Total hip arthroplasty

Indications for THA are guided by pain, functional impairment and radiographic findings (NIH 1995). However, no international consensus exists for surgical indications. Optimum surgical results are obtained through careful patient selection. Risk factors for complications should be evaluated, along with the benefits of THA, and patient expectations of the procedure should also be evaluated. Hip replacements are being performed in more active and younger patients than ever before, while life expectancy of patients is increasing at the same time. This combination means that the total number of THAs is raising worldwide on a yearly basis (Pivec et al. 2012).

There are four different fixation options for THA: 1) fully cementless design of both the femur and acetabulum components, 2) fully cemented design of both components, 3) hybrid design with a cemented stem and cementless acetabulum component, and 4) reverse hybrid design with a cemented cup and cementless stem. Bearing surface options for the THA components are MoP, ceramic-on-ceramic, metal-on-ceramic and MoM while bearing surfaces of the HRA are MoM (Figures 2 and 3).

Figure 2. Cementless metal-on-metal total hip arthroplasty components (Biomet M2a38 acetabulum component and Biomet BiMetric stem; Biomet, Warsaw, IN, USA).

Figure 3. Hip resurfacing arthroplasty components (Acetabulum component: Biomet ReCap;

Biomet, Warsaw, IN, USA and femoral stem: BioPro Resurfacing, Port Huron, MI, USA).

THA has shown good and excellent long-term outcomes, but the revision surgery burden has remained unchanged over the last several decades (Callaghan et al. 2000, Berry et al. 2002, SHAR 2015, AOANJRR 2016). The Charnley low-friction THA was the first device distributed worldwide and had excellent survivorship (78%) at the 35-year follow-up (Callaghan et al.

2000). Cemented THA survivorship has reached 83% at the 20 to 25-year follow-up (Petheram et al. 2016). Cementless THA have been used most often, with favourable survival results of 60–95% depending on the component used and the bearing surfaces and follow-up times (SHAR 2011, AOANJRR 2016). Some cementless stem designs have survivorship as high as 95% at the 20-year follow-up (Khanuja et al. 2011). The 10-year hybrid implant survival has been reported to be 91.6% in patients aged 65 to 74 and 93.9% in patients 75 and older at the 10-year follow-up (Mäkelä et al. 2014). Reverse hybrid survival rates have been reported to be 94% at the 10-year follow-up (Wangen et al. 2017).

Restoration of the normal anatomy of the hip at the THA provides better clinical function and abductor strength and reduces wear debris (Sakalkale et al. 2001, Yamaguchi et al. 2004).

Failure to restore the normal anatomy following THA is a risk for complications. Typical complications associated with the failure of restoration are dislocation, muscle weakness, leg-length discrepancy, impingement and early loosening of the implant (Kelley 1994, Asayama et al. 2005, Konyves et al. 2005). To improve the precision of the anatomical reconstruction, the size of the femoral head was increased and modularity of the components was introduced in the late 1990s.

2.3.1.1 Large diameter head metal-on-metal total hip arthroplasty

The first MoM articulation in THA was used in 1938 by Wiles (Wiles 1958). The McKee-Farrar prosthesis was the first cemented MoM THA in the early 1960s with acceptable results (McKee et al. 1966). The McKee-Farrar MoM THA had a higher early failure rate than Charnley MoP articulation THA, however at 20 years follow-up the survival rates were almost similar, with 77% (MoM) and 73% (MoP), respectively (Jacobsson et al. 1996). MoM THAs were widely used in the 1960s, but were later phased out in the mid-1970s and replaced by MoP bearings. The reason for this change was the higher loosening rates for MoM bearings compared to MoP bearings. There were also concerns about biological reactions related to alloy constituents (Benson et al. 1975, Dumpleton et al. 2005). Large diameter head hip resurfacing arthroplasty THA using Cobalt-Chromium (CoCr) articulation was re-introduced in early 1990s and became popularised in the late 1990s (McMinn et al. 1996, Mont et al. 2008, Malviya et al. 2010) (Figures 2 and 4). One reason for the increased use of LDH HRA and later also THA MoM bearings was the modest long-term results associated with the MoP bearings due to wear and osteolysis, especially among young and active individuals. Other reasons for the popularity of LDH THA and HRA were based on the proposed increased functional ability as a consequence of the larger head size and reduced risk of dislocation.

Figure 4. Large head MoM total hip arthroplasty in postoperative antero-posterior (1) and medio-lateral (2) native radiographs.

Problems related to MoM bearings include peri-articular fluid collections, soft tissue masses and gluteal muscle necrosis (Ollivere et al. 2009, Langton et al. 2011); these findings are called ARMD. The factors affecting the amount of wear debris are component size and positioning, diameter clearance, which allows fluid to lubricate between bearing surfaces, and the roughness of the surfaces (Mont et al. 2007). Metal ions and wear debris are potentially mutagenic, but because of the long latency period of cancers it is not possible to draw robust conclusions (Malviya et al. 2010). There is some evidence that the high levels of CoCr could cause hematopoietic malignancy (Wagner et al. 2012). Patients exposed to high circulating concentrations of cobalt may rarely develop neurological damage, hypothyroidism and cardiomyopathy which may not resolve even after removal of the implant (Bradberry et al.

2014). Large amounts of metallic wear debris lead to synovitis and osteolysis (Shimmin et al.

2008, Malviya et al. 2010). Further on, soft tissue reactions like ARMD may cause a failure of the implant due to the formation of peri-articular masses called pseudotumours (Langton et al.

2010, Bosker et al. 2012). This wear process of MoM prostheses can be monitored by following

systemic cobalt and chromium ion levels in serum and whole blood because elevated ion levels are associated to local ARMD and pseudotumours (De Pasquale et al. 2014). It is recommended to follow cobalt and chromium ion whole blood levels in every second year and if they raise over 5 µg/l, a metal artefact-reducing sequences magnetic resonance imaging (MARS-MRI) should be utilized to detect pseudotumours and ARMD (Lainiala et al. 2014, SAY 2015, Kwon et al. 2016). There are common indications for revision surgery following MoM THA including periprosthetic fracture, implant loosening, pseudotumour and pain, however threshold for revision surgery due ARMD and elevated whole blood metal ion levels is to some extent unclear (Haddad et al. 2011, Matharu 2017). Guidelines for MoM follow-up are evolving continually, however economical aspects should be also considered when choosing the screening level (Reito et al. 2016).

The number of LDH THAs and HRAs increased rapidly in the late 2000s. In 2008 approximately 35% of all hip replacements in the United States had MoM articulation while in the United Kingom HRA represented approximately 10% of all implanted hip devices (Bozic et al. 2009, NJR 2012). The use of MoM bearings decreased sharply in the early 2010s due to serious complications noted 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).