Development and Finite Element (FE) analysis of a novel short hip stem concept.

Introduction: In order to improve the anchorage behavior of short hip stems, this development project aims at designing a short hip stem concept that preserves the femoral neck and minimizes interference with the physiological stress distribution of the femur. The new design will be evaluated according to ISO 7206-4 which is the standard for testing Implants for surgery. Methods: Basic CAD models based on an established short stem prosthesis were created and evaluated using finite element analysis. The best design was further developed to achieve a more deformable stem while maintaining stability. The model was validated through in vitro testing. Results: The "H-Beam" short stem showed a higher degree of deformation of approximately 142-144% compared to the established short stem. The FE model had a relative error of 0.98% and 1.07% compared to the in vitro tests. An operating procedure was outlined for this new short stem design. Discussion: The FE model is deemed valid due to small differences in comparison to in vitro testing. The short-stem prosthesis is more flexible and can be easily adapted to individual anatomy during surgery. The prosthesis length is similar to conventional prostheses, but the new stem design could allow better and faster osteointegration while preserving the cancellous bone structure.

Retrospective clinical and X-ray-based outcome analysis of a short-stem hip arthroplasty taking into account the operative learning curve over 7 years in the 3-year control course.

INTRODUCTION: Self-monitoring is crucial to work progressively with a high-quality standard. A retrospective analysis is a valuable tool for studying the postoperative outcome of a prosthesis and for evaluating the learning process for the surgeon. MATERIALS AND METHODS: The learning process of one surgeon was analysed in 133 cases of hip arthroplasty. These were divided into seven groups representing the surgical years 2008-2014. Over the course of 3 postoperative years, a total of 655 radiographs were analysed at regarding three radiological quality parameters (centrum-collum-diaphyseal angle (CCD angle), intramedullary fit&fill ratio (FFR), and migration) and ancillary outcome parameters (Harris Hip Score (HHS), blood loss, operating time, and complications). This period was divided into five times: 1st-day post-op, 6 M, 12 M, 24 M, and 36 M. Bivariate Spearman's correlation analysis and pairwise comparisons were performed. RESULTS: The total collective achieved a proximal FFR of over 0.8. The distal prosthesis tip migrated and was located on the lateral cortex within the first months. The CCD angle initially showed a variation with a subsequent constant course. The HHS showed a significant increase (p < 0.001) to over 90 points postoperatively. Over time, the operating time and blood loss decreased. Intraoperative complications existed only at the beginning of the learning phase. A learning curve effect can be determined for almost all parameters by comparing the subject groups. CONCLUSIONS: Operative expertise was shown to gain through a learning curve, whereby postoperative results corresponded to the system philosophy of the short hip stem prosthesis. The distal FFR and the distal lateral distance could represent the principle of the prosthesis, which overall could be an interesting approach for verification of a new parameter.

Short Stem for Total Hip Arthroplasty (THA) - Overview, Patient Selection and Perspectives by Using the Metha® Hip Stem System.

Short stem hip replacement has not only gained attention but also significance over the past decades. However, there still remains uncertainty regarding the correct indications for these stems. Even companies, producing implants, have varying recommendations that are more likely based on a well-meant advice than on statistically evaluated data. Thus, it is important to evaluate the advantages and disadvantages of a short stem prosthesis. The goal of this paper is to reveal some of the existing uncertainty in this field, by analyzing the Metha® short hip stem system. This paper does not only focus on general aspects but also discusses some more specific problems, such as avascular necrosis and post-rheumatic diseases, as well as hip dysplasia and coxarthrosis. The aim is also to convey the opportunity to indicate this type of implant for elderly and obese patients as well as for femoral misalignments following post-Perthes disease, post-traumatic deformities or other malpositions of the hip.

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.

How screw connections influence the primary stability of acetabular cups under consideration of different bone models.

INTRODUCTION: Good osteointegration of implants requires sufficient primary stability. Aim of this study was to examine the influence of screw fixation on the primary stability of press-fit cups. METHODS: Two press-fit acetabular cups were tested with regard to the influence and number of screws and their primary stability. RESULTS: For the relatively thin-walled Allofit®-S cup, an influence of the number of screws on the different forms of movement could be demonstrated. CONCLUSIONS: We see a clear influence of the cup wall thickness on the elastic deformability and accordingly on the primary stability of the examined cups.

Outcome of short- to medium-term migration analysis of a cementless short stem total hip arthroplasty using EBRA-FCA: a radiological and clinical study.

INTRODUCTION: Stress shielding may lead to aseptic loosening which is a common reason for implant failure. An established method to identify implants with risk of implant failure caused by aseptic loosening is to measure early migration of the stem with the "Ein Bild Roentgen Analyse" femoral component analysis (EBRA-FCA). Therefore, the aim of this study was to measure the migration of a cementless short stem prosthesis via EBRA-FCA to predict the future performance. MATERIALS AND METHODS: A total collective of 71 patients were treated with a cementless short hip stem prosthesis. Indications for surgery were primary coxarthrosis, dysplasia coxarthrosis or femoral head necrosis. After surgery, the patients were followed-up immediately after 3, 6, 12, 24, 36, and 48 months and X-ray images for EBRA-FCA measurements were taken. Axial caudal migration as well as the varus/valgus tilting of the prosthesis was determined. Possible influencing factors like BMI, age, diagnosis, gender or Harris Hip Score (HHS) on the migration of the stem were assessed. RESULTS: HHS increased significantly direct postoperatively (p < 0.001). At the second follow-up the average caudal migration was 0.42 ± 0.52 mm (range: 0.00-2.85 mm) (p < 0.001). A total of 14 patients underwent a caudal migration greater than 1.5 mm until 48 months. The initial varus and valgus tilts within the first 3 months were significant (p < 0.001). No correlations between BMI, age, diagnosis, gender or HHS and the migration as well as the tilting of the cementless short hip stem prosthesis were found. CONCLUSIONS: Although initial axial caudal migration as well as tilting tendencies in varus or valgus position can be detected, there is no marked migration of the examined prosthesis after the first 48 months. Likewise, no aseptic early loosening was detected throughout the study period, which indicates good osseointegration of the short stem prosthesis.

In vitro examination of the primary stability of three press-fit acetabular cups under consideration of two different bearing couples.

BACKROUND: For preclinical statements about the anchoring behavior of prostheses, the primary stability of the prosthesis is of special importance. It was the aim of this study to examine and compare the relevant relative micromotions of three different acetabulum prostheses by introducing three-dimensional torques. METHODS: The cups were implanted under standard conditions into an anatomical artificial bone model. Three-dimensional torques were applied to the acetabular cups. Taking into account the resulting frictional moments of two different bearing couples, ceramic-on-ceramic and ceramic-on-polyethylene, the relative micromotions of the cups were recorded as maximum total micromotion, translational and rotational micromotion, and the primary stability values of the three cups were compared. RESULTS: Relative micromotion of all cup models was always significantly smaller with the CoC bearing couples than with the CoP bearing couples (p < 0.001). The rotational micromotion was always lower (p < 0.001) than the translational micromotion, and the rotational as well as the translational micromotions were each always lower than the maximum total micromotion (p < 0.001, p < 0.010). The thinnest-walled cup system always showed the largest relative micromotions. CONCLUSION: The results of our study can be interpreted as indicating that the low relative micromotions of all cups - irrespective of the use of CoC or CoP bearing couples - are within an acceptable range favoring secondary osseointegration of the implants. Furthermore, we were able to show that the cup wall thickness and the surface quality of the cup systems have an influence on the primary stability and the elastic deformability of the examined cup systems.

Development of periprosthetic bone mass density around the cementless Metha® short hip stem during three year follow up-a prospective radiological and clinical study.

PURPOSE: The purpose of this study was to check the concept of the cementless Metha® short hip stem in order to find out whether proximal physiological load transfer can be achieved. METHODS: Fourty-three patients were included. Epidemiological factors were established. The Harris Hip Score was determined and measurement of bone mass density as well as osteodensitometric and radiological measurements was carried out pre-operatively, post-operatively, and after six, 12, 24, and 36 months. RESULTS: Harris Hip Score improved from 55.9 ± 12.4 pre-operatively to 94.8 ± 8.2 after 36 months (p < 0.001). After initial reduction of bone density in zones 1 and 7 up to six months post-operatively, there was a steady approximation of bone density to the initial values (p < 0.05). CONCLUSION: The Metha® short hip stem shows good clinical results. Furthermore, there is an increase of bone density in the proximal zones 1 and 7 between six and 36 months serving as a sign of physiological load transfer.

Age-related osseointegration of a short hip stem: a clinical and radiological 24 months follow-up.

PURPOSE: The purpose of this study was to examine potential differences between patients under and over 60 years who underwent a total short hip stem arthroplasty in a 24-month follow-up in a clinical setting. METHODS: Sixty seven patients were included in this prospective study and divided in two groups. In the younger cohort 39 patients ≤ 59 years and in the older cohort 28 patients ≥ 60 years were included. Clinical and radiological examinations of the patients took place preoperatively, postoperatively and after 3, 6, 12 and 24 months. The Harris Hip Score (HHS) was raised as clinical examination. Changes of bone mass density (BMD), stress shielding, reactive lines, spot welds and sclerosing were examined as radiological analysis. RESULTS: The HHS improved from 53.6 ± 8.2 preoperative to 93.2 ± 9.6 in the younger cohort and for the older cohort from 57.6 ± 14.8 to 94.1 ± 7.6 after 24 months. BMD growth was detectable in regions of interest (ROI) 3 and 6 for both groups. Different results between the cohorts could only be detected in ROIs 4 and 5. The older cohort showed a loss of BMD of 5.95% in ROI 4 and 3.17% in ROI 5 after 24 months, whereas the younger cohort showed no loss of BMD in both ROIs. CONCLUSIONS: No significant differences or any influences of osseointegration and clinical outcome of the short hip stem for both groups were detectable. Lower losses (ROIs 1 and 2) and higher growths (ROIs 3, 4, 5) were discovered in younger patients and the HHS improved in both groups. Reasons for this result could be age, sex or the level of activity of the patients, but in both groups the short hip stem shows comparable results in clinical as well as in radiological examination. We conclude that the short hip stem seems to be an adequate treatment also for older patients.

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 MX 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 cerclages on primary stability of tumor megaprostheses subjected to distal femur defects.

BACKROUND: Purpose of this experimental study was to investigate the influence of cerclages on the primary stability of the MUTARS system using distally fractured synthetic femora. METHODS: 4 MUTARS prostheses were implanted in synthetic femora respectively. Groups consisted of 4 intact bones, 4 fractured with cerclages and 4 fractured bones without cerclages. Spatial micromovements were measured with a high-precision rotational setup. FINDINGS: The order from the weakest to the strongest torque transmission of the intact bones was rm1-rm4-rm2-rm3 (p=0.011) and of the fractured bones with cerclages rm4-rm1-rm3-rm2 (p=0.013). The MUTARS stems broke out of the fractured femoral shaft by removing cerclages (p<0.001) and by the influence of bone defect A (p<0.001). Overall micromovements of the intact bones were lower than those of the fractured bones without cerclages (p<0.001) and overall micromovements of the fractured bones with cerclages were lower than those of bones without cerclages (p<0.001). INTERPRETATION: Due to high press-fit at the proximal and distal isthmus region fissural fractures of the femur may occur. This should always be taken into account. It is advisable to secure them and provide a prophylaxis for these fissural fractures by means of cerclages.

Influences of fit and fill following hip arthroplasty using a cementless short-stem prosthesis.

INTRODUCTION: The purpose of this study consisted in examining the effects of fit and fill ratio of the Metha prosthesis (BBraun, Aesculap, Tuttlingen, Germany) on radiological and clinical outcomes at a follow-up of 1 year. METHODS: 40 patients were included. Fit and fill ratio measurements, radiological and clinical examinations were performed preoperatively and postoperatively. Correlations were established between fit and fill ratio, and potential factors like sex, age, body mass index, Harris Hip Score and changes of radiological signs. RESULTS: The whole cohort (100%) had a tight fit and fill ratio (>0.8) at the proximal level and at each follow-up. "Champagne-flute" configuration provoked high distal tight-fit and fill ratio. Poor distal fit and fill ratio compared to the proximal and the mid-stem level was measurable at each follow-up (p < 0.05). Correlations between fit and fill ratio and preoperative femur configurations were detectable. CONCLUSIONS: Implanting the Metha prosthesis induces tight fit and fill ratio at the proximal and coated sections. Preoperative femur configuration should be considered to achieve best fit and fill situation and therefore excellent primary stability. In most cases "normal" and "stove-pipe" configurations provide good proximal fit and fill. Since "champagne-flute" configuration induces undesirable tight distal fit and fill ratio the size of the Metha (®) stem should be adequately increased to achieve a more proximal load transmission.

Sports activities before and after medial unicompartmental knee arthroplasty using the new Heidelberg Sports Activity Score.

PURPOSE: The purpose of our study was to investigate the change of sports activities before and after medial unicompartmental knee arthroplasty (UKA) with the use of the Heidelberg Sports Activity Score (HAS). METHODS: The Heidelberg Sports Activity Score (HAS) as well as FFb-H-OA, Oxford-12-Score, Tegner, UCLA and ACS were carried out to assess sports activities in 157 patients with medial osteoarthritis of the knee joint before and after UKA. The newly developed HAS also records the important duration and the sporting activity. RESULTS: Patients practiced sports in a more deliberate manner after UKA. Hiking, cycling and swimming were the sports most increased after surgery. Patients ≤65 years of age were sportier than those >65 years. Men were sportier than women, but all became sportier postoperatively. Patients with a high body mass index (BMI) are less sporty at first, but then increase their sports activity after surgery. CONCLUSIONS: HAS is an effective and valid assessment scale for evaluating sports activities before and after knee replacement.

Length of clinically proven cemented hip stems: state of the art or subject to improvement?

PURPOSE: The purpose of this study was to identify the shortest possible length of an established cemented hip stem in order to reduce stress shielding and optimise its bending behaviour. METHODS: Twenty-five prototypes from the same batch (five for each stem length) were included. Lengths resulted from the original length (100 %) and four distal shortenings to 94, 88, 83 and 78 % of original length. For standardised implantations, synthetic femurs were prepared. Relative movements under axial torque and stem bending under varus-valgus torque applications were investigated consecutively. Analyses of variance (ANOVA) were applied to detect differences between lengths. RESULTS: The maximum torque transfer occurred at the level of the lesser trochanter (p < 0.01). In particular, for the 78 % version, relative motion of the stem tip increased significantly compared with all other lengths (p = 0.02-0.04). Comparable findings were made for mean overall movements. In regards to varus-valgus torque, both the distal tip and the proximal shoulder always bent in the same direction following the femoral deflection. For the 83 % version bending of stem's shoulder was increased compared with the other length (p = 0.01 - 0.02), whereas tip bending was always comparable (p = 0.45-0.91). CONCLUSIONS: Our data show that the intention to preclinically optimise the mechanical behaviour of cemented stems by modifying their length seems to be a reasonable option. Advantages are decreased inner bone stiffening to reduce stress shielding and a lower cement volume, which is involved in cellular interactions.

Changes of periprosthetic bone density after a cementless short hip stem: a clinical and radiological analysis.

PURPOSE: The purpose of this study was to examine the concept of proximal load initiation of a total short-stemmed hip arthroplasty (Metha BBraun, Aesculap, Tuttlingen, Germany) on the basis of bone variations by means of osteodensitometric dual energy X-ray absorptiometry and radiologic measurements. METHODS: After power analysis 40 patients were included in this study. DXA examination, radiological and clinical follow-up was performed pre-operatively and postoperatively. Socio-demographic relevancies of bone progression and radiological changes were raised epidemiologically and clinically. RESULTS: Improvement of the Harris hip score from 54.7 points preoperative to 96.7 points postoperative was detected (p < 0.01). Loss of summarized overall net average bone mass density (netavg BMD) could only be manifested after six months compared to the netavg BMD of the postoperative measurement (p < 0.01). After six and 12 months BMD atrophy was shown mainly in regions of interest (ROI) 1, 4 and 7. Positive correlations between changes of BMD and age, male sex and BMI were detectable. No stem had to be revised. CONCLUSIONS: The Metha implant shows excellent osseointegration at the coated area of the stem without factors of aseptic loosening in the short term. This study shows that parameters like age, sex and BMI influence BMD progression and stress shielding. Metha implant shows excellent results especially in young patients with good bone stock.