A review of pre-clinical testing of femoral stem subsidence and comparison with clinical data

Biomechanics of a calcar loading and a shortened tapered femoral stem: Comparative in-vitro testing of primary stability and strain distribution

Purpose

The most common femoral short stems available on the market can, in principle, be divided with regard to their anchoring concepts into a calcar loading and a shortened tapered design. The purpose of this study was to compare the primary stability and stress-shielding of two short stems, which correspond to these two different anchoring concepts.

Methods

Using seven paired fresh frozen human cadaver femurs, primary axial and rotational stabilities under dynamic load (100–1600 N) were evaluated by miniature displacement transducers after 100,000 load cycles. Changes in cortical strains were measured before and after implantation of both stem types to detect implant-specific load transmission and possible stress-shielding effects.

Results

Reversible and irreversible micromotions under dynamic load displayed no significant differences between the two implants. Implantation of either stem types resulted in a reduction of cortical strains in the proximal femur, which was less pronounced for the calcar loading implant.

Conclusions

Both short stems displayed comparable micromotions far below the critical threshold above which osseointegration may disturbed. Neither short stem could avoid proximal stress-shielding. This effect was less pronounced for the calcar loading short stem, which corresponds to a more physiological load transmission.

Acoustic analysis to monitor implant seating and early detect fractures in cementless THA: An in vivo study

The initial stability of cementless total hip arthroplasty (THA) implants is obtained by an interference fit that allows osseointegration for a long term secondary stability of the implant. Yet, finding the insertion endpoint that corresponds to an appropriate initial stability is currently often based on a number of subjective experiences of the orthopedic surgeon, which can be challenging. In order to assist the orthopedic surgeons in their pursuit to find this optimal initial stability, this study aims to determine whether the analysis of sound that results from the implant insertion hammer blows can be used to objectively monitor the insertion process of cementless THA implants. An in vivo study was conducted. The experimental results revealed vibro-acoustic behavior sensitive to implant seating, related to the low frequency content of the response spectra. This sensitive low-frequency behavior was quantified by a set of specific vibro-acoustic features and metrics that reflected the power and similarity of the low-frequency response. These features and metrics allowed monitoring the implant seating and their convergence agreed well with the endpoint of insertion as determined by the orthopedic surgeon. Intraoperative fractures caused an abrupt and opposite change of the vibro-acoustic behavior prior to the notification of the fracture by the orthopedic surgeon. The observation of such an abrupt change in the vibro-acoustic behavior can be an important early warning for loss of implant stability. The presented vibro-acoustic measurement method shows potential to serve as a decision supporting source of information as it showed to reflect the implant seating.

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.

Influence of different anteversion alignments of a cementless hip stem on primary stability and strain distribution

BACKGROUND

Stem anteversion in total hip arthroplasty is well known to have a high impact on dislocation, but empirical data regarding the clinical and biomechanical influence is lacking. Therefore, we evaluated the impact of different anteversion alignments on the primary stability and strain distribution of a cementless stem.

METHODS

The cementless CLS Spotorno stem was implanted in 3 different groups (each group n = 6, total n = 21) with different anteversion alignments: reference anteversion (8°), +15° torsion in anteversion (+23°), -15° torsion in retroversion (-7°) using composite femurs (Sawbones). Primary stability was determined by 3-dimensional micromotions using a dynamic loading procedure simulating walking on level ground. Additionally, surface strains were registered before and after stem insertion in the 3 different groups, using one composite femur for each group (total n = 3).

FINDINGS

The micromotion measurements did not show a significant difference between the 3 evaluated alignments. Moreover, determination of the strain distribution did also not reveal an obvious difference.

INTERPRETATION

This biomechanical study simulating walking on level ground indicates that there is no considerable influence of stem ante-/retroversion variation (±15°) on the initial stability and strain distribution when evaluating the cementless CLS Spotorno in composite femora.

Comparisons of the surface micromotions of cementless femoral prosthesis in the horizontal and vertical levels: a network analysis of biomechanical studies

BACKGROUND

Numerous quantitatively biomechanical studies measuring the fixation stability of femoral stem using micromotions at the bone-implant interfaces in different directions and levels remain inconclusive. This network meta-analysis performed systematically aims to explore the rank probability of micromotions at the bone-implant interfaces based on biomechanical data from studies published.

METHODS

Two electronic databases, PubMed/MEDLINE and Embase, were utilized to retrieve biomechanical studies providing the data of micromotions at the bone-stem interfaces. After screening and diluting out, the studies that met inclusion criteria will be utilized for statistical analysis. In order to contrast the stability of commonness and differences of the different parts of the femoral stem, the horizontal and vertical comparison of micromotions at the bone-implant interfaces were conducted using the pooled evaluation indexes including the mean difference (MD) and the surface under the cumulative ranking (SUCRA) curve, while inconsistency analysis, sensitivity analysis, subgroup analyses, and publication bias were performed for the stability evaluation of outcomes.

RESULTS

Screening determined that 20 studies involving a total of 249 samples were deemed viable for inclusion in the network meta-analysis. Tip point registered the highest micromotions of 13 measurement points. In the horizontal level, the arrangements of 4 measurement points at the proximal (P1-P4), middle (P5-P8) and distal part of the stem (P9-P12) were P1 = P2 = P3 = P4, P7 > P8 > P6 = P5 and P10 ≥ P12 = P9 = P11, respectively. In the vertical level, the arrangements of 3 measurement points at the anterior, posterior, medial, and lateral directions was P9 > P5 = P1, P10 > P6 > P2, P11 > P7 > P3, and P12 > P8 > P4, respectively.

CONCLUSION

The network meta-analysis seems to reveal that the distal part of the femoral stem is easier to register higher micromotion, and tip point of femoral stem registers the highest micromotions.

Two Different Methods to Measure the Stability of Acetabular Implants: A Comparison Using Artificial Acetabular Models

The total number of total hip arthroplasties is increasing every year, and approximately 10% of these surgeries are revisions. New implant design and surgical techniques are evolving quickly and demand accurate preclinical evaluation. The initial stability of cementless implants is one of the main concerns of these preclinical evaluations. A broad range of initial stability test methods is currently used, which can be categorized into two main groups: Load-to-failure tests and relative micromotion measurements. Measuring relative micromotion between implant and bone is recognized as the golden standard for implant stability testing as this micromotion is directly linked to the long-term fixation of cementless implants. However, specific custom-made set-ups are required to measure this micromotion, with the result that numerous studies opt to perform more straightforward load-to-failure tests. A custom-made micromotion test set-up for artificial acetabular bone models was developed and used to compare load-to-failure (implant push-out test) with micromotion and to assess the influence of bone material properties and press-fit on the implant stability. The results showed a high degree of correlation between micromotion and load-to-failure stability metrics, which indicates that load-to-failure stability tests can be an appropriate estimator of the primary stability of acetabular implants. Nevertheless, micromotions still apply as the golden standard and are preferred when high accuracy is necessary. Higher bone density resulted in an increase in implant stability. An increase of press-fit from 0.7 mm to 1.2 mm did not significantly increase implant stability.

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.

A comparative assessment of two designs of hip stem using rule-based simulation of combined osseointegration and remodelling

The stem–bone interface of cementless total hip arthroplasty undergoes an adaptive process of bone ingrowth until the two parts become osseointegrated. Another important phenomenon associated with aseptic loosening of hip stem is stress-shielding induced adverse bone remodelling. The objective of this study was to preclinically assess the relative performances of two distinct designs of hip stems by addressing the combined effect of bone remodelling and osseointegration, based on certain rule-based criteria obtained from the literature. Premised upon non-linear finite element analyses of patient-specific implanted femur models, the study attempts to ascertain in silico outcome of the hip stem designs based on an evolutionary interfacial condition, and to further comment on the efficacy of the rule-based technique on the prediction of peri-prosthetic osseointegration. One of the two hip stem models was a trade-off design obtained from an earlier shape optimization study, and the other was based on TriLock stem (DePuy). Both designs predicted similar long-term osseointegration (∼89% surface), although trade-off stem predicted higher post-operative osseointegration. Proximal bone resorption was found higher for TriLock (by ∼110%) as compared to trade-off model. The rule-based technique predicted clinically coherent osseointegration around both stems and appears to be an alternative to expensive mechanobiology-based schemes.

Short stems in total hip replacement: evidence on primary stability according to the stem type

BACKGROUND:

As the prevalence of total hip replacement is increasing in younger patients, less invasive implants (short stems) are becoming more favourable. However, despite the advantages of these stems, clinical results with a follow-up of more than 10 years are limited to a very few stem designs. There has been an increase in publications recently - mechanical and clinical studies - concerning the primary stability of short stems. Primary stem stability is an important factor as it reflects final stem stabilisation and is related to the clinical results of the prosthesis.

METHOD:

We conducted a systematic review of the literature to retrieve evidence concerning primary implant stability in short stems - as expressed by implant micromotion and stem subsidence - according to our previously proposed short-stem classification.

RESULTS:

Mechanical in vitro studies on stem micromotion are very few and limited to type 2 "partial collum" short stems. The results are comparable to those of stems with a known long-term excellent clinical course. Clinical results concerning stem migration patterns are also limited to some of the commercially available short stems. Although comparative studies are very few, the results for most of the short stems are similar to those of standard stems.

CONCLUSION:

There are promising results concerning biomechanical studies of the initial micromotion of short stems, as well as clinical results of stem migration patterns. Long-term clinical studies are needed in order to confirm these findings. The existing literature concerns very few of the many commercially available implants.

Varus malalignment of cementless hip stems provides sufficient primary stability but highly increases distal strain distribution

BACKGROUND

Varus position of cementless stems is a common malalignment in total hip arthroplasty. Clinical studies have reported a low rate of aseptic loosening but an increased risk for thigh pain. This in vitro study aimed to evaluate these clinical observations from a biomechanical perspective.

METHODS

A conventional cementless stem (CLS Spotorno) was implanted in a regular, straight (size 13.75) as well as in a varus position (size 11.25) in 6 composite femora (Sawbones), respectively. Primary stability was assessed by recording 3-dimensional micromotions under dynamic load bearing conditions and stress shielding was evaluated by registering the surface strain before and after stem insertion.

FINDINGS

Primary stability for stems in varus malposition revealed significantly lower micromotions (p < 0.05) for most regions compared to stems in neutral position. The greatest difference was observed at the tip of the stem where the straight aligned implants exceeded the critical upper limit for osseous integration of 150 μm. The surface strains for the varus aligned stems revealed a higher load transmission to the femur, resulting in a clearly altered strain distribution.

INTERPRETATION

This biomechanical study confirms the clinical findings of a good primary stability of cementless stems in a varus malposition, but impressively demonstrates the altered load transmission with the risk for postoperative thigh pain.

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.

Biomechanical comparison of a novel castless arthrodesis plate with T-plate and cross pin techniques for canine partial carpal arthrodesis

Objectives: To describe a novel canine castless partial carpal arthrodesis plate (par-CA) and its ex vivo biomechanical comparison with T-plate and cross pinning techniques for canine partial carpal arthrodesis.

Methods: The three implant systems were applied to three cohorts of six forelimbs from Greyhounds euthanatized for reasons unrelated to the study. Intercarpal and carpometacarpal palmar fibrocartilage and ligaments were sectioned. Potentiometers were applied between the radial carpal and third metacarpal bones to measure micromotion, and limbs were loaded at 30% of bodyweight at 1 Hertz for 10,000 cycles on a servo-hydraulic universal testing machine. Following assessment of micromotion, limbs were loaded to failure at 20 mm/s and ultimate strength, ultimate displacement, and stiffness were measured.

Results: The T-plate (p <0.01) and par-CA (p <0.01) had reduced micromotion relative to the cross pin constructs but there was no significant difference between the control, T-plate and par-CA constructs. There was no significant difference in ultimate strength between constructs. Ultimate displacement was reduced in the plated constructs. Stiffness did not differ between constructs.

Clinical significance: The novel par-CA construct was biomechanically similar to the T-plate and both were superior to cross pins in resisting micromotion. There was no difference in load at failure between constructs. The par-CA plate permits radial and ulnar carpal bone compression, a more distal location of the plate to limit impingement, and placement of screws in two metacarpal bones; features which may offer clinical benefits over T-plate fixation.

Influence of undersized cementless hip stems on primary stability and strain distribution

INTRODUCTION

Undersizing of cementless hip stems is a risk factor for aseptic loosening and early subsidence. The purpose of this study was to evaluate the effects of undersized stems and determine whether a biomechanical study can predict the clinical results.

MATERIALS AND METHODS

Three consecutive sizes of a clinically proven stem (CLS Spotorno) were implanted into six composite femora (size large, Sawbones^®), respectively. According to the Canal Fill Index (CFI), two stems (size 11.25 and 12.5) were undersized (CFI < 80%) and one stem (size 13.75) had an appropriate size (CFI > 80%). The primary stability was evaluated by measurement of 3-dimensional (3D)-micromotions under physiological adapted load and surface strains were recorded before and after implantation to detect stress-shielding processes.

RESULTS

Both undersized stems revealed significantly higher micromotions in all regions compared to the appropriate stem. The highest micromotions were registered at the distal tip of the three stem sizes. The changes in surface strain did not show a significant difference between the three stem sizes, but the highest strain reduction was observed proximally indicating a tendency for stress shielding.

CONCLUSIONS

This study confirms the clinical assumption that undersized stem result in a significantly reduced primary stability. Furthermore, in vitro studies allow to determine the effects of undersizing and stress shielding processes.

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.

Stable migration pattern of an ultra-short anatomical uncemented hip stem: a prospective study with 2 years radiostereometric analysis follow-up

INTRODUCTION

Shorter, anatomically shaped and proximally loading stems have been developed to achieve better stress distribution and be more bone preserving. The purpose of this prospective study was to evaluate the migration pattern of the Proxima™ ultra-short uncemented stem using radiostereometric analysis (RSA), and to review the literature regarding the migration of short stemmed hip arthroplasty.

METHODS

25 patients (28 hips) with hip osteoarthritis received a Proxima stem during total hip arthroplasty (THA). To measure stem migration, repeated RSA examinations were done during a 2 year follow up period. The patients were evaluated with the hip specific (HOOS) and the generic health (EQ5D) scores up to 1 year, and clinically for 6 years postoperatively.

RESULTS

Almost all migration occurred within the first 3 months, with mean subsidence of 0.22 mm and varus rotation of 1.04°, being the primary effect variables. After the third postoperative month and up to the 2 year RSA follow up no further significant migration occurred. The outcome scores showed substantial improvement after 1 year. No revisions were performed or indicated for any stem after a mean clinical follow up of 72.1 months.

CONCLUSIONS

Like many other uncemented stems, the Proxima showed early migration up to 3 months hereafter osseointegration seems to have occurred. The achieved stability and clinical outcomes indicate favorable early results for this stem in younger patients who have good bone quality and average BMI. We found, however, the surgical technique to be slightly more demanding compared to conventional stems owing to the unique implant design that necessitates specific adjusted femoral cutting and broaching procedures.

Acoustic pattern evaluation during cementless hip arthroplasty surgery may be a new method for predicting complications

BACKGROUND

Although surgeons must perform implantation of the cementless stem during total hip arthroplasty (THA) without complications, assessment is left to the surgeon's intuitive judgement, which could contain inter/intra-observer bias variety. We therefore asked (1) whether the sound created during the stem implantation could be evaluated objectively and (2) whether those sounds are correlate to the complication specific to the cementless stems. Our hypothesis is that the sounds produced during stem insertion could be quantified and related to the complications.

PATIENTS AND METHOD

In 71 THAs, we quantified the sound produced during stem insertion and investigated the relationship between these sounds and the occurrence of intraoperative fracture and subsidence.

RESULTS

The sound data were divided into two patterns: Patterns A and B. The difference between the peak value (dB) at the most common frequency (near 7 kHz) and the second most common frequency (near 4 kHz) of strikes during the final phase of implantation in Patterns A and B showed a significant difference. Adverse events on intraoperative fracture and subsidence were significantly less common in patients with Pattern A than in those with Pattern B (six of 42 hips with Pattern A and 13 of 29 hips with Pattern B, p = 0.004). Pattern A in predicting a clinical course without those adverse events was 69.2% and the specificity was 68.4%. Positive and negative predictive values were 85.7% and 44.8%, respectively.

CONCLUSION

The sound generated during stem insertion was quantified. Those sound patterns were associated with complications.

A biomechanical testing system to determine micromotion between hip implant and femur accounting for deformation of the hip implant: Assessment of the influence of rigid body assumptions on micromotions measurements

BACKGROUND

Accurate pre-clinical evaluation of the initial stability of new cementless hip stems using in vitro micromotion measurements is an important step in the design process to assess the new stem's potential. Several measuring systems, linear variable displacement transducer-based and other, require assuming bone or implant to be rigid to obtain micromotion values or to calculate derived quantities such as relative implant tilting.

METHODS

An alternative linear variable displacement transducer-based measuring system not requiring a rigid body assumption was developed in this study. The system combined advantages of local unidirectional and frame-and-bracket micromotion measuring concepts. The influence and possible errors that would be made by adopting a rigid body assumption were quantified. Furthermore, as the system allowed emulating local unidirectional and frame-and-bracket systems, the influence of adopting rigid body assumptions were also analyzed for both concepts. Synthetic and embalmed bone models were tested in combination with primary and revision implants. Single-legged stance phase loading was applied to the implant - bone constructs.

FINDINGS

Adopting a rigid body assumption resulted in an overestimation of mediolateral micromotion of up to 49.7μm at more distal measuring locations. Maximal average relative rotational motion was overestimated by 0.12° around the anteroposterior axis. Frontal and sagittal tilting calculations based on a unidirectional measuring concept underestimated the true tilting by an order of magnitude.

INTERPRETATION

Non-rigid behavior is a factor that should not be dismissed in micromotion stability evaluations of primary and revision femoral implants.

Influence of different sizes of composite femora on the biomechanical behavior of cementless hip prosthesis

BACKGROUND

For the biomechanical evaluation of cementless stems different sizes of composite femurs have been used in the literature. However, the impact of different specimen sizes on test results is unknown.

METHODS

To determine the potential effect of femur size the biomechanical properties of a conventional stem (CLS Spotorno) were examined in 3 different sizes (small, medium and large composite Sawbones®). Primary stability was tested under physiologically adapted dynamic loading conditions measuring 3-dimensional micromotions. For the small composite femur the dynamic load needed to be adapted since fractures occurred when reaching 1700N. Additionally, surface strain distribution was recorded before and after implantation to draw conclusions about the tendency for stress shielding.

FINDINGS

All tested sizes revealed similar micromotions only reaching a significant different level at one measurement point. The highest micromotions were observed at the tip of the stems exceeding the limit for osseous integration of 150μm. Regarding strain distribution the highest strain reduction after implantation was registered in all sizes at the level of the lesser trochanter.

INTERPRETATION

Specimen size seems to be a minor influence factor for biomechanical evaluation of cementless stems. However, the small composite femur is less suitable for biomechanical testing since this size failed under physiological adapted loads. For the CLS Spotorno osseous integration is unlikely at the tip of the stem and the tendency for stress shielding is the highest at the level of the lesser trochanter.

A randomised trial comparing a short and a standard-length metaphyseal engaging cementless femoral stem using radiostereometric analysis

This was a randomised controlled trial studying the safety of a new short metaphyseal fixation (SMF) stem. We hypothesised that it would have similar early clinical results and micromovement to those of a standard-length tapered Synergy metaphyseal fixation stem. Using radiostereometric analysis (RSA) we compared the two stems in 43 patients. A short metaphyseal fixation stem was used in 22 patients and a Synergy stem in 21 patients. No difference was found in the clinical outcomes pre- or post-operatively between groups. RSA showed no significant differences two years post-operatively in mean micromovement between the two stems (except for varus/valgus tilt at p = 0.05) (subsidence 0.94 mm (sd 1.71) vs 0.32 mm (sd 0.45), p = 0.66; rotation 0.96° (sd 1.49) vs 1.41° (sd 2.95), p = 0.88; and total migration 1.09 mm (sd 1.74) vs 0.73 mm (sd 0.72), p = 0.51). A few stems (four SMF and three Synergy) had initial migration > 1.0 mm but stabilised by three to six months, with the exception of one SMF stem which required revision three years post-operatively. For most stems, total micromovement was very low at two years (subsidence < 0.5 mm, rotation < 1.0°, total migration < 0.5 mm), which was consistent with osseous ingrowth. The small sample makes it difficult to confirm the universal applicability of or elucidate the potential contraindications to the use of this particular new design of stem.

Cite this article: Bone Joint J 2015; 97-B:595–602.

A novel method to assess primary stability of press-fit acetabular cups

Initial stability is an essential prerequisite to achieve osseointegration of press-fit acetabular cups in total hip replacements. Most in vitro methods that assess cup stability do not reproduce physiological loading conditions and use simplified acetabular models with a spherical cavity. The aim of this study was to investigate the effect of bone density and acetabular geometry on cup stability using a novel method for measuring acetabular cup micromotion. A press-fit cup was inserted into Sawbones® foam blocks having different densities to simulate normal and osteoporotic bone variations and different acetabular geometries. The stability of the cup was assessed in two ways: (a) measurement of micromotion of the cup in 6 degrees of freedom under physiological loading and (b) uniaxial push-out tests. The results indicate that changes in bone substrate density and acetabular geometry affect the stability of press-fit acetabular cups. They also suggest that cups implanted into weaker, for example, osteoporotic, bone are subjected to higher levels of micromotion and are therefore more prone to loosening. The decrease in stability of the cup in the physiological model suggests that using simplified spherical cavities to model the acetabulum over-estimates the initial stability of press-fit cups. This novel testing method should provide the basis for a more representative protocol for future pre-clinical evaluation of new acetabular cup designs.

Initial stability of an uncemented femoral stem with modular necks. An experimental study in human cadaver femurs

BACKGROUND

Uncemented implants are dependent upon initial postoperative stability to gain bone ingrowth and secondary stability. The possibility to vary femoral offset and neck angles using modular necks in total hip arthroplasty increases the flexibility in the reconstruction of the geometry of the hip joint. The purpose of this study was to investigate and evaluate initial stability of an uncemented stem coupled to four different modular necks.

METHODS

A cementless femoral stem was implanted in twelve human cadaver femurs and tested in a hip simulator with patient specific load for each patient corresponding to single leg stance and stair climbing activity. The stems were tested with four different modular necks; long, short, retro and varus. The long neck was used as reference in statistical comparisons. A micromotion jig was used to measure bone-implant movements, at two predefined levels.

FINDINGS

A femoral stem coupled to a varus neck had the highest value of micromotion measured for stair climbing at the distal measurement level (60μm). The micromotions measured with varus and retro necks were significantly larger than motions observed with the reference modular neck, P<0.001.

INTERPRETATION

The femoral stem evaluated in this study showed acceptable micromotion values for the investigated loading conditions when coupled to modular necks with different lengths, versions and neck-shaft angles.

Minimum 10 year survivorship analysis of a partially coated hydroxyapatite tapered femoral stem in elderly patients with an average age over 75

This is a retrospective, non-comparative study of 212 consecutive patients who underwent Total Hip Arthroplasty with an uncemented hydroxyapatite (HA) coated stem system from November 1997 to March 2000. The objective of the study was to analyze the performance of the implant at a minimum of 10 years in older patients (mean age 79.6 years). The Kaplan-Meier survivorship of the femoral stem at 10 years was 100%, and 97.5% for the whole prosthesis. The mean Merle d'Aubigné clinical score improved from 4.4 ± 2.1 points pre-operatively to 13.39 ± 3.77 points at final follow-up (p<0.05), and the mean VAS score for thigh pain was 1.25. The radiographic analysis showed that there were no significant radiolucent lines or osteolysis compromising the fixation of the implant.

Early migration characteristics of a 180° porous-coated cup with 1-mm press fit

INTRODUCTION

Evaluation of early cup movement is an important diagnostic tool to predict the likelihood of long-term implant loosening and clinical failure. The investigated cementless cup is clinically proven over 10 years, but there is a paucity of information that accurately describes the migration characteristics of this component.

MATERIALS AND METHODS

We retrospectively analysed the clinical outcome and migration behaviour of 60 Pinnacle 100 shells after an average 3.8-year follow-up (range 2.1-5.4 years). For migration measurement, EBRA (Einzel-Bild-Röntgen-Analyse) digital software was applied. Clinical assessment was performed using the HHS, the UCLA score and the SF-36 health survey.

RESULTS

The clinical outcome showed excellent results with a mean HHS of 95.4 (SD 7.1) and mean UCLA of 6.9 (SD 1.3). All implants were radiologically stable within the observation period and none of the cups was at risk for aseptical loosening. EBRA analysis revealed a mean total migration of 1.4 mm (SD 0.9) (95 % CI 1.1-1.6) at 3 years. Eight cups migrated more than 1 mm within the first three postoperative months, thereafter the migration curves flattened down.

CONCLUSION

Surgeons may expect to find a variable amount of early migration when using the Pinnacle cup. To our knowledge, these are the first results, which show an early "impaction" of a cementless cup, followed by subsequent osseointegration. We believe that an appropriate long-term outcome of the investigated cup is ensured. The data of the present investigation will provide clinicians with useful baseline information with which to compare new cup designs.

Simultaneous and multisite measure of micromotion, subsidence and gap to evaluate femoral stem stability

The initial stability of cementless femoral components is crucial for the long-term success of total hip arthroplasty. This has been reported in animal and clinical studies. Until now, the stability was evaluated by the measurement of relative micromotion on a few simultaneous locations around the stem in cadaveric experiments. This paper presents an extended experimental setup to measure simultaneously local micromotion, subsidence and gap on hundreds of points at the bone-stem interface. This technique we applied to anatomical and straight stems in three pairs of cadaveric femurs. Measurements were in agreement with typically reported values. Conversely to other methods, which measure micromotion between implant and bone anchoring points of the measuring device, our method provides local micromotion between stem surface and adjacent bone surface. The observed variation of micromotion at the peri-implant surface confirms the importance of this simultaneous measure on a lot of points around the implant.

A new technique to measure micromotion distribution around a cementless femoral stem

The interfacial micromotion is closely associated to the long-term success of cementless hip prostheses. Various techniques have been proposed to measure them, but only a few number of points over the stem surface can be measured simultaneously. In this paper, we propose a new technique based on micro-Computer Tomography (μCT) to measure locally the relative interfacial micromotions between the metallic stem and the surrounding femoral bone. Tantalum beads were stuck at the stem surface and spread at the endosteal surface. Relative micromotions between the stem and the endosteal bone surfaces were measured at different loading amplitudes. The estimated error was 10 μm and the maximal micromotion was 60 μm, in the loading direction, at 1400 N. This pilot study provided a local measurement of the micromotions in the 3 direction and at 8 locations on the stem surface simultaneously. This technique could be easily extended to higher loads and a much larger number of points, covering the entire stem surface and providing a quasi-continuous distribution of the 3D interfacial micromotions around the stem. The new measurement method would be very useful to compare the induced micromotions of different stem designs and to optimize the primary stability of cementless total hip arthroplasty.

Using 'subcement' to simulate the long-term fatigue response of cemented femoral stems in a cadaver model: could a novel preclinical screening test have caught the Exeter matt problem?

Previously, cement was formulated with degraded fatigue properties (subcement) to simulate long-term fatigue in short-term cadaver tests. The present study determined the efficacy of subcement in a 'preclinical' test of a design change with known clinical consequences: the 'polished'-to-'matt' transition of the Exeter stem (revision rates for polished stems were twice those for matt stems). Contemporary stems were bead blasted to give Ra = 1 microm (matt finish). Matt and polished stems were compared in cadaver pairs under stair-climbing loads (three pairs of size 1; three pairs of size 3). Stem micromotion was monitored during loading. Post-test transverse sections were examined for cement damage. Cyclic retroversion decreased for polished stems but increased for matt stems (p < 0.0001). The implant size had a substantial effect; retroversion of (larger) size-3 stems was half that of size-1 stems, and polished size-3 stems subsided 2.5 times more than the others. Cement damage measures were similar and open through-cracks occurred around both stems of two pairs. Stem retroversion within the mantle resulted in stem-cement gaps of 50-150 microm. Combining information on cyclic motion, cracks, and gaps, it was concluded that this test 'predicted' higher revision rates for matt stems (it also implied that polished size-3 stems might be superior to size-1 stems).

Early migration characteristics of a hydroxyapatite-coated femoral stem: an RSA study

Measurement of early stem subsidence can be used to predict the likelihood of long-term femoral component loosening and clinical failure. Data that examines the early migration pattern of clinically proven stems will provide clinicians with useful baseline data with which to compare new stem designs. This study was performed to evaluate the early migration pattern of a hydroxyapatite-coated press-fit femoral component that has been in use for over ten years. We enrolled 30 patients who underwent THA for osteoarthritis. The median age was 70 years (range, 55-80 years). Patients were clinically assessed using the Harris hip score. Radiostereometric analysis was used to evaluate stem migration at three to four days, six months, one year and two years. We observed a mean subsidence of 0.73 mm at six months, 0.62 mm at one year and 0.58 mm at two years and a mean retroversion of 1.82° at six months, 1.90° at one year and 1.59° at two years. This data suggests that subsidence is confined to the first six months after which there was no further subsidence. The results from this study can be compared with those from novel cementless stem designs to help predict the long-term outcome one may expect from new cementless stem designs.

Biomechanical evaluation of two types of short-stemmed hip prostheses compared to the trust plate prosthesis by three-dimensional measurement of micromotions

BACKGROUND

Stemless and short-stemmed hip prostheses have been developed to preserve femoral bone stock. While all these prostheses claim a more or less physiological load transfer, clinical long-term results are only available for the stemless thrust plate prosthesis. In this study, the in vitro primary stability of the thrust plate prosthesis was compared to two types of short-stemmed prostheses. In addition to the well-established Mayo prosthesis, the modular Metha prosthesis was tested using cone adapters with 130 degrees and 140 degrees neck-shaft-angles.

METHODS

The prostheses were implanted in composite femurs and loaded dynamically (300-1700 N). Three-dimensional micromotions at the bone-prosthesis interface were measured. In addition, the three-dimensional deformations at the surface of the composite femur were measured to gain data on the strain distribution.

FINDINGS

For all tested prostheses, the micromotions did not exceed 150 microm, the critical value for osteointegration. The thrust plate prosthesis revealed similar motions as the short-stemmed prostheses. The short-stemmed prosthesis with the 130 degrees cone tended to have the highest micromotions of all tested short-stemmed prostheses. The thrust plate prosthesis revealed the lowest alteration of bone surface deformation after implantation.

INTERPRETATION

The comparably low micromotions of the thrust plate prosthesis and the short-stemmed prostheses should be conducive to osseous integration. The higher alteration of load transmission after implantation reveals a higher risk of stress shielding for the short-stemmed prostheses.