Avascular necrosis of the femoral head (ANFH) is a relevant disease of the hip joint, and usually affects patients in the third to fifth decade of life and men are affected about five times as often as women (Merschin et al., 2018). Approximately 5000 to 7000 and 10,000 - 20,000 new patients with ANFH are diagnosed each year in Germany and in the United State, respectively (Roth et al., 2016, Aldridge and Urbaniak, 2004). This pathology results from disruption of the subchondral blood supply to the femoral head bone and is one of the main reasons for total hip arthroplasty (THA) (Adelani et al., 2013; Drescher et al., 2021). The treatment options including core decompression, osteotomies, nonvascularized and vascularized bone grafting, and joint reconstruction, depend on disease stage, the lesion extend and site, the presence of unilateral or bilateral involvement, as well as patient age and general health (Steinberg et al., 1996, Assouline-Dayan et al., 2002, Mont et al., 2006). Due to the limited survival time of hip arthroplasties, hip preserving surgical techniques should be preferred for treatment of ANFH in younger patients. For treatment of early stage ANFH, core decompression (CD) represents a surgically simple and minimally invasive method and is the most commonly used. CD is performed by drilling one or multiple channels into the necrotic lesion from the lateral subtrochanteric region of femur. The procedure may be combined with either nonvascularized (Keizer et al., 2006, Helbig et al., 2012) or vascularized bone grafting (Meizer et al., 2005, Babhulkar, 2009). Reported success rates of core decompression are up to 88 %, 77 % and 73 % for stage I, II and III ANFH according to the Ficat classification, respectively (Stulberg et al., 1991, Fairbank et al., 1995, Song et al., 2007, Atilla et al., 2019).

Favourable clinical outcomes in terms of lower revision rates have been described in the literature for the so-called "Trapdoor procedure". A 2 × 2 cm window is created from the articular cartilage in the femoral head over the necrotic area allowing for necrotic bone removal with a high-speed burr. The lesion is then filled with a bone graft, and the window is replaced and fixed with absorbable pins. In 10 hips for a cohort of 9 teenagers treated by the trapdoor procedure combined with containment osteotomy and after a mean follow-up of 53 months, Ko et al. (1995) reported none of the hips required further procedures. Mont et al. (1998) assessed the use of this technique in 30 patients who had Ficat stage III or IV ANFH, and showed promising results, with good or excellent outcomes in 86 % of the hips after a mean follow-up of 56 months. Rosenwasser et al. (1994) described the lightbulb technique that is nearly identical to the trapdoor procedure, in which the removal of necrotic lesion and bone grafting were performed through a cortical bone window made at the femoral head-neck junction. They observed a hip survival rate of 87 % after a mean follow-up of 12 years in a cohort of 13 patients who had Ficat stage II or III disease. Mont et al. (2003) found similar success rate with the lightbulb technique, reporting 86 % of 21 hips with Ficat stage II or III ANFH were clinically successful at a mean follow-up of 48 months. However, the disadvantage of the trapdoor and lightbulb procedures compared to core decompression is that they are more invasive and time-consuming. Furthermore, full weight bearing is not allowed until after 3 months (Mont et al., 1998, Mont et al., 2003, Landgraeber et al., 2017, Yildiz et al., 2018, Landgraeber and Jäger, 2020).

Advanced Core Decompression (ACD) is a relatively new option for minimally invasive treatment of ANFH that is based on the conventional core decompression technique, but is performed by using a percutaneous expandable reamer. This technique consists of removing the necrotic tissue and subsequently refilling the defect and drill canal with the resorbable bone graft substitute PRO-DENSE® (Wright Medical™, Arlington, TN, USA) (Civinini et al., 2012, Landgraeber et al., 2013, Landgraeber et al., 2015, Tran et al., 2014, Tran et al., 2016, Tran et al., 2022). The advantage of ACD is that patients are able to bear weight at an early postoperative stage, with a low risk of femoral fracture. In a cohort of 72 hips with Steinberg stage 1, 2, or 3 ANFH, Classen et al. (2017) showed a mid-term hip survival rate of 67 % which was nearly the same as the survival rate of 65 % reported for conventional CD (Mont et al., 1996, Mont et al., 2006). The main reasons for those ACD results were not superior to those of the conventional CD may be interpreted as: i) a complete removal of necrotic lesion was not achieved (Landgraeber et al., 2015, Classen et al., 2017), and ii) the bone graft PRO-DENSE® was only osteoconductive and too stiff that did not enhance osseointegration (Tran et al., 2016). On the first attempt to improve clinical outcomes of ACD, the technique was modified by using autologous bone derived from the femoral neck instead of the synthetic bone graft PRO-DENSE®. After a mean follow-up of 30 months for 29 hips with Steinberg stage 2 ANFH, an overall success rate of 75.9 % was reported by the modified ACD which seems to be favorable to those of the original one (Landgraeber et al., 2017). This preliminary result encourages us to further modify ACD technique. Since the success of the trapdoor and lightbulb procedures is most likely attributed to the complete removal of the necrosis (Landgraeber et al., 2017), we propose to combine ACD with the lightbulb procedure to get advantages of both methods, in which ACD is performed from an entrance point at the femoral head-neck junction. This study aims at evaluating the biomechanical behavior of the femora treated by the combined Lightbulb-ACD technique (LACD) as the basis for clinical application. To do this, we developed subject-specific finite element (FE) models of femora that are able to predict the femoral fracture risk under daily activity loading conditions. Biomechanical testing was additionally performed to confirm the FE results.