Background: Bacteria form a biofilm on the surface of orthopaedic devices, causing persistent and infection. Little is known about biofilms formation on bone grafts and bone substitutes. We analyzed various representative materials regarding their propensity for biofilm formation caused by Staphylococcus aureus.

Methods: As bone graft beta-tricalciumphosphate (b-TCP, CyclOsTM) and as bone substitute a tantalum metal mesh (trabecular metalTM) and PMMA (Pala-cosTM) were investigated. As test organism S. aureus (strain ATCC 29213) was used. Test materials were incubated with bacterial solution of 105 colony-forming units (cfu)/ml at 37°C for 24 h without shaking. After 24 h, the test materials were removed and washed 3 times in normal saline, followed by sonication in 50 ml Ringer solution at 40 kHz for 5 minutes. The resulting sonication fluid was plated in aliquots of 0.1 ml onto aerobe blood agar with 5% sheep blood and incubated at 37°C with 5% CO2 for 24 h. Then, bacterial counts were enumerated and expressed as cfu/ml. All experiments were performed in triplicate to calculate the mean ± standard deviation. The Wilcoxon test was used for statistical calculations.

Results: The three investigated materials show a differing specific surface with b-TCB> trabecular metal> PMMA per mm2. S. aureus formed biofilm on all test materials as confirmed by quantitative culture after washing and sonication. The bacterial counts in sonication fluid (in cfu/ml) were higher in b-TCP (5.1 x 106 ± 0.6 x 106) and trabecular metal (3.7 x 106 ± 0.6 x 106) than in PMMA (3.9 x 104 ± 1.8 x 104), p< 0.05.

Conclusion: Our results demonstrate that about 100-times more bacteria adhere on b-TCP and trabecular metal than on PMMA, reflecting the larger surface of b-TCP and trabecuar metal compared to the one of PMMA. This in-vitro data indicates that bone grafts are susceptible to infection. Further studies are needed to evaluate efficient approaches to prevent and treat infections associated with bone grafts and substitutes, including modification of the surface or antibacterial coating.

Background: Antibiotic-loaded spacers and cement nails are commonly used in patients undergoing a two-stage implant exchange procedure for treatment of prosthetic joint infection (PJI). During re-implantation 2–6 weeks after implant removal, tissue specimens are collected to document successful eradication of infection. However, these specimens have limited sensitivity, especially in patients receiving antimicrobial treatment. We investigated the value of sonication of removed spacers and cement nails.

Methods: We prospectively included patients in whom a spacer or cement nail was removed from January 2007 through April 2009 during a two-stage exchange procedure. The removed temporary device was sonicated in a container with Ringer’s solution in an ultrasound bath for 5 min at 40 kHz (as described in NEJM2007;357:654). The resulting sonication fluid was cultured aerobically and anaerobically for 10 days. In parallel, > 2 tissue samples were collected for conventional cultures on blood agar plates and enrichment broth. PJI was defined as visible purulence, acute inflammation on histopathology, sinus tract or significant microbial growth in tissue or implant sonication cultures.

Results: In this ongoing study, 28 spacers and 10 cement nails from patients with confirmed PJI were included (median age 75 y; range 49–86 y). All devices were impregnated with antibiotics (gentamicin and/or vancomycin) and were placed in the hip (n=21), knee (n=9) or shoulder joint region (n=7). At the time of explantation, the following pathogens were isolated: coagulase-negative staphylococci (n=19), Staphylococcus aureus (n=7), Streptococcus agalactiae (n=3), Propionibacterium acnes (n=5) and mixed infection (n=4). All patients received systemic antibiotics for a median of 19 days (range 11–42 days) before removal of the spacer/nail. At the time of re-implantation, tissue cultures were negative in all 38 patients, whereas sonication cultures showed growth of Propionibacterium acnes in 2 of 38 patients (5%) with a hip and shoulder spacer, both in significant numbers (150 and 550 colonies/ml sonication fluid, respectively). These organisms were probably present as mixed infection already at the time of explantation, but were missed due to overgrowth due to another organism (S. aureus in one patient and coagulase-negative staphylococci in another). Both patients were not initially treated for the Propionibacterium acnes infection, but the treatment was given after re-implantation.

Conclusion: Sonication of removed spacers is a suitable approach to identify persistent infection in patients with a two-stage exchange. Sonication may replace the current standard approach consisting of multiple tissue specimens in order to document successful eradication of infection.

Background: Negative pressure wound treatment is increasingly used through a Vacuum-Assisted Closure (VAC) device in complex wound situations. For this purpose, sterile polyurethane (PU) and polyvinyl alcohol (PVA) foam dressings are fitted to the wound size and covered with an adhesive drape to create an airtight seal. Little information exists about the type and quantity of microorganisms within the foams. Therefore, we investigated VAC foams after removal from the wound using a validated method (sonication) to detect the bacterial bioburden in the foam consisting as microbial biofilms.

Methods: We prospectively included VAC foams (PU and PVA, KCI, Rümlamg, Switzerland) without antibacterial additions (e.g. silver), which were removed from wounds in patients with chronic ulcers from January 2007 through December 2008. Excluded were patients with acute wound infection, necrotizing fasciitis, underlying osteomyelitis or implant. Removed foams from regular changes of dressing were aseptically placed in a container with 100 ml sterile Ringer’s solution. Within 4 hours after removal, foams were sonicated for 5 min at 40 kHz (as described in NEJM2007;357:654). The resulting sonication fluid was cultured at 37°C on aerobic blood agar plates for 5 days. Microbes were quantified as No. of colony-forming units (CFU)/ml sonication fluid and identified to the species level.

Results: A total of 68 foams (38 PU and 30 PVA) from 55 patients were included in the study (median age 71 years; range 33–88 years, 57% were man). Foams were removed from the following anatomic sites: sacrum (n=29), ischium (n=18), heel (n=13), calves (n=6) and ankle (n=2). The median duration of being in place was 3 days (range, 1–8 days). In all 68 foams, bacteria were found in large quantities (median 105 CFU/ml, range 102–7 CFU/ml sonication fluid. No differences were found between PU and PVA foams. One type of organisms was found in 11 (16%), two in 17 (24%) and 3 or more in 40 (60%) foams. Gram-negative rods (Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa) were isolated in 70%, followed by Staphylococcus aureus (20%), koagulase-negative staphylococci, streptococci (8%), and enterococci (2%).

Conclusion: With sonication, a high density of bacteria present in VAC foams was demonstrated after a median of 3 days. Future studies are needed to investigate whether antimicrobial-impregnated foams can reduce the bacterial load in foams and potentially improve wound healing.

Introduction: The original Müller straight stem (MSS, Zimmer^®; Winterthur, Switzerland) is made out of CoNiCr and showed excellent 20 year results but later modifications of the stem proved to be inferior. Aim of this study was to analyse the effect of shape, material and surface roughness on aseptic loosening of cemented Müller type straight stems.

Materials and patients: Between 1984 and 1996 a total of 926 THR were operated with four different versions of cemented Müller type straight stems and followed prospectively in the in-house register at our institution. Two different shapes of cemented Müller type straight stems (MSS and SL), both made out of two different alloys (CoNiCr and TiAl), were included in this study. All four versions differed in surface roughness (MSS CoNiCr Ra 1.0μm (satin); SL CoNiCr Ra 1.2μm (satin); MSS TiAl Ra 2.0μm (rough); SL TiAl Ra > 2.0μm (rough)). Survival analysis was done using Kaplan-Meier curves with aseptic loosening as endpoint, risk factors were tested with regression analysis.

Results: The 4 groups did not differ in age, gender and diagnosis, the mean follow up was 11.4 (0.1 to 23.0) years. Survival with aseptic loosening as endpoint was 97.7% (MSS CoNiCr), 96.4% (SL CoNiCr), 82.5% (MSS TiAl) and 67.4% (SL TiAl), respectively, at ten years. At final follow up all four groups differed significantly (p=0.044 Log rank test). Increasing roughness increased the risk for aseptic loosening and the harder CoCr had better survival than TiAl. results were significantly worse for the combination of the soft TiAl with a rough surface (SL TiAL, Ra > 2.0μm). For both stem designs the MSS shape had better survival than the SL shape (p=0.001)

Discussion: Wear modalities (abrasion vs. fretting) in the cement-stem interface are directly correlated with surface roughness, the amount of the released particles correlates with loosening. All examined stems had a roughness exceeding the limit of Ra =0.4 μm, producing abrasive wear with a higher volume of wear particles as compared to established polished stems which show fretting wear. Stem roughness and hardness are more important for long-term survival than some modifications in the shape.

Conclusion: Cemented Müller type straight stems should have a polished surface (Ra < 0.4 μm) and be made out of a hard material (CoNiCr) to minimize surface wear. This might be true for all types of cemented stems.

Background: Bacteria form biofilms on the surface of orthopaedic devices, causing persistent infections. Monitoring biofilm formation on bone grafts and bone substitutes is challenging due to heterogeneous surface characteristics. We analyzed various bone grafts and bone substitutes regarding their propensity for in-vitro biofilm formation caused by S. aureus and S. epidermidis.

Methods: Beta-tricalciumphosphate (β-TCP, Chro-nOsTM), processed human spongiosa (TutoplastTM) and PMMA (EndobonTM) were investigated. PE was added as a growth control. As test strains S. aureus (ATCC 29213) and S. epidermidis RP62A (ATCC 35984) were used. Test materials were incubated with defined bacterial solution (105 colony-forming units (cfu)/ml) at 37°C for 24 h without shaking. After 24 h, the test materials were removed and washed 3 times in PBS, followed by a standardised sonication protocol (Trampuz et al. 2007, NEJM). The resulting sonication fluid was plated in aliquots of 100μl onto aerobe blood agar with 5% sheep blood and incubated at 37°C with 5% CO2 for 24 h. Bacterial counts were enumerated and expressed as cfu/ml. Sonicated samples were transferred to a microcalorimeter (TA Instrument) and heat flow at 37°C was continuously monitored over a 24h period with a precision of 0.0001°C and a sensitiviy of 200μW. All experiments were performed in triplicates to calculate the mean ± standard deviation. ANOVA analysis was used for statistical calculations.

Results: For S. aureus bacterial counts (log10 cfu/ sample) were significantly higher (p< 0.001) for the porous (β-TCP 7.67 ± 0.17, Tutoplast 7.65 ± 0.15) than for the solid samples (PMMA 6.12 ± 0.18, PE 5.17 ± 0.22). Bacterial density (log10 cfu/surface) was 10^1–10^2 times higher for the S. epidermidis than for the S. aureus. In calorimetry the shape of the heat flow curves was characteristic for the individual strain and was not influenced by the test materials. The time to detection (TTD) was shortest for β-TCP for both strains and TTD was always shorter for S. aureus than S. epidermidis with corresponding material. Cfu/sample calculated from the calorimetric data was concordant with the standard culturing method.

Conclusion: Our results demonstrate biofilm formation with both strains on all tested materials. The calorimetry in all cases was able to detect quantitatively the amount of biofilm. Further studies are needed to see whether calorimetry is a suitable tool also to monitor approaches to prevent and treat infections associated with bone grafts and bone substitutes.

Introduction: The biological activity of PE-particles released due to wear is an established risk-factor for osteolysis and loosening after Total Hip Arthroplasty (THA). Cup position and orientation might have an effect on the risk of impingement and wear, thus contribute to the risk of aseptic loosening in the long-term what should be studied.

Methods: Between 1984 and 1987 a total of 149 cemented total hips (Müller all-poly cup, Müller straight stem, 32 mm head) have been implanted. All implants had a standardised clinical and radiological follow-up. The pre- and postoperative centre of rotation of the hip and the orientation of the cup were determined. Migration, linear wear and direction of wear were measured twice with standard Methods: and the digital EBRA method. Wear-volume was calculated, taking direction of wear and cup orientation into account. Radiographs were analysed for progressive osteolysis and loosening.

Results: 1 patient was lost to follow-up, 47 had died, 7 had been revised before 10 years follow-up. 18 patients had a missing or poor final radiograph, leaving 75 hips for long-term analysis. 41 were in male patients, mean age was 66.2 (+/− 11.0) years, mean follow-up 15.4 (+/−4.1) years. Mean inclination was 40.7° (+/− 7.1), mean anteversion was 14.8° (+/−8.4) And the mean cup positioning was 3.8 mm (+/− 4.3) medial and 5.3 mm (+/− 3.5) cranial.

Osteolysis was found in 36 cups, 18 of them have been revised.

The average linear wear was 1.1 (+/− 0.9) mm, the average wear volume 798.7 (+/−622.3) mm3, the linear wear rate 0.07 (+/−0.06) mm/year and the volumetric wear rate 54.5 (+/− 43.2) mm3/year.

Younger patients had increased linear wear rates (p=0.035). Osteolysis of the cup, cup migration and cup revision were correlated with linear and volumetric wear (all p=< 0.001).

There was no correlation of the cup position, inclination and anteversion with osteolysis, loosening and any of the wear parameters. Volume calculation did not provide further information.

Conclusion: We found a strong correlation between wear and loosening of PE cups, but cup position and orientation did not affect osteolysis and loosening. Thus in contrast to hard-hard bearings polyethylene is a forgiving bearing surface and improvement of the cup orientation (e.g. due to navigation) will not result in increased cup survival, as long as extreme positioning errors are avoided.

Introduction: Changes of the proximal femur like oste-olysis, stress shielding and osteopenia are frequently observed after total hip arthroplasty (THA). Such find-ings might be considered as risk factors for aseptic loosening and later revision. Cortical thinning is observed of healthy femora too and it is questioned whether the effect of the implant can be discriminated from age-related changes.

Aim of our study was to analyze cortical bone changes in prosthetic hips with time and compare those changes with the contra lateral non operated femur.

Materials and Methods:From 1984–87 165 hips were operated with a cemented Muller straight stem. Regular clinical and radiological follow up was scheduled after 1, 2, 5, 10, 15 years. We included only patients operated for osteoarthritis without revision and complete follow-up of more than 10 years. 37 THA hips in 35 patients remained for inclusion in the study. The mean follow-up was 16±4,6 years. Thickness of cortices was measured medially and laterally in 6 locations according to the 2nd to 6th Gruen zones and mean cortical thinning was calculated. The measurements were taken on standardized anterior-posterior x-rays of the pelvis. All measurements were analyzed with Image Access 4 Software calibrated with the reference to 32 mm femoral head.

10 patients were not operated on the contralateral hip and were measured in standardized manner in the same locations as in THA femurs.

Results: All included patients had pain free hips and did not require revision surgery at the last follow-up. Mean cortical thinning was 0,17±0,15 mm/year and it was mostly expressed in mid part of the stem (Gruen 2 and 6 zones). Most thinning occurred within the first 5 years (0,32±0,34 mm/year), later thinning was slower (0.09±0,37 mm/year). For the group with non operated contralateral hip mean thickness loss in THA hip was 0,2±0,17 mm/year and there was thickness loss of the contralateral femur too (0,03±0,12 mm/year), being much less as compared to the operated side (p< 0.001).

Conclusions: Loss of cortical thickness in THA hip with the Muller straight stem is frequently observed in long term and is not associated to expression of clinical symptoms and subsequent revision surgery. The effect is pronounced in the first postoperative years, mainly being explained by stress shielding. Additionally there is cortical thinning due to ageing, being much less than the influence of the implant. Thinning of the cortical bone must not be interpreted a sign of aseptic loosening.

Introduction: Since its introduction in 1977 the Müller straight stem and its various copies have become one of the most often used cemented stems worldwide (about 1.1 million stems, figure quoted by manufacturer). Though, there is still a lack of long-term follow-up data.

Material/Methods: A consecutive series of 165 primary hip replacements (161 patients) with the original forged Müller straight stem (CoNiCr) was operated between July 1984 and June 1987 and followed prospectively. Mean age at operation was 68.9 years (25.6 to 86.3 years). 70 stems were implanted in female patients. Operation was done in supine position through a transgluteal approach with no trochanteric osteotomy. All stems were cemented with a second-generation technique (distal plug, cement syringe). The head diameter was 32 mm diameter, 134 heads were out of metal, 31 out of ceramic. 151 hips had a cemented polyethylene cup (52 with armament screws). 13 were combined with an acetabular reinforcement ring (Müller ring) and one with an anti-protrusion cage (Burch-Schneider).

Clinical and radiological follow-up was planned at 4 months 1, 2, 5, 10, 15 and 20 years. Clinical follow-up included a standardised examination and the completion of an IDES form. Cumulative survival rates were calculated by Kaplan-Meier analysis. Radiographs were analysed for osteolysis according to Gruen et al. (zones 1–7) and radiolucent lines.

Results: 3 patients (3 stems) were lost to follow-up (two postoperative and one after 16.8 years), 103 patients (with 106 hips) had died without revision and 55 patients (56 hips) remained for follow-up. 15 stems were revised, 11 for aseptic loosening (9 in combination with the cup), two for infection and two for other reasons. 13 further patients had isolated cup revision. Survival with aseptic loosening of the stem as endpoint was 86% (95%-CI: 82 to 90%) at 20 years.

The median HHS at the last follow-up was 80 points (range 30–98 points).

31% of the non-revised 41 stems showed osteolysis, most of them in zone 7 (21.9%). Two stems with isolated cup revision had a continuous radiolucent line. Incomplete radiolucent lines appeared most frequently in zone 1. Cup revision was associated with increased radiological changes on the femoral side (p=0.094, Mann-Whitney U-test).

Conclusion: The Müller straight stem shows excellent clinical and radiological results in the long-term, comparable to those achieved with other well established cemented and non-cemented stems. From our data further clinical use of the Muller straight stem can highly be recommended.

Introduction: The acetabular component in primary THA is placed on pathologic bone stock. Little is known about histology and its impact on aseptic cup loosening. We therefore investigated undecalcified acetabular bone biopsies taken prior to primary THA and at the time of revision surgery. Study design: In a prospective study, starting 01/1993, a bone biopsy of the acetabulum out of zone 1 according to De Lee and Charnley is taken in each primary THA in a standardized manner. In case of revision surgery of the cup, another biopsy is taken again. Between 01/1993 and 08/1997 351 cementless titanium shells with screws cups have been implanted at our institution. The average age at operation was 68 years (range 29–90 years). In 88% osteoarthritis was the primary diagnosis. Biopsies were classified as normal bone (type I), increased sclerosis (type II), restless bone (type III) or dead bone (type IV) according to a classification developed in our institution. Biopsies with a special entity such as rheumatoid arthritis are grouped «others». The initial diagnosis on the pre-operative X-ray was correlated to the results of the biopsies. Signs for aseptic loosening of the cup were defined on the complete x-ray series. All explanted cups were analysed for signs of impingement or increased wear.

Results: 8 cups (8 patients) had to be revised until August 2004 (average age at revision 63.6years, range 49.6–77.8 years). The initial diagnosis was osteoarthritis in 5 cases, twice avascular necrosis of the femoral head and 1 rheumatoid arthritis. 5 times we found metal-on-metal, 3 times PE-ceramic bearings. We found one of each biopsies in type I and type IV, 2 in type II and type III. 2 biopsies had to be grouped «others». At time of revision one biopsy initially grouped as type III had to be reclassified as «others». 2 cups were revised due to late haematogenous infection. 3 cases showed a direct relation between the result of the biopsy and failure of the cup. In 1 case we found surgical failure, though impingement and a type IV biopsy as combined reasons for failure. 2 cups (2 patients) revised after 1.2 years, immunhistological reaction due to metal-on-metal could have been the reason for failure (still under investigation).

Discussion: Surgical failures and implant related factors are accused as major reasons for loosening of the cup in THA, patient related factors often neglected. The acetabular bone quality, as one patient related factor, showed in almost half of the cases analysed a correlation with later acetabular loosening. One should therefore be aware of acetabular bone quality as one factor for loosening of the cup when monitoring acetabular implants, especially when newly introduced.

Introduction: The debris of standard Polyethylene in Total Hip Arthroplasty (THA) has been responsible for aseptic loosing and osteolysis in many patients. Wear rates for Polyethylene-Ceramic are analysed around 10 to 20 mm3 per year, those of PE-metal even 5 times more. Dislocation of primary THA is also common and problematic. Using a PE insert with a dorsal rim lowers the risk of dislocation but could be a potential risk for impingement if not correctly placed. Aim of this study was to value of reduced risk for dislocation in terms of increased risk for impingements. Study design: Between 1989 and 2003, 1107 non-cemented SL cups with Polyethylene on Ceramic were implanted in 953 patients, 422 (44%) implanted in women. All PE inserts were with dorsal rim. Average age at implantation was 69.9 years (range 39 to 90). In all cases, a cemented stem was implanted. Of those 1107 primary THA, 100 were revised for aseptic loosening. In 11 cases both components were revised, 7 times only the cup was replaced and for the 82 revisions remaining, only the stem was revised. In all cases the PE insert was removed and replaced. All inserts were analysed macroscopically for impingement and the volume worn off was calculated. Dislocation risk of PE couplings with rim was compared to standard PE couplings without rim. Results: In 18 PE inserts of 100 with rim, signs of impingement were found, ranging from slight (considered as creep not wear) to massive. An average yearly wear rate of 43 mm^3 (range 7 to 119) was calculated. Median survival time of the inserts without impingement was 5.2 years (range 0.7 to 11.4) of those with impingement it was reduced to 4.5 years (range 1.1 to 13.8). No statistical difference calculated (Mann-Whitney-U test, p=0.28) . In 2 of all 1107 cases (0.2%) operative revision of dislocation was performed whereas in 6 of the 598 cases (1%) with PE cups without rim, operative intervention was performed. Discussion: The use of a PE insert with dislocation protection rim holds a potential danger if not correctly placed. Though an earlier failing of THA with PE impingement was found, it was not significant and overpowered by the fact that most loosenings were due to cemented titanium stems with their great potential for early loosening.