Research

Bone-preserving stem designs

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In the modern age of total hip arthroplasty (THA), there has been considerable interest in developing an implant that is bone-preserving, and is associated with small resections and physiological loading to maintain bone stock. Hip resurfacing is one such implant with mixed results, and is not without controversy, particularly with the recent Medicines and Healthcare products Regulatory Agency (MHRA) guidelines in mind. Another development is that of the short femoral stem. In this design, load transfer is more physiological compared with more traditionally designed stems, potentially reducing stress shielding around the proximal part of the stem. The authors of this paper from Seoul (South Korea) highlight the problems of assessing short femoral stems as their size and shape is so variable, making direct comparisons very difficult.¹ However, they wished to highlight a feature they had identified in a femoral stem which they had been using that was a shortened tapered version of a conventional stem, the TRI-LOCK (Depuy International Ltd, Leeds, UK). This study came about as the authors had noticed lateral cortical atrophy in Gruen zone I and blunting of the cut surface in zone VII. This was a retrospective study of 72 consecutive patients with a mean age of 48.2 years who underwent 80 THAs. Follow-up was 37.3 months with a detailed radiological analysis. The authors demonstrated that a total of 61 cases (76.3%) had either an intra-cortical osteolytic lesion (IOL) or thinning of the lateral cortex of greater than 10%. In 37 cases (46.3%), the lateral cortical thickness was less than 20%. With univariate analysis, the authors identified that cortical thickness appeared to correlate with a low body mass index (BMI). With a cortical thinning of more than 20%, there appeared to be a correlation with low BMI, gender and operation time. While the clinical significance of this is perhaps uncertain, there was one case of a periprosthetic fracture in a patient with lateral cortical reduction of 33.2%. This study will be of interest to hip surgeons as there has been increasing popularity in these ‘bone-conserving’ femoral stems. As with hip resurfacing implants, not all short femoral stems are the same, and the type of cortical thinning identified with this study may not be seen with all short femoral stems. Following the results of this study, the authors’ institution no longer uses shortened tapered femoral stems in patients with a BMI lower than 23.3 kg/m² or in younger patients, as it seems unwise to expose them to unnecessary risks. The clear message from this study is that not all short femoral stems are the same. As with the different types of hip resurfacings, there may be some significant losers with some short femoral stem designs.

Knee osteoarthritis: what cost?

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The American Academy of Orthopaedic Surgeons (AAOS) Clinical Practice Guidelines for the treatment of knee osteoarthritis (OA) provide non-operative recommendations of physical therapy, non-steroidal anti-inflammatories (NSAIDs) and tramadol, as does the UK-centric NICE guidance. Despite clear guidance documents, non-recommended treatments for OA are in common usage, including hyaluronic acid (HA) injections, corticosteroid (CS) injections, knee braces, wedge insoles and opioids. Many of these options are expensive and may put the patient at increased risk of infection and/or additional complications following surgery. In the era of value-based medicine, we are encouraged to consider the ratio of benefit to cost when making clinical choices. Researchers from Iowa City, Iowa (USA) used the PearlDiver programme (Pearl Diver, Inc., Colorado Springs, Colorado) to interrogate billing data and establish the cost of non-operative knee OA treatments administered during the year prior to total knee arthroplasty (TKA).³ Of 86 081 patients who underwent primary TKA in this analysis, only 56 690 (65.9%) received at least one of the treatments during the year prior to their TKA. Using the reimbursement fees paid by the insurance provider (both private and Medicaid/Medicare plans purchased through Humana, Inc. were included in the study), the aggregate costs of care per treatment type, per patient and per single episode were determined. Hyaluronic acid and CS injections made up more than 50% of all treatment costs, with HA injections accounting for nearly 30% of the total cost for all patients. The mean cost per patient over the course of the one-year period was highest for those treated with HA injections ($822), followed by physical therapy ($405) and knee bracing ($331). The mean cost for any single treatment episode was highest for knee bracing ($331), followed by HA injections ($294) and physical therapy ($84), NSAIDs and tramadol (the only AAOS recommended interventions) made up only 12.2% of the total cost for non-operative treatment of knee OA. The data to support HA and CS injections, knee braces, wedge insoles and opioids are mixed. In fact, recent evidence strongly suggests a higher risk of infection following HA injections, and limited pain relief and increased risk of complications following opioid administration. Knee OA is both a physically and financially demanding condition. The evidence here shows that costs related to non-inpatient, non-surgical procedures prior to TKA could be reduced by 45% if non-recommended interventions are avoided. This study raises a timely discussion on the balance between evidence and value-based medicine, particularly regarding cost reduction, in the year prior to TKA.

Vancomycin and ceftazidime in bone cement?

The two-stage exchange arthroplasty is considered the benchmark for the treatment of periprosthetic joint infection (PJI). As the World Health Organization (WHO) identifies antibiotic resistance as one of the major global health threats, interest is growing in methods of topical antibiotic delivery. In two-stage revision arthroplasty, an antibiotic-loaded bone cement spacer is implanted during the first stage of the procedure, delivering high-dose local antibiotics. If an organism has not been identified, either gram-positive or gram-negative organisms may be the culprit, and in some cases no organism is identified at all. To provide adequate antibiotic cover, broad-spectrum formulation or combination is often administered as a best guess. The research team from Taoyuan (Taiwan) designed their in vitro model to compare the efficacy of a variety of antibiotic combinations.⁴ They tested the following combinations: vancomycin and ceftazidime; vancomycin and imipenem; vancomycin and aztreonam; teicoplanin and ceftazidime; teicoplanin; or teicoplanin (not available in North America) and aztreonam, against methicillin- and imipenem susceptible Staph. aureus (MSSA), methicillin-resistant Staph. aureus (MRSA), Staph. epidermidis, P. aeruginosa, and E. coli. The authors mixed simplex bone cement with various antibiotic mixtures (at a ratio of 8 g antibiotic to 40 g cement) into cylindrical test specimens, and daily antibiotic release was calculated over the course of 60 days. Antibiotic activity was evaluated using a microtube dilution assay – bacterial growth associated with various concentrations of the antibiotics was compared visually and against a positive control. Elution testing showed that high doses of vancomycin and ceftazidime provided the best antibacterial activity against MSSA, MRSA, Staph. epidermis, P. aeruginosa, and E. coli for as long as, or longer than, all other combinations tested. Eight patients were diagnosed with a knee PJI and then enrolled in the in vivo portion of this study. All of the patients were treated with vancomycin- and ceftazidime-loaded cement spacers. The bioactivity of the joint fluid collected following implantation of the spacer (bacterial species identified were MSSA, MRSA, Enterococcus faecalis and Serratia marcescens) was evaluated for antibiotic concentration. Total antibiotic concentration reached over 500 μg/mL in some cases, without any toxic systemic effects. The results of this study suggest a possible alternative to the usual combination of vancomycin and an aminoglycoside (gentamicin or tobramycin) for antibiotic-loaded cement spacers. Longer follow-up is certainly required in an in vivo setting to determine the efficacy of the vancomycin/ceftazidime combination over time; however, the in vitro results look promising.