Francesc Angles MD1, Jorge H. Nuñez MD2 and Pablo Castillón MD1 1 Hospital Mutua de Terrasa. Departament Especialitats Medicoquirúrgiques. Universitat de Barcelona, Spain 2 Vall d’Hebron University Hospital, Barcelona, Spain Unstable THA should not be treated with any special device before knowing the etiology of instability. Larger femoral head size improves stability by increasing the jump distance. Routine use of large‐diameter femoral heads have become more popular because of the associated improvement in stability.1 Femoral head size has increased over time, from 22 mm heads in the 1960s to 36 mm heads in the last decade. According to the majority of the registries, the most common femoral head diameters are 32 and 36 mm.2 Femoral head size has also been shown to improve range of motion and function.3 The quality of literature addressing the effect of the bearing surface and the head size is highly variable with level I–IV evidence. There are some randomized trials; however, the majority of the outcome papers are multicenter cohort studies or single‐center cohort studies and in vitro studies. Larger heads increase impingement‐free range of movement between components and have the ability to offer longer neck options improving the possibility to obtain adequate soft tissues tension.4 Cinotti et al. reported the effect of head size on impingement in both optimally and nonoptimally positioned acetabular components, and found limited benefits to increasing head size beyond 32 mm.5 Howie et al. in a randomized controlled trial (RCT) demonstrated a significantly lower dislocation rate at one year for the 36 mm head group (0.8%) compared to a 28 mm head (4.4%) in primary THA.6 Another RCT which included 32, 36, and 40 mm heads concluded at five years after surgery that a larger femoral head group had a significantly lower risk for dislocation.7 Kostensalo et al., with data obtained from the Finnish Arthroplasty Register analyzed 4379 primary THA procedures concluded that 32 mm, 36 mm, and >36 mm were associated with a lower risk of revision due to dislocation compared with 28 mm heads.8 The Dutch Arthroplasty Register reported a 58% higher risk of revision due to dislocation for THA performed with 22–28 mm head compared with 32 mm.9 In the setting of the Nordic Arthroplasty Register Association database, Tsikandylakis et al. analyzed 186 231 metal‐on‐polyethylene THA (head size 28 mm, 32 mm, or 36 mm). They found in an adjusted Cox regression mode that patients with 28 mm heads had a higher risk of revision for dislocation (hazard ratio [HR] = 1.67; 95% confidence interval [CI]: 1.38–1.98) compared with 32 mm, whereas there was no difference between patients with 36 mm (HR = 0.85; 95% CI: 0.70–1.02) and 32 mm heads.10 Over the last years, the use of large‐diameter replacement femoral heads in THA has increased.2,11 Large femoral heads provide a wider impingement‐free range of motion and also increase the jump distance, improving stability and reducing the risk of dislocation.12 However, one of the main concerns when it comes to larger femoral heads is the longevity of the bearing surface. Bearing wear and head size cannot be examined irrespective of the bearing surface as different materials have different bearing friction properties. The conventional polyethylene has a greater risk of wear, but the relatively recent development of hard‐on‐hard bearings and the introduction of cross‐linked polyethylene has led to the revision of this concept. The quality of the literature addressing bearing wear is variable with level I–IV evidence. Most are cohort studies, national hip arthroplasty registries, and in vitro studies. The success of THA has been limited by periprosthetic osteolysis related to particulate polyethylene wear debris, but highly cross‐linked polyethylene (XLPE) was developed to decrease polyethylene wear and decrease osteolysis. Engh et al. conducted a prospective, randomized study of 236 patients (XLPE group: 116 patients/non‐XLPE group: 114 patients) and concluded that the XLPE liners have a95% wear rate reduction compared with the mean wear rate of the non‐XLPE, and the incidence of osteolysis was lower in the XLPE group.13 Assuming the outer diameter of the acetabular shell is kept the same, larger diameter bearings require accordingly thinner polyethylene liners. Johnson et al. considered that the minimum thickness could be reduced to 3.9 mm with XLPE,14 but in another study, by Girard et al., it was concluded that given the current data on wear and fatigue resistance surgeons should comply with the traditional 6 mm thickness, even with XLPE liners.15 Wear, in relation to larger femoral heads, could be linear and volumetric. It has been shown that linear wear rates of less than 0.1 mm per year have been associated with a low incidence of osteolysis, and in long‐term clinical studies no differences have been demonstrated in linear wear rates between 26, 28, 32, 36 and 40 mm heads when metal‐on‐cross‐linked polyethylene (MoXLPE) are used.16,17 However, surgeons and manufacturers should focus on decreasing volumetric wear. Currently, there is no agreed‐on threshold with respect to volumetric wear rates and osteolysis. Cross et al. proposed that a volumetric wear of 40 mm3/yr could eliminate osteolysis and up to 80 mm3/yr could be tolerated.18 Lachiewicz et al. in 2009 found no association between femoral head size and the linear wear rate, but observed an association between larger (36 and 40 mm) head size and increased volumetric wear rate and total volumetric wear.19
27 Head Size in Total Hip Arthroplasty
Clinical scenario
Top three questions
Question 1: In patients undergoing THA, does larger femoral head size, compared to smaller head size, result in improved stability?
Rationale
Clinical comment
Available literature and quality of the evidence
Findings
Resolution of clinical scenario
Question 2: In patients undergoing THA, do certain bearing couples, compared to others, result in better outcomes depending on femoral head size?
Rationale
Clinical comment
Available literature and quality of the evidence
Findings
Polyethylene bearing
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
