Neck fracture following total hip arthroplasty is a rare complication. Neck fractures are expected to occur more frequently, as more and more young patients will undergo THA. Most of these patients will have their hip stem in place for a long period. Young patients receiving THA will probably have to receive revision at some time point. The systematic review provides an overview on the available literature, in both primary and revision cases. Neck fractures occurred on average 7 years (SD 4.2 years) after hip stem placement. Several risk factors, both in head-neck fractures and neck-shoulder fractures, were identified.
Neck fracture is etiologically multifactorial and this study identified some potential risk factors. In the head-neck region group, (mechanically-assisted crevice) corrosion and the use of cobalt-chromium long-skirted or large-size femoral heads were frequently reported. Trunnionosis in the head-neck group might play an important role. Neck-shoulder fracture was frequently associated with some specific implant-related characteristics, such as introducer holes, sharp etches and laser etching. Several implant-, patient-, and surgeon-related factors could increase the risk of neck fractures. Two fractures were located in mid-neck and these fractures were added into the neck-shoulder group [9, 10]. It must be mentioned that this classification is arbitrary.
The most commonly reported implant-related risk factors were the use of large-size heads (> 40 mm), skirted heads, corrosion and design flaws. Crevice corrosion is caused by a cascade starting with fretting at the head-neck coupling leading to wear and disruption of the passive oxide layer. Severe corrosion caused by fretting results in reduced contact between the head-neck connection and leads to channels for fluid ingress followed by a stagnant body fluid in the crevice. Here, a chemical reaction takes place, forming hydrogen chloride. The hydrogen chloride decreases the local solution pH, damaging the metal, causing loss of material and pits at the trunnion . The material loss could possibly lead to a fatigue fracture of the neck. Previous studies have identified several risk factors for corrosion. An important risk factor for corrosion was the use of cobalt-chromium alloy femoral heads on a titanium or stainless steel trunnion. Growing evidence in literature showed that ceramic heads reduced this risk . Nonetheless, cobalt-chromium alloy femoral heads are still most frequently employed in total hip arthroplasty. In the last decade in Europe, there has been a trend towards the use of ceramic heads. Thereby, the trunnion material plays an important role. Less rigid titanium alloy stems (e.g., Accolade TMZF stem (Stryker Orthopedics, Mahwah, New Jersey, USA)) were introduced to decrease stress shielding and femoral bone loss around prosthesis . However, a less rigid trunnion leads to more micromotions, corrosion and gross trunnion failures . Other implant-related risk factors were laser etching of the neck, introducer/extraction holes, metallurgic flaws, small-diameter necks and sharp corners at the neck.
Patient-related risk factors are correlated to more intense use of the implant. The included patients had a relatively young age of 65 years at the time of fracture and most had an active lifestyle, leading to more micromotions and more crevice corrosion at the head-neck interface . Also, male gender, high body weight and greater BMI were patient-related risk factors for implant fractures.
One study reported that damage caused by the Hohmann retractors during revision surgery might have caused damage to the trunnion, leading to fracture. No other specific surgeon-related risk factors were mentioned in the other included studies.
In primary THA, the femoral head should be assembled well-centered on a clean and dry taper [13, 14]. Contaminated trunnions decrease the torsional resistance and increase fretting . Ceramtec (CeramTec, AG, Plochingen, Germany) advises use of a single moderate hammer blow, which was in contrast to recent studies advising use of higher assembly forces to decrease wear [13, 16]. Higher assembly forces could increase the stability of the head-neck junction, but might damage the ceramic head. Generally, a 4 kN blow is advised.
In revision surgery, coaxial removal of the head, cleaning of the trunnion and removal of all corrosive products are important. Corrosive products on the trunnion will lead to a suboptimal head fit, causing micromotions and crevice corrosion. Placing back ceramic heads with a taper sleeve on damaged trunnions will lower the risk of ceramic fracture. During revision surgery, the femoral neck should be protected from scratches by the surgical instruments.
Moreover, the choice of material is an important factor in preventing corrosion. The use of ceramic heads instead of cobalt-chromium heads leads to lower wear rates . Mixing and matching of components from different brands is discouraged by the manufacturers, because it could lead to a trunnion-head mismatch and thereby increased fretting and wear. Remarkably, a Dutch database study in 2016 did not find any differences in medium-term revision rates in the mixed-component groups . However, the National Joint Registry of England and Wales showed higher failure rates if a head and a femoral stem from different manufacturers were used . Fallahnezhad et al have shown a decreased torsional strength of the head-neck junction in the case of angular mismatch between the head and the neck connection . Also, Mueller et al concluded that using head and necks from different manufacturers could lead to less taper connection strength . However, with Corail (DePuy Synthes, Warsaw, IN, US) and Bicontact (B. Braun Aesculap, Tuttlingen, Germany) stems, mixing and matching led to a stronger connection, which was possibly attributed to better taper angles and higher quality femoral heads from competing companies . Bitter et al found that taper mismatches, which could be caused by mixing and matching, led to more wear than a perfect fit, especially a tip fit is increasing the amount of wear . Generally, mixing and matching different components could lead to an unstable head-neck connection and therefore we advise using head and stems from the same manufacturer to ensure a stable situation and reduce fretting and wear at the head-neck junction.
First, the low quality of evidence of the included studies did not allow us to perform statistical analysis. Few studies offered detailed information on the implant characteristics and microscopic analysis, which was not conducted in all cases. Another limitation of this review is that the conclusions are limited due to the relatively small number of identified cases. Finally, the restriction of our systematic review to include English, German, French and Dutch language studies may have resulted in language-related bias.