of synthetic cancellous bone density on the mechanical behaviour of synthetic femurs. Thirty left, large, fourth-generation composite femurs were mounted onto an Instron material testing system. The femurs were divided evenly into five groups each containing six femurs, each group representing a different synthetic cancellous bone density: 0.08, 0.16, 0.24, 0.32, and 0.48 g/cm3. Femurs were tested non-destructively to obtain axial, lateral, and torsional stiffness, followed by destructive tests to measure axial failure load, displacement, and energy. Experimental results yielded the following ranges and the coefficient of determination for a linear regression ( R2) with cancellous bone density: axial stiffness (range 2116.5–2530.6 N/mm; R2 = 0.94), lateral stiffness (range 204.3–227.8 N/mm; R2 = 0.08), torsional stiffness (range 259.9–281.5 N/mm; R2 = 0.91), failure load (range 5527.6–11 109.3 N; R2 = 0.92), failure displacement (range 2.97–6.49 mm; R2 = 0.85), and failure energy (range 8.79–42.81 J; R2 = 0.91). These synthetic femurs showed no density effect on lateral stiffness and only a moderate influence on axial and torsional stiffness; however, there was a strong density effect on axial failure load, displacement, and energy. Because these synthetic femurs have previously been experimentally validated against human femurs, these trends may be generalized to the clinical situation. This is the first study in the literature to perform such an assessment.

Hip resurfacing arthroplasty: a series of 140 consecutive hips with a minimum five year follow-up. A clinical, radiological and histological analysis

We reviewed the clinical and radiological outcome at a minimum of five years of 140 consecutive metal-on-metal hip resurfacing (HR) arthroplasties in 132 patients. The mean follow-up was 73 months (62 to 105). Revision of either component was defined as failure. The average Harris hip score (HHS) was 58.6 (25 to 88) preoperatively and 94.4 (60 to 100) at the latest follow-up (p<0.0005). Average University of California Los Angeles (UCLA) activity score was 3.1 (1 to 9) pre-operatively and 6.7 (2 to 10) post-operatively (p<0.0005). The survival after 6 years was 97.8% overall and 98.5% (excluding a post traumatic femoral neck fracture). These good medium-term results suggest that HR is an effective procedure in young and active patients.

Complications after metal-on-metal hip resurfacing arthroplasty

This article determines the incidence and cause of the complications commonly associated with metal-on-metal hip resurfacing implants and the proposed methods to prevent these complications. The literature available in PubMed was reviewed. Complication rates after hip resurfacing are low, and the procedure has shown both safety and efficacy in the hands of surgeons trained in specialized centers. Proper surgical technique can further reduce the incidence of femoral neck fracture, component loosening, and abnormal wear of the prosthesis. A more systematic detection of adverse local tissue reactions is needed to provide accurate assessments of their prevalence.

Birmingham hip resurfacing: a minimum follow-up of ten years

We report the survival, radiological and functional outcomes of a single surgeon series of his first 144 consecutive Birmingham hip resurfacing procedures (130 patients) at a minimum of ten years. There were ten revisions during this time. Although no patients were lost to follow-up some did not complete the scoring assessment or undergo radiological assessment at ten years. The ten-year survival for male patients was 98.0% (95% confidence interval 95.2 to 100). The ten-year survival for the total cohort with aseptic revision as the endpoint was 95.5% (95% confidence interval 91.8 to 99.0) and including revisions for sepsis was 93.5% (95% confidence interval 89.2 to 97.6). The median modified Oxford hip score at ten years was 4.2% (interquartile range 0 to 19) and the median University of California, Los Angeles score was 7.0 (interquartile range 5.0 to 8.0). This study confirms the midterm reports that metal-on-metal hip resurfacing using the Birmingham Hip provides a durable alternative to total hip replacement, particularly in younger male patients wishing to maintain a high level of function, with low risk of revision for at least ten years.

Metal-on-metal hip resurfacing: a critical review

In this article, a concise review of the current literature on metal-on-metal hip resurfacing (MoMHR) is given. In contrast to conventional total hip arthroplasty, older age, female sex and small femoral head sizes predispose to failure. Neck fracture and metal wear-related complications account for the most frequent reasons for re-operations. Although the long-term consequences of metal ion release remain unknown, the increasing prevalence of soft tissue related problems with potentially devastating functional consequences in this younger patient group are of concern. Outcome after revision for metal wear related failure of MoMHR is poor. In our opinion, patients with this device should be managed in dedicated centers with facilities for data collection and monitoring. The majority of proposed advantages of MoMHR cannot be supported by the published evidence.

A biomechanical investigation of implant alignment and femoral neck notching with the Birmingham Mid-Head Resection

The Birmingham Mid-Head Resection (BMHR) is a bone-conserving, short-stem alternative to hip resurfacing for patients with compromised femoral head anatomy. The current study examined the effect of coronal implant alignment and femoral neck notching on proximal femoral strength with the BMHR. Neither relative valgus nor varus implant alignment had a significant impact on femoral strength compared to neutrally aligned, matched, paired cadaveric specimens. A 5-mm superior neck notch significantly weakened BMHR-implanted synthetic femurs compared to unnotched controls, whereas a 2-mm notch did not significantly affect ultimate failure load. Relative valgus alignment had a protective effect on a full-cortical-thickness superior neck notch. Mid-head resection arthroplasty may be more forgiving to minor preparatory errors than a typical hip resurfacing.

The biomechanical analysis of three plating fixation systems for periprosthetic femoral fracture near the tip of a total hip arthroplasty

Background

A variety of techniques are available for fixation of femoral shaft fractures following total hip arthroplasty. The optimal surgical repair method still remains a point of controversy in the literature. However, few studies have quantified the performance of such repair constructs. This study biomechanically examined 3 different screw-plate and cable-plate systems for fixation of periprosthetic femoral fractures near the tip of a total hip arthroplasty.

Methods

Twelve pairs of human cadaveric femurs were utilized. Each left femur was prepared for the cemented insertion of the femoral component of a total hip implant. Femoral fractures were created in the femurs and subsequently repaired with Construct A (Zimmer Cable Ready System), Construct B (AO Cable-Plate System), or Construct C (Dall-Miles Cable Grip System). Right femora served as matched intact controls. Axial, torsional, and four-point bending tests were performed to obtain stiffness values.

Results

All repair systems showed 3.08 to 5.33 times greater axial stiffness over intact control specimens. Four-point normalized bending (0.69 to 0.85) and normalized torsional (0.55 to 0.69) stiffnesses were lower than intact controls for most comparisons. Screw-plates provided either greater or equal stiffness compared to cable-plates in almost all cases. There were no statistical differences between plating systems A, B, or C when compared to each other (p > 0.05).

Conclusions

Screw-plate systems provide more optimal mechanical stability than cable-plate systems for periprosthetic femur fractures near the tip of a total hip arthroplasty.

A custom-made guide-wire positioning device for hip surface replacement arthroplasty: description and first results

Background

Hip surface replacement arthroplasty (SRA) can be an alternative for total hip arthroplasty. The short and long-term outcome of hip surface replacement arthroplasty mainly relies on the optimal size and position of the femoral component. This can be defined before surgery with pre-operative templating. Reproducing the optimal, templated femoral implant position during surgery relies on guide wire positioning devices in combination with visual inspection and experience of the surgeon. Another method of transferring the templated position into surgery is by navigation or Computer Assisted Surgery (CAS). Though CAS is documented to increase accurate placement particularly in case of normal hip anatomy, it requires bulky equipment that is not readily available in each centre.

Methods

A custom made neck jig device is presented as well as the results of a pilot study.

The device is produced based on data pre-operatively acquired with CT-scan. The position of the guide wire is chosen as the anatomical axis of the femoral neck. Adjustments to the design of the jig are made based on the orthopedic surgeon's recommendations for the drill direction. The SRA jig is designed as a slightly more-than-hemispherical cage to fit the anterior part of the femoral head. The cage is connected to an anterior neck support. Four knifes are attached on the central arch of the cage. A drill guide cylinder is attached to the cage, thus allowing guide wire positioning as pre-operatively planned.

Custom made devices were tested in 5 patients scheduled for total hip arthroplasty. The orthopedic surgeons reported the practical aspects of the use of the neck-jig device. The retrieved femoral heads were analyzed to assess the achieved drill place in mm deviation from the predefined location and orientation compared to the predefined orientation.

Results

The orthopedic surgeons rated the passive stability, full contact with neck portion of the jig and knife contact with femoral head, positive. There were no guide failures. The jig unique position and the number of steps required to put the guide in place were rated 1, while the complexity to put the guide into place was rated 1-2. In all five cases the guide wire was accurately positioned. Maximum angular deviation was 2.9° and maximum distance between insertion points was 2.1 mm.

Conclusions

Pilot testing of a custom made jig for use during SRA indicated that the device was (1) successfully applied and user friendly and (2) allowed for accurate guide wire placement according to the preoperative plan.

Assessment of accuracy and reliability in preoperative templating for hip resurfacing arthroplasty

The current study investigated the accuracy and reliability of hip resurfacing component selection based on digital preoperative templating. Four surgeons made a template of preoperative radiographs on 2 occasions for acetabular and femoral components in 50 randomly selected hip resurfacing patients. Component selection reliability was variable among surgeons (kappa = 0.16-0.73) and fair between surgeons (kappa = 0.23-0.32). The average percentage of agreement for the acetabular component was 47% (range, 32%-64%) and for the femoral component was 54% (range, 38%-70%). Surgeons tended to underestimate implant size if the correct implant was not chosen (acetabular, 29%; femoral, 32%). Selection of an undersized femoral component may lead to femoral neck notching or varus implant alignment. This study emphasizes the need for intraoperative verification of preoperative templating results to ensure optimal implant selection in hip resurfacing.

The influence of the size of the component on the outcome of resurfacing arthroplasty of the hip

The survivorship of contemporary resurfacing arthroplasty of the hip using metal-on-metal bearings is better than that of first generation designs, but short-term failures still occur. The most common reasons for failure are fracture of the femoral neck, loosening of the component, osteonecrosis of the femoral head, reaction to metal debris and malpositioning of the component. In 2008 the Australian National Joint Registry reported an inverse relationship between the size of the head component and the risk of revision in resurfacing hip arthroplasty. Hips with a femoral component size of ≤ 44 mm have a fivefold increased risk of revision than those with femoral components of ≥ 55 mm irrespective of gender. We have reviewed the literature to explore this observation and to identify possible reasons including the design of the implant, loading of the femoral neck, the orientation of the component, the production of wear debris and the effects of metal ions, penetration of cement and vascularity of the femoral head. Our conclusion is that although multifactorial, the most important contributors to failure in resurfacing arthroplasty of the hip are likely to be the design and geometry of the component and the orientation of the acetabular component.

Increased strain in the femoral neck following insertion of a resurfacing femoral prosthesis

The cortical strains on the femoral neck and proximal femur were measured before and after implantation of a resurfacing femoral component in 13 femurs from human cadavers. These were loaded into a hip simulator for single-leg stance and stair-climbing. After resurfacing, the mean tensile strain increased by 15% (95% confidence interval (CI) 6 to 24, p = 0.003) on the lateral femoral neck and the mean compressive strain increased by 11% (95% CI 5 to 17, p = 0.002) on the medial femoral neck during stimulation of single-leg stance. On the proximal femur the deformation pattern remained similar to that of the unoperated femurs.

The small increase of strains in the neck area alone would probably not be sufficient to cause fracture of the neck However, with patient-related and surgical factors these strain changes may contribute to the risk of early periprosthetic fracture.

Two-year migration results of the ReCap hip resurfacing system-a radiostereometric follow-up study of 23 hips

There has been renewed interest for metal-on-metal hip resurfacing due to improved design and manufacturing of implants, better materials, and enhanced implant fixation. In contrast to conventional total hip replacements, only a few clinical hip resurfacing trials using radiostereometry (RSA) have been reported, and solely for the Birmingham hip resurfacing arthroplasty. The purpose of this RSA trial was to describe the migration pattern of a new hip resurfacing system (ReCap) within the first two years after primary surgery. Twenty-six patients underwent total hip replacement. The patients were followed-up for up to 24 months and were evaluated with the use of radiostereometric measurements. The prosthesis showed mean translations and rotation close to zero. Maximum translation was seen along the transverse axis in the medial direction (0.13 mm). No statistically significant translation or rotation was seen at two-years follow-up, (t-test, p <0.05, translation or rotation).

The effect of load application rate on the biomechanics of synthetic femurs

Biomechanical investigations are increasingly using commercially available synthetic femurs as surrogates for human cadaveric femurs. However, the rate of force application in testing these artificial femurs appears to be chosen arbitrarily without much consideration to their visco-elastic time-dependent nature. The aim of this study, therefore, was to examine the effect of loading rate on the mechanical behaviour of synthetic femurs. Ten left, medium, fourth-generation composite femurs (Model 3403, Pacific Research Laboratories, Vashon, WA, USA) were fixed distally into cement-filled steel cubic chambers for mounting into a mechanical tester. In randomized order, each of the ten femurs was loaded at rates of 1, 2.5, 5, 7.5, 10, 20, 30, 40, 50, and 60 mm/min to obtain axial, lateral, and torsional stiffness. Axial stiffness showed an aggregate average value of 1742.7 ± 174.7 N/mm with a high linear correlation with loading rate ( R2 = 0.80). Lateral stiffness yielded an aggregate average value of 56.9 ± 10.2 N/mm and was linearly correlated with loading rate ( R2 = 0.85). Torsional stiffness demonstrated an aggregate average value of 176.9 ± 14.5 N/mm with a strong linear correlation with loading rate ( R2 = 0.59). Despite the high correlations between stiffness and speed, practically this resulted in an overall average difference between the lowest and highest stiffness of only 4 per cent. Moreover, no statistical comparisons between loading rates for axial, lateral, or torsional test modes showed differences ( p ≤ 0.843). Future biomechanical investigators utilizing these synthetic femurs need not be concerned with loading rate effects over the range tested presently. This is the first study in the literature to perform such an assessment.

Performance of the resurfaced hip. Part 1: The influence of the prosthesis size and positioning on the remodelling and fracture of the femoral neck

Hip resurfacing is an established treatment for osteoarthritis in young active patients. Failure modes include femoral neck fracture and prosthesis loosening, which may be associated with medium-term bone adaptation, including femoral neck narrowing and densification around the prosthesis stem. Finite element modelling was used to indicate the effects of prosthesis sizing and positioning on the bone remodelling and fracture strength under a range of normal and traumatic loads, with the aim of understanding these failure modes better. The simulations predicted increased superior femoral neck stress shielding in young patients with small prostheses, which required shortening of the femoral neck to give an acceptable implant—bone interface. However, with a larger prosthesis, natural femoral head centre recreation in the implanted state was possible; therefore stress shielding was restricted to the prosthesis interior, and its extent was less sensitive to prosthesis orientation. With valgus orientation, the implanted neck strength was, at worst, within 3 per cent of its intact strength. The study suggests that femoral neck narrowing may be linked to a reduction in the horizontal femoral offset, occurring if the prosthesis is excessively undersized. As such, hip resurfacing should aim to reproduce the natural femoral head centre, and, for valgus prosthesis orientation, to avoid femoral neck fracture.

Imageless computer navigation without pre-operative templating may lead to malpreparation of the femoral head in hip resurfacing

The computed neck-shaft angle and the size of the femoral component were recorded in 100 consecutive hip resurfacings using imageless computer-navigation and compared with the angle measured before operation and with actual component implanted. The reliability of the registration was further analysed using ten cadaver femora. The mean absolute difference between the measured and navigated neck-shaft angle was 16.3° (0° to 52°). Navigation underestimated the measured neck-shaft angle in 38 patients and the correct implant size in 11. Registration of the cadaver femora tended to overestimate the correct implant size and provided a low level of repeatability in computing the neck-shaft angle.

Prudent pre-operative planning is advisable for use in conjunction with imageless navigation since misleading information may be registered intraoperatively, which could lead to inappropriate sizing and positioning of the femoral component in hip resurfacing.

A simple technique for alignment in total hip resurfacing arthroplasty: technical note and preliminary report

The functioning and survival of hip resurfacing arthroplasty depends on correct positioning and alignment of the implant. Correct positioning of the femoral alignment wire with respect to the femoral neck is the key to avoiding complications. Although the surgeon must align the wire in two planes, we can only control one plane at a time without changing position or relying on the indications of an assistant. Independent placement of two parallel alignment wires, one for varus-valgus orientation and another for version orientation, will help to determine two planes, the valgus sagittal plane and the version coronal plane, at the intersection of which both the optimum point of entry into the femoral head and the orientation line of the femoral alignment wire can be established. The marks on the neck and head and Kirschner wires following these marks define the planes. This simple technique allows us to reduce surgery time, minimize errors, and speed up the learning curve. It can be used with any type of resurfacing arthroplasty.