[Unicondylar knee arthroplasty-trigger for outpatient arthroplasty]. / Teilprothesen ­ Trigger zur ambulanten Endoprothetik?

The combination of a reduction in surgical trauma in unicondylar knee arthroplasty compared to total knee arthroplasty and the introduction of a standardised enhanced recovery concept leads to a pre-, peri- and postoperative improvement in the patient's condition, which results in a reduction of the length of stay in hospital. In healthy, motivated patients, day-case or outpatient surgical treatment is possible under these circumstances.

Tourniquet does not affect intraoperative kinematics during total knee arthroplasty: Results of a prospective study using a robotic assistance system.

PURPOSE: Tourniquet use during total knee arthroplasty (TKA) remains controversial. There are limited data demonstrating the effect of tourniquet use on flexion and extension gaps. The use of a tourniquet can theoretically affect the kinematics of the knee joint, specifically the extension and flexion gaps and the laxity, by mechanically compressing the soft tissues including the muscles above the knee joint. Therefore, this study was designed to prospectively evaluate changes in flexion and extension gaps with and without the use of a tourniquet. METHODS: The following prospective study included 50 consecutive patients who underwent TKA using a surgical robot. The inclusion criteria were advanced osteoarthritis (OA) and varus-alignment or valgus-alignment <3° (hip-knee-ankle angle, standing long-leg X-ray), and the exclusion criteria were BMI >35 kg/m2 and mechanical axis in >3° valgus. A CR-TKA was performed, and the medial and lateral gaps (in mm) throughout the full range of motion in 10° increments were recorded. The procedure was conducted both with and without an applied tourniquet (350 mmHg). RESULTS: No significant differences were observed in the medial joint space. By contrast, the lateral gap showed significant differences in 10-20° of flexion (with a tourniquet 1.9 mm vs. without a tourniquet 2.1 mm, p = 0.018), 20-30° (1.6 vs. 1.8 mm, p = 0.02), 100-110° (0.9 vs. 1.1 mm, p = 0.021), and 110-120° (0.8 vs. 1 mm, p = 0.038). Thus, at the above degrees of flexion on the lateral side, there was a decrease in the mean of 0.2 mm with the use of a tourniquet. CONCLUSION: Although the use of a tourniquet showed a detectable change in the lateral gap in four 10° segments of flexion, clinical relevance with an average difference of 0.2 mm is not achieved. Thus, the use of a tourniquet in TKA can still be advocated based on the presented data. LEVEL OF EVIDENCE: Level I.

Defining 'undersizing' in short-stem total hip arthroplasty: the importance of sufficient contact with the lateral femoral cortex.

INTRODUCTION: Undersizing is 1 of the main reasons for early implant failure. Adequate sizing in short-stem total hip arthroplasty can be challenging and, so far, lacks key decision criteria. METHODS: We included 191 calcar-guided short stems. All patients underwent standardised digital anteroposterior imaging pre- and post-surgery and during follow-up. Preoperative planning was performed digitally. Planned stem sizes were retrospectively assessed and compared with the implanted sizes. Additionally, adequate sizing was analysed by determining whether the stem made intraoperative contact with the lateral distal femoral cortex. Implant migration was assessed by Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis 5 years after surgery. Influence of different Dorr types and postoperative centrum-collum-diaphyseal angle (CCD) categories on lateral femoral cortical contact were analysed. Additionally, the Harris Hip Score (HHS) was assessed at final follow-up. Stem-revision rate was documented. RESULTS: Implanted stems were at least 2 sizes smaller than those at the preoperative planning in 49 (25.7%) cases. The stem made contact with the lateral distal femoral cortex in only 130 hips (68.1%). Mean subsidence was significantly higher in the no-contact group (2.07 mm, range -7.7 to 1.7) than in the contact group (1.23 mm, range -4.5 to 1.8) at the final follow-up (p = 0.0018). Stems at least 2 sizes smaller than those at preoperative planning showed a significantly higher prevalence of non-contact (46.9% vs. 26.8%) (p = 0.009). Those undersized stems were more likely found in varus hips. No influence of the Dorr classification and the different CCD categories on the probability of achieving sufficient cortical contact was found. HHS showed no intergroup differences. CONCLUSIONS: Stems that did not make intraoperative contact with the lateral femoral cortex showed significantly increased axial migration at mid-term follow-up. Thus, the investigated criteria regarding the definition of undersizing in short-stem THA should be acknowledged. No obvious mid-term consequences were noted regarding revision rate. Long-term results are mandatory.

The epicondylar ratio can be reliably determined in both computed tomography and X-ray.

PURPOSE: One of the key factors to the successful revision of total knee arthroplasty (rTKA) is the reconstruction of the joint line, which can be determined using the epicondylar ratio (ER). The measurement is established in X-ray and MRI. However, it is not known whether computed tomography (CT) allows a more reliable determination. The objective was to assess the reliability of the ER in CT and to determine the correlation between the ER in CT and a.p. X-ray of the knee. METHODS: The ER was determined on X-ray and CT images of a consecutive series of 107 patients, who underwent rTKA. Measurements were made by two blinded observes, one measured twice. The inter- and intraobserver agreement, as well as the correlation between the two methods, were quantified with the Intraclass Correlation Coefficient. RESULTS: The average lateral ER was 0.32 (± 0.04) in X-ray and 0.32 (± 0.04) in CT. On the medial side, the average ER was 0.34 (± 0.04) in X-ray and 0.35 (± 0.04) in CT. The interobserver agreement for the same imaging modality was lateral 0.81 and medial 0.81 in X-ray as well as lateral 0.74 and medial 0.85 in CT. The correlation between the two methods was lateral 0.81 and medial 0.79. CONCLUSIONS: The ER can be reliably determined in X-ray and CT. Measurements of the two image modalities correlate. Prior to rTKA, the sole use of the X-ray is possible.

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.

Abductor Muscle Force after Straight-Stem Compared to Short-Stem Total Hip Arthroplasty through a Modified Direct Lateral Approach: Functional Assessment of 70 Consecutive Patients of a Randomized Controlled Clinical Trial.

Because of preservation of proximal femoral bone stock and minimized soft tissue trauma, short-stem implants are becoming increasingly important in total hip arthroplasty (THA). The postulated advantage regarding the functional outcome has not been verified. We hypothesized an increased abductor muscle strength by the use of a short-stem design. Seventy consecutive patients of a randomized clinical trial were included. Of these, 67 patients met the inclusion criteria after 12 months. Thirty-five patients received a standard straight stem and 32 patients a short-stem femoral component. All surgeries were performed by a modified direct lateral approach. Isometric muscle strength of the hip abductors was evaluated preoperatively 3 and 12 months after surgery. Harris hip score (HHS) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores were evaluated. After three months, there were no differences between the two groups; the abductor force was comparable to the preoperative initial values. After 12 months, a significant increase in muscle strength for the short stem patient group compared to preoperative baseline values was measured (straight-stem THA, 0.09 Nm/kg ± 0.4, p = 0.32; short-stem THA, 0.2 Nm/kg ± 0.3, p = 0.004). Comparison of the 12-month postoperative total HHS and WOMAC revealed no significant differences between both groups. A significant increase in hip abductor muscle strength 12 months after short-stem THA compared to conventional-stem THA was observed.

Biomechanics of a cemented short stem: a comparative in vitro study regarding primary stability and maximum fracture load.

PURPOSE: In total hip arthroplasty, uncemented short stems have been used more and more frequently in recent years. Especially for short and curved femoral implants, bone-preserving and soft tissue-sparing properties are postulated. However, indication is limited to sufficient bone quality. At present, there are no curved short stems available which are based on cemented fixation. METHODS: In this in vitro study, primary stability and maximum fracture load of a newly developed cemented short-stem implant was evaluated in comparison to an already well-established cemented conventional straight stem using six pairs of human cadaver femurs with minor bone quality. Primary stability, including reversible micromotion and irreversible migration, was assessed in a dynamic material-testing machine. Furthermore, a subsequent load-to-failure test revealed the periprosthetic fracture characteristics. RESULTS: Reversible and irreversible micromotions showed no statistical difference between the two investigated stems. All short stems fractured under maximum load according to Vancouver type B3, whereas 4 out of 6 conventional stems suffered a periprosthetic fracture according to Vancouver type C. Mean fracture load of the short stems was 3062 N versus 3160 N for the conventional stems (p = 0.84). CONCLUSION: Primary stability of the cemented short stem was not negatively influenced compared to the cemented conventional stem and no significant difference in fracture load was observed. However, a clear difference in the fracture pattern has been identified.

Mid-term gender-specific differences in periprosthetic bone remodelling after implantation of a curved bone-preserving hip stem.

BACKGROUND: The implant-specific periprosthetic bone remodelling in the proximal femur is considered to be an important factor influencing the long-term survival of cementless hip stems. Particularly data of gender-specific differences regarding bone-preserving stems are very rare in literature and mainly limited to short-term investigations. Therefore, we investigated at mid-term one arm of a prospective randomised study to evaluate if there is an influence of gender on implant-specific stress shielding after implantation of a curved bone preserving hip stem (Fitmore) 5 years postoperatively. HYPOTHESIS: We hypothesised there will be no gender-specific differences in periprosthetic bone remodelling. PATIENTS AND METHODS: A total of 20 female and 37 male patients underwent total hip arthroplasty using the Fitmore stem. Clinical, radiological as well as osteodensitometric examinations were performed preoperatively, 7 days and 3, 12 and 60 months postoperatively. Clinical data collection included the Western Ontario and McMaster Universities Arthritis Index (WOMAC) and the Harris Hip Score (HHS). Periprosthetic bone mineral density (BMD) was measured using Dual Energy X-ray Absorptiometry (DXA) and the periprosthetic bone was divided into 7 regions of interest (ROI) for analysis. The results at 3, 12 and 60 months were compared with the first postoperative measurement after 7 days to obtain a percentage change. RESULTS: Periprosthetic BMD showed a decrease in all 7 ROIs for both groups 5 years postoperatively referred to the baseline value, except ROI 3 (0.8%, p=0.761), representing the distal lateral part of the stem, and ROI 5 (0.3%, p=0.688), representing the distal medial part of the stem in the male cohort. Significant gender differences were found in ROI 1 (-16.0% vs. -3.5%, p=0.016) and ROI 6 (-9.9% vs. -2.1%, p=0.04) in favour of the male patients. Clinical results showed no significant gender differences 5 years postoperatively with regard to WOMAC (mean 0.4 (±0.8, 0-3.3) in women vs. 0.3 (±0.8, 0-4.2) in men, p=0.76) and HHS (mean 93.0 (±9.7, 66.0-100.0) in women vs. 93.9 (±11.5, 53.0-100.0) in men, p=0.36). CONCLUSION: Proximal stress shielding was observed independent of gender 5 years postoperatively. However, there was a significantly lower bone loss proximal lateral and medial below the calcar in male patients, indicating a more physiological load transfer. [ClinicalTrials.gov identifier: NCT03147131 (Study ID D.3067-244/10). Registered 10 May 2017 - retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT03147131?term=Bieger&draw=2&rank=1] LEVEL OF EVIDENCE: IV; prospective study without control group.

Mid-term migration pattern of a calcar-guided short stem: A five-year EBRA-FCA-study.

BACKGROUND: Short-term results of several short-stem designs have indicated early axial migration. Mid- and long-term results for most designs are lacking. The objective of this study was to evaluate the mid-term migration pattern of a calcar-guided short stem five years postoperative. METHODS: Implant migration of 191 calcar-guided short stems was assessed by Ein-Bild-Roentgen-Analysis Femoral-Component- Analysis (EBRA-FCA) 5 years after surgery. Migration pattern of the whole group was analyzed and compared to the migration pattern of implants potentially being "at hazard" with a subsidence of more than 1.5 mm at 2 years postoperatively. Influence of preoperative Dorr types (A vs. B vs. C), age (<70 vs. >70 years), gender (female vs. male), weight (<90 kg vs. >90 kg), BMI (<30 vs. >30) and uni-vs. bilateral procedures on mid-term migration pattern was analyzed. Additionally outcome of varus- and valgus stem alignment was assessed. RESULTS: Mean axial subsidence was 1.5 mm (SD 1.48 mm) at final follow-up. Two years after surgery 73 short stems were classified "at hazard". Of these stems, 69 cases showed secondary stabilisation in the following period, whereas 4 cases presented unstable with more than 1 mm of further subsidence. Stem revision was not required neither in the group of implants with early stabilisation nor the group with pronounced early onset migration. Male gender and heavy-weight patients had a significant higher risk for axial migration, as well as extensive valgus stem alignment, whereas for Dorr type B, compared to A, no statistical difference could be observed. CONCLUSIONS: In most cases, even in the group of stems being "at hazard", settling could be documented. While different Dorr types did not show a statistically significant impact on axial migration, particularly in male and heavy-weight patients the risk of continuous subsidence is increased. In those 4 cases with further migration, undersizing of the stem could be recognized. At present, clinical consequences are still uncertain.

Periprosthetic Bone Mineral Density Changes After Implantation of a Curved Bone Preserving Hip Stem Compared to a Standard Length Straight Stem: 5-Yr Results of a Prospective, Randomized DXA-Analysis.

INTRODUCTION: The objective of this prospective, randomized study was to evaluate differences in periprosthetic bone mineral density changes in the proximal femur after implantation of a cementless bone preserving stem (Fitmore) compared to a cementless straight stem (CLS Spotorno). METHODOLOGY: Periprosthetic bone mineral density was measured in 140 patients (aged 33-74 yr) before surgery, 7 d, 3, 12, and 60 mo postoperatively, using dual energy X-ray absorptiometry. The 1-yr results have already been published. The results of the first postoperative measurement served as the baseline value. Because of the different length of the 2 implants we used adapted Gruen zones to divide the periprosthetic bone in seven regions of interest with comparable length. Clinical results were recorded using the Western Ontario and McMaster Universities Arthritis Index and the Harris hip score. RESULTS: Clinical findings showed comparable results in both groups at all follow-ups (p > 0.05). A total of 5 yr after surgery, the comparison of periprosthetic bone mineral density changes showed significant differences in regions of interests 3 (CLS -3.9% vs Fitmore -0.2%, p < 0.001) and 5 (CLS -3.6% vs Fitmore -1.3%, p = 0.0028) between both implants. CONCLUSION: Despite the fact that no exclusive proximal load distribution could be observed, proximal periprosthetic bone loss in the proximal femur was less pronounced after implantation of the bone preserving stem than with the straight stem.

Mid-term migration analysis of a femoral short-stem prosthesis: a five-year EBRA-FCA-study.

INTRODUCTION:: The objective of this study was to evaluate the mid-term migration pattern of a femoral short stem. METHODS:: Implant migration of 73 femoral short-stems was assessed by Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis (EBRA-FCA) 5 years after surgery. Migration pattern of the whole group was analysed and compared to the migration pattern of implants "at risk" with a subsidence of more than 1.5 mm 2 years postoperative. RESULTS:: Mean axial subsidence was 1.1 mm (-5.0 mm to 1.5 mm) after 60 months. There was a statistical significant axial migration until 2 years postoperative with settling thereafter. 2 years after surgery 18 of 73 Implants were classified "at risk." Nevertheless, all stems showed secondary stabilisation in the following period with no implant failure neither in the group of implants with early stabilisation nor the group with extensive early onset migration. CONCLUSION:: In summary, even in the group of stems with more pronounced early subsidence, delayed settling occurred in all cases. The determination of a threshold of critical early femoral short stem subsidence is necessary because of the differing migration pattern described in this study with delayed settling of the Fitmore stem 2 years postoperatively compared to early settling within the first postoperative year described for conventional stems.

[Acetabular Revision: Reconstruction of Cavitary and Segmental Defects with Special Consideration of Modular Highly Porous Acetabular Revision Systems]. / Azetabuläre Revision: Rekonstruktion kavitärer und segmentaler Defekte unter besonderer Berücksichtigung modularer hoch poröser Pfannensysteme.

Due to the increasing number of primary hip arthroplasty procedures, revision surgery will be an increasing challenge. The surgeon is frequently confronted with important acetabular defects. Careful preoperative planning is essential to achieve functional and long-term success. Treatment options include porous hemispheric cups, porous metal augments, oblong cups, cemented implants, rings, allografts, patient specific implants and component combinations. In addition to large hemispherical cups, modular porous metal revision systems are increasingly being used to treat severe acetabular bone loss. This article provides an overview of relevant literature and implant-specific features of different reconstruction strategies.

The epicondylar ratio can be reliably used on X-ray of the knee to determine the joint line.

PURPOSE: The epicondylar ratio (ER) is used to restore the individual joint line (JL), especially in revision total knee arthroplasty. It was first described in magnetic resonance imaging (MRI) but is usually applied to a.p. radiographs of the knee for preoperative planning. The objective of the current study was to define reliable landmarks in MRI and X-ray images of the knee, which allow comparison of the image modalities. Furthermore, the correlation of the measured ER in MRI and X-rays of the knee was calculated. METHODS: A consecutive series of 87 patients who underwent an arthroscopical intervention of the knee were included into the present study. The lateral epicondyle was defined as the most lateral and distal prominence. On the medial side, the measurement was aligned to the epicondylar sulcus. The medial and lateral ER were calculated by dividing the perpendicular distance from the JL to the epicondyle by the transepicondylar distance. One observer determined the ER twice to calculate the intramethod intraobserver agreement, and a second observer obtained the intramethod interobserer agreement. The ER obtained from X-ray and MRI was compared to calculate the intermethod correlation. RESULTS: The average lateral ER was 0.29 on X-ray versus 0.28 on MRI. The average medial ER was 0.33 and 0.33, respectively. Intramethod agreement ranged from 0.66 to 0.88 and intermethod correlation from 0.49 to 0.57. CONCLUSIONS: The ER can be determined reliably on MRI and X-ray images of the knee. The correlation of the ER in MRI and X-ray is fair.

Biomechanics of a cemented short stem: Standard vs. line-to-line cementation techniques. A biomechanical in-vitro study involving six osteoporotic pairs of human cadaver femurs.

BACKGROUND: Short-stem total hip arthroplasty (THA) potentially offers advantages compared to conventional THA, including sparing bone and soft tissue and being a facilitated and less traumatic implantation. However, the indication is limited to patients with sufficient bone quality. Cemented short-stem THA might provide an alternative to conventional cemented THA. To date, no cemented short stem is available on the market. METHODS: In the present in vitro study, primary stability of a new cemented short stem was evaluated, comparing standard (undersized stem) versus line-to-line (same-sized stem) cementing techniques, using six pairs of human cadaver femurs. Primary stability, including reversible micromotion and irreversible migration, was assessed in a dynamic material-testing machine. Fracture load was tested and fracture pattern analyzed. FINDINGS: Both cementation techniques (standard vs. line-to-line) displayed comparable results with respect to primary stability without any statistical differences (micromotion: 17.5 µm vs. 9.6 µm (p = 0.063); migration: 9.5 µm vs. 38.2 µm (p = 0.188)). Regarding fracture load, again, no difference was observed (3670 N vs. 3687 N (p = 0.063)). In all cases, proximal fractures of Vancouver type B3 occurred. INTERPRETATION: The present in vitro study demonstrates that the line-to-line cementation technique, which is favourable regarding the philosophy of short stem THA, can be further pursued in the course of the development of a cemented short stem. Further investigations should address how well the cemented short stem compares to well-established cemented straight-stem designs.

Vascular calcifications on the preoperative radiograph: harbinger of tourniquet failure in patients undergoing total knee arthroplasty?

OBJECTIVE: Vascular calcifications on the preoperative radiograph of patients scheduled for total knee arthroplasty (TKA) often give rise to concern, as their clinical relevance remains uncertain. The aim of this study was to investigate whether these vascular calcifications-especially medial artery calcifications (MACs), which increase arterial stiffness-were associated with tourniquet failure and thus with increased intraoperative blood loss. MATERIALS AND METHODS: A total of 765 patients who underwent primary TKA with a tourniquet (cuff pressure 350 mmHg) between 2009 and 2011 were screened for vascular calcifications on the preoperative radiograph. Vascular calcifications were classified into intimal and medial artery calcifications. Intraoperative blood loss of patients with and without MAC was compared, and a mixed linear regression model was used to adjust for the presence of several confounding factors (e.g., obesity, operating time). RESULTS: None of the 50 (6.5%) patients with MAC showed signs of tourniquet failure. Intraoperative blood loss of patients with MAC was not significantly elevated compared to the overall study group (p = 0.592) even when corrected for the presence of several confounding factors. CONCLUSION: We found no evidence that vascular calcifications seen on the preoperative radiograph might be associated with tourniquet failure. However, surgeons should be aware of this possibility as tourniquet failure might become more common considering the ongoing trend toward minimizing cuff pressures.

Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.

INTRODUCTION: The principle of implanting a calcar-guided short stem consists of an individual alignment alongside the medial calcar providing the ability of reconstructing varus and valgus anatomy in a great variety. However, still, there are broad concerns about the safety of extensive varus and valgus positioning in regard to stability, bony alterations, and periprosthetic fractures. MATERIALS AND METHODS: 216 total hip arthroplasties using a calcar-guided short stem (optimys, Mathys Ltd.) in 162 patients were included. Depending on postoperative CCD angle, hips were divided into five groups (A-E). Varus- and valgus tilt and axial subsidence were assessed by "Einzel-Bild-Roentgen-Analyse"(EBRA-FCA, femoral component analysis) over a 2-year follow-up. The incidence of stress-shielding and cortical hypertrophy as well as clinical outcome [Harris Hip Score (HHS)] were reported. RESULTS: Postoperative CCD angles ranged from 117.9° to 145.6° and mean postoperative CCD angles in group A-E were 123.3°, 128.0°, 132.4°, 137.5°, and 142.5°, respectively. After 2 years, the mean varus/valgus tilt was -0.16°, 0.37°, 0.48°, 0.01°, and 0.86°, respectively (p = 0.502). Axial subsidence after 2 years was 1.20, 1.02, 1.44, 1.50, and 2.62 mm, respectively (p = 0.043). No periprosthetic fractures occurred and none of the stems had to be revised. Rates of stress-shielding and cortical hypertrophy as well as HHS showed no significant difference between the groups. CONCLUSIONS: Valgus alignment results in increased subsidence but does not affect the clinical outcome. There is no difference in stress shielding and cortical hypertrophy between the groups. The authors recommend long term monitoring of valgus aligned stems.

One-stage bilateral versus unilateral short-stem total hip arthroplasty: comparison of migration patterns using "Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis".

PURPOSE: The hypothesis of this study was that femoral implant migration would not differ between simultaneous bilateral or unilateral short-stem THA. METHOD: Implant migration of 202 femoral short-stems (100 unilateral and 102 one-stage bilateral cases) in 151 patients was assessed by "Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis" in a two years follow-up (2.0-3.0 years). Migration patterns of unilateral and simultaneous cases were analysed and compared. RESULTS: There was no difference between the two groups regarding age, body mass index and gender. After two years mean subsidence of all 202 implants was 1.43 mm (-6.5 mm to 2.0 mm). After initial subsidence of 0.37 mm per month within the first six weeks, the mean monthly migration was reduced to 0.02 mm between one and two years post-operative. There was no statistical difference in mean migration between unilateral (1.34 mm) and simultaneous bilateral (1.51 mm) THA (p = 0.33). CONCLUSION: In summary, two years post-operative there was no difference in the amount of mean implant subsidence between unilateral compared to simultaneous bilateral short-stem THA. This suggests that regarding implant fixation simultaneous bilateral short-stem THA is as safe and successful as a solely unilateral intervention.

Primary stability of a shoulderless Zweymüller hip stem: a comparative in vitro micromotion study.

BACKGROUND: The Zweymüller stem design has proven long-term stability with a 20-year survival rate of over 90 %. Primary stability necessitates implant-bone micromotions below 150 µm, otherwise bony ingrowth is negatively influenced. METHODS: Using fresh paired human femurs, we investigated a modification of the Zweymüller-type stem design with reduced proximal lateral shoulder in reference to primary stability. Relative motion between the implant and the cortical bone as well as the irreversible implant migration was investigated under dynamic loading (100-1600 N) over 100,000 cycles using miniature displacement transducers. RESULTS: Micromotions were below the critical threshold for both implants at all measurement points. Axial reversible and irreversible micromotions were not influenced by reducing the shoulder of the prosthesis. Resistance against rotational moments was less pronounced after reduction of the shoulder without statistical significant results. CONCLUSIONS: Reducing the proximal shoulder of the Zweymüller-type stem design does not negatively influence axial stability but might negatively influence rotational stability. Even though, comparable results still suggest a reasonable resistance against rotational forces.