Variation of the impact duration during the in vitro insertion of acetabular cup implants

Revisiting two thousand hinge fractures in open wedge high tibial osteotomy with a fifty years review: the oscillating saw cannot replace the traditional "ear-hand" dialogue between osteotome and hammer to estimate the elastic modulus of bone

BACKGROUND

Hinge fracture on the lateral part of the tibia (LHF) is a common complication of medial Open Wedge High Tibial Osteotomy (OWHTO). Many factors have been described as risks for these fractures, but no study has compared an osteotome or an oscillating saw to prevent LHF following OWHTO.

METHODS

This "propensity-score-matched" (PSM) study was conducted from data obtained in the literature from 1974 to November 2024. A total of 10,368 knees with OWHTO were identified. After 1:1 matching based on correction amount, posterior slope change, surgeon's experience, the osteotome and oscillating groups comprised 2760 knees each.

RESULTS

Among the 5520 knees of the PSM population, the prevalence of LHF was 6.1% in the osteotome alone group (168 cases), and 22% in the oscillating saw group (607 cases). The osteotome group had a significant lower prevalence of hinge fracture than the oscillating saw group (OR, 0.23; 95% CI, 0.19 to 0.27; p < 0.0001) and a lower rate of clinically relevant hinge fractures with revision (OR, 0.34; 95% CI, 0.25 to 0.45; p < 0.001.

DISCUSSION

The osteotome may be an appropriate method for preventing hinge fractures following OWHTO.

An Instrumented Hammer to Detect the Rupture of the Pterygoid Plates

PURPOSE

Craniofacial osteotomies involving pterygomaxillary disjunction are common procedures in maxillofacial surgery. Surgeons still rely on their proprioception to determine when to stop impacting the osteotome, which is important to avoid complications such as dental damage and bleeding. Our group has developed a technique consisting in using an instrumented hammer that can provide information on the mechanical properties of the tissue located around the osteotome tip. The aim of this study is to determine whether a mallet instrumented with a force sensor can be used to predict the crossing of the osteotome through the pterygoid plates.

METHODS

31 osteotomies were carried out in 16 lamb skulls. For each impact, the force signal obtained was analysed using a dedicated signal processing technique. A prediction algorithm based on an SVM classifier and a cost matrix was applied to the database.

RESULTS

We showed that the device could always detect the crossing of the osteotome, sometimes before its occurrence. The prediction accuracy of the device was 94.7%. The method seemed to be sensitive to the thickness of the plate and to crack apparition and propagation.

CONCLUSION

These results pave the way for the development of a per-operative decision support system in maxillofacial surgery.

Using an Instrumented Hammer to Predict the Rupture of Bone Samples Subject to an Osteotomy

Osteotomies are common procedures in maxillofacial and orthopedic surgery. The surgeons still rely on their proprioception to control the progression of the osteotome. Our group has developed an instrumented hammer that was shown to provide information on the biomechanical properties of the tissue located around the osteotome tip. The objective of this study is to determine if this approach may be used to predict the rupture of a bone sample thanks to an instrumented hammer equipped with a force sensor. For each impact, an indicator τ is extracted from the signal corresponding to the variation of the force as a function of time. A linear by part regression analysis is applied to the curve corresponding to the variation of τ as a function of the distance d between the tip of the osteotome and the end of the sample. The experiments were conducted with plywood and bovine trabecular bone samples. The results show that τ starts increasing when the value of d is lower than 2.6 mm on average, which therefore corresponds to a typical threshold detection distance between the osteotome tip and the sample end. These findings open new paths for the development of this instrumented surgical hammer.

A parametric numerical analysis of femoral stem impaction

Press-fitted implants are implanted by impaction to ensure adequate seating, but without overloading the components, the surgeon, or the patient. To understand this interrelationship a uniaxial discretised model of the hammer/introducer/implant/bone/soft-tissues was developed. A parametric analysis of applied energy, component materials and geometry, and interactions between implant and bone and between bone and soft-tissues was performed, with implant seating and component stresses as outcome variables. To reduce the impaction effort (energy) required by the surgeon for implant seating and also reduce stresses in the hardware the following outcomes were observed: Reduce energy per hit with more hits / Increase hammer mass / Decrease introducer mass / Increase implant-bone resistance (eg stem roughness). Hardware stiffness and patient mechanics were found to be less important and soft tissue forces, due to inertial protection by the bone mass, were so low that their damage would be unlikely. This simple model provides a basic understanding of how stress waves travel through the impacted system, and an understanding of their relevance to implantation technique and component design.

Modal Analysis of the Ancillary During Femoral Stem Insertion: A Study on Bone Mimicking Phantoms

The femoral stem primary stability achieved by the impaction of an ancillary during its insertion is an important factor of success in cementless surgery. However, surgeons still rely on their proprioception, making the process highly subjective. The use of Experimental Modal Analysis (EMA) without sensor nor probe fixation on the implant or on the bone is a promising non destructive approach to determine the femoral stem stability. The aim of this study is to investigate whether EMA performed directly on the ancillary could be used to monitor the femoral stem insertion into the bone. To do so, a cementless femoral stem was inserted into 10 bone phantoms of human femurs and EMA was carried out on the ancillary using a dedicated impact hammer for each insertion step. Two bending modes could be identified in the frequency range [400-8000] Hz for which the resonance frequency was shown to be sensitive to the insertion step and to the bone-implant interface properties. A significant correlation was obtained between the two modal frequencies and the implant insertion depth (R² = 0.95 ± 0.04 and R² = 0.94 ± 0.06). This study opens new paths towards the development of noninvasive vibration based evaluation methods to monitor cementless implant insertion.

Anatomical subject validation of an instrumented hammer using machine learning for the classification of osteotomy fracture in rhinoplasty

Osteotomies during rhinoplasty are usually based on the surgeon's proprioception to determine the number and the strength of the impacts. The aim of this study is to determine whether a hammer instrumented with a force sensor can be used to classify fractures and to determine the location of the osteotome tip. Two lateral osteotomies were realized in nine anatomical subjects using an instrumented hammer recording the evolution of the impact force. Two indicators τ and λ were derived from the signal, and video analysis was used to determine whether the osteotome tip was located in nasal or frontal bone as well as the condition of the bone tissue around the osteotome tip. A machine-learning algorithm was used to predict the condition of bone tissue after each impact. The algorithm was able to predict the condition of the bone after the impacts with an accuracy of 83%, 91%, and 93% when considering a tolerance of 0, 1, and 2 impacts, respectively. Moreover, in nasal bone, the values of τ and λ were significantly lower (p < 10^-10) and higher (p < 10^-4) than in frontal bone, respectively. This study paves the way for the development of the instrumented hammer as a decision support system.

Validation of an Instrumented Hammer for Rhinoplasty Osteotomies: A Cadaveric Study

Background: Osteotomies during rhinoplasty are usually based on surgeon's proprioception to determine the number, energy, and trajectory of impacts. Objective: The first objective was to detect the occurrence of fractures. The second objective was to determine when the thicker frontal bone was encountered by the osteotome. Materials and Methods: An instrumented hammer was used to measure the impact force during lateral osteotomies on nine human anatomic specimens. A prediction algorithm was developed using machine learning techniques, to detect the occurrence of fractures, and the proximity of the osteotome to the frontal bone. Results: The algorithm was able to predict the occurrence of fractures and the proximity to the frontal bone with a prediction rate of 83%, 91%, and 93% when allowing for an error of 0, 1, and 2 impacts, respectively. The location of the osteotome in the frontal bone was predicted with an error of 7.7%. Conclusion: An osteotomy hammer measuring the impact force when performing lateral osteotomies can predict the occurrence of fractures and the proximity to the frontal bone, providing the surgeon with instant feedback.

Using an impact hammer to perform biomechanical measurements during osteotomies: Study of an animal model

Osteotomies are common surgical procedures used for instance in rhinoplasty and usually performed using an osteotome impacted by a mallet. Visual control being difficult, osteotomies are often based on the surgeon proprioception to determine the number and energy of each impact. The aim of this study is to determine whether a hammer instrumented with a piezoelectric force sensor can be used to (i) follow the displacement of the osteotome and (ii) determine when the tip of the osteotome arrives in frontal bone, which corresponds to the end of the osteotomy pathway. Seven New Zealand White rabbit heads were collected, and two osteotomies were performed on their left and right nasal bones using the instrumented hammer to record the variation of the force as a function of time during each impact. The second peak time τ was derived from each signal while the displacement of the osteotome tip D was determined using video motion tracking. The results showed a significant correlation between τ and D (ρ² = 0.74), allowing to estimate the displacement of the osteotome through the measurement of τ. The values of τ measured in the frontal bone were significantly lower than in the nasal bone (p<10^-10), which allows to determine the transition between the nasal and frontal bones when τ becomes lower than 0.78 its initial averaged value. Although results should be validated clinically, this technology could be used by surgeons in the future as a decision support system to help assessing the osteotome environment.

Characterization of Acetabular Cup Insertion Forces in Cancellous Bone Proxy for Validation of an Invasive Sensing Model and Development of Automatic Prosthesis Installation Device: A Preliminary Study

Total hip replacement is a widespread medical procedure, with over 300,000 surgeries performed each year in the United States alone. The vast majority of total hip replacements utilize press fit fixation. Successful seating of the implant requires a delicate balance between inserting the implant deep enough to obtain sufficient primary stability, while avoiding fracture of bone. To improve patient outcomes, surgeons need assistive technologies that can guide them as to how much force to apply and when to stop impacting. The development of such technology, however, requires a greater understanding of the forces experienced in bone and the resulting cup insertion and implant stability. Here, we present a preliminary study of acetabular cup insertion into bone proxy samples. We find that as the magnitude of force on the acetabular cup increases, cup insertion and axial extraction force increase linearly, then nonlinearly, and finally plateau with full insertion. Within the small nonlinear zone, approximately 90% of both cup insertion and extraction force are achieved with only 50% total energy required for full seating, posing the question as to whether full seating is an appropriate goal in press-fit arthroplasty. For repeated impacts of a given energy, cup displacement and force experienced in bone (measured force profile—MFP) increase correspondingly and reach a plateau over a certain number of impacts (number of impacts to seating—NOITS), which represents the rate of insertion. The relationship between MFP and NOITS can be exploited to develop a force feedback mechanism to quantitatively infer optimal primary implant stability.

Dynamic trial fitting by an expanding trial cup does not jeopardize primary acetabular component stability

BACKGROUND

Trial fitting of the acetabular component in uncemented total hip replacement is traditionally done by trial cups. Since trial cups do not resemble the real press-fit obtained by the definitive cup, a dynamic trial inserter, called the X-pander ®, was developed to mimic the real amount of press-fit. However, the concern is raised of losing the initial press-fit by using the X-pander® due to pre-expansion of the acetabulum. The purpose of this study was to assess if there is a difference in primary stability between both methods.

METHODS

A biomechanical randomized study was performed with bovine calf acetabula, with randomization between either using the X-pander® or the traditional trial cups to assess primary stability. The primary outcome was the force needed to achieve lever out of the implanted cup (Anexys, Mathys or Trident, Stryker), measured in Newton meter (Nm) with a biomechanical testing set up.

FINDINGS

In total, 54 cups (19 Anexys, 35 Trident) were inserted and tested after randomized trial fitting. Overall mean lever out was 45.1 Nm (SD 14.6) for the X-pander® group and 45.0 Nm (SD 14.5) for the trial cups group. After adjustment for potential confounders (cup size and type) mixed model analysis did not reveal a significant difference in lever out force between both testing devices (mean 1.0 Nm, 95%CI (-5.9; 8.0), p = .77).

INTERPRATION

Initial press-fit of the implanted cup is not lost by pre-expansion as done with dynamic trial fitting with the X-pander®.

Using an Impact Hammer to Estimate Elastic Modulus and Thickness of a Sample During an Osteotomy

Performing an osteotomy with a surgical mallet and an osteotome is a delicate intervention mostly based on the surgeon proprioception. It remains difficult to assess the properties of bone tissue being osteotomized. Mispositioning of the osteotome or too strong impacts may lead to bone fractures which may have dramatic consequences. The objective of this study is to determine whether an instrumented hammer may be used to retrieve information on the material properties around the osteotome tip. A hammer equipped with a piezo-electric force sensor was used to impact 100 samples of different composite materials and thicknesses. A model-based inversion technique was developed based on the analysis of two indicators derived from the analysis of the variation of the force as a function of time in order to (i) classify the samples depending on their material types, (ii) determine the materials stiffness, and (iii) estimate the samples thicknesses. The model resulting from the classification using support vector machines (SVM) learning techniques can efficiently predict the material of a new sample, with an estimated 89% prediction performance. A good agreement between the forward analytical model and the experimental data was obtained, leading to an average error lower than 10% in the samples thickness estimation. Based on these results, navigation and decision-support tools could be developed and allows surgeons to adapt their surgical strategy in a patient-specific manner.

Time-dependent Viscoelastic Response of Acetabular Bone and Implant Seating during Dynamic Implantation of Press-fit Cups

Deformation of an acetabular cup implant during cementless implantation is indicative of the radial compressive forces, and such of the initial implant fixation strength. Stress relaxation in the surrounding bone tissue following implantation could reduce the deformation of the cup and thus primary implant fixation. The aim of this study was therefore to determine the early shape change of the implanted cup immediately after implantation with different press-fit levels and whether recording the force during cup impaction can be used to estimate initial cup fixation. Cup implantations into porcine acetabulae (n=10) were performed using a drop tower. The force induced by the drop weight and cup seating after each impact was recorded. Deformation of the implanted cup was determined with strain gauges over a period of 10min. Lever-out torques were measured to assess the initial fixation strength. Stress relaxation in the bone caused a reduction in cup deformation of 13.52±4.06% after 1min and 29.34±5.11% after 10min. The fixation strength increased with a higher force magnitude during impaction (R_s²=0.810, p=0.037). Reduction of the radial compressive forces due to stress relaxation of the surrounding bone should be considered during press-fit cup implantation in order to compensate for the reduced fixation strength over time. In addition, recording the implantation force could help to estimate initial fixation strength.

Ex vivo estimation of cementless femoral stem stability using an instrumented hammer

BACKGROUND

The success of cementless hip arthroplasty depends on the primary stability of the femoral stem. It remains difficult to assess the optimal number of impacts to guarantee the femoral stem stability while avoiding bone fracture. The aim of this study is to validate a method using a hammer instrumented with a force sensor to monitor the insertion of femoral stem in bovine femoral samples.

METHODS

Different cementless femoral stem were impacted into five bovine femur samples, leading to 99 configurations. Three methods were used to quantify the insertion endpoint: the impact hammer, video motion tracking and the surgeon proprioception. For each configuration, the number of impacts performed by the surgeon until he felt a correct insertion was noted N_surg. The insertion depth E was measured through video motion tracking, and the impact number N_vid corresponding to the end of the insertion was estimated. Two indicators, noted I and D, were determined from the analysis of the time variation of the force, and the impact number N_d corresponding to a threshold reached in D variation was estimated.

FINDINGS

The pullout force of the femoral stem was significantly correlated with I (R² = 0.81). The values of N_surg, N_vid and N_d were similar for all configurations.

INTERPRETATION

The results validate the use of the impact hammer to assess the primary stability of the femoral stem and the moment when the surgeon should stop the impaction procedure for an optimal insertion, which could lead to the development of a decision support system.

A cadaveric validation of a method based on impact analysis to monitor the femoral stem insertion

The success of cementless hip arthroplasty depends on the primary stability of the femoral stem (FS). It remains difficult to assess the optimal impaction energy to guarantee the FS stability while avoiding bone fracture. The aim of this study is to compare the results of a method based on the use of an instrumented hammer to determine the insertion endpoint of cementless FS in a cadaveric model with two other methods using i) the surgeon proprioception and ii) video motion tracking. Different FS were impacted in nine human cadaveric femurs. For each configuration, the number of impacts realized when the surgeon felt that the FS was correctly inserted was noted N_surg. For each impact, the insertion depth E was measured and an indicator D was determined based on the time-variation of the force. The impact number N_vid (respectively N_d), corresponding to the end of the migration phase, was estimated analyzing the evolution of E (respectively D). The respective difference between N_surg, N_vid and N_d was similar and lower than 3 for more than 85% of the configurations. The results allow a validation of the use of an impact hammer to assess the moment when the surgeon should stop the impaction, paving the way towards the development of a decision support system to assist the surgeon.

Dependence of the primary stability of cementless acetabular cup implants on the biomechanical environment

Biomechanical phenomena occurring at the bone–implant interface during the press-fit insertion of acetabular cup implants are still poorly understood. This article presents a nonlinear geometrical two-dimensional axisymmetric finite element model aiming at describing the biomechanical behavior of the acetabular cup implant as a function of the bone Young’s modulus Eb, the diametric interference fit ( IF), and the friction coefficient µ. The numerical model was compared with experimental results obtained from an in vitro test, which allows to determine a reference configuration with the parameter set: μ*  = 0.3, [Formula: see text], and IF*  = 1 mm for which the maximal contact pressure tN = 10.7 MPa was found to be localized at the peri-equatorial rim of the acetabular cavity. Parametric studies were carried out, showing that an optimal value of the pull-out force can be defined as a function of μ, Eb, and IF. For the reference configuration, the optimal pull-out force is obtained for μ  = 0.6 (respectively, Eb = 0.35 GPa and IF = 1.4 mm). For relatively low value of µ ( µ < 0.2), the optimal value of IF linearly increases as a function of µ independently of Eb, while for µ > 0.2, the optimal value of IF has a nonlinear dependence on µ and decreases as a function of Eb. The results can be used to help surgeons determine the optimal value of IF in a patient specific manner.

Reflection of an ultrasonic wave on the bone-implant interface: Effect of the roughness parameters

Quantitative ultrasound can be used to characterize the evolution of the bone-implant interface (BII), which is a complex system due to the implant surface roughness and to partial contact between bone and the implant. The aim of this study is to derive the main determinants of the ultrasonic response of the BII during osseointegration phenomena. The influence of (i) the surface roughness parameters and (ii) the thickness W of a soft tissue layer on the reflection coefficient r of the BII was investigated using a two-dimensional finite element model. When W increases from 0 to 150 μm, r increases from values in the range [0.45; 0.55] to values in the range [0.75; 0.88] according to the roughness parameters. An optimization method was developed to determine the sinusoidal roughness profile leading to the most similar ultrasonic response for all values of W compared to the original profile. The results show that the difference between the ultrasonic responses of the optimal sinusoidal profile and of the original profile was lower to typical experimental errors. This approach provides a better understanding of the ultrasonic response of the BII, which may be used in future numerical simulation realized at the scale of an implant.

Effect of interposed tissue and contamination on the initial stability of a highly porous press-fit acetabular cup

For biologic fixation, press-fit acetabular cups should have initial stability with minimal micromotion and osteoconductive surfaces in contact with bone. Inadequate exposure potentially influences initial stability by increasing the possibility of soft tissue interposition and contamination at the implant-tissue interface. A sawbone model was used to examine how interposed tissue and contamination influence initial cup stability. Seven groups (n = 4) were tested with varying levels of interposed fatty and fibrous tissue placed around the rim of the cup. 54 millimeter in diameter highly porous hemispherical acetabular cups (Stryker, Mahwah NJ) and 54 mm reamed cavities in sawbone blocks were used. Shells were seated and maximum lever out force was recorded for each sample. Cups with fibrous tissue spaced evenly along the rim had a lever out force that was 150% of the control (107 ± 6 vs. 150 ± 12N, p = 0.005), and fatty tissue contamination had a lever out force that was 140% of the control (143 ± 18 vs. 107 ± 6N, p = 0.04). Cups with fibrous tissue placed eccentrically along the rim had a lever out force that was double the control 107 ± 6 N vs. 200 ± 15 N (p = 0.001). Surprisingly, fatty tissue contamination and fibrous tissue interposition at the rim increased initial stability. The eccentrically interposed tissue forced the opposite pole of the cup into the bone, resulting in a more secure press-fit. However, soft tissue interposition decreases implant/bone apposition, and the effect on long term fixation is unknown. Statement of Clinical Significance: Soft tissue interposition between the bone and cup may provide higher initial stability, but its long-term effects are unknown. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Monitoring cementless femoral stem insertion by impact analyses: An in vitro study

The primary stability of the femoral stem (FS) implant determines the surgical success of cementless hip arthroplasty. During the insertion, a compromise must be found for the number and energy of impacts that should be sufficiently large to obtain an adapted primary stability of the FS and not too high to decrease fracture risk. The aim of this study is to determine whether a hammer instrumented with a force sensor can be used to monitor the insertion of FS. Cementless FS of different sizes were impacted in four artificial femurs with an instrumented hammer, leading to 72 configurations. The impact number when the surgeon empirically felt that the FS was fully inserted was noted N_surg. The insertion depth E was assessed using video motion tracking and the impact number N_vid corresponding to the end of the insertion was estimated. For each impact, two indicators noted I and D were determined based on the analysis of the variation of the force as a function of time. The pull-out force F was significantly correlated with the indicator I (R² = 0.67). The variation of D was analyzed using a threshold to determine an impact number N_d, which is shown to be closely related to N_surg and N_vid, with an average difference of around 0.2. This approach allows to determine i) the moment when the surgeon should stop the impaction procedure in order to obtain an optimal insertion of the FS and ii) the FS implant primary stability. This study paves the way towards the development of a decision support system to assist the surgeon in hip arthroplasty.

Reflection of an ultrasonic wave on the bone-implant interface: A numerical study of the effect of the multiscale roughness

Quantitative ultrasound is used to characterize and stimulate osseointegration processes at the bone-implant interface (BII). However, the interaction between an ultrasonic wave and the implant remains poorly understood. This study aims at investigating the sensitivity of the ultrasonic response to the microscopic and macroscopic properties of the BII and to osseointegration processes. The reflection coefficient R of the BII was modeled for different frequencies using a two-dimensional finite element model. The implant surface roughness was modeled by a sinusoidal function with varying amplitude h and spatial frequency L. A soft tissue layer of thickness W was considered between bone tissue and the implant in order to model non-mineralized fibrous tissue. For microscopic roughness, R is shown to increase from around 0.55 until 0.9 when kW increases from 0 to 1 and to be constant for kW > 1, where k is the wavenumber in the implant. These results allow us to show that R depends on the properties of bone tissue located at a distance comprised between 1 and 25 μm from the implant surface. For macroscopic roughness, R is highly dependent on h and this dependence may be explained by phase cancellation and multiple scattering effects for high roughness parameters.

Influence of soft tissue in the assessment of the primary fixation of acetabular cup implants using impact analyses

BACKGROUND

The acetabular cup (AC) implant primary stability is an important determinant for the success of cementless hip surgery but it remains difficult to assess the AC implant fixation in the clinic. A method based on the analysis of the impact produced by an instrumented hammer on the ancillary has been developed by our group (Michel et al., 2016a). However, the soft tissue thickness present around the acetabulum may affect the impact response, which may hamper the robustness of the method. The aim of this study is to evaluate the influence of the soft tissue thickness (STT) on the acetabular cup implant primary fixation evaluation using impact analyses.

METHODS

To do so, different AC implants were inserted in five bovine bone samples. For each sample, different stability conditions were obtained by changing the cavity diameter. For each configuration, the AC implant was impacted 25 times with 10 and 30 mm of soft tissues positioned underneath the sample. The averaged indicator I_m was determined based on the amplitude of the signal for each configuration and each STT and the pull-out force was measured.

FINDINGS

The results show that the resonance frequency of the system increases when the value of the soft tissue thickness decreases. Moreover, an ANOVA analysis shows that there was no significant effect of the value of soft tissue thickness on the values of the indicator I_m (F = 2.33; p-value = 0.13).

INTERPRETATION

This study shows that soft tissue thickness does not appear to alter the prediction of the acetabular cup implant primary fixation obtained using the impact analysis approach, opening the path towards future clinical trials.

Ex Vivo Evaluation of Cementless Acetabular Cup Stability Using Impact Analyses with a Hammer Instrumented with Strain Sensors

The acetabular cup (AC) implant stability is determinant for the success of cementless hip arthroplasty. A method based on the analysis of the impact force applied during the press-fit insertion of the AC implant using a hammer instrumented with a force sensor was developed to assess the AC implant stability. The aim of the present study was to investigate the performance of a method using a hammer equipped with strain sensors to retrieve the AC implant stability. Different AC implants were inserted in five bovine samples with different stability conditions leading to 57 configurations. The AC implant was impacted 16 times by the two hammers consecutively. For each impact; an indicator I_S (respectively I_F) determined by analyzing the time variation of the signal corresponding to the averaged strain (respectively force) obtained with the stress (respectively strain) hammer was calculated. The pull-out force F was measured for each configuration. F was significantly correlated with I_S (R² = 0.79) and I_F (R² = 0.80). The present method has the advantage of not modifying the shape of the hammer that can be sterilized easily. This study opens new paths towards the development of a decision support system to assess the AC implant stability.

Influence of anisotropic bone properties on the biomechanical behavior of the acetabular cup implant: a multiscale finite element study

Although the biomechanical behavior of the acetabular cup (AC) implant is determinant for the surgical success, it remains difficult to be assessed due to the multiscale and anisotropic nature of bone tissue. The aim of the present study was to investigate the influence of the anisotropic properties of peri-implant trabecular bone tissue on the biomechanical behavior of the AC implant at the macroscopic scale. Thirteen bovine trabecular bone samples were imaged using micro-computed tomography (μCT) with a resolution of 18 μm. The anisotropic biomechanical properties of each sample were determined at the scale of the centimeter based on a dedicated method using asymptotic homogenization. The material properties obtained with this multiscale approach were used as input data in a 3D finite element model to simulate the macroscopic mechanical behavior of the AC implant under different loading conditions. The largest stress and strain magnitudes were found around the equatorial rim and in the polar area of the AC implant. All macroscopic stiffness quantities were significantly correlated (R² > 0.85, p < 6.5 e-6) with BV/TV (bone volume/total volume). Moreover, the maximum value of the von Mises stress field was significantly correlated with BV/TV (R² > 0.61, p < 1.6 e-3) and was always found at the bone-implant interface. However, the mean value of the microscopic stress (at the scale of the trabeculae) decrease as a function of BV/TV for vertical and torsional loading and do not depend on BV/TV for horizontal loading. These results highlight the importance of the anisotropic properties of bone tissue.

Development of an acoustic measurement protocol to monitor acetabular implant fixation in cementless total hip Arthroplasty: A preliminary study

In cementless total hip arthroplasty (THA), the initial stability is obtained by press-fitting the implant in the bone to allow osseointegration for a long term secondary stability. However, finding the insertion endpoint that corresponds to a proper initial stability is currently based on the tactile and auditory experiences of the orthopedic surgeon, which can be challenging. This study presents a novel real-time method based on acoustic signals to monitor the acetabular implant fixation in cementless total hip arthroplasty. Twelve acoustic in vitro experiments were performed on three types of bone models; a simple bone block model, an artificial pelvic model and a cadaveric model. A custom made beam was screwed onto the implant which functioned as a sound enhancer and insertor. At each insertion step an acoustic measurement was performed. A significant acoustic resonance frequency shift was observed during the insertion process for the different bone models; 250 Hz (35%, second bending mode) to 180 Hz (13%, fourth bending mode) for the artificial bone block models and 120 Hz (11%, eighth bending mode) for the artificial pelvis model. No significant frequency shift was observed during the cadaveric experiment due to a lack of implant fixation in this model. This novel diagnostic method shows the potential of using acoustic signals to monitor the implant seating during insertion.

What every surgeon should know about Ceramic-on-Ceramic bearings in young patients

Based on the exceptional tribological behaviour and on the relatively low biological activity of ceramic particles, Ceramic-on-Ceramic (CoC) total hip arthroplasty (THA) presents significant advantagesCoC bearings decrease wear and osteolysis, the cumulative long-term risk of dislocation, muscle atrophy, and head-neck taper corrosion.However, there are still concerns regarding the best technique for implantation of ceramic hips to avoid fracture, squeaking, and revision of ceramic hips with fracture of a component.We recommend that surgeons weigh the potential advantages and disadvantages of current CoC THA in comparison with other bearing surfaces when considering young very active patients who are candidates for THA. Cite this article: Hernigou P, Roubineau F, Bouthors C, Flouzat-Lachaniette C-H. What every surgeon should know about Ceramic-on-Ceramic bearings in young patients. EFORT Open Rev 2016;1:107-111. DOI: 10.1302/2058-5241.1.000027.

Assessing the Acetabular Cup Implant Primary Stability by Impact Analyses: A Cadaveric Study

BACKGROUND

The primary stability of the acetabular cup (AC) implant is an important determinant for the long term success of cementless hip surgery. However, it remains difficult to assess the AC implant stability due to the complex nature of the bone-implant interface. A compromise should be found when inserting the AC implant in order to obtain a sufficient implant stability without risking bone fracture. The aim of this study is to evaluate the potential of impact signals analyses to assess the primary stability of AC implants inserted in cadaveric specimens.

METHODS

AC implants with various sizes were inserted in 12 cadaveric hips following the same protocol as the one employed in the clinic, leading to 86 different configurations. A hammer instrumented with a piezoelectric force sensor was then used to measure the variation of the force as a function of time produced during the impact between the hammer and the ancillary. Then, an indicator I was determined for each impact based on the impact momentum. For each configuration, twelve impacts were realized with the hammer, the value of the maximum amplitude being comprised between 2500 and 4500 N, which allows to determine an averaged value IM of the indicator for each configuration. The pull-out force F was measured using a tangential pull-out biomechanical test.

RESULTS

A significant correlation (R2 = 0.69) was found between IM and F when pooling all data, which indicates that information related to the AC implant biomechanical stability can be retrieved from the analysis of impact signals obtained in cadavers.

CONCLUSION

These results open new paths in the development of a medical device that could be used in the future in the operative room to help orthopedic surgeons adapt the surgical protocol in a patient specific manner.

Finite element model of the impaction of a press-fitted acetabular cup

Press-fit surgical procedures aim at providing primary stability to acetabular cup (AC) implants. Impact analysis constitutes a powerful approach to retrieve the AC implant insertion properties. The aim of this numerical study was to investigate the dynamic interaction occurring between the hammer, the ancillary and bone tissue during the impact and to assess the potential of impact analysis to retrieve AC implant insertion conditions. A dynamic two-dimensional axisymmetric model was developed to simulate the impaction of the AC implant into bone tissue assuming friction at the bone-implant interface and large deformations. Different values of interference fit (from 0.5 to 2 mm) and impact velocities (from 1 to 2 m.s^-1) were considered. For each configuration, the variation of the force applied between the hammer and the ancillary was analyzed and an indicator I was determined based on the impact momentum of the signal. The simulated results are compared to the experiments. The value of the polar gap decreases with the impact velocity and increases with the interference fit. The bone-implant contact area was significantly correlated with the resonance frequency (R ² = 0.94) and the indicator (R ² = 0.95). The results show the potential of impact analyses to retrieve the bone-implant contact properties.

Ex vivo estimation of cementless acetabular cup stability using an impact hammer

Obtaining primary stability of acetabular cup (AC) implants is one of the main objectives of press-fit procedures used for cementless hip arthroplasty. The aim of this study is to investigate whether the AC implant primary stability can be evaluated using the signals obtained with an impact hammer. A hammer equipped with a force sensor was used to impact the AC implant in 20 bovine bone samples. For each sample, different stability conditions were obtained by changing the cavity diameter. For each configuration, the inserted AC implant was impacted four times with a maximum force comprised between 2500 and 4500 N. An indicator I was determined based on the partial impulse estimation and the pull-out force was measured. The implant stability and the value of the indicator I reached a maximum value for an interference fit equal to 1 mm for 18 out of 20 samples. When pooling all samples and all configurations, the implant stability and I were significantly correlated (R(2) = 0.83). The AC implant primary stability can be assessed through the analysis of the impact force signals obtained using an impact hammer. Based on these ex vivo results, a medical device could be developed to provide a decision support system to the orthopedic surgeons.

Development of a non-invasive diagnostic technique for acetabular component loosening in total hip replacements

Current techniques for diagnosing early loosening of a total hip replacement (THR) are ineffective, especially for the acetabular component. Accordingly, new, accurate, and quantifiable methods are required. The aim of this study was to investigate the viability of vibrational analysis for accurately detecting acetabular component loosening. A simplified acetabular model was constructed using a Sawbones(®) foam block. By placing a thin silicone layer between the acetabular component and the Sawbones block, 2- and 4-mm soft tissue membranes were simulated representing different loosening scenarios. A constant amplitude sinusoidal excitation with a sweep range of 100-1500 Hz was used. Output vibration from the model was measured using an accelerometer and an ultrasound probe. Loosening was determined from output signal features such as the number and relative strength of observed harmonic frequencies. Both measurement methods were sufficient to measure the output vibration. Vibrational analysis reliably detected loosening corresponding to both 2 and 4 mm tissue membranes at driving frequencies between 100 and 1000 Hz (p < 0.01) using the accelerometer. In contrast, ultrasound detected 2-mm loosening at a frequency range of 850-1050 Hz (p < 0.01) and 4-mm loosening at 500-950 Hz (p < 0.01).

In Vitro Evaluation of the Acetabular Cup Primary Stability by Impact Analysis

The implant primary stability of the acetabular cup (AC) is an important parameter for the surgical success of press-fit procedures used for the insertion of cementless hip prostheses. In previous studies by our group (Mathieu, V., Michel, A., Lachaniette, C. H. F., Poignard, A., Hernigou, P., Allain, J., and Haiat, G., 2013, “Variation of the Impact Duration During the in vitro Insertion of Acetabular Cup Implants,” Med. Eng. Phys., 35(11), pp. 1558–1563) and (Michel, A., Bosc, R., Mathieu, V., Hernigou, P., and Haiat, G., 2014, “Monitoring the Press-Fit Insertion of an Acetabular Cup by Impact Measurements: Influence of Bone Abrasion,” Proc. Inst. Mech. Eng., Part H, 228(10), pp. 1027–1034), the impact momentum and duration were shown to carry information on the press-fit insertion of the AC within bone tissue. The aim of the present study is to relate the impact momentum recorded during the AC insertion to the AC biomechanical primary stability. The experimental protocol consisted in testing 13 bovine bone samples that underwent successively series of 15 reproducible mass falls impacts (5 kg, 5 cm) followed by tangential stability testing. Each bone sample was tested with different hole sizes in order to obtain different stability configurations. The impact momentum and the tangential primary stability reach a maximum value for an interference fit equal to around 1 mm. Moreover, a correlation between the impact momentum and the stability was obtained with all samples and all configuration (R2 = 0.65). The implant primary stability can be assessed through the measurement of the impact force signal analysis. This study opens new paths for the development of a medical device which could be used as a decision support system to assist the surgeon during the insertion of the AC implant.

Monitoring the press-fit insertion of an acetabular cup by impact measurements: influence of bone abrasion

Press-fit procedures used for the insertion of cementless hip prostheses aim at obtaining optimal implant primary stability. We have previously used the measurement of impact duration to follow the insertion of the acetabular cup implant within bone tissue. The aim of this study was to investigate the variation of the value of the impact momentum due to successive insertions of the acetabular cup into bone tissue. The results obtained with impact momentum and contact duration measurements were compared. A total of 10 bovine bone samples were subjected to three successive procedures consisting of 10 reproducible impacts (3.5 kg falling 40 mm). Each procedure aimed at inserting the acetabular cup implant into the same bone cavity. The time variation of force during each impact was recorded by a force sensor, allowing the measurement of the impact duration (I 1) and momentum (I 2). The value of I 2 increased as a function of the impact number and reached a constant value after N 2 = 5.07 ± 1.31 impacts. Moreover, statistical analyses show that N 2 decreased significantly as a function of the number of experiments, which may be due to abrasion phenomena at the bone-implant interface. Abrasion phenomena led to a faster insertion of the acetabular cup when the implant had been previously inserted into the same bone cavity. An empirical analytical model considering a flat punch configuration to model the bone-implant contact conditions was used to understand the trend of the variation of I 2 during the insertion of the acetabular cup. The measurement of the force during impacts is useful to assess the bone-implant interface properties, but needs to be validated in the clinic to be useful for orthopaedic surgeons intra-operatively.

Does surface roughness influence the primary stability of acetabular cups? A numerical and experimental biomechanical evaluation

Most acetabular cups implanted today are press-fit impacted cementless. Anchorage begins with the primary stability given by insertion of a slightly oversized cup. This primary stability is key to obtaining bone ingrowth and secondary stability. We tested the hypothesis that primary stability of the cup is related to surface roughness of the implant, using both an experimental and a numerical models to analyze how three levels of surface roughness (micro, macro and combined) affect the primary stability of the cup. We also investigated the effect of differences in diameter between the cup and its substrate, and of insertion force, on the cups' primary stability. The results of our study show that primary stability depends on the surface roughness of the cup. The presence of macro-roughness on the peripheral ring is found to decrease primary stability; there was excessive abrasion of the substrate, damaging it and leading to poor primary stability. Numerical modeling indicates that oversizing the cup compared to its substrate has an impact on primary stability, as has insertion force.