The mean difference between intra-observer measurements ranged from -2.3 mm to 1.3 mm (95% CI -3.8 – 3.8 mm) and the mean difference in measured neck-shaft angle was -2.3° (95%
CI -3.8 – -0.8°). The mean difference between inter-observer measurements ranged from -0.2 mm to 4.0 mm (95% CI -1.7 – 5.8 mm) and the mean difference in measured neck-shaft angle was -2.5° (95% CI -5.7 – 0.7°). Pitman’s test revealed that there were no statistically significant differences in agreement and the reliability of intra- or inter-observer measurements.
6 Discussion
THA is one of the most successful interventions in modern medicine. The rapid development of hip surgery in the 20th century with continuing new innovations has associated to an enormous amount of publications and knowledge concerning hip surgery. The restoration of anatomy and biomechanics has been a fundamental goal of THA. To achieve this goal, a number of improvements in surgical technique, implant design and postoperative rehabilitation have increased the effectiveness and success of this procedure. Over one million primary and revision hip arthroplasties are performed worldwide annually and operations are performed in more active and younger patients than ever before (Pivec et al. 2012). The age profile of patients with THA has changed in the last two decades from elderly to late middle-aged. Surgeons performing THA are therefore facing the challenge of meeting high patient expectations and ensuring excellent outcomes while at the same utilising evidence-based and cost-effective implants and techniques in every day practice. A good overview of the current concepts in THA scenery is necessary when the surgery is trying to fill these requirements in every clinical practice.
LDH MoM articulation is a good example of a new technological innovation which was brought into the field of hip replacements with great expectations. Hip surgeons were focused on the improvements in functional outcomes and the longevity of these implants, so awareness of the possible complications related to MoM bearings was put aside at the same time. The introduction of MoM THAs in the late 1990s was performed in a rush and the amount of THAs implanted yearly was trending upwards year after year until it declined sharply in the early 2010s due to serious complications noticed by national and international arthroplasty registers (MHRA 2010, Smith et al. 2012, AOANJRR 2014). National and international orthopaedic organisations announced guidelines stating that LDH MoM THA and HRA device use should be discontinued (MHRA 2010, AOANJRR 2014, SAY 2015).
Cementless femoral components have become the preferred standard for femoral fixation in the United States due to their excellent durability and survivorship at the ten- to twenty-year follow-up (Lombardi et al. 2009). As a result of this success, approximately 60–90% of the THAs performed yearly in the United States use cementless femoral components (Khanuja et al. 2011). The survivorship of cementless femoral components has been excellent, varying from 88% to 91% at the ten-year follow-up (Carlson et al. 2017). The use of cementless acetabular components has gained popularity over the past decade, despite the fact that several publications have shown inferior survivorship compared to cemented acetabular components (Pakvis et al. 2011, Mäkelä et al. 2014). The reasons for the increased use of cementless acetabular components are their straightforward implantation, the ability to adjust component position and the variety of bearing and liner options. However, overall survival of cemented implants for THR is higher in patients aged over 65 years (Mäkelä et al. 2014). Recent hip register data from the National Joint Registry of England and Wales shows that today the most commonly used used operation type overall is cemented MoP with rate of 30.4% of all primary THRs (NJR 2017). The Finnish arthroplasty register shows that use of cemented THR in Finland has decreased in last ten years from 40% (year 2006) to 10% (year 2016) and cementless implants are nowdays the most common THR with rate of 59% in year 2016 (FAR 2016).
Common reasons for THR failure are mechanical failures, such as femoral neck fracture in the HRAs, and bearing-related problems such as soft tissue reactions and osteolysis (Berry 2002). However, study heterogeneity, inconsistent outcome definitions and non-standardised outcome measures make the comparison of arthroplasty studies a challenge (Marshall et al.
2014). In a systemic review, Marshall et al. showed that revisions are more frequent and occur
earlier in HRA and that the most common complications related to HRA are fractures, heterotopic ossification and component mismatch (Marshall et al. 2014). Common reasons for the revision of THA are heterotopic ossification, osteolysis, excessive polyethylene wear and infection (Marshall et al. 2014).
In this study we examined a few of the most common complications related to cementless THA in more detail. These studied complications were intraoperative calcar fractures, early aseptic loosening of cementless acetabulum components and dislocations of LDH MoM THAs and HRAs.
6.1 MIDTERM SURVIVAL (I)
Short and midterm results of LDH MoM bearings were excellent and LDH gave inherent stability of the articulation (McLaughlin et al. 1997, Treacy et al. 2011). However, studies with longer follow-up periods have shown high failure rates for MoM bearing THAs (Daniel et al.
2004, Berend et al. 2010, Carrothers et al. 2010, Langton et al. 2010, Mäkelä et al. 2010, Varnum et al. 2015). Our study findings are similar to those previous studies, while our mid-term revision rates are comparable to those of previous studies in which the prevalence of revision for any reason varied from 1.7–4.1% (Jameson et al. 2012, Sidler-Maier et al. 2015). In our study, the revision rate was higher in the HRA group (6.3%) than in the THA group (4.5%) and the results showed that HRAs had a 1.5-times higher hazard ratio for revision than LDH THAs. In a recent study based on data from the Nordic Arthroplasty register at the six-year follow up, the risk rate of revision for any reason was 1.5-times higher for MoM bearings compared with metal-on-polyethylene bearings (Varnum et al. 2015). It has been previously shown that female gender is a risk factor for early revision in MoM arthroplasties (Jameson et al. 2012). National Joint Registry for England and Wales has report inferior survivorship of HRAs in females and reason for this seems to be smaller implant head size compared to males (Smith et al. 2012). Our study findings support this, as revisions in both groups were performed more often in females with smaller implant head size than males. Interestingly, the specific component brand had no effect on the risk of revision or intraoperative complications in the THA or HRA groups.
ARMED has been the most common reason for the revision of LDH MoM THA and HRA at the 8-year follow-up in Australia and also in other studies with smaller sample sizes (Daniel et al. 2004, Berend et al. 2010, Carrothers et al. 2010, Langton et al. 2010, Mäkelä et al. 2010, AOANJRR 2014, Varnum et al. 2015). Although both LDH THA and HRA have large head size MoM bearings, LDH THAs have had higher revision rates due to ARMED, probably caused by corrosion at the taper junction (Carrothers et al. 2010). In our study, the most common reason for revision in the THA group was prosthetic fracture (1.2%). The number of peri-prosthetic fractures was comparable between ours and other previous studies in which the incidence of postoperative fracture varied between 0.07% and 18% (Kennedy et al. 1998, Langton et al. 2008). The second most common reason for revision in our study was aseptic loosening of either component (1.7%). Our results are similar to those of the NARA study, in which aseptic loosening was the reason for revision in 1.9% of cases (Varnum et al. 2015).
Fracture of the femoral neck was the most common reason for revision in the HRA group, with a prevalence of 1.7%. In previous studies, the prevalence of femoral neck fracture varied from 0.96–1.98% (Berry 2002, Langton et al. 2008, Langton et al. 2011).
In the radiological analysis, we noticed that revised THAs and HRAs had smaller anteversion angles and larger inclination angles than surviving THAs and HRAs. Previously, it was shown that component orientation is important to ensure optimal survivorship of THA patients and our results support this finding (Malviya et al. 2010). Measurements of the anteversion and inclination angles include all studied components, as well as acetabular
components which loosened early, which caused a scattering of the results and can be seen in the dot plot chart (Figure 16). Acetabulum component malposition is a factor that contributes to a higher dislocation rate, edge loading, excessive metal surface wear, and pseudotumours (Kennedy et al. 1998, Langton et al. 2011). The increased inclination and anteversion angles have also been shown to correlate with higher serum metal ion levels following MoM arthroplasties (Langton et al. 2008, Langton et al. 2010, Langton et al. 2011).