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

Influence of different CCD angles on osseointegration and radiological changes after total hip arthroplasty of a triple wedge shape cementless femoral stem: a prospective cohort study

PURPOSE

The purpose of this study was to evaluate the osseointegration and radiological outcomes in patients after total hip arthroplasty, hypothesizing different load patterns with one cementless stem design and different CCD angles (CLS Spotorno femoral stem 125° vs 135°).

METHODS

All cases of degenerative hip osteoarthritis fulfilling strict inclusion criteria were treated with cementless hip arthroplasty between 2008 and 2017. Ninety-two out of one hundred six cases were clinically and radiologically examined three and 12 months after implantation. Two groups with each 46 patients were rendered prospectively and compared in clinical (Harris Hip Score) and radiological outcome.

RESULTS

At final follow-up, no significant difference regarding Harris Hip Score was detected between the two groups (mean 99.2 ± 3.7 vs. 99.3 ± 2.5; p = 0.73). Cortical hypertrophy was found in none of the patients. Stress shielding was seen in a total of 52 hips (n = 27 vs. n = 25; 57% of the 92 hips). No significant difference regarding stress shielding was detected when comparing both groups (p = 0.67). Significant bone density loss was detected in Gruen zone one and two in the 125° group. The 135° group showed significant radiolucency in Gruen zone seven. No overall radiological loosening or subsidence of the femoral component was observed.

CONCLUSION

According to our results, the use of a femoral component with a 125° CCD angle versus a 135° CCD did not result in a different osseointegration and load transfer with a clinically relevant significance.

Stress distributions of the short stem and the tapered wedge stem at different alignments: a finite element analysis study

BACKGROUND

The mechanical effects of stem length reduction and stem alignment on the surrounding femur remain unknown. This study directly compared the stress distribution on the surrounding femur of existing tapered wedge stems and short stems and examined the properties of stress distribution at different stem alignments in three dimensions.

METHODS

Finite element analysis was conducted for standing and walking. The cementless stem was appropriately sized to ensure adequate contact with the medial cortical bone line that contours the medullary cavity. The stem neck axis was aligned with the femoral neck axis in the mid-position and placed in 2° of the varus and valgus, 3° of flexion and extension, and 10° and 40° of anteversion.

RESULTS

Regardless of stem length, the trend of stress distribution was similar. The short stem generated less stress around the stem than the tapered wedge stem. In the coronal plane, the effect of varus and valgus deflection was small. In the sagittal plane, the stress generated around the stem was higher in the extended position than in the flexed position. In the horizontal plane, the stress generated around the stem was higher when the stem anteversion was smaller.

CONCLUSIONS

Depending on the design, short stems can reduce the stress on the surrounding bone, compared to a longer tapered wedge with similar stress distribution. Additionally, a short stem can reduce the effect of the varus position. Stems should be placed to achieve stable initial fixation while noting that stresses increase with extension and reduced anteversion.

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.

Long-term Results of the Thrust Plate Prosthesis, 21-year Follow-up: A Case Report and Literature Review

Introduction:

The thrust plate prosthesis (TPP) is a type of cementless hip replacement. Aimed to preserve femoral diaphyseal bone, it was favored by some orthopedic surgeons in younger patients as they could potentially undergo multiple revision arthroplasties during their lifetime. Of particular note, the preserved diaphyseal bone allows for the implantation of a subsequent primary total hip arthroplasty (THA).

Case Report:

We reported on a 64-year-old male patient who underwent thrust plate prosthesis (TPP) implantation 21 years ago (1999) for the treatment of primary osteoarthritis (OA) of the right hip joint. At 21-year follow-up, he had not developed any post-operative complications, and he reported a SF12 score of 32 and Oxford hip score of 47/48.

Conclusion:

TPP proved to be successful clinically and radiologically, especially in the young patient. It loads directly to the cortex of the proximal femoral metaphysis as in a native hip joint. It is a bone preserving prosthesis which allows for good bone stock in the case of revision surgery that was a victim of commercial considerations.

Comparison of Short Stems Versus Straight Hip Stems: A Biomechanical Analysis of the Primary Torsional Stability

Cementless straight stems show very good survival rates. However, the more distal force application of straight stems may lead to release-related proximal stress-shielding. Nevertheless, this technical brief had the objective of conducting a biomechanical in vitro analysis comparing short stems with established straight stems with respect to their primary torsional stability. Two cementless short hip stems and three cementless straight hip stems were implanted in n = 5 synthetic femora each. Torsional torques were applied into the hip stems at a continuous interval of ±7 Nm. Micromotions were measured by six inductive extensometers on four different measurement levels. At the proximal measuring point, significantly smaller relative micromotions of the CLS® prosthesis could be detected compared to all other stem models (p < 0.05). In all stem models, smallest relative micromotions were found at the metaphyseal/diaphyseal measuring point. Only at the measuring point of the distal tips of the straight stems, statistically significantly lower relative micromotion of the CLS® stem compared to the Trendhip® stem could be found (p < 0.01). All the investigated stems generally display a rather comparable anchoring pattern and an almost physiological force application. Since the comparatively long straight stems present an anchoring pattern nearly identical to that of the examined short stems, a shortening of the established straight stems could be taken into consideration. This would offer the advantage of minimally invasive surgery and bone-saving resection as well as preservation of cancellous bone in case a revision would become necessary.

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.

Metaphyseal anchoring short stem hip arthroplasty provides a more physiological load transfer: a comparative finite element analysis study

BACKGROUND

Short stem total hip arthroplasty (SHA) preserves femoral bone stock and is supposed to provide a more natural load transfer compared to standard stem total hip arthroplasty (THA). As comparative biomechanical reference data are rare we used a finite element analysis (FEA) approach to compare cortical load transfer after implantations of a metaphyseal anchoring short and standard stem in native biomechanical femora.

METHODS

The subject specific finite element models of biomechanical femora, one native and two with implanted metaphyseal anchoring SHA (Metha, B. Braun Aesculap) and standard THA (CLS, Zimmer-Biomet), were generated from computed tomography datasets. The loading configuration was performed with an axial force of 1400 N. Von Mises stress was used to investigate the change of cortical stress distribution.

RESULTS

Compared to the native femur, a considerable reduction of cortical stress was recorded after implantation of SHA and standard THA. The SHA showed less reduction proximally with a significant higher metaphyseal cortical stress compared to standard THA. Moreover, the highest peak stresses were observed metaphyseal for the SHA stem while for the standard THA high stress pattern was observed more distally.

CONCLUSIONS

Both, short and standard THA, cause unloading of the proximal femur. However, the metaphyseal anchoring SHA features a clearly favorable pattern in terms of a lower reduction proximally and improved metaphyseal loading, while standard THA shows a higher proximal unloading and more distal load transfer. These load patterns implicate a reduced stress shielding proximally for metaphyseal anchoring SHA stems and might be able to translate in a better bone preservation.

Comparative analysis of the biomechanical behavior of two different design metaphyseal-fitting short stems using digital image correlation

BACKGROUND

The progressive evolution in hip replacement research is directed to follow the principles of bone and soft tissue sparing surgery. Regarding hip implants, a renewed interest has been raised towards short uncemented femoral implants. A heterogeneous group of short stems have been designed with the aim to approximate initial, post-implantation bone strain to the preoperative levels in order to minimize the effects of stress shielding. This study aims to investigate the biomechanical properties of two distinctly designed femoral implants, the TRI-LOCK Bone Preservation Stem, a shortened conventional stem and the Minima S Femoral Stem, an even shorter and anatomically shaped stem, based on experiments and numerical simulations. Furthermore, finite element models of implant-bone constructs should be evaluated for their validity against mechanical tests wherever it is possible. In this work, the validation was performed via a direct comparison of the FE calculated strain fields with their experimental equivalents obtained using the digital image correlation technique.

RESULTS

Design differences between Trilock BPS and Minima S femoral stems conditioned different strain pattern distributions. A distally shifting load distribution pattern as a result of implant insertion and also an obvious decrease of strain in the medial proximal aspect of the femur was noted for both stems. Strain changes induced after the implantation of the Trilock BPS stem at the lateral surface were greater compared to the non-implanted femur response, as opposed to those exhibited by the Minima S stem. Linear correlation analyses revealed a reasonable agreement between the numerical and experimental data in the majority of cases.

CONCLUSION

The study findings support the use of DIC technique as a preclinical evaluation tool of the biomechanical behavior induced by different implants and also identify its potential for experimental FE model validation. Furthermore, a proximal stress-shielding effect was noted after the implantation of both short-stem designs. Design-specific variations in short stems were sufficient to produce dissimilar biomechanical behaviors, although their clinical implication must be investigated through comparative clinical studies.

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.

Is the strain pattern of conventional stems negatively affected by a previously short stem THA? An experimental study in cadavaric bone

BACKGROUND

A short stem hip arthroplasty can be revised in many cases using a conventional stem. Furthermore, in some cases the implantation of a short stem is intended, but intraoperatively reasons may lead to the decision to implant a conventional stem after previous preparation of a short stem.

OBJECTIVE

In both cases it is questionable if the anchorage of a conventional stem is negatively affected by the previous preparation of a short stem. In clinical practice mid- or long-term follow up for these special cases hardly exist.

METHODS

The strain patterns for the conventional Bicontact stem in primary implantation and after preparation of the proximal femur for a METHA short stem were tested biomechanically in three pairs of cadaveric femora.

RESULTS

The strain patterns for the conventional Bicontact after preparation of the METHA short stem were similar to conditions after testing the conventional stem in primary conditions.

CONCLUSIONS

These data lead to the consequence that in clinical practise the implantation of a conventional stem after preparation of a short stem and even after revision of a short stem is possible without increased risk of loosening or long-term stress-shielding.

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

BACKGROUND

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

METHODS/DESIGN

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

DISCUSSION

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

TRIAL REGISTRATION

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

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.

Analysis of the elastic bending characteristics of cementless short hip stems considering the valgus alignment of the prosthetic stem

BACKGROUND

The resultant hip force causes a varus torque which must be compensated by a shear force couple depending on the stem alignment of the prosthesis. Since the prosthesis is substantially less flexible than the bone, the interior of the femur is stiffened over the entire prosthesis length. The present study thus aims at analyzing short-stem prostheses for its elastic bending characteristics, considering inappropriate valgus alignment of the prosthetic stem.

METHODS

Five short stem prostheses were implanted each in synthetic femora in a standardized manner - in neutral and valgus stem alignments. Bending movements were recorded applying a tilting torque M_X of ±3.5 Nm in medio-lateral direction. Variance analyses and Friedman tests were used. A P-value <.05 was considered statistically significant.

FINDINGS

Bending movements b₁-b₆ showed significant differences (P < .05). It could be shown that different stem alignments (P < .05) and different measuring points had a highly significant influence (P < .001) on the relative movements. Compared to the AIDA®, the MiniHip™ as well as the Metha® stiffened the femur to a higher degree (P < .001).

INTERPRETATION

Regarding the elastic bending behavior we see a relevant influence of the stems´ design. We conclude that the short-stem principle does not necessarily require the shortest possible prosthesis but rather a long and thin extending stem tip to optimize the lever ratios, ensuring a more physiological bending behavior of the femur. In addition, without sufficient anchoring of the prosthesis, the valgus stem alignment could favor tilting of the implant and should therefore be avoided.

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.

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.

Full-field measurement of micromotion around a cementless femoral stem using micro-CT imaging and radiopaque markers

A good primary stability of cementless femoral stems is essential for the long-term success of total hip arthroplasty. Experimental measurement of implant micromotion with linear variable differential transformers is commonly used to assess implant primary stability in pre-clinical testing. But these measurements are often limited to a few distinct points at the interface. New techniques based on micro-computed tomography (micro-CT) have recently been introduced, such as Digital Volume Correlation (DVC) or markers-based approaches. DVC is however limited to measurement around non-metallic implants due to metal-induced imaging artifacts, and markers-based techniques are confined to a small portion of the implant. In this paper, we present a technique based on micro-CT imaging and radiopaque markers to provide the first full-field micromotion measurement at the entire bone-implant interface of a cementless femoral stem implanted in a cadaveric femur. Micromotion was measured during compression and torsion. Over 300 simultaneous measurement points were obtained. Micromotion amplitude ranged from 0 to 24µm in compression and from 0 to 49µm in torsion. Peak micromotion was distal in compression and proximal in torsion. The technique bias was 5.1µm and its repeatability standard deviation was 4µm. The method was thus highly reliable and compared well with results obtained with linear variable differential transformers (LVDTs) reported in the literature. These results indicate that this micro-CT based technique is perfectly relevant to observe local variations in primary stability around metallic implants. Possible applications include pre-clinical testing of implants and validation of patient-specific models for pre-operative planning.

Comparison of Specific Femoral Short Stems and Conventional-Length Stems in Primary Cementless Total Hip Arthroplasty

There are several reported disadvantages with conventional-length femoral stems in cementless total hip arthroplasty (THA). Therefore, various efforts have been made to develop a specific femoral short stem to improve physiologic bone remodeling at the femoral aspect of a cementless THA. However, there are potential disadvantages with specific femoral short stems, such as malalignment, inadvertent subsidence, and potential proximal femoral fracture. Therefore, the authors quantitatively compared radiographic and clinical outcomes as well as component-specific complications between 2 groups of patients following primary cementless THA. A matched comparison was made between specific femoral short stems (n=50) and conventional-length femoral stems (n=50) in cementless THA procedures performed between January 2008 and January 2012. Patients were matched for age, sex, body mass index, height, surgical approach, and surgeon. No significant differences were found between the 2 groups in mean postoperative radiographic outcomes, functional outcomes, or complications. Both groups showed satisfactory performance at 5-year follow-up. Specific femoral short stems resulted in a higher incidence of malalignment and subsidence and a lower incidence of thigh pain and proximal bone resorption compared with conventional-length femoral stems. Although longer follow-up is required, specific femoral short stems may have clinical and radiographic advantages with equivalent perioperative complications relative to conventional-length femoral stems. However, this technique requires proper patient selection in combination with careful preoperative planning and meticulous surgical technique.

Comparative Analysis of the Biomechanical Behaviour of Two Cementless Short Stems for Hip Replacement: Linea Anatomic and Minihip

A comparative study between two stems (Linea Anatomic and Minihip) has been performed in order to analyse the differences in their biomechanical behaviour, concerning stem micromotions and load transmission between stem and bone. From the corresponding finite element models, a parametric study was carried out to quantify ranges of micromotions taking into account: friction coefficient in the stem-bone interface, press-fit and two types of gait cycle. Micromotions were evaluated for each stem at six different levels along repeated gait cycles. An initial and marked stem subsidence at the beginning of the simulation was observed, followed by an asymptotic decrease due to friction forces. Once migration occurs, a repeated reversible cyclic micromotion is developed and stabilized as gait cycle times are simulated. The general motion pattern exhibited higher amplitude of micromotion for Minihip compared to Linea stem. The load transmission mechanism was analyzed, identifying the main internal forces. The results show higher local forces for Minihip stem up to 80% greater than for Linea stem. The differences of design between Minihip and Linea conditioned different distributions of load, influencing the posterior stress-shielding. Consequently, short stems require high bone stock and quality should, being indicated for young patients with high bone quality.

The Short-term Clinical Outcome of Total Hip Arthroplasty Using Short Metaphyseal Loading Femoral Stem

Purpose

The purpose of this study was to retrospectively evaluate the short-term clinical and radiological outcomes of total hip arthroplasty (THA) with short metaphyseal loading femoral stem.

Materials and Methods

We retrospectively reviewed the records of 56 cases in 47 patients who had undergone THA with short metaphyseal loading femoral stem from April 2010 to December 2011. There were 20 males and 27 females. The mean age was 54 years (range, 26-77 years). The average follow up period was 4.6 years. Clinical results were evaluated by Harris hip scores (HHS) before the operation and at the last follow-up. Radiographic analysis was done by evaluating osteolysis, loosening, stress shielding, and alignement.

Results

The mean HHS significantly improved from 45 (range, 15-58) preoperatively to 98 (range, 85-100) at the last follow-up. In radiographic analyses, there was no evidence of osteolysis or loosening around the stems and the acetabuluar components. All cases showed rounding off of the calcar, grade 1 bone resorption of the proximal femur. With regard to implant alignment, 51 femoral component (91.1%) were in neutral position, and 5 (8.9%) were in varus position. There was 1 intraoperative fracture treated by cerclage wiring and no case was revised during follow-up period.

Conclusion

Although longer follow-up is needed to confirm the durability of the short metaphyseal loading femoral stem, this short stem might provide stable fixation without diaphyseal fixation and demonstrated good clinical result at mean 4.6 year short term follow-up.

A prospective randomized radiographic and dual-energy X-ray absorptiometric study of migration and bone remodeling after implantation of two modern short-stemmed femoral prostheses

BACKGROUND

The aim of this prospective randomized study was to analyze migration and strain transmission of the Metha™ and Nanos™ femoral prostheses.

MATERIALS AND METHODS

Between 1 January 2011 and 2 April 2013, 50 patients were randomized to receive short-stemmed femoral prostheses. Metha™ stems were implanted in 24 patients (12 female, 12 male; mean age 58.7 years; mean body mass index [BMI] 27.4) and Nanos™ stems in 26 patients (10 female, 16 male; mean age 59.7 years; mean BMI 27.1). Longitudinal stem migration, varus-valgus alignment, changes of center of rotation (COR), femoral offset and caput-collum-diaphyseal angle, leg length discrepancy, periprosthetic radiolucent lines incidence, and dual-energy X-ray absorptiometry (DEXA) scans were analysed after an average of 98 and 381 days.

RESULTS

There was no significant change of varus-valgus alignment or clinically relevant migration of the Metha™ or Nanos™ prostheses during postoperative follow-up. After 12.3 months, the DEXA scans showed small but significant differences of bone mineral density in Gruen zones 1 (minus ~8 %) and 6 (plus ~9 %) for the Metha™ and in Gruen zone 1 (minus ~14 %) for the Nanos™ (paired t test). Visual analog scale (VAS) and Harris Hip Score (HHS) improved significantly for both implants (Nanos™/Metha™ 12.3 months postoperatively HSS 96.5/96.2; VAS 0.7/0.8, respectively). COR or offset did not change significantly after surgery.

CONCLUSIONS

Neither implant showed signs of impaired osseointegration. DEXA demonstrated proximally located load transfer with only moderate proximal stress shielding.

LEVEL OF EVIDENCE

II.

An integrated CAD/CAM/robotic milling method for custom cementless femoral prostheses

Aseptic loosening is the primary cause of cementless femoral prosthesis failure and is related to the primary stability of the cementless femoral prosthesis in the femoral cavity. The primary stability affects both the osseointegration and the long-term stability of cementless femoral prostheses. A custom cementless femoral prosthesis can improve the fit and fill of the prosthesis in the femoral cavity and decrease the micromotion of the proximal prosthesis such that the primary stability of the custom prosthesis can be improved, and osseointegration of the proximal prosthesis is achieved. These results will help to achieve long-term stability in total hip arthroplasty (THA). In this paper, we introduce an integrated CAD/CAM/robotic method of milling custom cementless femoral prostheses. The 3D reconstruction model uses femoral CT images and 3D design software to design a CAD model of the custom prosthesis. After the transformation matrices between two units of the robotic system are calibrated, consistency between the CAM software and the robotic system can be achieved, and errors in the robotic milling can be limited. According to the CAD model of the custom prosthesis, the positions of the robotic tool points are produced by the CAM software of the CNC machine. The normal vector of the three adjacent robotic tool point positions determines the pose of the robotic tool point. In conclusion, the fit rate of custom pig femur stems in the femoral cavities was 90.84%. After custom femoral prostheses were inserted into the femoral cavities, the maximum gaps between the prostheses and the cavities measured less than 1 mm at the diaphysis and 1.3 mm at the metaphysis.

Reproducing the hip center with a femoral neck-retaining implant

Because of the recent trend for total hip arthroplasty in younger patients, more bone-preserving implants have been designed, based partly on the premise that maintenance of more bone stock would allow revision with standard primary implants. Another possible advantage of an implant that retains the femoral neck is that it may allow anatomical recreation of the center of the femoral head, femoral neck anteversion, anterior tilt, and caput-collum-diaphysis (CCD) angle, as well as femoral offset and leg length, without the use of modular implants. To determine whether the proximal femoral anatomy could be recreated, this study used computed tomography data and 3-dimensional modeling from 10 cadaver hips in 6 whole body specimens. Three femoral neck resection levels were investigated: 2 mm, 5 mm (recommended resection), and 10 mm from the base of the femoral neck. Results showed that the center of the femoral head, femoral neck anteversion, and CCD angle could all be recreated with available implant sizes, without modularity, within 2° and 1 mm on average. The addition of a modular neck provided no advantage in recreation of the hip center or other anatomical parameters. Use of a short metaphyseal femoral neck retaining-type of stem should allow restoration of anatomical parameters without the use of an exchangeable modular femoral neck.

Fixation of the shorter cementless GTS™ stem: biomechanical comparison between a conventional and an innovative implant design

INTRODUCTION

Conventional cementless total hip arthroplasty already shows very good clinical results. Nevertheless, implant revision is often accompanied by massive bone loss. The new shorter GTS™ stem has been introduced to conserve femoral bone stock. However, no long-term clinical results were available for this implant. A biomechanical comparison of the GTS™ stem with the clinically well-established CLS(®) stem was therefore preformed to investigate the targeted stem philosophy.

MATERIALS AND METHODS

Four GTS™ stems and four CLS(®) stems were implanted in a standardized manner in eight synthetic femurs. A high-precision measuring device was used to determine micromotions of the stem and bone during different load applications. Calculation of relative micromotions at the bone-implant interface allowed the rotational implant stability and the bending behavior of the stem to be determined.

RESULTS

Lowest relative micromotions were detected near the lesser trochanter within the proximal part of both stems. Maximum relative micromotions were measured near the distal tip of the stems, indicating a proximal fixation of both stems. For the varus-valgus-torque application, a comparable stem bending behavior was shown for both stems.

CONCLUSION

Both stems seem to provide a comparable and adequate primary stability. The shortened GTS™ design has a comparable rotational stability and bone-implant flexibility compared to a conventional stem. This study demonstrates that the CLS(®) stem and the GTS™ stem exhibit similar biomechanical behavior. However, a clinical confirmation of these experimental results is still required.

Changes in strain patterns after implantation of a short stem with metaphyseal anchorage compared to a standard stem: an experimental study in synthetic bone

Short stem hip arthroplasties with predominantly metaphyseal fixation, such as the METHA® stem (Aesculap, Tuttlingen, Germany), are recommended because they are presumed to allow a more physiologic load transfer and thus a reduction of stress-shielding. However, the hypothesized metaphyseal anchorage associated with the aforementioned benefits still needs to be verified. Therefore, the METHA short stem and the Bicontact® standard stem (Aesculap, Tuttlingen, Germany) were tested biomechanically in synthetic femora while strain gauges monitored their corresponding strain patterns. For the METHA stem, the strains in all tested locations including the region of the calcar (87% of the non-implanted femur) were similar to conditions of synthetic bone without implanted stem. The Bicontact stem showed approximately the level of strain of the non-implanted femur on the lateral and medial aspect in the proximal diaphysis of the femur. On the anterior and posterior aspect of the proximal metaphysis the strains reached averages of 78% and 87% of the non-implanted femur, respectively. This study revealed primary metaphyseal anchorage of the METHA short stem, as opposed to a metaphyseal-diaphyseal anchorage of the Bicontact stem.

Biomechanics of lateral plate and pedicle screw constructs in lumbar spines instrumented at two levels with laterally placed interbody cages

BACKGROUND CONTEXT

The lateral transpsoas approach to interbody fusion is gaining popularity because of its minimally invasive nature and resultant indirect neurologic decompression. The acute biomechanical stability of the lateral approach to interbody fusion is dependent on the type of supplemental internal fixation used. The two-hole lateral plate (LP) has been approved for clinical use for added stabilization after cage instrumentation. However, little biomechanical data exist comparing LP fixation with bilateral pedicle screw and rod (PSR) fixation.

PURPOSE

To biomechanically compare the acute stabilizing effects of the two-hole LP and bilateral PSR fusion constructs in lumbar spines instrumented with a lateral cage at two contiguous levels.

STUDY DESIGN

Biomechanical laboratory study of human cadaveric lumbar spines.

METHODS

Eighteen L1-S1 cadaveric lumbar spines were instrumented with lateral cages at L3-L4 and L4-L5 after intact kinematic analysis. Specimens (n=9 each) were allocated for supplemental instrumentation with either LP or PSR. Intact versus instrumented range of motion was evaluated for all specimens by applying pure moments (±7.5 Nm) in flexion/extension, lateral bending (LB) (left+right), and axial rotation (AR) (left+right). Instrumented spines were later subjected to 500 cycles of loading in all three planes, and interbody cage translations were quantified using a nonradiographic technique.

RESULTS

Lateral plate fixation significantly reduced ROM (p<.05) at both lumbar levels (flexion/extension: 49.5%; LB: 67.3%; AR: 48.2%) relative to the intact condition. Pedicle screw and rod fixation afforded the greatest ROM reductions (p<.05) relative to the intact condition (flexion/extension: 85.6%; LB: 91.4%; AR: 61.1%). On average, the largest interbody cage translations were measured in both fixation groups in the anterior-posterior direction during cyclic AR.

CONCLUSIONS

Based on these biomechanical findings, PSR fixation maximizes stability after lateral interbody cage placement. The nonradiographic technique served to quantify migration of implanted hardware and may be implemented as an effective laboratory tool for surgeons and engineers to better understand mechanical behavior of spinal implants.

Subject-specific finite element simulation of the human femur considering inhomogeneous material properties: a straightforward method and convergence study

In numerical finite element (FE) simulations of human bones subject-specific models are necessary to reproduce the physiological conditions, which include the determination of inhomogeneous material properties from computed tomography (CT) scans and their implementation in the numerical model. In the present approach common software packages are directly used for the entire simulation process from segmentation of CT scans, surface reconstruction, mesh generation, calculation of mean element densities to FE simulation. The influence of the mesh discretisation level on the maximum displacement, the total system energy and the principal surface stress distribution of eight human femurs was analysed. Both the maximum displacement and the total system energy showed typical convergence behaviour towards an asymptotic value with decreasing element size. The principal surface stress distribution followed similar qualitative trends at all mesh discretisation levels studied for the same femur. However, the stress distributions did not converge with decreasing element size and still differed significantly between the two smallest element sizes studied of approximately 2mm and 1mm. The magnitude of convergence differed among the individual femurs. Thus, individual convergence studies in terms of local stress or strain distributions are necessary for accurately predicting local stress and strain values in subject-specific FE bone models.

The influence of resection height on proximal femoral strain patterns after Metha short stem hip arthroplasty: an experimental study on composite femora

PURPOSE

The number of candidates for a total hip arthroplasty (THA) is steadily increasing, while the average patient age is decreasing for primary THA. The rise in THA is mainly due to excellent clinical outcomes and the extended longevity of modern implants. Short stem arthroplasties with predominantly metaphyseal fixation such as the Metha® stem are suggested for young patients. It is hypothesised that the more physiological load transfer of these devices reduces stress shielding, which in turn may reduce the risk of aseptic loosening. However, patients with femoral deformities often require a deviation of the resection height. To this end, our aim was to evaluate how resection height influences strain patterns in order to characterise possible limits for short stem implantation.

METHODS

Biomechanical testing using ten strain gauges on synthetic bone illustrated the strain patterns of three different resection heights (0, +5 and +10 mm) for the Metha stem.

RESULTS

The greatest differences in strains were displayed at the "high" (most proximal) resection height (+10 mm) when compared to the non-implanted strain pattern. At the medial calcar, the strain was 143% for +10 mm, 96% for +5 mm and 94% for 0 mm. Overall, discrepancies were less for deeper resections.

CONCLUSIONS

The deeper the resection, the more similar the strain patterns are when compared to a non-implanted synthetic bone. Changes in strain patterns are induced by variation in the varus/valgus positioning of the implant and by different offsets.

Bone remodeling after total hip arthroplasty with a short stemmed metaphyseal loading implant: finite element analysis validated by a prospective DEXA investigation

In total hip arthroplasty (THA), short stemmed cementless implants are used because they are thought to stimulate physiological bone remodeling and reduce stress shielding. We performed a numerical investigation on bone remodeling after implantation of a specific short stemmed implant using finite element analysis (FEA). Overall bone mass loss was 2.8% in the entire femur. Bone mass decrease was mostly found in the proximal part of the calcar and in the greater trochanter due to the vast cross section of the implant, probably leading to stress shielding. In the diaphysis, no change in the apparent bone density was proven. The assumptions made agreed well with bone remodeling data from THA recipients who underwent dual-energy X-ray absorptiometry. However, the clinical investigation revealed a bone mass increase in the minor trochanter region that was less pronounced in the FEA. Further comparisons to other stem designs must be done to verify if the relative advantages of the investigated implant can be accepted.

Experimental validation of numerically predicted strain and micromotion in intact and implanted composite hemi-pelvises

The failure mechanisms of acetabular prostheses may be investigated by understanding the changes in load transfer due to implantation and using the analysis of the implant–bone micromotion. Computational finite element (FE) models allow detailed mechanical analysis of the implant–bone structure, but their validity must be assessed as a first step, before they can be employed in preclinical investigations. In this study, FE models of composite hemi-pelvises, intact and implanted with an acetabular cup, were experimentally validated. Strains and implant–bone micromotions in the hemi-pelvises were compared with those predicted by the equivalent FE models. Regression analysis indicated close agreement between the measured and FE strains, with a high correlation coefficient (0.95–0.98), a low standard error (SE) (36–53 µε) and a low error in regression slope (7%–11%). Measured micromotions along three orthogonal directions were small, less than 30 µm, whereas the FE-predicted values were found to be less than 85 µm. Although the trends were similar, the deviations are due to artefacts in experimental measurement and additional imperfections in recreating experimental loading and boundary conditions in the FE model. This supports the FE model as a valid predictor of the measured strain in the composite pelvis models, confirming its suitability for further computational investigations on acetabular prostheses.

Revision of hip resurfacing arthroplasty with a bone-conserving short-stem implant: a case report and review of the literature

Introduction

Suitable treatment of early failure of total hip replacement is critical in younger patients, as bone stock is lost and the functional outcome is impaired.

Case presentation

We report the case of a 56-year-old Caucasian woman with early failure of hip resurfacing arthroplasty. While revision is usually performed with a conventional hip implant, this case report describes for the first time a revision procedure with a bone-conserving short-stem hip implant.

Conclusions

Our approach allows further conservation of femoral bone stock and provides a long-term solution to the patient, which maintains the possibility of using a conventional hip implant should a second revision become necessary.

Experimental Validation of Finite Element Models of Intact and Implanted Composite Hemipelvises Using Digital Image Correlation

A detailed understanding of the changes in load transfer due to implantation is necessary to identify potential failure mechanisms of orthopedic implants. Computational finite element (FE) models provide full field data on intact and implanted bone structures, but their validity must be assessed for clinical relevance. The aim of this study was to test the validity of FE predicted strain distributions for the intact and implanted pelvis using the digital image correlation (DIC) strain measurement technique. FE models of an in vitro hemipelvis test setup were produced, both intact and implanted with an acetabular cup. Strain predictions were compared to DIC and strain rosette measurements. Regression analysis indicated a strong linear relationship between the measured and predicted strains, with a high correlation coefficient (R = 0.956 intact, 0.938 implanted) and a low standard error of the estimate (SE = 69.53 με, 75.09 με). Moreover, close agreement between the strain rosette and DIC measurements improved confidence in the validity of the DIC technique. The FE model therefore was supported as a valid predictor of the measured strain distribution in the intact and implanted composite pelvis models, confirming its suitability for further computational investigations.

Migration analysis of a metaphyseal anchored short-stem hip prosthesis

BACKGROUND AND PURPOSE

Metaphyseal anchored short-stem hip implants were designed to improve load transmission and preserve femoral bone stock. Until now, only few outcome data have been available and migration studies are one of the few ways of obtaining data that are predictive of implant survival. We therefore evaluated a metaphyseal anchored short-stem hip implant by Ein Bild Roentgen Analyse femoral component analysis (EBRA-FCA).

PATIENTS AND METHODS

First, the EBRA-FCA method was validated for the short-stem hip implant. Then 80 of the first 100 consecutive implants were evaluated after at least 2 years. Clinical assessment was performed using the WOMAC and the UCLA score.

RESULTS

After 2.7 (2.0-4.2), years none of the implants had been revised and by that time the stems had subsided by a mean of 0.7 mm (SD 1.8) (95% CI: 0.3-1.1). Of the 80 implants, 78 were stable after 2 years, with 74 being primary stable and 4 showing secondary stabilization after initial subsidence. Continuous migration was seen in only 2 patients. The clinical outcome showed good results with a mean WOMAC of 11 (SD 13) and a mean UCLA score of 7.3 (SD 2.0).

INTERPRETATION

The metaphyseal anchored short-stem hip implant showed good functional results and a high degree of stability after 2 years. The outcome is comparable to that of clinically proven conventional hip implants and if the results are confirmed by long-term studies, short-stem hip arthroplasty might be an alternative for young patients requiring hip replacement.

Experimental evaluation of new concepts in hip arthroplasty

In this thesis we evaluated two different hip arthroplasty concepts trough in vitro studies and numerical analyses. The cortical strains in the femoral neck area were increased by 10 to 15 % after insertion of a resurfacing femoral component compared to values of the intact femur, shown in an in vitro study on human cadaver femurs. There is an increased risk of femoral neck fracture after hip resurfacing arthroplasty. An increase of 10 to 15 % in femoral neck strains is limited, and cannot alone explain these fractures. Together with patient specific and surgical factors, however, increased strain can contribute to increased risk of fracture. An in vitro study showed that increasing the neck length in combination with retroversion or reduced neck shaft angle on a standard cementless femoral stem does not compromise the stability of the stem. The strain pattern in the proximal femur increased significantly at several measuring sites when the version and length of neck were altered. However, the changes were probably too small to have clinical relevance. In a validation study we have shown that a subject specific finite element analysis is able to perform reasonable predictions of strains and stress shielding after insertion of a femoral stem in human cadaver femurs. The usage of finite element models can be a valuable supplement to in vitro tests of femoral strain pattern around hip arthroplasty. Finally, a patient case shows that bone resorption around an implant caused by stress shielding can in extreme cases lead to periprosthetic fracture.

Clinical and radiographical results of 179 thrust plate hip prostheses: 5-14 years follow-up study

INTRODUCTION

The thrust plate hip prosthesis (TPP) is a bone-reserving prosthesis for cementless fixation at the metaphysis of the proximal femur. We retrospectively evaluated the results of 162 patients (179 hips) who underwent hip arthroplasty using TPP.

PATIENTS AND METHODS

Eighty-three patients (87 hips) suffered from osteoarthritis of the hip joint (OA group), 79 patients (92 hips) from osteonecrosis of the femoral head (ON group). The mean age at surgery was 55 years in the OA group and 47.4 years in the ON group. The mean follow-up period was 97 months in the OA group and 104 months in the ON group. For these patients, we evaluated the results clinically and radiographically.

RESULTS

The mean Merle d'Aubigne's score improved from 8.2 to 16.9 in the OA group and from 9.1 to 16.6 in the ON group at the final follow-up. Early mechanical loosening of TPP was observed in two hips of OA and one hip of ON. In one patient of ON, bilateral TPPs had to be removed 5 years postoperatively because of infection. Two female patients with ON suffered from a spontaneous femoral fracture below the tip of the lateral plate. Kaplan-Meier survivorship using TPP removed for any reason as the end point was 97.7% in the OA group and 90.3% in the ON group after 13 years.

CONCLUSION

The middle-term results of the TPP were satisfactory if the indication for the TPP and the operative procedure were appropriate. The TPP is a useful and safe prosthesis for relatively young patients with not only osteoarthritis of the hip but also osteonecrosis of the femoral head.

Primary stability and strain distribution of cementless hip stems as a function of implant design

BACKGROUND

Short stem prostheses have been developed to preserve the femoral bone stock. The purpose of this study was to evaluate the stress-shielding effect in the proximal femur as well as the micromotion between bone and implant as a measure of primary stability for a new short stem in comparison to a clinically successful short stem and a straight stem.

METHODS

Using paired fresh human femurs, stress shielding was examined by using tri-axial strain gage rosettes. The strain distribution of the proximal femur was measured before and after implantation of three cementless prostheses of different design concepts and stem lengths. Furthermore, interface motion and rotational stability were investigated under dynamic loading (100-1600 N) after 100,000 load cycles using inductive miniature displacement transducers.

FINDINGS

A reduction of longitudinal cortical strains in the proximal femur was displayed for all three implants. The reduction was less pronounced for the shorter stem implants, however. Interface motion was below the critical threshold of 150 μm at almost all measuring points for all three stems, with a tendency for greater rotational stability in the shorter stem implants.

INTERPRETATION

The new short stem prosthesis displayed reduced stress shielding and comparable primary stability to an established short stem and a conventional shaft design. Shortening the stem did not negatively influence primary stability. The clinical implications of these findings remain to be proven.

Is there an increased stem migration or compromised osteointegration of the Mayo short-stemmed prosthesis following cerclage wiring of an intrasurgical periprosthetic fracture?

AIM

Short-stemmed prostheses are increasingly regarded as implants of first choice in primary THA. As a result of the press-fit fixation in the femoral metaphysis, the occurrence of intraoperative fractures were reported. The aim of this study was to analyze the postoperative results of the Mayo short-stem prosthesis following treatment of an intrasurgical femur fracture with cerclage wiring.

PATIENTS AND METHODS

From 1999 to 2005, in 38 patients (18 females, 20 males; mean age 56 years; mean BMI = 27) with the diagnosis of coxarthrosis in whom a Mayo short-stemmed prosthesis has been implanted, an intraoperative fracture was observed. The fractures were treated with cerclage wiring (1 cerclage, n = 32; 2 cerclages, n = 5; 3 cerclages, n = 1). Postoperatively, all patients were prescribed mobilization without weight-bearing (floor contact) on the treated leg for 6 weeks. Using the Wristing software, longitudinal stem migration and varus-valgus femoral stem alignment were examined digitally in anteroposterior X-rays taken immediately after surgery, after 6 weeks and on average after 5.7 years (Zeh et al., Z Orthop Unfall 149:200-205, 2011). Additionally, the incidence of periprosthetic radiolucent lines was captured in the anteroposterior X-rays and assigned to the Gruen zones. Additionally, a DEXA scan was performed. The X-rays of a matched control group after the implantation of a Mayo prosthesis without femur fracture were analyzed by the same method.

RESULTS

There was no significant migration of the Mayo prosthesis in the study or control groups during postoperative follow-up (t test, P > 0.05). The cerclage group compared with the control group showed a statistically significant valgus tilt of 1.5° on average during the follow-up, which is regarded to be clinically not relevant. The frequency of occurrence of radiolucent lines was not statistically different (chi-square test, χ = 0.42, P = 0.51). DEXA scans showed no differences of the bone mineral density in the Gruen zones compared with a historical control group.

CONCLUSION

After wiring of an intrasurgical fracture, no disadvantage could be proven for Mayo prosthesis regarding stem migration and varus-valgus alignment. Furthermore, due to the absence of differences in the occurrence of radiolucent lines and the same results in the DEXA scan, an unimpaired osseointegration is assumed.

Biomechanical evaluation of different offset versions of a cementless hip prosthesis by 3-dimensional measurement of micromotions

BACKGROUND

Cementless hip prostheses with different offsets are frequently used to restore the rotation center of the hip. However, a rising offset is theoretically associated with a potential risk for increased interface stresses and early loosening.

METHODS

To assess this potential risk for cementless stems, the primary stability of the CLS Spotorno stem was examined with respect to three different femoral neck versions (125°, 135° and 145°) measuring 3-dimensional micromotions. For this purpose 18 stems were implanted in composite femurs and tested dynamically using physiological loading conditions considering the necessary adaptation according to the different offsets. Additionally the deformations at the surface of the composite femur were recorded to draw conclusions about the tendency for stress shielding.

FINDINGS

The micromotions of the different offset versions were not significantly different. The highest values were obtained at the tip of the stems, even exceeding the critical limit for osseous integration of 150μm. Compared to untreated composite femurs the alteration of the deformations at the surface remained relatively low. A significant difference was only observed in the ventral measurement points.

INTERPRETATION

According to the measured micromotions no offset version of the CLS Spotorno can be declared as superior. The assumption that the varus version is characterized by extended interface stresses could not be confirmed. Furthermore, it could be demonstrated that according to the principle of proximal load transfer of the CLS Spotorno stem an osseous integration of the distal part cannot be expected and that the risk for stress shielding appears to be relatively low.

Extensive Risk Analysis of Mechanical Failure for an Epiphyseal Hip Prothesis: A Combined Numerical—Experimental Approach

There has been recent renewed interest in proximal femur epiphyseal replacement as an alternative to conventional total hip replacement. In many branches of engineering, risk analysis has proved to be an efficient tool for avoiding premature failures of innovative devices. An extensive risk analysis procedure has been developed for epiphyseal hip prostheses and the predictions of this method have been compared to the known clinical outcomes of a well-established contemporary design, namely hip resurfacing devices.

Clinical scenarios leading to revision (i.e. loosening, neck fracture and failure of the prosthetic component) were associated with potential failure modes (i.e. overload, fatigue, wear, fibrotic tissue differentiation and bone remodelling). Driving parameters of the corresponding failure mode were identified together with their safe thresholds. For each failure mode, a failure criterion was identified and studied under the most relevant physiological loading conditions. All failure modes were investigated with the most suitable investigation tool, either numerical or experimental.

Results showed a low risk for each failure scenario either in the immediate postoperative period or in the long term. These findings are in agreement with those reported by the majority of clinical studies for correctly implanted devices. Although further work is needed to confirm the predictions of this method, it was concluded that the proposed risk analysis procedure has the potential to increase the efficacy of preclinical validation protocols for new epiphyseal replacement devices.

Femoral neck cut level affects positioning of modular short-stem implant

A trend in total hip arthroplasty surgery has been to design more bone-preserving procedures, especially for younger patients. This study investigated the final implant positioning of a short metaphyseal femoral neck type of implant to determine whether leg length, caput collum diaphysis (CCD) angle, and offset could be re-created with different levels of femoral neck resection. Ten cadaveric hips in 6 whole-body specimens were used, with 3 fiducial markers to allow registration of computer navigation points to computed tomography scan data. Three femoral neck resection levels were investigated: 0 mm, +5 mm (the recommended level of resection), and +10 mm from the base of the femoral neck. Results showed that the CCD angle was significantly higher with 0-mm neck cut and the offset was lower, whereas the highest neck cut had longer leg-length results. Surgeons who use a short metaphyseal stem need to realize the importance of a proper femoral neck cut to restore anatomic parameters as well as the possible benefit of computer-assisted surgery to restore these anatomic parameters during surgery.