Patient and surgical variability in the primary stability of cementless acetabular cups: A finite element study

Abstract

Aseptic loosening is the most common indication for revision of cementless acetabular cups and often depends on the primary stability achieved following surgery. Cup designs must be capable of achieving primary stability for a wide variety of individuals and surgical conditions to be successful. Typically, preclinical finite element (FE) testing of cups involves assessing the performance in a single patient and under a limited set of idealized conditions. The aim of this study was to assess the effect of patient and surgical parameters on the primary stability of an acetabular cup design in a set of subject‐specific FE models. Interference fit was varied in a representative set of 12 patient‐specific models of the implanted hemipelvis. Linear mixed models showed a significant association with micromotion for interference fit (P < .0001), acetabular bone elastic modulus (P < .001), native acetabular diameter (P = .03), and the interference fit‐elastic modulus interaction (P = .01). There were no significant associations between the polar gap and any of the parameters considered. The significant interference fit‐elastic modulus interaction suggests that increasing the interference fit in patients with low bone quality leads to a greater reduction in micromotion than in patients with higher bone quality. However, the significant association between percentage bone yielding and interference fit (P < .0001) suggests a higher periacetabular fracture risk at higher interference fits. This work supports the development of preclinical testing of cup designs for the broad range patients and surgical conditions a cup may face following surgery.

Determinants of the primary stability of cementless acetabular cup implants: A 3D finite element study

“…Polar gaps values are found to be between 0 and 2483 µm in this study, which is in agreement with the findings of other clinical [32,42,49], experimental [26,29] and FE studies [1,40,54]. The polar gap is found to increase with larger interference fit [29], as well as with bone stiffness and friction coefficient.…”

“…Furthermore, it is commonly used as a determinant of primary stability in in vitro studies [13,18,21,27]. Moreover, the polar gap between the ACI and the hip is also frequently used in clinical studies [21,30,40] in order to assess the ACI stability. Large gaps indicate improper seating of the ACI and can affect the quality of long-term fixation due to the formation of fibrous tissue in the peri-implant region [5], the formation of low-quality bone tissue or even inhibit bone growth [22,32,41].…”

“…Several studies suggest that the interference fit is one of the most important factors in order to achieve adequate fixation [10,28,47] and that increased under-reaming can compensate low bone stiffness [21,40]. As low interfacial friction may destabilize the implant [21] and thus has to be avoided, an adequate interference fit according to the bone quality must be chosen instead.…”

“…Consequently, more accurate models of the human pelvis have been developed to model the contact behavior of the ACI. These models can be subdivided into two categories: case-specific modeling [19] and multi-factorial studies [1,2,18,21,40]. A case-specific model is usually used to investigate a unique situation e.g., a specific condition or geometrical anomaly, whereas a multifactorial study aims to generate various cases representative of a population and may require a large number of analyses [8].…”

The limit of tolerable micromotion for implant osseointegration: a systematic review

“…Failed osseointegration leads to aseptic implant loosening, implant failure and the need for expensive revision surgery, both financially and in terms of quality of life 12 . Thus, much research time and resource has been invested to ensure new implant designs and surgical techniques result in acceptable/improved micromotion at the bone-implant interface 13 – 22 . There even exists an ASTM standard (F2537–06) to ensure micromotion is measured accurately and repeatably.…”