Hip & Pelvis

Trends in hip bearing surface

In recent years, there has been increasing debate regarding the choice of bearing surface in total hip arthroplasty (THA). Traditionally, a metal head on a conventional polyethylene liner (MoP) was the default bearing, but there were some concerns about the polyethylene wear and long-term osteolysis. This spawned the development of alternative bearings, such as metal-on-metal (MoM) and ceramic-on-ceramic (CoC), as well as the refinement of MoP with highly cross-linked polyethylene liners, with either a ceramic or metal femoral head. In the current financial climate, our choice of bearing is coming under increasing scrutiny, and rightly so. This commendable study from Iowa City, Iowa (USA) looks at the current trends in the choice of bearings in THA and focuses particularly on the impact of issues related to MoM bearings, the perceived success of highly cross-linked polyethylene, and implant cost.¹ Between 2007 and 2015, the authors reviewed the implant choices made in a total of 28 504 primary THAs with a male:female split of 59:41. There were gender differences apparent in bearing choices: in the females, 16.3% received a MoM bearing; 47.6% metal-on-polyethylene (MoP); 32.6% ceramic-on-polyethylene (CoP); and 3.5% received a CoC bearing. This was compared with 18.1% of the males receiving a MoM bearing, 44% for MoP, 34.2% for CoP and 3.7% for CoC. There were also differences, as would be expected, in the median age for each bearing as follows: 65 to 70 for MoM; 70 to 74 for MoP; 65 to 69 for CoP; and 70 to 74 for CoC. There was a significant increase in the use of CoP from 6.4% in 2007 to 52% in 2015, which mirrored a decline in the use of MoP. There was also a decrease in the use of CoC and MoM over the same period. The decrease in the use of MoP appeared to be a recent phenomenon, decreasing from its peak use in 2012 at 53.6% to 39.8% in 2015. Patients over 65 years of age are more likely to receive a MoP bearing as opposed to any other bearing combination. Although this was an American study, there were some interesting observations that would be globally applicable. Age was an independent predictor of bearing choice, as was a patient’s private health insurance status. Gender was not found to be a significant factor. The changes in choice of bearing, particularly in the last five years, have been quite dramatic, with a shift away from MoM and MoP bearings to CoP. The most likely reasons for this shift include the adverse tissue reactions reported in the case of MoM, as well as the positive mid-term results of highly cross-linked polyethylene. The move away from metal heads to ceramic, combined with a polyethylene acetabular liner, could be due to the concerns of corrosion at the head-neck junction seen with metal heads. In addition, the implant breakages experienced with the earlier generation of ceramic heads appear to have been reduced significantly with more modern designs. Is the real choice of bearing in THA between CoP and MoP? The most significant factors that appear to influence this choice are age and whether the patient has private health insurance. The authors suggest that, with increasing reports of corrosion at the metal head-neck junction, a CoP bearing combination should be the choice in all patients, regardless of age or cost.

Total hip arthroplasty after fixation of minimally displaced femoral neck fractures

X-ref

The population is ageing and the prevalence of fragility fractures continues to increase. The standard of care for patients with a valgus-impacted or minimally displaced fractured neck of femur is in situ fixation with either three cancellous screws or a sliding hip screw. Most fractures managed this way heal fully and the patients return to their pre-fracture state; however, around 10% require further surgery due to nonunion, avascular necrosis (AVN), or degenerative changes. The solution for most of these patients is a total hip arthroplasty (THA). While there have been many studies analysing the conversion of failed open reduction and internal fixation of a displaced fractured neck of femur, none have looked at the conversion of failed in situ fixation of a minimally displaced fracture. This retrospective study of 62 patients from Rochester, Minnesota (USA) reports the outcomes of those aged over 65 years with a minimally displaced fracture treated initially with in situ fixation and who subsequently required conversion to a THA.² Follow-up was 5.5 years and, during this period, a total of 13 patients were treated with a hip hemiarthroplasty as they were deemed to be too high a risk for THA, and a total of 44 patients were reported with two years of clinical follow-up. The most common indication for conversion to a THA was osteonecrosis of the femoral head (44%), followed by post-traumatic degenerative changes (35%), and nonunion (21%). Patients had the screws removed at the time of the THA and screw tracks were filled with autogenous bone graft. Two patients died within 90 days of the surgery, two patients sustained an intra-operative periprosthetic fracture, and there were two post-operative dislocations that were both treated with closed reduction. Four patients developed wound healing or infection problems that required further surgery. Although survivorship analysis was excellent, with 97% free of any surgery at five years, and the mean clinical improvement was significant, there is a burden of complications that is higher than one would expect in a primary hip series. However, this is notably better than most series of THAs performed after failed fixation of displaced fractured neck of femur. The authors made no apology for the fact that higher-risk patients in their study had a hip hemiarthroplasty as opposed to a THA and suggested that this emphasised good patient selection. The authors highlighted some useful intra-operative tips such as dislocation of the hip with the metalwork in situ in order to reduce the torque on the femur through stress risers associated with the screw tracks. Although there is no evidence for bone grafting the screw tracks, the authors felt that this was good practice in their experience. In addition, where possible, the authors elected to use larger head sizes and suggested that this may also contribute to their low dislocation rate.

Surgical correction of cam deformity and degenerative process within the hip joint

X-ref

There has been a rapid expansion in the field of hip preservation surgery in the last ten years and, with it, an ever-increasing range of interventions that can be performed, many of them without any long-term evidence for efficacy, particularly in the field of hip arthroscopy. This has attracted some scepticism, particularly when it has been observed that not all cam deformities are symptomatic and not all develop into arthritis. This excellent paper from Ottawa (Canada) attempts to identify some objective evidence that surgical correction of a cam deformity can have a positive impact on the degenerative process in patients with symptomatic cam deformity.³ This prospective study includes ten patients with cam-femoroacetabular impingement (cam-FAI) who underwent hip preservation surgery including arthroscopy, surgical dislocation, and a mini-arthrotomy. The mean follow-up was 24.5 months. The authors report significant clinical improvement in terms of pain, activities of daily living, sports, and recreational activities over the two years of the study. Additionally, the cartilage and subchondral bone of the hip was assessed using quantitative CT and T1r MRI biomarkers. Although this was a small sample, the authors did observe a decrease in the bone mineral density, suggesting restoration of normal joint mechanics as well as a decrease in T1r values post-operatively. The decrease in T1r values, the authors argue, represents an improvement in the proteoglycan content that could possibly reflect ‘healthier’ articular cartilage. The authors used the posterosuperior quadrant as an internal reference and were able to show that the T1r values appeared to stabilise in the anterosuperior quadrant post-operatively. This in vivo evidence suggests that there was a local process responsible for mechanical overload as opposed to a systemic factor driving the degenerative process of FAI. Change in T1r values appeared to correlate with the clinical improvement noted on the functional scores post-operatively, again with the caveat that this is just ten patients in the report. The reduction in bone mineral density (BMD) in the anterosuperior quadrant was an interesting observation noted post-operatively. As the authors suggest, the increase in BMD in the anterosuperior rim is thought to be due to the early degenerative change but also to the repetitive impact of a cam against the acetabulum. The reduction in the BMD post-operatively would therefore suggest that the abnormal mechanical load had been addressed by the surgical correction. One can only speculate as to whether patients with asymptomatic cams also have an increased BMD in the anterosuperior rim. However, previous studies have suggested that there is a strong correlation between the severity of the cam deformity and the acetabular BMD. Changes in the BMD and proteoglycan content appeared to occur in the same regions of interest, which may further suggest the role of increased BMD (subchondral bone ‘stiffening’) and cartilage degeneration. Perhaps the increase in BMD relates to the altered biomechanics and the body’s attempt to protect the hip from the high compression load applied by the impingement. This is an extremely thought-provoking study that highlights some of the huge advances in articular cartilage imaging in the last few years, and suggests some possible objective evidence on how hip preservation surgery can improve hip biomechanics. There must be some caution exercised in interpreting data on such a small sample size; however, to recruit widespread support for hip preservation surgery, it is this type of objective evidence, along with a durable post-operative clinical improvement, that is needed.

How many cultures to diagnose total hip and knee arthroplasty infections?

This is a great paper for all those involved in the diagnosis of infection in the periprosthetic joint. The authors from Philadelphia, Pennsylvania (USA) set out to establish how many specimens are required to rule out, or rule in, deep tissue infection effectively.⁴ The difficulty, of course, is that as the number of cultures increases, although the sensitivity also increases, so too does the false positive rate. These authors based their paper on 113 consecutive cases of infected total hip and knee arthroplasties. The revisions were known to be infected, and the cultures undertaken at the time of revision surgery form the basis of this report. Overall, 85% of patients (n = 63/74) had a positive culture from samples taken at the time of surgery. The odds of a positive culture from fluid were 75% while the odds of tissue cultures yielding a positive result were 67%. The authors identified the optimal number of cultures needed to yield a positive result as four (but still with limited specificity = 0.61 and sensitivity = 0.63). Increasing the number of samples would increase sensitivity but at the cost of reduced specificity.